青海南部蛇绿岩中辉长岩锆石SHRIMP U-Pb定年和岩石地球化学特征  

Zircon SHRIMP U-Pb dating and lithogeochemistry of gabbro from the ophiolite in southern Qinghai Province

作　　者：段其发[1,2] 王建雄[2] 白云山[2] 姚华舟[2] 何龙清[2] 张克信[1,3] 寇晓虎[1,3] 李俊 

机构地区：[1]中国地质大学地球科学学院,湖北武汉430074 [2]宜昌地质矿产研究所,湖北宜昌443003 [3]中国地质大学地质过程与矿产资源国家重点实验室,湖北武汉430074 [4]湖北省地球物理勘查技术研究院,湖北武汉430056

出　　处：《中国地质》2009年第2期291-299,共9页Geology in China

基　　金：中国地质调查局1:25万曲麻莱县幅区域地质调查项目(200313000006)资助。

摘　　要：青海南部蛇绿岩呈构造岩片产于晚三叠世浅变质细碎屑岩中。扎河地区出露的辉长岩为蛇绿岩的组成单元,属低钾拉斑系列,其锆石SHRIMP U-Pb年龄为(239.8±3.1)Ma,表明岩石形成于中三叠世早期。地球化学特征上,岩石富MgO、Al2O3,低K2O、TiO2;REE总量低,(La/Yb)N=1.10～4.40,LREE略显富集,δEu=1.50～2.41,具有明显正铕异常;大离子亲石元素(Rb、Ba、Sr、Th、U)富集、高场强元素(Nb、Zr、Ti)亏损,反映岩浆源区曾遭受到俯冲组分的影响。综合分析认为,扎河辉长岩岩石具有板内和弧岩浆岩的双重属性,其形成环境为弧后盆地,表明中三叠世早期巴颜喀拉盆地发生过一次拉张事件。在区域上,它是甘孜-理塘蛇绿岩的组成部分。Ophiolites occur as lenses or slices within the Late Triassic metamophic sedimentary rocks in southern Qinghai Province, where no research work has been done before. The gabbro exposed in the Chaggur area is a unit of the ophiolite and belongs to the low-K tholeiitic series. The SHRIMP U- Pb age of zircons from gabrro is （239.8±3.1）Ma, suggesting that the re-spreading time of the Bayanhar ocean was most likely at the early stage of Middle Triassic. The gabrro is characterized by high MgO, Al2O3 and low K2O, TiO2. All rock samples have low REE abundances and show similar REE distribution patterns characterized by slight LREE enrichment with very distinct positive Eu anomalies. （La/Yb）N ratios are 1.10- 4.40, whereas δEu values are 1.50- 2.41. The samples are enriched in large ion lithophile elements （LILE） and depleted in Nb, Zr and Ti in the trace spider diagram, implying that the magma source was affected by input of some components from the subduction zone. It is thus considered that lithogeochemistry of the Chaggur gabrro was characterized by intraplate and arc volcanic rocks, and hence the gabrro was formed in a back-arc basin setting and should belong to the western segment of the Garze-Litang ophiolite melange.

关 键 词：锆石SHRIMP U—Pb定年 岩石地球化学 辉长岩 中三叠世 弧后盆地 青海 

分 类 号：P597.3[天文地球—地球化学]