HR: 0800h
AN: U21B-0419    [Abstracts]
TI: Strongly foliated garnetiferous amphibolite clasts in ophiolitic melanges, Yarlung Zangbo Suture Zone, Tibet; Early Cretaceous disruption of a back-arc basin?
AU: * Guilmette, C
EM: carl.guilmette.1@ulaval.ca
AF: Departement de Geologie et de Genie Geologique, Universite Laval, Universite Laval, Quebec, Qc G1K7P4, Canada
AU: Hebert, R
EM: rejean.hebert@ggl.ulaval.ca
AF: Departement de Geologie et de Genie Geologique, Universite Laval, Universite Laval, Quebec, Qc G1K7P4, Canada
AU: Wang, C
EM: chshwang@cugb.edu.cn
AF: Research Center for Tibetan Plateau Geology, China University of Geoscience, 29 Xueyuan Road, Haidian District, Beijing, 100083, China
AU: Indares, A D
EM: aindares@mun.ca
AF: Earth Science Department, Memorial University, Memorial University, St-John's, NL A1C5S7, Canada
AU: Ullrich, T D
EM: tullrich@eos.ubc.ca
AF: Department of Earth and Ocean Sciences, The University of British Columbia, The University of British Columbia, Vancouver, BC V6T1Z4, Canada
AU: Dostal, J
EM: jdostal@SMU.CA
AF: Department of Geology,Saint Mary's University, Saint Mary's University, Halifax, NS B3H3C3, Canada
AU: Bedard, E
EM: emilie.bedard.3@ulaval.ca
AF: Departement de Geologie et de Genie Geologique, Universite Laval, Universite Laval, Quebec, Qc G1K7P4, Canada
AB: Metre to decameter-size clasts of amphibolite are found embedded in ophiolitic melanges underlying the Yarlung Zangbo Suture Zone Ophiolites, South Tibet, China. These ophiolites and melanges occur at the limit between Indian and Tibetan-derived rocks and represent remnants of an Early Cretaceous intraoceanic supra-subduction zone domain, the Neo-Tethys. In the Saga-Dazuka segment (500 km along-strike), we discovered new occurrences of strongly foliated amphibolites found as clasts in the ophiolitic melange. In garnet-free samples, hornblende is green-blue magnesio-hornblende and cpx is low-Al diopside. In garnet- bearing samples, garnet is almandine with a strong pyrope component (up to 30 mol%) whereas coexisting hornblende is brown Ti-rich tschermakite and clinopyroxene is Al-diopside. Plagioclase composition was ubiquitously shifted to albite during a late metasomatic event. Geochemistry of these rocks indicates that their igneous protoliths crystallized from a slightly differentiated tholeiitic basaltic liquid that did not undergo major fractionation. Trace element patterns reveal geochemical characteristics identical to those of the overlying ophiolitic crust. These are 1) trace element abundances similar to that of N-MORBs or BABBs, 2) a slight depletion of LREE and 3) a moderate to strong Ta-Nb negative anomaly and a slight Ti anomaly. Such characteristics suggest genesis over a spreading center close to a subduction zone, possibly a back-arc basin. Step-heating Ar/Ar plateau ages were obtained from hornblende separates. All ages fall in the range of 123-128 Ma, overlapping the crystallization ages from the overlying ophiolite (126-131 Ma). Pseudosections were built with the THERMOCALC software in the system NCFMASH. Results indicate that the observed assemblage Hb+Pl+Gt+Cpx is stable over a wide range of P-T conditions, between 10-18 kbars and at more than 800°C. Measured mineral modes and solid solution compositions were successfully modeled, indicating equilibrium between 11-13 kbars and 825-850°C, corresponding to high-P granulite facies conditions. In a general way, the geochemistry of the strongly foliated amphibolite clasts suggests that their igneous protolith probably crystallized within the same supra-subduction zone as the crustal rocks from the overlying ophiolite. Then some of these rocks were entrained to mantle depth and were rapidly exhumed, most likely along a lithospheric scale thrust fault underneath the ophiolite. This event corresponds with the end of magmatic activity within the ophiolitic crust and mantle and could be regarded as the inception of a subduction plane at the spreading ridge of a back-arc basin. The whole package was later on obducted over the Indian passive margin, at about 70 Ma. Such a model suggests that closure of the oceanic domain separating India from Eurasia implied disruption of at least one arc-back-arc system, thus requiring at least one early intraoceanic collision or major plate movement reorganization prior to the Late Cretaceous obduction.
DE: 1065 Major and trace element geochemistry
DE: 1115 Radioisotope geochronology
DE: 3042 Ophiolites (8140)
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 3651 Thermobarometry
SC: Union [U]
MN: 2007 Fall Meeting