HR: 0800h
AN: V11A-1417    [Abstracts]
TI: An Early Paleozoic HP granulite-garnet peridotite unit in the south Altyn Tagh, North Tibet: Petrological and SHRIMP dating constraints
AU: * Zhang, J
EM: zhangjx@ccsd.org.cn
AF: Institute of Geology, CAGS, 26 Baiwanzhuang Road, Beijing, 100037 China
AU: Yang, J
EM: yangjsui@ccsd.org.cn
AF: Institute of Geology, CAGS, 26 Baiwanzhuang Road, Beijing, 100037 China
AU: Mattinson, C
EM: cgm@pangea.stanford.edu
AF: Department of Geological and Environmental Sciences, Stanford University, Stanford, Stanford, CA 94305-2115 United States
AB: Petrological investigations and SHRIMP dating were undertaken on a newly recognized HP granulite unit in the Bashiwake area of the south Altyn Tagh, western China. The HP granulite unit consists of bearing-kyanite felsic granulites, mafic granulites, sapphirine-bearing garnet clinopyroxenite and associated garnet peridotites. The predominant felsic granulites are quartz + Ternary feldspar (present perthite)-rich rocks containing various amounts of garnet, kyanite, plagioclase, biotite, rutile, spinel and corundum, and minor zircon, apatite. The mafic granulites enclosed directly in felsic granulites contain a peak assemblage of Grt + CpxI + Ternary feldspar (now mesoperthite) + Qtz + Rt. The growth of new clinopyroxene (CpxII), Opx and Pl in the matrix was related to decompression. The bearing-sapphirine garnet clinopyroxenite occurs as mafic layers in garnet peridotite. Petrographical data suggests a peak assemblage of Grt + CpxI + Ky + Rt. The early kyanite is inferred by the symplectite of Spr + Cor + Pl ,b Spl formed during decompression. Garnet peridotite contains an assemblage of Grt + Ol + Opx + Cpx. The fibrous kelyphites composed of Opx + Cpx + Spl around garnet suggests decompression. The peak assemblage of garnet + kyanite + ternary feldspar + quartz in the felsic granulite give a P-T conditions of around 925-995,aC and 20.0-24.5kbar, consistent with the estimates of T = 950-1020,aC and P = 18.5-25.3 kbar obtained from the peak assemblage of mafic granulite, and T = 870-1050,aC and P = 18.5-27.3 kbar of garnet peridotite within error. A medium-high pressure granulite facies overprint (780-820,aC, 9.5-12 kbar) is well defined by the formation of secondary CpxII ,b Opx + Pl at expense of garnet and early Cpx in the mafic graunlites, as well as by growth of Spl and Pl at expense of garnet and ky in the felsic granulite. The separated zircons from felsic granulite, sapphirine-bearing metabasites and garnet peridotite were measured on SHRIMP II iron microprobe. Most zircons are spherical, multifaceted garins, and some of the zircons contain mineral inclusions which are consistent with peak assemblages observed in thin section. The metamorphic zircons from the felsic granulite yielded an average age, for individual grain spots, of 496 ,b 9 Ma (mean of 11). Metamorphic grains from sapphirine-bearing metabasite and garnet peridotite gave the same ages of 497 ,b 11Ma (mean of 11) and 501 ,b 16 Ma (mean of 12), respectively. Therefore, the age of ŸŒ500 Ma is interpreted as dating the HP/UHT metamorphism. It is identical to ages determined for eclogites from the western segment of the south Altyn Tagh. This suggests that south Altyn Tagh region experienced the same crustal thicken event that resulted in the formation of eclogites and HP granulites in different areas, respectively. The compatible P-T paths of the HP granulites and associated garnet peridotites from the Bashiwake area of the south Altyn Tagh and their metamorphic ages around 500 Ma argues for a shared metamorphic history.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting