地球科学  2018, Vol. 43 Issue (4): 1278-1292.   PDF    
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南阿尔金陆块科克萨依新元古代花岗岩成因及地质意义
陈红杰1,2, 吴才来1, 雷敏1, 郭文峰1, 张昕1, 郑坤1, 高栋1, 吴迪2     
1. 中国地质科学院地质研究所, 中国地质调查局大陆动力学研究中心, 北京 100037;
2. 中国地质大学, 北京 100083
摘要:南阿尔金陆块是阿尔金造山带的重要组成部分.大量新元古代花岗岩出露于南阿尔金亚干布阳-帕夏拉依裆-科克萨依一带.这些花岗岩记录了与Rodinia超大陆汇聚有关的动力学信息,因此对其进行研究有利于对阿尔金造山带演化历史的认识和理解.选取了科克萨依花岗岩岩体进行了岩相学、地球化学、锆石U-Pb年代学和Hf同位素组成的研究.研究结果表明:(1)科克萨依二长花岗岩的主要矿物有:石英、钾长石、斜长石、黑云母和白云母;花岗岩的锆石U-Pb年龄为947~945 Ma.(2)地球化学特征显示,岩石具有高SiO2(71.54%~74.69%)、高Na2O+K2O(6.33%~7.40%),低CaO(1.59%~2.00%),低MgO(0.43%~0.61%)和TiO2(0.25%~0.37%)的特征,相对富钾,K2O/Na2O比值为1.02~1.71,A/CNK在1.10~1.14之间,属高钾钙碱性系列的过铝质花岗岩.富集Rb、Th、K、La等元素,亏损Nb、Ta、P、Ti等元素;轻稀土富集而重稀土亏损,具有明显的负Eu异常.(3)锆石εHft)为-4.09~+3.87之间,二阶段模式年龄tDM2为1.6~2.0 Ga.这些特征表明科克萨依二长花岗岩是古老地壳富长石贫黏土的(变)杂砂岩部分熔融形成的S型花岗岩.结合相邻地区新元古代花岗岩类的地球化学、同位素特征及阿尔金区域构造资料,认为科克萨依二长花岗岩形成于新元古代时期,是碰撞造山环境下的产物,是Rodinia超大陆汇聚碰撞过程的响应.
关键词花岗岩    地球化学    U-Pb年代学    Hf同位素特征    南阿尔金陆块    
Petrogenesis and Implications for Neoproterozoic Granites in Kekesayi Area, South Altyn Continent
Chen Hongjie1,2 , Wu Cailai1 , Lei Min1 , Guo Wenfeng1 , Zhang Xin1 , Zheng Kun1 , Gao Dong1 , Wu Di2     
1. Centre for Continental Dynamics, China Geological Survey, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
2. China University of Geosciences, Beijing 100083, China
Abstract: The South Altyn continental block is an important geological unit of the Altyn Tagh orogenic belt. Numerous Neoproterozoic granites outcrops in the South Altyn continental block, and are mainly located in Paxialayidang-Yaganbuyang-Kekesayi area. These granites provide indispensable dynamics information of the Rodinia supercontinent aggregation in Neoproterozoic. Therefore, the study of granites can help us to understand the formation and evolution history of the Altyn Tagh orogenic belt. In this paper, Kekesayi granitic pluton was studied by means of petrography, geochemistry, zircon U-Pb chronology and Hf isotopic analyses. The results are as follows. (1) Main minerals of Kekesayi monzonitic granite are:quartz+K-feldspar+plagioclase+biotite+muscovite. Zircon U-Pb dating shows that the granite was emplaced in 947-945 Ma. (2) Geochemistry characteristics show high SiO2 (71.54%-74.69%), K2O+Na2O (6.33%-7.40%) contents and low CaO (1.59%-2.00%), MgO (0.43%-0.61%) and TiO2 (0.25%-0.37%) contents, with K2O/Na2O ratios of 1.02-1.71 and A/CNK ratios of 1.10-1.14, showing a typical high-K calc-alkaline series with peraluminous features. Meanwhile, the granite is also enriched in Rb, K, Th and La, and depleted in Nb, Ta, Sr and Ba, with negative Eu anormalies and relative enrichment in LREE. (3) εHf(t) values range from -4.09 to +3.87 while two-stage model ages (tDM2) vary in 1.6-2.0 Ga. It is argued that the Kekesayi monzonitic granites were derived from partial melting of the meta-grey wackes of Late Paleoproterozoic to Early Mesoproterozoic ancient crustal materials. In combination with other Neoproterozoic granite features, the petrogenesis and isotopic geochronology indicate that the Kekesayi monzonitic granite was formed in collisional orogeny setting and may have been triggered by the assemblage of Rodinia supercontinent in Neoproterozoic.
Key Words: granite    geochemistry    U-Pb chronology    Hf isotopic characteristics    South Altyn continent    

0 引言

花岗岩记录了造山带的演化历史,前人提出了多种花岗岩成因模型来反映不同的形成机制(Bonin, 2007).在造山带演化的不同阶段(洋壳俯冲和陆陆碰撞阶段)均可产出大规模的花岗岩基(Milani et al., 2015).陆陆碰撞阶段导致大陆地壳的部分熔融产生同碰撞花岗岩岩浆作用,而同碰撞岩浆作用是多样的,例如埃达克岩、低Sr/Y钠质熔岩、石英闪长岩以及S型花岗岩(Song et al., 2015).S型花岗岩主要产于碰撞或碰撞后环境,具有高的铝饱和指数A/CNK>1.1,高SiO2含量,低Na2O(<3.2%)含量(Chappell and White, 2001).S型花岗岩的形成与大陆碰撞造山作用密切相关,对理解造山带演化及地壳生长具有重要意义(Zhao et al., 2017).

Grenville期一系列的造山事件(1 300~900 Ma)将不同的大陆块体和微陆块体拼接成了Rodinia超大陆(Hoffman, 1991; Moores, 1991; Li et al., 1995, 2008),随后受超级地幔柱的影响而发生裂解(Santosh et al., 2009).阿尔金造山带位于塔里木克拉通南缘,作为Rodinia超大陆的重要组成部分,塔里木克拉通记录了Rodinia超大陆汇聚(1.05~0.90 Ga)和裂解(0.82~0.74 Ga)事件(Lu et al., 2008).近年来的研究表明,阿尔金造山带中部的南阿尔金陆块出露大面积新元古代花岗岩质岩石及高压变质岩(王永和等,2004覃小峰等,2008),这些新元古代花岗质岩石均具有同碰撞性质(王超等,2006; Yu et al., 2013a王立社等,2015李琦等,2015).此外,与阿尔金相邻的东昆仑-柴北缘-祁连地区也相继有发现新元古代早期岩浆事件的报道,这些新元古代岩浆事件表明该时期岩浆活动范围较为广泛,与我国西部地区不同陆块之间的汇聚有关,也是对西部地区克拉通基底形成及全球Rodinia超大陆汇聚事件的响应(陆松年,1998郭进京等,1999梅华林等,1999陈能松等,2006; Wan et al., 2006于胜尧等,2011张建新等,2011孟繁聪等,2013; Yu et al., 2013a, 2013b; Wang et al., 2013).

受古生代造山事件叠加的影响,阿尔金造山带中元古代至新元古代的地壳演化过程仍不清楚.新元古代岩浆活动的性质及分布,与Grenville全球造山事件和Rodinia超大陆的关系,并没有得到很好的限制.本文以南阿尔金陆块科克萨依岩体作为研究对象,在详实的野外调查基础上,对其进行岩石学、年代学及同位素地球化学等方面的研究,并结合区域地质构造特征,探讨阿尔金地区新元古代岩浆作用及其与构造演化,以及与罗迪尼亚超大陆汇聚事件的关系.

1 区域地质背景

阿尔金山地处青藏高原北缘,介于塔里木板块、柴达木微板块、阿拉善地块之间,是塔里木盆地和青藏高原的天然分界线,被认为是塔里木克拉通变质基底的主要出露区域之一(张建新等,2011).阿尔金造山带经过多期的岩浆活动的强烈改造,在新元古代至早古生代经历了与板块汇聚有关的地壳深俯冲高压超高压变质作用(车自成等,1995王永和等,2004刘永顺等,2009),其后又遭受中-新生代的大型走滑断裂的改造,表明阿尔金造山带是由不同构造层次、不同时期和不同构造环境的地质体组成的复合造山带(刘良等, 1996, 1999许志琴等,1999张建新等,1999; Zhang et al., 2001; Cui, 2011吴才来等,2016).前人根据造山带内不同地质体的岩石学、地球化学、同位素年代学特征,自北向南依次划分为阿北变质地体、北阿尔金蛇绿混杂岩带、中阿尔金地块、南阿尔金高压超高压带(即南阿尔金陆块)、南阿尔金早古生代蛇绿混杂岩带(许志琴等,1999; Liu et al., 2009, 2012吴才来等, 2014, 2016).

南阿尔金陆块出露的地层主要为下元古界阿尔金岩群和青白口系索尔库里群.阿尔金岩群属变质结晶基底,是一套浅海相碎屑岩、火山岩、碳酸盐岩建造;索尔库里群属结晶基底之上的变质过渡基底,是一套陆块边缘裂陷海盆碎屑岩、火山岩、碳酸盐岩建造.前人将前阿尔金岩群解体,厘定出新元古代-早古生代阿尔金杂岩(于海峰等,2002).根据1:25万苏吾什杰幅区域地质调查报告成果及最新研究进展(西安地质矿产研究所, 2003, 新疆1:25万苏吾什杰幅区域地质调查报告),阿尔金杂岩主要由新元古代中酸性侵入岩、古元古代变质表壳岩和新元古代末-早古生代初期高压超高压变质岩地质体的多类构造岩片组成(王永和等,2004覃小锋等,2008刘永顺等,2009于胜尧等,2011).南阿尔金陆块出露的新元古代花岗质岩石侵入到“阿尔金岩群”变质杂岩中,变质杂岩由夕线石榴片麻岩、蓝晶石榴片麻岩及石榴斜长角闪岩等角闪岩相为主的岩石组成(于海峰等,2002).本文的研究区域位于南阿尔金陆块阿尔金岩群的东南缘,帕夏拉依裆以东的科克萨依-宝丰铁矿一带(图 1).

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图 1 阿尔金造山带地质图及研究区地质简图 Fig. 1 Geological sketch map of Altyn Tagh orogenic belt and geological sketch map of study area 图a据Liu et al.(2012);b据吴才来等(2016);c据西安地质矿产研究所(2003)编新疆1:25万苏吾什杰幅区域地质调查报告.TRB.塔里木盆地;QL.祁连山;QDB.柴达木盆地;WKL.西昆仑;EKL.东昆仑;HMLY.喜马拉雅山
2 岩体地质及岩相学特征

南阿尔金陆块新元古代花岗岩主要分布于亚干布阳-盖里克-科克萨依一带,科克萨依岩体位于南阿尔金陆块的东南缘(图 1).岩体侵入下元古界阿尔金岩群变质表壳岩中(于海峰等,2002),后被奥陶纪的帕夏拉依裆岩体侵入.阿尔金岩群中的变质表壳岩主要为斜长云母片岩,根据岩体的形成时代、岩石类型、侵入关系等,可以将该区新元古代花岗岩划分为环形山岩体、盖里克岩体和科克萨依岩体等几个单元(李琦等,2015王立社等,2015).本文选取的科克萨依岩体位于阿尔金左行走滑断裂北侧的阿尔金构造杂岩带上,盖里克岩体以东,玉普阿勒岩体以北.

科克萨依岩体主要岩性为二长花岗岩,呈灰白色-灰色(图 2a),表面具球状风化,含钾长石斑晶,眼球状中细粒结构,片麻状构造,局部地方塑性变形较强烈,并发育石英脉(图 2b).结合野外露头观察和室内镜下分析,主要矿物组合为:石英约25%~35%,钾长石约25%~35%,斜长石约20%~30%,黑云母约3%~5%,白云母约1%~3%(图 2c).石英呈半自形-他形,具有波状消光;微斜长石呈半自形宽板状,具有格子双晶,晶体中包裹有石英、云母等细小晶体;斜长石半自形,晶体表面较为浑浊,绢云母化较为强烈;黑云母呈褐色-浅黄色片状,局部绿泥石化;白云母多色性明显,两种云母均呈断续片状定向排列(图 2d);主要副矿物有磁铁矿、磷灰石等.

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图 2 科克萨依岩体野外及镜下照片 Fig. 2 Outcrops and photomicrographs of Kekesayi granite a.科克萨依岩体野外露头;b.新元古代花岗岩野外露头;c, d.二长花岗岩镜下照片(正交偏光);Pl.斜长石;Mc.微斜长石;Qz.石英;Bi.黑云母;Ms.白云母
3 分析方法 3.1 全岩化学分析

岩石粉末碎样和化学全分析工作由中国地质科学院地球物理地球化学勘查研究所中心实验室完成.将岩石样晒干,经无污染粗碎、细碎至小于0.5 cm的岩块后混匀,用四分法缩分出100 g左右样品,在恒温干燥箱中45 ℃烘干,用高铝瓷球磨机研磨至200目筛(小于0.074 mm),测定各种化学元素含量.其中常量元素氧化物及Sr、Ba、Zn、Rb、Nb、Zr、Ga等微量元素含量使用X荧光光谱仪测定;稀土元素和其他微量元素使用高分辨等离子质谱测定.H2O+、CO2、LOI等指标的分析测试方法分别参照GB/T14506.2-1993、GB9835-1988、LY/T1253-1999标准执行.分析测试时插入国家一级标准物质控制分析准确度,按样品总数的5%抽取检查样品编成密码进行重复分析,以及对异常点进行再次重复分析以控制分析测试精密度.经统计检验,分析结果的检出限、准确度、精密度和报出率等指标都满足或优于《地质矿产实验室测试质量管理规范》中的相关要求.大多数元素测试精度为5%,少量小于10×10-6的元素测试精度为10%.

3.2 锆石U-Pb定年

锆石的分选工作由河北廊坊区域调查研究院完成,将样品破碎至40~80目,使用常规的重液浮选和电磁分离方法,然后在双目镜下手工挑选出锆石;根据锆石的自形程度、颜色、形态及透明度等特征进行分类,选出有代表性的锆石进行制靶.拍摄透反射照片及阴极发光照片等程序由中国地质科学院地质研究所大陆动力学实验室完成.激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)锆石原位微区U-Pb定年工作在中国科学技术大学壳幔物质与环境重点实验室完成,仪器型号为ICP-MS ELAN DRC-Ⅱ型.激光剥蚀的斑束直径为32~44 μm,激光剥蚀样品的深度约为20~40 μm,实验以He为载气,结合锆石反射、透射照片,避开锆石内部裂隙和包裹体,采样方式为单点剥蚀,每测定4个样品点后测定标样一次.锆石U-Pb同位素组成分析采用91500国际标准锆石作为外标,206Pb/238Pb年龄的加权平均值误差为±2σ;元素含量测定采用NIST SRM610作为外标,29Si作为内标.U/Pb比值数据处理采用Ladating@Zrn,普通Pb同位素校正处理使用Andersen et al.(2002)方法,校正后的数据使用美国Berkeley地质年代中心编制的ISPLOT和SQUID程序(Ludwig, 2003),锆石的加权平均年龄及谐和图用Isoplot程序获得.

3.3 锆石Hf同位素

锆石Hf同位素的分析是在前述锆石U-Pb同位素的基础上完成的.测试分析在中国地质科学院地质研究所LA-MC-ICP-MS实验室进行,采用的仪器为Neptune Plus型多接收等离子质谱和GeoLasPro 193nm激光剥蚀系统(LA-MC-ICP-MS).实验过程中采用He作为剥蚀物质载气,剥蚀直径为44 μm,测定时使用锆石国际标样GJ-1作为参考标样.分析过程中锆石标样GJ-1的176Hf/177Hf测试加权平均值为0.282 007±0.000 025(2σ).计算初始176Hf/177Hf时,Lu的衰变常数采用1.865×10-11 a-1(Scherer et al., 2001),锆石εHf(t)值计算时采用球粒陨石Hf同位素值176Lu/177Hf=0.036,176Hf/177Hf=0.282 785(Bouvier et al., 2008).在Hf的地幔模式年龄计算中,亏损地幔176Hf/177Hf现在值采用0.253 25,176Lu/177Hf采用0.038 4(Griffin et al., 2000),地壳模式年龄计算时采用平均地壳的176Hf/177Hf=0.015(Griffin et al., 2002).相关仪器运行条件及详细分析流程见侯可军等(2007).

4 分析结果 4.1 地球化学特征

本文对科克萨依新元古代二长花岗岩8个样品进行了主量和微量元素分析,其详细分析结果见附表 1.主量元素分析结果显示:SiO2含量为71.54%~74.69%,Al2O3含量为13.15%~14.24%,Na2O含量为2.65%~3.14%,K2O含量为3.20%~4.67%,Na2O+K2O含量为6.33%~7.40%,FeOT含量为2.29%~2.94%,MgO含量为0.43%~0.61%,CaO含量为1.59%~2.00%,TiO2含量为0.25%~0.37%(附表1).根据SiO2-Na2O+K2O分类图,科克萨依二长花岗岩样品全落入花岗岩区(图 3a),岩石属高钾钙碱性系列(图 3b).铝饱和指数A/CNK值介于1.10~1.14,显示强过铝质特征(图 3c).

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图 3 科克萨依新元古代花岗岩体全岩SiO2-Na2O+K2O (a),SiO2-K2O (b)和A/CNK-A/NK图解(c) Fig. 3 Whole-rock SiO2 vs. Na2O+K2O classification diagram (a), SiO2 vs. K2O diagram (b) and A/CNK vs. A/NK diagram (c) of Kekesayi Neoproterozoic granites 图a据Maniar and Piccoli(1989);图b据Middlemost(1994);图c据Martin et al.(2005)

科克萨依二长花岗岩稀土配分模式整体与亚干布阳花岗岩(另文发表)较为一致,表现为轻稀土富集、重稀土相对亏损的右倾型(图 4a).REE总量为129.58×10-6~171.2×10-6,LREE/HREE为5.27~8.17,平均6.70.(La/Yb)N为4.32~9.79,表明轻、重稀土之间的分馏程度较高,(Gd/Lu)N为0.91~1.91,表明LREE元素内部具有一定程度分馏.而样品HREE元素内部分馏程度较低,δCe为0.98~1.04,此外均表现出中等负Eu异常,δEu为0.48~0.58(附表1).微量元素原始地幔标准化蛛网图中(图 4b),科克萨依二长花岗岩整体趋势与亚干布阳花岗岩类似.富集K、Th、Rb、U等元素,亏损Nb、Ta、Sr、Ba和Ti等元素.Eu负异常暗示岩浆在形成过程可能存在斜长石的分离结晶或源区有斜长石的残留,Ti的亏损可能与钛铁矿的分离结晶作用有关.

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图 4 科克萨依花岗岩全岩稀土配分模式(a)和微量元素蛛网图(b) Fig. 4 Chondrite-normalized REE patterns (a) and primitive mantle-normalized spider diagrams (b) for the granites in Kekesayi area 标准化值据Sun and McDonough(1989)
4.2 锆石U-Pb年代学

本文用于锆石U-Pb同位素定年的两个样品采自科克萨依岩体.对2个花岗岩样品进行了锆石U-Pb定年,测点均选择锆石结晶时生成的环带部位,分析结果如下.

样品12CL125-3锆石的CL图像显示呈灰黑色,晶形较好,主要为长柱状,个别呈椭圆状,具清晰岩浆振荡环带,锆石长约100~200 μm,长宽比约为2:1~3:1,个别锆石边缘具有暗黑色边,可能是受后期变质作用改造的结果(图 5a).对该样品一共19个点进行了测试,测点结果表明Th含量为5.83×10-6~30.69×10-6,U含量为26.23×10-6~172.35×10-6,Th/U比值为0.05~0.39,平均为0.21,显示岩浆锆石的特征(附表2).通过锆石内部的206Pb/238U进行年龄计算,锆石年龄变化于957~924 Ma之间,加权平均年龄为945±13 Ma(MSWD=0.09)(图 5a).

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图 5 科克萨依地区花岗岩体锆石谐和曲线 Fig. 5 Concordia plots of the granites in Kekesayi area

样品12CL126-3锆石的CL图像显示呈灰黑色,晶形完整,锆石长约100~250 μm,长宽比约为2:1~1.5:1,主要呈椭圆状,个别为长柱状,具清晰岩浆振荡环带,个别锆石边缘具有暗黑色边,可能是受到后期变质作用改造的结果(图 5b).对该样品共进行了17个点的测试,测点结果表明Th含量为5.87×10-6~21.01×10-6,U含量为24.81×10-6~199.01×10-6,Th/U比值为0.05~0.85,平均值为0.28,显示岩浆锆石的特征(附表2).通过锆石内部的206Pb/238U进行年龄计算,锆石年龄变化于959~936 Ma之间,加权平均年龄为947.5±7.3 Ma(MSWD=0.16)(图 5b).

4.3 锆石Lu-Hf同位素

对科克萨依岩体二长花岗岩两个样品(12CL125-3、12CL126-3)分别进行锆石原位Lu-Hf同位素分析,共计35个点(附表3).分析结果如下.

样品12CL125的176Yb/177Hf比值是0.037 092~0.082 695,176Hf/177Hf范围为0.282 178~0.282 318,176Lu/177Hf比值在0.001 143~0.002 571之间,平均值小于0.002.对应的εHf(t)变化于-1.36~+3.87,根据锆石U-Pb年龄计算的二阶段模式年龄(tDM2)变化介于1.56~1.89 Ga.样品12CL126的176Yb/177Hf比值是0.027 723~0.067 292,176Hf/177Hf范围为0.282 100~0.282 264,176Lu/177Hf比值在0.000 809~0.002 007之间,平均值小于0.002.对应的εHf(t)变化于-4.10~+2.45,根据锆石U-Pb年龄计算的二阶段模式年龄(tDM2)变化介于1.66~2.08 Ga(图 6).176Lu/177Hf<0.002,代表锆石形成后放射性成因Hf同位素积累较少,因此176Hf/177Hf比值基本上可以代表锆石结晶时同位素体系中Hf同位素组成.锆石的fLu/Hf(s)=-0.92~-0.98,与大陆铁镁质地壳的fLu/Hf(s)=-0.34相比明显较小,所以二阶段模式年龄可以代表源区物质从亏损地幔抽取的时间(第五春荣等,2007).

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图 6 科克萨依花岗岩体锆石Hf同位素组成 Fig. 6 Hf isotopic compositions of zircons from granites in Kekesayi area
5 讨论 5.1 岩石成因与源区特征

S型花岗岩常含有白云母、石榴石、堇青石等过铝质矿物,岩石常为过铝质,A/CNK>1.1,且K2O>Na2O(Sylvester, 1998).科克萨依二长花岗岩样品富碱(Na2O+K2O=7.48%~8.89%),K2O>Na2O,A/CNK>1.1,CIPW标准计算均出现刚玉(1.58%~2.10%),属强过铝质钙碱性花岗岩.此外,岩石中含过铝质矿物白云母,以及锆石中含有继承锆石,以上特征均指示科克萨依二长花岗岩为过铝质S型花岗岩.过铝质S型花岗岩一般形成于区域挤压环境下或者与俯冲相关的背景下(Barbarin, 1996; Collins, 1998; Douce and Harris, 1998; Douce, 1999; Healy et al., 2004; Chen et al., 2014);实验岩石学表明,在一定的温度压力条件下,多种源岩的部分熔融均可以产生过铝质花岗质熔体(Rapp et al., 1991; Rapp and Watson, 1995; Johannes and Holtz, 1996; Winther and Newton, 1996),熔体的成分变化则取决于初始熔融物质的成分、温度和压力,以及初始物质的含水量差异(Hansen et al., 2002).因此,判别强过铝质花岗岩的源岩性质成为判别构造环境的关键.

科克萨依二长花岗岩的Th/U比值(6.62~11.63)平均为9.14,稍高于上地壳平均值4.2;Zr/Hf比值为26.2~28.38,稍低于上地壳的Zr/Hf比值(~37)(Gao et al., 1998);Nb/Ta比值为8.9~13.2,与地壳岩石的比值(11~13)非常接近(Barth et al., 2000),这些特征显示其源岩具有地壳沉积岩的特征.Mg#值的高低是判断岩浆熔体来源的重要指标,当Mg#<40时认为是地壳部分熔融形成的熔体,而Mg#较高的则可能有幔源物质的加入(Rapp and Watson, 1995).科克萨依二长花岗岩的Mg#值在25.00~28.49之间,平均为27.25,同样指示该套花岗岩主要来源于地壳物质的部分熔融.

通过C/MF-A/MF图解(图 7a)可以看出,样品均落入杂砂岩的部分熔融范围;在Al2O3+FeOT+MgO+TiO2-Al2O3/(FeOT+MgO+TiO2)判别图解(图 7b)中,样品也全落在砂质岩的范围内(Douce, 1999).岩石的FeOT/MgO比值为4.48~5.35,Al2O3/TiO2比值为37.75~51.91,平均为44.7,K2O/Na2O比值为1.02~1.71,这些地球化学特征显示与地壳沉积岩部分熔融形成的S型花岗岩相似(路凤香和桑隆康,2002),暗示其源岩为陆壳沉积的砂质岩(Sylvester, 1998).实验岩石学表明,CaO/Na2O的比值主要受控于源区长石/黏土成分的比例,富长石、贫黏土的砂屑岩熔融形成的过铝质花岗岩CaO/Na2O一般大于0.3.科克萨依花岗岩的CaO/Na2O比值为0.53~0.71,平均为0.62,类似于澳大利亚Lachlan褶皱带上的S型花岗岩(图 7c).在Rb/Sr-Rb/Ba图解中(图 7d)样品都落入砂质岩熔融区域内.结合Rb-Th具有负相关的特征(图 8a)、CaO-FeOT+MgO-(Al2O3-(Na2O+K2O))图解(图 8b)及较低的Mg#值,笔者认为该套二长花岗岩的源岩为杂砂岩.

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图 7 科克萨依地区花岗质岩体C/MF-A/MF图解(a),Al2O3+FeOT+MgO+TiO2-Al2O3/(FeOT+MgO+TiO2)图解(b),Al2O3/TiO2-CaO/Na2O图解(c)和Rb/Sr-Rb/Ba图解(d) Fig. 7 Geochemical diagrams of granites in Kekesayi area, C/MF vs. A/MF (a), Al2O3+FeOT+MgO+TiO2 vs. Al2O3/(FeOT+MgO+TiO2) (b), Al2O3/TiO2 vs. CaO/Na2O (c), Rb/Sr vs. Rb/Ba (d) 图a据Alther et al.(2000);图b据Douce(1999);图d据Sylvester(1998).数据来源:Himalayan淡色花岗岩据Searle and Fryer(1986); Inger and Harris(1993); Ayres and Harris(1997).Lachlan S型花岗岩据Chappell and Simpson(1984); Healy et al.(2004)
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图 8 Rb-Th图解(a)和CaO-FeOT+MgO-(Al2O3-(Na2O+K2O))图解(b) Fig. 8 Diagram of Rb-Th (a) and diagram of CaO-FeOT+MgO-(Al2O3-(Na2O+K2O))(b) 图a据Chappell et al.(1999);图b据White and Chappell(1977)

锆石饱和温度可以近似代表花岗岩近液相线的温度,Watson and Harrison(1983, 2005)通过高温实验获得锆石溶解度模拟公式TZr(℃)={12 900×[lnDZr(496 000/熔体)+0.85M+2 195] }-273.15,公式中Zr为分配系数,M为(Na+K+2×Ca)/(Al×Si)的阳离子含量比值.计算结果表明,科克萨依二长花岗岩M值为1.25~1.31,锆石温度为775~796 ℃.当地壳中发生低温(<800 ℃)部分熔融产生岩浆时,源区应有水的加入,而岩浆中的水通常来自于黑云母、白云母及角闪石等矿物的脱水反应(Best and Christiansen, 2001).科克萨依二长花岗岩具有较高的SiO2(平均72.97%)、Na2O+K2O(平均6.87%)含量、A/CNK>1.1(平均1.14),及较低的FeOT+MgO+TiO2含量(平均3.4%),暗示其可能是由云母分解脱水而引发部分熔融(Patiňo Douce, 2005).

科克萨依二长花岗岩的Sr含量(69.23×10-6~93.19×10-6)较低,存在δEu负异常以及Nb亏损,暗示在岩浆形成过程中可能存在斜长石的分离结晶作用或源区有斜长石的残留.Yb的含量高低可能与形成的深度有关,并且可以反馈源区是否存在石榴石的残留(张旗等,2006).科克萨依二长花岗岩具有高Yb(2.46×10-6~5.07×10-6)和Y(22.83×10-6~38.75×10-6)特征,La/Yb比值为6.02~13.65,暗示源区未残留石榴石,部分熔融程度应在石榴石稳定区以上(Defant et al., 2002).根据实验岩石学,低Sr高Yb型花岗岩残留相为斜长石,通常在熔体与斜长石平衡条件下形成,与浙闽型花岗岩具有相似的特征(张旗等,2010).脱水部分熔融的实验表明,在1.6 GPa时残留相出现石榴石(Rapp et al., 1991),由于残留相没有石榴石,推断该二长花岗岩的形成压力较低.科克萨依二长花岗岩的εHf(t)主要集中于-4.09~+3.87,二阶段模式年龄(tDM2)集中于1.56~2.08 Ga(图 6),表明科克萨依花岗岩主要源于古元古代-中元古代的地壳物质.

5.2 构造环境与意义

阿尔金地区新元古代花岗岩多具有同碰撞花岗岩的性质,并被认为与Rodina超大陆的汇聚事件有关(王超等,2006覃小锋等,2008李琦等,2015王立社等,2015).阿尔金造山带西段环形山锆石U-Pb年龄为928±7.7 Ma,被认为是产于Rodinia超大陆汇聚背景下,地壳沉积岩部分熔融形成的S型花岗岩(王立社等,2015);而阿尔金造山带中部帕夏拉依档盖里克岩体锆石的U-Pb年龄为886.5±5 Ma,形成于同碰撞构造环境,是地壳角闪岩部分熔融形成的产物(李琦等,2015).

笔者对采自科克萨依二长花岗岩体的两个样品进行了锆石U-Pb定年分析,分析结果显示岩体形成于945~947 Ma,代表该岩体的结晶年龄,表明该岩体形成于新元古代.岩石的微量元素Rb含量为214.8×10-6~242.8×10-6,Nb和Ta含量分别为7.89×10-6~11.20×10-6和0.70×10-6~1.10×10-6,Nb/Ta=8.6~11.3,总体具有高Rb、低Yb、Nb、Ta的特征,类似于同碰撞型花岗岩(Pearce et al., 1984).在R1-R2构造环境判别图解(图 9a)中,显示样品与同碰撞构造作用有关,在岩石Nb-Y判别图解中(图 9b),科克萨依岩体与亚干布阳岩体样品具有相似的特征,基本全落入同碰撞区域.结合科克萨依花岗岩岩体的锆石U-Pb年龄,笔者认为科克萨依岩体可能形成于同碰撞环境,是Rodinia超大陆汇聚阶段不同板块之间碰撞的产物.

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图 9 岩石R1-R2构造环境判断图解(a)和岩石Nb-Y图解(b) Fig. 9 R1-R2 discrimination diagram (a) and Nb-Y diagram (b) 图a底图据Batchelor and Bowden(1985);图b底图据Pearce et al.(1984).①地幔分异产物;②板块碰撞前;③碰撞隆起后;④造山晚期;⑤非造山;⑥同碰撞;⑦造山期后

在与阿尔金相邻的昆仑、祁连山、柴北缘等造山带均有新元古代花岗岩的报道,且都卷入了早古生代末期构造-热事件.如祁漫塔格花岗岩(940 Ma)(孟繁聪等,2013)、北山柳园与榴辉岩共生的花岗质岩石(880 Ma)(梅华林等,1999)、祁连山响河花岗岩(917±12 Ma)(郭进京等,1999)和柴北缘沙柳河一带的花岗片麻岩(~900 Ma)(于海峰等,1999)等,这些花岗岩被认为产于同碰撞构造背景,是我国西部陆块汇聚碰撞的反映.此外,部分学者认为新元古代早期阿尔金-祁连-柴达木北缘和塔里木板块可能是同一个块体,在Grenville造山的后期阶段(950~900 Ma),暂时连接到中国华南板块的北部或Rodinia的西部,这些地区出露的新元古代花岗岩可能源于活动大陆边缘的中-基性火成岩(Yu et al., 2013a).目前阿尔金构造带还没有确切的新元古代变质事件的证据,但是在阿尔金造山带中部出露很多早古生代花岗岩与榴辉岩(张建新等,1999刘良等, 2002, 2003张安达等,2004王超等,2006),暗示这些新元古代花岗质岩石可能遭受到古生代构造事件的影响.

中元古代(1.3~1.8 Ga)期间阿尔金地区存在塔里木地块、中阿尔金微陆块、柴达木地块几个古老地块(覃小锋等,2006).这些陆块在新元古代发生了汇聚碰撞,最终导致塔里木地块、柴达木地块及中阿尔金微陆块的拼合,并引起大陆边缘沉积物部分熔融形成花岗岩.正是这次大规模碰撞造山事件,导致塔里木变质基底的最终固结(Lu et al., 2008).本文通过对阿尔金南缘科克萨依岩体的岩石学、地球化学、同位素年代学的研究认为,科克萨依二长花岗岩形成于新元古代时期(947~945 Ma)Rodinia超大陆汇聚阶段.南阿尔金陆块中类似的新元古代岩浆活动是Grenville全球性造山事件的结果(刘永顺等,2009; Yu et al., 2013a; Wang et al., 2013).

6 结论

(1) 科克萨依二长花岗岩,主要由石英、钾长石、斜长石、黑云母、白云母等矿物组成.地球化学特征显示其具有高SiO2、高Na2O+K2O含量,低CaO、MgO含量,K2O/Na2O比值为1.59~2.61,A/CNK>1.1,属过铝质高钾钙碱性系列,由地壳杂砂岩部分熔融形成的.

(2) 锆石U-Pb年龄为947.5~945 Ma,反映了本区新元古代时期发生过强烈的构造-热事件;锆石Hf同位素结果显示:εHf(t)为-4.09~3.87,tDM2为1.56~2.08 Ga,表明其源岩主要源于古元古代-中元古代的地壳.

(3) 科克萨依二长花岗岩是新元古代时期碰撞造山的产物,是Grenville全球造山事件和Rodinia超大陆汇聚事件的响应.

致谢 感谢两位匿名评审人提出的宝贵意见!

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