祁连山西段新元古代晚期花岗质片麻岩成因及LA-ICP-MS锆石U-Pb定年  

Petrogenesis and LA-ICP-MS zircon U-Pb dating of Late Neoproterozoic granitic gneisses in western Qilian Mountain

作　　者：李猛[1] 王超[1] 李荣社[1] 彭岩[1] 

机构地区：[1]国土资源部岩浆作用成矿与找矿重点实验室/中国地质调查局西安地质调查中心,陕西西安710054

出　　处：《地质通报》2015年第8期1438-1446,共9页Geological Bulletin of China

基　　金：中国地质调查局项目(编号:1212010610102);中国地质调查局百名青年地质英才计划;陕西省科技计划项目(编号:2014KJXX19)

摘　　要：祁连山西段吊大坂新元古代花岗质片麻岩出露于北大河岩群二云母片麻岩中。地球化学特征显示,吊大坂新元古代花岗质片麻岩主要为碱性、弱过铝质花岗岩（A/CNK=1.04～1.12）。岩石主体Rb含量较低,Sr和Ba含量高,K/Rb值介于179～225之间,说明吊大坂新元古代花岗质片麻岩不是高分异花岗岩,但明显的负Eu异常（Eu/Eu＊=0.25～0.35）,说明该岩浆经历了弱的分离结晶作用。在（Al2O3＋TFe O＋Mg O＋Ti O2）-Al2O3/（TFe O＋Mg O＋Ti O2）图解中,未经历强烈分异的花岗质岩石样品点落入地壳杂砂岩或中性岩浆岩源区。未经历强烈分异的样品富集强不相容元素,并且在Y-Nb和（Y＋Nb）-Rb图解中,主体落入板内花岗岩区域。吊大坂花岗质片麻岩LA-ICP-MS锆石206Pb/238U年龄为736±5Ma,该年龄被解释为花岗质片麻岩的形成时代,说明祁连山存在750～730Ma的岩浆作用记录,可能为新元古代晚期伸展背景下的产物。祁连山新元古代岩浆活动记录了新元古代早期汇聚、新元古代晚期伸展到裂解的长期演化过程。Neoproterozoic granitic gneisses from the Diaodaban area of the western Qilian Mountain outcrop in two-mica gneiss of the Beidahei Group. The granitic gneisses are mainly alkaline and peraluminous, with A/CNK values of 1.04～1.12. Their low Rb values and high Sr and Ba values as well as K/Rb ratios of 179～225 show that the rocks are not highly fractionated granite;nevertheless, the rocks have obvious negative Eu anomaly （Eu/Eu=0.25～0.35）, indicating that the magma experienced weak fractionation crystallization. The diagram of （Al2O3＋TFeO＋MgO＋TiO2）-Al2O3/（TFeO＋MgO＋TiO2） suggests that the protolith of the rocks was sourced from graywacke or mafic-intermediate igneous rocks. Samples are enriched in incompatible elements and have geochemical characteristics of the intraplate granite in Y-Nb and （Y＋Nb）-Rb diagrams. LA-ICP-MS zircon U-Pb dating of the granitic rocks yielded a weighted mean age of 736 ± 5Ma, suggesting that there is a magmatic record of 750～730Ma in the Qilian Mountain, probably resulting from intraplate extension in the late Neoproterozoic. The Neoproterozoic magmatic activities in the Qilian Mountain recorded a long-term evolution history from early Neoproterozoic convergence through Late Neoproterozoic extension to breakup.

关 键 词：LA-ICP-MS锆石U-PB年龄 花岗质片麻岩 新元古代 祁连山西段 

分 类 号：P534.31[天文地球—古生物学与地层学] P597.3[天文地球—地质学]