秦祁结合部清水-张家川基性岩形成时代和构造归属探讨  

Discussion on the age and tectonic affinity of the mafic rocks in Qingshui-Zhangjiachuan of the conjunction area between the Qinling and Qilian orogenic belts

作　　者：付长垒[1,2] 闫臻 王秉璋[2] FU ChangLei;YAN Zhen;WANG BingZhang(Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;Key Laboratory of the Northern Qinghai-Tibet Plateau Geological Processes and Mineral Resources,Qinghai Geological Survey Institute,Xining 810012,China)

机构地区：[1]中国地质科学院地质研究所,北京100037 [2]青海省地质调查院,青藏高原北部地质过程与矿产资源重点实验室,西宁810012

出　　处：《岩石学报》2019年第10期3141-3160,共20页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41702239、41672221、41872241);中国地质调查局地质调查项目(DD20190006、DD20160201-04、DD20160022-02)联合资助

摘　　要：中央造山系北缘发育完整的早古生代"弧-盆"体系,而其洋内弧盆体系还是陆缘弧盆体系属性的确定,可为原特提斯域古板块构造格局和中央造山系早期造山过程的恢复提供依据。秦岭-祁连结合部清水-张家川地区出露的与原特提斯洋俯冲作用相关的基性岩形成时代和构造归属的探究,是解决中央造山系早古生代洋-陆转换过程以及东西链接等科学问题的重要内容之一。清水-张家川地区主要出露块状、枕状玄武岩以及少量辉绿岩墙和硅质岩夹层,玄武岩和辉绿岩明显富集Th、LREE,亏损Nb、Ta、Zr、Hf和Ti元素,而Th/Nb比值则分为1. 09~2. 04和0. 18~0. 73两组,分别与岛弧和弧后裂谷熔岩相一致。另外,这些基性岩具较高的Th/Yb和εNd(t)(+4. 4^+4. 8)值,显示岩浆来自受洋壳沉积物混染的亏损地幔源区。最新LA-ICP-MS锆石U-Pb测年结果显示辉绿岩墙形成于500±3Ma。结合区域岩石地层资料,综合分析表明清水-张家川地区基性岩形成于寒武纪,早于晚奥陶世酸性火山岩和侵入岩。它们与其东、西侧北秦岭和北祁连构造带内的寒武纪-早奥陶世洋内弧和弧后盆地岩浆岩共同构成了原特提斯洋北缘早古生代洋内弧-盆体系。The tectonic affinity of Early Paleozoic arc-basin system in the northern margin of the Chinese Central Orogenic Belts is vital to reconstruct their tectonic framework and orogen. Mafic rocks are well exposed in Qingshui-Zhangjiachuan of the conjunction area between the Qinling and Qilian orogenic belts,which are closely related to subduction of the Proto-Tethyan ocean. Their ages and tectonic affinities are important to reveal the Early Paleozoic ocean-continent transitional processes and the interconnection relationship between the east and west belts within the Chinese Central Orogenic Belts. Voluminous massive and pillow basalts and minor dolerite dykes and chert interlayers occur in the Qingshui-Zhangjiachuan area. Geochemical data demonstrate that basalts and dolerites display strong enrichment in Th and LREE,and apparent depletion in Nb,Ta,Zr,Hf and Ti. Their Th/Nb ratios are 1. 09 ~ 2. 04 and 0. 18~ 0. 73,respectively,which are consistent with those of island arc tholeiite and backarc rift-related lavas. These mafic rocks have relatively high Th/Yb and εNd( t) values( + 4. 4 ~ + 4. 8),suggesting a depleted mantle source contaminated by subducted sediment.New LA-ICP-MS zircon U-Pb dating of dolerite dyke yields a weighted mean206 Pb/238 U age of 500 ± 3 Ma. Combined with regional lithostratigraphy data,it is concluded that mafic rocks in the Qingshui-Zhangjiachuan area mainly formed in Cambrian,prior to Late Ordovician felsic volcanic and intrusive rocks. These rocks,together with Cambrian-Early Ordovician intra-oceanic arc and back-arc basin igneous rocks in the eastern North Qinling belt and western North Qilian belt,further constitute an Early Paleozoic intra-oceanic arc-basin system within the northern margin of the Proto-Tethyan ocean.

关 键 词：早古生代 洋内弧-盆体系 秦祁结合部 中央造山系 原特提斯洋 

分 类 号：P542[天文地球—构造地质学] P588.124[天文地球—地质学]