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_Journal of Structural Geology 32 (2010) 606-622_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Orientation-related deformation mechanisms of naturally deformed amphibole in amphibolite mylonites from the Diancang Shan, SW Yunnan, China Shuyun Cao a,b,*, Junlai Liu b, Bernd Leissa Geoscience Centre, University of Göttingen, Göttingen 37077, Germany b State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences, Beijing 100083, China Article history: Received 2 February 2009; Received in revised form 11 March 2010; Accepted 29 March 2010; Available online 3 April 2010 Keywords: Amphibole mylonites Microand sub-microstructures Fabric analysis Twinning nucleation recrystallization Diancang Shan of Yunnan Abstract Sheared amphibolite rocks from the Diancang Shan high-grade metamorphic complex along the Ailao Shan-Red River shear zone, southwestern Yunnan, China, show typical mylonitic microstructures. The mylonites are characterized by porphyroclastic microstructures and the ultramylonites are highly lineated with alternating amphibole-and quartzofeldspathic domains. Microstructural analysis and P-T estimation suggest that the amphibole grains in the mylonitic rocks are deformed and dynamically recrystallized at amphibolite facies. In the mylonitic amphibolites, there are two types of amphibole porphyroclasts: type I “hard” and type II “soft”. They have their crystallographic orientations subnormal and subparallel to the stretching lineation of the rocks, respectively. The two types of porphyroclasts show distinct deformation microstructures and sub-microstructures formed by various deformation mechanisms, which contribute in different ways to the generation of the fine-grained matrix. Shape preferred orientation analysis, misorientation analysis of the two types of porphyroclasts and new fine grains around them further prove the generation of the fine grains in matrix from the type II porphyroclasts. The type I “hard” porphyroclasts are deformed mainly by mechanical rotation, work hardening and intragranular microfracturing. In contrast, the deformation of the type II “soft” porphyroclasts is mainly attributed to crystalline plasticity, i.e., twinning, dislocation creep and dynamic recrystallization. During the deformation of the type II porphyroclasts, the (100) slip system plays a dominant role during deformation and grain size reduction of amphibole. Twinning along the active (100) slip system, in combination with dislocation creep (gliding and climbing), governs the nucleation of subgrains and formation of dynamically recrystallized fine grains, a process here named Twinning Nucleation Recrystallization. Crown Copyright © 2010. Published by Elsevier Ltd. All rights reserved. 1. Introduction Amphibole is one of the most important components in middle to lower crustal rocks. The deformation behavior of amphibole provides important clues to the understanding of mechanical, rheological and physical properties of the crust (e.g., Drury and Ural, 1990; Stönitz, 1993; Barruol and Kern, 1996; Berger and Stönitz, 1996; De Meer et al., 2002; Kitamura, 2006; Dá Azpiroz et al., 2007; Tatham et al., 2008). Amphibole typically shows brittle to ductile behavior under a wide range of deformation P-T conditions, as revealed by studies on naturally and experimentally deformed amphiboles (e.g., Rooney et al., 1970, 1975; Biermann, 1981; Hacker and Christie, 1990; Berger and Stönitz, 1996; Dá Azpiroz et al., 2007). * Corresponding author at: Geoscience Centre, University of Göttingen, Göttingen 37077, Germany. Tel.: +49 551 3912624; fax: +49 551 3914624. E-mail addresses: scao@gwdg.de (S. Cao), jliu@cugb.edu.cn (J. Liu). Early studies on deformation of amphibole have focused on slip systems and deformation twinning in experimentally and naturally deformed amphibole crystals. The main slip systems determined in amphibole are (100) _001_, (010) _001_ and (hk0) _001_ (e.g., Rooney et al., 1970, 1975; Dollinger and Blacic, 1975; Skrotzki, 1992; Jiang and Skrotzki, 1996). Experimental studies reveal that deformation twins are commonly (101) and rarely (100) (e.g., Rooney et al., 1970, 1975; Buck, 1970; Hacker and Christie, 1990). In naturally deformed amphibole, however, (100) twinning (e.g., Dollinger and Blacic, 1975; Biermann, 1981; Biermann and Van Roermund, 1983; Kenkmann, 2000; Imon et al., 2002) predominates over (101) and other twinning systems (e.g., Morrison-Smith, 1976; Allison and La Tour, 1977; Cumbest et al., 1989; Hacker and Christie, 1990). Observations on naturally deformed amphiboles have also revealed variations of deformation mechanisms at different P-T conditions (e.g., Allison and La Tour, 1977; Nyman et al., 1992; Babai and La Tour, 1994; Leiss et al., 2002; Imon et al., 2002, 2004; Dá Azpiroz et al., 2007). It is generally accepted that amphibole is deformed by brittle processes and concomitant breakdown reactions under greenschist facies (e.g., Brodie and Rutter, 1985) or even at amphibolite facies (Lafrance and Vernon, 1993), e.g., by microfracturing (e.g., along (110) cleavage plane, Allison and La Tour, 1977; Nyman et al., 1992) or mass transfer (Brodie and Rutter, 1985; Stönitz, 1993; Shelley, 1994; Imon et al., 2002, 2004). Dominance of dislocation activity, subgrain formation and dynamic recrystallization may occur at higher temperature and pressure conditions (Biermann and Van Roermund, 1983; Cumbest et al., 1989; Skrotzki, 1992), although it is inferred that amphibole generally does not deform significantly by intracrystalline plasticity below 650-700°C in the presence of fluid (Berger and Stönitz, 1996). In polyphase rocks, mineral phases (e.g., plagioclase) mostly behave in different ways during deformation. It is found that at approximately 700°C and up to 900 MPa deformation of amphibole grains in monophase layers is characterized by crystal plasticity, but in the two-phase mixtures by granular flow (Kruse and Stönitz, 1999). Therefore, the formation of deformation microstructures and variation of deformation mechanisms of the polyphase rocks may be attributed to the leading role of chemical disequilibrium. Detailed studies on amphibole deformation at low-pressure conditions suggest that dissolution-precipitation creep and cataclastic flow, rigid body rotation within a weaker plagioclase matrix, and subgrain rotation recrystallization may explain the deformation and dynamic recrystallization at different conditions (Dá Azpiroz et al., 2007). The texture types of amphibole and plagioclase in the polyphase rocks mostly result from varying deformation modes of the different mineral phases (Leiss et al., 2002). From the above discussions, there still exist significant disagreements on the deformation behavior of amphibole under different conditions. It appears that the high temperature deformation of amphibole is a complex process and the dominant deformation mechanisms are still unclear. The present study on amphibolite mylonites collected from Diancang Shan, SW Yunnan, China, provides new results for and constraints on the deformation and dynamic recrystallization of amphibole at amphibolite facies conditions, by applying OM (Optical Microscope), TEM (transmission electron microscope), EPMA (electron probe microanalysis) and EBSD (electron backscattered diffraction) techniques. Microstructural, submicrostructural and LPO (lattice preferred orientation) texture studies reveal that the amphibolite mylonites and ultramylonites developed from amphibolite rocks by progressive shearing. Different mineral phases in the rocks show distinct deformation characteristics. Amphibole and plagioclase grains are intensely deformed with obvious grain size reduction, but quartz grains are recrystallized dominantly by grain growth. Initial crystallographic orientations of amphibole grains from the host rocks have strong effects on the behavior of the grains during deformation. Most fine grains are generated by combined twinning and dislocation creep of parent amphibole grains with specific preferred crystallographic and shape orientations. Twinning along (100), in combination with dislocation creep (gliding and climbing) governs the nucleation of subgrains and formation of dynamically recrystallized fine amphibole grains. 2. Geological setting and structural analysis Large scale left-lateral strike-slip shearing along the Ailao Shan-Red River shear zone in SW China plays important roles in accommodating the southeastward extrusion of the Indochina block during Indian-Eurasian collision (e.g., Zhong et al., 1990; Lacassin et al., 1993; Leloup et al., 1993, 1995; Burchell and Wang, 2003; Scherrer et al., 1994). Over 500 km of southeastward displacement of the Indochina block relative to the Yangtze-South China block occurred along the shear zone (e.g., Leloup et al., 2001; Liu et al., 2006; Morley, 2007) (Fig. 1a). Several blocks of middle to lower crustal metamorphic rocks are exposed along the Ailao Shan-Red River shear zone. They appear as lens-shaped bodies and are fault-bounded from non-metamorphic Paleozoic to Cenozoic rocks to the east and weakly deformed Paleozoic and Mesozoic meta-sedimentary rocks to the west. The Diancang Shan metamorphic complex is a typical such block, which is composed of three structural units: the central high strain shear zone, the western low-grade metamorphic volcanic-sedimentary sequence in the Lanping basin, and the eastern superimposed retrograde metamorphic belt (Liu et al., 2008) (Fig. 1b). 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