甘肃北山勒巴泉南一带新元古代变质侵入岩的地质地球化学特征及成因探讨

在甘肃北山勒巴泉一带赋存一套变质侵入岩。岩性主要为闪长质片麻岩、石英闪长质片麻岩、英云闪长质片麻岩、花岗闪长质片麻岩及二长花岗质片麻岩。Rb-Sr全岩等时线定年获得似斑状中细粒花岗闪长质片麻岩年龄值727±50Ma,锆石Pb-Pb定年获得中细粒花岗闪长质片麻岩年龄值766±49Ma,结合其与围岩的接触关系及强变形变质特征,形成时代为南华纪中晚期。其岩石地球化学特点为:岩石A12O3CaO+Na2O+K2O,A/CNK=1.30~1.38,属过铝质类型;σ=2.08~3.78,NK/A=0.32~0.61,属钙碱性系列。在微量元素组成上,富集LREE和大离子亲石元素(LILE),而亏损HREE、高场强元素(Nb、Ta、Yb),Eu异常具弱—明显Eu负异常—弱的Eu正异常(δEu=0.24~1.20)。这些花岗岩类总体具有俯冲的构造背景,岩浆的产生可能与北山地区在新元古代末古陆壳裂解作用有关。

GEOCHEMISTRY AND GENESIS OF NEOPROTEROZOIC METAMORPHIC INTRUSION IN THE SOUTH TO LABAQUAN,BEISHAN REGION OF GANSU

In the south to Lebaquan, Beishan region of Gansu Province, there be occurred a set of metamorphic intrusion which consist of diorite gneiss, quartz-diorite gneiss, tonalite gneiss, granodiorite gneiss and monzonitic gneiss. According to the age of 727±50Ma determined by Rb-Sr whole rock isochron dating for middle-fine granular granodiorite gneiss, the age of 766±49Ma by Pb-Pb dating, and analysis of the contact to wall rock and deformation and metamorphic features, the intrusion should be occurred at middle to late stage of Southchina Period. Geochemical study reveal that these granitoid formed under subduction process, the formation of magma may be related to late Neoproterozoic continental crustal splitting.