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CONSTITUTION AND METAMORPHISM OF THE SONGDUO GROUP IN THE SOUTH MARGIN OF GANDISE-B TYPE MOUNTAIN CHAIN IN TIBET

ZHANG Sen qi 1, WANG Bing zhang 1, WANG Jin 1, YANG Yan xing 1,HU Jian min 2 (1.Qing hai Institute of Geology and Exploration,Xining 810012,China; 2 Xian University of Engineering, Xian 710054,China) [  
The Songduo Group is one constitute part of the basement in the Gandise B type mountain chain in Tibet. It developed into an active root thrusting super microlithou during late Triassic and late Tertiary when Jarlungzangbojiang subduction super collison stage. It is volcanic sediments formed in small oceanic basin and along continental margin in the north margin of Gondwana continent. Previous work divided Songduo Group from top downward into the Chasagang Formation, Mabuku Formation, Leilongku Formation, and the Luomaling ultra mefic rock belt. Our furthur studies show that these formations are structurely contacted. In fact, they are not single stratigraphic unit, but a structure rock assemblage. They consist of geological mass of different character and various sources, and underwent metamorphism of lower grade greenschist facies to lower grade amphibolite facies. The Chasagang Formation is dominated by metamorphic basic volcanic rocks of T MORB composition. The Mabuku Formation, schist and quartzite of the Leilongku Formation are equivalent to muddy sand flysh formation and quartz flysh formation. Distinguished with multiple meathod, the Luomaling ultramefic rock is considered to be formed in small oceanic basin of extensional setting. It is concluded that the Songduo Group is volcanic sediment formed in a limited oceanic basin. [
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