地球科学  2018, Vol. 43 Issue (4): 1219-1236.   PDF    
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东昆仑五龙沟地区晚志留世A型花岗岩成因:U-Pb年代学、地球化学、Nd及Hf同位素制约
王艺龙1,2, 李艳军1,3, 魏俊浩1,3, 李欢1, 韩玉4, 周红智1, 黄啸坤1, 柯坤家1     
1. 中国地质大学资源学院, 湖北武汉 430074;
2. 辽宁省地质勘查院, 辽宁大连 116100;
3. 固体矿产勘查国家级实验教学示范中心, 湖北武汉 430074;
4. 青海省第一地质矿产勘查院, 青海平安 810600
摘要:东昆仑造山带东段五龙沟金矿田内首次发现晚志留世A型花岗岩体.对其开展了锆石U-Pb年代学、岩石地球化学、Nd及Hf同位素研究,探讨岩体成因和构造背景.岩体LA-ICP-MS锆石U-Pb定年结果为420±3 Ma,为晚志留世岩浆活动产物.岩石具有高SiO2(76.0%~78.4%)、K2O(4.64%~5.22%)和Na2O(2.93%~3.25%)含量,低FeOT(0.98%~1.45%)、MgO(0.11%~0.22%)和CaO(0.27%~0.79%)含量特征.样品富集大离子亲石元素(Rb、K、La)和LREE,亏损高场强元素(Nb、P、Ti)和HREE,具有强烈的Eu负异常(Eu/Eu*=0.09~0.12).该岩体104×Ga/Al比值为3.09~3.15,具有A型花岗岩的特征.全岩εNdt)=-2.5~-2.2,对应的二阶段模式年龄tDM2(Nd)=1 339~1 365 Ma.锆石εHft)=-2.8~+2.1,二阶段模式年龄tDM2(Hf)=1 269~1 583 Ma.地球化学、Nd及Hf同位素揭示该岩体为软流圈地幔部分熔融形成的幔源岩浆与其诱发的古老地壳物质混合形成.构造判别图解指示岩体具有A2型花岗岩特征,形成于后碰撞伸展构造环境.结合和勒冈那仁和冰沟A型花岗岩体,认为东昆仑地区至少在晚志留世已进入伸展阶段.
关键词A型花岗岩    晚志留世    岩石成因    后碰撞伸展    东昆仑    岩石学    地球化学    
Origin of Late Silurian A-Type Granite in Wulonggou Area, East Kunlun Orogen: Zircon U-Pb Age, Geochemistry, Nd and Hf Isotopic Constraints
Wang Yilong1,2 , Li Yanjun1,3 , Wei Junhao1,3 , Li Huan1 , Han Yu4 , Zhou Hongzhi1 , Huang Xiaokun1 , Ke Kunjia1     
1. Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China;
2. Geological Exploration Institute of Liaoning Province, Dalian 116100, China;
3. National Demonstration Center for Experimental Mineral Exploration Education, Wuhan 430074, China;
4. Qinghai No.1 Academy of Geology and Mineral Exploration, Ping'an 810600, China
Abstract: Late Silurian A-type granite is reported for the first time in the Wulonggou gold district, eastern segment of the Kunlun orogenic belt. This paper presents (1) laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb age for the Wulonggou A-type granite to determine precisely the time of the magmatism, (2) geochemical, Nd and Hf isotope data for the Wulonggou A-type granite to constrain the petrogenesis and tectonic setting. The LA-ICP-MS U-Pb analyses of zircon from the Wulonggou A-type granite yielded a weighted mean 206Pb/238U age of 420±3 Ma, indicating that it was emplaced in the Late Silurian. All rock samples are enriched in SiO2 (76.0%-78.4%), K2O (4.64%-5.22%) and Na2O (2.93%-3.25%), but with relatively low concentrations of FeOT (0.98%-1.45%), MgO (0.11%-0.22%) and CaO (0.27%-0.79%). Samples are also enriched in large ion lithophile elements (Rb, K, La) and light rare earth elements, but depleted in high field strength elements (Nb, P, Ti) and heavy rare earth elements with strong negative Eu anomalies (Eu/Eu*=0.09-0.12). The 104×Ga/Al ratios of the Wulonggou A-type granites vary from 3.09 to 3.15. Mineralogy and geochemistry of the rocks show an affinity with aluminous A-type granite. The εNd(t) values of whole-rock range from -2.5 to -2.2 with corresponding two-stage Nd model ages ranging from 1 339 to 1 365 Ma. The εHf(t) values of zircon vary from -2.8 to +2.1 and two-stage Hf model ages range from 1 269 to 1 583 Ma. The Nd and Hf isotopic data imply that different source materials have contributed to the magma genesis. Integrated geological, geochemical and isotopic data suggest that Wulonggou A-type granite is most likely generated via the mixture of asthenosphere-derived magma and the ancient crustal materials. The A-type granite was marked by A2 type and was formed in post-collisional extensional tectonic environment. In combination with the A-type granites such as those in the Helegangnaren and Binggou, it is concluded that the eastern Kunlun area had begun its post-orogenic extension stage since Late Silurian, and this stage continued at least about 34 Ma.
Key Words: A-type granite    Late Silurian    petrogenesis    post-collisional extensional setting    East Kunlun orogen    petrology    geochemistry    

东昆仑造山带位于青海省中西部,与其周缘地区共同构成秦祁昆造山系,是中央造山带的重要组成部分(许志琴等,2006;杨经绥等,2010).自中元古代以来,东昆仑经历了多期洋陆转换、弧-弧和弧-陆碰撞过程(施彬和刘力,2014).已有研究显示东昆仑地区至少经历了加里东期和海西-印支期两个重要的造山旋回(姜春发等,1992;潘桂棠等,2002).分别与原特提斯洋和古特提斯洋俯冲消减密切相关.对于东昆仑地区原特提斯洋的打开、扩张和闭合时限已有较多研究.原特提斯洋在早寒武世时已处于打开和扩张阶段(Yang et al., 1996;陆松年,2002),洋壳的俯冲消减从早寒武世末一直持续到早志留世初期(刘彬等,2013a; Zhang et al., 2014),而与后碰撞相关的构造岩浆事件则多为早-中泥盆世(赵振明等,2008;刘彬等,2012).但是,对于东昆仑地区原特提斯后造山伸展阶段的起始时间仍存在不同认识.朱云海等(2005)通过对诺木洪地区玄武岩(419±5Ma)和变火山岩(401±6Ma)的研究,认为早泥盆世时洋壳仍处于俯冲消减阶段;但刘彬等(2012)则认为在早泥盆世时东昆仑地区为后碰撞伸展阶段,这一伸展阶段甚至始于早志留世(锆石SHRIMP U-Pb年龄441±5Ma,王晓霞等,2012).可见对于区域构造由碰撞挤压向伸展转换时限还需要更多的证据.A型花岗岩因其对伸展构造背景特殊的指示意义,被广泛用来探讨造山过程中构造体系转换时限(Collins et al., 1982; King et al., 1997;吴锁平等,2007;贾小辉等,2009;丰成友等,2012).因此,与原特提斯造山过程有关的A型花岗岩的出现将会为研究这一伸展时限提供一种重要的制约因素.目前东昆仑地区报道的A型花岗岩主要集中于中-晚三叠世,如小红山(212±7Ma,陈丹玲等,2001)、长山(220±1Ma,丰成友等,2012)、野马泉(213±1Ma,高永宝等,2014)、于沟子(210±1Ma,钱兵等,2015)等碱性花岗岩体,形成于后碰撞或造山后伸展构造环境,较为准确地限定了晚古生代-早中生代东昆仑造山带后造山伸展阶段的时限.近年来,国内外学者通过LA-ICP-MS锆石U-Pb同位素研究在东昆仑地区识别出大量中志留世-中泥盆世岩浆事件(刘彬等, 2012, 2013a; Li et al., 2013; Zhang et al., 2014;李希等,2014;王涛等,2016;严威等,2016).这些岩体形成于原特提斯洋俯冲-碰撞阶段,由地幔分异、壳幔相互作用或下地壳部分熔融形成.但有关A型花岗岩的研究却鲜有报道.目前仅报道了冰沟(391±3Ma,刘彬等,2013a)和和勒冈那仁(425±7Ma, Li et al., 2013)两个A型花岗岩体.该地区,尤其是冰沟以西地区仍缺乏该时期A型花岗岩的成因及构造背景研究.

五龙沟金矿田位于东昆仑造山带的东段,区内大面积发育晚二叠世-中三叠世和晚三叠世花岗岩(260~244Ma和~215Ma, Ding et al., 2014;罗明非等,2015;栗亚芝等,2015).尽管也有晚志留世的二长花岗岩(420~418Ma,陆露等,2013;严威等,2016),但仍无该时期A型花岗岩的报道.项目组首次发现矿区南部出露有晚志留世A型花岗岩体.本文对其开展LA-ICP-MS锆石U-Pb年代学、系统的岩石学和同位素地球化学研究,探讨岩石成因及成岩构造背景,以期为东昆仑地区原特提斯构造演化提供参考及制约依据.

1 地质背景及岩石学特征

东昆仑造山带夹持于柴达木盆地和松潘-甘孜地块两大构造单元之间,地质构造演化历史复杂.区内构造线总体呈NWW向展布,由北向南分别以昆北、昆中和昆南3条区域性深大断裂为界,划分为昆北地体、昆南地体、巴颜喀拉地体等主要构造单元(图 1a, 1b).其中,昆北地体位于昆北断裂带和昆中断裂带之间,该地体内出露大面积古-中元古代金水口群变质基底及加里东期-印支期侵入岩(图 1b).金水口群是东昆仑造山带最古老的变质岩系(莫宣学等,2007),总体为一套角闪岩相-麻粒岩相深变质岩系(陈能松等,1998),由下部古元古代白沙河组和上部中元古代小庙组组成.白沙河组主要由片麻岩和斜长角闪岩、片岩等组成(陈有炘等,2011).而小庙组主要由石英岩、片岩、片麻岩、大理岩等组成(陆露等,2013).带内前寒武纪变质岩从下到上表现出渐进的变质作用(Liu et al., 2005).金水口群麻粒岩的研究表明其经历了早古生代(460Ma)麻粒岩相变质作用及随后(402Ma)与碰撞后伸展相关的深熔事件(张建新等,2003).说明昆北地体变质基底在早古生代造山作用过程中发生了活化(王国灿等,2004).区内岩浆活动强烈,不同类型岩浆岩广泛发育,以加里东期-印支期侵入岩及少量镁铁质-超镁铁质杂岩为主.加里东期侵入岩主要为奥陶纪-泥盆纪二长花岗岩(王晓霞等,2012;陆露等,2013;严威等,2016).印支期侵入岩主要为晚二叠世-三叠纪花岗闪长岩、闪长岩、花岗斑岩等(高永宝等,2014;钱兵等,2015;张炜等,2016).已报道的镁铁质-超镁铁质杂岩多出露在祁漫塔格、清水泉和诺木洪地区(Chen et al., 2002;张亚峰等,2010;朱小辉等,2010;岳维好等, 2013, 2017;王冠等,2014).

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图 1 东昆仑构造位置图(a),东昆仑地区地质简图(b)和红旗沟岩体地质简图(c) Fig. 1 Geotectonic framework (a), geological map of the East Kunlun orogen belt (b) and simplified geological map of the Hongqigou granite (c) 图a据Xu et al.(2001);图b修编自Zhang et al.(2014)

红旗沟矿段是五龙沟矿田中红旗沟-深水潭金矿床Ⅺ矿带的重要部分之一.矿体赋存于沿晚志留世花岗岩体与新元古代小庙组接触部位的破碎蚀变带中,带宽1~20m.矿带北侧发育一条宽5~20m的辉长岩脉,延伸超过500m.岩脉锆石U-Pb年龄为419±4Ma (项目组未发表数据).而所研究的A型花岗岩体位于红旗沟矿段南侧,空间上呈NW向展布.与新元古代及古生代地层均为侵入接触关系,地表出露面积约4.2km2(图 1c).该A型花岗岩体岩性单一,为浅肉红色正长花岗岩,中粗粒结构,块状构造(图 2a),主要由碱性长石(50%~55%)、斜长石(10%~15%)、石英(20%~25%)和黑云母(5%~7%)组成.碱性长石为半自形-自形板状,发育条纹结构(正条纹),为条纹长石,局部包裹斜长石和石英,粒径约为1~5mm (图 2b).斜长石灰白色,半自形-自形板状,可见明显聚片双晶,表面略见绢云母化、粘土化,根据显微镜下(010)∧Np′最大消光角法,测定An约为18~25,属于更长石,粒径约为0.5~2.0mm(图 2b).石英呈灰色,主要为他形,少量为半自形,波状消光明显,粒径约为0.6~1.0mm.黑云母呈半自形片状,具淡绿色-深褐色多色性,多充填于长石和石英间隙中,粒径约为0.05~0.10mm(图 2b).副矿物组合为磷灰石、锆石、磁铁矿等.岩相学特征与冰沟正长花岗岩(刘彬等,2013a)基本相似.

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图 2 五龙沟地区红旗沟正长花岗岩野外照片(a)和镜下照片(正交偏光)(b) Fig. 2 Field pictures (a) and microphotographs (b) of the Hongqigou syenogranite from the Wulonggou area Kfs.碱性长石;Pl.斜长石;Bi.黑云母;Q.石英
2 测试方法 2.1 锆石U-Pb定年及Hf同位素

用于锆石LA-ICP-MS定年样品WSMG-1采自新鲜露头(N 36°12′4″,E 95°56′28″)(图 1c).样品破碎挑选由河北省区域地质调查研究所完成.分选过程遵循标准程序,经破碎、重磁分选之后,在双目镜下选择晶形完好并且纯净透明的锆石制靶.制靶之后磨蚀至锆石核部出露,并进行透射光、反射光和阴极发光(CL)照相.

锆石U-Pb定年在南京大学内生金属矿床成矿机制研究国家重点实验室激光剥蚀等离子质谱仪(LA-ICP-MS)上完成.激光剥蚀系统为New Wave 213nm,ICP-MS为Agilent 7500a,激光束斑直径采用32μm.测试过程中使用He作为载气.锆石U-Pb同位素分馏采用GJ-1(Jackson et al., 2004)进行校正,并选用锆石标样Mud Tank作为内标,控制分析精度.锆石样品同位素比值及相关元素含量处理采用GLITTER (ver.4.4)软件,并使用Andersen(2002)编制的软件进行普通铅校正.U-Pb年龄谐和图和加权平均年龄的计算采用Isoplot 3软件(Ludwig, 2003).

锆石原位Hf同位素组成的分析在南京大学内生金属矿床成矿机制研究国家重点实验室利用New wave UP193激光剥蚀系统和Neptune Plus MC-ICP-MS进行联机测试.仪器的测试条件及数据的采集可参见Wu et al.(2006).根据锆石大小使用的激光束斑直径为40μm.采用He作为剥蚀物质的载气,将剥蚀物质从激光剥蚀系统传送到MC-ICP-MS,并在进入MC-ICP-MS之前与Ar气混合,形成混合气.用176Lu/175Lu=0.02658和176Yb/173Yb=0.796218(Chu et al., 2002)进行同量异位干扰校正176Lu和176Yb对176Hf的干扰,计算测定样品的176Lu/177Hf和176Hf/177Hf比值.样品测定过程中获得标准锆石GJ-1的176Hf/177Hf=0.282016±0.000008(n=36,2σ),与参考值176Hf/177Hf=0.282013±0.000019(2σ)(Elhlou et al., 2006)一致.

2.2 全岩地球化学测试

室内对岩石样品进行岩相学鉴定后,挑选出5件新鲜无蚀变或蚀变较弱的样品,粉碎至200目以下,然后进行主、微量元素和Sm-Nd同位素测试.主量元素分析在澳实矿物实验室集团澳实分析监测(广州)有限公司采用ME-XRF06方法分析完成.分析流程如下:先称取0.9g样品,煅烧后加入Li2B4O7-LiBO2助熔物,充分混合,然后放置在自动熔炼仪中,使之在1050~1000℃之间熔融;熔融物倒出后形成扁平玻璃片,再用XRF荧光光谱仪分析,分析精度优于5%.微量元素和Sm-Nd同位素在南京大学内生金属矿床成矿机制研究国家重点实验室完成.微量元素用ICP-MS测定(型号为Finnigan Element Ⅱ),详细分析方法参考高剑峰等(2003),分析精度优于5%.Sm-Nd同位素用TIMS(型号为Finnigan Triton TI)分析测试,树脂分离和质谱测试方法见濮巍等(2005).Nd以H3PO4作为激发剂,将提纯后的Nd涂于Re带上后上机测试,测试过程中采用146Nd/144Nd=0.7219进行质量分馏校正.Nd同位素标样STD-1的测定值为143Nd/144Nd=0.512099±0.000007(1σn=5).

3 结果 3.1 锆石U-Pb年代学

样品WSMG-1锆石为无色-浅黄色透明状,颗粒以短轴状和等轴状为主,粒径多集中在100~150μm.CL图像显示所测锆石发育有明显的振荡环带(图 3a),指示其岩浆成因.选择韵律环带明显的锆石,进行了15个点的定年分析.U-Pb同位素定年测试结果见表 1.所测锆石Th和U含量分别为124×10-6~460×10-6和148×10-6~660×10-6,Th/U比值为0.49~0.97.在锆石U-Pb谐和图上(图 3b),数据点均落在谐和线上,指示锆石形成后U-Pb同位素体系处于封闭环境.测点206Pb/238U年龄介于417±6Ma~423±5Ma,加权平均年龄为420±3Ma(MSWD=0.1).该结果代表了正长花岗岩的形成年龄.这一数据与矿田内已获得的二长花岗岩体年龄一致(418~420Ma,陆露等,2013;严威等,2016),为晚志留世岩浆活动的产物.

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图 3 红旗沟正长花岗岩样品(WSMG-1)典型锆石CL图及U-Pb定年结果 Fig. 3 Cathodoluminescence images for zircons of sample WSMG-1 showing sites of U-Pb (solid circles) and Hf (dashed circles) analyses 实线圈和虚线圈分别代表U-Pb和Hf同位素分析测试点
表 1 五龙沟地区红旗沟正长花岗岩(WSMG-1)锆石LA-ICP-MS U-Pb定年分析数据 Table 1 Zircon LA-ICP-MS U-Pb data of the Hongqigou syenogranite sample (WSMG-1) from Wulonggou area
3.2 岩石地球化学特征

5件样品主量和微量元素测试结果及特征值列于表 2.样品具有高SiO2(76.0%~78.4%),富K2O(4.64%~5.22%),贫MgO(0.11%~0.22%)、TiO2(0.12%~0.17%)和P2O5(0.01%~0.02%)特征.全碱含量较高,K2O+Na2O=7.57%~8.47%,K2O/ Na2O=1.55~1.61.SiO2-K2O图解上样品点落入高钾钙碱性岩区域(图 4a).岩石Al2O3含量为11.0%~12.3%,CaO含量为0.27%~0.79%,A/CNK为0.97~1.06,显示准铝-弱过铝质花岗岩特征(图 4b).

表 2 红旗沟正长花岗岩主量元素(%)、微量元素和稀土元素(10-6)分析结果 Table 2 Major elements (%), trace and REE elements (10-6) data of the Hongqigou syenogranite
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图 4 红旗沟正长花岗岩K2O-SiO2(a)和A/NK-A/CNK(b)关系 Fig. 4 K2O vs. SiO2 (a) and A/NK vs. A/CNK (b) diagrams for the Hongqigou syenogranite 图a底图据Collins et al.(1982);图b底图据Maniar and Piccoli(1989).数据来源:和勒冈那仁碱长花岗岩据Li et al.(2013);冰沟正长花岗岩据刘彬等(2013a)

稀土元素总量(∑REE)为235.4×10-6~266.9×10-6,均值为253.3×10-6.轻稀土元素(LREE)含量为209.1×10-6~239.3×10-6,重稀土元素(HREE)含量为26.0×10-6~32.8×10-6,LREE/HREE为7.13~8.66(表 2).显示岩石轻稀土元素分馏明显,重稀土元素分馏相对微弱.球粒陨石标准化配分图解显示轻稀土元素富集,重稀土元素相对亏损的右倾“V”形配分模式.样品(La/Yb)N=7.7~12.4,(Gd/Yb)N=1.5~1.9,具有强烈的负Eu异常(0.09~0.12).微量元素方面,岩石具有富集大离子亲石元素(LILE、Rb、K、La)和相对亏损高场强元素(HFSE、Nb、P、Ti)的特征.样品104×Ga/Al比值为3.09~3.15(均值为3.11),Zr+Nb+Ce+Y=321.9×10-6~373.4×10-6.原始地幔标准化蛛网图可见Rb、Th、U、K等元素的正异常和Ba、Nb、P、Ti等元素的负异常(图 5a).稀土和微量元素配分特征与东昆仑地区晚志留世-中泥盆世A型花岗岩(Li et al., 2013;刘彬等,2013a)一致.

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图 5 红旗沟正长花岗岩稀土元素球粒陨石标准化配分图(a)和微量元素原始地幔标准化蛛网图(b) Fig. 5 Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace element patterns (b) for the Hongqigou syenogranite 标准化数据引自Sun and McDonough(1989).数据来源同图 4
3.3 Nd及Hf同位素

本次研究对3件样品进行了全岩Sm-Nd同位素分析,测试结果列于表 3.样品143Nd/144Nd=0.512280~0.512320,用锆石U-Pb年龄进行Sm-Nd同位素计算获得εNd(t)变化于-2.5~-2.2,对应的两阶段Nd模式年龄tDM2(Nd)=1339~1365Ma.

表 3 红旗沟正长花岗岩全岩Sm-Nd同位素分析结果 Table 3 Sm-Nd isotopic compositions of the Hongqigou syenogranite

在锆石U-Pb定年基础上,利用LA-MC-ICP-MS对完成了U-Pb年龄测试的15颗锆石进行了15个点的Lu-Hf同位素分析,结果列于表 4.样品176Yb/177Hf比值为0.021057~0.046558,176Lu/177Hf比值为0.000792~0.001642,176Hf/177Hf比值为0.282441~0.282580.以对应的锆石U-Pb年龄计算出锆石εHf(t)=-2.8~+2.1,两阶段Hf模式年龄tDM2(Hf)=1269~1583Ma.

表 4 红旗沟正长花岗岩(WSMG-1)锆石Hf同位素分析结果 Table 4 Hf isotopic data for sample WSMG-1 from the Hongqigou syenogranite
4 讨论 4.1 岩石成因

红旗沟正长花岗岩属高钾钙碱性铝质岩石,矿物组成以条纹长石、石英和少量斜长石为特征;地球化学组分富Si、Na和K,贫Ca、Mg,高(K2O+Na2O)/Al2O3、FeOT/MgO值,明显富集LILE和REE,强烈亏损Sr、Ba、P、Ti、Eu,稀土元素配分曲线呈右倾“V”形分布.矿物学及地球化学特征与贾小辉等(2009)总结的A型花岗岩及东昆仑地区晚志留世-早泥盆世A型花岗岩一致(图 5).样品104×Ga/Al比值为3.09~3.15,高于Whalen et al.(1987)提出的A型花岗岩判定标准(104×Ga/Al=2.6).在Whalen et al.(1987)的判别图解中,所有样品点均落入A型花岗岩区域(图 6).样品中未发现石榴石、白云母和堇青石等富铝矿物,总体为准铝-弱过铝质(A/CNK=0.97~1.06),与典型强过铝质S型花岗岩(A/CNK>1.1,Chappell and White, 1992)有一定的区别.并且样品具低的P2O5含量(0.01%~0.02%)和高的Na2O含量(2.93%~ 3.25%),也明显区别于高分异S型花岗岩(Chappell and White, 1992).另外,FeOT含量、Zr含量、形成温度及判别图解等可以用来区分高分异Ⅰ型和A型花岗岩(Whalen et al., 1987; King et al., 1997;贾小辉等,2009).A型花岗岩全铁(FeOT)含量高,一般大于1.00%,且形成温度较高(>800℃),而高分异Ⅰ型花岗岩一般小于1.00%,形成温度较低(均值764℃) (贾小辉等,2009).本文所研究的红旗沟正长花岗岩体FeOT含量为1.09%~1.61%,主量元素锆石饱和温度计计算温度为782~800℃(表 2),表明岩体最低成岩温度高于782℃,与上述A型花岗岩特征一致.但是样品Zr含量为150×10-6~178×10-6,低于King et al.(1997)所研究的澳大利亚Lachlen褶皱带的A型花岗岩(301×10-6).这可能与岩石的分异程度有关,随着分异程度的增加,铝质A型花岗岩的Zr含量会发生显著下降.King et al.(1997)指出一些高分异铝质A型花岗岩Zr含量甚至低于200×10-6.样品Rb、Sr含量分别为208×10-6~221×10-6和48.8×10-6~56.5×10-6,显示分异程度高.综上所述,红旗沟正长花岗岩应归属于高分异铝质A型花岗岩.

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图 6 红旗沟正长花岗岩成因类型判别图解 Fig. 6 Petrogenesis discrimination diagrams for the Hongqigou syenogranite 底图据Whalen et al.(1987);数据来源同图 4

目前国内外学者对A型花岗岩的成因仍有不同认识,主要有幔源岩浆的结晶分异作用、幔源岩浆与壳源岩浆的混合作用以及地壳物质的部分熔融等(贾小辉等,2009).本文所研究的红旗沟正长花岗岩高硅(SiO2 =76.0%~78.4%)、富钾(K2O=4.64%~5.22%),相对富集LILE,而明显亏损HFSE,Zr/Hf=26.0~40.4(平均29.1),与地壳岩石Zr/Hf值较为接近(Zr/Hf=33,Taylor and McLennan, 1985),这些特征均表明地壳岩石参与了成岩.昆北地体基底为金水口群(王国灿等,2007),已有研究显示其为古-中元古代岩系(王国灿等,2004;任军虎等,2010;陈有炘等,2011).五龙沟矿田420~418Ma二长花岗岩中的1861Ma和1666Ma古元古代继承锆石的发现也表明该时期古元古代基底物质参与了成岩作用(陆露等,2013).另外,昆北地体内中-晚志留世岩体εHf(t)值表现出随时代变新逐渐减少的趋势,显示岩体中古老地壳组分递增的特点.然而,夏日哈木镁铁质-超镁铁质岩体(423±1Ma,王冠等,2014)、色德日(417±3Ma,岳维好等,2017)、果洛龙洼(416±4Ma,岳维好等,2013)及红旗沟(419±4Ma,项目组未发表数据)等基性岩脉的存在,为东昆仑地区晚志留世幔源物质成岩提供了证据.红旗沟正长花岗岩Nd和Hf两阶段模式年龄tDM2(Hf)=1269~1538Ma,tDM2(Nd)=1339~1365Ma,位于东昆仑基底和岩体成岩年龄之间,暗示岩浆演化过程中存在地幔物质的参与.红旗沟正长花岗岩εNd(t) =-2.5~-2.2,与红旗沟辉长岩εNd(t)值(-5.1~-1.2,项目组未发表数据)基本一致,但略高于冰沟正长花岗岩(εNd(t) =-4.7~-3.8,刘彬等,2013a).在εNd(t)-t图解上,样品明显位于昆北地体基底演化域之上(图 7b).同时,样品εHf(t)=-2.8~+2.1,略低于原特提斯洋俯冲阶段形成的胡晓钦镁铁质岩石(刘彬等,2013b)和Yikehalaer花岗闪长岩(Li et al., 2015)Hf同位素组成,但与红旗沟辉长岩(εNd(t)=-2.8~-10.9,项目组未发表数据)及后碰撞或造山后伸展背景形成的跃进山辉长岩(刘彬等,2012)、猴头沟二长花岗岩(严威等,2016)具有基本一致的Hf同位素组成(图 7a).全岩Nd同位素及锆石Hf同位素结果表明红旗沟A型花岗岩体源区应由地幔和基底物质混合而成.至于幔源物质性质,基本确定为软流圈或亏损地幔(罗照华等,2002;刘彬等, 2012, 2013a, 2013b; Zhang et al., 2014; Li et al., 2015).甚至罗照华等(2002)认为东昆仑造山带产生幔源岩浆底侵作用的深部动力学机制可能是俯冲结束与碰撞开始时的板片断离作用.Zhang et al.(2014)通过对三铜沟岩体的研究,指出430Ma左右昆中洋俯冲板片断离,导致软流圈地幔物质上涌交代上覆地幔楔,并引发地壳物质部分熔融.因此,笔者认为可能正是软流圈地幔部分熔融形成的幔源岩浆与其诱发的古老地壳物质混合这一岩浆作用过程形成了红旗沟晚志留世A型花岗岩.

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图 7 红旗沟正长花岗岩锆石Hf同位素组成图解(a)和t-εNd(t)图解(b) Fig. 7 Hf isotopic compositions of zircons (a) and t-εNd(t) (b) diagrams for the Hongqigou syenogranite 数据来源:东昆仑基底据余能等(2005);胡晓钦镁铁质岩石据刘彬等(2013b);Yikehalaer花岗闪长岩据Li et al.(2015);猴头沟二长花岗岩据严威等(2016);跃进山辉长岩据刘彬等(2012);冰沟正长花岗岩据刘彬等(2013a);红旗沟辉长岩为项目组未发表数据
4.2 成岩构造背景

东昆仑地区至少经历了两个阶段的造山运动,与新元古代-早古生代原特提斯洋和石炭纪晚期-三叠纪古特提斯洋俯冲消减密切相关.这两次完整的构造旋回奠定了东昆仑地区现今构造格局.早寒武世东昆仑原特提斯洋已处于打开和扩张阶段(522~518Ma,Yang et al., 1996;陆松年, 2002).中寒武世时洋壳开始进入俯冲阶段(莫宣学等,2007),随着洋壳持续的俯冲作用,形成了如都兰可可沙地区石英闪长岩(515±4Ma,张亚峰等,2010)、镁铁质-超镁铁质杂岩(509±7Ma,冯建赟等,2010)、清水泉麻粒岩(508±8Ma,李怀坤等,2006)、旺尕秀辉长杂岩(468±2Ma,朱小辉等,2010)、具有岛弧特征的祁漫塔格鸭子泉闪长岩(480±3Ma,崔美慧等,2011)、敖洼得花岗闪长岩(454±2Ma,陈加杰等,2016)和清水泉变质火山岩(448±4Ma,Chen et al., 2002)等一系列与洋壳俯冲相关的弧岩浆岩.关于洋壳闭合时限,尽管王涛等(2016)认为五龙沟二长花岗岩(438±3Ma)形成于同碰撞环境,但大多研究者认为该时期东昆仑地区仍处于原特提斯洋壳俯冲消减晚阶段.施彬和刘力(2014)指出灶火沟早志留世(437~434Ma)花岗闪长岩和二长花岗岩为原特提斯洋壳俯冲消减阶段产物;胡晓钦镁铁质岩(438±2Ma,刘彬等,2013b)和Yikehalaer花岗闪长岩(436±7Ma,Li et al., 2015)也形成于俯冲作用晚阶段; Zhang et al.(2014)通过对三铜沟岩体(432~427Ma)的研究,认为~430Ma原特提斯洋俯冲消减基本结束.目前比较统一的认识是东昆仑造山带早-中泥盆世处于造山后伸展阶段(谌宏伟等,2006;赵振明等,2008;刘彬等,2013a;王冠等,2013),但对于其起始时限仍存在较大争议.红旗沟正长花岗岩形成于晚志留世,与昆北地体内晚志留世-中泥盆世A型花岗岩具有极为相似的岩相学及地球化学特征(图 5a, 5b),岩石判别图解显示为A型花岗岩(图 6).在Rb-(Y+Nb)判别图解中,样品均落于火山弧、同碰撞与板内花岗岩交界部位(图 8a),这一范围也是后碰撞花岗岩的投影区域(Pearce, 1996; Frster et al., 1997),而非板内裂谷等构造环境.Rb/30-Hf-3Ta图解也显示其具有后碰撞花岗岩特征(图 8b),代表了后碰撞张性构造环境(Harris et al., 1986).R1-104×Ga/Al和Nb-Y-3Ce图解进一步确定岩体位于PA或A2型花岗岩区域(图 9a, 9b),形成于伸展构造环境(Eby, 1992; Hong et al., 1996).岩体的主量及微量元素特征表明该区晚志留世时应处于后碰撞伸展构造背景.近年来的研究表明,东昆仑造山带存在大量晚志留世岩浆活动记录,如阿牙克二长花岗岩(角闪石40Ar/39Ar年龄421±2Ma,郝杰等,2003)、白干湖钾长花岗岩和二长花岗岩(422±3Ma和421±4Ma,李国臣等,2012)、祁漫塔格黑云母二长花岗岩和钾长花岗岩(421±3Ma和421±2Ma,郝娜娜等,2014)、五龙沟地区正长花岗岩和二长花岗岩(420~417Ma,陆露等,2013;李希等,2014;严威等,2016).这些花岗质岩石类型以正长花岗岩和二长花岗岩为主,形成于后碰撞伸展(郝杰等,2003;李国臣等,2012;李希等,2014;郝娜娜等,2014)或造山后伸展背景(严威等,2016).甚至晚志留世黑海地区过铝质花岗岩(424~421Ma)也形成于伸展构造背景(施彬等,2016).东昆仑地区还发育有晚志留世基性侵入岩(423~416Ma,岳维好等, 2013, 2017;王冠等,2014及项目组未发表数据),与同时代的花岗质岩石构成“双峰式”侵入岩组合,形成于后碰撞伸展背景(王冠等,2014).此外,东昆仑地区东段也发育有和勒冈那仁(425±7Ma,Li et al., 2013)和本文的红旗沟等晚志留世A型花岗岩,以及冰沟A型花岗岩(391±3Ma,刘彬等,2013a).这些伸展背景下形成的侵入岩的出现表明,至少在晚志留世末期东昆仑地区已进入伸展阶段(严威等,2016),而不是前人认为的早-中泥盆世.而且这一阶段至少持续了34Ma.

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图 8 红旗沟正长花岗岩Rb-(Y+Nb)(a)和Rb/30-Hf-Y-3Ta(b)构造环境判别图解 Fig. 8 Rb vs. Y+Nb (a) and Rb/30-Hf-Y-3Ta (b) discrimination diagrams of the Hongqigou syenogranite 图a底图据Pearce(1996);图b底图据Harris et al.(1986)数据来源同图 4
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图 9 红旗沟正长花岗岩R1-104×Ga/Al(a)和Nb-Y-3Ga(b)图解 Fig. 9 R1 vs. 104×Ga/Al (a) and Nb-Y-3Ga (b) diagrams of the Hongqigou syenogranite 图a据Hong et al.(1996);图b据Eby(1992).数据来源同图 4
5 结论

(1) 红旗沟正长花岗岩锆石LA-ICP-MS U-Pb年龄为420±3Ma,为晚志留世岩浆活动产物.

(2) 岩相学、地球化学、Nd及Hf同位素表明红旗沟正长花岗岩为A型花岗岩,由软流圈地幔部分熔融形成的幔源岩浆与其诱发的古老地壳物质混合形成.

(3) 红旗沟正长花岗岩形成于原特提斯后碰撞伸展背景,表明东昆仑造山带至少在晚志留世末期已进入伸展阶段.

致谢 感谢三位匿名审稿人在论文评审过程中提出的宝贵修改意见和编委会对本文的帮助.论文撰写过程中得到了中国地质大学(武汉)陈加杰博士的帮助,野外工作得到了青海省第一地质矿产勘查院陈建林的大力支持,在此一并表示感谢!

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