西藏仲巴地体中433Ma伸展热事件：波库二云母花岗岩锆石年代学、地球化学和Hf同位素制约

433 Ma extension-related magmatism in the Zhongba terrane,southern Tibet:constraints from U-Pb zircon chronology,geochemical and Hf isotopic compositions of Boku two-mica granite

喜马拉雅造山带中发育多期次淡色花岗岩,其成因与构造转换密切相关。波库二云母花岗岩位于仲巴地体西段,锆石U-Pb年龄为433.4±2.0Ma,εHf(t)=-11.18^-8.75。全岩地球化学显示具有较高的SiO2(72.38%~73.10%)、Al2O3(15.14%~15.38%)、CaO(1.54%~1.77%)、Sr(141×10^-6~174×10^-6)、Ba(386×10^-6~574×10^-6)含量和δEu值(平均0.8),以及较低K2O/Na2O(0.87~1.11)和Rb/Sr(0.86~1.09)比值,A/CNK=1.64~1.77,随着Ba含量的增加,Rb/Sr比值保持不变。这些特征表明波库二云母花岗岩为变泥质岩发生水致白云母部分熔融的产物,与原特提斯洋向冈瓦纳大陆北缘俯冲后仲巴地体-特提斯喜马拉雅地体与冈瓦纳大陆亲缘性的陆块发生碰撞后板片断离伸展环境下的岩浆热事件相关,与特提斯喜马拉雅429Ma吉隆白云母脱水部分熔融形成的淡色花岗岩的成因不同表明它们发生部分熔融作用的物理化学条件不同,亦指示仲巴地体与特提斯喜马拉雅地体在早古生代可能具有不同的构造演化环境。

Paleozoic and Cenozoic leucogranites are commonly distributed in the Himalayan orogenic belt,and their genesis and geochemical compositions are closely related to various tectonic settings.Boku two-mica granite pluton is located in the western Zhongba terrane.LA-ICPMS analysis on zircon grains yielded an average U-Pb age of 433.4±2.0Ma(n=17)with a range ofεHf(t)values from-11.18to-8.75.Boku leucogranites show relativelyhigh contents of SiO2(72.38%~73.10%),Al2O3(15.14%~15.38%),CaO(1.54%~1.77%),Sr(141×10^-6~174×10^-6)and Ba(386×10^-6~574×10^-6),high ratios of A/CNK(1.64~1.77),slightly highδEu values with an average of 0.80,but low ratios of K2O/Na2O(0.87~1.11)and Rb/Sr(0.86~1.09).They have a trend of nearly constant Rb/Sr ratios despite large variations in Ba concentrations.All these geochemical features indicate that the Boku two-mica leucogranite is peraluminous-type granite and derived from fluid-fluxed partial melting muscovite in metasedimentary sources,they probably formed in an extensional setting due to slab delamination and asthenospheric upwelling.Different from the contemporaneous Gyirong leucogranites in Tethyan Himalaya derived from dehydration partial melting,they formed in different physical and chemical condition,indicting that the Zhongba terrane and Tethyan Himalaya terrane may have been in different tectonic environments during the Early Silurian.