TSK 11 Göttingen 2006 Almeida et al. Microfabrics and textures of quartzites of the Indiavaí- Lucialva shear zone, SW Amazonian Craton: prelimi- nary conclusions Poster Harrizon Lima de Almeida1 Amarildo Salina Ruiz1 Axel Vollbrecht2 Introduction The purpose of this contribution is to present preliminary results regarding the kinematics and deformation con- ditions of the Indiavaí-Lucialva Shear Zone, based on the analysis of the texture and microfabrics of related quartzites. Geological setting The Amazonian Craton consists of older Archean cores related to a larger ac- cretionary event represented by the for- mation of several province belts (Teix- eira et al. 1989; Tassinari et al. 2000). The tectonic evolution of the SW Amazonian Craton, in particular, has been attributed the formation of an accretionary complex, Proterozoic in age (Rodonia-San Ignacio Orogeny). Although that division was based on geochronologic data, lineaments have been proposed as structural limits be- tween several terranes. Saes (1999) pro- posed that the Santa Helena and Jauru (Alto Jauru, Geraldes et al., 2001) ter- ranes were separated by the Indiavai- Lucialva shear zone (ILSZ). The ILSZ constitutes an important mega struc- ture which deformed units of the Jauru terrain and parts of the limit east of the Santa Helena terrain (Fig. 1). Based 1 Dep. Geologia Geral, Universidade Fed- eral de Mato Grosso-ICET/DGG 2 Geowis- senschafliches Zentrum der Georg-August-Uni- versität Göttingen (GZG) on field studies and Ar-Ar muscovite ages, Ruiz (2005) and Ruiz et al. (2005) have demonstrated that the ILSZ was formed during an extensional tectonic event, possibly associated with the re- arrangement of collided blocks related to the Sunsás Orogeny (ca. 900Ma). The Indiavaí–Lucialva Shear Zone The Indiavaí–Lucialva Shear Zone (ILSZ) strikes NW-SE over a dis- tance of about 100 km separating magmatic rocks from a metavulcanic- metasedimentary sequence. In the studied area (Fig. 1) rocks which display a distinct mylonitic fabric fre- quently contain feldspar porphyroclasts and garnet. In addition, quartz-rich tectonites, former quartzites, developed along the ILSZ. The mylonitic foliation (smyl) of these mylonitic quartizites, marked by aligned muscovite flakes and strongly flattened small quartz grains, is subparallel to the margins of the shear zone which strikes 315–330° and dips with angles between 60° and 70° to the east-northeast. Shear sense criteria are usually abundant mainly in gneisses which have retained evidence of mylonitization at mesoscale. More- over, the rocks show a well-developed stretching lineation which is defined mainly by elongated grains of quartz and muscovite laths. The dip angles of this lineation typically vary between about 50° and 70°. The ILSZ clearly exhibits a normal fault kinematic, i.e. a top-to-the-NE movement, jux- taposing the Santa Helena Batholith against metavulcanic-metasedimentary sequences and orthogneisses of the basement. 1 Almeida et al. TSK 11 Göttingen 2006 Figure 1: Geological sketch map of the SW Amazonian Craton. The stereoplots display the poles to the mylonitic foliation smyl (A) and stretching lineation L (B), measured within the ILSZ; modified after Ruiz (2005). Microfabrics In all the analyzed quartzites, quartz forms polycrystalline aggregates con- sisting of inequigranular grains with very irregular interlobate grain bound- aries. The average quartz grain size scatters between 0.3 and 1.4mm (Fig. 2A). The occurrence of dissec- tion microstructures (Urai et al. 1986) with the consequent formation of iso- lated ‘island’ grains, has also been observed in the several thin sections (Fig. 2A). Such microstructures are in- terpreted as resulting from fast mi- gration of grain boundaries, correlated with the regime 3 microstructure of Hirth & Tullis (1992) which indicates high-temperature (HT) conditions dur- ing mylonitisation. In addition, other features pointing to grain boundary mi- gration like dragging and pinning mi- crostructures (Jessel, 1987) have been observed. Microstructures represent- ing a subsequent intracrystalline defor- mation include deformation bands and irregular subgrains. Frequently, two sets of deformation bands are developed (‘chessboard patterns’; Fig. 2B) indicat- ing HT crystal plasticity (e.g., Kruhl, 1996). Muscovite occurs as solid inclu- sions in quartz or along the quartz grain boundaries. In the first case, muscovite consists of boudinaged single crystals elongated parallel to the mylonitic fo- liation (Fig. 2C). It also displays defor- mation (kink) bands, particulary within larger grains. Kinematic indicators, e.g. mica and tourmaline displaying distinct asymmetric fish-structures, point to a top-to-the-ENE (normal) sense of shear (Fig. 2D). Altogether, grain boundary migration has to be assumed here as the main mechanism of dynamic recrystal- lization. Quartz textures As a start, quartz c-axis textures (crystallographic-preferred orienta- 2 TSK 11 Göttingen 2006 Almeida et al. Figure 2: Deformation microstructures in mylonitic quartzite A. Quartz grains with irregular shape and lobate boundaries. The arrow points to an isolated ‘island’ grain. B. Two sets of deformation bands in quartz (‘chessboard patterns’). C. Tour- maline (top) and muscovite grains (middle) showing boudinage. D. Tourmaline micro fish (arrow) documenting a top-to-the-east sense of shear when reoriented to the field. tions) have been measured for three selected quartzite mylonite samples (Fig. 3). The quartz c-axis patterns are complex with a more or less pronounced asymmetry with respect to the reference directions (smyl and L) indicating a pre- dominance of non-coaxial deformation. The c-axis maxima close to smyl suggest that prism slip in or [c], which is generally attributed to HT deformation regimes (e.g. Mainprice et al. 1986), has significantly contributed to the texture development. Only sample C shows, in addition, a significant concentration of -axes sub-normal to smyl pointing to basal slip at lower temperatures. Intermediate positions of maxima are probably related to combined prism and basal slip in connection with complex deformation geometries (e.g. Garbutt & Teyssier, 1991). According to Stipp et al. (2002) complex pole figure of this Figure 3: Quartz c-axis textures of three selected quartzite mylonites (A, B, C) from the ILSZ; horizontal line-trace of smyl; dots — direction of L; trace of smyl is ori- ented with respect to geographic directions; Schmidt net, lower hemisphere. kind are typical for HT deformation of quartz polycrystals. As related to the field, the obliquity of the patterns indicates a top-to-the-NE sense of shear. Conclusions Based on both characteristic quartz mi- crostructures and quartz c-axis textures the knowledge about the deformation conditions within the ILSZ could be improved. The microfabrics and in- tracrystalline structures of quartz in- dicate HT deformation with dominant grain boundary migration recrystalliza- tion and subsequent weak plastic defor- mation producing two sets of deforma- tion bands in many grains. Deformation in a HT deformation regime is also con- firmed by typical quartz c-axes textures which, in addition, reflect a strong com- ponent of non-coaxial strain. Acknowledgements The research was supported by CNPQ Grant 350565/2004-0 and FAPEMAT Grant 3.2.2.33/02-2004. 3 Almeida et al. TSK 11 Göttingen 2006 References Garbutt JM & Teyssier C (1991) Prism - slip in quartzites of the Oakhurst Mylonite Belt, California. Journal Structural Geology 13: 657–666 Geraldes MC, Van Schmus W, Condie KC, Bell S, Teixeira W & Babinski M (2001) Pro- terozoic geologic evolution of the SW part of the Amazonian Craton in Mato Grosso state, Brazil. Precambrian Research 11: 91–128 Hirth G & Tullis J (1992) Dislocation creep regime in quartz aggregates. Journal Struc- tural Geololy 2: 145–159. Jessel MW (1987) Grain-boundary migra- tion microstructure in a naturally deformed quartzite. 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