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新疆温泉南部哈尔达坂石英二长岩年代学、地球化学及构造意义
张宇昕1,2 亚夏尔·亚力坤1,2 张博文1,2 展新忠1,2 侯丹凤1,2
(1. 新疆大学,地质与矿业工程学院 乌鲁木齐 830049;2. 新疆大学,中亚造山带大陆动力学与成矿预测实验室 乌鲁木齐 830049)
摘 要 西天山赛里木微地块北缘,哈尔达坂铅锌矿矿区中南部的石英二长岩,呈岩脉状就位于中元古界哈尔达坂群灰岩、白云质灰岩和微晶灰岩中。SHRIMP U-Pb 法测得锆石206Pb/238U加权平均年龄378.8±7.2 Ma(MSWD=0.56),表明其形成于中泥盆世早期。岩石地球化学特征显示,其具有偏铝质—钙碱性(A/CNK=0.88~1.44)、富碱(K2O+Na2O=7.08%~8.49%)的特征,富集大离子亲石元素Rb、Cs、Th、K,贫Eu、Ba、Sr和Ti、Nb、Ta、P等高场强元素。轻稀土略富集,重稀土略亏损,配分模式为右倾海鸥型;且轻、重稀土内部分馏较弱,具弱负Eu异常(δEu=0.47~0.97);锆石饱和温度为(841℃~913℃);且有高10000*Ga/Al(~2.8,>2.6)和较高Y/Nb比值(3.03~3.81,>1.2)。指示哈尔达坂石英二长岩具有A2型花岗岩的元素地球化学特征。综合前人研究:判断哈尔达坂石英二长岩形成于中泥盆世的岛弧环境,并可能与新疆北部泥盆纪的北天山洋脊俯冲有关,由洋脊俯冲造成板片窗作用使得软流圈地幔上涌,并熔融下地壳并形成近似“板内伸展环境”特色的花岗质岩石。
关键词 A型花岗岩 锆石U-Pb年代学 赛里木微地块 洋脊俯冲 板片窗
Geochronology, geochemistry and tectonic significance of quartz monzonite in Hardaban, southern Wenquan, Xinjiang
Abstract: Quartz monzonite in the central and southern part of the Haerdaban Pb, Zn deposit in the northern margin of the Sayram micro block, West Tianshan, is in the form of dyke and is located in the limestone, dolomitic limestone and microcrystalline limestone of the Middle Proterozoic Haerdaban Group. The SHRIMP zircon U-Pb dating yielded a 206Pb/238U age of 378.8±42 Ma (MSWD=0.56), which is interpreted as the formation age of the Early-Middle Devonian. Geochemical characteristics of the rocks show that they are aluminous calc alkaline (A/CNK=0.88~1.44) and alkali rich (K2O+Na2O=7.08%~8.49%), rich in large ion lithophile elements Rb, Cs, th, K, and poor in Eu, Ba, Sr and high field strength elements such as Ti, Nb, Ta, P. The samples exhibit a slight enrichment of LREE as well as negative anomalies of Eu (δEu=0.47~0.97), Also present “Sea-gull” type rare earth element pattern. Finally, these samples are characterized by the high saturation temperature of zircons (841℃~913℃), high 10000*Ga/Al(~2.8,>2.6) and especially Y/Nb(3.03~3.81,>1.2). It indicates that the Haerdaban quartz monzonite have the elemental geochemical characteristics of type A2 granites. Comprehensive petrology, petrogeochemistry and previous studies: It is believed that the Hardaban quartz monzonite veins were formed in the island arc environment of the Middle Devonian and may be related to the subduction of the Devonian ridges in northern Xinjiang. The subduction caused slab windows to upwell the asthenospheric mantle and melt the lower crust to form granite with characteristics similar to “intraplate extensional environment”.
Keywords A-tpye granites, Zircon U-Pb geochronology, Sayram block, Ocean ridge subduction, Slab windows
中亚造山带位于西伯利亚克拉通、塔里木克拉通、华北克拉通和东欧克拉通之间,是全球最典型的增生型造山带之一(Xiao et al., 2008)。西天山造山带位于中亚造山带最南端,作为全球范围内显生宙大陆地壳增生最显著的地区(Xiao et al.,2003;薛春纪, 2014a,2014b),其形成于古亚洲洋及其附属洋盆长期俯冲和增生作用的环境,并形成一系列微陆块、岛弧、大洋高原、增生杂岩和蛇绿岩的组合(Xiao and Santosh,2014;肖文交等,2019)。
早古生代,伊犁—哈萨克斯坦板块历经多期次裂解、闭合并呈现多陆块及多岛弧的古亚洲洋演化。晚古生代,伊犁—哈萨克斯坦板块北侧准噶尔—巴尔喀什微板块沿依连哈比尔尕深断裂发生裂解,形成北天山洋盆并进入俯冲、闭合构造阶段(左国朝等,2008;解洪晶等,2013a)。关于北天山洋的闭合时间,存在较多争议:如晚泥盆世(肖序常, 1992)、早石炭世(Xia et al., 2012)、早石炭世晚期(李永军等,2010)、晚石炭世(Han et al.,2010)、晚石炭—早二叠世(Zhang et al., 2016)、早二叠世(Xiao et al., 2013;Xiao and Santosh, 2014)。
哈尔达坂地区位于西天山北部,挟于伊犁中天山板块与准噶尔板块之间。区域内发育大量花岗质岩体,其中A型花岗岩出露尤为显著(唐功建等,2008;李晓英,2013;解洪晶等,2013a,2013b;郝增元等,2018;齐天骄等,2018;张博文等,2018;林涛等,2019;展新忠,2019;周勇等,2020)。A型花岗岩作为一种特殊岩石类型,通常产于造山后或板内构造环境中,如岩石圈减薄、大陆裂谷以及碰撞后伸展作用,因此A型花岗岩通常可指示其伸展构造背景(Eby,1992;Bonin,2007)。另外,A型花岗岩还可产于洋脊俯冲导致的板片窗作用下(Thorkelson and Breitsprecher,2005;李贺等,2011;Zheng et al.,2018;Windley and Xiao,2018)。板片窗的打开使得软流圈物质上涌带来大量的热,并在俯冲带形成一个近似的“板内伸展”环境。这类“板内伸展”环境的岩石组合,其源区可能为弧的中下地壳、俯冲洋壳、地幔橄榄岩和软流圈物质等(沈晓明等, 2010)。
脉岩作为深成岩浆活动的末梢,常常充填于侵入体的补给通道内,可反映母岩浆或原生岩浆自岩浆房上涌的过程(罗照华等,2006)。此外脉岩还可反演其源区的物质组成以此揭示有关壳幔演化的重要信息(Westerman et al.,2003)。这为大陆地壳演化、花岗岩形成构造背景识别、花岗岩岩浆的分异与成矿效应等诸多方面研究奠定了基础。
因此,通过赛里木微板块内新发现的脉岩系列来进行北天山洋俯冲—洋盆收缩起始时限及机制的补充尤为关键。故本文通过对哈尔达坂石英二长岩脉岩相学、年代学及全岩地球化学研究,探讨其形成时代、岩石成因及构造背景,并结合邻近赛博岩体、喇嘛苏岩体的研究,以期通过岩石组合耦合关系,为赛里木微地块晚古生代洋壳俯冲的构造演化提供新的理论依据。
图1 中亚造山带构造纲要图(a. 据Gao et al., 2009修改),伊犁—中天山构造简图(b. 据Qian et al., 2009修改);赛里木微地块区域地质图(c. 据冯京等,201修改1)
Fig. 1 Simplified geological map of the Central Asian orogenic belt (a, modified after Gao et al., 2009); Tectonic sketch map (b, modified after Qian et al., 2009); Regional geological map of Sayram massif (c, modified after Feng et al., 2009)
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