摘要:
本文对位于青藏高原拉萨地体东南缘林芝杂岩中的两类石榴角闪岩进行了详细的地球化学和锆石U-Pb 年代学研究。这两类石榴角闪岩分别为硅不饱和的含十字石石榴角闪岩和硅饱和的含石英石榴角闪岩。含十字石石榴角闪岩矿物组合为十字石、石榴子石、角闪石、钠云母、绿泥石、斜长石。石英石榴角闪岩矿物组合为石榴子石、角闪石、石英、斜长石、黑云母。岩石学及变质相平衡研究表明两类石榴角闪岩均经历了高压角闪岩相变质作用。含十字石石榴角闪岩和石英石榴角闪岩具有 MORB 的地球化学特征,锆石U-Pb年代学分析获得了800~200 Ma的206Pb/238U年龄范围,出现了~430 Ma、~268 Ma和~216 Ma年龄峰值。~430 Ma年龄可能和拉萨地体岩浆活动有关,~268 Ma变质年龄和~216 Ma变质年龄和拉萨地块经历的高压变质作用有关。其中~268 Ma年龄和拉萨地块内部松多高压带榴辉岩的峰期变质年龄一致,而~216 Ma年龄和榴辉岩的围岩含石榴子石片岩年龄一致。对比该区域的年代学研究成果,这表明林芝杂岩体不仅经历了中新生代的变质和岩浆再造活动,还经历了古特提斯洋闭合,南北拉萨地块发生碰撞的晚二叠世的高压变质作用和三叠纪的中压变质作用。
Abstract:
In this paper, the authros studied the geochemistry and chronology of garnet amphibolites distributed in the Nyingchi region within the southeastern segment of Lhasa terrane. The studied samples consist of staurolite-bearing garnet amphibolites and quartz-bearing garnet amphibolites, which experienced the high-pressure amphibolite facies metamorphism. The staurolite-bearing garnet amphibolite has a mineral association of garnet, amphibole, staurolite, chlorite, plagioclase, mica and minor ilmenite and apatite. The quartz-bearing garnet amphibolites has a mineral assemblage of quartz, garnet, and amphbolite. Petrological and geochemistry studies show that the protoliths of the garnet amphibolites are characterized by MORB affinity. Detrital zircons from the paragneiss yielded a206Pb/238U age range of 800~202 Ma. With main populations at ~430 Ma, ~268 Ma, and ~216 Ma. ~268 Ma and ~216 Ma metamorphic ages are related to the HP metamorphism of the Lhasa Block. ~268 Ma age is similar to the age of peak metamorphic eclogite in Sumdo, and ~216 Ma is similar to the age of the garnet-bearing schist, i.e., the host rock of eclogite. In combination with regional chronology of Lhasa Block, the authors hold that the Nyingchi Complex not only experienced the metamorphism and magmatic activities in Cenozoic but also experienced the UHP metamorphism in Permian and the HP metamorphism in Triassic following the closure of the Paleo-Tethyan Ocean and the collision between the south Lhasa terrane and the north terrane.
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