International Geologiical Congress - Oslo 2008

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MPI-02 Integrated perspectives on the accretion of oceanic crust

 

Ophiolites of the yarlung zangbo suture zone southern Tibet

 

Réjean Hébert, Université Laval (Canada)
Guillaume Lesage, Université Laval (Canada)
Carl Guilmette, Université Laval (Canada)
Émilie Bédard, Université Laval (Canada)
Chengshan Wang, Beijing University of Geosciences (China)
Jaroslav Dostal, St. Mary's University (Canada)
Thomas David Ullrich, University of British Columbia (Canada)
 

 

Reinvestigation of the 2500+ km long contact zone by various investigators has shown that the YZSZ comprise rocks from Permian to Miocene in age and metamorphic intensities from high-grade to very-low grade. The Permian and Triassic rocks are made of limestone associated with radiolarite showing affinities with Indian continental margin and deep Tethyan ocean floor, respectively. Two ophiolite series are recognized. One series is Mid- to Late Jurassic (150-177 Ma) in age and ill-defined. It is probably derived from the destruction of a marginal basin comprising an intraoceanic arc and fore-arc settings. The second series is Lower Cretaceous (120-130 Ma) and represents the destruction of a marginal basin comprising an arc-back-arc system. These ophiolites are spatially associated with ophiolitic mélanges and flysch respectively representing the reworking of the Cretaceous ophiolites and Indian continental margin and the Tethyan ocean floor.

Amphibolite and garnet amphibolite blocks (123-130 Ma) found within the ophiolitic mélange share the same geochemical attributes with Series 2-ophiolites. Their protoliths were probably generated at a back-arc spreading center and metamorphosed in a nascent subduction zone at depth around 50 km. Some radiometric ages suggest events 80 Ma and 90 old represent the entry of Indian continental margin into the intra-oceanic subduction zone. However these ages seem to be very rare throughout the whole suture zone and are therefore considered as resulting from local metamorphic events. Some alkaline igneous rocks (131-144 Ma) within the flysch could represent Kerguelen OIB plume products. The study of igneous blocks and the sedimentary matrix suggests a continuous passive margin model. The Miocene (11-17 Ma) post-collisional ultrapotassic rocks discovered in 2006-2007 result from the collapse of the Tibet Plateau accommodated by E-W extensional regime. They carry crustal xenoliths of metamorphic origin representing a window through the deep crustal section underlying the YZSZ. The geochemistry of these shoshonitic intrusives shows strong subduction components resulting from the metasomatism of the mantle wedge over the subduction zones accomodating the closure of Neo-Tethys basin.
Suture zones are defined as narrow tectonic bands within orogenic belts allowing direct marking of the contact of two major terranes originally distant by several tenths to hundreds of km. They are separated by intermediate domains occurring as highly deformed remnants or sheets. These domains could represent the roots of nappes or thrust sheets. According to this general definition, the Yarlung Zangbo Suture Zone (YZSZ) should represent the contact between India and Eurasian plates once separated by the large Tethys Ocean or one of its derived sub-basins such as the Neo-Tethys basin.

 

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