东天山卡拉塔格复式岩体的时代、成因及构造意义

Age,Petrogenesis and Tectonic Implications of the Kalatage Intrusive Rocks in the Eastern Tianshan,Xinjiang

东天山位于中亚造山带最南部,记录了中亚造山带晚期的俯冲增生过程,然而早古生代一些关键地质演化过程仍存在争议,特别是大南湖-头苏泉带的构造属性、俯冲起始时间及俯冲极性。本文选取大南湖-头苏泉带北缘的卡拉塔格复式岩体开展了系统的年代学和地球化学工作,以揭示该带早古生代构造演化过程。锆石U-Pb定年结果显示,花岗岩、花岗闪长岩和辉长闪长岩分别形成于434±4 Ma、442±2 Ma和445±4 Ma,表明该复式岩体主要形成于早志留世。花岗岩和花岗闪长岩均显示低磷(P_(2)O_5<0.1%)、富钠(K_(2)O/Na_(2)O≤0.5)、准铝质-弱过铝质(A/CNK=0.57~1.24,0.91~1.03)的钙碱性I型花岗岩特征。样品具有较低的Sr/Y(1.9~4.5,8.1~11.4)和(La/Yb)N值(1.6~2.4,2.4~3.0),较高的ε_(Hf)(t)(+12.7~+15.5,+11.0~+14.7)和ε_(Nd)(t)值(+7.0~+7.5,+7.1~+7.7),以及年轻的二阶段Hf模式年龄(426~589Ma,485~722Ma)和Nd模式年龄(559~603Ma,555~598Ma),表明花岗岩和花岗闪长岩是年轻下地壳部分熔融的产物。辉长闪长岩相对富集大离子亲石元素(LILE)、高度不相容元素,亏损高场强元素(HFSE),显示典型的与俯冲相关的岩浆的地球化学特征;其较高的MgO含量(5.26%~5.54%),亏损的ε_(Hf)(t)(+12.1~+15.7)和ε_(Nd)(t)值(+7.1~+7.3),说明母岩浆来源于亏损地幔。辉长闪长岩高的Ce/Nb(11.5~16.1)和低的Th/La值(0.11~0.13)指示地幔源区经历了板片流体的交代作用。综合前人对该地区早古生代岩浆活动的研究成果,我们认为大南湖-头苏泉带岩浆弧早志留世处于康古尔洋向北俯冲的洋内弧环境,且俯冲开始时间不晚于早志留世。

The Eastern Tianshan(ET)is located at the southernmost part of the Central Asian Orogenic Belt(CAOB)and has recorded the late stage of the subduction-accretion process of the CAOB.However,some key tectonic evolution processes of the ET in the Early Paleozoic remains controversial,such as the tectonic property,the onset of subduction,and the subduction polarity of the Dananhu-Tousuquan belt.In order to reveal the Early Paleozoic tectonic evolution of the Dananhu-Tousuquan belt,we carried out a systematical study on zircon U-Pb age and whole-rock geochemistry of the Kalatage intrusions.Our dating results indicate the granite,granodiorite,and gabbro-diorite were intruded at 434±4 Ma,442±2 Ma,and 445±4 Ma,respectively.These ages suggest that the Kalatage intrusions were mainly generated in the Early Silurian.The granite and granodiorite samples exhibit characteristics of calc-alkaline I-type granites with low P2O5(<0.1%),high Na2O(K2O/Na2O≤0.5),and metaluminous to weakly peraluminous geochemistry(A/CNK=0.57–1.24 and 0.91–1.03,respectively).They have very low Sr/Y(1.9–4.5 and 8.1–11.4,respectively)and(La/Yb)N(1.6–2.4 and 2.4–3.0,respectively),higherεHf(t)(+12.7–+15.5 and+11.0–+14.7,respectively)andεNd(t)(+7.0–+7.5 and+7.1–+7.7,respectively),and more juvenile two stage Hf(426–589 Ma and 485–722 Ma,respectively)and Nd model ages(559–603 Ma,and 555–598 Ma,respectively),indicating that they are the products of partial melting of the juvenile lower crust.Gabbro-diorite is characterized by enrichments in large ion lithophile elements(LILE)and highly incompatible elements,and depletions in high field strength elements(HFSE),displaying typical geochemical features of a subduction-related origin.They have low SiO2(54.17%–55.39%)and high MgO contents(5.26%–5.54%),positiveεHf(t)(+12.1–+15.7)andεNd(t)values(+7.1–+7.3),together with their relatively young Hf and Nd model ages(429–589 Ma and 634–669 Ma,respectively),suggesting that their parental magmas were derived from depleted mantle.The gabbro-diorite samples have relatively high Ce/Nb(11.5–16.1)and low Th/La(0.11–0.13),indicative of a magmatic source metasomatized by slab-derived fluids.Combining with the previous results of the Early Paleozoic magmatic rocks in this area,we suggest that the Dananhu-Tousuquan igneous arc were formed in an intra-oceanic arc environment triggered by the northward subduction of the Kanggur ocean plate,and the initiation subduction of the Kanggur plate is no later than the Early Silurian.