青藏高原Pb同位素地球化学及其意义

Pb Isotopic Geochemistry of Tibetan Plateau and Its Implications

根据青藏高原不同构造单元基底片麻岩、花岗岩类和火山岩等不同类型岩石的486套Pb同位素数据的整理和分析,发现青藏高原岩石圈存在3种主要类型,即亏损Pb同位素的特提斯洋地幔域端元、富集Pb同位素的喜马拉雅成熟大陆地壳端元和青藏高原北部的过渡型Pb同位素的地幔端元。这3类地球化学端元与前人通过Sr-Nd同位素研究获得的3类端元一致。拉萨地块内部不同类型岩石的Pb同位素地球化学特征指示出两类岩浆作用,一类是特提斯洋岩石圈俯冲消减再循环和亏损地幔物质注入导致的亲特提斯洋型岩浆作用,另一类是与类似于喜马拉雅大陆地壳物质加入导致的富集地幔源区有关的超钾质岩浆作用。岩浆作用的Pb同位素地球化学记录了特提斯洋俯冲消减作用和随后发生的印度大陆向北拼合、碰撞和俯冲过程,也记录了大规模的壳幔相互作用对高原岩石圈演化与隆升的贡献。

Pb isotopic compositions of basement, granitoids and volcanic rocks in Tibetan Plateau, with a total data sets of 486, were compiled and analyzed. Three different types of main geochemical crustal or mantle geochemical end-members were identified by their Pb characteristics. The first one is the Neo-Tethyan mantle reser- voir represented by Yarlung Zangpo ophiolite, showing low Pb features; the second one is the Himalayan continental crust represented by the basement and granitoids from Tethyan and Higher Himalayas with the most enriched Pb compositions; the third one is the North Tibetan Plateau enriched mantle source region, with a medium enriched Pb compositions. These three end-members classification is very consistent with the former result by Sr-Nd isotopic studies. A detailed discrimination for the rocks from Lhasa block allowed us to separate them into two groups. The first group, including the I-type Gangdese granitoid plutons, Linzizong volcanic rocks and adakitic ore-bearing porphyries, etc. , shows strong Tethyan oceanic crust affinity, with significant source contribution of the recycled Tethyan oceanic crust or depleted mantle. The second group, solely represented by the uhrapotassic volcanics found in western Lhasa block, exhibits an obvious input of the Himalayan continental crust to their mantle source regions. In Lhasa block, the main processes of Tethyan subduction and thereafter India-Asia collision were all recorded by Pb isotopic geochemistry in magmatism during different stages. The crust-mantle interaction among the above end-members has played an important role in the evolution and uplift of the Tibetan Plateau.