甘肃北山地区古堡泉辉绿岩脉地球化学特征及其地质意义  

Geochemical Characteristics of Gubaoquan Dolerite Dykes in Beishan Area of Gansu,China and Their Geological Significance

作　　者：高文彬 钱壮志[1,2] 徐刚 段俊[1] 师震[1] 马博骋 杨涛 GAO Wen-bin;QIAN Zhuang-zhi;XU Gang;DUAN Jun;SHI Zhen;MA Bo-cheng;YANG Tao(School of Earth Science and Resources,Chang'an University,Xi'an 710054,Shaanxi,China;Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits of Ministry of Natural Resources,Xi'an 710054,Shaanxi,China;Northwest Geological Exploration Institute of China Metallurgical Geology Bureau,Xi'an 710119,Shaanxi,China)

机构地区：[1]长安大学地球科学与资源学院,陕西西安710054 [2]自然资源部岩浆作用成矿与找矿重点实验室,陕西西安710054 [3]中国冶金地质总局西北地质勘查院,陕西西安710119

出　　处：《地球科学与环境学报》2020年第5期622-636,共15页Journal of Earth Sciences and Environment

基　　金：国家自然科学基金项目(41872076);陕西省自然科学基础研究计划项目(2019JQ-674);中央高校高新技术研究培育项目(300102279203)。

摘　　要：甘肃北山地区古堡泉辉绿岩脉主体侵位于奥陶纪—泥盆纪花岗闪长岩中,少量穿插于早古生代地层中,主要有辉绿岩和辉绿玢岩两种岩石类型,主要矿物为斜长石和辉石,此外还有少量角闪石、黑云母等次要矿物及磁铁矿等副矿物。锆石U-Pb年代学结果表明辉绿岩结晶年龄为(285.0±4.7)Ma,与北山地区广泛分布的辉绿岩脉群形成时代相近。古堡泉辉绿岩εNd(t)为6.39~6.78,表明岩浆来自类似亏损地幔的源区;全岩Sr-Nd同位素模拟计算表明岩浆侵位过程中遭受了10%~15%上地壳物质混染。辉绿岩的轻、重稀土元素含量比值为1.72~2.17,在球粒陨石标准化稀土元素配分模式中呈平坦型;微量元素具明显的Nb-Ta负异常。其稀土和微量元素特征不同于塔里木地块与地幔柱活动有关的巴楚辉绿岩,而与北山地区东部音凹峡和西部坡东辉绿岩具有相似的地球化学特征。结合已有研究成果认为,古堡泉辉绿岩形成于古亚洲洋闭合之后的伸展环境,是岩石圈拆沉导致软流圈减压部分熔融形成玄武质岩浆作用的产物。Gubaoquan dolerite dykes are predominantly intruded into Ordovician-Devonian granodiorites,and a few are interspersed in Early Paleozoic strata.The dykes are divided into two different rock types,including dolerite and sillite.The mainly minerals are plagioclase and pyroxene,with minor amphibole,biotite and some accessory minerals.The results of zircon U-Pb dating yield an age of(285.0±4.7)Ma,indicating the emplacement time of dolerite dykes,which is coeval to the formation age of dolerite dykes in other places of Beishan area.The εNd(t)of Gubaoquan dolerite varies from 6.39 to 6.78,indicating that the parental magma is derived from a depleted mantle source.The whole rock Sr-Nd isotopic simulation suggests that the magma undergoes a 10%-15% of contamination by upper crust materials during emplacement process.The LREE/HREE ratio of dolerite varies from 1.72 to 2.17,and the chondrite-normalized REE pattern displays a flat pattern.The primitive mantle-normalized trace element spider diagram shows obvious negative anomalies of Nb-Ta.The characteristics of rare earth and trace elements are different from Bachu dolerite associated with mantle plume activities in Tarim block,but are similar with Yin'aoxia(east part)and Podong(west part)dolerites in Beishan area.Integrated with the existing research results,it is suggested that Gubaoquan dolerite dykes are formed after the closure of Paleo-Asian ocean,as the result of basaltic magmatism caused by the decompression and partial melting of asthenosphere due to lithosphere delamination and break-off in an extensional environment.

关 键 词：岩石地球化学 SR-ND同位素 岩浆源区 地壳混染 构造环境 辉绿岩脉 早二叠世 甘肃 

分 类 号：P595[天文地球—地球化学] P597[天文地球—地质学]