藏北聂荣地区早侏罗世末期的岩浆混合作用及构造意义

Magma mixing of late Early Jurassic age from Nyainrong,northern Tibet and its tectonic significance

藏北聂荣岩体的岩浆源区性质和岩石成因还未能得到很好约束,直接限制了对班公湖-怒江缝合带早侏罗世构造岩浆演化历史的认识。为探讨这一问题,本文报道了聂荣岩体中的花岗质岩石及其闪长质包体的岩石学、全岩主量元素和微量元素地球化学、锆石U-Pb定年和Lu-Hf同位素分析结果。一件闪长质包体样品的锆石U-Pb年龄为175.0±1.3Ma,与其寄主花岗岩类样品的年龄(174.5±1.4Ma,175.9±0.4Ma)同期。聂荣岩体中～175Ma的寄主岩石均属于偏铝质到过铝质(A/CNK=1.04～1.16)钾玄岩系列的花岗闪长岩到碱性花岗岩,标准刚玉分子数为0.6%～2.1%,包括I型和S型两种成因类型,富集大离子亲石元素、轻稀土元素和Pb,锆石εHf(t)值为-9.8～-3.7,Hf同位素地壳模式年龄为1.45～1.84Ga,主要来源于成熟地壳物质的重熔;闪长质包体在成分上主要属偏铝质钾玄岩系列的二长闪长岩,锆石εHf(t)值变化范围大(-10.3～-1.2),Hf同位素亏损地幔模式年龄介于0.87～1.22Ga,其来源可能与古老岩石圈地幔物质的部分熔融有关。聂荣地区在早侏罗世末期很可能发生了来源于成熟地壳物质深熔或重熔的熔体(占大比例)和岩石圈地幔来源的幔源岩浆之间的混合作用,混合过程中幔源组分的输入使S型熔体向I型熔体转化。在～175Ma侵位的聂荣岩体很可能是这种由岩浆混合作用形成的母岩浆再经历一定程度的分离结晶作用形成的。结合碎屑锆石研究成果,本文暂时主张聂荣岩体可能是与由班公湖-怒江洋壳北向俯冲所引起的聂荣微陆块和羌塘地体间碰撞相关的产物,这可能指示班公湖-怒江洋壳沿聂荣微陆块南侧的分支,在侏罗纪早期既发生了南向俯冲(俯冲于拉萨地体之下),又发生了北向俯冲(俯冲于羌塘地体之下)等地球动力学过程。

The nature of magma source region and petrogenesis of the Nyainrong pluton in northern Tibet remain poorly constrained,consequently raising uncertainties for understanding the tectonomagmatic evolution of the Bangong-Nujiang suture zone during the Early Jurassic. To explore these issues,this paper reports the petrological,whole-rock geochemical,zircon U-Pb dating,and zircon Lu-Hf isotopic data of host granites and dioritic enclaves from the Nyainrong pluton. A dioritic enclave sample yields zircon U-Pb age of 175.0±1.3Ma,which is coeval with the host granitoid samples (174.5±1.4Ma,175.9±0.4Ma). The host granitoids are metaluminous to peraluminous,and shoshonitic,ranging from granodiorite to alkali-granite in rock type. They consist of I-type and S-type,and have A/CNK of 1.04~1.16,and normative corundum (C-norm) of 0.6%~2.1 %,and are enriched in large LILE,LREE and Pb. These rocks are characterized by negative zircon εHf(t) of-9.8 to-3.7,with Hf crustal model ages of 1.45~1.84Ga,indicative of remelting of mature crust. The dioritic enclaves are mainly monzodioritic,metaluminous,and shoshonitic. These rocks are heterogeneous in zircon εHf(t) values (-10.3^-1.2),with Hf mantle model ages of 0.87~1.22Ga,indicating a derivation of partial melting of ancient lithosphere mantle materials. Magma mixing between anatexis or remelting of mature crust-derived materials (significant crustal input) and ancient lithosphere mantle-derived materials is most likely occurred in the Nyainrong region in the late Early Jurassic,in which the shift from S-type melts to I-type melts can be attributed to the enhanced contributions from mantle-derived materials. Such hybrid melts with subsequent fractional crystallization are responsible for the generation of the ~175Ma Nyainrong pluton. The data reported here,in combination with the data of detrital zircons in the literature,allow us to propose tentatively that the ~175Ma magma mixing identified in the Nyainrong pluton could be related to the collision between the Nyainrong microcontinent and Qiangtang Terrane following the northward subduction of the Bangong-Nujiang Ocean seafloor. Our results imply in turn that the Bangong-Nujiang Ocean seafloors may have subducted southward towards beneath the Lhasa Terrane and also subducted northward towards beneath the Qiangtang Terrane.