南天山拜城县波孜果尔A型花岗岩类锆石U-Pb定年及其Lu-Hf同位素组成*

2014-04-10 01:22刘春花吴才来郜源红雷敏秦海鹏李名则
岩石学报 2014年6期
关键词:锆石岩浆A型

刘春花 吴才来 郜源红 雷敏 秦海鹏 李名则

大陆构造与动力学国家重点实验室,中国地质科学院地质研究所,北京 100037

新疆拜城县波孜果尔A型花岗岩类岩体位于塔里木地台北缘及邻区的近东西向碱性侵入岩带上,主要岩石类型为霓石钠闪石英碱长正长岩、霓石钠闪碱长花岗岩、黑云母碱长正长岩。全岩SiO2=68.97%~74.14%,Na2O+K2O=9.67%~11.19%,Al2O3=13.72%~15.26%,Fe2O3=0.18%~1.41%,FeO=0.91%~1.51%,CaO=0.35%~0.63%。稀土元素总量较高,ΣREE=298×10-6~1286×10-6,平均706×10-6,轻稀土富集,重稀土亏损,强烈的Eu负异常,呈“右倾海鸥型”的稀土元素配分模式。富Nb、Ta、Zr、Hf等高场强元素,亏损Ba、K、Sr等大离子亲石元素,Zr+Nb+Ce+Y=936×10-6~3684×10-6,平均1813×10-6。为A1型花岗岩。岩体形成于早二叠纪。锆石LA-ICP-MS U-Pb年龄为287.7~291.6Ma,平均289.8Ma,岩体形成后,在279.1~282Ma左右经历了后期热液流体的改造。锆石εHf(t)值为-6.3~9.0,两阶段模式年龄(tDM2)跨越古元古代晚期-新元古代中期,主要集中在中元古代。岩浆平均温度832~839℃,形成于非造山的板内构造环境,且具高温、无水、低氧逸度的成岩特点。该岩体具有壳幔混源的特点。

A型花岗岩类;锆石U-Pb定年;Lu-Hf同位素;波孜果尔;南天山

1 引言

中亚造山带(CAOB)(或阿尔泰构造拼贴体)(图1a)是西伯利亚古陆与中朝、塔里木板块(包括东欧板块,即卡拉库姆板块)之间的古亚洲洋形成、演化、消亡而形成的巨型缝合带,属典型的增生型造山带,为全球显生宙陆壳增生与改造最显著的大陆造山带,其显著特点是发育大量的岩浆岩(Jahnetal., 2000; 韩宝福等, 2006; 夏林圻等, 2006),其中石炭-二叠纪岩浆岩尤为发育(杨天南等, 2006; 朱志新等, 2006; 陈汉林等, 2006; 李锦轶等, 2006; 张招崇等, 2009; 童英等, 2010; 黄河等, 2011; Xiaoetal., 2008, 2010; 韩宝福等, 2006)。中亚南天山造山带(图1b)是中亚造山带的重要组成部分,位于伊犁-哈萨克斯坦板块和塔里木-卡拉库姆板块之间,其主体属于塔里木板块北缘,经乌兹别克斯坦、塔吉克斯坦、吉尔吉斯斯坦和哈萨克斯坦延伸至我国新疆和甘肃交界(高俊等,2006;韩宝福等,2006)。它是晚前寒武纪就已形成的南天山洋盆在其后的俯冲消减闭合过程中,两侧陆块及其间的增生杂岩等逐渐拼贴碰撞而形成的碰撞造山带(高俊等,2006)。该带以费尔干纳右行走滑断裂(TFF)(图1a)为界,分为东西两侧,中国南天山主要在东侧,是塔里木板块与哈萨克斯坦板块之间的开阔洋盆关闭形成的碰撞带(李锦轶等,2006)。南天山在大地构造位置上包括了伊犁-中天山陆块的南缘和塔里木板块北缘(图1b)(何国琦等,2001)。

近年来许多学者从构造、地层、古生物、岩石、地球化学和同位素年代学等角度对南天山造山带进行了研究,并讨论了南天山的区域构造格局和演化过程,但是南天山古洋盆闭合(碰撞造山)时间仍存在争议,有早石炭世末(何国琦等,2001;李永军等,2010)、晚石炭世末(高俊等,2006,2009;黄河等,2010)、中二叠世以前(王超等,2007)、晚二叠世之后(苟龙龙和张立飞,2009)、二叠纪末-三叠纪初(李曰俊等,2009)等。存在上述分歧的主要原因之一是缺乏对岩浆岩属性及其形成时代的精确测定;另外,对南天山后碰撞阶段中酸性岩的形成过程中是否有幔源物质加入,目前尚无定论(黄河等,2010)。南天山波孜果尔位于塔里木盆地北缘南天山晚古生代聚合带和伊犁-中天山微板块之间(图1b),距拜城县城正北直距约43km,该岩体地质工作程度较低。波孜果尔碱性花岗岩类是一典型的A型花岗岩类(刘春花等,2012,2013),一般认为,A型花岗岩的发育是造山运动结束的标志(张增杰等,2003)。本文通过该岩体的全岩化学全分析、锆石LA-ICP-MS U-Pb定年及其Lu-Hf同位素研究,旨在探讨波孜果尔A型花岗岩类的构造背景、岩石成因、源区性质以及限定南天山古洋盆闭合(碰撞造山)的时限。

2 地质背景

塔里木盆地北缘碱性岩带西起阿图什、东到尉犁以东,长约1100km(图1b)(邹天人和李庆昌,2006)。波孜果尔A型花岗岩体位于此碱性岩带上,呈小岩株状近东西向展布,岩体出露长约4.45km,宽0.4~1.2km(平均约0.8km左右),面积约3.56km2(图1c)。该岩体主要侵入志留纪穷库什太组的大理岩中,已出露的岩体全岩矿化(邹天人和李庆昌,2006),是一个大型-超大型的REE-Nb-Ta-Zr矿床(徐海明等,2009*徐海明, 王军, 张磊等. 2009. 新疆拜城县波孜果尔铌、钽矿地质报告. 中国地质科学院矿产资源研究所)。

波孜果尔地区的地层属天山地层区南天山分区,哈里克套小区(图1c)。出露的地层主要为古生代志留纪穷库什太组(S3q)、石炭纪干草湖组(C1g)和二叠纪小提坎里克组(P1x)。区内的火山岩较为发育,火山活动大致可分为志留纪、早石炭世和早二叠世三个旋回。侵入岩不甚发育,仅在中部发育上述A型花岗岩体。

区内发育两个小型褶皱。褶皱①是位于F1断层南侧的小型背斜,背斜轴呈近东西向展布,轴面倾角近于直立,规模16km以上,南翼地层倾向南,倾角70°左右,北翼地层倾向北,倾角40°~70°不等。褶皱②是位于F3和F4断层之间的小型背斜,背斜轴呈北东东至南西西向展布,倾向南,倾角75°~80°,规模大于12km,南翼倾向南,倾角30°左右,北翼倾向北,倾角50°左右。

近东西向的F1和F2断裂(阿克牙伊利亚克塔格断裂)为区内的主要断裂,长160km以上,断层面产状倾向北,倾角70°左右,为压性断层,有多期次活动的特点,沿断裂带断续分布较多的构造破碎岩,并形成较多的绿泥石化、褐铁矿化等蚀变现象。北东东-南西西向F3和F4断裂主要分布于区内南侧一带,规模较小,沿断层带断续分布较多的构造破碎岩,断层面倾向北,倾角60°左右,为压扭性断层。北东-南西向F5断裂一般为F1、F2主干断裂的次级断裂,规模较小,断层平直,断层破碎带不发育,为压扭性断层。

岩体的围岩常发生绿帘石化、绿泥石化及萤石化。绿帘石呈细粒状或隐晶质,集合体呈细脉状或薄膜状分布,常与绿泥石共生,并伴有绢云母、碳酸盐等矿物。绿泥石呈片状、纤维状集合体分布。萤石多呈紫色、黑紫色、黑色等,它形粒状、团块状或粗细不等的脉状,沿裂隙交代充填,为热液作用晚期的产物。

图1中亚造山带及其邻区大地构造略图(a,据黄河等,2011)、塔里木盆地北缘的碱性岩分布图(b,据邹天人和李庆昌,2006)和波孜果尔一带区域地质简图(c,据新疆维吾尔自治区地质局区域地质调查大队,1982*新疆维吾尔自治区地质局区域地质调查大队. 1982. 1:20万区域地质调查却响幅地质图. 中国地质图制印厂)

TFF-塔拉斯-费尔干纳走滑断裂;YCSF-伊犁-中天山微板块南缘断裂带;TNF-塔里木北缘断裂带;1-第四纪洪积层;2-第三纪砾岩、砂砾岩、砂岩、泥岩;3-白垩纪砂质泥岩、粉砂岩、长英质砂岩、砾岩;4-侏罗纪泥岩、泥质粉砂岩、泥质页岩、泥灰岩、砂岩、炭质页岩、石英砂岩、粉砂岩、砾岩、粗砂岩、煤层;5-三叠纪砾岩、中粒砂岩、泥质粉砂岩夹粗砂岩、细砾岩;6-二叠纪小提坎立克组酸性熔岩、凝灰岩及碎屑岩建造;7-石炭纪干草湖组浅-滨海相碳酸盐及碎屑岩建造;8-志留纪大理岩、灰岩、石英片岩、混合岩、变质粉砂岩;9-志留纪穷库什太组片理化泥质粉砂岩、大理岩、结晶灰岩、绢云母石英片岩、安山玢岩,辉绿岩、英安斑岩;10-海西中晚期A型花岗岩;11-断层;12-砾岩;13-大理岩;14-研究区

Fig.1Sketch geological map showing the main tectonic unites of Central Asian Orogenic Belt (a, after Huangetal., 2011), map of distribution of alkaline rocks in the northern margin of the Tarim Basin (b, after Zou and Li, 2006) and regional geological map of Boziguoer (c)

TFF-Talas Fergan strike-slip fault; YCSF-fault zone in the southern margin of Yili-Central Tianshan microplate; TNF-fault zone in the northern margin of Tarim; 1-Quaternary diluvial layer; 2-Tertiary conglomerate, glutenite, sandstone and mudstone; 3-Cretaceous sandy shale, siltstone, felsic sandstone and conglomerate; 4-Jurassic mudstone, argillaceous siltstone, argillaceous shale, marl, sandstone, carbonaceous shale, quartz sandstone, siltstone, conglomerate, grit stone and coalbed; 5-Triassic conglomerate, medium-grained sandstone, argillaceous siltstone fold grit stone and conglomerates; 6-Permian Xiaotikanlike Formation acidic lava, tuff and clastic rock; 7-Carboniferous Gancaohu Formation shallow and marine carbonate and clastic rock; 8-Silurian marble, limestone, quartz schist, migmatite and metamorphic siltstone; 9-Silurian Qiongkushitai Formation schistose argillaceous siltstone, marble, crystalline limestone, sericite quartz schist, andesitic porphyry, diabase and dacite porphyry; 10-Late Hercynian A-type granites; 11-fault; 12-conglomerate; 13-marble; 14-study area

3 岩石学特征

该岩体属于A型花岗岩类,可细分为如下3种岩石类型(刘春花等,2012),岩石学特征如下:

霓石钠闪石英碱长正长岩(图2a-c),灰白色,细粒结构,块状构造。主要矿物包括钠长石(58%~70%),钾长石(14%~17%),石英(5%~8%);次要矿物钠铁闪石(8%~10%),霓石(2%~5%),黑云母(1%~2%)。钾长石的Or平均96.94%,Ab平均2.46%,An平均0.6%。钠长石的Ab平均98.68%,Or平均0.92%,An平均0.41%。钠铁闪石中SiO2平均49.69%,FeO平均33.97%,Na2O平均7.92%,K2O平均1.54%。霓石中SiO2平均52.9%,FeO平均30.49%,Na2O平均14.58%。黑云母中SiO2平均35.69%,TiO2平均2.03%,Al2O3平均10.06%,FeO平均36.99%,K2O平均8.89%,属于铁叶云母。副矿物主要包括锆石、星叶石、烧绿石、钍石、萤石、独居石、氟碳铈镧矿、磷钇矿等(刘春花等,2013)。

霓石钠闪碱长花岗岩(图2d-f),灰白色,细粒结构,块状构造。主要矿物钠长石(35%~45%),钾长石(22%~25%),石英(22%~25%);次要矿物钠铁闪石(10%~12%),霓石(1%~3%),含极少量的锂云母。钾长石的Or平均97.6%,Ab平均2.19%,An平均0.2%。钠长石的Ab平均98.38%,Or平均1.21%,An平均0.42%。钠铁闪石中SiO2平均51.25%,FeO平均31.25%,Na2O平均9.18%,K2O平均1.83%。霓石中SiO2平均52.55%,FeO平均31.13%,Na2O平均14.42%。副矿物主要包括锆石、星叶石、烧绿石、钍石、萤石、独居石、氟碳铈镧矿、磷钇矿等(刘春花等,2013)。

黑云母碱长正长岩(图2g-i),肉红色,细粒结构,块状构造。主要矿物钠长石(40%~55%),钾长石(26%~30%),石英(10%~14%);次要矿物黑云母(7%~12%),钠铁闪石(2%~4%)。钾长石的Or含量平均97.19%,Ab平均2.23%,An平均0.58%。钠长石的Ab平均98.45%,Or平均0.83%,An平均0.72%。钠铁闪石中SiO2平均50.18%,FeO平均33.34%,Na2O平均8.11%,K2O平均1.71%。霓石中SiO2平均52.38%,FeO平均31.82%,Na2O平均14.27%。黑云母中SiO2平均34.95%,TiO2平均2.3%,Al2O3平均13.2%,FeO平均34.43%,K2O平均8.93%,属于铁叶云母。副矿物主要包括锆石、星叶石、烧绿石、钍石、萤石、独居石、氟碳铈镧矿、磷钇矿等(刘春花等,2013)。

4 分析方法

4.1 全岩化学分析

岩石样品破碎和化学全分析在河北省区域地质矿产调查研究所完成,氧化物用X荧光光谱仪3080E测试,执行标准分别为:Na2O、MgO、Al2O3、SiO2、P2O5、K2O、CaO、TiO2、MnO、Fe2O3、FeO按GB/T 14506.28—1993标准;H2O+按GB/T 14506.2—1993标准;CO2按GB 9835—1988标准;LOI按LY/T 1253—1999标准。分析的相对标准偏差小于2%~8%。稀土元素La、Ce、Pr、Nd、Sm、Eu、Gd、Tb、Dy、Ho、Er、Tm、Yb、Lu、Y和微量元素Cu、Pb、Hf、Ta、Sc、Cs、V、Co、Ni用等离子质谱(ICP-MS)Excell测试,执行标准为DZ/T 0223—2001;微量元素Sr、Ba、Zn、Rb、Nb、Zr、Ga用X荧光光谱仪2100测试,执行JY/T 016—1996标准。分析精度大多数元素可达到10-8,少量元素为10-6(Zr、Ba)和10-7(Hf、Nb),其相对标准偏差小于10%。

4.2 锆石LA-ICP-MS U-Pb定年分析

图2 波孜果尔A型花岗岩类正交偏光显微镜照片和背散射(BSE)图像(a、b、c)-霓石钠闪石英碱长正长岩,半自形细粒结构,霓石与钠铁闪石共生,锆石呈半自形;(d、e、f)-霓石钠闪碱长花岗岩,花岗结构,霓石与钠铁闪石共生,含少量锂云母,锆石为半自形-自形;(g、h、i)-黑云母碱长正长岩,叶片状的铁叶黑云母呈团簇状分布,锆石为半自形-自形.Q-石英;Ab-钠长石;Kf-钾长石;Bi-黑云母;Aeg-霓石;Arf-钠铁闪石;Zr-锆石;Lpd-锂云母;Aph-星叶石Fig.2 Orthogonal polarizing photomicrographs and BSE images of A-type granitoids(a, b, c)-aegirine or arfvedsonite quartz alkali feldspar syenite has subhedral fine-grained texture, it could be observed that aegirine associated directly with arfvedsonite, zircon shows subhedral in shape; (d, e, f)-aegirine or arfvedsonite alkali feldspar granite shows granitic texture and aegirine associated directly with arfvedsonite, it containing a small amount of lepidolite, zircon shows subhedral-euhedral in shape; (g, h, i)-biotite alkali feldspar syenite, leaf-shaped siderophyllite distributed in clusters, zircon shows subhedral-euhedral in shape; Q-quartz; Ab-albite; Kf-K-feldspar; Bi-biotite; Aeg-aegirine; Arf-arfvedsonite; Zr-zircon; Lpd-lepidolite; Aph-astrophyllite

野外分别采集样品11CL901、11CL902、11CL903和11CL904各约2kg,破碎至80~120目,使用常规的重液浮选和电磁分离方法,最后人工挑选出锆石,由河北省区域地质矿产调查研究所完成。然后在双目镜下根据锆石颜色、自形程度、形态和透明度等特征进行分类,挑选出具有代表性的锆石,制成锆石靶,由中国地质科学院地质研究所大陆动力学实验室完成。拍摄正交偏光、反射光和阴极发光照片。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:ICP-MS ELAN DRC-Ⅱ型。激光剥蚀束斑直径在60~44μm,激光剥蚀样品的深度为40~20μm。实验中采用He作为剥蚀物质的载气,采样方式为单点剥蚀,每测定4个样品点后测定标样一次。锆石年龄采用标准锆石91500作为外部标准物质,元素含量采用NIST SRM610作为外标,用29Si作为内标来消除激光能量在点分析过程中以及分析点之间的漂移。U/Pb比值数据处理使用软件LaDating@Zrn,校正Pb同位素处理使用软件ComPb corr#3-18(Anderson, 2002),校正后的数据使用美国Berkeley地质年代学中心Kenneth R. Ludwig编制的ISOPLOT和SQUID程序(Ludwig, 2003)计算年龄。加权平均年龄计算及谐和图采用Isoplot(ver 3.0)完成。

4.3 锆石Lu-Hf同位素测定

锆石Lu-Hf同位素测试在中国地质科学院矿产资源研究所国土资源部成矿作用与资源评价重点实验室完成,所用仪器为Neptune多接收等离子质谱和Newwave UP213紫外激光剥蚀系统(LA-MC-ICP-MS),实验过程中采用He作为剥蚀物质载气,根据锆石大小,剥蚀直径采用55μm或40μm,测定时使用国际上通用的锆石标样GJ1作为参考物质,分析过程中锆石标准GJ1的176Hf/177Hf测试加权平均值为0.282015±8(2σ,n=10)。相关仪器运行条件及详细分析流程见侯可军等(2007)。

5 结果

5.1 岩石地球化学

波孜果尔A型花岗岩类样品的主量元素变化特征为:SiO2=68.97%~74.14%,Al2O3=13.72%~15.26%、Fe2O3=0.18%~1.41%、FeO=0.91%~1.51%、CaO=0.35%~0.63%、Na2O+K2O=9.67%~11.19%(表1),在TAS岩石分类图解(图3)上,11CL902落在碱性石英二长岩区,其余样品落在碱性花岗岩区。K2O/Na2O比值为0.63~0.93,反映岩体富钠;里特曼指数δ=2.99~4.79,铝饱和指数A/CNK=0.89~0.97。图3和图4表明该岩体属碱性到过碱性系列,考虑到岩石均含碱性暗色矿物,因此,主体为过碱性系列。

表1波孜果尔A型花岗岩类的主量元素(wt%)、微量元素(×10-6)分析结果

Table 1Major element (wt%) and trace element (×10-6) analysis of A-type granitoids from boziguoer

样品号11CL90111CL90211CL90311CL904样品号11CL90111CL90211CL90311CL904SiO271.2268.9771.3774.14ΣREE478.51286759.7298.3TiO20.100.100.060.06LREE381.51193660.4213.9Al2O315.1215.2613.9513.72HREE97.0393.0499.2984.38Fe2O30.501.411.390.18LREE/HREE3.9312.826.652.53FeO0.911.371.511.03(La/Yb)N3.6211.027.831.74MnO0.030.100.090.02δEu0.070.080.120.05MgO0.050.030.030.03δCe0.901.460.800.92CaO0.630.550.350.40Cr11.7812.6612.5311.65Na2O5.526.696.465.24Ni4.453.843.913.92K2O5.124.504.054.43Cu12.1011.6111.1710.98P2O50.020.010.020.01Pb85.13238.773.76138.0H2O+0.430.410.300.31Zn215.9455.3517.7314.6H2O-0.130.140.070.13As2.601.602.000.50LOI0.730.820.640.68Sb0.271.440.180.06Mg#9.164.173.685.61Se0.060.070.060.03DI96.7595.0194.0897.39Te0.020.020.010.02A/NK1.030.960.931.02Ga44.0360.4951.1445.92A/CNK0.960.910.890.97Ge1.281.361.301.58SI0.420.240.240.31Sc1.261.181.451.33δ3.994.793.882.99V9.077.246.257.55K2O/Na2O0.930.670.630.85Cd0.200.580.510.04La94.44288.0217.249.23Hg8.008.00186.010.00Ce173.3737.5302.696.05Rb569.61445867.3705.9Pr21.8141.2831.0612.51Sr13.705.3011.1011.00Nd72.86106.290.2943.21Cs59.9139.2835.2144.23Sm18.6718.9718.5712.68Ba26.9619.3927.3728.67Eu0.390.480.730.20Zr562.91945712.9413.8Gd17.1218.9218.9712.34Nb320.9826.5169.9257.9Tb4.123.784.023.18Ta10.4644.708.6622.37Dy26.3523.5325.5721.42Hf16.9451.8037.5117.02Ho6.715.856.555.54Mo2.470.390.340.37Er17.9716.3117.9215.57Nb/Ta30.6618.4919.6111.53Tm3.573.453.713.52Zr/Hf33.2337.5419.0024.31Yb18.7118.7519.9020.27Rb/Ba21.1374.5231.6924.62Lu2.472.462.642.54Rb/Sr41.58272.6478.1464.17Y174.2175.8214.5168.3Zr+Nb+Ce+Y123136841400936

注:Mg#=mol(MgO)/mol(MgO+FeO)×100

图3 波孜果尔A型花岗岩类(Na2O+K2O)-SiO2分类图解(据Middlemost, 1994)Fig.3 (Na2O+K2O)-SiO2 diagram of A-type granitoids from boziguoer (after Middlemost, 1994)

图4 波孜果尔A型花岗岩类A/NK-A/CNK图(据Rollinson, 1993)Fig.4 A/NK-A/CNK diagram of A-type granitoids from boziguoer (after Rollinson, 1993)

样品稀土元素总量ΣREE=298×10-6~1286×10-6(表1),平均706×10-6,LREE/HREE=2.53~12.82,平均6.48,表明稀土总量较高,且以轻稀土富集和重稀土亏损为特征。球粒陨石标准化配分曲线为右倾海鸥型(图5);轻重稀土中等分馏,(La/Yb)N=1.74~11.02,平均6.05。δEu=0.05~0.12,强烈的负Eu异常。δCe=0.80~1.46,除11CL902外,其他3个样品具有微弱的负Ce异常。

图5 波孜果尔A型花岗岩类稀土元素球粒陨石标准化分布型式图(球粒陨石标准化值据Sun and McDonough, 1989)Fig.5 Chondrite-normalized REE diagram of A-type granitoids from Boziguoer (chondrite data from Sun and McDonough, 1989)

样品Rb/Sr比值为41.58~272.64(表1),平均114;Rb/Ba比值为21.13~74.52,平均38;Zr含量414×10-6~1945×10-6,平均909×10-6;Zr+Nb+Ce+Y=936×10-6~3684×10-6,平均1813×10-6。微量元素原始地幔标准化蛛网图(图6)上,Rb/Sr、Rb/Ba比值较高,富Nb、Ta、Zr、Hf等高场强元素,亏损Ba、K、Sr等大离子亲石元素,样品的微量元素总体趋势一致,表明三种岩石是同源岩浆演化的产物。

图6 波孜果尔A型花岗岩类微量元素原始地幔标准化微量元素蛛网图(原始地幔标准化值据Sun and McDonough, 1989)Fig.6 Primaitive-mantle-normalized trace element spider diagram of A-type granitoids from Boziguoer (primitive mantle data from Sun and McDonough, 1989)

5.2 锆石LA-ICP-MS U-Pb定年

11CL901(黑云母碱长正长岩)取样坐标:N 43°13′12″,E 81°54′03″,H 3909m。锆石为自形,四方柱{100}和{110}面相对四方双锥{111}面发育,粒度0.2×0.1mm~0.4×0.2mm,长宽比为3:1~1:1。阴极发光(CL)照片显示锆石核部颜色发黑,大部分锆石核部无明显的环带,少部分锆石核部具有振荡环带(点2、16、27、33和40);锆石边部有一圈很窄的形状不规则的白边(图7a)。该样品共测定了28颗锆石的28个点(表2),从锆石LA-ICP-MS U-Th-Pb同位素分析结果看,锆石的U、Th含量变化较大,分别为:56×10-6~1345×10-6(平均501×10-6)和8×10-6~2340×10-6(平均447×10-6),U和Th相关性较好(R2=0.730)(图8a),Th/U值为0.15~1.74,除点1(0.18)、点10(0.15)、点23(0.21)和点28(0.16)外,其余Th/U比值均大于0.3。锆石的206Pb/238U年龄为280±6Ma~313±8Ma(表2),除点27(313±8Ma)和点38(300±7Ma)外,其余均小于300Ma。在206Pb/238U-207Pb/235U谐和图(图9a)上,锆石的年龄有18个点紧邻谐和线,10个点在谐和线之下且离谐和线较远,但是这些年龄都均匀地分布在不一致线上,与谐和线的下交点年龄为286.3±3.5Ma(MSWD=0.93)。选择紧邻谐和线的18个点计算其加权平均年龄为287.7±2.9Ma(MSWD=0.84),与下交点年龄在误差范围内基本一致。点28(280±6Ma)和点10(284±6Ma)位于锆石白边上,其平均值与下文11CL902和11CL903锆石白边年龄一致。

图7 波孜果尔A型花岗岩类锆石的阴极发光CL图像Fig.7 Zircon cathodoluminescence (CL) images of A-type granitoids from Boziguoer

11CL902(霓石钠闪碱长花岗岩)取样坐标:N 42°13′11″,E 81°54′11″,H 4010m。锆石为半自形至自形,粒度0.1×0.1mm~0.2×0.18mm,长宽比为1.5:1~1:1。CL照片显示11CL902锆石特征与11CL901相似,但较11CL901的白边宽(图7b)。该样品共测定了26颗锆石的28个点(表2),其中点1、8、10、14、16、31、32位于锆石的白边上,其余均位于核部或者核边交界处。该样品锆石的U、Th含量分别为:54×10-6~841×10-6(平均400×10-6)和5×10-6~585×10-6(平均185×10-6),U和Th相关性较好(R2=0.735)(图8b),Th/U值为0.08~0.84,15个点小于0.3,13个点大于0.3。206Pb/238U-207Pb/235U谐和图(图9b)年龄分布整体趋势和11CL901相似。锆石白边上7个测点的206Pb/238U年龄在275±4Ma~285±7Ma,加权平均年龄为279±4Ma(MSWD=0.43),下交点年龄282.2±5.5Ma(MSWD=0.24)(图略)。锆石核部的206Pb/238U年龄跨度较大,在275±4Ma~312±5Ma(图7b、表2),选择核部年龄较一致的点3、4、5、6、12、20、23、25、28和点29,其加权平均年龄为290.6±2.8Ma(MSWD=0.29),下交点年龄288±8Ma(MSWD=0.27)(图略)。所有测点的不一致线和谐和线的下交点年龄278±3Ma(MSWD=1.03),选择紧邻谐和线的12个点计算其加权平均年龄为280.9±2.6Ma(MSWD=0.82),与锆石白边上7个测点的平均年龄在误差范围内基本一致。

表2波孜果尔A型花岗岩类锆石LA-ICP-MS U-Th-Pb同位素分析结果

Table 2Zircon LA-ICP-MS U-Th-Pb dating results of A-type granitoids from Boziguoer

测点号Pb*ThU(×10-6)Th/U207Pb/206Pb207Pb/235U206Pb/238U206Pb/238U(Ma)测值绝对误差1s测值绝对误差1s测值绝对误差1s测值绝对误差1s11CL901黑云母碱长正长岩11CL901-119.54653590.180.13300.0060.82580.0470.04480.001282911CL901-213.021422580.550.06830.0040.42240.0240.04520.001285511CL901-595.76234013451.740.09250.0030.58190.0250.04520.001285611CL901-835.964795980.800.16120.0070.98210.0600.04510.0022841011CL901-934.023916590.590.06090.0040.38190.0220.04480.001283611CL901-102.458560.150.05640.0040.34520.0230.04500.001284611CL901-1179.80174811011.590.13320.0070.86550.0460.04670.001294611CL901-1338.625956750.880.08140.0060.51850.0370.04580.001288711CL901-1520.522433890.630.08440.0070.54520.0480.04620.001291711CL901-1623.143584160.860.06290.0040.40760.0260.04680.001295611CL901-198.24471550.300.10700.0090.69210.0600.04660.001293911CL901-2041.394437490.590.08480.0060.55230.0400.04670.001294711CL901-227.09531370.390.08420.0070.54070.0500.04650.002293911CL901-2310.91522430.210.05660.0040.36170.0210.04570.001288611CL901-2436.324846600.730.08560.0050.54910.0320.04600.001290611CL901-2547.908246881.200.16340.0111.02190.0760.04510.001284611CL901-2631.154014410.910.20360.0131.34670.0940.04730.001298811CL901-279.91631720.370.09640.0080.66430.0520.04970.001313811CL901-284.84191140.160.05610.0040.34290.0220.04450.001280611CL901-2954.819019130.990.08490.0050.55180.0290.04690.001295611CL901-3125.022524030.630.13740.0110.89390.0620.04690.001296711CL901-3213.301052820.370.07000.0040.43690.0260.04470.001282611CL901-3315.441793010.600.05980.0040.38990.0250.04670.001294611CL901-3550.738938641.030.07980.0050.52510.0370.04700.001296711CL901-3623.592753930.700.14640.0100.95810.0650.04590.001289711CL901-3724.992794180.670.15270.0110.99650.0750.04640.001292711CL901-3824.833644240.860.13260.0100.88180.0690.04760.001300711CL901-4042.535098120.630.09880.0050.61100.0300.04520.001285711CL902霓石钠闪碱长花岗岩11CL902-12.535600.080.05630.0050.35060.0270.04520.001283811CL902-228.11984110.240.17970.0091.26370.0730.04830.001304811CL902-320.38943690.260.12500.0050.81210.0350.04610.001291611CL902-444.015606670.840.13870.0040.91110.0270.04650.001293411CL902-519.841143980.290.07970.0030.51210.0160.04560.001288411CL902-634.343005770.520.12120.0050.78610.0320.04590.001289511CL902-740.374797640.630.07110.0020.45050.0140.04490.001283411CL902-82.359540.170.05040.0060.30840.0380.04440.001280911CL902-93.399600.150.10260.0110.66830.0680.04740.0022991111CL902-104.2714990.140.05890.0040.35540.0220.04520.001285711CL902-1233.372625520.470.13420.0050.86660.0300.04580.001289411CL902-1340.572865800.490.16840.0071.21070.0600.04960.001312511CL902-142.5414570.240.05910.0040.36090.0190.04510.001282511CL902-153.3114790.180.05710.0020.34880.0120.04360.001275411CL902-1625.062054760.430.11320.0040.69570.0200.04420.001279411CL902-1723.041194390.270.10650.0040.67600.0250.04490.001283411CL902-1918.98694060.170.07660.0020.47860.0140.04470.001282411CL902-2045.195857410.790.10190.0030.65880.0180.04590.001289411CL902-2140.783147600.410.09670.0060.62220.0400.04480.001283311CL902-2337.094846070.800.11420.0060.73890.0420.04610.0012905

续表2ContinuedTable2测点号Pb*ThU(×10-6)Th/U207Pb/206Pb207Pb/235U206Pb/238U206Pb/238U(Ma)测值绝对误差1s测值绝对误差1s测值绝对误差1s测值绝对误差1s11CL902-249.40392050.190.07240.0030.45930.0160.04520.001282411CL902-2556.333448410.410.17600.0071.13020.0510.04580.001289511CL902-2614.73663190.210.06960.0030.44010.0150.04520.001281411CL902-2730.292374380.540.18950.0071.29080.0500.04880.001307411CL902-2825.431684730.360.10240.0070.67240.0490.04640.001292511CL902-2934.472565670.450.14670.0050.97290.0320.04680.001295411CL902-314.74161130.140.06150.0030.37490.0180.04420.001276611CL902-324.4218990.180.06710.0030.41680.0190.04420.001275411CL903霓石钠闪碱长花岗岩11CL903-16.8024.641630.150.05200.0020.32160.0100.04400.001278411CL903-33.969.29950.100.05040.0020.32690.0120.04520.001285411CL903-44.1025.93890.290.07620.0030.46630.0160.04450.001281511CL903-51.773.24100.330.08210.0030.52360.0160.04580.001289511CL903-65.2845.261040.430.05430.0020.37200.0130.04840.001305511CL903-73.8339.24710.550.06620.0030.44680.0160.04790.001302511CL903-823.78262.833560.740.20720.0101.30290.0450.04610.001291911CL903-96.8825.261420.180.08540.0040.56200.0280.04580.001289411CL903-107.1323.891680.140.05130.0020.32580.0110.04510.001285411CL903-1139.51419.057620.550.06440.0020.39850.0120.04410.001278511CL903-134.1530.98690.450.13680.0080.86740.0480.04510.001284611CL903-142.078.79400.220.07790.0040.53010.0270.04830.001304611CL903-1518.8479.373340.240.10820.0040.74340.0280.04890.001308511CL903-166.3920.901480.140.06160.0020.38180.0130.04440.001280411CL903-177.4824.831760.140.05040.0020.31540.0100.04470.001282411CL903-1821.0768.933280.210.14810.0081.05570.0580.05000.001315611CL903-191.937.88450.180.05710.0030.34720.0190.04410.001278511CL903-211.8012.48280.450.17220.0111.08510.0640.04600.0022901011CL903-224.1422.04890.250.06170.0030.38990.0160.04560.001288511CL903-233.8049.42780.630.05730.0030.34250.0130.04330.001273511CL903-2431.93157.334620.340.16870.0051.18540.0360.05030.001316511CL903-2511.6666.042080.320.10410.0040.70130.0280.04830.001304511CL903-261.173.77270.140.05830.0040.34870.0230.04370.001276611CL903-273.6930.41760.400.05530.0020.35180.0130.04600.001290511CL903-2823.81189.014670.400.08220.0030.52510.0150.04600.001290411CL903-297.7323.981810.130.05370.0020.33250.0130.04470.001282411CL903-306.8721.971600.140.05900.0030.36000.0140.04410.001278411CL903-316.5517.721570.110.05480.0020.33390.0110.04410.001278411CL903-326.5622.441570.140.05360.0020.32430.0120.04360.001275411CL904霓石钠闪石英碱长正长岩11CL904-121.37555140.110.05470.0030.33290.0170.04410.001278611CL904-358.4791910120.910.08530.0040.52790.0280.04430.001279611CL904-436.17993000.330.04610.0040.27960.0260.04400.001278511CL904-534.921058170.130.05030.0030.31950.0190.04540.001286611CL904-66.03291280.230.06680.0060.40250.0300.04580.001289811CL904-771.26161110821.490.05640.0030.36660.0200.04650.001293611CL904-829.303345810.570.07520.0040.46450.0240.04430.001279611CL904-939.913436990.490.10040.0060.66360.0370.04720.001297611CL904-1040.413885650.690.17550.0141.16330.0940.04730.001298811CL904-1135.744325090.850.15640.0101.02530.0650.04700.001296811CL904-1359.498009880.810.08960.0050.58470.0310.04680.0012956

图8 波孜果尔A型花岗岩类锆石的Tu和U关系图Fig.8 Relationship between Th and U in the zircons of A-type granitoids from Boziguoer

图9 波孜果尔A型花岗岩类锆石的U-Pb协和线图和平均年龄图Fig.9 Zircon U-Pb concordia diagrams and averaged age of A-type granitoids from Boziguoer

11CL903(霓石钠闪碱长花岗岩)取样坐标:N 42°13′10″,E 81°54′20″,H 4098m,与11CL902岩性相同。锆石为半自形至自形,粒度0.1mm×0.1mm~0.4mm×0.2mm,长宽比为2:1~1:1,与11CL901相似(图7c)。该样品共测出了28颗锆石的29个点(表2)。该样品锆石的U、Th含量分别为:10×10-6~762×10-6(平均179×10-6)和3.24×10-6~419×10-6(平均60×10-6),U和Th相关性较好(R2=0.770)(图8c),Th/U值为0.10~0.74,17个点小于0.3,12个点大于0.3。在206Pb/238U-207Pb/235U谐和图(图9c)上,有8个样品点偏离谐和线和不一致线较远,锆石白边的206Pb/238U年龄在273±5Ma~285±4Ma,都分布在下交点附近。选择核部年龄较一致的点5、8、9、21、22、27和点28,其加权平均年龄为289.4±3.8Ma(MSWD=0.028),下交点年龄288.8±5.3Ma(MSWD=0.032)(图略)。所有测点的下交点年龄278.3±6.5Ma(MSWD=3.1),选择紧邻谐和线的15个点进行平均,其加权平均年龄280.5±2.2Ma(MSWD=1.08),也是锆石白边的平均年龄。

11CL904(霓石钠闪石英碱长正长岩)取样坐标:N 43°13′12″,E 81°54′33″,H 3852m。锆石为半自形至自形,粒度0.1×0.1mm~0.3×0.1mm,长宽比为2.5:1~1:1,CL照片显示锆石的白边不发育(图7d)。该样品共测出了25颗锆石的25个点(表2)。该样品锆石的U、Th含量分别为:128×10-6~1785×10-6(平均708×10-6)和29×10-6~3371×10-6(平均788×10-6),U和Th相关性较好(R2=0.650)(图8d),Th/U值为0.11~2.86,4个点小于0.3,21个点大于0.3。点1、3、4、8、14、19和点31的206Pb/238U年龄在277±7Ma~282±6Ma,其加权平均年龄为279.1±4.3Ma(MSWD=0.073),下交点年龄278±5.8Ma(MSWD=0.014)(图略),与11CL902和11CL903的白边年龄在误差范围内一致。除上述7颗锆石外,其余18颗锆石的206Pb/238U年龄在286±6Ma~298±8Ma,在206Pb/238U-207Pb/235U谐和图(图9d)上,这18颗锆石的下交点年龄289.7±4.5Ma(MSWD=0.20),选择紧邻谐和线的15个点计算其加权平均年龄为291.6±2.9Ma(MSWD=0.24)(图9d)。

5.3 Lu-Hf同位素

本文对4个样品进行了Lu-Hf同位素分析,除11CL902测点选择在锆石的白边外,其余3个样品测点均选择在锆石的黑核上。锆石Lu-Hf同位素分析结果见表3,锆石的fLu/Hf(s)值为-0.73~-0.98,显著小于大陆镁铁质地壳fLu/Hf(s)=-0.34(Amelinetal., 1999),因此两阶段模式年龄能更真实地反映其源区物质从亏损地幔抽取的时间(第五春荣等,2007)。11CL901(14个测点)锆石黑核的176Yb/177Hf值0.075238~0.526770,176Lu/177Hf值0.001160~0.006886,176Hf/177Hf值为0.282537~0.282879,其εHf(t)值-2.7~9.0,两阶段Hf模式年龄(tDM2)为733~1475Ma。11CL902(18个测点)锆石白边的176Yb/177Hf值0.073100~0.560419,176Lu/177Hf值0.000953~0.006851,176Hf/177Hf值为0.282491~0.282783,其εHf(t)值-4.9~5.3,tDM2为964~1613Ma。11CL903(19个测点)锆石黑核的176Yb/177Hf值0.046000~0.722249,176Lu/177Hf值0.000644~0.009050,176Hf/177Hf值为0.282427~0.282871,其εHf(t)值-6.3~8.2,tDM2为787~1706Ma。11CL904(17个测点)锆石黑核的176Yb/177Hf值0.098242~0.396574,176Lu/177Hf值0.001136~0.004998,176Hf/177Hf值为0.282505~0.282874,其εHf(t)值-3.5~9.2,tDM2为723~1529Ma。所有测点176Lu/177Hf比值较低(为0.000644~0.009050),因此由176Lu衰变生成的176Hf极少,表明绝大多数锆石在形成以后,仅具有较少的放射性成因Hf的积累,因而所测定的176Hf/177Hf值可以代表该锆石形成时的176Hf/177Hf值(吴福元等,2007)。四个样品tDM2的年龄跨越古元古代晚期-新元古代中期,主要集中在中元古代,εHf(t)正、负值均有(图10)。

6 讨论

6.1 岩体的时代

波孜果尔A型花岗岩类锆石CL图像表明,核为黑色,代表其可能是在一种高U的环境下结晶形成的,后来受到流体的改造,形成了白色的热液交代边。结合锆石的CL图像,11CL901的Th/U值小于0.3的测点均位于锆石的白边上;11CL902和11CL903白边的测点Th/U值均小于0.3,部分黑核的Th/U值也小于0.3;11CL904大部分测点的Th/U值均大于0.3。一般认为岩浆成因锆石的Th/U值大于0.3,变质重结晶的锆石则小于0.1(Hoskin and Ireland, 2000),说明这4个样品锆石核是岩浆结晶成因。新增生的白边Th/U值小于0.3是锆石受流体直接交代改造的有力证据,流体对锆石边部的改造也会间接影响锆石核部,导致了核部某些测点的Th/U值小于0.3。在U-Pb谐和图上,部分测点在谐和线上,部分测点由于存在不同程度的铅丢失而偏离谐和线,但每个样品的谐和与不谐和数据都构成了一条较好的不一致线。11CL901~11CL904黑核的平均年龄分别为287.7±2.9Ma(MSWD=0.84)、290.6±2.8Ma(MSWD=0.29)、289.4±3.8Ma(MSWD=0.028)、291.6±2.9Ma(MSWD=0.24),可以解释为岩浆结晶的年龄,这组年龄在误差范围内基本一致,代表该岩体不同岩性的岩石为同期岩浆事件的产物。11CL902和11CL903白边的平均年龄分别为280.9±2.6Ma(MSWD=0.82)和280.5±2.2Ma(MSWD=1.08),11CL901锆石白边的测点10、28的平均年龄为282±6Ma,11CL904的白边测点1、3、4、8、14、19、31的平均年龄为279.1±4.3Ma(MSWD=0.073),不同样品白边年龄在误差范围内基本一致,可解释为后期热液流体改造的热事件年龄,同时也说明不同岩性的岩石经历了同期热液流体的改造。

表3波孜果尔A型花岗岩类锆石Lu-Hf同位素分析结果

Table 3Zircon Lu-Hf isotopic results of A-type granitoids from Boziguoer

样品号t(Ma)176Yb177Hf2σ176Lu177Hf2σ176Hf177Hf2σεHf(0)εHf(t)2σtDM1(Ma)tDM2(Ma)fLu/Hf(s)11CL901-311CL901-411CL901-611CL901-711CL901-1111CL901-1211CL901-1311CL901-1611CL901-2211CL901-2311CL901-2611CL901-2811CL901-2911CL901-302880.0752380.0009830.0011600.0000120.2825660.000021-7.3-1.20.79751381-0.970.3160890.0070400.0047570.0000700.2825430.000038-8.1-2.71.311141475-0.860.3658090.0007390.0056990.0000220.2826250.000037-5.20.11.310131302-0.830.2460740.0051590.0039110.0000590.2825820.000033-6.7-1.11.210271378-0.880.1381280.0011360.0017210.0000180.2826690.000027-3.62.41.08421156-0.950.3691450.0065240.0041980.0000440.2828100.0000481.36.91.7685869-0.870.5267700.0029760.0058300.0000250.2828790.0000423.89.01.5607733-0.820.1790650.0046520.0021770.0000620.2826980.000028-2.63.31.08101096-0.930.4301160.0007870.0058710.0000160.2827790.0000380.25.41.3771959-0.820.1853090.0049910.0029160.0000900.2826420.000024-4.61.20.99101232-0.910.4844940.0037240.0068860.0000610.2826140.000045-5.6-0.61.610701342-0.790.3249170.0033420.0039450.0000410.2826460.000041-4.51.11.49311235-0.880.2090500.0034350.0025780.0000490.2825370.000028-8.3-2.51.010561464-0.920.2209860.0026690.0028470.0000260.2826700.000031-3.62.21.18671168-0.9111CL902-111CL902-211CL902-311CL902-511CL902-711CL902-911CL902-1011CL902-1111CL902-1211CL902-1311CL902-1411CL902-1511CL902-1711CL902-1811CL902-2011CL902-2111CL902-2511CL902-262810.1542000.0021230.0022110.0000370.2825410.000021-8.2-2.40.710391454-0.930.2827290.0029360.0036380.0000520.2827460.000029-0.94.61.07711010-0.890.4356340.0126140.0054520.0001320.2826580.000034-4.01.11.29551229-0.840.3564470.0143570.0046410.0001470.2825650.000035-7.3-2.01.210771429-0.860.4238330.0044660.0057390.0000410.2827020.000032-2.52.61.18921133-0.830.4765010.0050430.0062350.0000400.2824910.000039-10.0-4.91.412501613-0.810.5604190.0051810.0068510.0000600.2827830.0000450.45.31.6787964-0.790.1546070.0040880.0019580.0000440.2825460.000028-8.0-2.21.010251439-0.940.0909390.0030520.0013040.0000270.2825710.000025-7.1-1.20.99721376-0.960.2052940.0108760.0025620.0001220.2826730.000029-3.52.21.08551160-0.920.2119640.0017370.0027350.0000180.2826270.000035-5.10.61.29271265-0.920.1823360.0037590.0025510.0000410.2826110.000029-5.70.01.09461299-0.920.0731000.0010740.0009530.0000140.2825660.000023-7.3-1.30.89691382-0.970.3782160.0131800.0046110.0001360.2827160.000037-2.03.31.38401089-0.860.2790680.0047180.0036860.0000740.2825380.000045-8.3-2.81.610881478-0.890.2036790.0024390.0028490.0000380.2826000.000032-6.1-0.41.19701328-0.910.1783200.0021280.0023060.0000190.2826370.000030-4.81.01.19021240-0.930.1969610.0012030.0026400.0000100.2825510.000032-7.8-2.11.110361435-0.9211CL903-111CL903-211CL903-311CL903-711CL903-911CL903-1011CL903-1111CL903-1211CL903-1311CL903-1711CL903-1811CL903-2011CL903-2111CL903-2211CL903-2311CL903-2411CL903-2511CL903-2611CL903-272890.3051990.0039530.0035310.0000380.2825740.000048-7.0-1.31.710291392-0.890.1972460.0030320.0021880.0000400.2826390.000028-4.71.21.08971229-0.930.4426640.0132740.0047160.0001360.2826890.000039-2.92.51.48851147-0.860.2168100.0068900.0023610.0000740.2824270.000043-12.2-6.31.512091706-0.930.1589550.0011950.0019730.0000150.2826280.000029-5.10.91.09061250-0.940.1539750.0016550.0019550.0000220.2825510.000029-7.8-1.81.010171423-0.940.1359140.0043150.0017980.0000540.2825320.000025-8.5-2.50.910401464-0.950.6810850.0062930.0088610.0000810.2828710.0000463.58.21.6684787-0.730.7222490.0059830.0090500.0000600.2827700.000045-0.14.51.68701017-0.730.2283430.0087630.0027490.0000990.2826420.000030-4.61.21.19061229-0.920.3979460.0068970.0049800.0000690.2826390.000048-4.70.71.79701262-0.850.6500220.0053740.0075670.0000810.2827960.0000450.95.81.6782939-0.770.4609460.0113110.0054080.0001120.2828030.0000391.16.41.4721898-0.840.2596910.0033630.0033400.0000560.2825900.000039-6.4-0.71.49981352-0.900.3367590.0141610.0042680.0001480.2825880.000034-6.5-1.01.210281367-0.870.1092150.0005380.0014760.0000050.2826040.000022-5.90.20.89281298-0.960.1818950.0030870.0023020.0000460.2826020.000026-6.0-0.10.99541314-0.930.1866900.0051070.0021590.0000660.2825340.000030-8.4-2.51.010481465-0.930.0460000.0020080.0006440.0000230.2825320.000020-8.5-2.30.710091451-0.98

续表3

Continued Table 3

样品号t(Ma)176Yb177Hf2σ176Lu177Hf2σ176Hf177Hf2σεHf(0)εHf(t)2σtDM1(Ma)tDM2(Ma)fLu/Hf(s)11CL904-111CL904-211CL904-411CL904-511CL904-611CL904-711CL904-911CL904-1011CL904-1111CL904-1411CL904-1611CL904-1711CL904-1811CL904-2311CL904-2411CL904-2511CL904-262920.3233360.0012650.0035810.0000130.2828400.0000422.48.11.5627790-0.890.2729290.0071840.0031290.0000730.2827190.000044-1.93.91.58001059-0.910.2840520.0036630.0034220.0000390.2827010.000041-2.53.21.58341103-0.900.1695500.0010120.0020570.0000040.2825620.000019-7.4-1.40.710041398-0.940.3955450.0070340.0043960.0000770.2828740.0000353.69.21.2588723-0.870.0982420.0010340.0011360.0000100.2825240.000019-8.8-2.60.710341473-0.970.1826570.0017480.0020680.0000150.2825680.000033-7.2-1.21.29951384-0.940.1864480.0016070.0021600.0000090.2825050.000031-9.5-3.51.110901529-0.930.3493980.0069080.0046400.0000810.2825970.000042-6.2-0.71.510261351-0.860.1542530.0024800.0021760.0000410.2826950.000040-2.73.31.48141100-0.930.1780300.0015110.0023410.0000180.2827060.000040-2.33.61.48021078-0.930.2217840.0006700.0027640.0000140.2826290.000033-5.10.81.29261258-0.920.3799240.0053660.0049980.0000660.2827930.0000390.86.21.4727913-0.850.3532060.0111440.0043550.0001430.2827260.000040-1.63.91.48191058-0.870.3965740.0039290.0048230.0000390.2828490.0000362.78.21.3637787-0.850.2891840.0051560.0036560.0000460.2827560.000037-0.65.11.3756981-0.890.1169170.0007490.0014870.0000140.2825710.000034-7.1-1.01.29761372-0.96

图10 波孜果尔A型花岗岩类锆石206Pb/238U-εHf(t)和εHf(t)同位素组成图Fig.10 206Pb/238U-εHf(t) and εHf(t) isotopic compositions diagrams of A-type granitoids from Boziguoer

前已述及,有关南天山古洋盆闭合的(碰撞造山)时间存在较大的争议。A型花岗岩的发育是造山运动结束的标志(张增杰,2003),碱性侵入岩带的发育代表了伸展的构造背景(刘楚雄等,2004;童英等,2010)。姜常义等(1999)认为发生于早二叠世的伸展作用不限于南天山,而是涉及到更大的区域。黄河(2010)认为早石炭世时南天山洋因塔里木板块和哈萨克斯坦板块的碰撞而最终闭合,到二叠纪时,南天山地区已经进入后碰撞晚期,由于应力释放出现伸展拉张环境。波孜果尔A型花岗岩类岩体形成时代287.7~291.6Ma,属于早二叠纪,表明在此之前南天山古洋盆已经闭合,碰撞造山已经结束,表明碰撞最晚发生在晚石炭世,早二叠纪已经进入后碰撞演化阶段。另外目前获得的该碱性岩带上依兰里克岩体的黑云霞石歪长伟晶岩单颗粒年龄为273Ma,黑英山克其克果勒岩体中的霓霞正长岩单颗粒锆石U-Pb年龄为275Ma、霓霞岩全岩U-Pb年龄为266.5Ma,塔木西岩体的黑云霓辉花岗岩单颗粒锆石U-Pb年龄为275Ma(刘楚雄等,2004;邹天人和李庆昌,2006),说明中二叠世伸展背景下的碱性岩浆作用非常活跃。

图11 波孜果尔A型花岗岩类Zr/Hf、Nb/Ta与Mg#相关图解Fig.11 Zr/Hf-Mg# and Nb/Ta- Mg# diagrams of A-type granitoids from Boziguoer

6.2 岩浆物质来源

花岗岩中经常含有多组年龄不同的锆石,反映了花岗岩多组分来源的特征(吴福元等,2007),由于不同来源的锆石其Hf同位素组成可能不同,并且锆石的Lu-Hf同位素体系有着很高的封闭温度,且相对于U-Pb同位素体系更不易被后期流体、热事件改造(Harrisonetal., 2005),即使在麻粒岩相变质的条件下,锆石仍可保留原始的Hf同位素组分,这使得锆石可以记录岩浆源区的信息。锆石的微区176Hf/177Hf比值结合锆石U-Pb年龄和微量元素分析,可对锆石的成因演化提供重要信息,因此,我们通常采用Hf同位素的基本原理来研究锆石的来源,进而讨论其源区演化和伴随的地质过程。

花岗质岩石主要来源于地壳岩石的部分熔融(吴福元等,2007)。4个样品的68个测点中εHf(t)值有29个负值(-0.1~-6.3平均-1.8),对应的tDM2为1314~1706Ma,平均1428Ma;38个正值(0.1~9.0平均3.7),对应的tDM2为723~1302Ma,平均1072Ma(图10);tDM2有12个值<1000Ma(新元古代),56个值>1000Ma(中元古代-古元古代)。εHf(t)为负值时,对应的tDM2值普遍较大,表明锆石主要来自古元古代-中元古代古老地壳的部分熔融;εHf(t)为正值时,对应的tDM2值普遍较小,表明锆石可能含有中元古代-新元古代亏损地幔物质。以上说明波孜果尔A型花岗岩类的源区成分的复杂性,既有古元古代-中元古代古老地壳物质,也有中元古代-新元古代亏损地幔物质。11CL902反映的锆石白边的176Hf/177Hf和εHf(t)信息与其余3个样品代表的锆石黑核的信息基本一致;4个样品锆石原位微区稀土配分模式一致,总体表现出LREE亏损和HREE富集的特征,并具有明显的负Eu异常,无Ce的正负异常(Liuetal., 2013),这些都说明后期流体对锆石的改造不大,锆石Lu-Hf体系基本还属于一个封闭的环境。

A型花岗岩类具有深源浅成的特点,Green(1995)认为幔源岩浆和壳源岩浆的Nb/Ta分别约17.5和11~12之间,Dostal and Chatterjee(2000)认为球粒陨石的Zr/Hf约36;本文4个样品Nb/Ta(图11)分别为30.66、18.49、19.61和11.53,Zr/Hf(图11)分别为33.23、18.49、19.61和11.53;波孜果尔A型花岗岩类的另外24个样品(Liuetal., 2013)显示黑云母碱长正长岩(图11◇)Zr/Hf为28.74~58.04,平均38.02;霓石钠闪碱长花岗岩(图11□)中Zr/Hf为16.69~40.7,平均28;霓石钠闪石英碱长正长岩(图11○)Zr/Hf在28.31~49.73,平均38.93。Nb/Ta除一个样品7.23外都大于11,Zr/Hf除16.69、19、49.66、49.73和58.04外,其他集中在24.31~42.82,表明波孜果尔A型花岗岩类岩浆物质来源较深。邹天人和李庆昌(2006)认为与碱性岩有关的稀土矿床的物质主要来自上地幔。锆石Hf同位素数据说明波孜果尔A型花岗岩类源区岩浆是壳幔混源的。因此,本文认为波孜果尔A型花岗岩类源区岩浆是由古元古代-中元古代古老地壳和中元古代-新元古代亏损地幔物质混合而成的。

6.3 构造环境

图12 波孜果尔A型花岗岩类Yb-Ta和Y-Nb图解(据Pearce et al., 1984)syn-COLG-同碰撞型;VAG-火山弧型;WPG-板块内部型;ORG-洋中脊型;11CL901-●;11CL902-■;11CL903-▲;11CL904-◆(下同)Fig.12 Yb-Ta and Y-Nb diagrams of A-type granitoids from Boziguoer(after Pearce et al., 1984)syn-COLG-syn-Collision Granite; VAG-Volcanic Arc Granite; WPG-Within Plate Granite; ORG-Oceanic Ridge Granite; 11CL901-●; 11CL902-■; 11CL903-▲; 11CL904-◆(following the same)

图13 波孜果尔A型花岗岩类Y-Nb-Ce、Y-Nb-Ga×3的三角图解(据Eby, 1992)A1-非造山A型花岗岩;A2-造山后A型花岗岩Fig.13 Y-Nb-Ce、Y-Nb-Ga×3 diagrams of A-type granitoids from Boziguoer(after Eby, 1992)A1-Anorogenic A-type granitoids; A2-Orogenic A-type granitoids

A型花岗岩产于伸展的构造背景得到了普遍认同(Loiselle and Wones, 1979; Pitcher, 1983; Whalenetal., 1987; Maniar and Piccoli, 1989; 洪大卫等, 1995; Eby, 1990, 1992; 吴锁平等, 2007)。张明山(1997)认为南天山哈尔克山-黑英山-库米什一带晚古生代形成了拉张裂陷环境。在39°N~43°N,76°E~94°E之间的塔里木地台北缘及邻区存在一条泥盆纪-二叠纪的近EW向带状展布的碱性侵入岩带,跨越了不同的构造单元,形成于非造山的拉张环境(刘楚雄等,2004),本文研究的岩体也在这一碱性岩带上。早二叠世碱性花岗岩在南天山呈带状大面积分布,可能揭示了不同构造背景下的伸展特点(童英等,2010)。波孜果尔A型花岗岩类黑云母电子探针成分在Abdel-Rahman(1994)的FeOT-MgO-Al2O3三角图上落在非造山碱性花岗岩区,在Wones and Eugster (1965) Fe3+-Fe2+-Mg2+三端员图解上,落在Ni-NiO(NNO)线之下,暗示岩石形成于高温无水且低氧逸度的环境(刘春花等,2012)。本文4个样品在Pearceetal.(1984)的Yb-Ta和Y-Nb图解构造判别图解上落在板内花岗岩区(图12);在Eby(l990, 1992)的Y-Nb-Ce、Y-Nb-Ga×3三角图上落在A1区(图13),一般认为,A1型花岗岩原始岩浆来源于地幔并产于大陆裂谷或地幔热柱、热点环境;而A2型花岗岩岩浆起源于地壳或由岛弧岩浆派生,产于碰撞后或造山期后的张性构造环境。可见,本文研究的波孜果尔A型花岗岩类属A1型,形成于非造山的板内构造环境。

6.4 岩石成因

A型花岗岩的成因主要有交代模式、岩浆混合作用、分异模式以及部分熔融模式4种,至今仍没有统一的认识。笔者认为波孜果尔A型花岗岩类是下地壳部分熔融和地幔物质加入形成的。

波孜果尔A型花岗岩类未见幔源岩浆的暗色微粒闪长质包体,也没有其它岩浆混合现象,且本文4个样品的SiO2含量很高(68.97%~74.14%),都说明在岩浆上升侵位和形成过程中,岩浆混合程度不高,混合作用的发生主要在源区岩浆形成的过程中。

该岩体的三类岩石岩石学和地球化学特征高度一致,且锆石LA-ICP-MS206Pb/238U年龄相近,稀土元素球粒陨石标准化图(图5)上,轻稀土分异微弱,重稀土几乎未分异,证明其经历了较弱的分异作用。岩性的差异可能是源区岩浆的成分本身存在差异引起的。通过结晶分离作用产生花岗质岩石必然有比花岗质岩体大得多的基性-超基性岩出露(黄河等,2011),而实际的野外现象并非如此。

姜常义等(1999)认为在南天山、准噶尔盆地、天山山脉和塔里木盆地的许多地方,在早二叠世末期发生了弥散型大陆伸展作用,推测这些地区的壳幔边界可能发生了大规模幔源岩浆的底侵作用,并推测底侵的幔源岩浆为碱长花岗岩浆的生成提供了热源,但没有提供物源。黄河等(2010)认为二叠纪时南天山地区在伸展拉张环境下,软流圈物质上涌导致了岩石圈地幔发生部分熔融形成富碱的基性岩浆,基性岩浆底侵导致中下地壳部分熔融,可能还伴随着基性岩浆与少量酸性岩浆的混合作用。幔源岩浆不可能直接演化为花岗质岩浆(张旗,2012),本文认为波孜果尔A型花岗岩类并不是上地幔基性岩浆直接结晶分异形成的,而是在二叠纪伸展背景下,塔里木地台北缘及邻区的岩石圈发生松弛,地壳减薄,软流圈物质上涌而最终导致了大规模岩浆作用的发生(即二叠纪大火成岩省的发育)。软流圈物质上涌使得上地幔较高的部位发生部分熔融,生成碱性玄武岩浆,碱性玄武质岩浆在底侵上升的过程中,通过分异作用,在下地壳附近形成碱性中性岩(粗面岩或安粗岩)(在玄武质岩浆上升时已脱水)。在之后的软流圈热源继续上涌时,碱性中性岩和古老地壳作为波孜果尔A型花岗岩类的源岩,其中低熔点组分发生部分熔融,并在岩浆源区发生岩浆混合作用,形成A型花岗质岩浆,A型花岗质岩浆上升侵位形成波孜果尔A型花岗岩类,估算的岩浆平均温度TREE=832℃和TZr=839℃(刘春花等,2012)可以大致代表岩浆源区温度,软流圈岩浆和幔源岩浆分别为A型花岗质岩浆的生成提供了热源和部分物源。

7 结论

(1)波孜果尔A型花岗岩类为A1型花岗岩,包括霓石钠闪石英碱长正长岩、霓石钠闪碱长花岗岩、黑云母碱长正长岩。各岩石的稀土总量较高,轻稀土富集,重稀土亏损,强烈的Eu负异常,微弱的Ce负异常,稀土元素球粒陨石标准化分布型式为“右倾海鸥型”,富集Nb、Ta、Zr、Hf等高场强元素,亏损Ba、K、Sr等大离子亲石元素。

(2)波孜果尔A型花岗岩体位于塔里木地台北缘及邻区泥盆纪-二叠纪近EW向带状展布的碱性侵入岩带上,形成于非造山的板内构造环境,与区域上该时期伸展的构造背景相一致。

(3)3种岩性的岩石为同期岩浆事件的产物,岩体形成时代为287.7~291.6Ma,属于早二叠世,南天山古洋盆闭合(碰撞造山)最晚发生在晚石炭世,早二叠世已经进入后碰撞演化阶段。岩体形成后,经历了同一期次(279.1~282Ma)的热液流体改造。

(4)波孜果尔A型花岗岩类是古老的下地壳部分熔融和亏损地幔物质加入形成的。在早二叠世伸展背景下,地壳减薄,软流圈物质上涌使得上地幔较高的部位发生部分熔融,生成碱性玄武岩浆,通过分异作用在下地壳附近形成碱性中性岩,在之后的软流圈热源继续上涌时,碱性中性岩和古老地壳作为源岩,其中低熔点组分发生部分熔融,并在源区进行岩浆混合,形成A型花岗质岩浆,A型花岗质岩浆上升侵位形成A型花岗岩类,岩浆源区温度大致在832~839℃。软流圈岩浆和幔源岩浆分别为A型花岗质岩浆的生成提供了热源和部分物源。

致谢中国科学技术大学壳幔物质与环境重点实验室侯振辉老师,中国地质科学院矿产资源研究所郭春丽老师、侯可军老师在实验过程中给予了大量的指导和帮助;中国地质调查局西安地质调查中心徐学义研究员和陈隽璐高级工程师对论文提出了宝贵的修改意见;在此一并表示衷心的感谢!

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