扬子板块西北缘早中泥盆世构造演化:来自略阳地区踏坡组岩石学、锆石年代学和微量元素组成的约束  

Tectonic evolution of the northwestern Yangtze block in central China during the Early-Middle Devonian:constraints from petrology,zircon U-Pb dating and trace element composition of the Tapo Formation in the Lüeyang area

作　　者：王训练[1] 周洪瑞[1] 王振涛[2] 沈阳[3] 于子栋 杨志华[1] WANG Xunlian;ZHOU Hongrui;WANG Zhentao;SHEN Yang;YU Zidongn;YANG Zhihua(School of the Earth Sciences and Resources,China University of Geosciences,Beijing,100083;MNR Key Laboratory of MetalLogeny and Mineral Assessment,Institute of Mineral Resources,CAGS,Beijing,100037;Museum,China University of Geosciences,Beijing,100083)

机构地区：[1]中国地质大学(北京)地球科学与资源学院,北京100083 [2]中国地质科学院矿产资源研究所,自然资源部成矿作用与资源评价重点实验室,北京100037 [3]中国地质大学(北京)博物馆,北京100083

出　　处：《地质学报》2019年第12期2997-3019,共23页Acta Geologica Sinica

基　　金：陕西省勉略宁地区基础地质调查项目（公益[2015]01-02）;国家重点研发计划第4课题（编号2017YFC0602704）资助的成果

摘　　要：碧口地块北缘下中泥盆统踏坡组的物源长期被认为来自碧口地块新元古界基底--碧口岩群,但一直缺少碎屑锆石物源数据的支持。本文对略阳地区泥盆系踏坡组不同层位的3个碎屑岩样品开展了系统的岩石学、锆石岩相学及其U-Pb定年和微量元素组成研究。锆石晶体特征对比分析显示,研究样品存在两种类型的锆石:普遍发育变质增生边的碎屑锆石(剖面北段样品)和不发育变质增生边的典型岩浆成因的碎屑锆石(剖面南、中段样品)。前者显示2个年龄峰值(~2.5 Ga主峰、~2.0 Ga次峰),还形成了2473±24 Ma的上交点年龄和359±84 Ma的下交点年龄,而后者两个样品有着一致的年龄峰值(~2.0 Ga主峰、~2.5 Ga次峰和~1.39 Ga微小峰值)。3个样品的谐和年龄均大于1.3 Ga,并不能限定踏坡组的沉积时代,且均不支持其物源主要来自碧口岩群的传统认识。碎屑锆石地球化学判别图解指示它们的源岩主要为形成于造山带环境中的花岗岩类(花岗闪长岩和英云闪长岩)、基性岩和钾镁煌斑岩。基于样品中的岩屑类型,结合区域地质、古流向、锆石年龄对比和源岩判别,认为踏坡组的原始物源来自位于扬子板块北缘的太古宙-古元古代基底(鱼洞子杂岩和崆岭杂岩),并且鱼洞子杂岩在踏坡组沉积后期曾大面积出露/抬升。同时表明碧口地块可能至少在早中泥盆世就与扬子板块拼合在一起了。The provenance of the Early-Middle Devonian Tapo Formation in the northern margin of Bikou Terrane has long been considered to have derived from the Meso-Neoproterozoic Bikou Group, but lack of support from the detrital zircon provenance data. In this paper, systematic petrology, zircon types, zircon LA-ICP-MS U-Pb dating and trace element compositions of the clastic rocks from three different lithologic sections of the Tapo Formation in the Lueyang area, northern margin of Yangtze were studied. The comparative analysis of zircon crystals shows that there are two types of zircons: detrital zircons with metamorphic accretion rims(upper lithologic section) and detrital zircons of typical magmatic genesis(lower and middle lithologic sections). The dating results show that the former has two age peaks(~2.0 Ga main peak and ~2.5 Ga sub-peak), upper and lower interception ages of 2473±24 Ma and 359±84 Ma, respectively;the latter has a ~2.5 Ga main peak, a ~2.0 Ga sub-peak and a ~1.39 Ga micro-peak. The concordant ages of three samples are older 1.3 Ga, which can not define the age of the Tapo Formation, and does not support the previous understanding that the provenance of the Tapo Formation was mainly derived from the Bikou Group. Geochemical discrimant diagram of zircon compositions shows that source rocks are granitoid(granodiorite and quartze diorite), mafic rock and lamproite formed mainly in a magmatic arc/orogenic environment. Based on the type of rock fragments, regional geology, zircon ages correlation and paleocurrent data, it is considered that the Tapo Formation originated from the Archean-Paleoproterozoic basement(Yudongzi and Kongling complexs) located in the northern margin of Yangtze and the Yudongzi complex was widely exposed/uplifted in the late deposition of the Tapo Formation. At the same time, this study indicates that the Bikou Terrane may have been amalgamated with the Yangtze plate at least as early as the Early-Middle Devonian.

关 键 词：碧口地块 泥盆系 踏坡组 碎屑锆石 物源 

分 类 号：P61[天文地球—矿床学]