敦煌地块新太古代晚期岩浆-构造热事件:来自东巴兔山地区TTG片麻岩的证据

.Late Neoarchean magmatism and tectono-thermal events in the Dunhuang Block,NW China:Evidence from TTG gneisses of the Dongbatu Mountains area

敦煌地块位于塔里木克拉通的东北缘,区内广泛出露前寒武纪岩石,主要由新太古代到古元古代TTG片麻岩、变质表壳岩及变基性-中酸性侵入岩组成,研究其中的TTG片麻岩对于探讨敦煌地块早前寒武纪大陆地壳的形成和演化具有重要的意义。本文对敦煌地块东巴兔山地区的太古宙TTG片麻岩进行了详细的野外调查、锆石U-Pb年代学、地球化学及Hf同位素分析,以查明其岩石成因和构造背景,进而探讨敦煌地块早前寒武纪构造演化过程。LA-ICP-MS锆石U-Pb定年结果表明,东巴兔山上口子和营湖峡地区的奥长花岗质片麻岩的形成年龄为2.6~2.5Ga,属于新太古代晚期岩浆作用的产物,之后经历了古元古代(~2.4Ga和~2.0Ga)和晚古生代(~350Ma)的两期变质事件。东巴兔山地区TTG片麻岩具有高的SiO 2(67.29%~76.20%)和Na 2O(4.94%~5.83%)含量,低的MgO(0.51%~1.57%)和K 2O(0.39%~1.30%)含量,以及中等Al 2O 3(14.09%~15.87%)含量,其铝饱和指数(A/CNK)值为0.96~1.04,Mg#值为50.2~66.1。TTG片麻岩具有低-中等的稀土元素含量(∑REE=69.6×10^(-6)~174×10^(-6)),轻、重稀土分馏明显((La/Yb)N=23.9~141),富集轻稀土,显示弱到明显Eu正异常(Eu/Eu=1.08~1.81),富集大离子亲石元素(如,Rb、Sr、Th、U等),亏损高场强元素(如,Nb、Ta、Ti等)。此外,这些岩石还具有高Sr(551×10^(-6)~824×10^(-6))、低Y(3.13×10^(-6)~8.98×10^(-6))含量,Sr/Y比值为66.8~215。锆石Hf同位素组成显示,东巴兔山地区TTG片麻岩具有负到正的εHf(t)值(-11.7~+8.3),但大部分为正值,对应的二阶段Hf模式年龄为3811~2521Ma。上述地球化学特征表明,敦煌地块东巴兔山地区新太古代晚期TTG片麻岩主体应为新生玄武质地壳物质部分熔融的产物,源区可能有石榴子石、角闪石和(或)金红石残留。结合已有的研究成果,本研究认为敦煌地块东巴兔山地区的新太古代TTG片麻岩形成于与俯冲相关的构造背景,代表了一期重要的地壳生长事件,之后经历了古元古代和古生代两期区域变质作用事件。

Dunhuang Block is located in the northeastern margin of the Tarim Craton.The study area extensively exposed the Precambrian rocks,mainly composed of TTG gneisses,metamorphic supracrustal rocks,and Paleoproterozoic meta-mafic and felsic intrusive rocks.The study of TTG gneisses can provide insight into understanding the formation and evolution of continental crust in the Dunhuang Block.In this contribution,detailed field investigation,zircon U-Pb geochronology,geochemistry and Hf isotopic analysis of the Dongbatu Mountains area TTG gneisses in the Dunhuang Block are carried out to ascertain the petrogenesis and tectonic setting,and then to discuss the Early Precambrian tectonic evolution of the Dunhuang Block.LA-ICP-MS zircon U-Pb data reveals that the trondhjemitic gneisses of the Shangkouzi and Yinghuxia area in the Dongbatu Mountains were formed at ca.2.6~2.5Ga,which belong to the product of Late Neoarchean magmatisms,and later underwent regional tectonic-thermal events at~2.4Ga,~2.0Ga and~350Ma.The Dongbatu Mountains area TTG gneisses exhibit high SiO 2(67.29%~76.20),Na 2O(4.94%~5.83%),low MgO(0.51%~1.57%),K 2O(0.39%~1.30%)contents,and moderate Al 2O 3(14.09%~15.87%)contents,high Mg#(50.2~66.1)value as well as A/CNK values of 0.96~1.04.TTG gneisses are characterized by low rare earth element(∑REE=69.6×10^(-6)~174×10^(-6))contents,variable(La/Yb)N values of 23.9~141,enrichment light REEs,weakly to significant positive Eu anomalies(Eu/Eu=1.08~1.81),and enriched in large-ion lithophile elements(e.g.,Rb,Ba,Th and U),but depleted in high-field-strength elements(e.g.,Nb,Ta and Ti).Moreover,these rocks display high Sr(551×10^(-6)~824×10^(-6)),low Y(3.13×10^(-6)~8.98×10^(-6))contents,and relatively high Sr/Y ratios of 66.8~215.Hf-in-zircon isotopes characteristics of the Dongbatu Mountains area TTG gneisses show negative to positiveεHf(t)values(-11.7~+8.3;mostly>0)with two-stage Hf model ages(t DM2)of 3811~2521Ma.The above geochemical characteristics indicate that the Neoarchean TTG gneisses from the Dongbatu Mountains area in the Dunhuang Block were derived from the partial melting of juvenile basaltic crust and a residue phase dominated by garnet,amphibolite and/or rutile,which formed in subducted-related or island arc tectonic setting.This study,combined with published regional geological data,suggests that the Neoarchean TTG gneisses in the Dongbatu Mountains area of Dunhuang Block were formed in the arc-magmatic tectonic setting associated with subduction,representing an important crustal growth event,followed by two regional metamorphic events in Paleoproterozoic and Paleozoic periods.