Coesite eclogite facies metamorphic tectonite: Evidence for ultrahigh pressure deformation |
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Authors: | Zhongyan Zhao Aiming Fang Liangjun Yu |
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Institution: | (1) LTE Lab, Institute of Geology and Geophysics, Chinese Academy of Sciences, 100029 Beijing, China |
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Abstract: | An ultrahigh pressure ductile shear mélange crops out on the beach of Yangkou Bay near Qingdao City. The mélange is composed
of weakly deformed blocks in a highly ductilly flow mylonites. Ultrahigh pressure metamorphic (UHPM) tectonite includes strongly
deformed eclogite and mylonitized eclogite. Coesite occurs in the tectonite as both interstitial mineral and inclusions in
garnet and omphacite, indicating that the deformation took place in the stability field of coesite (800–850°C, >30 GPa) in
the upper mantle. Coesite is rounded or short prismatic grains with undulatory extinction, and often fractured, suggesting
brittle deformation. Garnet is also characterized by brittle fractures and sometimes necked and slightly elongated. Omphacite
is elongated, with long axis preferred orientation. Undulatory extinction, subgrains and dynamically recrystallized grains
suggest plastic flow of omphacite. Ultrahigh pressure metamorphic tectonite was probably formed in the ductile shear zone
during the early stage of exhumation of the ultrahigh pressure metamorphic rocks. Its kinematic indicators point to the transport
direction of the UHPM slab during the early stage of exhumation. |
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Keywords: | metamorphic tectonite ultrahigh pressure metamorphic rocks coesite eclogite |
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