MICRO-PHASES AND MICROSTRUCTURES IN CLINOPYROXENE MINERALS FROM A RAPIDLY SUBDUCTED AND EXHUMED HIGH PRESSURE METAMORPHIC TERRAIN: HRTEM AND X-PEEM INVESTIGATIONS

Microstructures and micro-inclusions in minerals can provide information about the genesis and evolutionary history of host rocks. High-resolution TEM (HRTEM) and X-ray PhotoElectron Emission Microscopy (X-PEEM) are useful tools for studying microstructures and microchemistry of minerals, which can provide spatially resolved chemistry about micro-phases and interfaces in minerals. The sample for this study is from a garnet clinopyroxenite, a high pressure metamorphic rock in northern Dabie Mountains of China. The terrain hosting the sample was a product of the Triassic continental collision between the Sino-Korean and Yangtze cratons in East-central China. Previously, the rock was considered as a product of an ultra-high pressure metamorphism (Tsai & Liou, 2000, Am. Mineral. v.85, p.1-8). Recent study also shows that the eclogite-facies rocks re-equilibrated under granulite-facies conditions (Fu, et al., 2003, Lithos, v.70, p.293-319). The clinopyroxene (Wo₃₉En₃₇Jd₁₄Ac₆Fs₄) crystals contain micron to sub micron silica-rich (> 75% SiO₂ ) precipitates, some of them CO₂ (L)-bearing, in the core and exsolution lamella of a second generation pyroxene in the rim. HRTEM and X-PEEM results indicate that silica-rich glass precipitates are uniformly distributed and that the (Fe,Mg)-rich pyroxene lamellae and have P2₁ /c symmetry (the symmetry for pigeonite, not orthopyroxene). We suggest that the Al-bearing exsolution lamellae formed metastably (quickly). The silica-rich micro-phases inside the host pyroxene probably formed during fast transformation from amphibole into pyroxene, which may indicate rapid subduction of the continental plate. The preservation of silica-rich glass precipitates and the formation of (Fe,Mg)-rich clinopyroxene lamella indicate very fast cooling of the host rock. Our results indicate that the host rock (in paleo-continent plate in the Dabie Mountains) had experienced extremely rapid subduction and exhumation.