巴颜喀拉中部三叠纪称多高镁闪长岩的地球化学特征、岩石成因及其构造意义

GEOCHEMISTRY,PETROGENESIS AND TECTONIC IMPLICATIONS OF THE LATE TRIASSIC CHENDUO HIGH-Mg DIORITES IN THE CENTRAL BAYAN HAR TERRANE

本次研究通过对巴颜喀拉中部称多地区三叠纪高镁闪长岩开展系统的锆石U-Pb年代学、岩石学和地球化学综合研究,深入剖析其岩石成因,在此基础上与北羌塘地体北部同时期典型高镁闪长岩进行对比,进而为深入理解巴颜喀拉-松潘-甘孜地体三叠纪岩浆岩的成因及其地球动力学背景提供重要证据。通过锆石LA-ICP-MS U-Pb同位素测年方法获得该闪长岩的结晶年龄为216±2 Ma,与北羌塘地区三叠纪典型高镁闪长岩的形成年龄在误差允许范围内基本一致。岩石样品具有显著高的MgO(3.2%~5.03%)、Mg#(51%~58%)、Cr(75.8~127.6×10^-6)、Ni(10~41.5×10^-6)、YbN(6.53~8.51×10^-6)和Y(17.79~22.65×10^-6)以及相对低的(La/Yb)N和Sr/Y比值(分别为7.63~13.16和12.50~18.38),可与日本Steouchi地区赞歧质高镁安山岩和北羌塘三叠纪典型的高镁闪长岩成分类比。所有岩石样品轻重稀土分异较强,轻稀土(LREE)相对富集,并且具有中等程度的Eu负异常。样品在微量元素组成上相对于原始地幔要明显富集Th和REE,亏损Nb,Ta,P和Ti,整体上表现俯冲带岩浆岩的成分特征。综合岩石学、地球化学以及区域上多学科研究结果,可以确定研究区及周缘这些三叠纪高镁闪长岩很可能形成与板块俯冲有关的环境中,并且与与受俯冲增生楔沉积物熔体交代的地幔橄榄岩的部分熔融有关。

Comprehensive study of zircon U-Pb geochronology,petrology and geochemistry for the Triassic high-Mg diorite in the Chengduo area,central Bayan Har Terrane is carried out so as to unravel its petrogenesis.Correlation of high-Mg diorite in the Chengduo area with the contemporaneous high-Mg diorite developed in the north Qiangtang Terrane is made in order to provide evidence for the determination of genesis and geodynamic background of Triassic igneous rocks in the Bayankala-Songpan-Ganzi terrane.LA-ICP-MS zircon U-Pb dating of the high-Mg diorite yields a crystallization age of 216±2 Ma,close to the formation age of typical Triassic high-Mg diorite in the north Qiangtang terrane.The high-Mg diorite has high contents in MgO(3.2%-5.03%),Mg#(51%-58%),Cr(75.8-127.6×10^-6),Ni(10-41.5×10^-6),YbN(6.53-8.51×10^-6)and Y(17.79-22.65×10^-6),and low in(La/Yb)N and Sr/Y(7.63-13.16 and 12.50-18.38,respectively),similar to Sanukitic high-Mg andesites in the Steouchi area(Japan)and to typical Triassic high-Mg diorite in the north Qiangtang terrane.The high-Mg diorite displays strong differentiation of rare earth elements,relatively enrichment of light rare earth elements with moderate negative Eu anomaly.It exhibits enrichment of LREE,Th,and U,deplete of Nb,Ta,P and Ti in comparison with the primitive mantle.Combined with the results of petrology,geochemistry and regional multidisciplinary studies,it can be determined that the Triassic high-Mg diorite in the study area and its surrounding area probably formed in the environment of plate subduction and is related to the partial melting of mantle peridotite metasomatized by accretionary wedge-related sediment melt.