HR: 0800h
AN: U21B-0420    [Abstracts]
TI: Geology and Geochemistry of the Saga and Sangsang Ophiolite, Yarlung Zangbo Suture Zone, Southern Tibet
AU: * Bedard, E
EM: emilie.bedard.3@ulaval.ca
AF: Departement de geologie et genie geologique, Universite Laval, Quebec, QC G1K7P4, Canada
AU: Hebert, R
EM: rejean.hebert@ggl.ulaval.ca
AF: Departement de geologie et genie geologique, Universite Laval, Quebec, QC G1K7P4, Canada
AU: Guilmette, C
AF: Departement de geologie et genie geologique, Universite Laval, Quebec, QC G1K7P4, Canada
AU: Wang, C
AF: Research Center for Tibetan Plateau Geology, China University of Geosciences, 29 Xueyuan Road, Haidian District, Beijing, 100083, China
AU: Ullrich, T D
AF: Department of Earth and Ocean Sciences, The University of British Columbia, Vancouver, Vancouver, BC V6T1Z4, Canada
AU: Dostal, J
AF: Department of Geology, Saint Mary's University, Halifax, NS B3H3C3, Canada
AB: The Saga and Sangsang ophiolites belong to the E–W-trending Early Cretaceous ophiolite belt of the Yarlung Zangbo Suture Zone (YZSZ), resulting from the closure of the Neo-Tethys Ocean during the collision between the Indian and Eurasian plates. These ophiolitic massifs crop out west of Xigaze and presumably represent a western extention of the central portion of the YZSZ ophiolite belt. The Saga massif comprises fresh coarse- grained mantle tectonite (lherzolite), an ophiolite mélange (harzburgite ± dunite ± garnet- clinopyroxene -bearing amphibolite), a stripe of metamorphosed upper crust rocks and a sequence of uppermost crust rocks, such as chert, basaltic lavas and diabase. Sangsang ophiolite consists of harzburgitic ophiolite mélange and fine-grained mantle tectonites. Spinel chemistry suggests that Saga massif peridotites have abyssal peridotite affinities and have undergone only apparent low degrees of partial melting (5-12%) while Sangsang peridotites have both abyssal peridotite and fore-arc peridotite affinities and have underwent higher apparent degrees of partial melting (17-30%). The Ol-Sp equilibrum for Saga and Sangsang peridotites therefore suggest that Saga peridotites have a deepest mantle provenance (>20 kbar) than Sangsang peridotites (<15 kbar) and that the composition of Saga peridotites is similar to the composition of pre-oceanic peridotites while peridotites from the Sangsang massif are alike abyssal and subduction margin peridotites. Peridotites from both massifs show variable degrees of serpentinization (LOI:0.7-13.6%). Their Mg# vary between 0.89 and 0.91. Saga massif peridotites show distinctive flat REE-patterns with La/Yb_N ratios close to 1. Thus, the abundances of REE elements in these rocks suggest that their composition is similar to a chondritic mantle (or a refertilized mantle with chondritic liquids). The uppermost crust and metamorphosed upper crust rocks compositions correspond to basalt and basaltic andesite. They are slightly depleted in highly incompatible elements (LREE) with respect to the less incompatible elements (HREE), with La/Yb_N between 0.5 and 0.8 and with a slight negative Ta anomaly suggesting the presence of a subduction component. The abundances of incompatible elements in these mafic rocks are therefore quite similar to N-MORB or BABB. On the basis of along-strike correlation, age relationships and geochemical characteristics, we suggest that the Saga and Sangsang ophiolites belong to the same intraoceanic suprasubduction zone segment as others ophiolitic massifs to the east (Xigaze ophiolites). However, the geochemistry of mantle rocks from these two massifs is quite different compared to each other and compared to other eastern Yarlung Zangbo ophiolites suggesting contrasted petrogenetic histories. The Saga and Sangsang massifs may thus represent different portions of an arc – back-arc system configuration.
DE: 1021 Composition of the oceanic crust
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3640 Igneous petrology
DE: 8140 Ophiolites (3042)
SC: Union [U]
MN: 2007 Fall Meeting