陕西南秦岭曹坪复式花岗岩的成因-锆石U-Pb年代学及地球化学制约

Genesis of Composite Granite-Zircon U-Pb Chronology and Geochemical Constraints in Caoping,South Qinling of Shannxi

南秦岭曹坪复式花岗岩体U-Pb同位素年龄为204.6~206.6万年,时代为晚三叠-早侏罗世。岩石地球化学显示曹坪岩体为高钾钙碱性的准-过铝质I型花岗岩。微量元素Nb、Ti、Ba、Ta、P元素为负异常,Th、Pb、K、Rb元素为正异常,大离子亲石元素Sr、Ba相对亏损,K、Rb相对富集;稀土配分曲线显示轻稀土元素富集,铕正异常的相对平缓右倾曲线模式。Nb/La比值为0.63~1.21,Nb/Ta为7.3~11.9,显示物质来源为下地壳和地幔混合形成。曹坪岩体的形成是由秦岭造山带发生岩石圈的拆沉作用引起,在印支晚期秦岭造山带的岩石圈属性从挤压体制向伸展体制开始转换。

The isotopic age of composite granite mass U-Pb in Caoping,South Qinling is 2.046~2.066 million years,the age is late Triassic-Early Jurassic.Lithogeochemistry shows Caoping rock mass is high-K calc-alkaline quasi-I type peraluminous granite.In the trace amount,elements of Nb,Ti,Ba,Ta and P have negative anomaly,elements of Th,Pb,K and Rb have positive anomaly,the large ion lithophylic element Sr,Ba is relative loss,K,Rb is relatively enriched.The rare earth partition curve shows light rare earth element enrichment,and europium positive anomaly is the relative flat right-leaning curve mode.The specific value of Nb/La is 0.63-1.21,and Nb/Ta is 7.3-11.9,which shows the material sources are the mixed formation of the earth crust and mantle.The formation of Caoping rock mass was caused by delamination of lithosphere in Qinling orogenic belt,the properties of lithosphere of late Indosinian Qinling Orogenic Belt is converted from the squeeze system to the stretch system.