Geochimical Characteristics And Tectonic Environments Of The Yarlung Zangbo Ophiolites In Xigaze, Tibet

The Yarlung Zangpo ophiolite is one of the best preserved ophiolites in the world. They are remnants of the Neo-Tethys ocean-floor, marking a suture within Gondwanan in Mesozoic era. The study of petrology and geochemistry may reveal the evolution history and tectonic features. The Xigaze ophiolite in the center of the Yarlung Zangpo ophiolite is intact and has important implication on the research of evolution of Tethyan. In this paper, major and trace elements, as well as Sr-Nd, Re-Os and O isotopic compositions for the basic rocks (basalts, diabases and gabbros) from three sections of ophiolites in Xialu, Qunrang and Bailang of Xigaze area were studied.These rocks are charactered by low Ti, K, high Mg, Al, Ca, Na and LOI, implying the metasomatic process after the rocks formed in the ocean; the rocks are sub-alkaline basaltic rocks. Most of them is low-K tholeiitic, few of them is calc alkali basaltic rock.Their REE and trace elements are very similar to the N-MORB, but with enrichment of large ion lithophile elements (LILE) and depletion of high field strength elements (HFSE). The rocks show the characteristics from middle ocean ridge basalts to island arc basalts.These rocks are characterized by deleted mantle Sr-Nd isotopes (εNd (t):+6.2～+8.7,87Sr/86S:0.70313～0.70546). Strontium isotopic value toward to the high direction, lower rhenium and osmium content, volcanic arc basalt-like osmium isotope and high δ18O value (10.3～12.8‰) indicated that these rocks had undergone the seawater alteration, but without the contamination of continental crust. Sr-Re-Os-O isotopic compositions can’t reveal the Characteristic of the source region because of the seawater alteration and oceanic slab subduction..The Xigaze ophiolite is Supra Subduction Zone (SSZ) type oceanic crust, instead of the typical N-MOR type ophiolite. It may have occurred in back-arc or fore-arc tectonic settings in an oceanic slab subduction-related system.There are at least four different types of tectonic environment within the1500km-long Yalung Zangpo ophiolite. The four-type basalts are (1) continental arc,(2) mantle plume or hot spot,(3) MORB and Oceanic arc, and (4) typical oceanic arc. The presence of multiple tectonic settings rusulting from geochemical signatures which associated with the basaltic rocks within the Yalung Zangpo suture zone reconstructs a long-evolutional history of the Tethyan Ocean.