西秦岭关子镇蛇绿混杂岩的地球化学和锆石SHRIMP U-Pb年龄  

Geochemistry and zircon SHRIMP U-Pb ages of the Guanzizhen ophiolite complex,the Western Qinling orogen,China.

作　　者：李王晔[1] 李曙光[1] 裴先治[2] 张国伟[3] 

机构地区：[1]中国科学院壳幔物质与环境重点实验室中国科学技术大学地球和空间科学学院,合肥230026 [2]长安大学地球科学与国土资源学院,西安710054 [3]西北大学大陆动力学国家重点实验室,西安710069

出　　处：《岩石学报》2007年第11期2836-2844,共9页Acta Petrologica Sinica

基　　金：国家自然科学基金重点项目(40234041);西北大学大陆动力学国家重点实验室开放课题基金(06LCD06)资助。

摘　　要：西秦岭沿天水武山断裂带产出的关子镇蛇绿混杂岩由变玄武岩、变中-基性深成岩浆岩以及少量蛇纹岩组成。变玄武岩具有平坦或轻稀土略亏损的稀土配分型式((La/Yb)_N=0.63～1.24),且不存在Nb、Ta和Ti的负异常,表明它们形成于洋中脊环境。变中-基性深成岩浆岩包含两种类型:类型Ⅰ显示轻稀土富集((La/Yb)_N>2.2)、具有显著Nb负异常((La/Nb)_N>2.4)的特征,类似于岛弧火山岩;类型Ⅱ显示轻稀土亏损到略富集((La/Yb)_N=0.44～1.38)、无显著Nb负异常((La/Nb)_N<1.5),这些特征与上述变玄武岩类似,表明它们也是关子镇蛇绿岩的组成部分。蛇绿岩中辉长岩和闪长岩(该闪长岩属"大洋斜长花岗岩类")样品的锆石SHRIMP U-Pb年龄分别为534±9Ma和517±8Ma,这反映关子镇蛇绿岩可能为早-中寒武世古洋壳残片。由于西秦岭的天水武山断裂带是东秦岭商丹断裂带的西延并共同被认为是华北和华南陆块之间的主缝合线,上述年龄对理解该带及古秦岭洋的演化提供了重要制约。The Guanzizhen ophiolite complex occurred along the Tianshui-Wushan Fault in the Western Qinling orogen, is composed of meta-basalts, meta-intermediate-basic plutonites and minor serpentinite. The meta-basalts are characterized by slight light rare earth element (LREE) depletion or flat REE distribution (La/Yb) (N) = 0.63 - 1.24) with no Nb, Ta or Ti negative anomalies in spiderdiagram, indicating they were formed at the middle ocean ridge. The meta-intermediate-basic plutonites can be subdivided into two types. Type I, characterized by LREE enrichment ( ( La/Yb)(N) > 2. 2) and significant Nb negative anomalies in spider-diagram ( La/Nb)(N) > 2.4), is similar to island arc volcanic rocks. Type U, characterized by flat to LREE depletion BEE distribution (La/Yb) N = 0.44 - 1.38) with no Nb negative anomalies in spider-diagram ( ( La/Nb)(N) < 1. 5), is similar to those meta-basalts mentioned above, indicating they are belong to the Guanzizhen ophiolite. The SHRIMP U-Pb ages of 534 9Ma and 5178Ma for gabbro and diorite (this diorite is belong to 'oceanic plagiogranite') from the ophiolite, respectively, suggest that the Guanzizhen ophiolite is an early-middle Cambrian paleo-oceanic crust fragment. Since the Tianshui-Wushan Fault in the Western Qinling could be the westward extension of the Shang-Dan Fault in the eastern Qinling and both were considered to be the major suture between the North China Block and the South China Block, these ages provided an important constraint on evolution of the Qinling paleo-ocean and the Shang-Dan tectonic zone.

关 键 词：西秦岭 蛇绿岩 锆石SHRIMP U-PB定年 大洋斜长花岗岩类 

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