HR: 17:08h
AN: V14A-06 INVITED     [Abstracts]
TI: Garnet peridotites from the Sulu UHP terrane, China: A window insights into mantle and subduction processes
AU: * Zhang, R
EM: zhang@pangea.Stanford.EDU
AF: Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 United States
AU: Liou, J
AF: Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 United States
AU: Yang, J
AF: Institute of Geology, Chinese Academy of Geological Science, Beijing, 100037 China
AU: Tainfu, L
AF: Institute of Geology, Chinese Academy of Geological Science, Beijing, 100037 China
AB: Garnet peridotites from the Sulu UHP terrane of China occur as block and lens within granulite-amphibolite facies gneiss and contain lenses of clinopyroxenite and coesite-bearing eclogite. In addition to the assemblage Ol + Opx + Cpx + Grt, some peridotites contain additional hydrous phases (e.g., phlogopite, Ti-clinohumite and pargasite) and magnesite. The abundances of "fertile elements" (TiO$_{2}$, Al$_{2}$O$_{3}$, CaO and FeO) of the Sulu peridotites are lower than those of the primitive mantle and show negative correlation with MgO content. The garnet peridotites and pyroxenites preserved mantle $\delta$$^{18}$O values: +4.8 to +5.7 for Grt, +4.7 to +5.4 for Ol, +4.5 to 5.6 for Cpx and +5.7 to +5.8 $\permil$ for Opx. These characteristics suggest that the Sulu peridotites represent pieces of a depleted, residual mantle. Most peridotites with LREE-enrichment patterns, high $^{87}$Sr/$^{86}$Sr (0.7071-0.7100) and low $^{143}$Nd/$^{144}$Nd ratios (0.5122-0.5128) have been subjected to the effects of mantle metasomatism and crustal contamination. In Donghai, the site area of the Chinese Continental Scientific Drelling (CCSD), peridotites with or without magnesite are enriched in Th, U, Sr, Ba and LREE, and show Hf and Ti negative anomalies; these may be attributed to carbonatitic metasomatism. Zircon separates from peridotitic cores of the PP1 drill hole show concentric zoning without inherited cores indicating a metamorphic origin; they yield SHRIMP U-Pb age of 221 $\pm$ 3 Ma (weighted mean), consistent with the Dabie-Sulu regional UHP metamorphism. Polymorph transformation (OREN to CLEN) of orthopyroxene and exsolution microstructures in peridotitic minerals are common; these include Ilm in olivine, Qtz, Ilm, Phl and Grt + Ilm $\pm$ Amp in diopside, and Rt $\pm$ Cpx + Ap in garnet. These petrochemical and age data, microstructures and P-T estimates conclude the mantle-derived Sulu garnet peridotites were inserted into a Triassic continental subduction zone and experienced in situ UHP metamorphism together with subducted slab at 750-1000$\deg$C and 4-7 GPa with an extremely low thermal gradient of $<$ $5\deg$C/km.
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting