HR: 17:20h
AN: V14A-07    [Abstracts]
TI: Unusual garnet-clinopyroxenite core from the main hole of the Chinese Continental Scientific Drilling Project in Donghai
AU: * Tsujimori, T
EM: tatsukix@pangea.Stanford.EDU
AF: Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 United States
AU: Zhang, R Y
AF: Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 United States
AU: Liou, J G
AF: Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 United States
AU: Xu, Z Q
AF: Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037 China
AU: Yang, J S
AF: Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037 China
AB: The Chinese Continental Scientific Drilling (CCSD) in Donghai has reached 4360m depth in September, 2004 and recovered more than 80% core samples for scientific investigation. These core samples include both para- and ortho-gneisses with less than 25% eclogitic and garnet peridotitic rocks. Inclusions of coesite and coesite pseudomorph occur in garnet and omphacite of some eclogites and in zircon separates from many gneisses, eclogites and amphibolites; their occurrences suggest in-situ UHP metamorphism. One moderately-oriented garnet-clinopyroxenite (558.65m) occurs as thin layer within rutile-rich eclogites (90m thick) and contains abundant ilmenite (15vol%) together with granoblastic garnet (Pyr$_{29-32}$Alm$_{42-47}$Grs$_{16-21}$Spr$_{1-2}$Adr$_{2-7}$; X$_{Mg}$= 0.37-0.42), clinopyroxene (Jd$_{6-13}$Ac$_{13-24}$Aug$_{67-73}$CaTs$_{<4}$Opx$_{<3}$; X$_{Mg}$= 0.88-0.97) and minor amount of iron sulfide. Ilmenite occurs as either coarse-grained granoblast ($\sim$3mm) in matrix or inclusions in garnet and clinopyroxene; matrix ilmenite contains inclusions of garnet and clinopyroxene. Both matrix and inclusion ilmenite exhibit coarse exsolution lamellae (20-80$\mu$m) of hemoilmenite (Ilm$_{71-80}$) and ilmenohematite (Ilm$_{14-18}$); the hemoilmenite and ilmenohematite themselves carry further fine-scale exsolution lamellae of hematite and ilmenite ($<$2$\m$um), respectively. These textural evidences indicate that pre-exsolved ilmenite-hematite solid solution was in equilibrium with garnet and clinopyroxene. Considering those petrologic features and bulk compositions of surrounding eclogite cores, the garnet-clinopyroxenite may initially have been a titanomagnetite-rich ultramafic layer in a coherent Fe-Ti gabbro sequence. Original titanomagnetite may have been oxidized and recrystallized to form Fe$^{3+}$-rich ilmenite at an UHP condition, and subsequent cooling during exhumation exsolved hematite from ilmenite. The local ultramafic composition may have allowed the occurrence of ilmenite instead of rutile, and oxidation may play an important role to accommodate significant amounts of acmite (Ac$>$Jd) in clinopyroxene and andradite ($\sim$37mol%) in garnet.
DE: 3360 Remote sensing
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting