云开地体印支期构造转折——来自阳春二云母花岗岩的地球化学制约  

Indosinian Tectonic Transition in Yunkai Massif:Petrological and Geochemical Constraints from Two-mica Granite in Yangchun Area,South China

作　　者：麻志旺 郭锋[1,3] MA Zhiwang;GUO Feng(State Key Laboratory of Isotope Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,Guangdong,China;University of Chinese Academy of Sciences,Beijing 100049,China;CAS Center for Excellence in Deep Earth Science,Guangzhou 510640,Guangdong,China)

机构地区：[1]中国科学院广州地球化学研究所,同位素地球化学国家重点实验室,广东广州510640 [2]中国科学院大学,北京100049 [3]中国科学院深地科学卓越创新中心,广东广州510640

出　　处：《大地构造与成矿学》2023年第5期1183-1201,I0001-I0006,共25页Geotectonica et Metallogenia

基　　金：国家自然科学基金项目(42073032、41525006)资助。

摘　　要：云开地体位于古特提斯构造域、古太平洋构造域的交接部位,是研究华南印支期花岗岩构造背景的关键窗口。本文在总结前人资料的基础上,选择云开地体阳春二云母花岗岩开展了年代学与岩石地球化学研究。锆石U-Pb定年结果给出两组谐和年龄:一组为426.4±1.7Ma(MSWD=2.4,n=8)志留纪末,属加里东期;另一组为239.1±1.7Ma(MSWD=1.2,n=4)中三叠世,属印支期。独居石U-Pb定年结果为239.0±0.3Ma(MSWD=1.2,n=31),与第二组锆石U-Pb年龄一致,指示该岩体形成于中三叠世。阳春二云母花岗岩总体显示出S型花岗岩或过铝质花岗岩的特点,微量元素富集大离子亲石元素、亏损高场强元素,稀土元素配分模式相对平坦,具有明显的Eu负异常,全岩锆饱和温度低(725~747℃)。在同位素组成上,其全岩εNd(t)(-10.8~-9.4)、锆石εHf(t)(-13.2~-7.8)和独居石εNd(t)(-10.9~-8.4)三者之间相互吻合,并具有相似的亏损模式年龄(全岩tDM2(Nd)为1.8~1.9Ga、锆石tDM2(Hf)为1.8~2.1Ga、独居石tDM2(Nd)为1.7~1.9Ga),表明岩浆起源于古老再循环地壳物质的重熔,并经历了以斜长石和独居石为主的分离结晶作用。结合区域构造演化历史,我们认为阳春二云母花岗岩形成于从大洋俯冲到造山带垮塌的转折阶段:晚二叠世海相沉积和Ⅰ型花岗岩反映了古特提斯洋俯冲作用;早-中三叠世区域地壳发生强烈缩短加厚并诱发了深熔作用形成S型花岗岩,代表了同造山阶段;晚三叠世A2型花岗岩浆作用和陆相红层沉积指示造山带垮塌阶段。The tectonic background of the Indosinian granites in South China Block remains highly debated.The Yunkai massif,connecting the Paleo-Tethys and Paleo-Pacific tectonic domains,is a key site for studying the tectonic background of the Indosinian granitoids in South China.In this paper,we select a two-mica granitic pluton from the Yangchun area of the Yunkai Massif to conduct comprehensive researches including petrology,geochronology,and geochemistry.Zircon U-Pb dating yield two concordant 206Pb/238U ages of 426.4±1.7 Ma(MSWD=2.4,n=8)and 239.1±1.7 Ma(MSWD=1.2,n=4),respectively.A further monazite U-Pb dating gives only one concordant 206Pb/238U age at 239.0±0.3 Ma(MSWD=1.2,n=31),consistent with the younger zircon U-Pb age.We therefore conclude that the Yangchun two-mica granite was formed during Triassic(ca.239 Ma).The granite rocks belong to S-type granite with a peraluminous affinity.They also show enrichment in large ion lithophile elements and depletion in high field strength elements and relatively flat chondrite-normalized rare earth element patterns with negative Eu anomalies,and have low zircon saturation temperature of 725–747℃as well.The bulk-rock and monazite Nd and zircon Hf isotope analyses yield whole-rockεNd(t)values from–10.8 to–9.4,monaziteεNd(t)values from–10.9 to–8.4,and zirconεHf(t)values from–13.2 to–7.8,with depleted model age ranges of 1.8–1.9 Ga for whole rock tDM2(Nd),1.7–1.9 Ga for monazite tDM2(Nd)and 1.8–2.1 Ga for zircon tDM2(Hf).The consistent Nd and Hf isotope data among the bulk rock,monazite and zircon in the Yangchun two mica granite indicate that the granite was derived from ancient recycled crustal components.The parent magma experienced a dominated fractionation of plagioclase and monazite which resulted in the negative Sr and Eu anomalies and low LREE concentrations.Considering the regional geological background,we propose that the two-mica granite in the Yangchun area formed during the tectonic transition between the oceanic slab subduction and oroge

关 键 词：构造转折 岩石地球化学 二云母花岗岩 印支期 云开地体 

分 类 号：P595[天文地球—地球化学] P597[天文地球—地质学]