2019, Vol. 35
2. 中国地质大学(北京)地球科学与资源学院, 北京 100083;
3. 成都理工大学地球科学学院, 成都 610059;
4. 西藏自治区地质矿产勘查开发局第六地质大队, 拉萨 851400
2. School of Earth Sciences and Resources, China University of Geosciences, Beijing 10008;
3. School of Earth Sciences, Chengdu University of Technology, Chengdu 610059, China;
4. No.6 Geological Party, the Bureau of Geological Exploration and Mineral Resources, Lhasa 851400, China
青藏高原是研究陆陆碰撞等深部动力学过程的最佳天然实验室(Chung et al., 2009; Sun et al., 2015)，其多种岩浆岩的形成、侵位与复杂的地质演化历史密切相关(Chung et al., 2005)。西藏冈底斯成矿带是我国最具经济价值的成矿带之一(唐菊兴等, 2013)，根据其不同的成矿系列和矿床组合，可进一步划分为冈底斯斑岩铜矿带和矽卡岩型铅锌成矿带(Wang et al., 2017)。其中，冈底斯斑岩铜矿带中发育大量斑岩型Cu-Mo矿床和矽卡岩型Cu-Pb-Zn矿床(Zheng et al., 2015)，其矿床的形成与中新世高Sr/Y比值的埃达克质岩浆岩有关(Hou et al., 2011; Wu et al., 2016)。与冈底斯后碰撞岩浆活动有关的埃达克质岩广泛发育，东西延伸近1300km，主要分布于雅鲁藏布江和南拉萨地块一带，其侵位时代主要集中于26~9Ma(Guo et al., 2007a; King et al., 2007; Xu et al., 2010)。
西藏浦桑果矿床位于冈底斯成矿带中段，为近年来新发现的矽卡岩型富铜铅锌(钴镍)矿床。截止目前，矿区探获铜金属量大于10万吨(平均品位为1.42%)、铅+锌金属量大于20万吨(Pb+Zn平均品位为2.83%)、钴金属量大于250吨(平均品位140g/t)(刘祖军等，2012)。目前，该矿床的成矿作用(崔晓亮, 2013)、成矿物质来源(李壮等, 2018a)、金属硫化物组合(杨海锐, 2013)、矽卡岩矿物学特征(李壮等, 2018b)都作了详细的研究。然而，矿区相关的中酸性侵入岩体的岩石成因及深部动力学背景研究程度较低，因此，本文主要以浦桑果矿区的花岗闪长岩和闪长玢岩中酸性岩体为研究对象，利用LA-ICP-MS锆石U-Pb年代学、主微量元素地球化学、全岩Sr-Nd-Pb同位素及锆石Lu-Hf同位素分析手段，厘定矿区岩浆岩的侵位时代，分析岩石地球化学属性，初步探讨其岩石成因及深部动力学过程。
1 区域地质背景青藏高原主要由东西向拉长的不同块体汇聚而成，从南到北依次被划分为特提斯喜马拉雅、拉萨地块、羌塘地块和松潘-甘孜地块(Yin and Harrison, 2000)，且分别以雅鲁藏布江缝合带(IYSZ)、班公湖-怒江缝合带(BNSZ)、龙木错-双湖缝合带(LSSZ)和金沙江缝合带(JSSZ)为构造边界(图 1a; Zhu et al., 2011)。中生代-早新生代岩浆岩广泛发育于拉萨地块(图 1b)，并形成著名的冈底斯岩浆岩基(Zhu et al., 2013; Jiang et al., 2014; Liu et al., 2017)。Chen et al. (2012)根据不同类型岩石的锆石Hf同位素特征，进一步将拉萨地块划分为南拉萨地体、中拉萨地体和北拉萨地体，且分别被洛巴堆-米拉山断裂(LMF)和狮泉河-纳木错蛇绿岩带(SNMZ)分割(图 1b)(Wang et al., 2014a; Zhu et al., 2017)。其中，南拉萨地体主要由部分冈底斯岩浆岩基、林子宗群火山岩及少量沉积盖层组成(Zhang et al., 2010; Wu et al., 2014a)，且发育少量渐新世-中新世(30~8Ma)的钾质(超钾质)火山岩和后碰撞型埃达克质岩(Guo and Wilson, 2012; Hébert et al., 2014)，后碰撞型埃达克质岩主要以岩枝或岩脉侵位于沉积地层中(Chung et al., 2009)，部分埃达克质岩石构成与铜矿密切相关的含矿斑岩体(Hou et al., 2013)。
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图 1 青藏高原构造单元划分简图(a, 据Pan et al., 2012修改)及冈底斯中生代-新生代岩浆岩和中新世埃达克质岩(26~10Ma)分布图(b, 据Liu et al., 2017修改) 数据来源：1-朱诺(Gao et al., 2010; Zeng et al., 2017)；2-吉如(Zheng et al., 2014; Yang et al., 2016)；3-浦桑果(本文)；4-冲江(Hu et al., 2017)；5-达布(Wu et al., 2014b)；6-拉抗俄(Hou et al., 2004; Leng et al., 2016)；7-知不拉(Xu et al., 2016)；8-驱龙(Hu et al., 2015)；9-甲玛(Hou et al., 2004; Zheng et al., 2016)；图 6-图 12数据来源同此 Fig. 1 Geographic map showing the tectonic boundaries and units of the Tibetan Plateau (a, modified after Pan et al., 2012) and the distribution of the Mesozoic-Cenozoic magmatism and the major Miocene adakitic rocks (26~10Ma) and their ages in southern Tibet (b, modified after Liu et al., 2017) Data sources: 1-Zhu'nuo (Gao et al., 2010; Zeng et al., 2017); 2-Jiru (Zheng et al., 2014; Yang et al., 2016); 3-Pusangguo (this study); 4-Chongjiang (Hu et al., 2017); 5-Dabu (Wu et al., 2014b); 6-Lakang'e (Hou et al., 2004; Leng et al., 2016); 7-Zhibula (Xu et al., 2016); 8-Qulong (Hu et al., 2015); 9-Jiama (Hou et al., 2004; Zheng et al., 2016); the data sources in Fig. 6-Fig. 12 are the same as this figure |
西藏浦桑果矽卡岩型铜多金属矿床大地构造位置位于南冈底斯成矿带火山岩浆弧内(图 1b)。区域出露地层从侏罗系到第四系皆有分布，由老到新依次为昂杰组(C2a)、下拉组(P2x)、雄村组(J1-2x)、麻木下组(J2-K1m)、比马组(K1b)、楚木龙组(K1c)、塔克那组(K1t)、昂仁组(K1-2a)、设兴组(K2s)、秋乌组(E2q)、典中组(E1d)、年波组(E2n)、日贡拉组(E3r)、大竹卡组(E3-N1d)、芒乡组(N1m)、嘎扎村组(N2g)、宗当村组(N2z)及第四系(Q)。受南侧雅鲁藏布江缝合带、北侧班公湖-怒江缝合带构造影响，区内发育一系列近东西向断裂构造、南北向次级断裂构造及火山环形构造体系。区域内岩浆岩发育并广泛出露，主要包括晚三叠-中侏罗世花岗岩(215~175Ma)(张宏飞等, 2007)、晚侏罗-晚白垩世花岗岩和中酸性火山岩(160~80Ma)(朱弟成等, 2008)、古新世-始新世火山岩(70~40Ma)和渐新世-中新世中酸性闪长岩、二长花岗岩等(33~10Ma)(莫宣学等, 2003)。
2 矿床地质特征浦桑果矿床位于南木林县北西方向近30km处。矿区出露地层整体较简单，主要为早白垩世塔克那组、晚白垩世设兴组和始新世典中组。塔克那组岩性主要为灰岩、大理岩和火山碎屑岩；设兴组岩性主要为砂岩、泥岩、粉砂岩等；典中组主要为火山碎屑岩。其中，塔克那组为主要赋矿围岩。地层走向近东西，倾向北东，倾角约45°~60°。矿区构造主要为南北向断裂及褶皱构造(图 2a)。矿区岩浆岩发育并广泛出露，岩石类型包括矿区西侧的黑云母花岗闪长岩、矿区中部的闪长玢岩和北侧的辉长岩脉，其中，黑云母花岗闪长岩和闪长玢岩分别以岩基和岩株形式侵位于塔克那组中。
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图 2 浦桑果铜多金属矿床地质简图(a)及实测地层剖面图(b、c, 据刘祖军等, 2012①修改) Fig. 2 The simplified geological map (a) and the measured stratigraphic sections of the Pusangguo copper polymetallic deposit (b, c) |
① 刘祖军. 2012.西藏自治区南木林县浦桑果矿区铜多金属矿详查报告.拉萨:西藏自治区矿产勘查开发局第六地质大队
矿区共圈定5条矿体，自北向南依次编号为Ⅰ、Ⅱ、Ⅲ、Ⅳ和Ⅴ(图 2a)。矿体主要赋存于黑云母花岗闪长岩、闪长玢岩与塔克那组接触部位的矽卡岩化带中，矿体走向近东西，矿体形态呈层状、脉状及不规则状(图 2b)。Ⅰ号矿体为矿区的主矿体，矿体沿走向延伸近350m，沿倾向方向延伸近200m，铜铅锌金属资源量约22万吨，占整个矿床金属资源量的60%。矿石构造主要包括块状构造(图 3a)、浸染状构造(图 3b, c)，次为条带状构造(图 3d)和角砾状构造。矿石结构主要包括结晶结构(图 3e, f)和交代结构(图 3g)。矿石矿物主要为黄铜矿、闪锌矿、方铅矿、黄铁矿，次为辉砷镍钴矿、赤铁矿、斑铜矿、辉铜矿和极少量铋矿物(针硫铋铅矿和硫铋铅铜矿)(图 3h-l)。脉石矿物主要为石榴子石、透辉石、绿帘石、石英，次为硅灰石、绿泥石、角闪石和阳起石(图 3m-p)。
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图 3 浦桑果铜多金属矿床主要矿物组合及矿石组构特征 Py-黄铁矿；Ccp-黄铜矿；Sp-闪锌矿；Cob-辉砷镍钴矿；Hem-赤铁矿；Bn-斑铜矿；Cc-辉铜矿；Aik-针硫铋铅矿；Gn-方铅矿；Grt-石榴子石；Di-透辉石；Act-阳起石；Amp-角闪石；Ep-绿帘石；Chl-绿泥石；Qtz-石英 Fig. 3 Photographs and photomicrographs showing the main ore structure and textures in the mineral assemblages of the Pusangguo copper polymetallic deposit Py-pyrite; Ccp-chalcopyrite; Sp-sphalerite; Cob-cobaltite; Hem-hematite; Bn-bornite; Cc-chalcocite; Aik-aikinite; Gn-galena; Grt-garnet; Di-diopside; Act-actinolite; Amp-amphibole; Ep-epidote; Chl-chlorite; Qtz-quartz |
围岩蚀变类型主要为矽卡岩化、大理岩化和碳酸盐化，次为硅化、角岩化。其中，矽卡岩化与铜铅锌等矿化密切相关，且主要发育于矿区中酸性岩体与塔克那组灰岩及大理岩的接触部位。根据矿物组合、矿物相互穿插关系可知，浦桑果矿床的矿化过程与典型的岩浆热液接触交代型矽卡岩矿床的成矿过程类似。崔晓亮(2013)对矿区部分矽卡岩矿物(石榴子石、透辉石)及石英、方解石等脉石矿物进行了详细的包裹体研究，认为从进变质矽卡岩阶段至退变质矽卡岩阶段，成矿流体逐渐从高温(523~387℃)、高盐度(48.84% NaCleqv)转变为低温(198~186℃)、低盐度(5.18%~3.25% NaCleqv)状态。因此，可将浦桑果矿床的成矿过程划分为早期进变质矽卡岩阶段、退变质矽卡岩阶段、石英-硫化物阶段和晚期碳酸盐阶段四个成矿阶段。
3 样品采集及分析方法 3.1 样品特征用于本研究的岩石样品包括矿区广泛发育的黑云母花岗闪长岩(PLX5-6，29°35′17″N、89°26′20″E)和(PLX2-2，29°35′45″N、89°26′08″E)，及闪长玢岩露头(PLX5-9，29°35′32″N、89°26′40″E)和(PLX1-8，29°35′34″N、89°26′31″E)，岩石样品较新鲜，仅局部遭受风化，具体采样位置如图 2。黑云母花岗闪长岩颜色为灰白-浅棕色，岩石具中粗粒状结构，块状构造，主要组成矿物为斜长石(35%~40%)、石英(20%~25%)、钾长石(15%~20%)，次为少量黑云母(10%~15%)、角闪石(8%~10%)(图 4a)。斜长石呈板状晶体，粒径约1.5~2mm，普遍发育聚片双晶结构；钾长石发育卡式双晶结构，粒径约1~2.5mm；黑云母具明显的多色性，粒径约0.2~1mm。角闪石呈半自形-自形晶，粒径约0.5~1mm。副矿物为锆石、磁铁矿及磷灰石等(图 4b, c)。闪长玢岩样品颜色呈浅灰绿色，岩石具明显斑状结构，基质具细粒或隐晶质结构，块状构造，主要组成矿物为斜长石(55%~60%)、角闪石(30%~35%)，次为钾长石(5%~8%)、石英(8%~10%)，以及少量磷灰石、锆石、磁铁矿等副矿物(1%~2%)(图 4d)。斑晶主要由斜长石和角闪石组成，斑晶直径约0.5~2mm。基质主要为角闪石和斜长石，次为少量的石英、钾长石和黑云母(图 4e, f)。
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图 4 浦桑果铜多金属矿床黑云母花岗闪长岩和闪长玢岩样品的岩相学特征 (a-c)黑云母花岗闪长岩样品手标本特征及主要矿物组成；(d-f)闪长玢岩样品手标本特征及主要矿物组成.Hb-角闪石；Pl-斜长石；Kfs-钾长石；Qtz-石英；Bt-黑云母；Mt-磁铁矿 Fig. 4 Photographs and photomicrographs showing the petrographic characteristics of the Pusangguo biotite granodiorite and diorite porphyrite (a-c) hand-specimen and main mineral compositions of the biotite granodiorite; (d-f) hand-specimen and mineral compositions of the diorite porphyrite. Hb-hornblende; Pl-plagioclase; Kfs-K-feldspar; Qtz-quartz; Bt-biotite; Mt-magnetite |
样品的锆石分选在广州岩拓技术服务有限公司利用单矿物常规分离技术完成，制靶后在中国地质科学院矿产资源研究所电子探针实验室进行锆石透射光、反射光及阴极发光照相，优选环带发育良好、无裂痕的锆石进行U-Pb同位素定年。锆石U-Pb同位素定年在中国地质大学(北京)地质过程与矿产资源国家重点实验室激光剥蚀等离子质谱仪(LA-ICP-MS)微区分析实验室完成，激光剥蚀系统为美国Coherent公司的GeoLasPro 193准分子固体进样系统，ICP-MS为美国Thermo Fisher公司的X Series 2型四级杆等离子体质谱。测试中，激光斑束直径为32μm，频率为6Hz，采用He作为载气，Ar作为补偿气。采用美国国家标准参考物质NISTSRM610对仪器进行最佳化，并将其作为微量元素含量测定的外标。采用标准锆石91500为定年外标，采用标准锆石Mud Tank作为监控样品。在样品测试过程中每测定5个样品点测定两次标准锆石91500，每个样品的前20s为背景信号采集时间，样品信号采集时间为50s。测试完成后，采用软件ICPMSDataCal(Liu et al., 2008)对样品的测试数据进行后期处理，年龄计算及谐和图的绘制均采用Isoplot 3.0软件完成(Ludwig, 2003)。
3.3 锆石Lu-Hf同位素分析锆石Lu-Hf同位素分析的锆石点均挑选自LA-ICP-MS锆石U-Pb定年的有效点。锆石Hf同位素分析在中国地质科学院地质研究所大陆构造与动力学实验室完成。分析过程中采用配有193nm激光的Neptune多接收电感耦合等离子质谱仪进行测定，详细的操作过程及分析步骤见参考文献(Wu et al., 2006)。分析过程中，标准锆石GJ-1的176Hf/177Hf测试加权平均值分别为0.282285±13(n=35)。锆石εHf值的计算采用176Lu衰变常数为1.867×10-11a-1(Söderlund et al., 2004)，球粒陨石的176Hf/177Hf=0.282772，176Lu/177Hf=0.0332(Blichert-Toft and Albarède, 1997)，Hf亏损地幔二阶段模式年龄(tDM2)的计算采用平均陆壳的176Lu/177Hf比值0.015(Griffin et al., 2000)。
3.4 全岩主、微量元素分析对新鲜岩石样品进行无污染粉碎至200目，用于分析全岩主、微量元素及Sr-Nd-Pb同位素值。本文全岩主、微量元素及Sr-Nd-Pb同位素测定均在核工业北京地质研究院分析测试中心完成，主量元素分析采用XRF方法完成，标样为AB104L和AL104，分析精度为不小于1%；微量元素采用仪器Thermo Scientific X Series Ⅱ ICPMS分析测定，将样品粉末与2%含量的HNO3溶解后，使用NexION300D质谱仪进行测定；微量元素测定精度为不小于2‰。
3.5 全岩Sr-Nd-Pb同位素分析首先准确称量实验要求的全岩粉末(200目)50~100mg左右，使用纯化HF-HNO3-HCl溶样，之后加入纯化HCl使用Rb-Sr(AG50W-X12，200~400目)、Sr-Nd(LN树脂)交换柱进行分离提纯和元素提取。样品测试仪器型号为热电离质谱仪TIMS-ICPMS，数据以86Sr/88Sr=0.1194和146Nd/144Nd=0.7219校正作为分馏修正。在样品测试的整个过程中，所测定的Alfa Nd标样和NBS-987 Sr标样的Nd-Sr同位素比值，分别为143Nd/144Nd=0.512433±0.000008(±2σ)和87Sr/86Sr=0.710252±0.000015(±2σ)。
全岩铅同位素分析，首先将全岩粉末样(200目)与超纯的HNO3+HCl溶液混合，待干燥后，再与HBr+HNO3溶液混合。然后，将混合物装入一个含有50 Am的AG 1-X 8阴离子树脂的柱子上，并通过异丙基热电离质谱仪进行分析。204Pb/206Pb和208Pb/206Pb值分析精度为不少于0.005%。分析中采用标样NBS981进行校正(NBS981的208Pb/206Pb=2.164940±15, 207Pb/206Pb=0.914338±7, 204Pb/206Pb=0.0591107±2)。实验详细的分析步骤见参考文献(Wang et al., 2018)。
4 分析结果 4.1 锆石U-Pb年龄本文对2件黑云母花岗闪长岩和2件闪长玢岩样品分别开展了LA-ICP-MS锆石U-Pb定年。锆石U-Pb定年数据及计算结果详见表 1；锆石协和年龄图解见图 5。锆石CL形态特征显示，黑云母花岗闪长岩和闪长玢岩的锆石具相似性，锆石普遍呈灰白色，半自形-自形晶，主要呈长柱状晶体，少量呈短柱状，其长轴长度为110~320μm，长短轴之比多为1:1~3:1(图 5)。本文共计完成75粒锆石的U-Pb定年，U和Th含量变化均较大，U含量变化范围为41×10-6~2430×10-6，Th含量变化范围为53×10-6~11191×10-6，Th/U比值为0.4~8.9(>0.1)，且大部分锆石具明显振荡环带结构，属典型的岩浆锆石特征(Wu and Zheng, 2004)。
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表 1 浦桑果矿床黑云母花岗闪长岩和闪长玢岩的LA-ICP-MS锆石U-Pb年龄分析结果 Table 1 Zircon age data acquired by LA-ICP-MS methods for the biotite granodiorite and diorite porphyrite in the Pusangguo deposit |
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图 5 浦桑果铜多金属矿床黑云母花岗闪长岩(a、b)和闪长玢岩(c、d)的锆石U-Pb谐和年龄图 Fig. 5 The U-Pb concordia diagrams for zircons from the Pusangguo (a, b) biotite granodiorite and (c, d) diorite porphyrite |
黑云母花岗闪长岩样品(PLX2-2和PLX5-6)共完成37个锆石点分析，Th含量为53×10-6~630×10-6，U含量为41×10-6~445.8×10-6，Th/U比值为0.6~2.5，具典型的岩浆锆石特征。样品(PLX2-2)29个测点获得的206Pb/238U年龄加权平均结果为14.4±0.4Ma (MSWD=0.7)(图 5a)；样品(PLX5-6)的8个测点获得的206Pb/238U年龄加权平均结果为13.6±0.2Ma (MSWD=0.4)(图 5b)。闪长玢岩(PLX5-9和PLX1-8)共完成38个锆石点分析，Th含量为127×10-6~11191×10-6，U含量为150×10-6~2430×10-6，Th/U比值为0.8~8.9，具典型岩浆锆石特征。样品(PLX5-9)的12个测点获得的206Pb/238U年龄加权平均结果为13.6±0.1Ma (MSWD=0.9)(图 5c)；样品(PLX1-8)26个测点获得的206Pb/238U年龄加权平均结果为14.6±0.3Ma(图 5d)。4个岩石样品的平均加权年龄在误差范围内重叠，表明此年龄值可代表岩石的结晶年龄，岩体的侵位年龄与南拉萨地体后碰撞埃达克质岩石同时代(图 1b)，岩体侵位时代均为中新世。
4.2 全岩主、微量元素浦桑果黑云母花岗闪长岩和闪长玢岩的全岩主、微量元素含量列于表 2中。黑云母花岗闪长岩的烧失量为0.32%~0.48%，闪长玢岩的烧失量为0.32%~1.3%，说明岩石较新鲜基本未受到后期蚀变影响。黑云母花岗闪长岩SiO2含量为65.55%~67.3%，Al2O3含量为15.28%~15.85%，Fe2O3T含量为3.64%~4.14%，MgO含量为1.41%~1.74%，CaO含量为2.85%~3.76%，Na2O含量为3.39%~3.97%，K2O含量为3.86%~4.07%，Mg#为29~38。闪长玢岩SiO2含量为58.27%~60.66%，Al2O3含量为16.71%~17.23%，Fe2O3T含量为4.95%~6.01%，MgO含量为2.06%~2.93%，CaO含量为5.36%~6.25%，Na2O含量为4.08%~4.9%，K2O含量为2.07%~2.61%，Mg#值为32~36(表 2)。相较于闪长玢岩，黑云母花岗闪长岩具高SiO2、K2O和K2O/NaO(1.02~1.20)值，具低Al2O3、Fe2O3T、MgO、CaO含量和Mg#值特征。黑云母花岗闪长岩和闪长玢岩的A/CNK比值分别为0.93~1.01和0.76~0.90，具Ⅰ型花岗岩特征(Maniar and Piccoli, 1989)。
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表 2 浦桑果矿床黑云母花岗闪长岩和闪长玢岩的全岩主量(wt%)及微量元素(×10-6)分析结果 Table 2 Whole-rock major (wt%) and trace elements (×10-6) compositions of the biotite granodiorite and diorite porphyrite in the Pusangguo deposit |
岩石全碱TAS图解中(Le Maitre et al., 2002)(图 6a)，样品数据点均落在花岗闪长岩和闪长岩区域内，岩石类型与室内镜下鉴定结果一致。如图 6b所示，黑云母花岗闪长岩和闪长玢岩均位于高钾钙碱性区域内，表明岩石均属于高钾钙碱性系列，且黑云母花岗闪长岩钾含量明显高于闪长玢岩。其中，闪长玢岩明显亏损Y(10.4×10-6~12.4×10-6)和Yb(0.8×10-6~1.1×10-6)元素，富集Sr元素(687×10-6~1616×10-6)，具高Sr/Y比值(62.7~132)、La/Yb比值(27.4~34.8)弱负Eu异常特征(δEu=0.83~1.02)(表 2)。黑云母花岗闪长岩的地球化学特征与闪长玢岩具一定相似性。在Sr/Y-Y(图 6c)和(La/Yb)N-YbN(图 6d)图解中(Defant and Drummond, 1990; Petford and Atherton, 1996)，本文所有岩石样品数据点均落入埃达克岩区域内，指示浦桑果黑云母花岗闪长岩和闪长玢岩具典型埃达克质岩的地球化学属性。
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图 6 浦桑果铜多金属矿床黑云母花岗闪长岩和闪长玢岩的主微量元素图解 (a) SiO2-(Na2O+K2O)图解(Le Maitre, 2002)；(b) SiO2-K2O图解(Peccerillo and Taylor, 1976)；(c) Y-Sr/Y图解(Defant and Drummond, 1990)；(d) (La/Yb)N-YbN图解(Petford and Atherton, 1996) Fig. 6 The trace elements diagrams of the biotite granodiorite and diorite porphyrite in the Pusangguo copper polymetallic deposit (a) classification of total alkalis versus SiO2 (Le Maitre, 2002); (b) SiO2 vs. K2O diagram (Peccerillo and Taylor, 1976); (c) Sr/Y ratios vs. Y discrimination diagram (Defant and Drummond, 1990); (d) (La/Yb)N vs. YbN diagram (Petford and Atherton, 1996) |
浦桑果黑云母花岗闪长岩和闪长玢岩的球粒陨石标准化稀土元素配分图(图 7a)均表现出明显右倾特征，表明轻稀土元素与重稀土元素之间具明显的分馏特征，指示两种不同岩石类型可能具相同的岩浆来源。黑云母花岗闪长岩和闪长玢岩均具相对富集轻稀土元素(La、Ce、Pr、Nd、Sm、Eu)而亏损重稀土元素(Gd、Tb、Dy、Ho、Er、Tm、Yb、Lu)特征(图 7a)。原始地幔标准化微量元素蛛网图(图 7b)显示，相比于原始地幔，黑云母花岗闪长岩和闪长玢岩均具相对富集大离子亲石元素(Rb、Ba、Sr、Th、U等)而强烈亏损高场强元素(Ta、Nb、Ti、P等)；具中等负Eu异常(δEu=0.63~0.99)而无明显Ce异常特征(δCe=0.88~0.95)。
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图 7 浦桑果黑云母花岗闪长岩和闪长玢岩的球粒陨石标准化稀土元素配分图(a)和原始地幔标准化微量元素蛛网图(b)(标准化值据Sun and McDonough, 1989) Fig. 7 Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace element patterns (b) for the Pusangguo biotite granodiorite and diorite porphyrite (normalization values after Sun and McDonough, 1989) |
相比于其他斑岩-矽卡岩型矿床(如冲江、朱诺、吉如、知不拉、驱龙、甲玛、拉抗俄等)成矿密切相关的埃达克质侵入岩体，浦桑果矿床黑云母花岗闪长岩和闪长玢岩的主微量稀土元素地球化学特征具有高度相似性(图 6、图 7)，所有侵入岩体均表现为富集大离子亲石元素和轻稀土元素而相对亏损高场强元素和重稀土元素特征，且具明显的埃达克质岩石属性，指示其岩石可能具相同的岩浆来源。
4.3 全岩Sr-Nd-Pb同位素值浦桑果矿床黑云母花岗闪长岩和闪长玢岩全岩Sr-Nd-Pb同位素分析结果见表 3和图 8。黑云母花岗闪长岩和闪长玢岩样品的初始Sr、Nd、Pb同位素组成分别用其对应的岩体锆石U-Pb年龄值14.8Ma和14.6Ma进行校正计算。由表 3可知，浦桑果矿床黑云母花岗闪长岩和闪长玢岩全岩Sr-Nd-Pb同位素值相对较均一，其同位素值较接近。其中，黑云母花岗闪长岩样品的全岩初始(87Sr/86Sr)i比值为0.707050~0.707138，平均值为0.707091；初始(143Nd/ 144Nd)i值为0.512328~0.512376，平均值为0.512347；εNd(t)值为-5.69~-4.73，平均值为-5.3，二阶段模式年龄集中于1216~1293Ma，平均值为1262Ma。闪长玢岩样品的全岩初始(87Sr/86Sr)i比值为0.705198~0.706572，平均值为0.705864；初始(143Nd/144Nd)i值为0.512415~0.512605，平均值为0.512504；εNd(t)值为-3.98~-0.28，平均值为-2.24，二阶段模式年龄值为852~1154Ma，平均为1012Ma(图 8)。
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表 3 浦桑果矿床黑云母花岗闪长岩和闪长玢岩的全岩Sr-Nd-Pb同位素分析结果 Table 3 Whole-rock Sr-Nd-Pb isotopic compositions for the biotite granodiorite and diorite porphyrite in the Pusangguo deposit |
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图 8 浦桑果铜多金属矿床黑云母花岗闪长岩及闪长玢岩的全岩(87Sr/86Sr)i-εNd(t)图解 冈底斯中新世埃达克质岩(Gao et al., 2010; Xu et al., 2010; Hou et al., 2013)；白垩纪埃达克质岩(Zhu et al., 2009)；侏罗纪斑岩(杨志明等, 2011)；林子宗群火山岩(Mo et al., 2007; Gao et al., 2008)；雅鲁藏布江蛇绿岩(Xu and Castillo, 2004)；印度洋深海黏土(Benothman et al., 1989)；安多片麻岩(Harris et al., 1988)；拉萨上地壳(Ma et al., 2014)；拉萨下地壳(Wen et al., 2008) Fig. 8 The (87Sr/86Sr)i vs. εNd(t) diagram for the biotite granodiorite and diorite porphyrite in the Pusangguo copper polymetallic deposit Gangdese Miocene adakites (Gao et al., 2010; Xu et al., 2010; Hou et al., 2013); Cretaceous adakites (Zhu et al., 2009); Jurassic porphyries (Yang et al., 2011); Linzizong volcanics (Mo et al., 2007; Gao et al., 2008); Yarlung-Tsangpo ophiolite (MORB) (Xu and Castillo, 2004); Indian Ocean pelagic sediment (Benothman et al., 1989); Amdo orthogneiss (Harris et al., 1988); Lhasa upper crust (Ma et al., 2014); Lhasa lower crust (Wen et al., 2008) |
浦桑果黑云母花岗闪长岩和闪长玢岩的全岩Pb同位素列于表 3和图 9中。结果显示，黑云母花岗闪长岩的206Pb/204Pb，207Pb/204Pb和208Pb/204Pb值分别为18.551~18.565、15.683~15.694和39.009~39.026；初始(206Pb/204Pb)i，(207Pb/204Pb)i和(208Pb/204Pb)i值分别为18.531~18.545、15.682~15.693和38.969~38.988。闪长玢岩的206Pb/204Pb、207Pb/204Pb和208Pb/204Pb值分别为18.513~18.616、15.664~15.718和38.906~39.073；初始(206Pb/204Pb)i、(207Pb/204Pb)i和(208Pb/204Pb)i值分别为18.504~18.608、15.664~15.718和38.89~39.055。
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图 9 浦桑果铜多金属矿床黑云母花岗闪长岩和闪长玢岩的(207Pb/204Pb)i-(206Pb/204Pb)i (a)和(208Pb/204Pb)i-(206Pb/204Pb)i (b)图解(底图据Zhao et al., 2009修改) 南北冈底斯花岗岩(>40Ma)(Dong et al., 2008)；拉萨地体埃达克岩(Li et al., 2017)；北半球参考线值(NHRL)：208Pb/204Pb=1.209×206Pb/204Pb+15.627；207Pb/204Pb=0.1084×206Pb/204Pb+13.491 Fig. 9 Plots of (207Pb/204Pb)i vs. (206Pb/204Pb)i (a) and (208Pb/204Pb)i vs. (206Pb/204Pb)i (b) for the biotite granodiorite and diorite porphyrite in the Pusangguo copper polymetallic deposit (base map after Zhao et al., 2009) The northern and southern Gangdese granites (>40Ma) (Dong et al., 2008); the adakites of the Lhasa terrane (Li et al., 2017); Northern Hemisphere Reference Line (NHRL): 208Pb/204Pb=1.209×206Pb/204Pb+15.627; 207Pb/204Pb=0.1084×206Pb/204Pb+13.491 |
浦桑果黑云母花岗闪长岩和闪长玢岩的锆石Lu-Hf同位素分析结果见表 4和图 10。黑云母花岗闪长岩锆石初始176Hf/177Hf(i)值为0.282771~0.282834，平均值为0.282807；锆石176Yb/177Hf值为0.007365~0.014287，平均值为0.011199；锆石176Lu/177Hf值为0.000265~0.000407，平均值为0.000328，均小于0.0020，显示锆石在形成之后放射成因Hf的积累极为有限；锆石εHf(t)值为0.3~2.5，平均值为1.6；锆石一阶模式年龄值(tDM1)为582~669Ma，平均年龄为620Ma；锆石二阶模式年龄值(tDM2)为938~1081Ma，平均年龄为999Ma(图 10a)；fLu/Hf平均值为-0.99。闪长玢岩锆石初始176Hf/177Hf(i)值为0.282762~0.282934，平均值为0.282850；锆石176Yb/177Hf值为0.011415~0.196849，平均值为0.044386；锆石176Lu/177Hf值为0.000347~0.005160，平均值为0.001242，均小于0.0020，显示锆石在形成之后放射成因Hf的积累极为有限；锆石εHf(t)值为0~6.0，平均值为3.1；锆石一阶段模式年龄(tDM1)为448~690Ma，平均年龄为573Ma；锆石二阶模式年龄(tDM2)为712~1101Ma，平均年龄为901Ma(图 10b)；fLu/Hf值为-0.8~-0.99，平均值为-0.96。锆石Hf同位素投图结果(图 10c)显示，本文研究的浦桑果黑云母花岗闪长岩和闪长玢岩的锆石Hf同位素均位于亏损地幔与球粒陨石之间，且落于冈底斯中新世埃达克质岩的锆石Hf同位素范围内，指示浦桑果矿床的中酸性侵入岩体可能与冈底斯中新世埃达克质侵入岩体具有岩浆同源性及相似的地球动力学背景。
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表 4 浦桑果矿床黑云母花岗闪长岩和闪长玢岩的锆石Lu-Hf同位素分析结果 Table 4 Zircon Lu-Hf isotopic data for the biotite granodiorite and diorite porphyrite in the Pusangguo deposit |
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图 10 浦桑果铜多金属矿床黑云母花岗闪长岩和闪长玢岩的锆石εHf(t)-锆石U-Pb年龄图解 Fig. 10 The plot diagrams of zircon εHf(t) vs. U-Pb ages for the biotite granodiorite and diorite porphyrite in the Pusangguo copper polymetallic deposit |
埃达克岩是Defant and Drummond (1990)研究阿留申群岛新生代俯冲洋壳熔融产生的火山岩时提出的术语，用以概括具有特定地球化学属性的一套中酸性侵入岩和火山岩的组合，包括安山岩、英安岩、石英闪长岩、花岗闪长岩、石英二长岩、英云闪长岩、斜长花岗岩等。其地球化学特征是SiO2≥56%，Al2O3≥15%，MgO < 3%(少量>6%)，亏损重稀土元素(HREE)与Y(Y≤18×10-6)，高Sr(多数大于400×10-6)、La/Yb≥20、Sr/Y>40，一般具有正Eu异常特征(少数具弱的负Eu异常)(Defant and Kepezhinskas, 2001; Richard and Kerrich, 2007)。
浦桑果矿床黑云母花岗闪长岩和闪长玢岩均具高SiO2(58.3%~67.3%)、Al2O3(15.3%~17.2%)、Sr含量(599×10-6~1616×10-6)，低MgO含量(1.4%~2.9%)，高Sr/Y(48.2~132)和La/Yb(27.4~35.4)比值特征；亏损重稀土元素(HREE)和Y元素；具弱负Eu异常特征(表 2和图 7)，上述特征与典型埃达克岩的地球化学性质相似。此外，浦桑果黑云母花岗闪长岩和闪长玢岩在岩石地球化学组成方面与冈底斯斑岩铜矿带大多与斑岩-矽卡岩矿床相关的同时代侵入岩(Wu et al., 2014b; Zheng et al., 2014; Zeng et al., 2015; Hu et al., 2015, 2017; Xu et al., 2016; Yang et al., 2016)具相似的埃达克质岩地球化学性质，指示岩石可能具相似的岩浆来源和演化过程。
大量研究结果表明，冈底斯成矿带斑岩-矽卡岩铜矿床与 中新世埃达克质岩密切相关，岩石形成时代主要集中于10~23Ma(Zheng et al., 2007; 王保弟等, 2010; Hou et al., 2013)。目前，对冈底斯成矿带发育的埃达克质岩的岩浆起源及岩石成因主要观点如下：(1)残留俯冲特提斯洋壳板片地幔楔的部分熔融(Martin et al., 2005; Li et al., 2011; Hu et al., 2015)；(2)长英质岩浆与玄武质岩浆的岩浆混合作用(Castillo, 2006; Guo et al., 2007b)；(3)岩石圈地幔橄榄岩发生部分熔融作用(Xu et al., 2010; Jiang et al., 2014; Chen et al., 2015)；(4)拉萨地体加厚新生下地壳发生部分熔融(Xu et al., 2002; Guo et al., 2007a; Chung et al., 2009; Chen et al., 2011; Zhao et al., 2015; Hou et al., 2015; Tian et al., 2017; Li et al., 2017)。
研究表明，约50~42Ma时，特提斯洋俯冲板片开始下沉，发生板片断离作用，最终逐渐下沉到深部岩石圈地幔中(Kohn and Parkinson, 2002; Lee et al., 2009)。然而，浦桑果埃达克质中酸性侵入岩体(黑云母花岗闪长岩和闪长玢岩)形成时代均为14Ma左右，岩体侵位形成于后碰撞构造环境而非特提斯洋俯冲板片断离构造背景，因此，可排除由俯冲特提斯洋壳板片发生板片断离或部分熔融作用的岩石成因模型。Allègre and Minster (1978)指出岩石中La、Nd、Th、Sm、Y等不相容元素之间的比值可有效判别岩石是由岩浆部分熔融作用还是岩浆分离结晶作用形成。如图 11，浦桑果矿床黑云母花岗闪长岩和闪长玢岩侵入岩体Th、Nd、La等不相容元素比值之间均显示出较好的正相关关系，且与甲玛、驱龙等大多斑岩-矽卡岩矿床由部分熔融作用形成的埃达克质侵入岩表现趋势相似(图 11a-d)，指示岩石是由岩浆部分熔融作用而非同化混染或分离结晶作用形成。此外，Streck et al. (2007)研究认为由长英质和玄武质岩浆发生岩浆混合作用而形成的埃达克质岩石通常含较高MgO含量(>4.5%)和高Mg#指数(>66)，然而，本文研究的浦桑果侵入岩体均表现出具低MgO含量(1.41%~2.93%)和低Mg#指数(40.6~50.8)特征(表 2；图 12a, b)，这与长英质和玄武质岩浆混合作用形成的埃达克质岩石特征明显不符，亦可排除此种岩石成因模式。此外，地幔橄榄岩主要由辉石岩和辉石岩熔体组成，主要形成玄武质岩浆而非埃达克质岩浆，浦桑果埃达克质中酸性侵入岩体非岩石圈地幔橄榄岩的部分熔融作用而形成。
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图 11 浦桑果铜多金属矿床黑云母花岗闪长岩和闪长玢岩体的Th/La (a)、Th/Nd (b)、Th/Sm (c)和Th/Y (d)与Th相关性图 Fig. 11 Correlation diagrams of Th/La (a), Th/Nd (b), Th/Sm (c) and Th/Y (d) vs. Th for the biotite granodiorite and diorite porphyrite in the Pusangguo copper polymetallic deposit |
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图 12 浦桑果矿床及冈底斯斑岩铜矿带部分斑岩-矽卡岩型矿床埃达克质侵入岩体的SiO2-MgO图解(a)和SiO2-Mg#图解(b) 全称因为前文没有出现过 Fig. 12 MgO vs. SiO2 (a) and Mg# vs. SiO2 (b) diagrams for adakitic intrusions from the Pusangguo and other porphyry-skarn deposits in GPCB |
在Mg#与MgO和SiO2图解中(图 12a, b)，本文研究的所有样品点均落入起源于下地壳的埃达克质岩区域内，且浦桑果矿床黑云母花岗闪长岩和闪长玢岩的锆石Hf同位素值与冈底斯成矿带中新世埃达克质岩石的Hf同位素组成较为相似(表 4、图 10)，岩体的εNd(t)和(87Sr/86Sr)i较洋中脊玄武岩(MORB)亏损而较拉萨下地壳更富集(表 3、图 8)，Sr-Nd同位素值主要位于冈底斯中新世埃达克质侵入岩区域内(图 8)，这与起源于新生下地壳的埃达克质岩具相似的Sr-Nd同位素组成特征(Jiang et al., 2012; Li et al., 2017)。此外，在全岩Pb同位素图解(图 9a, b)中，所有浦桑果侵入岩的样品点均落在拉萨地体埃达克岩区域内。综上所述，浦桑果矿床黑云母花岗闪长岩和闪长玢岩中酸性侵入岩体可能主要起源于新生下地壳。
中新世时期(18~10Ma)，整个西藏地壳厚度增厚至约40~55km左右(Mo et al., 2007; Guan et al., 2012)，而研究证实地壳厚度在40~50km时，新生下地壳组成以榴辉岩和富含石榴石的角闪岩相为主，同样在(La/Yb)N-YbN图解中(图 6d)，浦桑果侵入岩数据点均位于角闪岩相至含石榴石(10%)角闪岩相区域内，指示岩体中存在含石榴石角闪岩相的残留熔体且主要以部分熔融作用形成埃达克质岩浆熔体(Rapp et al., 1999)。实验岩石学表明，加厚新生下地壳部分熔融作用形成的埃达克质岩通常含较低Mg#值和MgO、Cr、Ni含量(Wang et al., 2007)，如榴辉岩或角闪岩部分熔体(Martin et al., 2005)。浦桑果矿床埃达克质侵入岩体具低MgO含量、低Mg#指数及低Cr、Ni、Co等含量特征，具高K2O含量、高K2O/Na2O、高(La/Yb)N和高Sr/Y比值特征(表 2)，相似于由加厚新生下地壳部分熔融形成的岩浆岩特征(Liu et al., 2010)。
综上所述，浦桑果矿床中酸性侵入岩体的岩浆可能主要起源于拉萨地体加厚新生下地壳，且主要由新生下地壳中富含石榴石的角闪岩相发生部分熔融作用，形成具埃达克质岩地球化学属性的岩浆熔体，上涌至矿区有利构造部分发生侵位，形成埃达克质黑云母花岗闪长岩和闪长玢岩侵入体。
5.2 岩石地球动力学背景锆石U-Pb年代学表明，浦桑果矿床黑云母花岗闪长岩的侵位年龄为13.6±0.2Ma和14.4±0.4Ma；闪长玢岩形成年龄为13.6±0.1Ma和14.6±0.3Ma(表 1、图 5)，岩体均形成于中新世。岩体的侵位年龄与冈底斯大多数中新世大型-超大型斑岩或矽卡岩型矿床与成矿密切相关的侵入岩体形成时代一致，如朱诺矿床花岗斑岩(12.3±0.3Ma, Zeng et al., 2017)，吉如矿床斑岩(15.5±0.3Ma, Yang et al., 2016)，冲江矿床黑云母二长花岗斑岩(14.9±0.3Ma, Hu et al., 2017)，达布矿床二长花岗斑岩(14.6±0.3Ma, Wu et al., 2014)，拉抗俄矿床斑状花岗闪长岩(13.7±0.7Ma, Leng et al., 2016)，知不拉矿床花岗闪长岩(16.9±0.3Ma, Xu et al., 2016)，驱龙矿床二长花岗岩(16.6±0.5Ma, Hu et al., 2015)，甲玛花岗斑岩(15.9±0.5Ma, Hou et al., 2004)(各矿床位置如图 1b)，指示其矿床形成于相似的地球动力学背景之下。
前人对冈底斯中新世岩浆活动的地球动力学背景和过程研究，认为引起冈底斯中新世大规模埃达克质岩浆岩的主要地球动力学模型包括：(1)新特提斯洋俯冲板片发生断离或拆沉(Williams et al., 2001; Mahéo et al., 2002; Chung et al., 2003; Hou et al., 2004)；(2)印度大陆岩石圈的拆沉与软流圈对流上涌(Ji et al., 2009; Zhang et al., 2010; Zheng et al., 2014; Li et al., 2011; Xu et al., 2016)。新特提斯洋壳的拆沉或断离作用(50~42Ma)会直接导致岩石圈地幔物质大量上涌(Tian et al., 2017)，从而引起残留的岩石圈地幔发生部分熔融作用，引起大规模的以地幔岩石圈为主的岩浆热液活动，然而在整个冈底斯斑岩铜矿带并未发现中新世时期以地幔成因为主的大规模岩浆活动存在(Zheng et al., 2014)。此外，新特提斯洋俯冲板片发生拆沉，将形成以地幔橄榄岩等基性-超基性岩为主的岩石组合类型，岩石普遍具高MgO、高Cr、高Ni含量及高Mg#值特征(Williams et al., 2001)，如宁真埃达克质侵入岩体(Xu et al., 2002)。本文研究的浦桑果黑云母花岗闪长岩和闪长玢岩具埃达克质地球化学属性，侵入岩体普遍具低MgO、低Cr、低Ni含量和低Mg#值的特征(表 2)；岩体侵位形成时代均为中新世，而南拉萨地体在中新世时期(18~10Ma)为后碰撞伸展的构造背景(Chung et al., 2009; Lee et al., 2009)。综上所述，浦桑果侵入岩产出的构造背景和地球化学特征明显不符合由新特提斯俯冲洋壳发生断离或拆沉作用形成的埃达克质岩石特征，故可排除上述岩石成因观点，这与中新世吉如矿床埃达克质侵入岩(Yang et al., 2016)和冲江矿床埃达克质侵入岩体(Hu et al., 2017)的岩石成因观点相符合。
冈底斯成矿带广泛发育的南北向断裂带(24~10Ma)的形成与埃达克质岩浆岩的分布存在紧密的时空关系，Wang et al. (2014)认为，南拉萨地体经历了从碰撞挤压的构造背景(Hou et al., 2004; Ji et al., 2009; Liu et al., 2011; Chen et al., 2012)到中新世(18~10Ma)后碰撞伸展环境的构造转换(Zhao et al., 2009; Liu et al., 2011)。其中，~65Ma：新特提斯大洋板块俯冲作用停止(Hou et al., 2013; Zhao et al., 2015)。65~50Ma，早期低角度俯冲的新特提斯洋壳板片拖拽印度板块岩石圈进入俯冲带，导致印度大陆与亚洲大陆发生陆-陆碰撞，区域构造背景开始进入碰撞挤压的构造环境，并在拉萨地块形成大量的同碰撞型火山侵入岩体(图 13a)。50~42Ma，新特提斯洋壳板片发生板片断离，引起热的软流圈通过板片断离窗上涌(Guo et al., 2013; Zhao et al., 2015)，导致拉萨地块新生下地壳发生部分熔融，形成大面积分布的林子宗群火山岩和冈底斯岩基(Ji et al., 2012)(图 13b)。42~25Ma，印度板块向北低角度底垫于拉萨地块之下，导致南拉萨地体地壳增厚和喜马拉雅强烈挤压变形，印度板块上地壳与下地壳发生分离形成念青唐古拉山古老结晶基地；软流圈的持续上涌，导致加厚岩石圈的拆沉和地幔减薄，引起地壳的伸展和张性正断层的发育以及印度板块下地壳及岩石圈地幔部分发生拆沉(图 13c)，并释放大量流体和熔体交代拉萨地块岩石圈地幔中含石榴子石二辉橄榄岩形成富集岩石圈地幔(Mo et al., 2008; Tian et al., 2017; Liu et al., 2017)，形成钾质或超钾质火山岩(Liu et al., 2014; Hou et al., 2015)。
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图 13 浦桑果铜多金属矿床中新世埃达克质侵入岩体的岩石成因及地球动力学模型简图 Fig. 13 Schematic diagram showing the petrogenetic and geodynamical model for the Miocene adakitic intrusions in the Pusangguo copper polymetallic deposit |
25~18Ma，冈底斯成矿带区域构造背景主要为南北向挤压和东西向伸展的构造背景(Xu et al., 2016; Li et al., 2017)。软流圈不断上涌，诱发由低角度底垫印度板块释放的流体交代含石榴子石二辉橄榄岩形成的富集岩石圈地幔发生部分熔融作用，从而形成早期富碱且含Cu、Co等金属物质基性幔源岩浆(浦桑果辉长岩脉)。18~10Ma，基性幔源岩浆低侵至加厚新生下地壳，导致加厚新生下地壳中富含石榴子石的角闪岩相发生部分熔融，形成富含Cu-Pb-Zn、富水、碱性的高氧逸度闪长质熔体(埃达克质岩浆)，最终在有利的构造部位发生侵位，形成具埃达克质属性的黑云母花岗闪长岩和闪长玢岩侵入体，并在浦桑果矿区中酸性侵入岩体与塔克那组灰岩接触带，形成广泛的矽卡岩化和矽卡岩型铜多金属矿体(图 13d)。
5.3 埃达克质岩浆与斑岩-矽卡岩成矿系统尽管部分埃达克质岩浆与斑岩-矽卡岩型铜矿床密切相关(Mungall, 2002; 侯增谦等, 2003)，但并非所有埃达克质岩浆岩都能形成斑岩-矽卡岩型铜矿床，因此，埃达克质岩浆的含矿性一直都是学术界争论的热点话题(Defant and Kepezhinskas, 2001; Oyarzun et al., 2001; Hou et al., 2009)。此外，埃达克质岩浆的氧逸度会影响硫元素在硅酸盐熔体中的溶解和沉淀，从而直接影响亲铁和亲铜元素的溶解度(Botcharnikov et al., 2011; Richards, 2011)，含水且具较高氧逸度的埃达克质岩浆更有利于成矿，可直接形成斑岩-矽卡岩型矿床(Hou et al., 2009; Wang et al., 2014b)。
含水条件下，元素Ti更易寄主于金红石中而元素Nb更易寄主于角闪石中(Hou et al., 2004)，浦桑果埃达克质侵入岩体相对富集Th，U等轻稀土元素(LREE)而强烈亏损Nb，Ta，Ti等重稀土元素(HREE)(表 2、图 7a, b)，指示岩浆源区可能存在残留的金红石和角闪石，源区岩石可能为石榴角闪岩相(Mahoney et al., 1998)，这与(La/YbN)-YbN图解(图 6d)结果一致。研究认为，大陆板片不太可能产生高氧逸度的超临界流体(Bissig et al., 2003)，早期南拉萨地体深部向北俯冲的板片为特提斯洋壳板片，而非印度大陆板片(Owens and Zandt, 1997)。因此，南拉萨地体之下的岩石圈应该是含水的(Hou et al., 2004)，因来自早期俯冲大洋板块的高氧逸度超临界流体将直接被引入到下地壳中(Bissig et al., 2003)，指示岩浆源区可能受到洋壳俯冲作用的改造。Kay and Mpodozis (2001)研究证实由角闪岩相发生部分熔融作用形成的埃达克质岩Sm/Yb比值为5~7，浦桑果矿床埃达克质侵入岩的Sm/Yb比值为4.5~5.8，平均值为5.1(表 2)，表明在岩石发生部分熔融的过程中存在富含角闪石或石榴石的矿物集合体的分解作用。角闪石的分解将释放大量的流体，从而形成含矿的埃达克质岩浆，这对于形成斑岩-矽卡岩成矿系统极为有利(Reich et al., 2003; Hou et al., 2009)。浦桑果矿床埃达克质中酸性侵入岩浆主要由加厚新生下地壳发生部分熔融形成，富集Cu、Pb、Zn等成矿元素，大量地壳物质加入初始的埃达克质熔体中，从而导致金属成矿富集形成(Hou et al., 2009)；当含水埃达克质熔体处于高氧逸度状态时，金属元素可与矿物结晶熔化过程产生的大量挥发份相结合，从而发生金属元素的沉淀富集(Sillitoe and Thompson, 1998)。
6 结论基于浦桑果矿床中酸性侵入岩体(黑云母花岗闪长岩和闪长玢岩)的锆石U-Pb年代学、全岩主微量稀土元素地球化学特征、全岩Sr-Nd-Pb及锆石Hf同位素组成特征的研究，结合区域邻区矿床已有资料，可得出如下结论：
(1) LA-ICP-MS锆石U-Pb年代学表明，浦桑果矿床黑云母花岗闪长岩和闪长玢岩的侵位年龄均为13~14Ma，岩体形成时代为中新世。岩石地球化学特征表明，岩石普遍均具高K2O、SiO2和Sr含量，低Yb和Y含量，高(La/Yb)N和Sr/Y比值特征，具埃达克质岩的地球化学属性。
(2) 年代学、地球化学及同位素特征综合表明，浦桑果矿床的中酸性侵入岩体的岩浆主要起源于拉萨地块加厚新生下地壳的部分熔融，且与南拉萨地体大多数斑岩-矽卡岩型铜多金属矿床与成矿密切相关的埃达克质侵入岩具相似的岩浆起源，不同矿床可能具同源岩浆性质。
(3) 浦桑果矿床埃达克质中酸性侵入岩体主要形成于后碰撞伸展的构造背景，因碰撞挤压至后碰撞伸展背景的构造转换，导致拉萨地块岩石圈地幔发生部分熔融形成富含Cu、Co等金属的基性岩浆熔体，基性岩浆底侵加厚新生下地壳，引起加厚新生下地壳中富含石榴子石的角闪岩相发生部分熔融，从而形成闪长质熔体(埃达克质岩浆)，沿区域断裂等构造通道上涌，最终在浦桑果矿区有利构造位置侵位形成具埃达克质属性的黑云母花岗闪长岩和闪长玢岩侵入岩体，在接触带形成矽卡岩型铜多金属矿体。
致谢 野外工作中得到了西藏自治区地质矿产勘查开发局第六地质大队王茂丽工程师的大力支持；锆石U-Pb定年及主微量测试及全岩Sr-Nd-Pb同位素分析过程和锆石Lu-Hf同位素分析过程中分别得到了中国地质大学(北京)相鹏老师和北京核工业地质研究院刘牧老师和中国地质科学院地质研究所王铮老师的大力支持和耐心指导；匿名审稿人对本文提出了诸多宝贵意见和建议；贵刊主编和编辑认真评阅了本文；在此一并深表衷心的感谢！
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