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Fabrics of Migmatites and the Relationships Between Partial Journal of Structural Geology 48 (2013) 45e56 Contents lists available at SciVerse ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Fabrics of migmatites and the relationships between partial melting and deformation in high-grade transpressional shear zones: The Espinho Branco anatexite (Borborema Province, NE Brazil) Luís Gustavo F. Viegas a,b,*, Carlos J. Archanjo a, Alain Vauchez b a Instituto de Geociências, USP, rua do lago 562, 05508-080 São Paulo, SP, Brazil b Géosciences Montpellier, Place E. Bataillon, 34095 Montpellier Cedex 5, France article info abstract Article history: The Espinho Branco anatexite, located within a transcurrent, high-temperature shear zone in NE Brazil, Received 10 July 2012 was the subject of a comprehensive petrostructural study (Anisotropy of Magnetic Susceptibility e AMS, Received in revised form Anisotropy of Anhysteretic Remanence e AAR, Electron Backscatter Diffraction e EBSD) to evaluate the 13 December 2012 compatibility of different fabrics with the kinematics of melt deformation. Magnetite dominates Accepted 17 December 2012 susceptibilities larger than 1 mSI and biotite displays [001] lattice directions consistent with AMS k axes. Available online 27 December 2012 3 In contrast, migmatites with a susceptibility lower than 0.5 mSI and no visible mesoscopic foliation provide crystallographic fabrics distinct from AMS and AAR. However, AAR remains consistent with the Keywords: fi fi Migmatite regional strain eld. These results suggest that the correlation of eld, AMS and crystallographic fabrics is AMS not always straightforward despite the relatively simple organisation of the magnetic fabric in the EBSD anatexite. We conclude that AMS recorded the final stages of the strain field in the migmatite irrespective Borborema Province of its complex mesoscale structures and contrasting crystallographic fabrics. Petrofabrics Ó 2012 Elsevier Ltd. All rights reserved. NE Brazil 1. Introduction two-phase materials enhancing strain localisation in the liquid (magma) phase and promoting strain hardening in the solid (host Migmatites are composite igneous and metamorphic high- rock) phase; textures are commonly divided into solid-state in the grade rocks that record crustal flow processes at the roots of oro- metamorphic host and magmatic microstructures in the leuco- gens (Whitney et al., 2004). Migmatite petrology gives insights into somes or magma (Vigneresse et al., 1996; Vernon, 2000). the composition and differentiation of the middle to upper crust, More recently, new methodologies were tested with the aim of while migmatite structures register the deformation that is active shedding some light onto deformation patterns in migmatitic during orogenesis (Ashworth, 1985). bodies. Ferré et al. (2003) pioneered an AMS study on anatexites The petrostructural characteristics of migmatites have been and concluded that the apparent structural complexity at the studied for over than thirty years (Mehnert, 1968). Quantitative mesoscale masks a simple magmatic flow pattern that can be (Blumenfeld and Bouchez, 1988; Leitch and Weinberg, 2002), and mapped in detail using magnetic fabrics. This methodology was qualitative (Brown and Rushmer, 1997; Weinberg and Mark, 2008) followed by other workers (Denèle et al., 2007; Charles et al., 2009; work was extensively employed, encompassing field mapping, melt Archanjo et al., 2012) and coupled with crystallographic preferred topology and fabric analysis to characterise the structure and orientation (CPO) measurements of rock-forming minerals through reconstruct the emplacement and strain history and their rela- electron backscatter diffraction (Kruckenberg et al., 2010). These tionships with large-scale crustal deformation. However, the tools proved useful in establishing correlations between magnetic inherently complex geometry at the outcrop scale constantly and mesoscale structural fabrics. renders tectonic interpretations uncertain or ambiguous. Due to Migmatites emplaced in high-strain zones usually retain fabrics both their igneous and metamorphic nature, migmatites behave as consistent with the regional strain field (Brown, 1994; Paterson et al., 1998). However, the exact chronology between partial melting and deformation is difficult to establish due to the complex crosscutting relationships between the foliations and the meso- * Corresponding author. Géosciences Montpellier, Place E. Bataillon, 34095 Montpellier Cedex 5, France. scopic melt pockets (Rutter and Neumann, 1995; Rosenberg and E-mail addresses: [email protected], [email protected] (L.G.F. Viegas). Handy, 2005). 0191-8141/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jsg.2012.12.008 46 L.G.F. Viegas et al. / Journal of Structural Geology 48 (2013) 45e56 This paper presents a comprehensive petrostructural study on NE-trending Senador Pompeu shear zone. This suggests that the migmatites outcropping in the high-grade, transcurrent Patos shear Borborema shear zones form a crustal-scale branched system zone located in the Borborema Province, northeast Brazil. This shear (Vauchez et al., 1995; Oliveira, 2008). The Eastern Domain displays zone constitutes a major Neoproterozoic structure regarded as a gradual rotation of the mylonitic foliation from E-W to NE-SW, a crustal boundary dividing different tectonic terranes (Vauchez close to the Atlantic coastal deposits. The shear zone foliations et al., 1995; Van Schmus et al., 2008). The shearing is associated are mainly subvertical, and stretching lineations are subhorizontal, with partial melting, and migmatites display contrasting morphol- which is consistent with a dominant transcurrent dextral motion ogies, comprising foliation-parallel leucosome veins, randomly along the entire zone (Corsini et al., 1991). oriented nebulites and isotropic leucogranites. Combining field Anatexites are frequent in the Central Domain, forming lenses of studies with magnetic (AMS, AAR) and crystallographic fabric granitic domains mixed with “unmelted” material. They display (EBSD) measurements, we show that migmatite internal structures folded stromatic, schollen, schlieren and boudinage structures and can be mapped with good consistency throughout the shear zone derive mostly from the melting of the Paleoproterozoic basement. A even when melt pockets accumulate with no visible mesoscopic narrow medium-to-low temperature mylonite belt outlines the fabric. Furthermore, our study shows that detailed mapping of the southern margin of the shear zone (Fig. 2) and reworks the fabric of anatexites emplaced in shear zones is needed before orthogneisses and migmatites under lower amphibolite-green placing constraints on the melting and deformation relationships in schist facies conditions. Although they have the same fabric hot orogens. orientation and kinematics, it is not clear whether the high- and low-grade mylonites are coeval or were formed at different 2. Geological setting reworking episodes. Available geochronological studies of the Patos mylonites The Patos shear zone consists of a w600 km E-trending strike- include zircon U/Pb (TIMs) data and Sm/Nd model ages obtained in slip shear zone that deforms the Precambrian rocks of the Bor- the Central Domain (Costa, 2002). The zircons show strong isotopic borema Province (Fig. 1). It forms part of a continental-scale shear discordance but provide unconstrained upper intercept ages system that can be followed from NE Brazil to West Africa (Vauchez ranging from 2.2 Ga to 2.0 Ga, indicating that a Paleoproterozoic et al., 1995; Arthaud et al., 2008; De Witt et al., 2008). Three major protolith was involved in deformation and migmatisation. Whole- structural domains can be defined in the Patos shear zone. The rock Sm/Nd model ages range from 3.4 Ga to 2.6 Ga and also 40 39 Central Domain, located between the towns of Catingueira and suggest the presence of Archean sources. Ar /Ar ages range from fi Patos (Fig. 2), comprises E-trending mylonitic gneisses approxi- 540 Ma to 490 Ma and are attributed to nal cooling and late mately 30 km in width in structural continuity with the N-NE fabric exhumation of the shear zone (Monié et al., 1997; Corsini et al., of the Seridó belt. This led Corsini et al. (1991) to argue that the 1998). Patos-Seridó structure forms a mechanically coupled system in which transcurrent displacements are transferred to a transpres- 3. The Espinho Branco anatexite: field characteristics sional belt. The Western Domain consists of a duplex structure where NE- The Espinho Branco anatexite, located in the Central Domain, trending lenses of orthogneisses, metapelites and granitoids occupies an elliptical area of w25 km2 elongated in the E-W are bounded by E-trending mylonites that merge with the western direction. This migmatitic zone can be traced in structural Fig. 1. The Neoproterozoic Borborema shear zone system in NE Brazil. The box shows the study area located within the Patos shear zone. L.G.F. Viegas et al. / Journal of Structural Geology 48 (2013) 45e56 47 Fig. 2. Geological map of the connection zone between the Patos shear zone and the Seridó belt. continuity with the NE-trending Santa Luzia diatexite (Fig. 2). The Shape-preferred orientations of centimetre-scale K-feldspar mineralogical assemblage comprises K-feldspar, quartz, plagioclase grains and biotite flakes up to 2 mm define a mesoscopic steeply and biotite, with amphibole, zircon, titanite and opaques as dipping magmatic foliation (Fig. 3). This fabric is marked
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