The Development Of Sumdo Suture In The Lhasa Block, Tibet

The Qinghai-Tibet plateau is a composite continental fragment formed by collision of multiple terranes and island arcs. The Lhasa terrane, which is located in the central part of the plateau, is bounded by the Yarlung-Zangbo suture to the south and Bangong-Nujiang suture to the north. In 2006, an E-W-trending belt of (ultra)-high pressure eclogite was discovered in the Sumdo region of the Lhasa terrane. Careful field studies combined with petrological, geochemical and isotopic analyses show that the Sumdo eclogites mark a Carboniferous-Permian suture zone, at least 100 km long, containing ophiolite fragments, eclogites and Indosinian post-orogenic granitoids. This suture divides the Lhasa block into a northern and southern segment.Three lithological types of eclogite are recognized in the Sumdo suture zone; rutile eclogite, quartz eclogite and common eclogite. These are accompanied by garnet amphibolite and plagioclase amphibolite formed by retrograde metamorphism of the eclogites. The common eclogites were derived from mid-ocean ridge basalts, whereas the rutile eclogites were formed from oceanic island basalts and the quartz eclogites from lavas intermediate between MORB and OIB. LA-ICPMS U-Pb dating of zircon from the Sumdo eclogites indicates a Permian metamorphic age (260-270 Ma) and a Carboniferous protolith age of 303±4.8 Ma.The ophiolite fragments in the Sumdo suture zone are composed of the ultramafic rocks, MORB-type basalt, OIB-type basalt and island arc basaltic andesite, some of which are intruded by post-collisional granites. The Gongbupala ultramafic body, a typical tectonic block in the suture zone, is completely serpentinized. Its geochemical features suggest that it is composed of harzburgite, typical of depleted mantle peridotite. The MORB and OIB-type basalts crop out in the Chasagang Formation, and the basaltic andesite crops out in the Leilongku Formation, both of which make up the Sumdo Group..Zircons from the OIB-type basalt with typical magmatic characteristics yield an average U-Pb age of 306 (95%) Ma, suggesting formation in a Paleo-Tethyan basin in the Carboniferous. U-Pb dating of zircon from the basaltic andesites yielded a concordant age of 265±3.1Ma, similar to the metamorphic age (266-270 Ma) of the eclogites, suggesting formation during subduction of the oceanic crust. Indosinian granodiorite with an age of 194±4.3 Ma crops out north of the Sumdo suture. These granodiorites are similar to the late Indochina granites in the Lhasa block, and most likely formed during continent-arc collision or during closure of the Paleo-Tethyan Ocean.A four-stage model for the evolution of the Sumdo Paleo-Tethyan suture is proposed:1) From the Ordovician to the Devonian Gondwanaland was part of an epicontinental sea; 2) In the Carboniferous, continental rifting produced a local basin, which then evolved into a Paleo-Tethyan ocean basin, dividing the Lhasa block into two segments; 3) From the Permian to the early Triassic, Paleo-Tethyan oceanic crust was subducted northward; 4) In the middle Triassic and early Jurassic, the two fragments collided to form the modern Lhasa block.