陕西南秦岭柞水岩体锆石U-Pb年龄、岩石地球化学特征及其地质意义

Zircon U-Pb ages and petrochemical characteristics of Zhashui granitic plutons in South Qinling Orogen of Shaanxi and their geological significance

柞水岩体锆石LA-ICP-MS U-Pb测年结果为203.3±1.3 Ma,表明其形成于印支晚期。岩体属于高钾钙碱性系列的准铝质I型花岗岩。稀土配分曲线模式为轻稀土富集,铕负异常明显的相对平滑右倾曲线模式。微量元素明显富集Rb、K等大离子亲石元素及Zr、Hf等高场强元素,强烈亏损Ba、Sr、P、Ti元素,Nb、Ta表现为弱亏损,Nb/La比值为0.63～0.97;Nb/Ta为7.64～11.64,显示物质来源为下地壳和地幔混合而成。柞水岩体的形成是南秦岭造山带在主造山后期发生岩石圈拆沉,地幔物质上涌与下地壳岩浆混合作用的结果,显示在印支晚期南秦岭造山带岩石圈属性为从挤压向伸展转变。

The zircon LA-ICP-MS U-Pb dating on Zhashui granitic pluton is 203.3 ± 1.3 Ma, which demonstrates Late Triassic age. Quantitative analysis on the pluton indicates that it is the metalumious I-type granite of high-K calc-alkaline series in category. The chondrite-normalized REE patterns exhibit a gently right-dipping V- type curve with a Eu negative anomaly. Primitive mantle-normalized trace element spidergram shows enrichment of LILE （e. g. Rb, K）, HFSE （e. g. Zr, Hf）, and a severe depletion in Ba, Sr, P, Ti and lightly depletion in Nb, Ta. Nb/La ranging from 0. 63 to 0. 97, with Nb/Ta from 7.64 to 11.64, which indicates the magma as mixture of mantle and lower crust. We conclued that the formation of Zhashui granitic pluton was mantle material flowing up and mixing with lower crust magma, related to lithospheric delamination occurred in late main orogenic stage of South Qinling Orogen. And in this period, the tectonic nature of lithosphere in South terized by transition from compression to extention. Qinling Orogen was characterized by transition from compression to extention.