西秦岭天水地区流水沟岩浆杂岩LA-ICP-MS锆石U-Pb测年和岩石地球化学特征

摘要

流水沟岩浆杂岩分布于西秦岭北缘天水关子镇地区,主要由变辉长岩、变辉长闪长岩和变闪长岩组成.为确定其构造属性,探讨西秦岭北缘构造演化过程,对其进行了LA-ICP-MS锆石U-Pb测年和主量、微量、稀土元素分析.结果表明,岩浆杂岩体的锆石为岩浆成因,其形成时代为548.7Ma±1.8Ma(MSWD=0.56).流水沟岩浆杂岩具有SiO2变化范围大(52.47～66.95％),低TiO2,低碱,Mg#值变化范围较大的特征,主量元素在SiO2-氧化物变异图解上具有较好的线性相关性.稀土元素配分曲线总体呈近平坦型,LREE/HREE比值多数在1.14～2.60之间,轻重稀土元素分馏较弱,δEu介于0.65～1.90之间,大多数具有弱的负Eu异常；在微量元素蛛网图上,大离子亲石元素Cs、Sr、Th、U富集,Rb、K和高场强元素Nb、P、Ti、Y亏损,Ti/Y=113～253,Th/Yb=0.23～2.85,Th/Nb=0.09～1.00,Hf/Th=0.41～2.97,显示了流水沟岩浆杂岩形成于岛弧环境.流水沟岩浆杂岩可能反映在中央造山系中存在一期与“泛非造山运动”有关的构造热事件,为中央造山系在新元古代晚期一早古生代的构造演化提供了证据.%The Liushuigou igneous complex distributed in the northern margin of the West Qinling Mountains consists of metagabbro, metagabbro-diorite and metadiorite. In order to determine the structural properties and discuss the process of tectonic evolution, the authors carried out the LA-ICP-MS zricon U-Pb dating as well as major and trace elements analysis. The results show that the zircon is of magmatic origin, and the age of the igneous complex is 548.7±1.8Ma (MSWD=0.56). The Liushuigou igneous complex is characterized by a wide range of SiO2 (52.47%-66,95%), low TiO2, low ALK, a wider range of Mg#, and a positive correlation between oxide and SiO2 in variation diagrams. The REE patterns are flat, the ratios of LREE/HREE range from 1.14 to 2.60, LREE and rnHREE show low fractionation, and 8Eu values are between 0.65 and 1.90, mostly assuming negative anomalies, In the trace element spidergrams, there exist the enrichment of such LILEs as Cs, Sr, Th and U and depletion of Rb and K and such HFSEs as Nb, P, Ti and Y, the ratios of Ti/Y, Th/Yb, Th/Nb and Hf/Th are 113-253, 0.23-2.85, 0.09-1.00 and 0.41-2.97, respectively, suggesting that the igneous complex was formed in an island-arc setting. The Liushuigou igneous complex might indicate that there existed a " Pan-African orogeny" tecto no thermal event in the Central Orogenic System. The results obtained by the authors provide evidence for the tectonic evolution of the Central Orogenic System in Late Neoproterozic and Early Paleozoic period.