Van Schmus et al 2003, Notas de estudo de Geologia
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Van Schmus et al 2003, Notas de estudo de Geologia

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The Seridó Group of NE Brazil, a late Neoproterozoic pre- to syn-collisional basin in West Gondwana: insights from SHRIMP U2013Pb detrital zircon ages and Sm2013Nd crustal residence (TDM) ages
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doi:10.1016/S0301-9268(03)00197-9

Precambrian Research 127 (2003) 287–327

The Seridó Group of NE Brazil, a late Neoproterozoic pre- to syn-collisional basin in West Gondwana: insights from SHRIMP

U–Pb detrital zircon ages and Sm–Nd crustal residence (TDM) ages

W.R. Van Schmus a,∗, B.B. de Brito Neves b, I.S. Williams c, P.C. Hackspacher d, A.H. Fetter a, E.L. Dantas a,d, M. Babinski a,b

a Department of Geology, University of Kansas, Lawrence, KS, USA b Inst. Geociências, Univ. de São Paulo, São Paulo, SP, Brazil

c Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia d Inst. de Geociências e Ciências Exatas, Univ. Estadual Paulista, Rio Claro, SP, Brazil

Accepted 27 June 2003

Abstract

The Seridó Group is a deformed and metamorphosed metasedimentary sequence that overlies early Paleoproterozoic to Archean basement of the Rio Grande do Norte domain in the Borborema Province of NE Brazil. The age of the Seridó Group has been disputed over the past two decades, with preferred sedimentation ages being either Paleoproterozoic or Neoproterozoic. Most samples of the Seridó Formation, the upper part of the Seridó Group, have Sm–Nd TDM ages between 1200 and 1600 Ma. Most samples of the Jucurutu Formation, the lower part of the Seridó Group, have TDM ages ranging from 1500 to 1600 Ma; some basal units have TDM ages as old as 2600 Ma, reflecting proximal basement. Thus, based on Sm–Nd data, most, if not all, of the Seridó Group was deposited after 1600 Ma and upper parts must be younger than 1200 Ma.

Cathodoluminescence photos of detrital zircons show very small to no overgrowths produced during ca. 600 Ma Brasil- iano deformation and metamorphism, so that SHRIMP and isotope dilution U–Pb ages must represent crystallization ages of the detrital zircons. Zircons from meta-arkose near the base of the Jucurutu Formation yield two groups of ages: ca. 2200 Ma and ca. 1800 Ma. In contrast, zircons from a metasedimentary gneiss higher in the Jucurutu Formation yield much younger ages, with clusters at ca. 1000 Ma and ca. 650 Ma. Zircons from metasedimentary and metatuffaceous units in the Seridó Formation also yield ages primarily between 1000 and 650 Ma, with clusters at 950–1000, 800, 750, and 650 Ma. Thus, most, if not all, of the Seridó Group must be younger than 650 Ma. Because these units were deformed and meta- morphosed in the ca. 600 Ma Brasiliano fold belt during assembly of West Gondwana, deposition probably occurred ca. 610–650 Ma, soon after crystallization of the youngest population of zircons and before or during the onset of Brasiliano deformation.

The Seridó Group was deposited upon Paleoproterozoic basement in a basin receiving detritus from a variety of sources. The Jucurutu Formation includes some basal volcanic rocks and initially received detritus from proximal 2.2–2.0 Ga (Transamazonian) to late Paleoproterozoic (1.8–1.7 Ga) basement. Provenance for the upper Jucurutu Formation and all of the Seridó Formation was dominated by more distal and younger sources ranging in age from 1000 to 650 Ma. We suggest that the Seridó basin may have developed as the result of late Neoproterozoic extension of a pre-existing continental basement, with formation of small marine basins that were largely floored by cratonic basement (subjacent oceanic crust has not yet been found). Immature sediment was

∗ Corresponding author. Tel.: +1-785-864-5276. E-mail address: rvschmus@ku.edu (W.R. Van Schmus).

0301-9268/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0301-9268(03)00197-9

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initially derived from surrounding land; as the basin evolved much of the detritus probably came from highlands to the south (present coordinates). Alternatively, if the Patos shear zone is a major terrane boundary, the basin may have formed as an early collisional foredeep associated with south-dipping subduction. In any case, within 30 million years the region was compressed, deformed, and metamorphosed during final assembly of West Gondwana and formation of the Brasiliano–Pan African fold belts. © 2003 Elsevier B.V. All rights reserved.

Keywords: Seridó Group; Neoproterozoic; Detrital zircons; Geochronology; Brazil; Borborema Province

1. Introduction

The Borborema Province of northeast Brazil (Almeida et al., 1981) comprises the west-central part of a Pan African–Brasiliano tectonic collage that formed as a consequence of late Neoproterozoic (ca. 600 Ma) assembly of West Gondwana (Brito Neves et al., 2000). Most of the province consists of Paleoproterozoic to Archean basement blocks with Neoproterozoic metasedimentary and metavolcanic sequences forming major fold belts within it (Fig. 1). The eastern part of the province can be divided into three major tectonic domains: the Rio Grande do Norte domain (RGND), which is north of the Patos shear zone; a central domain (transversal zone of Ebert, 1970; medial shear corridor of Trompette, 1994; or Domain Zona Transversal (DZT), of Jardim de Sá, 1994) between the Patos and Pernumbuco shear zones; and a southern domain (SD) between the Pernambuco shear zone and the São Francisco craton (SFC). Central Ceará state constitutes a fourth major domain, and the NW part of Ceará state comprises the Médio Coreau domain (cf. Fetter, 1999).

The Seridó Group is a major metasedimentary se- quence in the eastern part of the Rio Grande do Norte domain (Figs. 1 and 2); the age of the Seridó Group is important because it is a classical area for the geology and mineral resources of the Borborema Province, and it can be used to help constrain ages of regional defor- mation. For example, if the Seridó Group is relatively young, then most, if not all, of the deformation de- scribed by Hackspacher et al. (1997) must be Brasil- iano deformation. On the other hand, if the Seridó Group is relatively old (ca. 2.0 Ga), then the defor- mation may represent more than one major orogenic event.

In recent years the authors obtained Sm–Nd whole-rock and U–Pb zircon data that they interpreted in terms of a Neoproterozoic age for the Seridó Group

(Van Schmus et al., 1995a,b; Van Schmus et al., 1997). This interpretation has been in conflict with interpreta- tions of field and regional geologic data that have been used to argue for a Paleoproterozoic age (ca. 2 Ga) for the Seridó Group (Jardim de Sá, 1994, 1995; Jardim de Sá et al., 1995, 1997). In this paper we summarize our earlier whole-rock Sm–Nd data and isotope dilu- tion U–Pb zircon data and present new SHRIMP U–Pb data for detrital zircons from four localities in the Seridó Group (Van Schmus et al., 1999, 2000). These data conclusively show that the Seridó Group was de- posited in the late Neoproterozoic, probably between 650 and 610 Ma, and we will discuss the implications of this result for the regional tectonic history.

2. Regional geology

The Borborema Province (Fig. 1) can be gener- alized as (a) basement gneiss and migmatite com- plexes, (b) deformed and metamorphosed supracrustal sequences, (c) Brasiliano granitic plutons; and (d) Brasiliano shear zones. The province was highly de- formed and metamorphosed about 600 Ma after for- mation of the Brasiliano tectonic collage. In pre-drift reconstructions this province is adjacent to similar Pan African rocks and structures in western Africa (cf. Trompette, 1994; Castaing et al., 1993, 1994; Toteu et al., 1994, 2001); thus, the Borborema Province represents the western part of a large tectonic collage that occupies much of northwestern Gondwana. This paper focuses on relationships in the Rio Grande do Norte domain; the general geology of the other do- mains was summarized by Brito Neves et al. (2000).

2.1. Basement complexes

The basement complex in the Rio Grande do Norte domain is comprised of the Rio Piranhas massif in the

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Fig. 1. General geologic map for NE Brazil showing the major structures and crustal subdivisions (modified from Van Schmus et al., 1995).

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Fig. 2. A portion of Rio Grande do Norte in NE Brazil showing major geologic units, with particular reference to the Seridó Group. Modified after DNPM (1998). The numbers refer to sample localities (Appendix A) as follows: 1, 95-109, 95-110, 95-111; 2, 95-106, 95-107, 95-108; 3, 95-105; 4, 93-26, 94-80, 94-81, 94-82, 94-83, 95-104; 5, 93-33, 93-34; 6, 93-35; 7, 93-03; 8, 94-77, 95-101; 9, 94-70, 94-71, 94-72, 94-73, 94-74, 94-75, 94-76, 95-102, 95-103, SED-Gf-55; 13, 93-42, 94-84; 14, 94-86, EC-61; 15, 94-87; 16, 95-112, 95-113; 17, 94-88; 18, 94-89; 19, 93-45; 20, 93-48; 21, MVULC; 22, SED-J-4, SED-J-10, 94-90, 94-92, 94-94, 94-95. Localities 10, 11, 12, 23, and 24 are not used for this report.

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west and the São José do Campestre massif in the east (Hackspacher et al., 1990; Dantas et al., 1998). Recent U–Pb and Sm–Nd studies of these rocks (Van Schmus et al., 1995a; Dantas, 1997) show that most of the Rio Piranhas and São José do Campestre gneisses are mid- dle Paleoproterozoic (Transamazonian, 2.2–2.0 Ga), although a 2.6–3.4 Ga Archean nucleus is well docu- mented in the São José do Campestre massif, west of Natal (Dantas et al., 1998). Sm–Nd crustal residence ages (TDM) for rocks with Transamazonian crystal- lization or metamorphic ages are typically 2.4–3.0 Ga (Van Schmus et al., 1995a; Dantas, 1997), indicating that these units were not wholly juvenile when they formed. Jardim de Sá (1994) also recognized several augen gneisses, designated by him as “G2 granites,” that are distinct from the other basement gneisses. These G2 augen gneisses occur in the central part of the Rio Grande do Norte domain, commonly be- tween older basement and adjacent units of the Seridó Group (Fig. 2), and published U–Pb zircon ages are ca. 1.9–2.0 Ga (Jardim de Sa et al., 1988; Legrand et al., 1991).

2.2. Supracrustal fold belts

Several metasedimentary and metavolcanic–meta- sedimentary basins in the Borborema Province are present as curvilinear fold belts (Fig. 1). The oldest ones are the 1.8–1.7 Ga Orós and Jaguaribeano fold belts in the eastern part of Ceará state (cf. Sá et al., 1995). Recent U–Pb and Sm–Nd data show that a ma- jor fold belt in the DZT and southern domain formed about 1.0 Ga (Kozuch, 2003; Kozuch et al., 1997a,b,c; Van Schmus et al., 1995a; Van Schmus et al., 1999), during an episode in NE Brazil now known as the Cariris Velhos orogenic cycle (Campos Neto et al., 1994; Brito Neves et al., 1995). Other metasedimen- tary regions may be younger than, coeval with, or older than the ca. 1 Ga fold belts, but in many cases we lack definitive geochronology.

The main fold belt north of the Patos shear zone is the Seridó fold belt (Fig. 2; Table 1). Until recently the Seridó fold belt in the Rio Grande do Norte do- main was considered correlative with the Piancó-Alta Brı́gida fold belt in the DZT and the Riacho do Pontal fold belt in the southern domain because of their ap- parent alignment from one domain to another across the Patos and Pernambuco shear zones (Fig. 1). New

U–Pb, sedimentological, and geochemical data show, however, that this correlation may not be valid, and the Seridó belt, strictly speaking, is primarily restricted to the Rio Grande do Norte domain. Sm–Nd crustal res- idence ages (TDM) for the metasedimentary rocks and cross-cutting plutons from the 1 Ga or younger fold belts in the DZT and the southern domain are typically 1.6–1.2 Ga (Van Schmus et al., 1995a; Kozuch, 2003). This shows that they were not wholly derived from ju- venile material, but contain a substantial contribution from older basement, either as reworked detritus or as crustal contamination in younger magmas.

2.3. Brasiliano deformation, metamorphism, and plutonism

The Borborema Province is a structural province whose extent is defined primarily on the limits of late Neoproterozoic deformation, metamorphism, and plu- tonism. This event is the Brasiliano orogeny in South America; age limits of this orogenic cycle have ranged from 400 to 900 Ma prior to development of good geochronologic constraints, and there is still signifi- cant debate among Brazilian geologists about how to define the beginning and end of this orogenic cycle. In this paper we will focus on the age of deformation and peak metamorphism in the Borborema Province that occurred ca. 600 Ma, roughly coeval with the struc- turally correlative Pan African fold belt in western Africa (cf. Toteu et al., 1994, 2001).

Over the past decade there have been many new U–Pb ages on zircon, monazite, and sphene (titanite) reported for Neoproterozoic rocks of the Borborema Province (Jardim de Sá, 1994; Van Schmus et al., 1995a; Fetter and Van Schmus, 1996; Dantas, 1997; Guimarães and Silva Filho, 1998; Guimarães et al., 1998; Kozuch, 2003; Fetter, 1999; Guimarães et al., 2000; Silva Filho et al., 2000; unpublished data of Van Schmus and co-workers). These will be presented in a separate paper defining the timing of deforma- tion, metamorphism, and plutonism in the province. In summary, the following timeline is relevant to the geochronology of the Seridó Group as presented in this paper.

The duration of deformation is best controlled by the ages of ca. 600 Ma Brasiliano plutons. These plu- tons can be recognized on the basis of field relation- ships as pre-, syn-, or post-tectonic in terms of the

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Table 1 Summary of stratigraphic relationships for Seridó Group and related Rocks

Ebert (1970) Santos and Brito Neves (1984) Archanjo and Salim (1986) Jardim de Sá (1994) This paper

Seridó Formation Seridó Group (Seridó Formation) (Parelhas conglomerate)

Seridó Formation (Seridó schists) (Parelhas conglomerate)

Seridó Group (<650 Ma) (Seridó Formation) (Equador quartzite) (Jucurutu Formation)

Unconformity Unconformity Unconformity Unconformity Equador Formation G2 granites (1.9 Ga) G2 granites (>1.9 Ga) Unconformity? Intrusive contact Intrusive contact

Jucurutu Group? (quartzites) (paragneisses) (marbles) (Equador quartzite)

Jucurutu Group (Equador quartzite) (quartzites) (paragneisses) (marbles)

G2 granites Seridó Group (>1.9 Ga) (Seridó Formation) (Equador quartzite) (Jucurutu Formation)

Unnamed units?

Unconformity Unconformity Intrusive contact Unconformity Unconformity Jucurutu Formation Unconformity

Basement complex Basement complex Basement complex Basement complex (2100–2200 Ma)

Basement complex (2100–2200 Ma and ≥2700 Ma)

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main deformational event. Several of the pre-tectonic plutons, now often gneissic granites, have ages of 640–610 Ma, indicating that deformation began after 620 Ma; plutons in the 620–640 Ma range are gener- ally found south of the Patos shear zone. Post-tectonic plutons commonly have ages of 580–570 Ma, indica- tion that the main deformation had ceased by 580 Ma. U–Pb crystallization ages of syn-deformational ig- neous rocks or U–Pb ages on metamorphic zircon, monazite, or sphene (titanite) suggest that the peak of thermal activity occurred about 610–590 Ma. Thus, the Brasiliano orogeny in the Borborema Province is best defined as 620–580 Ma, with a peak at 600 Ma.

This age in NE Brazil is coeval with ages re- cently reported for the Pan African orogenic peak in Cameroon, West Africa (Toteu et al., 1994, 2001), and it is fully consistent with correlation of these two regions. Many new results for Brasiliano deformation and metamorphism in other parts of Brazil are also ca. 600 Ma, but there are also some regional variations. In other words, the Brasiliano orogeny was probably not fully synchronous on a continental scale. These will not be discussed further here, but various chapters in the new summary of Brazilian geology (Cordani et al., 2000) review much of the data.

2.4. Brasiliano shear zones

The Borborema Province contains many major shear zones of Brasiliano age (Brito Neves et al., 1982; Jardim de Sá, 1994; Vauchez et al., 1995). Many of the shear zones, such as the Patos and Pernambuco shear zones (Fig. 1), can be traced into comparable shear zones in Africa (cf. Bertrand and Jardim de Sá, 1990; Toteu et al., 1990; Castaing et al., 1993, 1994; Trompette, 1994). The Patos and Pernambuco shear zones also divide the eastern part of the Bor- borema Province into its three major tectonic domains (Fig. 1). In many cases the shear zones represent major faults within former continental blocks, but in other cases they may represent major terrane or block boundaries (Brito Neves et al., 2000); metamorphic conditions ranging from medium to low grade reveal extensive crustal deformation at a variety of depths during the late Neoproterozoic (Vauchez et al., 1995). The Patos shear zone, in particular, could represent a major terrane boundary, and this is relevant to possi-

ble interpretations of the Seridó Group, as discussed in the following.

3. The Seridó Group and related units

3.1. The Seridó Group

The stratigraphy, nomenclature, and age of the Seridó Group have been problematic for three decades. Ebert (1970) ultimately recognized two main group- ings for supracrustal rocks of the region (Table 1): schists of the Seridó Formation, which unconformably overlie an older metasedimentary complex (Jucurutu Group). The Jucurutu Group in turn unconformably overlies the older basement gneiss complex. Santos and Brito Neves (1984) summarized subsequent vari- ations in stratigraphy up to that time. Based on their review, they generally recommended the scheme of Ebert, with a Seridó Group (consisting of the Seridó Formation and Parelhas conglomerate) uncon- formably overlying the Jucurutu Group (consisting of Equador quartzites and other units), which in turn unconformably overlies the basement complex. Archanjo and Salim (1986) interpreted the Seridó Formation (with basal Parelhas conglomerate) as un- conformably overlying the Equador Formation. They proposed a major break between the younger forma- tions and the Jucurutu Formation, with “G2” augen gneisses intrusive into the Jucurutu Formation. The Jucurutu Formation, in turn, unconformably overlies Paleoproterozoic basement.

Jardim de Sá (1994) considered the Seridó–Jucurutu strata as part of a single major succession, with lo- cal unconformities and interbedded quartzite units (Table 1). He referred to the Seridó and Jucurutu se- quences as formations within the Seridó Group and concluded that the boundary between the Seridó For- mation and Jucurutu Formation was gradational and conformable. The Equador quartzites were locally present near the transition from Jucurutu Forma- tion to Seridó Formation Jardim de Sá also inter- preted field relationships as showing that the augen gneiss (his G2 granite) was intrusive into at least the lower part of the Seridó Group. Radiometric ages of ca. 1.9 Ga for the G2 granites (Jardim de Sa et al., 1988; Legrand et al., 1991) therefore meant that the Seridó Group should be Paleoproterozoic

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(Jardim de Sá, 1994, 1995; Jardim de Sá et al., 1995, 1997).

Caby et al. (1995) generally followed the schemes of Santos and Brito Neves (1984) and of Archanjo and Salim (1986), with an older Jucurutu Group (Ju- curutu, Equador formations) unconformably overlain by a Seridó Group. In preliminary reports, several of us (Van Schmus et al., 1995b; Van Schmus et al., 1997) interpreted the entire Seridó Group as middle Neoproterozoic, generally following the stratigraphy of Jardim de Sá (1994), but discounting his interpreta- tions of the G2 granite. Archanjo and Legrand (1997) recently argued in favor of a Neoproterozoic age, al- though Jardim de Sá et al. (1995, 1997) still argued for a Paleoproterozoic age for these rocks.

In this paper we will continue to follow the strati- graphic nomenclature, but not the age assignments, of Jardim de Sá (1994). We will show, in fact, that this group was deposited in the late Neoproterozoic, after 650 Ma, not during the middle Neoproterozoic as previously argued (Van Schmus et al., 1995b; Van Schmus et al., 1997), and certainly not during the Pa- leoproterozoic.

3.2. Localities studied

Of the many units within the Seridó Group, we fo- cused on two main rock suites: (a) biotite schists and interlayered felsites of the Seridó Formation and (b) biotite gneisses and interlayered felsic units of the Ju- curutu Formation. We sampled these at several local- ities throughout the region as our work progressed (Fig. 2). We also sampled other specific units as tests of various competing models, as identified specifically in the following.

The Jucurutu Formation as currently defined is a complex assemblage of fine-grained quartz-rich bi- otite gneisses, calc-silicate schists and gneisses, mar- bles, mafic metavolcanic rocks, and meta-arkoses. The mafic metavolcanic rocks are most common near the base of the section. In at least one locality (between samples 95-112 and 95-113 at locality 16 southeast of São Vicente, Fig. 2), the Jucurutu Formation ap- peared to grade upward into the Seridó Formation with no obvious unconformity or intervening quartzite units.

The Seridó Formation is typically dominated by biotite-rich schists ranging from low-grade (upper

greenschist) facies phyllitic schists to high-grade schists that commonly contain garnet and, in several localities, cordierite and/or sillimanite. In low-grade schists the bedding is well preserved and shows incomplete to complete turbidite sequences, in- dicating that the original sediment was probably immature turbidites. In many outcrops of higher metamorphic grade, the general compositional lay- ering in the schists can be followed over tens of meters, indicating that a crude lithostratigraphy is preserved in spite of the metamorphism and deformation. At several localities we also found light-colored, sugary textured, fine-grained units in- tercalated with the schists; we believe that these originally may have been air-fall or water-laid tuffs.

We sampled several rock types within both the Seridó and Jucurutu formations for U–Pb (zircon) and Sm–Nd analyses. We also assembled a regional suite of Seridó Formation biotite schists and Jucu- rutu Formation biotite gneisses (Fig. 2; Appendix A), in addition to other rock types. We used these for Sm–Nd analyses to determine regional variations in εNd(t) values (t = 600 Ma) and corresponding crustal residence ages (TDM; DePaolo, 1981) that may reflect variations in provenance as well as set maximum limits to the depositional ages. We also made more detailed collections and suites of anal- yses at several localities, where it appeared that we had a good chance to obtain meaningful U–Pb zircon ages. The localities studied in more detail are:

3.2.1. Fazenda São Pedro This locality (4, Fig. 2) is an outcrop of bare rock

in the spillway of a small reservoir on Fazenda São Pedro, about 5 km east of the town of Pedra Preta. The schist units are subvertical and nearly planar; they consist of interlayered dark-gray biotite schists, buff colored fine-grained sandstones and siltstones, and two light-gray, sugary textured felsic units (inter- preted in the field as metavolcanic tuffs). We sampled both “tuffs” (93-26, 94-80), one siltstone (95-104), and one schist layer (94-81) for possible zircon extraction; we also collected several small samples of all units for Sm–Nd analyses. The schists at this locality are relatively low-grade; neither garnet nor cordierite was observed.

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3.2.2. SE of Barra de Santa Rosa This locality (20, Fig. 2) includes roadcuts on both

sides of highway BR 104, about 20 km SE of the turn-off to the town of Barra de Santa Rosa in the São José do Campestre massif. The metasedimentary units are subhorizontal due to thrusting and may in- clude isoclinal folds. The rocks consist of interlay- ered dark-gray biotite schists (93-48E, 93-48G) and light-gray, sugary textured felsic units interpreted as possible metavolcanic tuffs (93-48B, 93-48C). The schists at this locality are relatively high-grade; garnet and cordierite can be found readily on fresh boulders blasted from the roadcut. The felsic units are generally very coherent, rather than friable. We sampled three “tuff” varieties and one schist layer for possible zir- con extraction, plus several small samples of all units for Sm–Nd analyses.

3.2.3. North of Soleana (MVULC) This locality (21, Fig. 2), about 40 km east of Barra

de Santa Rosa, consists of several outcrops of typical dark-gray biotite schist of the Seridó Formation crop- ping out over an area of a few square kilometers. The schists at this locality are generally more deformed, with both regional foliation and smaller scale folds. Interlayered with the schists are several lighter gray, sugary textured, discontinuous, contorted felsic lay- ers interpreted by Dr. Emanuel Jardim de Sá (personal communication, 1996) as volcanogenic tuffs interlay- ered with the schists.

3.2.4. Town of Jucurutu Samples of typical fine-grained, dark-gray quartz–

biotite gneiss of the Jucurutu Formation were collected for Sm–Nd analyses in and within few kilometers of the town of Jucurutu (14, 15, Fig. 2), which is the type locality for this formation.

3.2.5. Rio Potengi (RP) Outcrops along Rio Potengi (9, Fig. 2) consist

of several units of Jucurutu Formation, including a basal conglomerate, unconformably overlying Paleo- proterozoic granitic and tonalitic basement gneisses. The Jucurutu gneisses at this locality are commonly silicified, taking on the appearance of fine-grained granitic gneisses; however, the structures of typi- cal Jucurutu gneisses are preserved in the silicified units. Outcrops of typical Seridó schist and nor-

mal Jucurutu gneiss occur several kilometers to the north.

3.2.6. Fazenda da Lapa (FL) Several units of the Jucurutu Formation occur in and

along the road to the west of the river on Fazenda da Lapa (22, Fig. 2). The most prominent units are felsic gneisses; zircons from one of these units (SED-J-10) represent several populations, and many are rounded, indicating that these units are probably meta-arkoses (Van Schmus et al., 1995a).

3.2.7. SW of Angicos (SWA) Jardim de Sá (1994) argued for an intrusive rela-

tionship between the Jucurutu Formation and his G2 augen gneiss, and we visited a locality SW of the town of Angicos (13, Fig. 2) where he believed that this relationship could be seen in the field. We collected samples of typical Jucurutu gneiss and Seridó schist that were not in contact with the G2 augen gneiss (2, 3, Fig. 2), and we also collected several gneissic to schistose xenoliths that occur within the G2 augen gneiss (1, 2, Fig. 2) and which are similar in appear- ance to typical Jucurutu Formation and Seridó Forma- tion units.

4. Analytical methods

Sm–Nd and U–Pb data summarized in Tables 3 and 4 and in Appendix B were obtained at the Iso- tope Geochemistry Laboratory (IGL), Department of Geology and Kansas University Center for Re- search, University of Kansas, Lawrence. SHRIMP data summarized in Table 4 and presented in detail in Appendix C were obtained at the Research School of Earth Sciences (RSES), Australian National Univer- sity (ANU), Canberra, ACT, Australia.

4.1. Sm–Nd analyses

Rock powders for Sm–Nd analysis were dissolved and REE were extracted using the general methods of Patchett and Ruiz (1987); isotopic compositions were measured with a VG Sector multi-collector mass spectrometer. External precision based on repeated analyses of our internal standard is ±40 ppm (2σ) or better; all analyses are adjusted for instrumental bias

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Table 2 Sm–Nd data for samples from outcrops assigned to the Serido Group

Sample ID Map number (Fig. 2)

Sample information Nd (ppm) Sm (ppm) 147Sm/144Nd 143Nd/144Nd ± 2 εNd(0) (today)

εNd(t) (600 Ma)

TDM (Ga)

Various outcrops of Seridó Schist 93-03 7 Quarry W of Natal 30.80 5.97 0.11725 0.512077 ± 12 −10.9 −4.9 1.53 93-33 5 W of Riachuelo 24.64 5.25 0.12872 0.512343 ± 09 −5.8 −0.5 1.26 93-34 5 W of Riachuelo 24.09 5.34 0.13400 0.512398 ± 10 −4.7 0.1 1.24 93-35 6 W Caicara do R.V. 31.37 6.19 0.11922 0.512071 ± 08 −11.1 −5.1 1.57 93-42 13 W Sta. Do Matos 24.66 5.35 0.13114 0.512469 ± 10 −3.3 1.7 1.06 93-45 19 W Picui 37.81 7.48 0.11955 0.512100 ± 08 −10.5 −4.6 1.53 93-48B 20 Barra de Sta. Rosa 22.14 4.82 0.13162 0.512385 ± 09 −4.9 0.1 1.22 93-48C 20 Barra de Sta. Rosa 22.11 4.67 0.12812 0.512364 ± 09 −5.4 −0.1 1.21 93-48C Repeat Barra de Sta. Rosa 22.12 4.70 0.12836 0.512377 ± 09 −5.1 0.2 1.19 93-48G 20 Serido schist 25.90 5.62 0.13122 0.512388 ± 07 −4.9 0.1 1.21 94-77 8 Serido schist 31.35 6.31 0.12168 0.512189 ± 09 −8.8 −3.0 1.41 94-84 13 At 93-42 21.46 4.65 0.13111 0.512386 ± 08 −4.9 0.1 1.21 94-88 17 S. José do Seridó 23.53 5.27 0.13547 0.512403 ± 11 −4.6 0.1 1.25 94-89 18 Cruzeta 22.86 5.05 0.13348 0.512404 ± 10 −4.6 0.3 1.22 95-113 16 W Currais Novos 20.91 4.55 0.13144 0.512365 ± 08 −5.3 −0.3 1.26

Various outrops of Jucurutu gneiss 94-86 14 E Jucurutu 31.27 6.01 0.11619 0.512108 ± 08 −10.3 −4.2 1.46 94-87 15 S Jucurutu 35.69 7.10 0.12024 0.512056 ± 08 −11.4 −5.5 1.61 95-101 8 Jucurutu gneiss 28.82 5.34 0.11211 0.512062 ± 10 −11.2 −4.8 1.47 95-112 16 W Currais Novos 22.83 4.25 0.11248 0.512041 ± 09 −11.6 −5.2 1.51 EC-61 14 Jucurutu city 32.26 5.97 0.11189 0.512066 ± 45 −11.2 −4.7 1.46

Angicos region 95-105 3 Serido schist 22.62 4.96 0.13253 0.512369 ± 11 −5.2 −0.3 1.26 95-106 2 Jucurutu gneiss 29.74 5.78 0.11759 0.512075 ± 10 −11.0 −4.9 1.53

G2 granite and inclusions in the granite 95-107 2 G2 granite 38.09 6.21 0.09857 0.511043 ± 09 −31.1 −23.6 2.67 95-108 2 Schist xenolith 35.34 5.73 0.09802 0.511048 ± 09 −31.0 −23.5 2.65 95-109 1 “Jucurutu”? 51.17 9.48 0.11199 0.511247 ± 11 −27.1 −20.7 2.71 95-110 1 Schist xenolith 40.56 6.63 0.09877 0.511034 ± 09 −31.3 −23.8 2.68 95-111 1 Schist xenolith 29.75 4.88 0.09921 0.511084 ± 11 −30.3 −22.9 2.63

Rio Potengi region 94-70 9 Jucurutu gneiss 24.31 4.70 0.11681 0.512089 ± 09 −10.7 −4.6 1.50 94-74 9 at SED-Gf-55 25.78 5.09 0.11944 0.512096 ± 09 −10.6 −4.7 1.53 SED-Gf-55 9 Jucurutu gneiss 26.49 5.14 0.11740 0.512098 ± 10 −10.5 −4.5 1.49 SED-Gf-55 9 Jucurutu gneiss 25.71 5.03 0.11824 0.512091 ± 10 −10.7 −4.7 1.52 94-76 9 Jucurutu gneiss 28.32 5.60 0.11958 0.512121 ± 08 −10.1 −4.2 1.49 95-102 9 Jucurutu gneiss 28.64 5.73 0.12094 0.512100 ± 10 −10.5 −4.7 1.55

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Jucurutu metavolcanic rocks 94-71 9 Amph. gneiss 15.48 3.56 0.13893 0.511384 ± 09 −24.5 −20.1 3.47 94-72 9 Felsic layer in 94-71 15.56 4.24 0.16487 0.511749 ± 07 −17.4 −14.9 – 94-73 9 Metadacite 18.66 3.94 0.12763 0.511206 ± 10 −27.9 −22.7 3.32

Basement 94-75 9 Granitic gneiss 58.10 9.93 0.10334 0.510779 ± 07 −36.3 −29.1 3.17 95-103 9 Tonalite gneiss 229.86 39.02 0.10265 0.510752 ± 10 −36.8 −29.6 3.19

Fazenda São Pedro (Seridó Formation) 94-81 4 Schist 28.13 5.83 0.12527 0.512257 ± 09 −7.4 −2.0 1.35 94-82 4 Schist 33.36 6.80 0.12326 0.512226 ± 09 −8.0 −2.4 1.38 94-83A 4 Adj. 94-83 29.36 5.85 0.12052 0.512286 ± 09 −6.9 −1.0 1.24 94-83B 4 50 m. W 94-83 29.79 5.61 0.11383 0.512172 ± 08 −9.1 −2.7 1.33 94-83C 4 50 m E 94-83 45.93 8.58 0.11288 0.512189 ± 09 −8.8 −2.3 1.29 95-104 4 Metasandstone 34.91 6.98 0.12096 0.512196 ± 10 −8.6 −2.8 1.39 93-26 4 “Tuff”-1 36.20 6.65 0.11102 0.511664 ± 10 −19.0 −12.5 2.05 94-80 4 “Tuff”-2 34.48 6.27 0.10091 0.511619 ± 09 −19.9 −13.2 2.09 94-83 4 Meta-arkose/dike? 4.58 0.70 0.09200 0.511010 ± 08 −31.8 −23.8 2.56

Fazenda da Lapa 94-90 22 Felsic gneiss 19.69 3.59 0.11008 0.511357 ± 09 −25.0 −18.4 2.49 94-92 22 “Jucurutu” Gneiss 47.58 8.69 0.11045 0.511313 ± 08 −25.8 −19.3 2.57 94-94 22 Felsic gneiss 101.97 14.89 0.08826 0.510892 ± 08 −34.1 −24.3 2.63 94-95 22 Felsic gneiss 20.45 3.62 0.10695 0.511350 ± 08 −25.1 −18.3 2.43 SED-J-4 22 Felsic gneiss 26.84 5.43 0.12233 0.511410 ± 09 −24.0 −18.3 2.75 SED-J-11 22 Felsic gneiss 2.47 0.63 0.15413 0.511666 ± 51 −19.0 −15.7 3.65

Note: 143Nd/144Nd normalized to 146Nd/144Nd = 0.72190. εNd(0) calculated relative to CHUR(0) = 0.512638. Model ages (TDM) were calculated according to the single-stage depleted-mantle model of DePaolo (1981). Primary ages used for εNd(t) are based on U/Pb ages were known or estimated (italics) based on regional geology.

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Table 3 Single-grain isotope dilution data for zircons with ages less than 1000 Maa

Fractionb Size (mg) Concentrationsc Observedd Ages ± 2 (Ma)e

U (ppm) Pb (ppm) 206Pb/204Pb 206Pb/238U 207Pb/235U 207Pb/206Pb

Seridó Formation BR93-48B-02 0.011 582 65 1417 715 ± 09 769 ± 10 930 ± 06 BR93-48E-01 0.002 171 24 402 746 ± 19 742 ± 27 729 ± 53 BR93-48E-02 0.004 71 10 525 733 ± 17 736 ± 18 744 ± 18 BR95-104-01 0.002 321 41 1247 729 ± 05 736 ± 05 757 ± 05 BR95-104-02 0.009 246 30 4425 686 ± 04 691 ± 05 708 ± 05 BR95-104-03 0.006 330 37 1710 624 ± 04 636 ± 04 680 ± 03 BR95-104-04 0.005 174 21 1061 669 ± 05 671 ± 05 675 ± 04 BR95-104-06 0.005 514 47 1598 536 ± 03 569 ± 03 705 ± 03 MVULC-02 0.005 144 19 621 694 ± 07 717 ± 10 788 ± 20 MVULC-03 0.006 352 27 922 475 ± 05 501 ± 08 621 ± 26 MVULC-04 0.009 518 46 929 528 ± 03 575 ± 03 764 ± 05

Jucurutu Formation EC-61-02 0.002 387 52 558 622 ± 04 630 ± 04 659 ± 04 EC-61-04 0.003 207 28 688 761 ± 04 808 ± 05 939 ± 05 EC-61-07 0.002 1849 184 428 522 ± 03 558 ± 04 705 ± 06 EC-61-08 0.002 1659 175 481 609 ± 03 636 ± 04 733 ± 05 EC-61-10 0.007 3682 362 1611 572 ± 03 609 ± 03 746 ± 02 EC-61-11 0.002 2669 266 936 592 ± 03 609 ± 04 673 ± 05

a See Appendix B for full details and analyses of older zircons. b See Appendix B for details on individual fractions. c Total U and Pb concentrations corrected for analytical blank. d Not corrected for blank or non-radiogenic Pb. e Ages given in Ma using decay constants recommended by Steiger and Jäger (1977). Uncertainties at 2σ.

determined by measurements of our internal standard; on this basis our analyses of La Jolla Nd average 0.511860±0.000010 for 143Nd/144Nd. Sm–Nd ratios are correct to within ± 0.5%, based on analytical un- certainties. Reference values of εNd(t) were calculated for t = 600 Ma, the age of peak metamorphism dur- ing the Brasiliano orogeny in this region (Van Schmus et al., 1995a; Dantas, 1997; Fetter, 1999). TDM re- sults following the depleted-mantle model of DePaolo (1981) are also presented in Table 2.

4.2. Thermal ionization mass spectrometry isotope dilution U–Pb analyses

Zircon fractions used for thermal ionization mass spectrometry–isotope dilution (TIMS–ID) analyses (Table 3; Appendix B) were air-abraded (Krogh, 1982), and individual grains were carefully selected by hand prior to dissolution to avoid cores or over- growths. Zircons were dissolved and Pb and U were

separated using procedures modified after Krogh (1973) and Parrish (1987). For small samples and single-crystal analyses, samples were total-spiked with a mixed 205Pb–235U tracer solution. Pb compo- sitions were corrected for mass discrimination as de- termined by analyses of NBS SRM-982 (equal-atom) Pb and monitored by analyses of NBS SRM-983 (ra- diogenic) Pb. Uranium fractionation was monitored by analyses of NBS SRM U-500. Uncertainties in Pb/U ratios due to uncertainties in fractionation and mass spectrometry for typical analyses are ±0.5%; in some instances weak signals (e.g. small single crystals) caused uncertainties to range up to ±2%. Radiogenic 208Pb, 207Pb, and 206Pb were corrected for modern blank Pb and for non-radiogenic origi- nal Pb corresponding to Stacey and Kramers (1975) model Pb for the approximate age of the sample. Decay constants used were 0.155125 × 10−9 per year for 238U and 0.98485 × 10−9 per year for 235U.

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4.3. SHRIMP U–Pb analyses

Two of us (W.R.V.S., I.S.W.) used the SHRIMP (Sensitive High Resolution Ion Microprobe) at the ANU to measure U–Pb ages on detrital zircons from several units of the Seridó Group. Zircons from each sample were mounted as a single-crystal layer in epoxy disks and polished with 1 m diamond to give a scratch-free surface. Each sample was photographed in both transmitted and reflected light, and scanning electron microscope (SEM) cathodoluminscence (CL) images were made for all grain mounts. Individual analytical spots for the SHRIMP analyses were typi- cally about 20 m in diameter. For this work we typ- ically chose interior, core-free and overgrowth-free, areas of individual crystals. No attempt was made to select a particular type of crystal in order to obtain as representative a population as possible.

Analytical methods and data reduction followed those detailed by Williams (1998a) and Stern (1997) using the AS-3 Duluth Gabbro standard (1099 Ma; Paces and Miller, 1993). In order to optimize the num- ber of grains analyzed, each grain was mass-scanned four times and the standard analyzed after every group of five to seven sample grains. This yields somewhat lower precision for each grain, but allows analysis of a greater number of grains and more representative population statistics.

The raw data were corrected for non-radiogenic Pb using 204Pb as a monitor and the composition of Broken Hill Pb (a common contaminant in Aus- tralian labs) to correct for non-radiogenic 206Pb, 207Pb, and 208Pb (“204-corrected” data; cf. Stern, 1997). The data were also processed to refine the common Pb correction by assuming concordance be- tween the 207Pb/235U and 206Pb/238U ages (so-called “207-corrected” data), which sometimes yields more precise and more accurate results for young zircons. Most 204-corrected results for zircons younger than 1000 Ma plot near concordia, and the differences be- tween the individual 206Pb/238U ages calculated with the 207-corrected method and those calculated with the 204-corrected method were generally small (1–3 million years). Therefore, only 204-corrected results are used here. Because of general concordancy of the results and better precision, 206Pb/238U ages are pre- ferred for zircons younger than 1200 Ma. For samples with ages greater than 1200 Ma, the 204-corrected

207Pb/206Pb ages are preferred (Appendix C), since they yield results that are more representative of the true ages for older zircons.

The 207Pb/235U and 206Pb/238U data from 204-corrrected analyses were plotted on conventional concordia diagrams (Figs. 6, 8, 10, and 13) using ANU software (“PLONK”) to show the distribution of the detrital populations over the full age range of all grains (600–2700 Ma). Probability distributions of 204-corrected 206Pb/238U ages for grains younger than 1200 Ma (Table 4) were calculated for three sam- ples (Fig. 11), also using ANU software (“NOBLE”). The probability distributions take into account not

Table 4 Summary of SHRIMP 206Pb/238U ages <1200 Ma for Seridó Group samples

Seridó Formation Jucurutu Formation

95-104 (age ± σ)

93-48G (age ± σ)

EC-61 (age ± σ)

SED-J-10 (age ± σ)

628 ± 16 655 ± 7 634 ± 13 684 ± 7 639 ± 8 665 ± 12 646 ± 6 639 ± 14 670 ± 11 647 ± 7 642 ± 26 671 ± 13 650 ± 10 648 ± 12 680 ± 7 650 ± 8 648 ± 6 681 ± 9 653 ± 9 649 ± 10 687 ± 17 655 ± 8 650 ± 10 693 ± 11 661 ± 23 652 ± 14 715 ± 9 663 ± 7 653 ± 10 717 ± 13 666 ± 11 653 ± 10 727 ± 6 671 ± 6 654 ± 7 736 ± 11 676 ± 12 666 ± 12 738 ± 20 677 ± 6 677 ± 32 739 ± 14 678 ± 12 678 ± 15 743 ± 5 682 ± 6 679 ± 12 744 ± 14 683 ± 13 679 ± 8 747 ± 10 685 ± 7 681 ± 11 748 ± 11 691 ± 5 688 ± 23 752 ± 6 708 ± 12 691 ± 21 754 ± 14 714 ± 14 723 ± 18 757 ± 6 715 ± 11 772 ± 21 761 ± 10 716 ± 20 779 ± 15 771 ± 10 724 ± 12 779 ± 14 779 ± 14 726 ± 11 780 ± 13 780 ± 11 728 ± 10 784 ± 14 782 ± 14 769 ± 8 788 ± 13 783 ± 10 808 ± 5 789 ± 18 802 ± 15 828 ± 8 790 ± 11 804 ± 8 837 ± 23 794 ± 11 805 ± 11 849 ± 10 799 ± 17 807 ± 15 856 ± 6 805 ± 41 808 ± 14 875 ± 11

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Table 4 (Continued )

Seridó Formation Jucurutu Formation

95-104 (age ± σ)

93-48G (age ± σ)

EC-61 (age ± σ)

SED-J-10 (age ± σ)

816 ± 17 821 ± 16 882 ± 29 826 ± 20 823 ± 11 913 ± 27 827 ± 12 837 ± 14 923 ± 37 839 ± 9 914 ± 21 924 ± 11 866 ± 12 956 ± 14 939 ± 16 894 ± 10 957 ± 7 955 ± 12 899 ± 27 958 ± 15 964 ± 11 903 ± 10 963 ± 12 971 ± 9 935 ± 13 979 ± 22 992 ± 15 944 ± 21 980 ± 14 993 ± 29 965 ± 18 994 ± 17 1000 ± 20 969 ± 17 995 ± 30 1002 ± 11 971 ± 18 1005 ± 17 1009 ± 13 977 ± 14 1020 ± 35 1013 ± 17 978 ± 11 1107 ± 29 1014 ± 15 982 ± 11 1015 ± 15 990 ± 20 1016 ± 10

1005 ± 18 1023 ± 8 1016 ± 22 1027 ± 22

1035 ± 21 1036 ± 17 1036 ± 16 1048 ± 10 1064 ± 12 1141 ± 13

Note: Only grains with ages less than 1200 Ma reported in this table; ages reported were calculated using “204-corrected” method (Stern, 1997), which makes no assumption about concordance. Use of the “207-corrected” method, which assumes concordancy, yields ages only slightly different (±1 to 4 million years) than those reported here. See text for discussion. Note that only one grain for sample SED-J-10 gave a young age. See Figs. 6, 8, 10 and 13 plus Appendix C for presentation of all analyses.

only the mean age for the sample, but also the analyt- ical uncertainty for each analysis. Histograms of the mean ages alone are superimposed on the probability distributions in Fig. 11.

5. Results

5.1. Early Sm–Nd and U–Pb results

Our first indications that the Seridó Group is not Paleoproterozoic came with U–Pb analyses of multi-grain and single-grain zircon fractions from a felsic unit within the Jucurutu Formation at Fazenda da Lapa. This unit (SED-J-10; Fig. 7 in Van Schmus

et al., 1995a) yielded detrital zircons from domi- nantly two age populations: ca. 2100–2200 Ma and ca. 1700–1800 Ma. More compelling indications of a young age for the Seridó Group came from Sm–Nd analyses on a regional suite of samples from both the Seridó and Jucurutu formations (Table 2; Figs. 3 and 4). These analyses commonly yielded crustal for- mation model ages (TDM; DePaolo, 1981) in the range of 1200–1600 Ma, with εNd(600 Ma) values that were distinctly less negative (from 0 to −5) than epsilon values for known Paleoproterozoic basement in the region (−18 to −23; Van Schmus et al., 1995a).

We also extracted zircons from several samples of the Seridó Formation U–Pb data for zircons from some of these units (Appendix B) defined an array (not shown) with a lower intercept at about 740 Ma and an upper intercept at about 2000 Ma; some samples were virtually on concordia at ca. 740 Ma. These re- sults convinced us (Van Schmus et al., 1995b) that the Seridó Group was Neoproterozoic, and we interpreted the depositional age of the Seridó Formation at about 740 Ma. In order to refine our conclusions, we pur- sued the problem further, and our more detailed re- sults are summarized in the following. Although each of the three approaches is, in our mind, definitive, we have presented them in order of increasing precision and detail.

5.2. Sm–Nd results for the Seridó Group

Sm–Nd data from many localities of Seridó For- mation in the region (Fig. 2) generally show typi- cal crustal Sm/Nd ratios with crustal formation ages between 1.2 and 1.6 Ga (Table 2; Fig. 3). These re- sults mean that the maximum depositional age for the Seridó Formation is about 1.2 Ga, assuming the source of the detritus came from depleted mantle, was rapidly eroded and deposited, and that the 147Sm/144Nd ra- tios (which are normal continental crust values of ca. 0.12–0.13) were not affected significantly by 600 Ma Brasiliano metamorphism and deformation. If the Nd data represent mixing between younger and older com- ponents (e.g. Arndt and Goldstein, 1987), then the de- positional age must be significantly less than 1.2 Ga. Although these samples represent a range of metamor- phic grade, from cordierite- and garnet-bearing schists at the upper end to lower grade, less deformed bi- otite schists with well preserved sedimentary bedding

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Fig. 3. Nd evolution diagram for rocks from the Seridó Group. Note that the data fall into three populations, having TDM ages of ca. 1.2, 1.4, and 1.6 Ga with εNd(t) at 800 Ma of +1.0 (A), −0.5 (B), and −4.0 (C), respectively. Also shown are fields for data from the Paleoproterozoic basement of the Rio Piranhas massif (RPM) and Archean basement of the São José do Campestre massif (SJCM; Van Schmus et al., 1995a; Dantas et al., 1998). The line labeled 93-83 represents a felsic unit interlayered with Seridó schists near Fazenda São Pedro (see text). The initial Nd composition for A, most radiogenic samples of the Seridó Formation, could represent detritus from a source with a similar composition, or it could represent a mixture of detritus from older basement and a late Neoproterozoic juvenile source. In any case it sets a maximum limit of ca. 1200 Ma for deposition of the Seridó Formation. The initial Nd compositions for (B) and (C) probably represent mixtures of detritus from older basement and younger sources.

structures at the lower end, the results do not show any correlation with metamorphic grade. The data fall into three groups, with TDM ranging from 1.2 to 1.6 Ga (Fig. 3), but we believe that these groups represent parts of a continuum, from detritus dominated by rel- atively younger sources to that with greater contribu- tions from older basement.

Several typical Jucurutu Formation gneisses from the Jucurutu region in the west and from Rio Potengi in the east have εNd(600 Ma) values near −5.0, with corresponding TDM ages of 1.5–1.6 Ga (Fig. 4). These values argue for a maximum depositional age of ca. 1.6 Ga. Because Jucurutu Formation samples did not

yield younger crustal residence ages (1.2–1.4 Ga), similar to those from the Seridó Formation, it is possible to conclude from Sm–Nd data alone that the Jucurutu Formation could be significantly older than the overlying Seridó Formation. Alternatively, in a mixing model the differences may only reflect variations in provenance.

The Sm–Nd results for the Seridó Group in gen- eral are similar to those found in the Pajeú-Paraı́ba, Piancó-Alto Brı́gida, and Sergipano fold belts south of the Patos shear zone (Fig. 1), which contain ca. 1.0 Ga Cariris Velhos metavolcanic, metasedimen- tary, and metaplutonic units having non-juvenile (at

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Fig. 4. Nd evolution diagram for rocks from the Jucurutu Formation. Also shown are fields for data from the most radiogenic samples of the Seridó Formation (A), Paleoproterozoic basement of the Rio Piranhas massif (RPM), and Archean basement of the São José do Campestre massif (SJCM; Van Schmus et al., 1995a; Dantas et al., 1998). Note that the most radiogenic samples from the Jucurutu Formation plot essentially with the least radiogenic samples of Seridó schist (C) and have similar εNd(t) for 650 Ma. These samples define a maximum age of ca. 1600 Ma for deposition of the Jucurutu Formation. Samples from the lower part of the Jucurutu Formation near Fazenda da Lapa plot close to results from the Paleoproterozoic basement and probably indicate that this part of the section is dominated by local basement detritus.

1 Ga) Nd compositions (TDM ages of 1.7–1.2 Ga; Van Schmus et al., 1995a; Kozuch et al., 1997a,b,c; Brito Neves et al., 1995). Thus, Sm–Nd data for the Seridó Group as a whole could also be satisfied by detritus exclusively from these, or similar, 1 Ga terranes. In this case the Jucurutu Formation had to be deposited more recently than 1 Ga. However, there are several other mixing models possible, and Nd data by them- selves do not yield tight constraints on the age of the Seridó Group other than to require that it is younger than 1.2–1.6 Ga.

5.3. Other Sm–Nd results

In several instances Sm–Nd data for units within the Seridó Group yielded somewhat older model ages.

These are discussed in the following, since they relate to provenance questions and to interpretations of field data.

5.3.1. Fazenda São Pedro At sample site 94-83, west of the Fazenda São Pe-

dro (4, Fig. 2), we sampled a felsic unit interlay- ered with typical Seridó schist. However, Sm–Nd data yield a much older model age than typical for the schists (2.6 Ga versus 1.2 Ga). To test whether the Nd model age is specific to this unit, we sampled the typ- ical Seridó schist on both sides of the unit, including one sample immediately adjacent to it. These sam- ples (94-83A, 94-83B, 94-83C; Table 2) yield typical Seridó schist model ages (1.24–1.30 Ga). This indi- cates that the old model age is unique to the felsic

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unit. We offer three possibilities: (a) the felsic unit is a meta-arkose with provenance from the cratonic basement; (b) the felsic unit is a metarhyolite whose magma was derived by melting subjacent Paleopro- terozoic basement having inherited Nd, or (c) the fel- sic unit is a sill whose magma was derived from the subjacent basement.

Outcrops of Jucurutu gneiss in Rio Potengi (9, Fig. 2) constitute a broad antiform, with a basal con- glomerate resting unconformably upon cratonic base- ment (SED-C-50, 95-103) that has a model age of 3.2 Ga (Table 2). Jucurutu gneiss samples are locally silicified, but their Nd data are similar to those for typ- ical Jucurutu gneisses elsewhere, including the type area near the town of Jucurutu (94-86, 94-87, EC-61). There are several metavolcanic units in the section at Rio Potengi (southernmost exposures), but they have much less radiogenic Nd, with mid-Archean model ages (Table 2). We interpret these results to indicate that the volcanic magmas were locally derived from the underlying São José do Campestre massif of mid- dle Archean age (Dantas et al., 1998), whereas the de- tritus in the paragneisses must have come from more distant, younger sources. We were not able to extract good zircon crystals from any of the metavolcanic samples. A Seridó schist sample (94-77) to the north of Rio Potengi yields a model age typical for other Seridó schists (Table 2), indicating that there is no major geochemical alteration affecting these samples.

5.3.2. Fazenda da Lapa Some units mapped as Jucurutu Formation such as

those at Fazenda da Lapa near the Rio Grande do Norte-Paraı́ba boundary east of Patos (22, Fig. 2), have much older model ages, similar to those from the Paleoproterozoic basement (Fig. 4). However, sin- gle detrital zircons from one of these units (SED-J-10) have well-defined 2200 and 1800 Ma populations (Van Schmus et al., 1995a; in the following). This indi- cates that the older Nd signatures at this locality are probably due to differences in provenance, rather than due to differences in depositional ages or to erroneous stratigraphic assignment of these units to the Jucurutu Formation.

5.3.3. SW of Angicos The Angicos area (13, Fig. 2) was one of the key

regions where relationships suggested that the Seridó

Group is intruded by ca. 1.9 Ga G2 granitoids (cf. Jardim de Sa et al., 1988; Jardim de Sá, 1994). The principal evidence in the Angicos region consists of Seridó-like and Jucurutu-like xenoliths within an au- gen gneiss (G2 granite). Both Rb–Sr and U–Pb data indicate a late Transamazonian age (ca. 1.95 Ga) for the augen gneiss (Jardim de Sa et al., 1988; Legrand et al., 1991), so that if the xenoliths in question are truly fragments of the Seridó Group, then the Seridó Group must predate the G2 granite.

Sm–Nd results for several xenolith samples (95-108, 95-109, 95-110, and 95-111) have old model ages (Table 2: TDM = 2.63–2.71). These are similar to that of the G2 granite (95-107: TDM = 2.67 Ga) and to samples of older schist and gneiss from the Rio Piranhas massif (Van Schmus et al., 1995a; Dantas, 1997); they are unlike the 1.2–1.6 Ga model ages found for typical Seridó schists and Jucurutu gneisses elsewhere. We also collected typical Seridó and Ju- curutu samples (95-105, 95-106) from outcrops of these formations near the G2 granite, and these yield model ages (1.26 and 1.53 Ga, respectively) that are equivalent to those of typical Seridó Group schists and gneisses elsewhere. We conclude, therefore, that the evidence for intrusion of the G2 granite into the Seridó Group in this region was due to miscorrelation of the xenolith samples in the granite. We propose, in- stead, that the Jucurutu Formation sits unconformably upon the G2 granite (they are sub-parallel throughout the region), and that the xenoliths in the G2 gran- ite are actually fragments of older, similar looking, schists and gneisses from the basement complex.

5.4. Isotope dilution U–Pb results

Our initial U–Pb results from Faz. São Pedro (FSP) and SE of Barra de Santa Rosa (BSR) yielded a discor- dia (not shown) with intercepts at ca. 740 Ma and ca. 2000 Ma, which we interpreted as the ages of detrital or xenocrystic components (ca. 2000 Ma) and the age of a syn-depositional volcanic component (740 Ma). We subsequently obtained additional isotope dilution zircon data from these two localities and from sev- eral other localities (Table 3; Appendix B). At the 93-48 locality we analyzed zircons from two types of rock: light-gray felsic layers 10–20 cm thick (93-48B, 93-48C) that are interlayered with the normal schists and typical schist (93-48E, 93-48G). At the Fazenda

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São Pedro locality we also analyzed zircons from two types of rock: two light-colored felsic layers 20–30 cm thick (93-26, 94-80) and a brownish, fine-grained silty sandstone (95-104) that are interlayered with the nor- mal schists. We also extracted zircons from biotite schist at a quarry in an outlier of Seridó Formation west of Natal (93-3; location 7, Fig. 2), from a bi- otite schist that crops out along highway BR-304, 18 km west of Riachuelo (93-33; location 6, Fig. 2), and from discontinuous felsic zones in otherwise nor- mal, amphibolite grade (garnet-bearing) Seridó schist from outcrops near the town of Solanea, Paraı́ba state (MVULC, location 21, Fig. 2).

Results of our TIMS–ID zircon analyses are in Table 3 and Appendix B. The zircons include a both multi-grain and single-grain fractions and present a wide variety of ages and discordance, with 207Pb/206Pb ages ranging from 2600 to 620 Ma (Appendix B). For the multi-grain analyses some of

Fig. 5. Cathodoluminescence photo of zircons from sample 95-104, a fine-grained sandstone interbedded with finer grained metagraywacke of the Seridó Formation (upper part of the Seridó Group). This sample is from a region of low metamorphic grade (greenschist to lower amphibolite), and there are no visible overgrowths on the zircons. Thus, the ages measured on this population must represent the primary ages of their sources, and the ages were not reset during the ca. 600 Ma. Brasiliano metamorphism. Photo is 500 m wide.

the results probably include mixtures of grains with diverse ages, and some of the single-grain analyses could include grains with cores, overgrowths, or both. Nonetheless, many of the analyses yield ages sub- stantially younger than 1000 Ma (Table 3). Assuming that Brasiliano metamorphic overgrowths do not con- tribute significantly to the data (a valid assumption, as shown in the following), then the host rocks must originally have been deposited in the Neoprotero- zoic. Furthermore, there are several fractions with 207Pb/206Pb ages less than the 740 Ma age inferred earlier (Van Schmus et al., 1995b; Van Schmus et al., 1997), suggesting that the depositional age is late Neoproterozoic.

Although the TIMS–ID results were much closer to the truth than originally realized, residual uncertain- ties about multi-grain mixtures, metamorphic over- growths, or metamorphic resetting did not permit firm conclusions about the depositional ages or provenance

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Fig. 6. Concordia plot for SHRIMP, 204-corrected, zircon analyses from sample 95-104. The detrital zircons have major populations at ca. 650, 790, and 970 Ma, indicating a depositional age ≤650 Ma. Regional tectonics and metamorphic ages require that the depositional age be >610 Ma.

of the units within the Seridó Group using these data alone.

5.5. SHRIMP U–Pb results

SHRIMP data greatly enhanced our understanding of the depositional age of the Seridó Group, its Sm–Nd isotopic characteristics, and the tectonic history of the region. Bulk zircon populations were extracted from four samples: 95-104, a fine-grained metasandstone from the low-grade Seridó Formation sequence at Fazenda São Pedro; 93-48G, a garnet-bearing biotite schist from the Seridó Formation sequence southeast of Barra de Santa Rosa; EC-61, a quartz-rich biotite gneiss from the Jucurutu Formation in the town of Jucurutu (type locality); and SED-J-10, a meta-arkose from Fazenda da Lapa (Van Schmus et al., 1995a). Fig. 5 shows cathodoluminescence (CL) images of zircons from 95-104, and Fig. 6 shows a conventional U–Pb concordia diagram for zircons from that sam- ple. Fig. 7 shows CL images of zircons from 93-48G, and Fig. 8 shows a conventional U–Pb concordia di-

agram for zircons from that sample. Fig. 9 shows CL images of zircons from EC-61, and Fig. 10 shows a conventional U–Pb concordia diagram for zircons from that sample. Fig. 11 shows both conventional histograms and probablity distribution curves for ages less than 1200 Ma (Table 4) for all three samples. Finally, Fig. 12 shows CL images of zircons from SED-J-10, and Fig. 13 shows a conventional U–Pb concordia diagram for zircons from that sample.

There are several important features about these data, but one of the basic aspects is that metamorphic overgrowths are very minor to absent for all zircons in all four samples (cf. Figs. 5, 7, 9 and 12), even where the metamorphic grade is relatively high. Thus, the ages obtained from both SHRIMP and TIMS–ID anal- yses are not significantly biased by Brasiliano meta- morphic overprints. The SHRIMP ages must be the true ages of the individual zircons for several reasons: (1) CL images clearly show that primary igneous zon- ing present in most grains; (2) many grains still have euhedral to subhedral crystal form; (3) metamorphic overgrowths cannot be seen for sample 95-104, and

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Fig. 7. Cathodoluminescence photo of zircons from sample 93-48G, a biotite-, garnet-, cordierite-schist from an outlier of the Seridó Formation in the SE part of the Rio Grande do Norte domain. In spite of the fact that this sample was metamorphosed at upper amphibolite facies, only incipient overgrowths (white, low-U areas) are present. Thus, the ages measured on this population represent the primary ages of their sources, and the ages were not reset during the ca. 600 Ma. Brasiliano metamorphism. Photo is 500 m wide.

Fig. 8. Concordia plot for SHRIMP, 204-corrected, zircon analyses from sample 93-48G. The detrital zircons have major populations at ca. 680, 740–800, and 970 Ma, indicating a depositional age ≤660 Ma. Regional tectonics and metamorphic ages require that the depositional age be >610 Ma.

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Fig. 9. Cathodoluminescence photo of zircons from sample EC-61, a fine-grained, siliceous meta-graywacke of the Jucurutu Formation (lower part of the Seridó Group). Althought this sample was metamorphosed at high grade, only incipient or thin overgrowths are present. Thus, the ages measured on the interiors of grains represent the primary ages of their sources, and the ages were not reset during the ca. 600 Ma. Brasiliano metamorphism. Photo is about 500 m wide.

Fig. 10. Concordia plot for SHRIMP, 204-corrected, zircon analyses from sample EC-61. The detrital zircon population has major clusters at ca. 650 and 970 Ma, with several grains having intermediate ages. The depositional age must be ≤650 Ma. Regional tectonics and metamorphic ages require that the depositional age be >610 Ma.

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Fig. 11. Histograms and probability distribution curves for 204-corrected 206Pb/238U ages of detrital zircon populations from samples 95-104, 93-48G, and EC-61. Data from Table 4. Ages greater than 1200 Ma are not shown. Note significant populations of zircons at ca. 650 Ma, which set maximum depositional ages for these units.

they are only present as thin, discontinuous rims (white due to low U) in 93-48G and EC-61, which are higher grade schist and gneiss, respectively. This conclusion is further supported by the work of Williams (1998a,b) and Williams and Chappell (1998), who reported data for detrital zircon suites from schists in the contact metamorphic aureole of an S-type granite in the Lach- land Fold Belt of SE Australia. SHRIMP U–Pb anal- yses were reported for zircon populations in several

metamorphic zones of increasing grade, starting with sub-greenschist grade metasedimentary rocks and end- ing with the xenocrystic cores in the granite itself. In all cases the zircon age distribution remained un- changed (cf. Williams, 1998a), showing conclusively that zircons are relatively insensitive to alteration dur- ing metamorphism up to and including anatexis. Once we accept the validity of the SHRIMP ages, we can address several other questions.

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Fig. 12. Cathodoluminescence photo of zircons from sample SED-J-10, a meta-arkose from the lower part of the Jucurutu Formation (lower unit of the Seridó Group). In spite of the fact that this sample was metamorphosed at amphibolite grade, only incipient overgrowths (bright areas) are present. Thus, the ages measured on this population represent the primary ages of their sources, and the ages were not reset during the ca. 600 Ma. Brasiliano metamorphism. Photo is 500 m wide.

The most important conclusion from the geochrono- logic data is that the youngest detrital zircons (Table 4; Fig. 11) average about 650 Ma, thus requiring that the depositional age is ≤650 ± 5 Ma. Since these rocks were deformed and metamorphosed about 600±10 Ma during the Brasiliano orogeny (Section 2.3), they must be older. Furthermore, both Seridó Formation sam- ples (95-104 and 93-48G) and the Jucurutu Formation sample (EC-61) yielded similar results, indicating that most of the sequence was deposited in a relatively short interval.

There are several other distinct age clusters among the younger (<1200 Ma) zircons (Fig. 11). These in- clude a cluster about 750 Ma in 93-48G, which influ-

enced earlier interpretations of the depositional age (Van Schmus et al., 1995b; Van Schmus et al., 1997), a population about 790 Ma in 95-104, and 950–1000 Ma zircons in all three samples. We believe the middle Neoproterozoic provenance is real, since most zir- cons plot near concordia and are based only on the 204-corrected method. In addition, samples 95-104 (Fig. 6), 93-48G (Fig. 8), and EC-61 (Fig. 10) include a few Paleoproterozoic to Archean grains, but they are not a major portion of the population.

SHRIMP data for sample SED-J-10 confirm earlier TIMS–ID results for that sample, but yield more pre- cise ages for the two main populations: 1798± 17 Ma and 2207 ± 12 Ma (Fig. 13). The TIMS–ID work

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Fig. 13. Concordia plot for SHRIMP, 204-corrected, zircon analyses from sample SED-J-10. The detrital zircon population has major populations at ca. 1800 and 2200 Ma. The depositional age must be ≤1800 Ma, inconsistent with an age of ca. 2000 Ma for the Serido Group. One grain has an age of ca. 680 Ma (Table 4) which, if indigenous to the sample, requires deposition <680 Ma. Regional tectonics and metamorphic ages require that the depositional age be >610 Ma.

included one grain with an age of about 1400 Ma, but this was not confirmed by the SHRIMP work. How- ever, the SHRIMP analyses included one grain with an age of 684± 7 Ma (Table 4), suggesting that rocks at this locality were also deposited less than 700 Ma. Because only one young grain was encountered, it should be used cautiously for setting depositional age limits. In any case, the cluster at 1798 ± 17 Ma pre- cludes these rocks being older than G2 granites in the region.

6. Discussion, interpretations, and conclusions

6.1. Age of Seridó Group

Sedimentary rocks must be younger than the youngest component contained within them. Since some of the detrital zircons have original ages of

ca. 650 Ma, then samples 95-104, 98-43, and EC-61 must all be younger than ca. 650 Ma. This includes not only the upper unit, the Seridó Formation, but also at least the upper part of the lower unit, the Jucurutu Formation; if the single grain in SED-J-10 with an age of 684 Ma (Table 4) is indigenous to that rock, then virtually all of the Seridó Group must be younger than 700 Ma. Because these rocks were deformed and metamorphosed during the Brasiliano orogeny, which peaked at ca. 600 Ma in the region, the Seridó Group must be older than this, and probably older than 610 Ma. Hence, the probable window for deposition of these rocks is 650–610 Ma. Although this might seem to be a relatively short window for deposition of the Seridó Group, deposition of several kilometers of siliciclastic sediment within a few mil- lion years in active tectonic regimes is not unusual in the modern world, and it should not be unusual in the Neoproterozoic world.

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6.2. General provenance of the Seridó Group

The Sm–Nd data for younger members of the Seridó Formation yield average crustal formation ages of 1.2–1.6 Ga for the constituent detritus. Since mixing of detritus from several sources is a possible explanation of such data (Arndt and Goldstein, 1987), there are two end-member models to explain the TDM ages: First, all the detritus could have come from a single source having a TDM of 1.2–1.6 Ga; that source could be juvenile rocks of those ages, or younger rocks with inherited component. An example of the latter option is rocks of the Cariris Velhos orogen, south of the Patos shear zone, which formed about 1.0 Ga but have TDM 1.2–1.6 Ga (Van Schmus et al., 1995a; Kozuch et al., 1997b; Kozuch, 2003). There are no known rocks in NE Brazil that could satisfy the first option. Second, the TDM ages could be artifacts of mixing of material of diverse ages, ranging from juvenile, late Neoproterozoic rocks to Archean base- ment. In this case, there are innumerable options for the ages of the mixing components (Figs. 3 and 4).

The SHRIMP zircon results clearly show that the second explanation is preferred for the Sm–Nd data in the Seridó Group: The TDM ages are the result of mixing material having a wide range of crystalliza- tion ages and crustal formation ages. Inspection of the SHRIMP results shows major contributions from ca. 650, 750–800, 950–1000, 1800, 2200 Ma, and Archean sources with scattered contributions from sources having other ages. Thus, the ranges in TDM ages for the Seridó Group are, in most cases, due to dominance of detritus from one or another source. The fact that the TDM ages are significantly greater than 1.0 Ga, whereas most of the zircons are younger than 1.0 Ga, is probably due to the fact that most igneous crustal rocks in NE Brazil have Sm–Nd model ages significantly greater than their crystallization ages (case in point: 1.0 Ga Cariris Velhos orhogneisses and metavolcanics with 1.4–1.6 Ga TDM ages, mentioned above).

Sm–Nd studies for several sedimentary basins have shown a general younging upward of the TDM model ages (Barovich et al., 1989; McLennan and Hemming, 1992; McLennan et al., 1995; Anderson and Samson, 1995; Boghossian et al., 1996). This has generally been attributed to dominance of proximal, old base- ment for lower units in the basin, followed by evolu-

tion to dominance from younger, more distal sources as the basin fills and the old, proximal basement is covered. We cannot assign exact stratigraphic position to our Seridó Group samples, but we believe that pro- gressive burial of the regional basement did prevail, so that the samples with the highest εNd(600) values and younger TDM ages represent the uppermost parts of the section. This is consistent with results from several of our samples (Fig. 14): SED-J-10 is near the base of the Jucurutu Group and contains mainly Paleoproterozoic zircons, which are readily available locally. Two samples collected 1 km apart across the contact between the Jucurutu Formation (95-112) and the overlying Seridó Formation (95-113) yield ages of 1.51 and 1.26 Ga, respectively that indicate the prove- nance for the Seridó Formation was overall slightly younger than that for the Jucurutu Formation. This conclusion is supported by the fact that many samples in the Seridó Formation have TDM ages of 1.4–1.2 Ga, whereas all our pelitic metasedimentary samples from the Jucurutu Formation have TDM ages of 1.5–1.6 Ga.

The Sm–Nd model ages for all pelitic metasedimen- tary rocks of the Seridó Group are less than 1.6 Ga. However, a few other units associated with the Seridó Group have distinctly older model ages, with TDM as old as 3.5 Ga (Table 3). For the outcrops at Fazenda da Lapa, the probable explanation is that the felsic gneisses, which are interpreted here as meta-arkoses, contain debris derived directly from the local Rio Piranhas massif basement, which typically has TDM ages of 2.4–2.6 Ga (Van Schmus et al., 1995a; Dantas, 1997). This is consistent with the detrital population being dominated by ca. 1.8 Ga and ca. 2.2 Ga zircons (Fig. 13; Appendix C). The 2.2 Ga population zir- cons can easily be from proximal sources; the nearest source for 1.8 Ga zircons is to the west in the 1.8 Ga Jaguaribeano belt, near Orós in Ceará state (Fig. 1; Sá et al., 1995), although there may be several other ca. 1.8 Ga cratonic sequences north of the Patos shear zone or on the São Franciso craton.

At Rio Potengi there are a few thin metavolcanic units near the base of the section (1, Fig. 14). These all yield very old Sm–Nd model ages (TDM 3.2–3.5 Ga), and we suggest that the magmas for these metavol- canic rocks were derived from subjacent crust and/or lithospheric mantle associated with the nearby São José do Campestre massif (Fig. 1), which has been shown to contain an Archean nucleus with rocks as

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Fig. 14. Sketch showing inferred relationships during deposition of the Seridó Group. 1: Paleoproterozoic and Archean basement. 2: Mafic, rift-related (?), volcanic rocks occur locally at or near the base of the section. 3: Arkosic units at or near the base of the section contain abundant Paleoproterozoic zircons and old Sm–Nd TDM model ages (ca. 2.6 Ga). 4: Finer grained siliciclastic sediments of the middle to upper Jucurutu Formation were derived mainly from more distal Neoproterozoic sources (0.65–1.0 Ga), with correspondingly younger detrital zircon suites and TDM ages. 5: Carbonate and calcsilicate rocks occur dispersed throughout the Jucurutu Formation. 6: Thin to very thick units of the Equador quartzite occur discontinuously between the Jucurutu Formation and the overlying Seridó Formation; locally the Seridó Formation unconformably overlies the Jucurutu Formation or the basement complex. 7: Units of the Seridó Formation generally have younger TDM model ages indicating younger provenance, although detrital zircon suites are similar to those from the upper Jucurutu Formation.

old as 3.4 Ga (Dantas, 1997; Dantas et al., 1998). Sim- ilarly, we suggest that the possible tuffs at Fazenda São Pedro (93-26, 94-80) contain a substantial base- ment component, and that felsic gneiss 94-83 may be a sill or rhyolite tuff also derived from subjacent base- ment. Thus, metaigneous rocks in the Seridó Group appear to be derived from subjacent Paleoproterozoic to Archean basement, whereas most of the detritus in the pelitic units (which dominate the section) appears to have come from distal Neoproterozoic sources.

6.3. Zircon provenance for metaturbidites of the Seridó Group

The Seridó Formation was originally a graywacke (turbidite) deposited upon underlying units of the Jucurutu Formation or directly upon older cratonic basement of the Rio Piranhas-São José do Campestre massifs. These basins were receiving detritus from Neoproterozoic volcanic, plutonic, or metamorphic

terranes having significant magmatism at ca. 650, 750–800, and 950–1000 Ma, in addition to minor con- tributions from 1800, 2200 Ma, and Archean sources. The older zircons are easily accounted for by the older basement complexes in the Borborema Province, as mentioned above. The sources for the Neoproterozoic zircons are more problematic.

The ca. 950–1000 Ma zircon population has a pos- sible source near the Seridó fold belt. This source is now represented by orthogneisses (metaplutons), fel- sic volcanic rocks, and felsic siliciclastic rocks of the Cariris Velhos orogen south of the Patos shear zone in Pernambuco and Paraı́ba states (Brito Neves et al., 1995; Bittar, 1998), which have zircon populations of 940–1000 Ma (Kozuch et al., 1997a,b,c; Van Schmus et al., 1999; Kozuch, 2003) and Sm–Nd model ages (TDM) ranging from ca. 1000–1600 Ma (Van Schmus et al., 1995a; Kozuch, 2003). If much of the sedi- ment in the Seridó Group was derived from the Cariris Velhos orogen, then it must have been transported

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northward across the present Patos shear zone to the Rio Grande do Norte domain. Therefore, if the Patos shear zone represents a major suture between terranes of the DZT and the Rio Grande de Norte domain, it must have been closed or nearly closed prior to de- position of the Seridó Group. It is also possible, of course, that the ca. 1000 Ma zircons came from a more distal source to the north, west, or east, but at present we do not know of any likely candidates.

The sources of the ca. 750–800 Ma detrital zircons in the Seridó Group likewise have not been identified in the RGND. Fetter (1999) and Santos et al. (1998) reported middle Neoproterozoic volcanic and plutonic rocks of approximately that age in the Médio Coreau domain of NW Ceará state (Fig. 1), and isolated occurrences of rocks with zircon ages 750–900 Ma have also been reported from the DZT, to the south of the Patos shear zone (Van Schmus et al., 1995b; Kozuch et al., 1997a,b,c), although they are not yet well studied. Pimentel and Fuck (1992) and Pimentel et al. (1997) reported 750–900 Ma magmatic ages from Neoproterozoic units in the Brasilia belt to the southwest, and Toteu et al. (2001) reported middle Neoproterozoic ages from Pan African terranes in northern Cameroon. Thus, there are several possible distal sources for zircons in the 700–900 Ma range, although none of them can be uniquely identified at this time.

The provenance of the ca. 650 Ma zircons which define the maximum depositional age for the Seridó Group is also presently unconfirmed. Zircons of this age or slightly younger (e.g. 630 Ma) were found by Van Schmus et al. (1999) in detrital zircon popula- tions from metasedimentary rocks in central Paraı́ba state, south of the Patos shear zone (Bittar, 1998; Sá et al., 1998), and which have been recently recognized as late Neoproterozoic. Kozuch (2003) determined TIMS–ID U–Pb ages of 620–630 Ma for zircons from some metavolcanic rocks within this sequence and from an undeformed rhyolitic dike in the nearby Teix- eira structural high. Guimarães et al. (1998, 2000) reported ages of ca. 640 Ma for deformed Brasiliano granites in the DZT. These could be suitable sources, within the margin of error for defining the age of the young population through the SHRIMP data, but the data are not compelling. On the other hand, these zircons also could have come from unknown, distal sources elsewhere in Gondwana.

6.4. Evolution of the Seridó depositional basin

The Seridó basin formed shortly before or during early stages of the continental collision that formed West Gondwana, namely between 650 and 610 Ma. The origin of the basin is not yet well defined and will not be explored in detail here, but it probably included some extensional phase in its early stages, as shown by the presence of mafic volcanic rocks at the base of the Jucurutu Formation (Fig. 14). In Rio Grande do Norte, this extension probably did not proceeded far enough to yield oceanic crust, since no juvenile, late Neoproterozoic oceanic or island arc complexes have been found in this part of the Borborema Province. Pimentel and Fuck (1992) and Pimentel et al. (1997) described juvenile Neoproterozoic oceanic or island arc complexes in the Brasilia belt to the west of the São Franciso craton, and Fetter et al. (2003) have argued that the Santa Quitéria complex (SQ, Fig. 1) in NW Ceará state is a continental-margin magmatic arc. The Santa Quitéria complex and the juvenile complexes of the Brasilia belt could be parts of a major convergent system along the western part of the São Francisco craton and Borborema Province. If so, then the Seridó basin and other late Neoproterozic basins in the east- ern part of the province could be related to foreland deposition associated with this convergent margin. It is also possible that the basin formed by intracratonic extension and deformation unrelated to any active con- tinental margin. In any case, detrital zircon and Nd iso- topic data show that the Serido basin developed only a few tens of million years before the main orogenesis that accompanied the assembly of West Gondwana.

Fig. 14 shows inferred relationships during deposi- tion of the Seridó Group. Regional mapping indicates that this group is underlain everywhere by Paleopro- terozoic to Archean basement of the Rio Grande do Norte domain (1) and thus started as an intracratonic basin. Mafic, rift-related (?), volcanic rocks (2) occur locally at or near the base of the section. Conglom- eratic or arkosic units (3) at or near the base of the section were derived mainly from proximal sources in the region and contain abundant Paleoproterozoic zircons and old Sm–Nd TDM model ages (ca. 2.6 Ga). Basal deposits quickly covered the proximal base- ment, and finer grained siliciclastic sediments of the middle to upper Jucurutu Formation (4) were derived mainly from more distal Neoproterozoic sources

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(0.65–1.0 Ga), with correspondingly younger detrital zircon suites and TDM ages.

Carbonate rocks (5) occur as marbles and calcsili- cate gneisses dispersed throughout the Jucurutu For- mation, but we have no isotopic data to constrain their origin. Thin to very thick units of the Equador quartzite (6) occur discontinuously between the Jucurutu For- mation and the overlying Seridó Formation and may be bounded by unconformities; there are no provenance data for these units at this time. Locally the Seridó For- mation unconformably overlies the Jucurutu Forma- tion with no quartzite present; the boundary probably represents a short hiatus in sedimentation. Although detrital zircon data do not allow for a long hiatus, an interval of erosion, non-deposition, and (or) deforma- tion up to 10 or 20 million years is possible.

Units of the Seridó Formation (7) generally have younger TDM model ages, indicating younger prove- nance. Detrital zircon populations are similar, and although there seems to be greater dominance of younger zircons in one of the Seridó Formation samples (95-105; Table 4), our data are not compre- hensive enough to draw broad conclusions regarding major changes in provenance. The Seridó Formation generally represents deeper water sedimentation, as indicated by turbidite depositional features, and lo- cally it overlies the basement complex directly. Thus, there was significant deepening and broadening of the basin as it evolved.

6.5. Conclusions

In summary, we conclude that:

1. The Seridó Group was probably deposited between 650 and 610 Ma, possibly between 640 and 620 Ma.

2. The detritus in the Seridó basin came from sources with ages as old as Archean and as young as 650 Ma, although some of the older zircons could have come from reworked younger sediments. The Sm–Nd model ages (TDM) for most metasediment samples represent mixing of Nd compositions from many sources, and the 1.2–1.6 Ga model ages do not relate to any specific major geologic events in NE Brazil.

3. The Seridó depositional basin and its subsequent deformation may have formed as a result of a short extensional and contractional tectonic cycle be-

tween 700 and 600 Ma. This tectonic cycle could have occurred in a back-arc extensional environ- ment adjacent to an active continental margin to the south. Alternatively, the tectonic cycle could have involved intracratonic rifting farther from a continental margin, followed by subsequent clo- sure of a small ocean basin to the south during the Brasiliano–Pan African orogenies and assembly of West Gondwana.

4. The presence of ca. 1000 Ma (Cariris Velhos) de- trital zircons north of the Patos shear zone sug- gests that the Rio Grande do Norte domain and the Domaine Zona Transversal were in sedimen- tological continuity soon after 650 Ma at the lat- est. If the Patos shear zone is a Brasiliano suture, closure must have occurred during early stages of the orogeny, with major deformation and metamor- phism occurring subsequently. Alternatively, the Patos shear zone may represent an older terrane boundary (post-Cariris Velhos) that was reactivated after 600 Ma during final transpressive stages of the Brasiliano–Pan African collision (Vauchez et al., 1995).

5. U–Pb ages of individual zircon grains in detrital populations can provide very detailed information about the depositional history of silicilastic se- quences and the sources of the detritus contained within them. These results can, in turn, be used to place important constraints on the tectonic evolu- tion of regions, and those constraints may not be attainable by any other methods.

Acknowledgements

We are indebted to many colleagues and students who have contributed to on-going field, petrologic, structural, and geochemical studies in the Borborema Province. Over the past decade the general geologic framework of NE Brazil has become much better known through their efforts, making our sampling for, acquisition of, and interpretation of isotopic data possible. Special thanks are given to Emanuel Jardim de Sá, Renauld Caby, Jaziel Sá, Carlos Archanjo, and Edilton dos Santos for useful, and sometimes lively, discussions of the problem. Reviewers’ comments by J. Bertrand and G. Gehrels helped significantly in preparing the final version of the manuscript.

W.R. Van Schmus et al. / Precambrian Research 127 (2003) 287–327 315

W.R.V.S. wishes to acknowledge support from the U.S. National Science Foundation (Grants EAR 9117594, 9316047, and 9614473). He also thanks the University of Kansas and the Research School of Earth Sciences, ANU, for logistical and analyti- cal support during his visit to the SHRIMP facility

Appendix A. Summary of sample locations (deg:min) and descriptions

Rio Grande do Norte (RN) and Paraı́ba (PB) states 93-03 7 (05:52.26S, 35:24.54W; RN). Biotite-rich migmatitic gneiss, in quarry. 93-26 4 (05:33.97 S, 36:04.35W; RN). Metagraywacke of Seridó Group. Outcrop north of road

at Faz. São Pedro. Sampled light gray, 0.3 m thick, layer (tuff?). 93-33 5 (05:45.05S, 35:57.11W; RN). Metagraywacke. Biotite–garnet schist with well preserved

bedding and crossbedding. Outcrop at crest of hill, 18 km west of Riachuelo. 93-34 5 (05:45.57S, 35:58.66W; RN). Biotite–garnet ± sillimanite ± cordierite schist. Roadcut

highway. 304, 22.5 km west of Riachuelo. 93-35 6 (05:44.59S, 36:04.9W; RN). Biotite schist-gneiss of Serido Group. Outcrop along

highway 304 about 8 km west of Caicara do Rio do Vente. 93-42 13 (05:58,54S, 36:41.94W; RN). Freshly blasted debris from pipleline trench. Serdio

Formation. Road to Santana do Matos—Jucurutu, 1 km east of dam. 93-45 19 (06:32.59S, 36:26.81W; PB). Serido Formation bio schist. Outcrop in stream, north of

road between Carauba and Picui, just east of Paraı́ba border. 93-48 20 (06:48.97S, 35:57.09W; PB). Outcrop of Seridó Formation on highway 104 about 20 km

southeast of Barra de Santa Rosa. Blasted pieces north of road. Mostly Biotite–garnet–cordierite schist; may include tuffs. B, C: possible tuffs; E, G: Biotite–garnet schist.

94-70 9 (06:08.15 S; 36:14.48 W; RN). Felsic gneiss outcrop in Rio Potengi (location of B.B.B.N. sample SED-G-APO).

94-71 9 (06:08.15S, 36:14.48W; RN). Amphibolitic gneiss outcrop in Rio Potengi a few meters upstream from 94-70 (metabasalt?).

94-72 9 (06:08.15S, 36:14.48W; RN). Felsic gneiss (meta-tuff?) interlayered with amphibolitic gneisses of 94-71; outcrop in Rio Potengi.

94-73 9 (06:08.15S, 36:14.48W; RN). Dacitic gneiss outcrop in Rio Potengi about 100 m upstream from 94-72; same location as B.B.B.N. sample SED-D-60.

94-74 9 (06:08.17S, 36:14.58W; RN). Fine-grained felsic gneiss outcrop in Rio Potengi, from same location as B.B.B.N. sample SED-Gf-55.

94-75 9 (06:08.21S; 36:14.69W; RN). Augen gneiss outcrop in Rio Potengi, same location as B.B.B.N. sample SED-C-50.

94-76 9 (06:08.21 S, 36:14.69W; RN). Granitic gneiss outcrop in Rio Potengi, 100 m downstream (north) of 94-75; same location as B.B.B.N. sample SED-Gn-RS.

94-77 8 (05:58.73S, 36:10.33W; RN). Typical Seridó schist from fresh roadcut on road west from São Tomé, east of turn to Rio Potengi.

94-80 4 (05:33.93S, 36:04.31W; RN). Light felsic layer (tuff?) in Seridó schists; outcrop in spillway of dam at Fazenda São Pedro, just north of road from João Camarão to Pedra Preta (similar to 93-26).

94-81 4 (05:33.93S, 36:04.31W; RN). Typical Seridó schist; outcrop in spillway of dam at Fazenda São Pedro, just north of road from João Camarão to Pedra Preta.

94-82 4 (05:33.93S, 36:04.31W; RN). Typical Seridó schist; outcrop in spillway of dam at Fazenda São Pedro, just north of road from João Camarão to Pedra Preta.

at ANU, where he was a Visiting Research Fel- low from February through May, 1999. B.B.B.N., P.C.H., and E.L.D. wish to acknowledge research grants from FAPESP, and M.B. B.B.B.N. and P.C.H. wish to thank CNPq for research fellow- ships.

316 W.R. Van Schmus et al. / Precambrian Research 127 (2003) 287–327

Appendix A. (Continued )

94-83 4 (05:34.69S, 36:05.40W; RN). Felsic unit in Seridó schists; outcrop in road bed, east slope of hill about 3 km west of Fazenda São Pedro; possible meta-rhyolite quartz–feldspar porphyry.

94-84 13 (05:58.54S, 36:41.94W; RN). Typical Seridó schist, fresh material from pipeline construction, south of road from Santana do Matos to Jucurutu, 1 km east of dam.

94-86 14 (06:02.32S, 37:00.32W; RN). Typical Jucurutu metasedimentary gneiss. Roadcut on highway east of Jucurutu, west of turn-off to north.

94-87 15 (06:05.74S, 37:03.83W; RN). Typical Jucurutu metasedimentary gneiss. Outcrop east side of road, about 7 km south of Jucurutu.

94-88 17 (06:25.95S, 36:51.33W; RN). “Low-grade” Seridó schist (banded sequence) from outcrop on north side of Caico-Cruzeta highway, 3 km east of São José do Seridó.

94-89 18 (06:24.64S, 36:47.98W; RN). Typical Seridó schist from outcrop north of highway just west of town of Cruzeta (south of dam).

94-90 (06:48.48S, 37:09.86W; RN). Strongly lineated felsic layer (meta-tuff? meta-arkose?) in Jucurutu metasediments; outcrops west of Fazenda da Lapa (B.B.B.N. location SED-J-2).

94-92 22 (06:48.43S, 37:10.05W; RN). Typical Jucurutu gneiss; outcrops west of Fazenda da Lapa. 94-94 22 (06:48.41S, 37:10.31W; RN). Strongly lineated felsic layer (meta-tuff? meta-arkose?) in

Jucurutu metasediments; outcrops west of Fazenda da Lapa (B.B.B.N. location SED-J-4). 94-95 22 (06:48.39S, 37:10.43W; RN). Strongly lineated felsic layer (meta-tuff? meta-arkose?) in

Jucurutu metasediments; outcrops west of Fazenda da Lapa (B.B.B.N. location SED-J-5). 95-101 8 (05:58.14 S; 36:09.63 W; RN). Jucurutu gneiss; outcrop on north side of road from São

Tomé to Rio Potengi about 200 m east of bridge. 95-102 9 (06:07.79 S; 36:14.89 W; RN). Jucurutu gneiss, Rio Potengi; northernmost Jucurutu outcrop

on west bank of river (weathered Seridó schist to north). 95-103 9 (06:08.21 S; 36:14.68 W; RN). Tonalitic gneiss in core of antiform, Rio Potengi; intruded by

augen gneiss of SED-C-50. 95-104 4 (05:33.93 S; 36:04.31 W; RN). Seridõ schist for detrital zircons, Faz. São Pedro (same

outcrop as 93-26, 94-79 to 94-82). 95-105 3 (05:43.11 S; 36:42.05 W; RN). Low-grade Seridó schist; outcrop along gravel road soutwest

from Angicos toward São Rafael. 95-106 2 (05:45.05 S; 36:45.35 W; RN). Jucurutu gneiss; outcrop along gravel road soutwest from

Angicos toward São Rafael. 95-107 2 (05:45.16 S; 36:45.78 W; RN). Granitic augen gneiss (G2) outcrop along gravel road

soutwest from Angicos toward São Rafael. 95-108 2 (05:45.16 S; 36:45.78 W; RN). Xenolith of Jucurutu (?) gneiss in granitic augen gneiss (G2)

outcrop (95-107) along gravel road soutwest from Angicos toward São Rafael. 95-109 1 (05:45.24 S; 36:47.43 W; RN). Sample of Jucurutu (?) gneiss in fault contact (intruded by?)

augen gneiss correlative with 95-107. Outcrop on north side of old railroad cut west of road SW of Angicos.

95-110 1 (05:45.24 S; 36:4.7.43 W; RN). Sample of Jucurutu (?) gneiss in fault contact (intruded by?) augen gneiss correlative with 95-107. Same outcrop as 95-109.

95-111 1 (05:45.24 S; 36:47.43 W; RN). Xenolith (?) of Jucurutu (?) gneiss in augen gneiss correlative with 95-107. Same outcrop as 95-109.

95-112 16 (06:13.58 S; 36:40.56 W; RN). Felsic/calcsilicate variety of Jucurutu gneiss a few meters west of contact with Seridó schist; outcrop on south side of highway, east of São Vicente toward Currais Novos.

95-113 16 (06:14.26 S; 36:39.97 W; RN). Seridó schist; outcrop about 1 km east of 95-112 on south side of highway between São Vicente and Currais Novos.

EC-61 14 Sample collected in town of Jucurutu, RN, by E. Dantas. MVULC 21 (06:41.81 S; 35:40.59 W; PB). MVULC provided by Jardim de Sá (1995). Fine-grained,

gray-feldspar porphyry (tuff?) in Serido Formation; outcrop along hillside and exposed in farm road, about 200–300 m. east of main road.

Field Number, location (Fig. 2), (latitude [degrees:minutes], longitude [degrees:minutes]; state: RN, Rio Grande do Norte; PB, Paraı́ba. Description, location, and comments.

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Appendix B. U–Pb Data for zircons from Seridó Group units in northeastern Brazil

Fractiona Size (mg)

Concentrationsb Observedc Atomic ratiosd Ages ± 2σ (Ma)e

U (ppm)

Pb (ppm)

206Pb/ 204Pb

206Pb/ 238U

207Pb/ 235U

207Pb/ 206Pb

206Pb/ 238U

207Pb/ 235U

207Pb/ 206Pb

93-3 (EC-29): Seridó schist near Natal (7, Fig. 2) 01: NM(1)el 0.005 283 54 507 0.15946 1.8822 0.08561 953 ± 08 1068 ± 11 1311 ± 10 02: M(−1)eu, cl 0.066 341 48 159 0.12390 1.0793 0.06318 753 ± 41 743 ± 46 714 ± 61 03: M(−1)el 0.040 67 12 1405 0.16104 1.8527 0.08344 963 ± 05 1064 ± 06 1280 ± 02 04: M(0) 0.040 195 68 368 0.27285 6.5257 0.17346 1555 ± 26 2049 ± 33 2591 ± 05 05: M(2) el 0.030 284 41 2676 0.13761 1.3221 0.06968 831 ± 05 855 ± 06 919 ± 03

93-26: Felsite in Seridó schist at Fazenda Sao Pedro (4, Fig. 2) 01: NM(1) 0.005 283 54 507 0.15946 1.8822 0.08561 954 ± 13 1075 ± 18 1329 ± 19 02: NM(0) 0.033 173 31 454 0.14566 1.6388 0.08160 877 ± 06 985 ± 08 1236 ± 06 03: M(0) 0.041 303 51 976 0.14727 1.7956 0.08843 886 ± 08 1044 ± 12 1392 ± 16 04: M(1) 0.079 523 66 1260 0.11369 1.2878 0.08215 694 ± 05 840 ± 06 1249 ± 03 05: M(2) 0.053 628 77 1154 0.11169 1.2883 0.08366 683 ± 05 841 ± 06 1285 ± 03

93-33: Seridó schist outcrop on highway BR 304, west of Riacheleu (5, Fig. 2) 01: M(−1) cl 0.011 402 66 1738 0.15238 2.4522 0.11671 914 ± 05 1258 ± 07 1907 ± 03 02: M(−1) rd 0.033 268 56 932 0.18795 2.9395 0.11343 1110 ± 11 1392 ± 16 1855 ± 04 03: M(0) st, cl 0.022 284 20 952 0.06260 0.74293 0.08607 391 ± 03 564 ± 04 1340 ± 04

93-48B: Felsic bed in schist. Outcrop of Seridó Formation on BR 104, SE of Barra de Santa Rosa (20, Fig. 2) 01: NM(−1)eu, cl 0.046 194 32 132 0.12823 1.1284 0.06382 778 ± 36 767 ± 38 736 ± 35 02: M(−1) rd, cl 0.011 582 65 1417 0.11725 1.1323 0.07994 715 ± 09 769 ± 10 930 ± 06 03: M(−1) el 0.035 248 82 296 0.27473 7.1469 0.18867 1565 ± 38 2129 ± 53 2730 ± 10 04: M(0) el 0.060 335 77 6790 0.22323 3.6452 0.11843 1299 ± 09 1560 ± 11 1933 ± 02

93-48C: Felsic bed in schist. Outcrop of Seridó Formation on BR 104, SE of Barra de Santa Rosa (20, Fig. 2) 01: NM(−2) 0.113 220 46 90 0.15341 2.2667 0.10716 920 ± 72 1202 ± 116 1751 ± 99 02: NM(−1) 0.109 340 48 2543 0.13349 1.4099 0.07660 808 ± 05 893 ± 06 1111 ± 03 03: M(0) 0.008 440 87 117 0.15670 2.1206 0.09815 938 ± 18 1155 ± 28 1589 ± 37

93-48E: Schist, outcrop of Seridó Formation on BR 104, SE of Barra de Santa Rosa (20, Fig. 2) 01: NM(0) eu, cl, s 0.002 171 24 402 0.12270 1.0763 0.06362 746 ± 19 742 ± 27 729 ± 53 02: M(0) eu, cl, s 0.004 71 10 525 0.12048 1.0645 0.06408 733 ± 17 736 ± 18 744 ± 18

94-80: Felsic bed in Seridó schist at Fazenda Sao Pedro (4, Fig. 2) 01: M(0) eu, cl 0.008 440 87 117 0.15670 2.1206 0.09815 938 ± 61 1156 ± 76 1589 ± 20

318 W

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(2003) 287–327

Appendix B. (Continued )

Fractiona Size (mg)

Concentrationsb Observedc Atomic ratiosd Ages ± 2σ (Ma)e

U (ppm)

Pb (ppm)

206Pb/ 204Pb

206Pb/ 238U

207Pb/ 235U

207Pb/ 206Pb

206Pb/ 238U

207Pb/ 235U

207Pb/ 206Pb

95-104: Meta-sandstone in schist at Fazenda São Pedro (4, Fig. 2) 01: NM(−3) el 0.002 321 41 1247 0.11970 1.0641 0.06447 729 ± 05 736 ± 05 757 ± 05 02: NM(−3) s 0.009 246 30 4425 0.11224 0.9746 0.06298 686 ± 04 691 ± 05 708 ± 05 03: M(−3) eu 0.006 330 37 1710 0.10161 0.87114 0.06218 624 ± 04 636 ± 04 680 ± 03 04: M(−3) eu 0.005 174 21 1061 0.10944 0.93611 0.06204 669 ± 05 671 ± 05 675 ± 04 05: M(−1) eu 0.007 392 53 3287 0.12791 1.33124 0.07549 776 ± 04 859 ± 05 1082 ± 03 06: M(−1) eu, s 0.005 514 47 1598 0.08667 0.75160 0.06289 536 ± 03 569 ± 03 705 ± 03

EC-61: Jucurutu gneiss, town of Jucurutu (14, Fig. 2) 01: NM(−1) eu, cl, s 0.005 309 73 2706 0.21584 2.9264 0.09833 1260 ± 06 1388 ± 07 1593 ± 03 02: NM(−1) eu, s 0.002 387 52 558 0.10125 0.8594 0.06156 622 ± 04 630 ± 04 659 ± 04 03: M(0) eu, py, cl 0.006 210 50 1719 0.22376 3.0712 0.09954 1301 ± 07 1426 ± 07 1616 ± 02 04: M(1) eu, py, tb, s 0.003 207 28 688 0.12529 1.2157 0.07038 761 ± 04 808 ± 05 939 ± 05 05: M(1) eu, tb, s 0.003 861 270 740 0.27128 4.5814 0.12248 1547 ± 20 1746 ± 24 1993 ± 02 06: M(2) sh, tb 0.005 3564 448 1723 0.11751 1.4613 0.09020 716 ± 04 915 ± 05 1430 ± 02 07: M(2) eu, br, s 0.002 1849 184 428 0.08440 0.7320 0.06290 522 ± 03 558 ± 04 705 ± 06 08: M(3) eu, tb, s 0.002 1659 175 481 0.09909 0.8710 0.06375 609 ± 03 636 ± 04 733 ± 05 09: M(3) eu, tb 0.007 2467 328 1637 0.12325 1.5840 0.09321 749 ± 04 964 ± 05 1492 ± 02 10: M(4) sh, py 0.007 3682 362 1611 0.09288 0.8215 0.06414 572 ± 03 609 ± 03 746 ± 02 11: M(4) eu, br, tb 0.002 2669 266 936 0.09618 0.8219 0.06198 592 ± 03 609 ± 04 673 ± 05

MVULC-AM-A: Felsic layers in Seridó Schist near Soleana (21, Fig. 2) 01: M(1) 0.009 322 39 975 0.11470 1.3242 0.08373 700 ± 08 856 ± 10 1286 ± 08 02: M(2) 0.005 144 19 621 0.11367 1.0254 0.06543 694 ± 07 717 ± 10 788 ± 20 03: M(2) 0.006 352 27 922 0.07641 0.6373 0.06049 475 ± 05 501 ± 08 621 ± 26 04: M(3) 0.009 518 46 929 0.08538 0.7614 0.06468 528 ± 03 575 ± 03 764 ± 05

a NM: non-magnetic, M: magnetic, numbers in parentheses indicate side tilt used on Franz separator at 1.5 A power; c: coarse fraction (>200 mesh); cl: internally clear grains; el: elongate grains; eu: euhedral grains; fr: externally frosted grains; rd: rounded grains; sh: short; st: stubby; tb: turbid; tr: transluscent; br: brown; pu: purple; py: pale yellow; ye: yellow; n/a: not abraded; s: single grain.

b Total U and Pb concentrations corrected for analytical blank. c Not corrected for blank or non-radiogenic Pb. d Radiogenic Pb corrected for blank and initial Pb; U corrected for blank. e Ages given in Ma using decay constants recommended by Steiger and Jäger (1977). Uncertainties at 2σ.

W.R. Van Schmus et al. / Precambrian Research 127 (2003) 287–327 319

Appendix C. SHRIMP age results for detrital zircon grains (“204Pb-corrected” data)

Grain.spot U (ppm)

Th (ppm)

Pba

(ppm)

206Pb/238U age (Ma) ± 1σ

207Pb/235U age (Ma) ± 1σ

207Pb/206Pb age (Ma) ± 1σ

208Pb/232Th age (Ma) ± 1σ

Sample 95-104: Fine-grained sandstone from Fazenda São Pedro 104-02.1 235 116 32 790 ± 11 776 ± 18 737 ± 60 718 ± 24 104-03.1 159 154 21 679 ± 12 665 ± 31 618 ± 129 674 ± 29 104-04.1 172 41 22 790 ± 18 763 ± 33 688 ± 117 673 ± 84 104-05.1 113 90 16 772 ± 21 818 ± 22 944 ± 48 809 ± 32 104-06.1 628 610 108 894 ± 10 932 ± 19 1024 ± 54 828 ± 20 104-07.1 112 90 21 971 ± 18 965 ± 27 952 ± 73 973 ± 44 104-08.1 142 134 18 649 ± 10 666 ± 29 726 ± 117 666 ± 22 104-09.1 92 53 31 1708 ± 46 1743 ± 32 1785 ± 37 1718 ± 72 104-10.1 133 61 23 1005 ± 18 982 ± 23 933 ± 61 982 ± 36 104-11.1 166 127 30 965 ± 18 988 ± 24 1040 ± 62 984 ± 33 104-12.1 124 77 22 969 ± 17 976 ± 24 992 ± 63 970 ± 57 104-13.1 118 105 17 780 ± 13 755 ± 39 679 ± 151 750 ± 30 104-14.1 435 112 71 982 ± 11 991 ± 16 1012 ± 41 964 ± 26 104-15.1 98 70 14 779 ± 15 775 ± 46 765 ± 174 730 ± 58 104-16.1 46 23 7 826 ± 20 811 ± 65 772 ± 240 849 ± 90 104-17.1 122 136 25 1016 ± 22 1021 ± 21 1032 ± 41 1019 ± 38 104-18.1 169 89 39 1255 ± 22 1249 ± 22 1239 ± 43 1285 ± 42 104-20.1 584 537 71 648 ± 6 662 ± 12 708 ± 47 623 ± 9 104-21.1 23 10 3 805 ± 41 775 ± 142 686 ± 591 765 ± 196 104-22.1 294 161 47 903 ± 10 917 ± 12 949 ± 28 897 ± 18 104-23.1 211 114 23 639 ± 14 591 ± 22 414 ± 92 588 ± 38 104-24.1 260 275 33 652 ± 14 644 ± 21 617 ± 78 655 ± 20 104-25.1 328 245 51 839 ± 9 859 ± 17 909 ± 53 815 ± 15 104-26.1 176 87 24 779 ± 14 769 ± 25 741 ± 86 754 ± 35 104-27.1 189 113 21 650 ± 10 644 ± 19 624 ± 78 640 ± 23 104-28.1 234 156 33 799 ± 17 777 ± 22 716 ± 65 758 ± 35 104-29.1 353 275 42 654 ± 7 650 ± 13 638 ± 50 642 ± 14 104-30.1 175 170 70 1864 ± 32 1863 ± 20 1861 ± 19 1847 ± 46 104-31.1 71 41 10 816 ± 17 748 ± 40 550 ± 155 776 ± 51 104-32.1 47 26 7 899 ± 27 791 ± 66 498 ± 256 746 ± 86 104-33.1 79 115 12 691 ± 21 708 ± 27 762 ± 82 689 ± 36 104-34.1 102 55 12 688 ± 23 691 ± 44 701 ± 169 683 ± 52 104-35.1 340 345 66 977 ± 14 972 ± 13 960 ± 25 981 ± 20 104-36.1 150 52 70 2331 ± 28 2298 ± 22 2267 ± 30 2311 ± 54 104-37.1 953 1628 144 679 ± 8 685 ± 9 704 ± 23 670 ± 10 104-38.1 463 228 51 639 ± 8 657 ± 10 719 ± 29 657 ± 14 104-39.1 317 253 38 653 ± 10 643 ± 19 610 ± 74 664 ± 24 104-40.1 194 105 32 935 ± 13 967 ± 16 1040 ± 38 870 ± 33 104-41.1 92 58 10 628 ± 16 598 ± 65 487 ± 327 577 ± 63 104-42.1 77 45 13 944 ± 21 935 ± 51 911 ± 160 950 ± 48 104-43.1 571 420 104 978 ± 11 994 ± 12 1031 ± 25 967 ± 17

320 W.R. Van Schmus et al. / Precambrian Research 127 (2003) 287–327

Appendix C. (Continued )

Grain.spot U (ppm)

Th (ppm)

Pba

(ppm)

206Pb/238U age (Ma) ± 1σ

207Pb/235U age (Ma) ± 1σ

207Pb/206Pb age (Ma) ± 1σ

208Pb/232Th age (Ma) ± 1σ

104-44.1 172 136 21 666 ± 12 672 ± 23 690 ± 85 638 ± 23 104-45.1 127 167 21 788 ± 13 760 ± 29 677 ± 106 767 ± 29 104-46.1 330 366 57 866 ± 12 885 ± 16 933 ± 41 856 ± 19 104-47.1 272 108 29 653 ± 10 657 ± 13 674 ± 42 653 ± 20 104-48.1 264 351 36 648 ± 12 665 ± 14 723 ± 41 655 ± 16 104-49.1 167 91 61 1843 ± 32 1866 ± 20 1891 ± 18 1843 ± 48 104-50.1 80 69 45 2482 ± 60 2479 ± 30 2477 ± 18 2457 ± 101 104-51.1 104 91 20 990 ± 20 989 ± 24 987 ± 56 988 ± 32 104-52.1 117 115 15 678 ± 15 666 ± 39 626 ± 165 667 ± 33 104-53.1 113 75 15 723 ± 18 690 ± 33 584 ± 128 719 ± 34 104-54.1 373 269 45 677 ± 32 674 ± 28 661 ± 42 668 ± 40 104-55.1 162 91 55 1757 ± 31 1785 ± 26 1818 ± 39 1730 ± 53 104-56.1 135 43 18 794 ± 11 776 ± 21 727 ± 70 808 ± 35 104-57.1 128 163 63 2063 ± 26 2068 ± 19 2073 ± 24 2113 ± 40 104-58.1 168 65 22 784 ± 14 771 ± 25 731 ± 88 737 ± 39 104-59.1 143 158 18 642 ± 26 610 ± 48 492 ± 205 610 ± 38 104-60.1 137 71 16 681 ± 11 646 ± 33 526 ± 146 635 ± 43 104-61.1 160 106 24 827 ± 12 822 ± 17 810 ± 50 847 ± 38

Sample 93-48G: Biotite schist from SE of Barra de Santa Rosa 48G-01.1 591 166 78 804 ± 8 798 ± 9 781 ± 21 804 ± 15 48G-02.1 163 58 21 761 ± 10 759 ± 19 751 ± 69 761 ± 23 48G-03.1 2366 229 357 957 ± 7 962 ± 6 973 ± 10 815 ± 19 48G-04.1 672 242 88 783 ± 10 772 ± 10 740 ± 27 769 ± 18 48G-05.1 42 31 8 1020 ± 35 975 ± 49 877 ± 135 1025 ± 67 48G-06.1 1079 206 130 757 ± 5 756 ± 6 754 ± 16 767 ± 13 48G-07.1 320 133 40 739 ± 14 728 ± 22 697 ± 76 733 ± 44 48G-09.1 155 99 32 1107 ± 29 1091 ± 26 1059 ± 48 1117 ± 43 48G-10.1 185 53 21 717 ± 13 704 ± 18 665 ± 60 685 ± 35 48G-11.1 96 74 18 1005 ± 17 1007 ± 18 1012 ± 36 1012 ± 29 48G-12.1 137 75 18 748 ± 11 750 ± 22 756 ± 80 746 ± 32 48G-13.1 51 18 9 995 ± 30 976 ± 51 933 ± 148 979 ± 85 48G-14.1 441 292 62 780 ± 11 777 ± 18 766 ± 58 778 ± 16 48G-15.1 243 172 108 2118 ± 26 2158 ± 18 2196 ± 22 2053 ± 39 48G-16.1 128 63 22 956 ± 14 960 ± 31 967 ± 92 963 ± 28 48G-17.1 237 300 32 665 ± 12 684 ± 14 750 ± 36 657 ± 18 48G-18.1 280 146 49 993 ± 17 988 ± 21 976 ± 52 1000 ± 29 48G-19.1 755 266 84 681 ± 9 673 ± 10 649 ± 28 671 ± 20 48G-20.1 563 281 68 727 ± 6 732 ± 8 749 ± 25 496 ± 16 48G-21.1 288 104 47 963 ± 12 960 ± 15 955 ± 37 908 ± 29 48G-22.1 141 85 20 805 ± 11 775 ± 21 690 ± 76 813 ± 29 48G-23.1 135 119 25 958 ± 15 979 ± 23 1027 ± 61 990 ± 25 48G-24.1 229 103 26 670 ± 11 658 ± 17 616 ± 60 669 ± 24

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Appendix C. (Continued )

Grain.spot U (ppm)

Th (ppm)

Pba

(ppm)

206Pb/238U age (Ma) ± 1σ

207Pb/235U age (Ma) ± 1σ

207Pb/206Pb age (Ma) ± 1σ

208Pb/232Th age (Ma) ± 1σ

48G-25.1 86 112 15 837 ± 14 841 ± 23 853 ± 71 824 ± 28 48G-26.1 143 76 59 2056 ± 26 2046 ± 18 2036 ± 22 2020 ± 53 48G-27.1 428 246 56 747 ± 10 746 ± 10 742 ± 23 737 ± 15 48G-28.1 65 37 9 807 ± 15 821 ± 30 857 ± 100 844 ± 31 48G-29.1 80 43 9 671 ± 13 665 ± 23 648 ± 90 683 ± 36 48G-30.1 65 17 26 2113 ± 57 2128 ± 32 2142 ± 20 2035 ± 104 48G-31.1 88 34 14 914 ± 21 873 ± 37 772 ± 117 892 ± 59 48G-32.1 108 124 17 779 ± 14 741 ± 27 626 ± 97 773 ± 35 48G-33.1 477 38 170 1995 ± 19 1991 ± 17 1987 ± 26 1972 ± 71 48G-34.1 127 16 15 736 ± 11 757 ± 16 820 ± 48 800 ± 57 48G-35.1 89 25 12 802 ± 15 797 ± 19 783 ± 52 793 ± 39 48G-36.1 306 360 40 655 ± 7 650 ± 13 630 ± 54 662 ± 11 48G-37.1 46 61 19 1757 ± 79 1766 ± 58 1777 ± 72 1741 ± 107 48G-38.1 763 456 100 752 ± 5 748 ± 7 736 ± 19 691 ± 11 48G-39.1 86 60 40 2199 ± 85 2173 ± 56 2148 ± 63 2184 ± 125 48G-40.1 392 35 43 715 ± 9 700 ± 11 651 ± 33 643 ± 46 48G-41.1 174 113 23 754 ± 14 724 ± 31 633 ± 119 742 ± 38 48G-42.1 118 108 45 1817 ± 39 1796 ± 25 1772 ± 23 1822 ± 57 48G-43.1 232 281 39 823 ± 11 794 ± 16 714 ± 53 788 ± 17 48G-44.1 1615 989 212 743 ± 5 734 ± 5 707 ± 14 734 ± 8 48G-45.1 619 268 71 680 ± 7 678 ± 10 672 ± 35 677 ± 13 48G-46.1 212 260 43 980 ± 14 982 ± 14 986 ± 31 968 ± 24 48G-47.1 512 342 64 693 ± 10 694 ± 10 695 ± 23 698 ± 17 48G-48.1 101 102 20 979 ± 22 1010 ± 21 1079 ± 35 1035 ± 33 48G-49.1 843 138 290 1896 ± 13 1942 ± 8 1992 ± 7 1896 ± 28 48G-50.1 861 601 342 1935 ± 15 1951 ± 9 1969 ± 7 1912 ± 23 48G-51.1 191 75 26 808 ± 14 789 ± 20 733 ± 65 790 ± 30 48G-52.1 93 35 11 739 ± 20 704 ± 45 597 ± 178 745 ± 53 48G-53.1 195 45 24 771 ± 10 758 ± 19 721 ± 68 710 ± 47 48G-54.1 207 135 30 821 ± 16 802 ± 22 750 ± 67 793 ± 35 48G-55.1 150 114 20 744 ± 15 738 ± 31 717 ± 115 728 ± 32 48G-56.1 206 127 78 1895 ± 21 1892 ± 14 1889 ± 15 1896 ± 53 48G-57.1 94 50 13 782 ± 15 781 ± 25 778 ± 81 740 ± 35 48G-58.1 76 96 51 2644 ± 41 2622 ± 23 2605 ± 22 2708 ± 63 48G-59.1 88 59 11 687 ± 17 685 ± 20 677 ± 56 689 ± 48 48G-60.1 313 141 184 2714 ± 26 2685 ± 14 2663 ± 12 2700 ± 44

Sample EC-61: Fine-grained biotite gneiss from Jucurutu EC61-01.1 69 61 23 1558 ± 61 1599 ± 39 1654 ± 27 1744 ± 107 EC61-02.1 230 87 28 728 ± 10 721 ± 17 701 ± 58 744 ± 33 EC61-03.1 715 777 90 646 ± 6 631 ± 12 578 ± 49 645 ± 9 EC61-04.1 860 100 92 691 ± 5 681 ± 7 648 ± 22 690 ± 21 EC61-05.1 848 609 104 677 ± 6 666 ± 8 631 ± 27 684 ± 13

322 W.R. Van Schmus et al. / Precambrian Research 127 (2003) 287–327

Appendix C. (Continued )

Grain.spot U (ppm)

Th (ppm)

Pba

(ppm)

206Pb/238U age (Ma) ± 1σ

207Pb/235U age (Ma) ± 1σ

207Pb/206Pb age (Ma) ± 1σ

208Pb/232Th age (Ma) ± 1σ

EC61-06.1 86 35 44 2486 ± 33 2451 ± 24 2422 ± 31 2474 ± 144 EC61-07.1 394 129 152 2014 ± 15 2034 ± 11 2055 ± 13 2062 ± 29 EC61-08.1 467 319 57 685 ± 7 687 ± 9 696 ± 27 688 ± 16 EC61-08.2 67 27 7 664 ± 19 673 ± 27 703 ± 92 709 ± 48 EC61-09.1 230 124 27 676 ± 12 668 ± 25 642 ± 98 638 ± 29 EC61-10.1 185 268 77 1772 ± 25 1853 ± 20 1945 ± 26 1718 ± 35 EC61-11.1 233 417 36 666 ± 11 675 ± 20 708 ± 72 681 ± 16 EC61-12.1 414 554 55 649 ± 10 657 ± 11 681 ± 28 636 ± 16 EC61-13.1 131 113 26 1036 ± 17 1012 ± 20 961 ± 48 1045 ± 29 EC61-14.1 97 49 12 724 ± 12 698 ± 24 616 ± 91 697 ± 47 EC61-15.1 139 209 32 1036 ± 15 1032 ± 15 1023 ± 29 1045 ± 25 EC61-16.1 49 10 6 716 ± 20 736 ± 34 797 ± 114 771 ± 115 EC61-17.1 112 185 26 1015 ± 15 997 ± 23 959 ± 62 1032 ± 26 EC61-18.1 899 435 100 655 ± 8 655 ± 8 653 ± 22 657 ± 15 EC61-19.1 124 140 24 913 ± 27 950 ± 25 1038 ± 42 1011 ± 45 EC61-20.1 148 169 20 683 ± 13 677 ± 25 658 ± 97 711 ± 20 EC61-21.1 121 116 23 992 ± 14 975 ± 31 937 ± 92 985 ± 25 EC61-22.1 265 53 39 924 ± 11 903 ± 20 853 ± 62 842 ± 66 EC61-23.1 925 248 122 808 ± 5 799 ± 7 775 ± 19 799 ± 10 EC61-24.1 170 109 20 650 ± 8 648 ± 10 644 ± 36 677 ± 15 EC61-25.1 389 280 149 1839 ± 14 1896 ± 9 1959 ± 10 2023 ± 33 EC61-26.1 257 442 61 1016 ± 10 1023 ± 11 1037 ± 27 1051 ± 14 EC61-27.1 256 93 30 714 ± 14 693 ± 19 627 ± 61 726 ± 30 EC61-28.1 156 97 19 678 ± 12 655 ± 16 578 ± 56 677 ± 23 EC61-29.1 403 85 63 964 ± 11 953 ± 16 928 ± 43 855 ± 45 EC61-30.1 87 93 28 1521 ± 38 1497 ± 34 1464 ± 55 1521 ± 59 EC61-31.1 196 125 81 2027 ± 27 2003 ± 21 1979 ± 28 1968 ± 41 EC61-32.1 167 116 36 1141 ± 13 1106 ± 24 1036 ± 64 1148 ± 29 EC61-33.1 80 69 15 1000 ± 20 1033 ± 19 1104 ± 34 1020 ± 44 EC61-34.1 48 27 7 837 ± 23 791 ± 41 665 ± 144 856 ± 53 EC61-35.1 554 884 80 663 ± 7 655 ± 11 629 ± 39 659 ± 10 EC61-36.1 199 118 34 955 ± 12 944 ± 13 917 ± 31 967 ± 23 EC61-37.1 114 47 70 2824 ± 45 2747 ± 21 2691 ± 12 2826 ± 72 EC61-38.1 43 16 6 882 ± 29 916 ± 28 998 ± 52 936 ± 50 EC61-39.1 563 306 103 1023 ± 8 1009 ± 8 979 ± 17 1031 ± 19 EC61-40.1 95 45 17 1014 ± 14 993 ± 26 948 ± 75 995 ± 48 EC61-41.1 361 231 68 1028 ± 14 1018 ± 15 996 ± 32 1041 ± 26 EC61-42.1 166 192 22 653 ± 9 660 ± 17 683 ± 66 658 ± 16 EC61-43.1 310 195 57 1013 ± 17 999 ± 13 970 ± 19 1037 ± 24 EC61-44.1 399 520 89 1048 ± 9 1034 ± 11 1004 ± 27 1051 ± 14 EC61-45.1 1914 2386 341 856 ± 6 865 ± 7 888 ± 19 877 ± 8 EC61-46.1 68 66 12 923 ± 37 889 ± 41 803 ± 103 954 ± 54 EC61-47.1 204 188 130 2674 ± 24 2671 ± 12 2668 ± 9 2749 ± 38

W.R. Van Schmus et al. / Precambrian Research 127 (2003) 287–327 323

Appendix C. (Continued )

Grain.spot U (ppm)

Th (ppm)

Pba

(ppm)

206Pb/238U age (Ma) ± 1σ

207Pb/235U age (Ma) ± 1σ

207Pb/206Pb age (Ma) ± 1σ

208Pb/232Th age (Ma) ± 1σ

EC61-48.1 272 103 38 828 ± 8 810 ± 12 761 ± 38 802 ± 20 EC61-49.1 293 154 33 647 ± 7 643 ± 16 629 ± 64 663 ± 16 EC61-50.1 229 137 44 1064 ± 12 1039 ± 14 989 ± 34 1032 ± 27 EC61-51.1 197 139 111 2528 ± 28 2503 ± 16 2482 ± 15 2566 ± 46 EC61-52.1 280 572 46 682 ± 6 667 ± 13 616 ± 50 683 ± 9 EC61-53.1 153 163 19 634 ± 13 618 ± 32 561 ± 143 639 ± 24 EC61-54.1 285 146 44 875 ± 11 857 ± 14 813 ± 37 877 ± 21 EC61-55.1 57 62 7 661 ± 23 652 ± 54 622 ± 230 671 ± 41 EC61-56.1 241 195 32 708 ± 12 699 ± 16 671 ± 49 736 ± 19 EC61-57.1 1171 215 123 671 ± 6 662 ± 7 633 ± 17 633 ± 16 EC61-58.1 522 230 91 1009 ± 13 985 ± 12 933 ± 24 996 ± 23 EC61-59.1 298 41 44 939 ± 16 922 ± 16 882 ± 34 818 ± 52 EC61-60.1 200 231 41 1002 ± 11 969 ± 22 894 ± 65 990 ± 18 EC61-61.1 78 36 14 993 ± 29 981 ± 26 953 ± 47 1003 ± 70 EC61-62.1 77 131 19 1035 ± 21 1066 ± 21 1130 ± 40 1069 ± 30 EC61-63.1 398 412 77 971 ± 9 982 ± 9 1006 ± 17 960 ± 17 EC61-64.1 53 45 10 1027 ± 22 1009 ± 54 970 ± 165 1013 ± 69 EC61-65.1 292 242 42 769 ± 8 758 ± 10 725 ± 31 783 ± 16 EC61-66.1 340 102 40 715 ± 11 708 ± 16 687 ± 53 725 ± 29 EC61-67.1 253 413 41 726 ± 11 711 ± 18 664 ± 61 751 ± 17 EC61-68.1 239 33 32 849 ± 10 826 ± 15 763 ± 44 799 ± 46

Sample SED-J-10: Meta-arkose from Fazenda da Lapa J10-01.1 54 44 19 1666 ± 43 1715 ± 33 1774 ± 44 1749 ± 91 J10-02.1 135 40 47 1809 ± 41 1986 ± 31 2177 ± 36 2091 ± 133 J10-03.1 26 22 9 1668 ± 66 1777 ± 69 1907 ± 113 1847 ± 116 J10-04.1 26 22 9 1668 ± 76 1697 ± 59 1733 ± 78 1712 ± 145 J10-05.1 309 113 71 1184 ± 17 1637 ± 40 2281 ± 78 1952 ± 152 J10-06.1 143 143 54 1741 ± 34 1758 ± 26 1777 ± 35 1775 ± 54 J10-07.1 133 102 59 2069 ± 41 2139 ± 29 2207 ± 33 2020 ± 69 J10-08.1 508 319 87 901 ± 11 1287 ± 19 2008 ± 38 982 ± 28 J10-09.1 588 221 92 844 ± 8 1230 ± 18 1988 ± 38 1265 ± 43 J10-10.1 30 23 11 1704 ± 64 1747 ± 58 1799 ± 89 1761 ± 121 J10-11.1 37 30 13 1692 ± 83 1643 ± 77 1579 ± 130 1478 ± 156 J10-12.1 59 49 20 1680 ± 43 1742 ± 44 1818 ± 73 1660 ± 76 J10-13.1 81 61 28 1729 ± 35 1761 ± 27 1800 ± 36 1704 ± 71 J10-14.1 49 44 18 1688 ± 38 1744 ± 53 1812 ± 100 1861 ± 71 J10-15.1 53 52 19 1706 ± 38 1740 ± 32 1782 ± 46 1695 ± 74 J10-16.1 41 36 14 1666 ± 56 1759 ± 47 1871 ± 65 1661 ± 101 J10-17.1 51 50 19 1768 ± 57 1793 ± 41 1822 ± 48 1850 ± 92 J10-18.1 146 86 70 2303 ± 31 2259 ± 19 2219 ± 20 2318 ± 55 J10-19.1 38 27 15 1911 ± 68 1860 ± 45 1804 ± 49 2010 ± 113 J10-20.1 68 41 21 1489 ± 42 1624 ± 32 1804 ± 36 1931 ± 82

324 W.R. Van Schmus et al. / Precambrian Research 127 (2003) 287–327

Appendix C. (Continued )

Grain.spot U (ppm)

Th (ppm)

Pba

(ppm)

206Pb/238U age (Ma) ± 1σ

207Pb/235U age (Ma) ± 1σ

207Pb/206Pb age (Ma) ± 1σ

208Pb/232Th age (Ma) ± 1σ

J10-21.1 84 87 34 1837 ± 29 1819 ± 24 1797 ± 35 1828 ± 50 J10-22.1 41 28 15 1790 ± 72 1788 ± 52 1786 ± 63 2042 ± 137 J10-23.1 50 49 20 1852 ± 62 1799 ± 47 1739 ± 65 1924 ± 97 J10-24.1 196 126 91 2195 ± 42 2204 ± 24 2213 ± 19 2211 ± 75 J10-25.1 189 132 88 2183 ± 23 2203 ± 15 2221 ± 16 2189 ± 53 J10-26.1 126 125 48 1784 ± 36 1793 ± 26 1804 ± 32 1789 ± 67 J10-28.1 118 60 57 2321 ± 32 2284 ± 19 2252 ± 18 2360 ± 61 J10-29.1 42 32 15 1773 ± 46 1798 ± 34 1828 ± 43 1745 ± 77 J10-30.1 29 23 12 1863 ± 96 1849 ± 68 1833 ± 80 1997 ± 189 J10-31.1 65 59 25 1800 ± 38 1780 ± 27 1757 ± 33 1786 ± 77 J10-32.1 39 28 14 1754 ± 73 1769 ± 65 1786 ± 100 1796 ± 158 J10-33.1 50 37 19 1836 ± 58 1829 ± 38 1822 ± 41 1982 ± 125 J10-34.1 169 108 75 2120 ± 42 2154 ± 29 2185 ± 33 2153 ± 71 J10-35.1 340 201 108 1646 ± 21 1694 ± 16 1754 ± 22 1585 ± 41 J10-36.1 114 76 58 2363 ± 55 2290 ± 30 2226 ± 26 2331 ± 90 J10-37.1 38 33 16 1911 ± 56 1851 ± 45 1784 ± 66 1911 ± 99 J10-38.1 22 6 10 2317 ± 102 2392 ± 64 2457 ± 64 2915 ± 269 J10-39.1 156 124 72 2118 ± 37 2162 ± 25 2204 ± 30 2187 ± 67 J10-40.1 40 45 17 1857 ± 73 1909 ± 61 1967 ± 85 1916 ± 119 J10-41.1 154 91 50 1661 ± 47 1707 ± 35 1764 ± 43 1814 ± 82 J10-42.1 230 133 99 2082 ± 24 2146 ± 16 2207 ± 16 2097 ± 43 J10-43.1 227 189 109 2205 ± 28 2199 ± 25 2193 ± 37 2182 ± 44 J10-44.1 283 90 61 1124 ± 67 1468 ± 84 2008 ± 138 2218 ± 306 J10-44.2 91 54 33 1812 ± 40 1779 ± 36 1742 ± 58 1778 ± 104 J10-45.1 191 117 89 2231 ± 42 2213 ± 22 2196 ± 14 2168 ± 66 J10-46.1 35 31 14 1878 ± 56 1879 ± 38 1881 ± 42 1968 ± 92 J10-47.1 50 32 17 1690 ± 61 1757 ± 46 1838 ± 55 1799 ± 102 J10-48.1 63 50 24 1824 ± 44 1843 ± 40 1864 ± 62 1889 ± 86 J10-49.1 133 108 51 1837 ± 24 1813 ± 18 1785 ± 24 1907 ± 47 J10-50.1 163 140 82 2288 ± 32 2239 ± 22 2195 ± 28 2255 ± 48 J10-51.1 145 81 69 2272 ± 27 2218 ± 17 2168 ± 20 2350 ± 53 J10-52.1 383 245 46 684 ± 7 686 ± 11 693 ± 40 674 ± 14 J10-52.2 416 237 45 621 ± 10 653 ± 13 765 ± 41 662 ± 20 J10-53.1 38 37 15 1834 ± 46 1826 ± 65 1816 ± 123 1976 ± 113 J10-54.1 120 78 45 1850 ± 42 1807 ± 30 1759 ± 38 1867 ± 74 J10-55.1 378 252 106 1386 ± 19 1729 ± 19 2174 ± 25 1553 ± 42 J10-56.1 34 28 13 1827 ± 72 1816 ± 49 1802 ± 55 2002 ± 119 J10-57.1 53 65 21 1771 ± 38 1758 ± 50 1742 ± 95 1771 ± 71 J10-58.1 21 22 8 1767 ± 89 1748 ± 93 1726 ± 161 2064 ± 181 J10-59.1 57 59 22 1817 ± 51 1836 ± 45 1857 ± 69 1838 ± 102 J10-60.1 153 171 63 1835 ± 26 1806 ± 26 1772 ± 43 1909 ± 46 J10-61.1 103 59 35 1729 ± 30 1801 ± 29 1885 ± 46 1779 ± 70

a Radiogenic Pb.

W.R. Van Schmus et al. / Precambrian Research 127 (2003) 287–327 325

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