Microstructures And Deformation Mechanisms Of High Temperature Mylonites In Diancangshan, Western Yunnan

This thesis focuses on macro-and micro-structural analysis of high temperature mylonitic rocks from Diancangshan Area, western Yunnan, combined with investigation on regional geological evolution of Diancangshan and adjacent areas. Emphasis is on application of different techniques to structural and microstructural analysis. OM is the basic technique for microstructural analysis, primary data of dimensional and crystallographic orientations and their significance for deformation analysis of rocks and minerals. Applying TEM (transmission electron microscope) submicrostructures and deformation mechanisms of hornblende, feldspar and quartz are studied, special emphasis is on hornblende grains with different initial orientation. Application of EBSD/SEM techniques, with TEM techniques, in analyzing the mechanisms of grainsize reduction and grain orientation forms the major part of the thesis. The evolution from porphyroclasts to matrix, patterns of crystal orientation are discussed in detail. A new mechanism of dynamic recrystallization of hornblende, i.e. twinning nucleation recrystallization is proposed from the present study. Microprobe component analyses of hornblende and plagioclase mineral pairs are finished, and temperatures and pressures of recrystallization and deformation are calculated and estimated using hornblende and hornblende/plagioclase geothermometer and geobarometer.Deformation characteristics, micro- and submicrostructures, deformation mechanisms and rheological behavior of rocks and minerals from high temperature mylonites in Diancangshan area studied. Further discussion includes their significance to rheology of rocks in middle crust and specific tectonic environment. It is concluded that 1) High temperature granitic and hornblende mylonites from Diancangshan show typical brittle-ductile transition nature. Different mineral phases in the rocks show distinct deformation characteristics. Quartz grains are of completely plastic or affected by oriented grain growth. Feldspar grains are dominated by crystal plastic deformation, and partly have brittle deformation characteristics. Hornblende grains show evidences for typical brittle-ductile transition. 2) Hornblende mylonites formed by intense shearing at lower amphibolite facies in middle crust has typical mylonitic microstructures, shown by coarse porphyroclasts and fine matrix grains. Hornblende porphyroclasts with different initial crystallographic orientations form two types of porphyroclasts, i.e. type I"hard"porphyroclasts and type II"soft"porphyroclasts. The occurrence of two types of porphyroclasts and their micro- and submicrostructures of the porphyroclasts and fine grained matrix provide powerful evidences for brittle-ductile transition behavior of hornblende grains. 3) A totally new mechanism of dynamic recrystallization, i.e. twinning nucleation recrystallization is proposed to interpret the dynamic recrystallization of hornblende crystals in the high temperature mylonites formed at the brittle-ductile transition. (100) [001] twin gliding and dislocation creep are mutually enhanced during twinning nucleation and forms column shaped subgrains and recrystallized new grains. 4) Microstructural analysis reveals that feldspar and hornblende rocks have typical brittle-ductile transition nature at middle crust. This implies that the middle crust is not a mechanically weak zone as suggested in the literature, but possess rocks and mineral phases with high mechanical strengths. 5) The high temperature deformation characteristics of the mylonites provide arguments for exhumation of the high grade metamorphic complex of Diancangshan. Combined with primary analysis of the superposed low temperaturemetamorphic belt to the east and low grade metamorphic rock belt to the west, the structural and microstructural characteristics of the high temperature mylonites provide proofs for extensional exhumation of the high grade metamorphic complexes in Xuenlongshan, Diancangshan, Ailaoshan and Day Nui Con Voi belt in Oligocene- Miocene period.