HR: 16:45h
AN: V54B-04    [Abstracts]
TI: Prograde and Retrograde Metamorphic Evolution of UHP Eclogites From the Main Hole of CCSD, China
AU: * Tong, L
EM: tong@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, 128, Academia Road, Section 2, Taipei, 11529 Taiwan
AU: Jahn, B
EM: jahn@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, 128, Academia Road, Section 2, Taipei, 11529 Taiwan
AU: Okamoto, K
V54B-04 AF: Institute of Earth Sciences, Academia Sinica, 128, Academia Road, Section 2, Taipei, 11529 Taiwan
AU: Iizuka, Y
V54B-04 AF: Institute of Earth Sciences, Academia Sinica, 128, Academia Road, Section 2, Taipei, 11529 Taiwan
AU: Xu, Z
V54B-04 AF: Institute of Geology, CAGS, 26 Baiwanzhuang Road, Beijing, 100037 China
AB: The Chinese Continental Scientific Drilling (CCSD) borehole in Donghai of eastern China, provides an excellent chance to study continental subduction and exhumation processes. Coesite-bearing rutile and phengite eclogite samples from the main borehole of 100-2000 m were examined, and three major evolutionary stages can be distinguished from mineral assemblages and textural relations. The three stages are: (1) Pre-peak prograde stage is indicated by the inclusion assemblages in garnet and omphacite such as amphibole, plagioclase, coesite/pseudomorph, rutile and phengite. This is probably associated with prograde metamorphism during early stage of subduction. (2) Peak UHP metamorphic stage is characterised by the development of texturally equilibrated mineral assemblage, such as garnet, omphacite, phengite, rutile, apatite, with or without K-feldspar, kyanite and coesite/pseudomorph. (3) Post-peak retrograde stage results in the extensive development of decompression textures. These include thin rim of amphibole around garnet grain, fine-grained symplectite assemblages of amphibole-plagioclase/epidote on omphacite, and biotite-plagioclase on phengite. Post-peak hydrous mineral assemblages may be attributed to retrograde metamorphism with fluid input during uplift of UHP metamorphic slab. In addition, the presence of interstitial K-feldspar and K-feldspar-albite symplectite on phengite implies that decompressional but localized partial melting might have occurred during the rapid exhumation of UHP slab. The fast exhumation is very likely responsible for the preservation of prograde amphibole-bearing inclusion assemblages. Some calcite-dominated veins cutting all fabrics may be related to brittle fracturing during the final stage whilst the uplift is close to shallow crustal levels. (Supported by NSC-Taiwan, NSC93-2116-M-001-022, NSC94-2116-M-001-088).
DE: 1042 Mineral and crystal chemistry (3620)
DE: 3625 Petrography, microstructures, and textures
DE: 3651 Thermobarometry
DE: 3654 Ultra-high pressure metamorphism
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005