Elsevier

Precambrian Research

Volume 298, September 2017, Pages 552-571
Precambrian Research

Geometry, kinematics and geochronology of the Sertânia Complex (central Borborema Province, NE Brazil): Assessing the role of accretionary versus intraplate processes during West Gondwana assembly

https://doi.org/10.1016/j.precamres.2017.07.006Get rights and content

Highlights

  • Top-to-W tectonic transport in the Sertânia Complex coeval with high-T metamorphism.

  • Neoproterozoic deposition of at least part of the Sertânia Complex.

  • Deformation and metamorphism related to the Brasiliano Orogeny.

  • Deformation style and detrital age spectra similar to the Surubim Complex.

  • The Sertânia and Surubim complexes do not belong to distinct terranes.

Abstract

Terrane accretion and intraplate deformation are both common processes occurring during the assembly of large continental masses. The relative roles of these mechanisms are at the heart of a debate concerning the tectonic evolution of the Borborema Province, northeastern Brazil. In the Central subprovince, four terranes or domains have been proposed. In the Alto Moxotó domain, paragneiss of the Sertânia Complex were considered to record a distinct structural and geochronological evolution compared to those in the adjacent Rio Capibaribe domain, in the East. Here, we report the results of a geological study conducted in the type area of the Sertânia Complex. Kinematic criteria and metamorphic mineral assemblages indicate that the main foliation record top-to-the-W tectonic transport under upper amphibolite facies conditions (650–700 °C). The main foliation was affected by overturned folds and upright to inclined folds; the latter two formed coevally with the development of transcurrent shear zones. Samples of mafic orthogneiss and paragneiss tectonically intercalated in the same outcrop were dated by LA-ICP-MS. The orthogneiss sample yields a crystallization age of 1978 ± 8 Ma. Detrital zircons in the paragneiss sample show a multimodal pattern with a well-defined age peak at c. 700 Ma, showing that the paragneiss is younger than 700 Ma and so the main foliation is also younger than this age. In turn, these findings suggest that the occurrence of exclusively Paleoproterozoic zircons in three other dated samples of paragneiss cannot be used to accurately define the deposition age of the detritus. The structural and geochronological characteristics of the Sertânia Complex are similar to those of the Surubim Complex in the Rio Capibaribe domain. Together with previous results showing that the basement of the Alto Moxotó and Rio Capibaribe domains is also similar, we interpret these sequences as having been deposited on a once contiguous tract of continental crust. An accretionary terrane model is thus not required to account for the tectonic evolution of this portion of the Borborema Province. Instead, a model of intracontinental deformation of a preexistent continent is fully supported by the data and also by the lack of evidence for oceanic subduction.

Introduction

Since the recognition that the North American Cordillera grew westward by accretion of terranes with distinct origins (Coney et al., 1980), the concept of terrane-style tectonics has been widely applied to ancient orogenic belts whose topographic expression has been erased long ago (e.g., Williams and Hatcher, 1982, Vaughan et al., 2005, Cawood and Buchan, 2007, Yuan et al., 2007, Wilhem et al., 2012). Precambrian examples, amongst many others, include the Svecofennian orogen (Korja and Heikkinen, 2005), the 1.9 Ga accretionary phase of the Hudsonian Orogeny (Berman et al., 2007), the Racklan and Forward orogens in northwestern Canada (Furlanetto et al., 2013), and, in the Gondwana realm, the Neoproterozoic Arabian-Nubian shield (e.g., Johnson et al., 2011, Robinson et al., 2014) and the Tuareg shield (Black et al., 1994, Caby, 2003, Bosch et al., 2016). Interpreting an orogen as accretionary has wide implications concerning the tectonic setting, paleogeographic reconstructions, and crustal growth models (Cawood and Buchan, 2007). More recently, it has also been increasingly recognized that important intracontinental deformation may occur away from contemporary plate boundaries (Aitken et al., 2013), both in modern (Cunningham, 2005, De Grave et al., 2007, Li and Li, 2007) and in ancient orogens (Nyman et al., 1994, Hand and Sandiford, 1999, Haines et al., 2001, Dickerson, 2003, Ferguson et al., 2004, Faure et al., 2009, Raimondo et al., 2010, Howard et al., 2015). Furthermore, deformation driven by ocean closures and continental collisions may affect areas of preexistent crust previously located in an intracontinental setting (e.g., Avouac and Burov, 1996, Hoepffner et al., 2006, Kroner and Romer, 2013, Villaseca et al., 2015), including the destabilization of large cratonic segments during periods of supercontinent formation (Abdelsalam et al., 2002, De Waele et al., 2006, Liégeois et al., 2013).

In this paper, we discuss the role that accretionary and intraplate processes had in the Neoproterozoic tectonic evolution of the Borborema Province, northeastern Brazil (Fig. 1), during the assembly of West Gondwana. The Borborema Province shows a complex geological evolution, with records of deformation and magmatic events spanning most of the Precambrian (see reviews in Brito Neves et al., 2000, Van Schmus et al., 2008, Neves, 2003, Neves, 2015). The province is divided into three subprovinces by the Patos and Pernambuco shear zones systems: Northern, Central (or Transversal), and Southern (e.g., Brito Neves et al., 2000, Van Schmus et al., 2008, Neves, 2003, Neves, 2015) (Fig. 1b). There is now a consensus that the build-up of the Archean to Paleoproterozoic basement of the province involved the collage of juvenile island arcs and preexisting crustal blocks (Souza et al., 2007, Souza et al., 2016, Dantas et al., 2013, Van Schmus et al., 2008, Van Schmus et al., 2011, Neves et al., 2015a, Santos et al., 2015), and that the Neoproterozoic Brasiliano (=Pan-African) Orogeny resulted from convergence of the São Franscisco-Congo and Amazonian-West Africa cratons, although there is disagreement concerning the width of the oceans involved (Araújo et al., 2014, Neves, 2015). In the Central subprovince, the main discussion concerns the role of terrane assembly during the Neoproterozoic. In one view, its formation occurred via accretion of terranes during two orogenic events: Cariris Velhos in the Early Neoproterozoic (c. 1000–920 Ma) and Brasiliano in the Late Neoproterozoic to early Cambrian (c. 640–550 Ma) (Brito Neves et al., 1995, Brito Neves et al., 2000, Santos and Medeiros, 1999, Santos et al., 2010, Lages and Dantas, 2016, Padilha et al., 2016). Alternatively, the central portion of the Borborema Province is regarded as part of a larger Paleoproterozoic continent that underwent essentially intracontinental deformation during the Brasiliano Orogeny (Mariano et al., 2001, Neves, 2003, Neves, 2015, Da Silva Filho et al., 2014, Bastow et al., 2015, Dias et al., 2015). In this view, the Cariris Velhos event is interpreted as purely extensional, with contractional deformation and metamorphism only occurring during the Brasiliano Orogeny (Neves, 2003, Neves, 2015, Guimarães et al., 2012, Guimarães et al., 2016).

Santos (1995) and Santos and Medeiros (1999) proposed that the Central subprovince resulted from the collage of four terranes (here called domains), which are, from east to west: Rio Capibaribe, Alto Moxotó, Alto Pajeú and Piancó-Alto Brígida (Fig. 1c, inset). The Sertânia Complex (Fig. 1c) was one of the main elements used in support of this proposition (Santos et al., 2004). This complex consists dominantly of migmatized garnet-biotite gneisses that constitute the main metasedimentary unit of the Alto Moxotó domain. The inferred Paleoproterozoic age of deposition of the Sertânia Complex (Santos et al., 2004) would distinguish it from supracrustal sequences in the adjacent Rio Capibaribe and Alto Pajeú domains, which have, respectively, late Neoproterozoic and early Neoproterozoic deposition ages (Neves et al., 2006, Neves et al., 2009, Van Schmus et al., 2011, Guimarães et al., 2012, Brito Neves et al., 2013, Da Silva Filho et al., 2014). The samples dated by Santos et al. (2004) were collected in the easternmost part of the Alto Moxotó domain. Here we report the results of a structural, metamorphic and geochronological work conducted in the type area of the Sertânia Complex (Fig. 1c). We describe the deformation and metamorphism of this sequence, assess the age(s) of deposition by carrying out new U-Pb analyses of detrital zircons, and compare its tectonic evolution with that of the Surubim Complex in the Rio Capíbaribe Domain. Based on these data, we discuss the regional implications for the geological evolution of the Central subprovince and evaluate if it is better explained by an accretionary or an intracontinental model.

Section snippets

Geological setting and previous geochronology

The main structural feature of the Central subprovince is a network of E-W- to ENE-WSW-striking dextral and NNE-SSW- to NE-SW-striking sinistral shear zones (Fig. 1c). The proposed limit between the Alto Moxotó domain and the Rio Capibaribe domain corresponds to the sinistral Congo shear zone and the dextral Coxixola shear zone (Santos and Medeiros, 1999, Rodrigues and Brito Neves, 2008, Miranda, 2010, Santos, 2012). The boundary between the Alto Moxotó and Alto Pajeú domains is less

General geology

The study area is in the northern part of the Sertânia Sheet (SC.24-E-II), encompassing the homonymous city (Fig. 2). Metasedimentary rocks of the Sertânia Complex consist dominantly of stromatic migmatites with medium- to coarse-grained mesosome (Fig. 3a). The abundance of biotite and garnet (porphyroblasts up to 2 cm-wide; Fig. 3b) and the common occurrence of sillimanite (Fig. 3b) in the mesosome indicate that the protoliths were probably sediments with an important pelitic component.

Samples and methods

Five samples were collected for the geochronological study (Fig. 2). Three samples (SE-04, SE-10 and SE-12) are from the mesosome of migmatized paragneiss. The two others are from metamafic (SE-8A) and metasedimentary (SE-8B) rocks found tectonically intercalated in the same outcrop (Fig. 8a and b). The four metasedimentary samples are very similar, containing garnet porphyroblasts (0.3–1 cm in diameter) in a biotite-rich, medium-grained matrix. The geographical coordinates of the outcrops where

Linking deformation phases to kinematics

Field and map-scale structures show that the study area underwent polyphased deformation (D1–D5). Phase D1 is only preserved as a folded biotite foliation in intrafolial folds related to the second deformation phase (D2) (Fig. 4a and b). This second deformation event is the most pervasive and was responsible for development of a regional foliation under upper amphibolite facies conditions (S2) (Fig. 7). Phase D3 is represented by overturned to recumbent, close to open folds that only locally

Conclusion

The main foliation in the Sertânia Complex was developed under estimated metamorphic temperatures of 650–700 °C, resulting in common migmatization. It is associated with a W-trending stretching lineation and kinematic indicators showing top-to-the-W tectonic transport. Our data do not allow establishing exactly the age of metamorphism, but it is certainly related to the Brasiliano Orogeny since the main foliation is shared by Paleoproterozoic orthogneiss and Neoproterozoic paragneiss, in which

Acknowledgements

This work was supported through funding from the Brazilian agency Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq; grant 472582/2011-9). We thank Adejardo da Silva Filho, Andres Bustamante and undergraduate students for their participation in field work; CPRM – Geological Survey of Brazil for making available aerogeophysical images; and Roberto Weinberg, an anonymous reviewer and associate editor Natasha Wodicka for constructive comments that helped to improve the paper.

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