念青唐古拉早白垩世侵入岩年代学、地球化学及岩石成因研究

Study on Chronology,Geochemistry and Petrogenesis of Early Cretaceous Intrusive Rocks in Nyainqentanglha Area

念青唐古拉早白垩世侵入岩主要由花岗闪长岩和石英闪长岩两类岩石组成。花岗闪长岩锆石LA-MC-ICPMSU-Pb年龄为133.20±0.92Ma,与石英闪长岩年代一致。花岗闪长岩富硅、富钾、贫磷,准铝质至弱过铝质,髙钾钙碱系列,Mg#平均为40,Nb/Ta比值为10.3,较高的[n(87Sr)/n(86Sr)]i(0.71947～0.72503),较低的εNd(t)(-12.6～-12.7),以及较老的亏损地幔模式年龄(约1927Ma),锆石饱和温度最高可达830℃,暗示了岩浆主要来自古老地壳的熔融,可能与幔源岩浆的底侵作用相关。石英闪长岩低硅、高镁(Mg#平均为50)、富铁、富钙、低碱,准铝质,钙碱系列至钾玄系列,Nb/Ta比值为20,Sr/Y比值为9,暗示了岩浆的成分主要为幔源。均富集轻稀土,亏损重稀土,弱至中等负铕异常;富集大离子亲石元素(K、Rb、Th),亏损高场强元素(Nb、Ta、P、Ti),具有活动陆缘钙碱岩系的微量元素分布特征。花岗闪长岩和石英闪长岩具有密切的岩石成因联系,石英闪长岩形成于富集俯冲带组分的地幔熔体在上升过程中混染了一定量壳源物质结晶分异的产物,花岗闪长岩则为底侵作用生产的大量壳源熔体与少量幔源熔体混合并发生一定程度分离结晶后的产物。结合前人的研究成果,推测早白垩世侵入岩与新特提斯洋西段向NE消减有关。

The Early Cretaceous intrusive rocks in the Nyainqentanglha area are mainly consisted of granodiorite and quartz diorite.The zircon LA-MC-ICPMS U-Pb dating for defines a crystallization age 133.20±0.92Ma of the granodiorite is the same as that of the quartz diorite.The granodiorite is riched in Si and K,poored in P,belongs to metaluminous to weak peraluminous,high-K calc-alkaline series,has average magnesium index of 40 and Nb/Ta ratio of 10.3,／[n（87^Sr）/n（86^Sr）／]i of 0.71947～0.72503,εNd（t） of-12.6～-12.7,tDM of about 1927Ma,and the highest magma temperature to 830℃ calculating by concentration of zirconium in whole-rock and zircon;which indicate the granodioritic magma is mainly originated from partial melting of ancient crust relating to underplating of mantle-derived magma.Quartz diorite is poored in Si and alkalinity,riched in Mg,Fe and Ca,belongs to metaluminous,calc-alkaline to shoshonite series,has Nb/Ta ratio of 20,Sr/Y ratio of 9;which indicate quartz–dioritic magma originated from partial melting of mantle.Both granodiorite and quartz diorite are LREE-rich,HREE-depletion,weak to medium Eu negative anomaly.And both of them have higher LILE concentrations,lower HFSE abundances,showing the same distribution characteristics of trace elements;which are similar to calc-alkaline rocks in island arc or active continental margin.The granodiorite and quartz diorite may be derived form the same magmatic processes.Quartz diorite originate from crystallization differentiation of mantle source magma assimilating and contaminating some crust materials;while granodiorite originate from crystallization differentiation of massous crust sourced magma mixing some mantle sourced magma.Connecting with the predecessors research results,it can be supposed that the petrogenesis may be related to the subducting towards north of Neo-Tethyan.