西秦岭北缘天水地区新元古代花岗质片麻岩地球化学特征及其形成环境

摘要

西秦岭北缘新元古代花岗质片麻岩位于天水地区分隔北秦岭造山带和北祁连构造带的新阳-元龙韧性剪切带中.花岗质片麻岩具高Si、高Al的特征,属高钾钙碱性系列,A/CNK在1.104～1.389之间,为硅、铝过饱和类型,属典型的强过铝质S型花岗质岩石.轻、重稀土元素分馏较强,具中等负Eu异常.岩石富集大离子亲石元素(Cs、Rb、Ba、Th、U、K、La等),强烈亏损高场强元素(Nb、Ta、Ti、Hf等),明显的Ba、P、Sr负异常,相对富集Zr.该花岗质岩石是一种典型的壳源成因类型,主要为上地壳中以成熟度较低、含泥质成分较高的杂砂岩、岩屑杂砂岩为原岩的古元古界秦岭岩群中含水矿物相脱水部分熔融形成的,可能存在少量的分离结晶作用.该花岗质片麻岩具有同碰撞型花岗岩的特征,可能是北秦岭微地块与相邻地块在新元古代早期发生汇聚的产物,是Rodinia超大陆在西秦岭地区汇聚的响应.%The Neoproterozoic granitic gneisses in Tianshui area are located in the Xinyang -Yuanlong ductile shear belt which is the boundary between the North Qinling orogenic belt and the North Qilian tectonic zone. Geochemistry, petrogenesis and tectonic setting of the granitic gneisses are discussed in this paper. The granitic gneisses are nch in SiO2 and A12O3, with A/CNK being 1.104~ 1.389, showing the supersaturation of Al and Si and suggesting the features of typical strongly peraluminous S-type granite. The ∑REE range from 128.27×10-6 to 242.09×10-6. Chondrite-normalized REE patterns show a nghtward inclination with medium negative rnEu anomalies. The trace element geochemistry is characterized by positive anomalies of Cs, Rb, Ba, Th, U, K, La, strongly-depleted negative anomalies of Nb, Ta, Ti, Hf, and evident negative anomalies of Ba, P, Sr. The granitic gneisses have features of crust-derived and typical continent-continent syn-collisional granites. The Paleoproterozoic Qinling Group whose protoliths were immature psam-mites containing higher pelitic compositions probably served as the source rocks. The generation of granitic gneisses resulted from dehydration melting due to the breakdown of micaceous minerals, with the possible existence of some fractional crystallization. The granitic gneisses suggest a collision between the North Qinling micro-block and its adjacent block in the Early Neoproterozoic period. It is a response of the assembly of Rodinia in West Qinling.