西昆仑塔什库尔干地块马尔洋一带中寒武世花岗岩的发现及地质意义

THE DISCOVERY AND GEOLOGICAL SIGNIFICANCE OF MID-CAMBRIAN GRANITES IN MAERYANG AREA,TASHKURGHAN MASSIF,WEST KUNLUN

西昆仑塔什库尔干地块马尔洋一带发育一期中寒武世花岗质岩浆活动,岩石类型为片麻状花岗闪长岩和二长花岗岩,LA-ICP-MS锆石U-Pb测年表明,其侵入时代分别为(506.0±6.8) Ma和(514.0±9.3) Ma。岩石SiO 2含量为67.83%~75.76%,具中高钾、准铝质—弱过铝质等特征;岩石稀土总量与平均陆壳相当(∑REE=90.85×10 -6 ~186.10×10 -6 ),轻、重稀土分馏程度较强((La/Yb) N=5.38~22.43),负Eu异常明显(δ Eu=0.42~0.71);微量元素以富集Rb、K、Ba、Th、U等大离子亲石元素和亏损Nb、Ta、P、Ti等高场强元素为特征。地质学及岩石地球化学特征指示该期花岗岩属准铝质到弱过铝质高分异I型花岗岩,锆石饱和温度指示其为高温岩浆岩,总体具有壳幔混源的特征,富云包体指示壳源成分占有较大的比例。该期花岗岩与邻区中—晚寒武世中酸性侵入岩指示塔什库尔干—甜水海地块南缘很可能存在一条中寒武世以来的岩浆弧带,其可能为在原特提斯(有限)洋盆于中寒武世向北俯冲、消减背景下幔源物质上涌底侵加热古老陆壳进而发生部分熔融的产物。这也预示着塔什库尔干地区在经过震旦纪—早寒武世伸展裂解阶段之后,于中—晚寒武世其大地构造环境及地球动力学背景发生了重大转折。

The paper reports a mid-Cambrian granitic magmatic activity which is comprised of gneissic granodiorite and monzogranite in Maeryang area of Tashkurgan massif, West Kunlun. LA-ICP-MS zircon U-Pb dating yields the emplacement ages of ca.(506.0±6.8) Ma and (514.0±9.3) Ma, respectively. The granites are characterized by moderate-high alkaline, quasi-aluminum to weak peraluminum, and a moderate range of SiO 2 (67.83%~75.76%). Addtionally, the total REE of granites is equivalent to the average continental crust (∑REE=90.85×10 -6 ~186.10×10 -6 ) and marked by obvious fractionation between light and heavy REE ((La/Yb)N=5.38~22.43) and negative Eu anomalies(δ Eu=0.42~0.71). The intrusions have geochemical features of enrichment of LILEs such as Rb, K, Ba, Th, U and depletion of HFSEs such as Nb, Ta, P, Ti. The geological and petrogeochemical characteristics of the granites indicate that they are quasi-aluminous to weakly peraluminous highly fractionated I-type granites. Zircon saturation temperature reflects that the mid-Cambrian granites can be assigned to the high-temperature magmatic rocks. The presence of mica-rich enclaves in the granites which were most likely derived from crust-mantle mixing magma indicates that the crust composition accounts for a large proportion. The intermediate acid magma intrusion during middle-late Cambrian in Tashkurghan and its adjacent region shows that there was a magmatic arc zone in the southern margin of Tashkurgan-Tianshuihai massif. The granites in Maeryang area might originate in the partial melting of ancient continental crust heated by upwelling and underplating mantle during the Proto-Tethys ocean northward subduction in mid-Cambrian. It also indicates that the tectonic environment and geodynamic background during the middle-late Cambrian period have taken a major turn after the Sinian-early Cambrian extension and cracking stage in the Tashkurgan area.