HR: 14:24h
AN: V13C-04    [Abstracts]
TI: Mineral and Fluid Inclusions in Zircon of UHP Metamorphic Rocks from the CCSD-Main Hole: a Record of Metamorphic Evolution and Fluid Activity
AU: * ZHANG, Z
EM: zzm@ccsd.org.cn
AF: Institute of Geology, CAGS, China, Baiwanzhuang Road 26, Beijing, 100037 China
AU: Xu, Z
EM: xuzhiqin@ccsd.org.cn
AF: Institute of Geology, CAGS, China, Baiwanzhuang Road 26, Beijing, 100037 China
AU: Zhang, J
EM: zzm@ccsd.org.cn
AF: Institute of Geology, CAGS, China, Baiwanzhuang Road 26, Beijing, 100037 China
AU: Xiao, Y
EM: yxiao@gwdg.de
AF: Geoscience Centre, Uni. Goettingen, Goldschmidtstr. 1, Goettingen, D-37077 Germany
AU: Hoefs, J
EM: jhoefs@gwdg.de
AF: Geoscience Centre, Uni. Goettingen, Goldschmidtstr. 1, Goettingen, D-37077 Germany
AU: Shen, K
EM: zzm@ccsd.org.cn
AF: Institute of Geological Sciences of Shandong, Jinan, Jinan, Shandong, 250013 China
AU: Liou, J
EM: liou@pangea.Stanford.EDU
AF: Dept. of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 United States
AB: The recovered cores from depths of 0-3000 m of the Chinese Continental Scientific Drilling (CCSD) Main Hole in Donghai, southern Sulu, comprise of eclogite, paragneiss, orthogneiss, schist and garnet peridotite. Zircon grains ranging from 50 to 300 micrometers in size from all rock types except ultramafic rocks were investigated by Raman, cathodoluminescence, and microprobe analysis. Based on their growth structure and mineral inclusion assemblages, zircons were divided into three types. Type I has an oscillatory zoned magmatic core with low-P mineral inclusions of Qtz, Pl and Ap, and a metamorphic rim with uniform luminescence and UHP inclusions of Grt, Omp, Phn, Coe and Rt. This type occurs in most eclogites and orthogneisses, and minor schists, suggesting that the host rocks have been derived from intrusive rocks, which have been subjected to UHP metamorphism. Type II is generally unzoned, and occurs in a few eclogites, paragneisses and schists. The cores of the zircons contain UHP inclusions of Grt, Omp, Phn, Coe and Rt, whereas the rims have HP inclusions of Grt, Omp and Qtz, suggesting that they are of metamorphic origin. The third type of zircons occurs only in some orthogneisses, and show euhedral, oscillatory zonations without a core-rim structure, typical for a magmatic origin; in these zircons only low-P mineral inclusions were found, implying that the host rock did not experience UHP metamorphism. In a few cases, magmatic cores of type I zircons contain coesite inclusions, which implies that former magmatic quartz has been transformed to coesite during UHP metamorphism. In addition, polycrystalline quartz pseudomorph inclusions after coesite and symplectite after omphacite were observed in the metamorphic rim, suggesting that UHP mineral inclusions have been replaced by low-P minerals during retrogression. Abundant H2O-CO2, H2O- or CO2-dominanted fluid inclusions occur isolated or clustered in the magmatic cores of some zircons, coexisting with low-P mineral inclusions. These primary fluid inclusions should have been trapped during magmatic crystallization. Rarely fluid inclusions occur together with UHP mineral inclusions in zircons of metamorphic origin. They have similar compositions to those within the magmatic cores of zircons. These data suggest that the UHP metamorphism occurred under relatively dry conditions and as a closed fluid system. Fluid components during UHP metamorphism were obviously inherited from their protoliths. This work is sponsored by 973-project 2003CB716501 and NSFC-40399142.
DE: 8045 Role of fluids
DE: 8110 Continental tectonics--general (0905)
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting