藏南拉轨岗日变质核杂岩核部花岗质片麻岩的地球化学特征及构造意义

摘要

拉轨岗日变质核杂岩花岗质片麻岩高SiO2、K2O,低Fe2O3、MgO、MnO,铝饱和指数A/CNK为1.07～1.19,含白云母、石榴子石等过铝质矿物,属高钾钙碱性过铝质岩石.稀土总量较高,轻稀土富集,轻、重稀土分馏明显,Eu负异常；富集大离子亲石元素Rb、Th、U,贫高场强元素Sr、Nb,Hf和Ba亏损,具有壳源S型花岗岩的特征.87Sr/86Sr初始比值ISr变化于0.709～0.7306,εNd(t)值较低,为-7.13 ～-8.97,且εNd(t)和ISr无明显相关关系,εSr(t)值较高,为80.65 ～379.(206Pb/204Pb)t=18.1062 ～18.8085,(207Pb/24Pb)t=15.6713 ～ 15.7901,(208Pb/204Pb)t=37.529 ～ 38.1815,表现出地壳特征的Sr、Nd、Pb同位素特征.岩浆源区可能是以粘土岩主,砂质岩占次要地位的沉积岩,经部分熔融形成的花岗质岩浆上升侵位形成.LA-ICPMS锆石U-Pb年龄为514Ma,是泛非造山事件在北喜马拉雅拉轨岗日一带的地质记录.岩石形成于由碰撞造山的挤压环境向后碰撞造山的伸展环境转化阶段,说明泛非碰撞造山事件在拉雅拉轨岗日一带可能结束,进入后碰撞造山的构造演化阶段.岩浆底侵可能使拉轨岗日变质核杂岩在该阶段已经初步隆起,对变质核杂岩的形成起了重要作用.%The granitic gneiss of Laguigangri metamorphic core complexes in southern Tibet is characterized by high SiO2 and K2 O, low Fe2O3 , MgO and MnO, with alumina saturation index ( A/CNK) of 1. 07 ~ 1. 19 and peraluminous minerals ( such as muscovite and garnet). These features suggest that the granitic gneiss belongs to the high-K calci-alkaline peraluminous rocks. It is obvious that the chondrite-normalized REE shows right inclined patterns with obvious negative Eu abnormity, simultaneously, the primitive mantle normalized trace element spidergrams are characterized by enriched LILE (such as Rb, Th and U) and depleted HFSE (such as Sr and Nb) , as well as evidently low contents of Ba and Hf, which are compatible with the characteristics of crustal derived S-type granites. The initial values of 87Sr/86Sr(ISr) are in a range of 0.709 ~0. 7306, the εSr(t) in a range of 80. 65 ~379 and εNd(t =514Ma) values of -7. 13 ~ -8.97, without significant correlation between εNd(t) and ISr. The ratios of (206 Pb/204 Pb), ( = 18. 1062~18.8085) , (207Pb/204Pb),( =15.7~15.8) and (208Pb/204Pb),( =37.5~38.2) also reveal crustal Pb isotopic features. Therefore, the granitic magma is originated from partially melting of clay and sandstone sedimentary rocks. With the U-Pb isotopic age of 514Ma, we believe that these rocks in Laguigangri formed during the transition period from syncollision ( compressional) to post collision (extensional environment) in Pan African orogenic events in the north Himalayan. This magmatic activity may play an important role in formation of the Laguigangri metamorphic core complex.