西藏冈底斯B型山链南缘松多群的构成及其变质变形特征

松多群是冈底斯B型山链变质基底的组成部分之一 ,并于晚三叠世—晚第三纪雅江缝合带俯冲超碰撞阶段演化为该B型山链活化根带逆冲推覆超岩片 ,系冈瓦纳大陆北缘于震旦—寒武纪小洋盆及陆缘火山沉积的产物。松多群岔萨岗组、马布库组、雷龙库组及罗马岭超镁铁质岩带 ,彼此间均为构造接触 ,实为一套构造岩石组合体 ,由时代相近而来源、性质不同的地质体构成 ,并经历了低绿片岩相—低角闪岩相变质作用和 4期主要构造变形。岔萨岗组主体为变质基性火山岩 ,化学成分相当TMORB ;马布库组、雷龙库组片岩、石英岩相当于被动陆缘泥砂质复理石建造及石英质复理石建造。罗马岭超镁铁质岩经综合分析判别则形成于伸展裂陷小洋盆构造环境。因而得出松多群为一较完整有限小洋盆火山沉积产物的结论。

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

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.