南秦岭大堡组奥陶纪洋岛玄武岩的识别及其构造意义:来自地球化学和年代学证据  

Identification of the Ordovician oceanic island basalts and their tectonic significance of the Dabao Formation in southern Qinling:Constraints from geochemistry and geochronology of oceanic island basalt.

作　　者：王涛[1] 王宗起[2] 闫臻[1] 闫全人[1] 张英利[2] 向忠金[1] 

机构地区：[1]中国地质科学院地质研究所,北京100037 [2]中国地质科学院矿产资源研究所,北京100037

出　　处：《岩石学报》2009年第12期3241-3250,共10页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(40602026;40772137);中国地质科学院地质研究所所长基金项目(J0720);地质调查项目(1212010611807;1212010711816);国家科技支撑计划课题(2006BAB01A11)资助

摘　　要：秦岭造山带以商-丹构造带为界分为北秦岭和南秦岭构造带。大堡组位于南秦岭构造带内,由灰色-深灰色的泥岩、粉砂岩和炭硅板岩,以及夹含炭硅质岩、灰岩和基性火山岩块体组成。在大堡组中出露的能干上和蚂蝗峡基性岩块夹于黑色的泥岩之中,岩石地球化学分析结果表明,SiO_2含量为41.53%～53.59%,富TiO_2为2.14%～3.58%,REE的总量为∑REE=282.3×10^(-6),轻稀土元素较重稀土元素富集[(La/Yb)_N=8.34],略显Eu正异常(Eu~*=1.13～1.25),具有较高的Ti/Y(300～622)和Zr/Y(3.8～7.4)比值。大离子亲石元素Rb、Ba和K相对富集,无明显的Nb和Ta异常,其Nb和Ta值为N-MORB标准值的10～20倍,为典型洋岛玄武岩(OIB)地球化学特征。锆石SHRIMPⅡU-Pb测年获得能干上和蚂蝗峡基性岩块体分别形成于446Ma和455Ma。这些资料进一步表明,奥陶纪时期南秦岭存在着古洋盆。The Qinling orogen is divided into northern and southern Qinling tectonic belts by Shangxian-Danfeng tectonic m lange. The Dabao Formation is located in the southern Qinling orogen and is composed of grey and dark grey mudstone, siltstone, silicic slate matrix and chert, limeatone and basalt blocks. The Nengganshang and Mahuangxia basalt blocks are contained within black mudstone matrix in Dabao Formation. Based on geochemical analysis, the Nengganshang and Mahuangxia basalt blocks have similar geochemical characteristics to those of OIB, with high SiO(2) (41. 53% similar to 53. 59%), high TiO(2)( 2. 14% similar to 3. 58%), Fe(2)O(3)(T) (10. 23% similar to 15. 64%), high REE (282. 3 x10(-6)) contents, high Ti/Y (300 similar to 622), Zr/Y (3. 8 similar to 7. 4), and show enrichment light REE [( La/Yb)(N) = 8. 34], weak Eu positive anomaly (Eu* =1. 13 similar to 1. 25), without abnormity in Nb and Ta, and enrichment in Rb, Ba and K. The zircon SHRIMP II U-Pb dating results yielded ages of 446Ma and 455Ma in Nengganshang and Mahuangxia basalt blocks. The data suggests the existence of an ocean basin in south Qinling during Ordovician.

关 键 词：南秦岭 奥陶纪 洋岛玄武岩 识别 构造意义 岩石地球化学 年代学 证据 southern QINLING tectonic significance QINLING orogen GEOCHEMICAL characteristics GEOCHEMICAL analysis 构造带 high zircon SHRIMP south QINLING 轻稀土元素 岩块 基性岩 

分 类 号：P58[天文地球—岩石学]