Evolution of the Neoproterozoic Sergipano orogenic belt, NE Brazil: detrital zircon geochronology and Sm-Nd isotopes on metasedimentary rocks unravel part of the story

 

 

Oliveira, E.P.¹; Carvalho, M.J.²; Nascimento, R.S.³; McNaughton, N.⁴

 

1. Instituto de Geociências-Unicamp, Brasil. elson@ige.unicamp.br, carvalho@ige.unicamp.br, rosemery@ige.unicamp.br

2. Centre for Global Metallogeny - UWA (nmcnaugh@cyllene.uwa.edu.au)

 

 

Abstract

 

The Sergipano is the main orogenic belt between the Borborema province and the São Francisco craton and it is divided into several lithostratigraphic domains. Sm-Nd model ages (T_DM) and detrital zircon U-Pb SHRIMP geochronology indicate that protoliths of clastic metasedimentary rocks from the Marancó and Macururé domains resulted mostly from erosion of late Mesoproterozoic to early Neoproterozoic areas (1050-950 Ma), while detritus for similar rocks from the Canindé domain came also from younger areas (ca. 700 Ma). Samples from the Vaza Barris domain show the greatest scatter of both T_DM and zircon ages amongst all domains, with important contributions from Neoproterozoic and Mesoproterozoic sources (630-1120 Ma and ca. 1900 Ma) and less significantly from Archaean sources. The Estância domain sandstone has zircon populations at ca. 570 Ma, 620 Ma and 960 Ma, with a few older grains. Our preliminary results support a model in which sediments of the Marancó and Macururé domains were deposited on a continental margin of the ancient Borborema plate before its collision with the São Francisco craton; the Canindé domain is likely to be an aborted Neoproterozoic rift assemblage within the Pernambuco-Alagoas massif (PEAL); and at least the uppermost units of the Estancia and Vaza Barris domains are foreland basins formed during collision of PEAL with the São Francisco craton.

 

Keywords: detrital zircon, geochronology, Sm-Nd isotopes, Sergipano belt, Brazil

 

Introduction

     The Sergipano belt is one of the most important Precambrian orogenic belts of Brazil not only because it was considered as evidence for continental drift (e.g. Allard & Hurst, 1969) but also because it contains several structural and lithologic domains that render it comparable to Phanaerozoic orogens. The Sergipano belt was formed by the continental collision between the Congo-São Francisco Craton and the Pernambuco Alagoas Massif (PEAL) during the Brasiliano/Pan-African orogeny (e.g. Brito Neves et al., 1977). It has been previously interpreted as a typical geosynclinal (e.g. Humphrey & Allard, 1968; Silva Filho & Brito Neves, 1979), then as a collage of lithostratigraphic domains (Davison & Santos, 1989; Silva Filho, 1998), or as a Neoproterozoic fold-thrust belt produced by inversion of a passive margin basin located at the northeastern portion of the São Francisco craton (Santos et al. 1998, D’El-Rey Silva, 1999).

     In spite of the controversial opinions on the tectonic evolution of the Sergipano belt, there is a general consensus about its subdivision into six lithostratigraphic domains from north to south: Canindé, Poço Redondo, Marancó, Macururé, Vaza Barris and Estância, each separated from the other by shear zones (Santos et al., 1988; Davison & Santos, 1989; Silva Filho, 1998). Silva Filho & Torres (2002) have suggested three additional domains: Rio Coruripe, Viçosa and Pernambuco-Alagoas.

     Several authors have mapped the Sergipano belt over the last three decades, but little has been done about sediment provenance and its potential to understanding regional evolution. Here we present whole-rock Sm-Nd isotope data and detrital zircon U-Pb ages for clastic units of the Sergipano belt. Our preliminary results indicate that detritus for metasedimentary units of the Macururé, Marancó and Canindé domains came mostly from Mesoproterozoic source areas, whereas for rocks of the Vaza Barris, Estância and Canindé domains they were supplied from Neoproterozoic sources in addition to Mesoproterozoic ones.

Domains lithology

     The Canindé domain comprises a meta-volcanic-sedimentary sequence (Novo Gosto unit), a sub-volcanic microgabbro-porphyritic quartz-diorite complex (Gentileza unit), the Canindé gabbro-leucogabbro complex, and granitoid bodies (Lajedinho, Boa Esperança, Curralinho, Sitios Novos, Garrote, Xingó and Serra do Catú). The Poço Redondo domain is composed of migmatites and several granitic intrusions (e.g. Serra Negra, Sitios Novos), while the Marancó domain contains a meta-volcanic-sedimentary sequence with peridotite and amphibolite lenses, intruded by the Serra Negra batolith and stocks of the Sitios Novos granite. The Macururé domain consists mostly of garnet mica-schists and phyllites with minor quartzite and marble, all intruded by granitic bodies and a few basic to ultrabasic sheets. Santos et al (1998) subdivided the Vaza Barris domain into the Miaba Group (Itabaiana quartzite/conglomerate, at the base, followed upwards by the Ribeirópolis phyllites, metagreywackes and chorite-schist, and the Jacoca metalimestone), Simão Dias Group (Jacaré metasiltites and Frei Paulo metagreywackes, phyllites and metarhythimites) and the Vaza-Barris Group (Palestina metaconglomerates and phyllites, and Olhos D’Água limestone). The Estância domain comprises, from base to top, sandstones and argilites of the Juetê Formation, dolomites and limestone of the Acauã Formation, sandstones and argillites of the Lagarto Formation, and sandstone and minor conglomerate lenses of the Palmares Formation.

Results and discussion

Sm-Nd data

     Twenty-four metasedimentary rocks from the Marancó and Canindé domains (Nascimento, 2005; Carvalho, 2005) and thirty-seven samples from the Macururé, Vaza Barris and Estância domains were analysed for Sm-Nd isotopes.

     The rationale of the Sm-Nd isotopes for fine-grained sedimentary rocks is that their composition reflects the homogenized average of the source areas from which they derived. In practice, the depleted-mantle Sm-Nd model age (T_DM) is theoretically the maximum age for deposition of a particular sedimentary layer.

     Sm-Nd data for clastic metasediments of the Marancó and Canindé domains indicate model ages respectively in the range 1.2-2.3 Ga and 1.0-1.6 Ga, the former with significant contributions from older sources than the latter; rocks from these two domains should have been deposited after 1.2 Ga or much later in case of the Canindé domain.

   Similar data for the Macururé, Vaza Barris and Estância domains provide interesting insights into their evolution. If these three southermost metasedimentary domains of the Sergipano belt were originally deposited as laterally continuous sequences derived from the ancient São Francisco/West Congo paleocontinent, then T_DM ages of their rock units should be similar to those from the nearby basement rocks, such as the high- and low-grade terranes of the Serrinha block, for which we have an enormous amount of data. Histograms of T_DM ages for these domains (Macururé 1.28-1.78 Ga; Vaza Barris 1.6-3.62 Ga; Estância domain 1.49-1.64 Ga) clearly call for contributions from sources much younger than the São Francisco/West Congo paleocontinent, such as Pernambuco-Alagoas Massif, to the north, Poço Redondo domain and possibly also from the Canindé domain. No T_DM age older than 1.8Ga is observed in both Estância and Macururé domains. On the other hand, the Vaza Barris domain shows the greatest variation amongst all domains. The following T_DM age ranges are available for rock-units of this domain: Itabaiana phyllite (1,88Ga), Ribeirópolis metapelites (2.48-3.62 Ga), Frei Paulo metapelites /metagreywackes (1.67-3.0 Ga), Palestina diamictites (1.67-2.32 Ga) and Olhos D’Água meta-limestones (2.12-2.19 Ga). These numbers indicate that, in addition to the above referred to younger sources; rocks from the São Francisco/West Congo paleocontinent and from the Itabaiana and Simão Dias domes are potential sources for sediments of the Vaza Barris domain.

Detrital zircon U-Pb SHRIMP data

     We present data for one quartzite from the Marancó domain (Minuim unit, after Carvalho, 2005), one metagreywacke from the Canindé domain (Novo Gosto unit, after Nascimento, 2005), one quartzite from the Macururé domain (southerly of Macururé town), one metagreywacke of the Frei Paulo Formation (Vaza Barris domain) and sandstone of the Lagarto Formation (Estância domain, quarry between Lagarto and Simão Dias). 204-corrected ²⁰⁶Pb/²³⁸U ages are used for zircons younger than 1.2 Ga, whereas 204-corrected ²⁰⁷Pb/²⁰⁶Pb ages are preferred for older zircons (cf. Stern, 1997). Age histograms for the samples indicate the following results:

-The Marancó quartzite protolith originated from massive erosion of ca. 980 Ma-old sources. Carvalho et al (this symposium) report a younger zircon population of 960 Ma and constrained the sediments deposition age to the time interval 960-952 Ma. A few Palaeoproterozoic and Archaean zircon grains are present. The 960-1000 Ma zircon population could be derived from erosion of the Cariris Velhos orogen to the north, where Brito Neves et al. (1995) and Kozuch et al. (1997) reported zircon ages in the range 950-1030 Ma

- The Canindé metagreywacke contains significant zircon populations at ca. 690 Ma, 710 Ma and 980 Ma; no zircon older than 1030 Ma was found. The youngest detrital zircon (625 Ma) is metamorphic (Th/U=0.04) and contrasts markedly with the remaining ones which have Th/U ratios higher than 0.5. These numbers set a maximum deposition age of 690 Ma for the Novo Gosto metagreywacke, with the possibility of deposition after 625 Ma. As for the Marancó quartzite, the older zircon population of the Novo Gosto metagreywacke is likely to have been eroded from Cariris Velhos source areas. The 710 Ma and 690 Ma zircon populations, on the other hand, have source areas in the Canindé domain or elsewhere. Santos et al. (1998) report an age of 715 Ma (Van Schmus and co-workers, unpublished U-Pb data) for the Garrote granitic sheet, which crops out between the Novo Gosto unit and the Poço Redondo domain. Rocks of the Gentileza unit and the Canindé gabbroic complex yielded U-Pb SHRIMP ages in the range 680-690 Ma (Nascimento, 2005). Therefore, there stand the possibility that the Novo Gosto unit was a short-lived volcanic-sedimentary basin, possibly rift-related, which underwent most of its deformation during the Brasiliano orogeny through collision of the Poço Redondo migmatitic massif with the Pernambuco-Alagoas Massif.

- The Macururé quartzite contains age populations at ca. 980 Ma, 1020 Ma and less significantly at 2030 Ma. Therefore, the protolith resulted dominantly from erosion of sources with ages around 1.0 Ga, with a few grains coming from Palaeoproterozoic and Archaean sources. Interestingly, no zircon grain younger than 900 Ma was observed; this means that sedimentation of the Macururé rocks took place well before the Neoproterozoic Brasiliano orogeny, possibly shortly after the Cariris Velhos orogeny.

- Data for the Frei Paulo metagreywacke (Vaza Barris domain) are more scattered, with significant zircon populations clustered around 630 Ma, 684 Ma, 922 Ma, 1028 Ma, 1118 Ma and 1873 Ma, with a few Mesoarchean and Neoarchaean grains. These numbers constrain the age of sediment deposition to no later than 630 Ma and highlights contributions from rocks of Mesoproterozoic and Neoproterozoic ages, the latter probably from rocks representative of the early stages of the Sergipano belt evolution. Indeed, rocks of the Gentileza unit and Canindé gabbro (680-690 Ma, Nascimento, 2005), Sitios Novos granodiorite (651 ± 6 Ma, Carvalho, 2005) and of sin-collisional granites (628 Ma, Bueno et al., this Symposium; Long et al., 2003) crop out respectively within the Poço Redondo, Canindé and Macururé domains, to the north.

- The results for sandstone of the Lagarto Formation are of prime importance, because recent suggestions for the stratigraphy of the Estância and Vaza Barris domains (Santos et al., 1998, D’el-Rey, Silva 1999) place the Lagarto Formation close to the base of the original basin, underneath or correlated to the Frei Paulo Formation. Detrital zircon ages for the Lagarto sandstone cluster mostly around 570 Ma, 620 Ma and 960 Ma, with a few Palaeoproterozoic and Archaean grains. The 960 Ma-old zircon population is very similar to the age of Cariris Velhos gneisses and metavolcanics reported by Brito Neves et al (1995) and Kozuch et al. (1997) and of the 952 Ma-old Serra Negra augen gneiss and 980-960 Ma-old migmatites from the Marancó-Poço Redondo domains (Carvalho et al., this Symposium). On the other hand, the two younger zircon populations are widely represented in high-K and ultrapotassic plutons from PEAL and the Transversal Zone of the Borborema Province (Ferreira et al., 1998; Silva Filho et al., 2002; Van Schmus et al., 2003), as well as in granites of the Macururé domain, farther north (e.g. Brito Neves et al., 2003). The 570 Ma-old zircon population constrains also a maximum age for deposition of the Lagarto original sediments. No similar zircon age has been reported in rocks from the northeastern portion of the São Francisco craton. Conversely, 580-570 Ma-old, post-tectonic granites are common in the Borborema Province. Furthermore, the minimum age for collision between the Pernambuco-Alagoas massif and the São Francisco craton can be constrained by sin- to late collisional granites of the Macururé domain, such as the post-D1, 628 Ma-old Camará tonalite, southerly of Gararu town (Bueno et al., 2005, this Symposium), and the post-D2, 625 Ma-old Cel. João Sá granite (Long et al., 2003). A Sm-Nd isochron of 570 Ma was obtained by us on garnet micaschist of the Macururé domain (near Gararu town), and Ar-Ar ages in the time span 636-581 Ma are now available for muscovite and amphibole from the main shear zones of the Sergipano belt (Araújo & Oliveira, 2004). Therefore, the 570-620 Ma age interval found in zircons of the Lagarto sandstone matches also the ages of collision and metamorphism of the Sergipano belt.

     Finally, the implications of our findings for the evolution of the Sergipano belt are numerous. For instance, the Lagarto and the Frei Paulo formations, and possibly also the uppermost units of the Vaza Barris and Estância domains should all be interpreted as deposited in foreland basins and no longer on a passive-margin of the São Francisco plate as envisaged in previous works (e.g. Santos et al. 1998; D’El-Rey-Silva, 1999). Similarly, the precursors of the Macururé and Marancó metasedimentary rocks are likely to have been deposited on a continental margin of a plate that collided later with the São Francisco craton.

 

Acknowledgments

EPO greatly thanks FAPESP (02/03085-2 and 02/07536-4) and CNPq (300845/91) for research grants. CAPES provided scholarships to MJC during his PhD studies at Unicamp and UWA, Australia. RSN thanks FAPESP (00/1294-1) for a PhD scholarship. Discussions with geologists from the Brazilian Geological Survey (CPRM) at Salvador greatly improved our understanding of the Sergipano belt.

 

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