Van Schmus et al 2003

Van Schmus et al 2003

(Parte 1 de 9)

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 Schmusa,∗, B.B. de Brito Nevesb, I.S. Williamsc, P.C. Hackspacherd , a Department of Geology, University of Kansas, Lawrence, KS, USA b Inst. Geociencias, Univ. de São Paulo, São Paulo, SP, Brazil c Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia d Inst. de Geociencias e Ciencias Exatas, Univ. Estadual Paulista, Rio Claro, SP, Brazil

Accepted 27 June 2003


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 1600Ma. Most samples of the Jucurutu Formation, the lower part of the Seridó Group, have TDM ages ranging from 1500 to 1600Ma; some basal units have TDM ages as old as 2600Ma, reflecting proximal basement. Thus, based on Sm–Nd data, most, if not all, of the Seridó Group was deposited after 1600Ma and upper parts must be younger than 1200Ma.

Cathodoluminescence photos of detrital zircons show very small to no overgrowths produced during ca. 600Ma Brasiliano 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. 2200Ma and ca. 1800Ma. In contrast, zircons from a metasedimentary gneiss higher in the Jucurutu Formation yield much younger ages, with clusters at ca. 1000Ma and ca. 650Ma. Zircons from metasedimentary and metatuffaceous units in the Seridó Formation also yield ages primarily between 1000 and 650Ma, with clusters at 950–1000, 800, 750, and 650Ma. Thus, most, if not all, of the Seridó Group must be younger than 650Ma. Because these units were deformed and metamorphosed in the ca. 600Ma Brasiliano fold belt during assembly of West Gondwana, deposition probably occurred ca. 610–650Ma, 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

JucurutuFormationincludessomebasalvolcanicrocksandinitiallyreceiveddetritusfromproximal2.2–2.0Ga(Transamazonian) tolatePaleoproterozoic(1.8–1.7Ga)basement.ProvenancefortheupperJucurutuFormationandalloftheSeridóFormationwas dominated by more distal and younger sources ranging in age from 1000 to 650Ma. 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: (W.R. Van Schmus).

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

288 W.R. Van Schmus et al./Precambrian Research 127 (2003) 287–327 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. 600Ma) 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 sequence 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 deformation. For example, if the Seridó Group is relatively young, then most, if not all, of the deformation described by Hackspacher et al. (1997) must be Brasiliano deformation. On the other hand, if the Seridó Group is relatively old (ca. 2.0 Ga), then the deformation 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 interpretations of field and regional geologic data that have been used to argue for a Paleoproterozoic age (ca. 2Ga) 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 dilution 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 deposited in the late Neoproterozoic, probably between 650 and 610Ma, and we will discuss the implications of this result for the regional tectonic history.

2. Regional geology

The Borborema Province (Fig. 1) can be generalized as (a) basement gneiss and migmatite complexes, (b) deformed and metamorphosed supracrustal sequences, (c) Brasiliano granitic plutons; and (d) Brasiliano shear zones. The province was highly deformed and metamorphosed about 600Ma after formation 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 domains 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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289 Fig. 1. General geologic map for NE Brazil sho wing the major structures and crustal subdi visions (modified from Van Schmus et al., 1995).

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

W.R. Van Schmus et al./Precambrian Research 127 (2003) 287–327 291 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 middle Paleoproterozoic (Transamazonian, 2.2–2.0Ga), although a 2.6–3.4Ga Archean nucleus is well documented in the São José do Campestre massif, west of Natal (Dantas et al., 1998). Sm–Nd crustal residence ages (TDM) for rocks with Transamazonian crystallization or metamorphic ages are typically 2.4–3.0Ga

(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 between older basement and adjacent units of the Seridó Group (Fig. 2), and published U–Pb zircon ages are ca. 1.9–2.0Ga (Jardim de Sa et al., 1988; Legrand et al., 1991).

2.2. Supracrustal fold belts

Several metasedimentary and metavolcanic–metasedimentary basins in the Borborema Province are present as curvilinear fold belts (Fig. 1). The oldest ones are the 1.8–1.7Ga 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 major fold belt in the DZT and southern domain formed about 1.0Ga (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 metasedimentary regions may be younger than, coeval with, or older than the ca. 1Ga 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 domain was considered correlative with the Piancó-Alta Brıgida fold belt in the DZT and the Riacho do Pontal fold belt in the southern domain because of their apparent 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 1Ga or younger fold belts in the DZT and the southern domain are typically 1.6–1.2Ga (Van Schmus et al., 1995a; Kozuch, 2003). This shows that they were not wholly derived from juvenile 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 plutonism. This event is the Brasiliano orogeny in South America; age limits of this orogenic cycle have ranged from 400 to 900Ma prior to development of good geochronologic constraints, and there is still significant 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. 600Ma, roughly coeval with the structurally 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 deformation, 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. 600Ma Brasiliano plutons. These plutons can be recognized on the basis of field relationships as pre-, syn-, or post-tectonic in terms of the

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Summary of stratigraphic relationships for Serid ´ o Group and related Rocks

Ebert (1970) Santos and Brito Ne ves (1984) Archanjo and Salim (1986) Jardim de S´ a (1994) This paper

Serid ´ o Formation Serid ´ o Group (Serid ´ o

Formation) (P arelhas conglomerate)

Serid ´ o Formation (Serid ´ o schists) (P arelhas conglomerate)

(Serid ´ o Formation) (Equador quartzite) (Jucurutu Formation)

(Parte 1 de 9)