西天山小哈拉军山富钛磁铁矿辉长岩的岩石成因及其构造环境  

Petrogenesis and tectonic setting of the Xiaohalajunshan titanomagnetite-rich gabbro,Western Tianshan Orogen

作　　者：贺鹏丽[1] 黄小龙[1] 杨帆[1] 王雪 李武显[1] HE PengLi;HUANG XiaoLong;YANG Fan;WANG Xue;LI WuXian(State Key Laboratory of Isotope Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China;College of Earth Science and Engineering,Shandong University of Science and Technology,Qingdao 266590,China)

机构地区：[1]中国科学院广州地球化学研究所,同位素地球化学国家重点实验室,广州510640 [2]山东科技大学地球科学与工程学院,青岛266590

出　　处：《岩石学报》2020年第7期2001-2016,共16页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41703033、41625007)资助。

摘　　要：天山造山带晚古生代构造争议部分源于对晚古生代岩浆岩的岩石成因及其构造环境认识不足。本文对西天山小哈拉军山辉长岩进行了系统的矿物学、岩石学、地球化学和年代学研究,以探讨其岩石成因、地幔源区特征及其形成的构造背景。锆石U-Pb定年分析结果显示小哈拉军山辉长岩形成于早二叠世早期(295±3Ma)。该辉长岩富含钛磁铁矿(含量高达8vol%以上),并且主要造岩矿物如斜长石、单斜辉石、角闪石均显示出富Fe-Ti的特征,表明其形成于富Fe-Ti的岩浆体系。岩相学特征显示,小哈拉军山辉长岩的斜长石结晶早于单斜辉石和角闪石,由角闪石成分估算的岩浆水含量相对较低,暗示了相对干的初始岩浆体系,并且单斜辉石的Al-Ti含量关系显示出板内岩浆的特征,均表明该辉长岩并非岛弧岩浆产物,应形成于拉张构造环境,因此西天山地区在早二叠世早期已处于碰撞后伸展阶段。另外,该辉长岩的全岩富集大离子亲石元素和轻稀土元素,相对亏损重稀土和高场强元素,全岩εNd(t)略亏损(2.34~3.30),Sr同位素比值变化较大(87Sr/86Sr=0.7045~0.7067),表明岩浆来源于富集的地幔源区,可能与造山带形成之前的俯冲板片组分的交代富集相关。由于西天山地区在早二叠世已处于造山后的伸展阶段,是塔里木大火成岩省范围内地幔柱活动最易于表现的地区,而小哈拉军山辉长岩与相邻的哈拉达拉富V-Ti磁铁矿层状辉长岩具有相似的岩浆源区和矿化特征,并具有密切的时空关系,很可能都为塔里木早二叠世地幔柱活动的产物。The dispute of tectonic evolution of the Tianshan Orogenic Belt in the Late Paleozoic is partly due to poor understanding of the petrogenesis and tectonic setting of coeval magmatic rocks.In this study,we reported systematic petrology,mineralogy,geochemistry,and chronology data for the Xiaohalajunshan gabbro in the Western Tianshan Orogen,in order to investigate its petrogenesis,mantle source,and tectonic settings.The zircon U-Pb dating results show that the Xiaohalajunshan gabbro was emplaced in the beginning of Early Permian(295±3Ma).The studied samples contain abundant titanomagnetite(up to 8%on the volume),and major rock-forming minerals such as plagioclase,clinopyroxene and amphibole are all enriched in Fe and Ti,denoting a Fe-Ti rich magmatic system.Plagioclase is an earlier phase than clinopyroxene and amphibole on the petrography,and the water contents estimated from the amphibole composition are relatively low,which all suggest that primary magma for the gabbro is relatively dry.In addition,the correlation between Al and Ti contents of clinopyroxene shows an intraplate magmatic trend.Therefore,the Xiaohalajunshan gabbro was derived from an extensional tectonic setting instead of an arc system,indicating the Western Tianshan was in a post-orogenic extensional setting at least since the beginning of Early Permian.On the other hand,the Xiaohalajunshan gabbro is enriched in large ion lithophile elements(LILE)and light rare earth elements(LREE)and relatively depleted in heavy rare earth elements(HREE)and high field strength elements(HFSE),together with its slightly positive εNd(t)value(2.34~3.30)and variable initial 87Sr/86Sr ratio(0.7045~0.7067),which indicates an enriched mantle source refertilized by the fluids from subducted oceanic plate.During the Early Permian,the Western Tianshan in a post-orogenic extensional setting is a weakened area within the Tarim large igneous province,and thus is in favor to the expression of mantle plume activities.As the Xiaohalajunshan gabbro and nearby Haladala V-Ti magn

关 键 词：辉长岩 钛磁铁矿 富铁岩浆 地幔柱 西天山造山带 

分 类 号：P588.124[天文地球—岩石学] P597.3[天文地球—地质学]