西藏八宿吉利地区新发现寒武纪变质花岗岩锆石U-Pb年龄、地球化学特征及其地质意义

The Cambrian metamorphic granites in the Jili area, Baxoi, Xizang:U-Pb dating, geochemical signatures and geological significance

关键词:  吉利地区  变质花岗岩  锆石LA-ICP-MS测年  原特提斯洋  寒武纪  西藏

Key words:  Jili  metamorphic granite  LA-ICP-MS zircon U-Pb dating  proto-Tethys  Cambrian  Xizang

摘要:

八宿吉利地区寒武纪变质花岗岩位于曲扎湖-提卡一带，主要由变质二长花岗岩和变质花岗闪长岩组成。这一新发现对于认识和恢复原特提斯构造历史演化具有重要意义。锆石CL图像显示变质花岗岩锆石为岩浆成因。锆石LA-ICP-MS测年得出片理化变质二长花岗岩年龄为503.7±4.7Ma、变质花岗闪长岩年龄为494.7±3.4Ma，表明该岩体形成时代属于寒武纪。通过岩石地球化学分析，变质二长花岗岩SiO₂含量介于69.87%~79.89%之间；变质花岗闪长岩SiO₂含量介于66.63%~70.15%之间。前者Al₂O₃含量变化于12.36%~14.82%，Na₂O含量为2.54%~7.16%，K₂O含量为0.15%~5.95%，K₂O/Na₂O=0.02~2.34；后者Al₂O₃含量变化于14.66%~15.41%，Na₂O含量为3.60%~5.63%，K₂O含量为0.77%~2.78%，K₂O/Na₂O=0.14~0.77，属于钙碱性-碱性过铝质花岗岩。在侵入岩构造环境Rb-（Y+Nb）判别图解、Rb-（Yb+Ta）判别图解中，样品均落入"火山弧花岗岩"区域中，表明其形成于大陆边缘火山弧环境。结合锆石测年结果及区域地质背景分析，认为吉利地区变质花岗岩形成于冈瓦纳大陆裂离卡穷微陆块阶段，同时表明原特提斯洋形成最早时限可追溯至寒武纪。

Abstract:

The Cambrian metamorphic granites in the Jili area, Baxoi, Xizang consist dominantly of metamorphic monzogranites and metamorphic granodiorites. The new findings of the metamorphic granites in this study will be of significance in the understanding and reconstruction of the tectonic evolution of the proto-Tethys. The cathodoluminescence images of the zircons indicate the magmatic origin of the metamorphic granites in this area. The LA-ICP-MS zircon U-Pb dating data gives the ages of 503.7±4.7 Ma for the schistositized metamorphic monzogranites, and 494.7±3.4 Ma for the metamorphic granodiorites, implying that the metamorphic granites were initiated during the Cambrian. Geochemically, the metamorphic monzogranites have the SiO₂ contents of 69.87% to 79.89%, Al₂O₃ contents of 12.36% to 14.82%, Na₂O contents of 2.54% to 7.16%, K₂O contents of 0.15% to 5.95%, and K₂O/Na₂O ratios of 0.02 to 2.34, whereas the metamorphic granodiorites have the SiO₂ contents of 66.63% to 70.15%, Al₂O₃ contents of 14.66% to 15.41%, Na₂O contents of 3.60% to 5.63%, K₂O contents of 0.77% to 2.78%, and K₂O/Na₂O ratios of 0.14 to 0.77, indicating that the above-mentioned metamorphic granites belong to the calc-alkaline to alkaline peraluminous granites. In the Rb vs. (Y+Nb) and Rb vs. (Yb+Ta) discrimination diagrams for the tectonic interpretation, all the granites samples are projected into the volcanic arc granite field, suggesting the continental-margin volcanic arc environments. It follows that the metamorphic granites in the Jili area were initiated in the stages of the Kaqiong micro-continent during the breakup of the Gondwana land, and the timing of the proto-Tethys may be traced back to the Cambrian.