冈底斯中段早侏罗世辉长岩-花岗岩杂岩体成因及其对新特提斯构造演化的启示:以日喀则东嘎岩体为例

Petrogenesis of the Early Jurassic gabbro-granite complex in the middle segment of the Gangdese belt and its implications for tectonic evolution of Neo-Tethys: A case study of the Dongga pluton in Xi'gaze

以冈底斯中段日喀则东嘎出露的早侏罗世辉长岩-花岗岩杂岩体为对象,进行了锆石U-Pb年龄和Hf同位素,以及全岩元素地球化学组成的系统测定,据此探讨了岩石的成因及其对新特提斯构造演化的启示。该杂岩体中辉长岩主要由角闪石和钙质斜长石组成,缺乏辉石;花岗岩主要为英云闪长岩、花岗闪长岩等构成的TTG岩石组合;花岗岩中普遍发育呈塑变形态的镁铁质包体。锆石LA-ICP-MS U-Pb定年结果显示,英云闪长岩和镁铁质包体的成岩年龄十分接近,且与辉长岩的年龄基本一致,均为177~180Ma。化学组成上,辉长岩低硅、富铝、贫碱,富轻稀土和大离子亲石元素,贫高场强元素,相似于高铝玄武岩。英云闪长岩贫碱、准铝、富钠,属钙碱性I型花岗岩。镁铁质包体具有与寄主岩相似的矿物组成和微量元素分布模式,二者均具有显著亏损的锆石Hf同位素组成,εHf(t)值分别为+11.4^+15.0和+14.4^+18.6。综合分析表明,早侏罗世冈底斯南缘应处于新特提斯洋板片俯冲的构造背景,其中辉长质侵入体为遭受俯冲板片析出流体交代作用的亏损地幔部分熔融的产物,花岗质岩石起源于初生地壳的部分熔融,镁铁质包体为辉长质岩浆与花岗质岩浆二者经混合作用的产物。结合对区内其它辉长质侵入体及相关镁铁质包体资料的全面分析,表明在新特提斯洋板片的整个俯冲过程中(>205~40Ma),冈底斯南缘应存在多次的基性岩浆底侵及其诱发的壳幔岩浆混合作用。

The gabbro-granite complexes developed in convergent plate margins provide an excellent opportunity to investigate the genetic relationships between acid and basic magmas,and their interactions within the intrusive environment. Such complexes are widely outcropped in the southern margin of the Gangdese magmatic belt. In this paper,we take the Early Jurassic gabbro-granite complex in Dongga near Xi＇gaze as an example,and conduct an integrated study including petrology,elemental geochemistry and zircon Hf isotopes,with the aims of understanding their petrogenesis and the implications for Neotethyan evolution. Lithologically,gabbroic rocks in the Dongga complex consist mainly of amphiboles and calcic plagioclases,and pyroxenes are rarely observed.Granitoids are a set of TTG association composed mainly of tonalites and granodiorites. Mafic enclaves with various plastic shapes are widely dispersed in the granitoids. Zircon LA-ICP-MS U-Pb dating demonstrates that the tonalites and the mafic enclaves share similar ages of 177 ~ 180 Ma with the gabbros. Chemically,the gabbros have low Si O2 and K2O ＋ Na2 O,but high Al2O3 contents,and are enriched in LREEs and LILEs,depleted in HFSEs,showing similar chemical features with the high-alumina basalts. The tonalites exhibit sub-alkaline,metaluminous,and Na-rich signatures,and are genetically of calc-alkaline I-type granites. The mafic enclaves share similar mineral assemblages and trace element distribution patterns with the host granitoids. Both the granitoids and the mafic enclaves have highly depleted zircon Hf isotopic compositions,with εHf（ t） values of ＋ 11. 4 ~ ＋ 15. 0 and ＋ 14. 4 ~ ＋ 18. 6,respectively. The integrated petrology and elemental and isotopic compositions suggest that during the Early Jurassic,the southern margin of the Gangdese belt was under the Neotethyan subduction setting. The gabbros were generated by the hydrous partial melting of depleted mantle metasomatized by fluids released from subducted oceanic slab. The granitoids were originated from partial melting of juvenile crustal materials. The mafic enclaves were produced by binary mixing between the gabbroic and granitic magmas. In combination with a comprehensively synthesis of the other gabbros and related mafic enclaves along the southern margin of the Gangdese belt,we propose that multiple underplating of mafic magmas and the induced magma mixing were occurred during the whole duration（ 205 ~ 40Ma） of the Neotethyan subduction.