摘要:本文通过岩相学、岩石地球化学、锆石U-Pb定年和Lu-Hf同位素组成分析等方法，对出露于北秦岭西段宝鸡岩体王家山一带的黑云母花岗岩和其中的包体进行了研究。结果表明，该花岗岩形成时代为187±2 Ma，属于高钾钙碱性—钾玄岩系列岩石，富集Rb、Th、U等大离子亲石元素以及Nb、Zr和Hf等高场强元素，亏损Ba、Sr和Eu，具有高的全岩锆石饱和温度（825℃～838℃），显示A型花岗岩特征，形成于造山后的板内环境，可能为秦岭岩群副变质岩与安山质岩石部分熔融的产物。暗色包体显示塑性流变特征，具有岩浆结构，发育针状磷灰石和具有复杂成分环带的更长环斑结构长石，是幔源岩浆注入酸性岩浆发生混合作用的产物，形成时代为191±2 Ma，其锆石Hf同位素组成变化范围较大，εHf（t）值介于-11.26～-2.51，主要为富集地幔部分熔融产物。综合本文及前人已有研究结果，认为～190 Ma的早侏罗世早期秦岭地区早中生代碰撞造山过程已经结束，区域开始逐渐进入板内伸展构造演化阶段。
关键词: 秦岭造山带/
早侏罗世早期/
A型花岗岩/
暗色包体/
板内伸展
Abstract:In this paper, we present systematic petrological, geochemical, zircon U-Pb ages and Lu-Hf isotopic data for the biotite granites and mafic enclaves therein from Wangjiashan area, Shaanxi Province. The results showed that the biotite granites were formed at 187±2 Ma and belong to the high-K calc-alkaline and shoshonitic series. They are enriched in large ion lithophile elements(Rb, Th and U) and high field strength elements(Nb, Zr and Hf), but have negative Ba, Sr and Eu anomalies. The rocks have high zircon saturation temperatures(825 ℃~838 ℃). They exhibit A-type granite affinity and formed in a intraplate environment, and granitic magma may formed through partial melting of andesitic rocks and paragneiss of the Qinling Group. Mafic enclaves in the granites were formed at 191±2 Ma. They are fine-grained in texture and ellipse in shape, characterized by the appearance of needle-like apatites and alkali feldspar megacrystals that surrounded by plagioclase rims. Zircons from mafic enclves have negative εHf(t) values ranging from -11.26 to -2.51, indicating that the magma should be derived from the enriched mantle sources, and their formation experienced mixing event between the mantle-derived basic magma and the crustal acidic magma. Based on these results above and previous studies, it is concluded that the Early Mesozoic collisional orogenic process in Qinling area had come to an end in the Early Jurassic, and the Qinling orogenic belt started its intraplate extension tectonics process at ca.190 Ma.
Key words:Qinling orogenic belt/
Early Jurassic/
A-type granite/
Mafic enclaves/
Intraplate extension
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