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西藏邦铺斑岩钼(铜)多金属矿床侵入岩锆石微量元素特征及其地质意义

2015-12-05李壮王立强张忠罗茂澄冷秋锋康浩

地球科学与环境学报 2015年6期
关键词:微量元素西藏

李壮 王立强 张忠 罗茂澄 冷秋锋 康浩然 孟展

摘 要:西藏邦铺斑岩钼(铜)多金属矿床位于冈底斯成矿带东段,矿区侵入岩期次和类型多样,主要包括成矿前期黑云母二长花岗岩、成矿期闪长玢岩和二长花岗斑岩及成矿后期石英二长斑岩和辉绿玢岩等5种。在系统总结前人研究成果基础上,利用LA-ICP-MS技术获得了矿区闪长玢岩岩体锆石微量元素特征。结果表明:邦铺矿区侵入岩锆石类型主要包括岩浆锆石和热液锆石;5种侵入岩球粒陨石标准化稀土元素配分模式显示具相似的特征,曲线左倾,整体较陡,表现为明显的Ce正异常和Eu负异常,轻稀土元素亏损,重稀土元素富集。通过锆石Ti含量地质温度计计算出侵入岩锆石结晶温度,显示出成矿期和成矿后期侵入岩体岩浆来源于在水近饱和条件下发生的部分熔融。根据锆石中特定微量元素和稀土元素特征变化,各侵入岩锆石均是从陆壳岩浆中结晶分异形成的,与锆石相对应的寄主岩石均形成于陆壳环境。

关键词:多金属矿床;侵入岩;微量元素;成因类型;锆石Ti含量地质温度计;结晶环境;西藏

中图分类号:P595 文献标志码:A

Zircon Trace Element Characteristics of Intrusions in Bangpu Porphyry

Mo(Cu) Polymetallic Deposit of Tibet and Their Geological Significance

LI Zhuang1, WANG Li-qiang2, ZHANG Zhong3, LUO Mao-cheng4, LENG Qiu-feng1,

KANG Hao-ran2, MENG Zhan1

(1. School of Earth Sciences, Chengdu University of Technology, Chengdu 610059, Sichuan, China;

2. Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China;

3. Tibet Tianyuan Mineral Exploration Co. Ltd., Shigatse 857000, Tibet, China;

4. School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China)

Abstract: The Bangpu porphyry Mo(Cu) polymetallic deposit is located in the eastern part of Gangdise metallogenic belt. The period and type of intrusive rocks in this area are various, mainly including biotite monzonitic granite during the early mineralization, diorite porphyrite and monzonitic granite during the mineralization, and quartz monzonitic porphyry and allgovite during the late mineralization. On basis of summarizing the previous research, the characteristics of zircon trace elements from diorite porphyrite in Bangpu deposit were studied by the LA-ICP-MS technology. The results show that the types of zircon from intrusive rocks mainly include magmatic and hydrothermal zircons; chondrite-normalized REE patterns of the above five types of intrusive rocks are similar, and the curves are left deviation and steep with the characteristics of positive Ce anomaly, negative Eu anomaly, LREE enrichment and HREE depletion. Crystallization temperatures of zircon from the above five intrusive rocks calculated by the Ti-in-zircon geothermometer show that the magma of intrusive rock during the mineralization and late mineralization are derived from the partial melting under the condition of nearly water saturation. According to the characteristics of special zircon trace element and REE, the zircons of the above five intrusive rocks are crystallized and differentiated from continental crust magma, and the host rocks corresponding to zircon form in the continental crust environment.

Key words: polymetallic deposit; intrusive rock; trace element; genetic type; Ti-in-zircon geothermometer; crystallization environment; Tibet

0 引 言

西藏冈底斯斑岩成矿带位于雅鲁藏布江碰撞结合带北侧,其形成和发展过程(尤其南冈底斯)记录了雅鲁藏布江特提斯洋壳向北俯冲,直至印度与亚洲大陆碰撞、碰撞后伸展过程的岩浆和构造演化事件[1-2],是青藏高原近年来发现的最具经济价值的成矿带之一[3-5]。目前,该成矿带上已发现10余处大型、超大型斑岩铜钼矿床(包括甲玛、驱龙、雄村、吉如等)[6-10]和许多矽卡岩型铅锌多金属矿床(包括蒙亚啊、洞中拉、拉屋等)[11-12],几乎都形成于青藏高原陆-陆碰撞之后(中新世)地壳伸展环境[13]或碰撞挤压-隆升伸展转换过渡构造环境[14-15]。西藏邦铺斑岩钼(铜)多金属矿床位于墨竹工卡县尼玛江热乡,隶属于冈底斯—念青唐古拉板片东段中部,是甲玛—驱龙铜钼多金属矿集区内一个十分重要的超大型斑岩钼(铜)矿床。自2007年起,该矿区详细的地质勘探和研究工作才正式拉开帷幕,并逐步细致深入。近年来,研究主要集中在该矿床成岩成矿时代[16-17]、成矿流体性质[18-19]、流体包裹体及同位素[20]、矿床成因[21-22]、成矿物质来源[23-25]、岩浆演化过程[26-27]等方面。

锆石是自然界一种最常见的副矿物,普遍存在于岩浆岩、沉积岩和变质岩中,并有着极其稳定的晶体结构,能在复杂的地质环境中完好地保存下来[28-29],且锆石中稀土元素和微量元素对岩浆源区性质和岩浆形成过程具有非常重要的指示作用[30]。基于此,笔者在前人研究基础上,通过系统总结邦铺矿区5种不同类型侵入岩的锆石微量元素和稀土元素特征,对不同类型侵入岩锆石成因类型进行探讨,并对侵入岩形成环境做出合理解释。

1 区域地质

自中生代以来,特提斯构造域就不断演化,并完好记录了特提斯洋盆裂解→扩张→俯冲→碰撞的完整演化过程,增生造山与碰撞造山连续发育,成矿作用丰富多彩[31]。古新世初,非洲大陆的北移造就了特提斯主体洋盆的闭合[32],而特提斯洋盆的闭合造成了印度、阿拉伯板块与欧亚大陆的碰撞[33]。Zhu等研究认为冈底斯带是与印度—欧亚大陆碰撞有关的新生代造山带[34],且经历了新生代主碰撞陆陆聚合(41~65 Ma)、晚碰撞构造转换(26~40 Ma)和后碰撞地壳伸展(0~25 Ma)等过程,最终演变为伸展拉张的陆内构造环境[35-38]。

研究区位于冈底斯—念青唐古拉板片南北界及雅鲁藏布江缝合带与班公湖—怒江结合带之间,东西两侧分别与西南“三江”构造带和帕米尔—喀喇昆仑构造带相连(图1)。该区侵入岩复杂多样,从基性的辉长岩到酸性的花岗岩都有发现,呈EW向带状展布,时间上可分为晚三叠世—侏罗纪(152~205 Ma)、白垩纪(80~109 Ma)、古新世—始新世(41~65 Ma)和渐新世—中新世(13~33 Ma)4个阶段[39],空间上可分为北带、中带和南带3个亚带[40],从而使研究区发生不同程度的深部地球构造圈层物质、能量交换等过程,并伴随着强烈的流体作用和成矿作用发生。

图件引自文献[27],有所修改

图1 西藏冈底斯成矿带东段主要铜多金属矿床分布

Fig.1 Distribution of Main Copper-polymetallic Deposits in the East Section of Gangdise Metallogenic Belt, Tibet

2 矿床地质

邦铺矿区地层出露较为简单,主要包括矿区南部下二叠统洛巴堆组(P1l)、矿区中部和北部的古近系典中组(E1d)及第四系(Q),典中组与洛巴堆组主要呈逆冲断层接触(图2)。矿区构造以断层为主,包括正断层和逆断层,另发育少量节理构造,其中矿区南部正断层为矽卡岩型铅锌矿的导矿容矿构造。矿区岩浆活动频繁,发育不同类型和期次的侵入岩体,包括中西部成矿期的二长花岗斑岩(年龄为(16.23±0.19)Ma)[41]、中西部成矿后期的石英二长斑岩(年龄为(15.43±0.12)Ma)[42]、北部成矿后期的基性辉绿岩(年龄为(15.21±0.29)Ma)[43]、中东部成矿期的闪长玢岩(年龄为(14.96±0.16)Ma)[17]以及南部成矿前期的黑云母二长花岗岩(年龄为(62.1±1.9)Ma)[44]。主成矿期的侵入岩体成岩年龄与早期辉钼矿成矿年龄((15.32±0.79)Ma)[16]较一致。在时间上,黑云母二长花岗岩体最早形成,其次为二长花岗岩体、石英二长斑岩和辉绿玢岩,最后为闪长玢岩;在空间上,各岩体均为隐伏岩体,相互之间无明显的穿插关系。二长花岗斑岩和闪长玢岩分别是钼、铜矿体的主要成矿岩体;石英二长斑岩与铅锌矿体成矿关系密切;黑云母二长花岗岩和辉绿玢岩不含矿。

蚀变主要由邦铺矿区中西部二长花岗斑岩岩体中心向上显示出典型的钾硅酸盐化带、黄铁绢云岩化带(局部泥化带)及青磐岩化蚀变带。钾硅酸盐化主要发育于二长花岗斑岩和闪长玢岩中;绢云母化主要发育于二长花岗斑岩内;青磐岩化主要发育于古近系典中组中,表现为广泛发育的绿帘石、绿泥石矿物组合。

矿化类型主要有邦铺矿区中部钼(铜)矿化、铜(钼)矿化和铅锌矿化。钼矿化主要发育于中西部二长花岗斑岩中;铜矿化主要发育于中东部闪长玢岩中;铅锌矿化发育于南部下二叠统洛巴堆组矽卡岩中及矽卡岩与大理岩的接触带内。

邦铺矿区矿石矿物成分较简单,主要有辉钼矿,黄铜矿,斑铜矿,黄铁矿及少量方铅矿、闪锌矿、磁铁矿、黝铜矿等;脉石矿物主要包括长石类、石英、黑云母,次为绢云母、绿泥石、绿帘石、方解石和硬石膏等。矿石结构主要包括他形晶结构、半自形—自形晶结构、交代结构、填隙结构等。矿石构造主要有浸染状构造、脉状构造,其次为团块状、条带状、网脉状构造等。

3 分析测试

用于分析测试的样品为闪长玢岩,采自平硐PD5205中,采样位置见图2。样品较新鲜,表面呈灰黑色至灰白色,具有斑状结构和块状构造。岩石斑晶主要由斜长石组成,局部可见少量黑云母斑晶;基质由斜长石及角闪石组成。斜长石多为半自形至自形,粒径大小不等,蚀变相对较弱。

锆石的原位微量元素分析在西北大学大陆动力学国家重点实验室进行。将采集的各侵入岩样品经人工破碎,采用磁选和浮选方法分选出锆石后,在双目镜下挑选出晶形和色泽较好、无裂隙、无包裹体的锆石颗粒,粘在双面胶上并用环氧树脂固定,待环氧树脂充分固化后,将锆石靶表面抛光,然后进行锆石

图件引自文献[41],有所修改

图2 邦铺矿区地质图

Fig.2 Geological Map of Bangpu Deposit

颗粒内部结构分析(反射光和透射光照相和阴极发光图像研究),最后进行锆石LA-MC-ICP-MS微量元素测定。LA-ICPMS分析设备为德国MicroLas公司生产的GeoLas200M激光剥蚀系统与Elan6100DRCICP-MS联机,激光束斑直径为30 μm,激光剥蚀深度为20~40 μm,采用的标准锆石为91500。锆石阴极发光(CL)图像采集设备为英国Gatan公司生产的场发射扫描电子显微镜附属的Gatan MonoCL3+阴极发光装置系统,图像采集工作条件为240 μA分析电流、10 kV电压。

4 分析结果

4.1 锆石特征

闪长玢岩样品中选出的锆石透明度一般,颜色少数较深,呈灰黑色,多为浅灰白色或无色(图3)。锆石颗粒大小不等,粒径大小多在70~150μm之间,个别颗粒较大。锆石呈长柱状、板状及浑圆状和不规则状,晶体长宽介于1∶1~2∶1。锆石自形程度较好,局部发育完整的柱面和锥面。其余4种侵入岩体锆石阴极发光图像表明锆石透明度均较好,大多呈浅灰白色,锆石颗粒粒径大小不等,局部发生破碎,大小相对变化。锆石自形程度较高,多呈长柱状、板柱状,晶体长宽不一,多发育较好的柱面和锥面。锆石具有明显的震荡环带和扇形环带,环带较窄[42-43,45],说明其在低温条件下结晶,微量元素扩散速度慢[46]。

图3 闪长玢岩锆石阴极发光图像

Fig.3 CL Images of Zircons from Diorite Porphyrite

4.2 稀土元素和微量元素分析

锆石微量元素和稀土元素分析结果见表1。闪长玢岩中Th含量(质量分数,下同)为(146.87~547.84)×10-6,平均值为326.53×10-6;U含量为(251.23~1 668.37)×10-6,平均值为720.32×10-6;w(Th)/w(U)值较小,为0.20~1.51。其余4种侵入岩体中w(Th)/w(U)值均较大,平均值均大于1。闪长玢岩中所有锆石测点稀土元素总含量为(427.06~964.13)×10-6,平均值为625.19×10-6;wLREE/wHREE值为0.03~0.44,平均值为0.13;Ce呈正异常(0.96~86.50),平均值为12.25;Eu呈负异常(0.12~0.35),平均值为0.24。黑云母二长花岗岩所有锆石测点稀土元素总含量为(599.22~2 773.13)×10-6,平均值为1 514.60×10-6;wLREE/wHREE值为0.01~0.08,平均值为0.03;Ce呈正异常(7.40~124.17),平均值为70.56;Eu呈负异常(0.12~0.57),平均值为0.34。二长花岗斑岩所有锆石测点稀土元素总含量为(348.83~649.26)×10-6,平均值为495.44×10-6;wLREE/wHREE值为0.11~0.24,平均值为0.14;Ce呈正异常(2.96~105.01),平均值为52.29;Eu呈负异常(0.31~0.37),平均值为0.35。

表1 侵入岩微量元素和稀土元素分析结果

Tab.1 Analysis Results of Trace and Rare Earth Elements from Intrusive Rocks

注:闪长玢岩数据为本文数据;石英二长斑岩数据引自文献[42];辉绿玢岩数据引自文献[43];二长花岗斑岩和黑云母二长花岗岩数据引自文献[45];测点号以A开头的样品为闪长玢岩,以B开头的为黑云母二长花岗岩,以C开头的为二长花岗斑岩,以D开头的为石英二长斑岩,以E开头的为辉绿玢岩;w(·)为元素或化合物含量;wREE为稀土元素总含量;wLREE为轻稀土元素总含量;wHREE为重稀土元素总含量;δ(Eu)为Eu异常;w(·)N为元素含量球粒陨石标准化后的值。石英二长斑岩所有锆石测点稀土元素总含量为(390.20~769.08)×10-6,平均值为517.30×10-6;wLREE/wHREE值为0.13~0.60,平均值为0.21;Ce呈正异常(1.54~132.63),平均值为43.71;Eu呈负异常(0.35~0.47),平均值为0.38。辉绿玢岩所有锆石测点稀土元素总含量为(416.43~1 197.24)×10-6,平均值为606.30×10-6;wLREE/wHREE值为0.10~0.85,平均值为0.19;Ce呈正异常(1.16~98.80),平均值为46.59;Eu呈负异常(0.26~0.42),平均值为0.36。

ws为样品含量;wc为球粒陨石含量;球粒陨石标准化值引自文献[47];图中线条对应不同样品

图4 侵入岩球粒陨石标准化稀土元素配分模式

Fig.4 Chondrite-normalized REE Patterns for Intrusive Rock

邦铺矿区5种不同类型侵入岩中,闪长玢岩Eu异常平均值最小,球粒陨石标准化稀土元素配分模式总体比较相似,曲线较陡[图4(a)~(e)],稀土元素含量从La至Lu之间迅速增加。除闪长玢岩外,其余4种侵入岩体均表现为明显Ce正异常和Eu负异常,轻稀土元素亏损,而重稀土元素富集。不同类型侵入岩球粒陨石标准化稀土元素配分模式平均值[图4(f)]也显示出具典型的Ce正异常和Eu负异常,轻稀土元素亏损,重稀土元素富集。

5 讨 论

5.1 锆石成因类型

锆石w(Th)/w(U)值常被用作判断其成因的标志,以区分岩浆、变质和热液环境。Th4+离子半径与Zr4+离子半径非常接近,电价相同,很容易进入锆石中;又因U4+离子半径比Th4+小,更接近Zr4+,所以锆石中U含量常高于Th[48]。邦铺矿区除闪长玢岩外,其余4种侵入岩体中锆石w(Th)/w(U)值都较高,平均值均大于1。黑云母二长花岗岩中锆石w(Th)/w(U)值为0.27~2.17,平均值为1.16;二长花岗斑岩中锆石w(Th)/w(U)值为1.10~1.34,平均值为1.26;石英二长斑岩中锆石w(Th)/w(U)值为1.09~1.46,平均值为1.29;辉绿玢岩中锆石w(Th)/w(U)值为1.17~1.65,平均值为1.37。不同成因锆石的Th、U含量及其比值差异较大,一般岩浆锆石w(Th)/w(U)值大于0.4,热液锆石w(Th)/w(U)值小于0.1[44,49-51]。据此观点可知,上述4种侵入岩锆石应属典型的岩浆锆石类。然而,Hidaka等研究发现一些典型岩浆锆石也会具有非常低的w(Th)/w(U)值[52],因此,不能仅仅根据锆石的w(Th)/w(U)值来区分锆石的成因类型。此外,赵振华通过研究锆石微量元素地球化学特征也证实:w(Th)/w(U)值主要反映的是母体的特点或形成时局部的甚至包括流体源区的化学环境[43]。因此,当用w(Th)/w(U)值来判定锆石成因时必须谨慎,应当与其他综合地球化学指标相结合最终作出合理判断。

Ferry等研究发现,利用锆石(Sm/La)N-(Lu/Gd)N图解和δ(Ce)-δ(Eu)图解可以比较准确地区分热液锆石和岩浆锆石[53]。据此方法对邦铺矿区侵入岩锆石作(Sm/La)N-(Lu/Gd)N图解和δ(Ce)-δ(Eu)图解。闪长玢岩锆石在图5(a)中有10个测点落入热液锆石区域,在图5(b)中有9个测点落入热液锆石区域,据此可判断闪长玢岩锆石主要为热液锆石类;二长花岗斑岩锆石在图5(a)中有10个测点落入岩浆锆石区域,在图5(b)中同样有10个测点落入岩浆锆石区域,因此,二长花岗斑岩锆石主要属于岩浆锆石类;黑云母二长花岗岩锆石在图5(a)中有10个测点落入岩浆锆石区域,在图5(b)中有11个测点落入岩浆锆石区域,因此,黑云母二长花岗岩锆石应主要为岩浆锆石类;石英二长斑岩锆石在图5(a)中有9个测点落入岩浆锆石区域,在图5(b)中同样有9个测点落入岩浆锆石区域,据此可推断此类锆石亦主要为岩浆锆石类;基性辉绿玢岩锆石在图5(a)中有10个测点落在岩浆锆石区域,在图5(b)中有9个测点落在岩浆锆石区域,据此可判定辉绿玢岩锆石主要为岩浆锆石类。

此外,(Sm/La)N-La图解和δ(Ce)-(Sm/La)N图解也可以很好地区分岩浆锆石和热液锆石。图6(a)中黑云母二长花岗岩锆石有8个测点落在岩浆锆石区域,3个测点落在岩浆锆石边界区域内,图6(b)中有8个测点落在岩浆锆石区域,3个测点落在岩浆锆石边界区域,因此,此类锆石主要属于岩浆锆石类;图6(a)中辉绿玢岩锆石有5个测点落在岩浆锆石区域,5个测点落在岩浆锆石边界区域内,图6(b)中有6个测点落在岩浆锆石区域,2个测点落在岩

底图引自文献[53]

图5 侵入岩(Sm/La)N-(Lu/Gd)N图解和δ(Ce)-δ(Eu)图解

Fig.5 Diagrams of (Sm/La)N-(Lu/Gd)N and δ(Ce)-δ(Eu) for Intrusive Rocks

底图引自文献[54]

图6 侵入岩(Sm/La)N-La图解和δ(Ce)-(Sm/La)N图解

Fig.6 Diagrams of (Sm/La)N-La and δ(Ce)-(Sm/La)N for Intrusive Rocks

浆锆石边界区域, 2个测点靠近岩浆锆石区域,因此,辉绿玢岩锆石主要为岩浆锆石类;二长花岗斑岩锆石在图6(a)中有5个测点落在岩浆锆石区域,4个测点落在岩浆锆石边界区域,在图6(b)中有8个测点落在岩浆锆石区域内,因此,二长花岗斑岩锆石主要属岩浆锆石类;石英二长斑岩锆石在图6(a)中有4个测点落在岩浆锆石区域,2个测点落在过渡区域(更靠近岩浆锆石区域),在图6(b)中有7个测点落入岩浆锆石区域,3个测点落入岩浆锆石边界区域,因此,石英二长斑岩锆石主要属于岩浆锆石类;闪长玢岩锆石在图6(a)中有6个测点落入热液锆石边界区域内,1个测点落在热液锆石区域内, 5个测点靠近热液锆石区域,在图6(b)中有6个测点落在热液锆石区域,2个测点落在热液锆石边界区域,因此,闪长玢岩锆石主要属热液锆石类。

综上所述,邦铺矿区闪长玢岩锆石主要为热液锆石类,其余4种侵入岩体(包括黑云母二长花岗岩、二长花岗斑岩、辉绿玢岩和石英二长斑岩)中锆石均主要属岩浆锆石类。

5.2 锆石Ti含量地质温度计

由于锆石自身的高度稳定性及锆石中Ti含量(w(Ti))与地质温度(T)密切相关,其关系呈对数线性变化,根据此种关系可以大致判断岩浆当时的结晶温度。近年来,Harrison等提出的锆石Ti含量地质温度计[55]引起了普遍关注。Watson等提出的锆石Ti含量地质温度计计算公式为:T=(5 080±30)/[(6.01±0.03)-lg w(Ti)]-273[56]。由于此温度计所表现出的简单实用性,引起了许多研究者的关注并被广泛应用,且Zheng等已经尝试着将此温度计应用于不同成因的锆石中[57]。

应用上述锆石Ti含量地质温度计计算出邦铺矿区不同类型侵入岩体中锆石的结晶温度(表2)。从表2可以看出,闪长玢岩锆石结晶温度为399.27 ℃~614.02 ℃,平均为546.92 ℃;黑云母二长花岗岩锆石结晶温度为638.65 ℃~905.12 ℃,平均为755.86 ℃;二长花岗斑岩锆石结晶温度为639.24 ℃~978.74 ℃,平均为732.21 ℃;石英二长斑岩锆石结晶温度为681.12 ℃~721.67 ℃,平均为691.11 ℃;辉绿玢岩锆石结晶温度为604.44 ℃~720.70 ℃,平均为691.59 ℃。

Richards等研究表明,含矿岩浆富水是形成斑岩矿床的关键因素[58-59]。周金胜等通过大量计算得到,绝大部分高温条件(大于750 ℃)下形成的岩浆岩锆石结晶温度均落在湿花岗岩固相线以上,低的锆石结晶温度(如680 ℃)表明岩浆经历了在水近饱和条件下发生的熔融过程[60]。据邦铺矿区侵入岩锆石结晶温度计算结果(表2)可以看出:二长花岗斑岩、辉绿玢岩和闪长玢岩中70%的锆石结晶温度小于700 ℃;石英二长斑岩中近90%的锆石结晶温度小于700 ℃;黑云母二长花岗岩锆石平均结晶温度大于750 ℃。因此,推测邦铺矿床成矿前期的侵入岩体岩浆来源于缺水条件下发生的部分熔融,而成矿期和成矿后的侵入岩体岩浆来源于在水近饱和条件下发生的部分熔融。

表2 侵入岩锆石Ti含量地质温度计计算结果

Tab.2 Calculation Results of Intrusive Rocks by the Means of Ti-in-zircon Geothermometer

注:T1~T5分别为黑云母二长花岗岩、二长花岗斑岩、闪长玢岩、石英二长斑岩和辉绿玢岩地质温度。5.3 岩浆结晶环境

锆石中微量元素可以很好地记录并揭示出寄主岩石的结晶环境。将锆石中微量元素U和稀土元素相结合,绘制U-Yb、U/Yb-Y图解;这2种图解可以明显区分洋壳成因锆石和陆壳成因锆石,进而区分锆石是从大陆还是从洋壳岩浆结晶,从而可以很好地揭示侵入岩锆石的源区[30]。依据此方法对邦铺矿区5种侵入岩锆石数据进行U-Pb和U/Yb-Y投图(图7)。从图7可以看出,邦铺矿区5种侵入岩所有锆石测点均落入陆壳区域,从而可以推测邦铺矿区5种侵入岩体锆石结晶环境为陆壳环境。

底图引自文献[30]

图7 侵入岩U/Yb-Y图解和U-Yb图解

Fig.7 Diagrams of U/Yb-Y and U-Yb for Intrusive Rocks

在区域上,整个冈底斯花岗岩具有年轻的模式年龄,岩石圈地幔部分熔融后又发生底侵[61]。地幔物质透过岩浆底侵-岩浆混合作用大量进入地壳,黑云母二长花岗岩与其同时代的林子宗群火山岩具有相同的起源,均为俯冲板片及卷入的陆源沉积物部分熔融形成[40]。大约在16 Ma,已经断裂且下沉的洋壳残体在下沉过程中不断被软流圈熔融形成二长花岗斑岩[45];石英二长斑岩可能为晚期含矿二长花岗斑岩的母岩浆演化而成[42]。岩浆源区带混染了越来越多的古老地壳物质成分,且成矿作用一般发生在岩浆-热液活动中后期。

周雄等通过锆石Hf同位素研究,发现石英二长斑岩Hf同位素组成特征[18]与钼(铜)矿区二长花岗斑岩锆石Hf同位素组成特征[45]一致,且在区域及相邻矿区也得到了证实。甲玛含矿岩浆在结晶分异过程中受到了外来物质的混染[62],驱龙含矿斑岩岩浆源区主要为沉积物熔体对岩石圈地幔交代而成,岩石圈地幔部分熔融而后发生底侵作用[61]。最终证实源区物质主要来源于年轻地幔的组分,且在后期岩浆侵位过程中受到了古老地壳物质的混染,从而结晶形成于陆壳区域。

综上所述,邦铺矿区侵入岩岩浆最初来源于地幔成分,随着部分熔融作用而后发生底侵作用,并在后期岩浆侵位上升过程中同时受到古老地壳物质的混染,并最终于陆壳环境结晶分异成不同类型的侵入岩体。

6 结 语

(1)西藏邦铺矿区闪长玢岩、二长花岗斑岩、黑云母二长花岗岩、石英二长斑岩及辉绿玢岩等5种不同类型的侵入岩稀土元素球粒陨石标准化配分模式总体呈现相似的特征,曲线均较陡。除闪长玢岩外,其余4种侵入岩锆石均表现为明显的Ce正异常和Eu负异常,轻稀土元素亏损,重稀土元素富集。

(2)邦铺矿区各侵入岩中锆石成因不尽相同,锆石成因类型主要包括岩浆锆石、热液锆石两种类型。其中,闪长玢岩中锆石主要为热液锆石类,其余4种侵入岩体中锆石主要属岩浆锆石类。

(3)根据锆石Ti含量地质温度计计算得到的结晶温度显示出矿区含矿二长花岗斑岩、闪长玢岩及石英二长斑岩体为在水近饱和条件下发生部分熔融形成的。U/Y-Y图解和U-Yb图解表明矿区侵入岩体主要结晶形成于近水饱和的陆壳环境中。

野外地质调查过程中得到了西藏天仁矿业公司的大力支持,试验过程中得到了西北大学大陆动力学国家重点实验室第五春荣、柳小明等老师的帮助,在此一并表示感谢!

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