南拉萨地块中部早侏罗世仁钦则花岗闪长岩成因及其地质意义
Petrogenesis and Geological Implications of Early Jurassic Granodiorites in Renqinze Area,Central Part of Southern Lhasa Subterrane

青藏高原拉萨地块南部新特提斯洋的打开和俯冲时间，目前还未得到统一的认识.南拉萨地块上广泛发育早中生代花岗质岩石，为探讨亚洲-印度大陆碰撞前的新特提斯洋俯冲过程提供了天然样品.对仁钦则地区一套花岗闪长岩进行了详细的年代学、元素和同位素地球化学分析.锆石U-Pb定年结果显示，这套花岗闪长岩主要形成于180 Ma左右，为早侏罗世岩浆活动产物.仁钦则花岗闪长岩具有相对较高的SiO2含量(62.77%～64.18%)、较低的K2O/Na2O(0.29～0.60)和A/CNK值( 0.90～ 0.98)，表明其属于I型钙碱性岩石系列.岩石明显富集Ba、U等大离子亲石元素而亏损Nb、Ta等高场强元素，具有弧岩浆岩的地球化学特征.花岗闪长岩具有较高的CaO含量、较低的全碱和Al2O3含量，与含水的变基性岩部分熔融形成的熔体成分一致.锆石Ti和全岩Zr饱和温度计结果显示花岗闪长岩可能形成于下地壳源区.仁钦则花岗闪长岩具有较低的( 87Sr/ 86Sr)i (0.703 671～0.703 794)、较高的εNd ( t )(5.41～5.66) 和锆石εHf ( t )值(12.6～14.8)，进一步表明其很可能为新生基性下地壳的部分熔融产物.综合分析认为，仁钦则花岗闪长岩主要形成于新特提斯洋北向俯冲背景之下，新特提斯洋的打开时间至少在晚三叠世之前.
The opening and subduction time of the Neo-Tethys ocean in the southern part of the Lhasa block on the Qinghai-Tibet Plateau ramains controversial.The widely developed Mesozoic granitoids in the southern Lhasa subterrane of Tibetan Plateau,are essential samples to explore the prolonged evolution of subduction process of the Neo-Tethys before collision between the Asia and India continents.Here,we present detailed zircon U-Pb dating,trace element and Hf isotope,wholerock major and trace elements,and Sr-Nd isotope data for the granodiorites from the Renqinze area,central part of southern Lhasa subterrane.The granodiorites were crystallized at ca.180 Ma,which belongs to Early Jurassic.They are characterized by relatively high SiO2 (62.77%-64.18%) contents and low K2O/Na2O (0.29-0.60) and A/CNK values (0.90-0.98).These geochemical characteristics are similar to I-type calc-alkaline rocks.Renqinze granodiorites are enriched in LILEs (e.g. Ba and U) and depleted in HFSEs (e.g. Nb and Ta), showing the geochemical affinity of arc-related magmatism.The granodiorites fall into the range of partial melts of metabasaltic rocks due to their high CaO,low total alkaline and Al2O3 contents.The accordant results of Ti-in-zircon and wholerock zircon saturation temperature suggest that the Renqinze granitoid rocks were derived from the lower continental crust.Moreover,the granodiorites show low 87Sr/ 86Sr(0.703 671-0.703 794), high εNd ( t )(5.41-5.66) and zircon εHf ( t )(12.6-14.8) values,indicating they were likely generated from partial melting of a juvenile mafic lower crust.The Renqinze granitodiorites represent the products of subduction of the Neo-Tethys ocean.The timing for the opening of the Neo-Tethys is at least before Late Triassic