岩石学报  2019, Vol. 35 Issue (7): 2124-2142, doi: 10.18654/1000-0569/2019.07.11   PDF    
引用本文
谢富伟, 郎兴海, 唐菊兴, 肖鸿天, 马笛. 2019. 拉萨地块南缘晚白垩世角闪辉长岩、花岗斑岩副矿物微量元素特征对成岩成矿的指示. 岩石学报, 35(7): 2124-2142, doi: 10.18654/1000-0569/2019.07.11  
Xie FW, Lang XH, Tang JX, Xiao HT and Ma D. 2019. Trace element geochemistry of zircon, apatite, and titanite of Late Cretaceous hornblende gabbro and granite porphyry in the southern Lhasa subterrane: Implications for petrogenesis and mineralization. Acta Petrologica Sinica, 35(7): 2124-2142(in Chinese with English abstract)  
拉萨地块南缘晚白垩世角闪辉长岩、花岗斑岩副矿物微量元素特征对成岩成矿的指示
谢富伟1, 郎兴海1, 唐菊兴2, 肖鸿天1, 马笛3     
1. 成都理工大学地球科学学院, 自然资源部构造成矿成藏重点实验室, 成都 610059;
2. 中国地质科学院矿产资源研究所, 自然资源部成矿作用与资源评价重点实验室, 北京 100037;
3. 中国地质大学, 北京 100083
2018-08-01 收稿; 2019-01-04 改回.
基金项目: 本文受国家重点研发计划课题（2018YFC0604106、2018YFC0604105）、中国地质调查局项目（DD20160346）、深地资源成矿作用与矿产预测重点实验室开放基金项目（ZS1911）联合资助
第一作者简介: 谢富伟, 男, 1990年生, 讲师, 矿物学、岩石学、矿床学专业, E-mail:xiefuwei@hotmail.com
通讯作者: 郎兴海, 男, 1982年生, 教授, 主要从事矿床学、矿产普查与勘探的教学和研究工作, E-mail:langxinghai@126.com.
摘要: 拉萨地块南缘记录了新特提斯洋俯冲到印度-欧亚大陆碰撞及碰撞后的岩浆作用，其中晚白垩世的岩浆作用对研究印度-欧亚大陆碰撞前成岩成矿作用具有重要的意义。本文以拉萨地块南缘100Ma的角闪辉长岩和68Ma花岗斑岩的锆石、磷灰石、榍石为研究对象，利用背散射、阴极发光（CL）、电子探针（EPMA）和LA-ICP-MS原位微区分析等方法，查明锆石、磷灰石、榍石的主、微量元素特征，进一步反演岩石源区性质、结晶历史及结晶条件，并对岩体含矿性进行评价，有助于探讨冈底斯成矿带晚白垩世岩浆成因机制和成矿潜力。研究结果表明，角闪辉长岩锆石初始饱和温度为598~626℃，锆石Ti结晶温度为645~758℃，磷灰石饱和温度为690~819℃，榍石Zr温度为602~778℃；磷灰石具有中等-弱的负铕异常（δEu=0.67）、富集LREE，早期高温阶段锆石的结晶主要受到磷灰石结晶影响，随着温度降低，受到少量榍石结晶的影响；角闪辉长岩中的锆石在低的Hf、温度较高时却具有较高的Th、U含量显示岩浆源区受到更多俯冲板片出溶流体的影响，磷灰石具有较高的（La/Sm）N值以及Sr含量低于主岩，显示岩浆源区均一、熔体富Cl特征。花岗斑岩的锆石初始饱和温度为704~736℃，锆石Ti温度为630~799℃，磷灰石饱和温度为846~891℃，结合锆石的Ce/Sm、Yb/Gd以及磷灰石较大的负铕异常（δEu=0.29），显示花岗斑岩中的锆石从高温到低温阶段都受到磷灰石和榍石的共同结晶影响，磷灰石的结晶受到斜长石影响；花岗斑岩中大部分锆石Ti结晶温度高于其初始饱和温度，磷灰石Sr-Ap/Sr-WR为0.78~1.45，具有较高的F/Cl（32.87~67.60）、低的（La/Sm）N，指示花岗斑岩岩浆源区不均一，受到多期岩浆熔体的脉冲式灌入并加入了更多镁铁质的岩浆熔体，其熔体具有低的Cl。此外，花岗斑岩锆石具有较高的Ce4+/Ce3+、磷灰石具有较高的SO3、熔体中更富S，指示花岗斑岩具有高的氧逸度和成矿潜力。本文研究结果表明结合锆石、磷灰石和榍石微量元素特征可有效指示岩浆岩的源区组成、结晶历史、结晶条件以及成矿潜力，为岩石学的研究提供了一个新的思路。
关键词: 角闪辉长岩    花岗斑岩    锆石    榍石    磷灰石    拉萨地块南缘    
Trace element geochemistry of zircon, apatite, and titanite of Late Cretaceous hornblende gabbro and granite porphyry in the southern Lhasa subterrane: Implications for petrogenesis and mineralization
XIE FuWei1, LANG XingHai1, TANG JuXing2, XIAO HongTian1, MA Di3     
1. MNR Key Laboratory of Tectonic Controls on Mineralization and Hydrocarbon Accumulation, College of Earth Science, Chengdu University of Technology, Chengdu 610059, China;
2. MNR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China;
3. China University of Geosciences, Beijing 100083, China
Abstract: The southern Lhasa subterrane recorded magmatism related to Neo-Tethyan subduction, collision and post-collision between the two continents of Indian and Eurasian. The Late Cretaceous magmatism is of great significance to the study of pre-collisional petrogenesis and mineralization of the Indo-Eurasian continent. In this paper, zircon, apatite, and titanite from the 100Ma hornblende gabbro and the 68Ma granite porphyry in the southern Lhasa subterrane are studied. The main and trace elements of zircon, apatite, and titanite are identified by means of Back Scattering, Cathodoluminescence (CL), Electron Probe Analysis (EPMA) and LA-ICP-MS in situ microanalysis. The purpose of this paper is to further identify the magma sources, the crystallization histories and conditions of the magmatic rocks, and also to evaluate their ore-bearing properties, which is helpful to explore the genetic mechanisms and metallogenic potential of the Late Cretaceous magmatism in the Gangdese magmatic belt. For the studied hornblende gabbro, the results show that its zircon saturation and crystallization temperatures vary from 598~626℃ and 645~758℃, respectively, the apatite saturation temperatures vary from 690~819℃, and the titanite crystallization temperatures vary from 602~778℃. The chondrite-normalized REE patterns of apatite grains from the hornblende gabbro show enriched LREE values with moderate to negative negative Eu anomalies (δEu=0.67). The zircon compositions from the hornblende gabbro were initially a result of high-temperature apatite and further cooling by minor titanite crystallization. The low Hf but high crystallization temperature zircon grains from the hornblende gabbro have high Th and U contents, showing the magma source of hornblende gabbro was modified by more slab-derived fluids. The magma source of hornblende gabbro is homogeneous, and the melt is rich in Cl as evidenced by the higher Sr contents and (La/Sm)N values in apatite relative to the host rocks. For granite porphyry, the zircon saturation and crystallization temperatures vary from 704~736℃ and 630~799℃, respectively, while the apatite saturation temperatures vary from 846~891℃. Combined with zircon Ce/Sm and Yb/Gd ratios and larger negative Eu anomalies of apatite (δEu=0.29), the results show the zircon compositions from granite porphyry were affected by the co-crystallization of apatite and titanite from high temperature to low temperature, and the crystallization of apatite grains were affected by plagioclase crystallization. Most zircons of the granite porphyry have higher crystallization temperatures than their zircon saturation temperatures combined with the apatite grains, since the later have low Sr-Ap/Sr-WR (0.78~1.45), high F/Cl (32.87~67.60) and low (La/Sm)N. The granite porphyry have heterogeneous magma source, which were affected by impulse magma melts and involved more mafic magma melts. Compared with hornblende gabbro, the melt of the granite porphyry has low Cl contents. In addition, the zircon grains have high Ce4+/Ce3+, high apatite SO3 and more S contents in the melt of granite porphyry, indicating that granite porphyry have high oxygen fugacity and ore-forming potential. The results of this study show that the trace element geochemistry of zircon, apatite and titanite can effectively identify the magma source composition, crystallization history, crystallization conditions and metallogenic potential of magmatic rocks, which provide a new idea for petrological research.
Key words: Hornblende gabbro    Granite porphyry    Zircon    Apatite    Titanite    The southern Lhasa subterrane    

副矿物的微量元素及同位素组成可被有效应用于岩石成因研究，揭示记录在岩浆岩中的地质演化历史信息(Bea, 1996; Guo et al., 1996; Schaltegger et al., 1999)。锆石、磷灰石和榍石是岩浆岩中最常见的副矿物。锆石(ZrSiO4)富集MREE、HREE、Hf、U、Th，并随温度(T)、压力(P)及共存熔体/流体相的组分的变化而变化(Hanchar and Van Westrenen, 2007)。因此，可被用于评价岩浆的结晶年龄、结晶温度、氧逸度、岩浆演化历史过程等(Ballard et al., 2002; Miller et al., 2003; Watson et al., 2006; Lu et al., 2016)；磷灰石(Ca5(PO4)3(Cl, F, Cl))富集轻稀土(LREE)，同样对其初始结晶时岩浆成岩或成矿系统中的微量元素特别敏感(Sha and Chappell, 1999; Belousova et al., 2001)，被广泛应用于岩石成因及矿产勘查研究(Sha and Chappell, 1999; Hoskin et al., 2000; Belousova et al., 2002; Cao et al., 2012; Miles et al., 2013; Zirner et al., 2015; Mao et al., 2016; Chu et al., 2009)；榍石(CaTi(SO4) O)中含有大量稀土元素(REEs)及高场强元素(HFSEs)(Tiepolo et al., 2002; Gao et al., 2012; Deng et al., 2015)，对熔体的温度、压力、氧逸度及熔体组分十分敏感((Piccoli et al., 2000; Frost et al., 2001; Tiepolo et al., 2002; McLeod et al., 2011)，同样被用于岩石成因研究(Piccoli et al., 2000; Frost et al., 2001; Tiepolo et al., 2002; Mazdab, 2009; Smith et al., 2009; McLeod et al., 2011; Che et al., 2013; Xu et al., 2015)。

拉萨地块南缘(30°N以南)白垩纪岩浆作用最早由Schärer et al.(1984)报道，在最近十年，大量白垩纪岩浆岩在该带发现，已成为冈底斯岩基的主要组成部分。Wen et al.(2008a)首次总结了拉萨地块南缘的岩浆作用，认为其主要形成于103~80Ma，并伴随着83~80Ma的埃达克质花岗闪长岩的岩浆活动而终止(Wen et al., 2008b)。但米林地区100~89Ma的紫苏花岗岩(Ma et al., 2013c)、努日地区96~91Ma的花岗岩(Chen et al., 2015)、泽当地区~92Ma的花岗闪长岩(Jiang et al., 2015)、克鲁地区93~91Ma的石英二长岩(Jiang et al., 2012)都具有埃达克岩地球化学属性。近些年，Ji et al.(2014)在拉萨地块南缘北缘发现80~66Ma的晚白垩世岩浆作用，该期岩浆作用在73~69Ma存在一个岩浆间隙期。结合拉萨地块南缘已发现的白垩纪岩浆作用年代学数据，Xie et al.(2018c)将拉萨地块南缘白垩纪岩浆作用划分为：(1)109~97Ma的弱岩浆作用期；(2)96~86Ma的岩浆作用“爆发期”；(3)85~74Ma的弱岩浆作用期；(4)68Ma的岩浆“爆发期”。我们在拉萨地块南缘发现了100Ma的角闪辉长岩和~68Ma的花岗斑岩，两类新发现的岩石分别与109~97Ma的新特提斯洋低角度北向俯冲和68Ma加厚地壳岩石圈拆沉作用有关(Xie et al., 2018c)。

我们以这两套新发现的岩石为研究对象，分别挑选出锆石、磷灰石、榍石单矿物，利用背散射、阴极发光(CL)、电子探针(EPMA)和LA-ICP-MS等手段进行原位微区分析，查明68Ma花岗斑岩和100Ma角闪辉长岩的锆石、磷灰石、榍石的主、微量元素特征，反演岩石源区性质、结晶历史、结晶条件(温度、压力、氧逸度、挥发分等)，并对岩体含矿性进行评价，研究结果有助于探讨印度-欧亚大陆碰撞前冈底斯成矿带晚白垩世成岩成矿作用，对碰撞前岩浆成因机制和成矿潜力的评价有重要意义。

1 地质背景及岩石学特征

拉萨地块位于班公湖-怒江缝合带和雅鲁藏布缝合带之间(Yin and Harrison, 2000; Song et al., 2019)，与柴达木、松潘-甘孜、羌塘和喜马拉雅地块一同构成了青藏高原(Allégre et al., 1984; Dewey et al., 1988; Yin and Harrison, 2000; Pan et al., 2012)。洛巴堆-米拉山断裂和狮泉河-纳木错混杂岩带将拉萨地块由北向南依次分为北拉萨地块、中拉萨地块和南拉萨地块(Zhu et al., 2011)。目前，拉萨地块中生代岩浆作用的时空分布已初步建立：晚三叠世-中侏罗世(Chu at el., 2006;董彦辉等，2006; 潘桂棠等, 2006; Ji et al., 2009; Zhu et al., 2011; Guo at el., 2013; Kang et al., 2014；Lang at el., 2014; Tang at el., 2015; Meng et al., 2016a, b)以及晚白垩世(Schärer et al., 1984; McDermid et al., 2002; Wen et al., 2008a, b; Ji et al., 2009, 2014; Zhang et al., 2010, 2014; 管琪等, 2011; Jiang et al., 2012, 2014, 2015; Ma et al., 2013a, b, c, 2015; Zhu et al., 2011; 梁华英等, 2010; Chen et al., 2015; 叶丽娟等, 2015)的岩浆作用主要分布在南拉萨地块；晚侏罗世的岩浆主要分布在中拉萨地块(Murphy et al., 1997; Volkmer et al., 2007; Zhu et al., 2011; 姜昕等, 2010; 杜德道等, 2011; 张晓倩等, 2012)；早白垩世的岩浆作用主要分布在中拉萨地块和北拉萨地块(孟繁一等, 2010; Zhu et al., 2011; Cao et al., 2016)。

本文研究区位于拉萨地块南缘(图 1a)。拉萨地块南缘主要由晚三叠世-中新世的冈底斯岩基(Ji et al., 2009; Zhu et al., 2011)、195~93Ma的桑日群火山沉积岩(康志强等, 2009, 2010; Kang et al., 2014)、193~174Ma的叶巴组火山-沉积岩(董彦辉等，2006; 潘桂棠等, 2006; Zhu et al., 2008)、69~43Ma的林子宗群火山岩(Coulon et al., 1986; Chung et al., 2005; He et al., 2007; Lee et al., 2009, 2012; Mo et al., 2007, 2008)组成(图 1b)。尽管拉萨地块南缘已发现大量的白垩纪岩浆岩，但已报道的岩体多集中在日喀则到米林之间，而且镁铁质岩浆岩较少。花岗斑岩位于谢通门县以南(图 2a)，为斑状结构，斑晶主要为斜长石(25%~40%)和石英(10%~15%)，基质为长英质(40%~55%)(图 3a, c)，副矿物主要为锆石、磷灰石和少量铁氧化物，不含榍石。角闪辉长岩位于大竹卡北东方向(图 2b)，主要由长石(40%~50%)和角闪石(35%~45%)组成(图 3b, d)，角闪石和斜长石粒径分布在0.5~2.5cm之间，副矿物有锆石、磷灰石、榍石和铁氧化物。

图 1 青藏高原构造格架(a，据Ji et al., 2009修改)及拉萨地块南缘白垩纪岩浆岩分布图(b) 数据来源：McDermid et al., 2002; Schärer et al., 1984; Chen et al., 2015; Ji et al., 2009, 2014; Jiang et al., 2012, 2014, 2015; Ma et al., 2013a, b, c, 2015; Wen et al., 2008a, b; Zhang et al., 2010, 2014; 管琪等, 2011; Zhu et al., 2011; 梁华英等, 2010; 叶丽娟等, 2015.年龄上标为数据编号(见Xie et al., 2018c) Fig. 1 Simplified tectonic sketch map showing the distribution of the Transhimalayan batholiths (a, modified after Ji et al., 2009) and distribution of Cretaceous magmatic rocks in the southern Lhasa subterrane (b) Age data from: McDermid et al., 2002; Schärer et al., 1984; Chen et al., 2015; Ji et al., 2009, 2014; Jiang et al., 2012, 2014, 2015; Ma et al., 2013a, b, c, 2015; Wen et al., 2008a, b; Zhang et al., 2010, 2014; Guan et al., 2011; Zhu et al., 2011; Liang et al., 2010; Ye et al., 2015.The superscripts are the data number (Xie et al., 2018c)

图 2 谢通门(a, 据周清山和苟金，1997①修改)和大竹卡(b，据胡敬仁，2002②修改)地区地质简图 Fig. 2 Simplified geological maps of the Xietongmen area (a) and the Dazhuqu area (b)

① 周清山，苟金. 1997. 1:200000谢通门县幅地质图

② 胡敬仁. 2002. 1:250000日喀则市幅地质图

图 3 谢通门花岗斑岩(a、c)和大竹卡角闪辉长岩(b、d)野外和显微镜下照片 Pl-斜长石; Qtz-石英; Hbl-角闪石 Fig. 3 Field photographs and representative microstructures of the Xietongmen granite porphyry (a, c) and the Dazhuqu hornblende gabbro (b, d) Pl-plagioclase; Qtz-quartz; Hbl-hornblende
2 分析方法

锆石、磷灰石、榍石单矿物的挑选在廊坊市科大岩石矿物分选技术服务有限公司进行。首先，将角闪辉长岩和花岗斑岩分别破碎至适当粒级，经经摇床、淘洗、电磁及重力分选，在角闪辉长岩中分离出锆石、榍石和磷灰石单矿物，在花岗斑岩中分离出锆石和磷灰石单矿物。再由双目镜下挑纯，将分选出的锆石、磷灰石和榍石清洗后分别制成环氧树脂样品靶。将锆石、磷灰石和榍石磨制抛光后用于背散射(BSE)、阴极发光(CL)、EPMA主量元素和LA-ICP-MS微量元素分析。花岗斑岩的锆石和磷灰石CL图像见图 4；角闪辉长岩的锆石CL、磷灰石CL和榍石BSE图像见图 5。

图 4 花岗斑岩锆石(a)和磷灰石(b)CL图像 Fig. 4 The representative cathodoluminescence (CL) images of zircon grains (a) and apatite grains (b) from granodiorite porphyrite

图 5 角闪辉长岩中锆石(a)和磷灰石(b)CL图像及榍石背散射图像(c) Fig. 5 The representative cathodoluminescence (CL) images of zircon grains (a) and apatite grains (b) and the backscattered electron images of the titanite grains (c) in the hornblende gabbro

磷灰石和榍石主量元素分析在天津地质调查中心进行，实验仪器为JEOL EPMA-1600。测试电压为25kV，电流为10nA。F、S、Cl、Fe元素分析背景信号时间为40s，Na、Mg、Al、Si、P、K、Mn、Ca、Ti、V元素背景时间为20s。主量元素的允许相对误差小于2%。以下天然矿物或作为标定矿物：磷灰石(P、Ca)、石英(Si)、萤石(F)、硬石膏(S), 硅铍铝钠石(Cl)、硬玉(Na)、镁铝榴石(Mn)、磁铁矿(Fe)、钛铁矿(Ti)、钾长石(K)、铬铁矿(Cr)。磷灰石主量元素分析结果见表 1，榍石主量元素分析结果见表 2。

表 1 角闪辉长岩和花岗斑岩中磷灰石电子探针分析结果(wt%) Table 1 EPMA of chemical composition (wt%) of apatite grains from the hornblende gabbro and granodiorite porphyrite samples

表 2 角闪辉长岩中榍石电子探针分析结果(wt%) Table 2 EPMA of chemical composition (wt%) of titanite grains from the hornblende gabbro samples

锆石的微量元素分析中国地质大学(北京)地质过程与矿产资源国家重点实验室矿床地球化学微区分析室完成。ICP-MS试验仪器为美国产Thermo Fisher X-Series Ⅱ型四极杆电感耦合等离子体质谱仪，激光剥蚀系统为美国产Geolas 193准分子固体进样系统。激光束斑直径为32μm，频率为8Hz，激光剥蚀以He作为载气，Ar为补偿气。NIST SRM 610作为微量元素含量测定的外标，锆石91500作为内标，锆石GJ1作为监控样品。每5个样品测点之间测量2次91500，每个测点分析时间为100s，包括20s的背景信号采集时间，50s的激光剥蚀时间。测试完成后，应用软件ICPMSDataCal对分析数据进行处理(Liu et al., 2008)。分析结果见表 3。

表 3 角闪辉长岩和花岗斑岩锆石LA-ICP-MS微量元素分析数据(×10-6) Table 3 LA-ICP-MS trace element (×10-6) data of zircon grains from hornblende gabbro and granodiorite porphyrite samples

磷灰石和榍石微量元素分析在中国地质科学院地球化学研究所矿床地球化学国家重点实验室完成。激光剥蚀系统为GeoLasPro，ICP-MS为Agilent 7700x，激光剥蚀以He作为载气，激光剥蚀直径为44μm，频率4Hz。每个测点分析时间为90s，包括15s的背景信号采集时间，55s的分析时间和20s的剥蚀后时间。对于磷灰石，NIST SRM 610和NIST SRM 612作为微量元素含量测定的外标，为了检测试验的准确度度和精度，分析完NIST SRM 610和NIST SRM后对磷灰石Madagascar和Durango进行分析。Madagascar和Durango的推荐值来自Mao et al. (2016)。NIST SRM 610、NIST SRM 612、Madagascar、Durango中微量元素的分析精度优于5%。43Ca作为LA-ICP-MS分析中的内标元素，Ca的浓度取自电子探针数据，当该测点无探针数据时，选取各样品中磷灰石中的平均Ca浓度。利用ICPMSDataCal对分析数据进行处理(Liu et al., 2008)。对于榍石，NIST SRM 610、NIST SRM 612、BHVO-2G、BIR-1G和BCR-2G作为微量元素含量测定的外标，KL2-G和ML3B-G作为质量监控，43Ca作为均一化元素，为了提高和保证LA-ICP-MS分析的准确性，我们对Si、Fe、Ca、Ti四个主量元素进行了LA-ICP-MS分析，并和相应测点的电子探针数据进行比对，发现主量元素分析最大误差低于7.9%。NIST SRM 610、NIST SRM 612、BHVO-2G、BIR-1G、BCR-2G作为未知样品的微量元素允许相对误差小于10%。KL2-G和ML3B-G的分析精度优于5%。以上结果说明本分析方法可靠并具有较高的准确度。数据处理同样利用ICPMSDataCal软件(Liu et al., 2008)。磷灰石微量元素分析结果见表 4，榍石微量元素分析结果见表 5。

表 4 角闪辉长岩和花岗斑岩磷灰石LA-ICP-MS微量元素分析数据(×10-6) Table 4 LA-ICP-MS trace element (×10-6) data of apatite grains from hornblende gabbro and granodiorite porphyrite samples

表 5 角闪辉长岩榍石LA-ICP-MS微量元素分析数据(×10-6) Table 5 LA-ICP-MS trace element (×10-6) data of titanite grains from hornblende gabbro samples
3 分析结果 3.1 锆石微量元素

角闪辉长岩锆石的Ti含量变化大，分布在4.26×10-6~15.94×10-6之间，平均值8.27×10-6；Th/U分布在0.77~1.82之间，平均值1.17。稀土总量(ΣREE)分布在129×10-6~972×10-6之间，平均值530×10-6；稀土配分模式上亏损LREE，富集HREE，LREE/HREE分布在0.02~0.04之间；具有较大的负铕异常，δEu分布在0.29~0.80之间，平均值0.53(δEu=2×EuN/(SmN+GdN))。

花岗斑岩Ti分布在0.25×10-6~10.25×10-6之间(平均值4.94×10-6)，Th/U分布在0.59~1.29之间(平均值0.90)，亏损LREE，但LREE含量较高(603×10-6~2424×10-6)，LREE/HREE分布在0.03~0.06之间，具有较大的负铕异常，δEu分布在0.21~0.51之间，平均值0.35。

3.2 磷灰石主、微量元素

角闪辉长岩和花岗斑岩中磷灰石的主要成分为P2O5、CaO、MnO、SiO2、SO3、F和Cl。角闪辉长岩中磷灰石P2O5较为均一，分布在39.90%~40.86%之间；CaO分布在54.88%~56.93%之间；MnO含量较低，分布在0.03%~0.20%之间；SiO2分布在0.07%~0.18%之间。具有较高的F(1.85%~3.04%)和较低的Cl(0.35%~1.43%)。此外含有少量(0.10%)的FeO、K2O、Na2O(表 1)。角闪辉长岩中磷灰石的ΣREE分布在1217×10-6~4184×10-6之间，平均值2521×10-6；Sr含量分布在465×10-6~666×10-6之间，平均值581×10-6；稀土配分模式富集LREE(1197×10-6~4011×10-6)、亏损HREE(20×10-6~173×10-6)，LREE/HREE分布在18.47~60.32之间。具有变化较大的负铕异常，δEu分布在0.44~1.01之间，平均值0.67。

花岗斑岩中的磷灰石含量P2O5分布在40.08%~41.38%之间，平均值40.63%；CaO分布在53.70%~56.57%之间；MnO含量较低，分布在0.02%~0.15%之间，平均值0.11%；SiO2分布在0.09%~0.29%之间。含有较高的F(1.69%~2.84%，平均值2.14%)，低的Cl(0.17%~0.72%，平均值0.36%)。花岗斑岩中磷灰石ΣREE含量较高，分布在2111×10-6~7457×10-6之间，平均值4561×10-6；Sr含量分布在325×10-6~603×10-6之间，平均值436×10-6；富集LREE，亏损HREE，LREE/HREE分布在7.03~10.79之间。具有较大的负铕异常，δEu分布在0.19~0.44之间，平均值0.29。

3.3 榍石主、微量元素

角闪辉长岩中榍石具有较为均一的CaO (25.64%~26.25%)、TiO2 (36.52%~38.28%)、SiO2(28.62%~29.65%)、V2O5 (0.37%~0.50%)、Al2O3(0.61%~0.86%)；变化的Fe2O3(0.79%~1.11%)；MnO含量较少(0.07%~0.12%)；少数榍石颗粒含有少量F(< 0.16%)。Sr含量分布在39×10-6~56×10-6之间，平均值44×10-6；Zr含量变化较大，分布在40×10-6~1066×10-6之间，平均值414×10-6；富集LREE，亏损HREE，LREE/HREE分布在7.03~10.79之间8.16~32.48之间。(La/Yb)N分布在7.53~50.7之间；具有明显的正铕异常，δEu分布在1.01~1.89之间，平均值1.28。Na/Ta比值分布在5.80~49.6之间，平均值23.5。

4 讨论 4.1 锆石微量元素地球化学

根据火成岩的全岩主量成分的M指数(M =(Na+K+2Ca)/(Al×Si))及Zr浓度可以估算锆石初始的饱和温度(Watson and Harrison, 1983)。本文角闪辉长岩和花岗斑岩的M指数分别为2.83~3.15及2.07~2.27(Xie et al., 2018c)，根据lnD=(-3.80-(0.85×M-1))+12900/T(D为锆石中Zr浓度与熔体中Zr浓度的比值，T为绝对温度)(Watson and Harrison, 1983)计算的角闪辉长岩的锆石初始饱和温度为598~626℃(平均值613℃，n=5)，花岗斑岩的锆石初始饱和温度为704~736℃(平均值717℃，n=4)。

由于矿物原位微区分析的发展，矿物微量元素温压计得到广泛的应用，其原理主要依据微量元素在矿物及其存在的熔体/流体相之间的分配系数遵循Nernst定律。由于锆石的稳定性及锆石中的Ti含量随着岩体的SiO2增加而降低的特征，锆石的Ti温度计经常用于温度计算(Watson et al., 2006)。Ti主要替换锆石中的Si而发生ZrSO4+TiO2=ZrTiO4+SiO2或TiO2+SiO2=TiSiO4的反应，因此锆石的Ti温度计受到SiO2和TiO2的活度影响，锆石的Ti温度计计算公式被修正为：log(Ti-in-zircon)=(5.711±0.072)-(4800+86)/T(K) -logaSiO2+logaTiO2(Ferry and Watson, 2007; Fu et al., 2008)。考虑到角闪辉长岩中存在榍石，不存在石英，取aSiO2=0.5，aTiO2=0.7，计算得到角闪辉长岩中锆石Ti温度分布在645~758℃之间，平均值为692℃(n=18)(表 3)。而花岗斑岩中仅少量榍石、金红石等含Ti矿物，存在石英，取aSiO2=1，aTiO2=0.6，计算得到花岗斑岩中锆石Ti温度分布在630~799℃之间，平均温度731℃(n=13)。随着角闪辉长岩和花岗斑岩的锆石Ti温度的增加，锆石中Hf、Th/U降低(图 6a, b)，与拉萨地块南缘侏罗纪侵入岩(Xie et al., 2018b)和智利El Salvador斑岩铜矿区的侵入岩(Lee et al., 2017)一致。

图 6 锆石T-Hf (a)、T-Th/U(b)、年龄-Ce4+/Ce3+(c)、Ce/Sm-Yb/Gd(d)、Hf-Th(e)及Hf-U(f)图解 Fig. 6 Diagrams of T vs. Hf (a), T vs. Th/U (b), age vs. Ce4+/Ce3+ (c), Ce/Sm vs. Yb/Gd (d), Hf vs. Th (e) and Hf vs. U (f) from zircon

上述结果表明角闪辉长岩的锆石Ti结晶温度低于其初始饱和温度，花岗斑岩的大部分锆石Ti温度却高于其初始饱和温度，说明这部分高锆石Ti温度的锆石来自于更早的(温度更高的)熔体(Lee et al., 2017)，花岗斑岩较大的温度变化范围也证实可能存在多期岩浆熔体的脉冲式灌入。引起锆石温度差异的原因包括岩浆的脉冲式灌入、岩浆的混合以及与围岩的同化混染作用，但花岗斑岩具有较为均一的正的Hf同位素组成(+9.05~+12.38)(Xie et al., 2018c)，因此引起温度变化的原因可能是岩浆的脉冲式灌入。

锆石中四价微量元素Hf、Th、U由于离子半径与Zr相似而容易替代Zr进入锆石晶格中，而相对三价的LREE，HREE也更容易替代Zr进入锆石，但Ce存在Ce3+、Ce4+两种价态，由于Ce4+离子半径更接近Zr4+，因此锆石表现为显著的正Ce异常。在熔体/流体更为氧化时，Ce以Ce4+为主，会替代更多的Zr4+，因此锆石中的Ce4+/Ce3+可用于计算锆石结晶时熔体/流体相的氧逸度(Ballard et al., 2002; Trail et al., 2012)。由于现有的光谱分析方法无法测定锆石中Ce4+/Ce3+比值，Ballard et al.(2002)利用晶体化学原理方法得出利用以下公式：Ce4+/Ce锆石3+=(Ce熔体-(Ce锆石/DCe(Ⅲ)))/((Ce锆石/DCe(Ⅵ)))- Ce熔体)，其中DCe(Ⅲ)为锆石和熔体中Ce3+的分配系数，DCe(Ⅵ)为Ce4+在锆石和熔体中的分配系数。根据矿物-熔体之间的分配系数和晶体化学热力学公式可知：lnDi=4πENA/RT(ri/3+r0/6)(ri-r0)2，其中E为杨氏模量，NA为阿伏伽德罗常数，ri为锆石中某微量元素的离子半径，r0为最优离子半径，R为气体常数，T为热力学温度。因此lnDi是(ri/3+r0/6)(ri-r0)2的线性函数。所以当Ce以Ce3+时，其分配系数落在三价稀土元素拟合的直线上，而以Ce4+存在时将落在四价元素(Hf、Th、U、Zr)拟合的直线上。根据上述原理，我们获得角闪辉长岩的锆石Ce4+/Ce3+分布在7~137之间，平均值62(n=18)；花岗斑岩的锆石Ce4+/Ce3+分布在112~626之间，平均值333(n=15)(表 3，图 6c)。角闪辉长岩较低的锆石Ce4+/Ce3+值指示其岩浆具有较低的氧逸度，而花岗斑岩具有较高的锆石Ce4+/Ce3+值指示其岩浆具有较高的氧逸度。

影响岩浆分离结晶过程中各单矿物微量元素的差异受控于：1)初始熔体中微量元素的含量；2)岩浆结晶时微量元素进入不同矿物相的晶体化学性质；3)不同结晶环境(如氧逸度、温度、压力等)导致的微量元素的分馏。在锆石的Th/U-Yb/Gd图中(图 6d)，角闪辉长岩和花岗斑岩中所有的锆石都从高温演化到低温(高的Th/U演化到低的Th/U)，Yb/Gd逐渐增大，说明两套岩体的锆石结晶都受到分离结晶的影响，而未受到与围岩的混染，因为受到围岩的混染影响，锆石的Th/U降低，Yb/Gd会保持不变或变化较小(Lee et al., 2017)。锆石中Ce/Sm和Yb/Gd的增加显示熔体中相对于LREE和HREE，MREE更为亏损。锆石中不同微量元素的差异受到榍石、磷灰石和角闪石的结晶影响(Grimes et al., 2015)。这是因为磷灰石富集LREE和MREE(Fujimaki, 1986；Chu et al., 2009)，榍石与磷灰石具有相似也富集MREE(Sha and Chappell, 1999)，而角闪石与锆石相似，富集HREE(Bea, 1996; Hanchar and Van Westrenen, 2007)。因此，当磷灰石、榍石、角闪石比锆石先结晶或同时结晶时，会影响锆石的微量元素组分(如Ce/Sm、Yb/Gd)。根据Ce/Sm-Yb/Gd图解(图 6d)(Lee et al., 2017)，角闪辉长岩早期高温阶段锆石的结晶主要受到磷灰石结晶影响，随着温度降低，受到少量榍石结晶的影响；而花岗斑岩中的锆石从高温到低温阶段都受到磷灰石和榍石的共同结晶影响。

正常的岩浆演化过程由于岩浆的分离结晶会使残余熔体的Th、U含量升高，Th/U降低(Miller and Wooden, 2004)。花岗斑岩随着锆石中Hf的升高(温度降低)，Th和U升高，Th/U降低(图 6e, f)，符合正常岩浆的演化。但在Hf-Th(图 6e)和Hf-U(图 6f)图解中角闪辉长岩中的锆石在低的Hf、温度较高时却具有较高的Th、U含量。前人研究表明如果锆石中Hf升高，温度降低，Th、U相对保持不变或降低，而不是像预期那样增加，可能是岩浆富含水流体，流体活动元素从俯冲板片出溶(Bailey and Ragnarsdottir, 1994)。因此，角闪辉长岩的岩浆相比于花岗斑岩更加富集含水流体。板片俯冲过程中大离子亲石元素(LILEs，如Rb、Ba、Sr、K、U)为流体活动性元素，而LREEs和高场强元素(HFSEs)为熔体活动性元素，在全岩的Th/Nb-Ba/Th图解中(图 7, Xie et al., 2018c)，同样显示角闪辉长岩岩浆源区受到更多俯冲板片出溶流体的影响(Elliott et al., 1997)。

图 7 角闪辉长岩和花岗斑岩的Th/Nb-Ba/Th图解(底图据Elliott et al., 1997修改) Fig. 7 The diagram of the whole-rock Th/Nb vs. Ba/Th ratios of the hornblende gabbro and granodiorite porphyrite (base map after Elliott et al., 1997)
4.2 磷灰石微量元素地球化学

Piccoli and Candela (1994)提出假设全岩中的SiO2和P2O5代表了初始熔体的SiO2和P2O5浓度，利用公式：T=(26400×CSiO2/100-4800)/((12.4×CSiO2/100-ln(CP2O5/100))-3.97)(T为绝对温度，CSiO2为初始熔体中SiO2浓度，CP2O5为初始熔体中P2O5的浓度)可以获得磷灰石的初始饱和温度(AST)。根据上述公式，获得角闪辉长岩的磷灰石AST分布在690~819℃(平均值756℃)；花岗斑岩的磷灰石AST分布在846~891℃(平均值865℃)。磷灰石的AST、锆石的初始饱和温度以及锆石的Ti结晶温度皆表明花岗斑岩岩浆熔体的温度高于角闪辉长岩岩浆熔体。磷灰石的AST皆高于锆石的饱和温度以及锆石的Ti结晶温度表明锆石的结晶受到了磷灰石早期结晶的影响，这与锆石的Ce/Sm-Yb/Gd图解(图 6d)得出的结论一致。

磷灰石的REE、Eu异常、Sr和卤族元素记录了岩浆演化中的地球化学过程，被用于评价地质背景、岩浆结晶历史、岩浆的类型(Sha and Chappell, 1999; Hoskin et al., 2000; Belousova et al., 2002; Chu et al., 2009; Miles et al., 2013; Zirner et al., 2015)，也可用于指导矿产勘查(Mao et al., 2016)。磷灰石的微量元素组成受到岩石的铝饱和指数(ASI)影响(Sha and Chappell, 1999; Chu et al., 2009)，但角闪辉长岩的ASI为0.71~0.77(平均值0.73)与花岗斑岩相似(ASI=0.70~0.75，平均值0.72)，说明角闪辉长岩与花岗斑岩中磷灰石微量元素差异不是因为ASI的不同造成的，这与拉萨地块南缘与新特提斯洋俯冲有关的侏罗纪侵入岩相似(Xie et al., 2018b)。

Prowatke and Klemme (2006)认为长英质岩浆中结晶的磷灰石ΣREE与镁铁质岩浆中磷灰石的ΣREE呈倍数增长。花岗斑岩中磷灰石的ΣREE为4561×10-6，而角闪辉长岩中磷灰石ΣREE为2520×10-6，说明两套岩体的磷灰石微量元素受到岩浆酸性程度的影响。角闪辉长岩和花岗斑岩中的磷灰石具有不同的稀土配分模式，角闪辉长岩中的磷灰石轻、重稀土分馏较大，LREE富集、HREE亏损，具有中-弱的负铕异常。磷灰石是富集LREE(Sha and Chappell, 1999)并常表现为负铕异常(Bea, 1996)的矿物。角闪辉长岩中磷灰石中等-弱的负铕异常(δEu=0.67)以及典型的富集LREE的特征可能是磷灰石结晶较早，未受到其他矿物结晶的影响。磷灰石较高的饱和温度以及锆石Ce/Sm-Yb/Gd图解(图 6d)也显示角闪辉长岩中的磷灰石为早期的结晶矿物相。在稀土配分模式图上，角闪辉长岩中的磷灰石的Yb、Lu等HREE元素微弱的升高也显示磷灰石结晶时没有其他富HREE矿物相(锆石、角闪石等)的结晶。相对于角闪辉长岩，花岗斑岩中的磷灰石轻、重稀土分馏程度降低，其LREE/HREE分布在7.03~10.79之间，比角闪辉长岩中磷灰石相应比值降低2~6倍，表明花岗斑岩中磷灰石结晶时熔体中具有更低的LREE或磷灰石结晶时受到富LREE矿物同时结晶的影响。在锆石的Ce/Sm-Yb/Gd图解(图 6d)中，花岗斑岩中的锆石结晶受到磷灰石和榍石共同结晶的影响。榍石与磷灰石相似，也常表现为富集LREE，并具有负铕异常(Pan et al., 1993)。因此，花岗斑岩中的磷灰石可能受到榍石在早期或同时结晶的影响，导致其轻、重稀土分馏程度降低。花岗斑岩中磷灰石还具有较大的负铕异常(δEu=0.29)，Xie et al. (2018b)认为磷灰石较大的负铕异常是由于斜长石早于或同时与磷灰石结晶。这是因为磷灰石的铕异常反映了熔体的氧化还原性质或长石的结晶影响(Bea, 1996)。但花岗斑岩中磷灰石相比于角闪辉长岩具有更高的锆石Ce4+/Ce3+比值，但却具有更大的负铕异常，说明其铕异常与熔体的氧逸度无关，而与斜长石的结晶有关。

实验证实磷灰石分配MREE的能力强于LREE和HREE(Watson and Green, 1981; Fujimaki, 1986)。花岗斑岩中磷灰石的(La/Sm)N(1.90~3.46)低于全岩(La/Sm)N(4.73~5.99)，(Yb/Sm)N(0.08~0.16)远低于全岩(Yb/Sm)N(0.35~0.41)；然而角闪辉长岩中磷灰石(La/Sm)N(5.08~23.58)却远高于全岩(La/Sm)N(2.19~2.52)(图 8a)。前人实验研究表明富Cl流体相的出溶会导致熔体中LREE的亏损能力强于MREE和HREE，磷灰石从富Cl流体相出溶后的熔体中结晶时会具有高的F/Cl(低Cl)、低的(La/Sm)N值(Flynn and Burnham, 1978; Keppler, 1996)。所以，花岗斑岩中磷灰石具有较高的F/Cl(32.87~67.60)、低的(La/Sm)N值可能是继承于高F/Cl熔体；但角闪辉长岩中磷灰石具有较高的(La/Sm)N值可能是因为未受到富Cl流体相出溶的影响，角闪辉长岩中磷灰石的Cl含量(0.10%~0.62%)也明显高于花岗斑岩中磷灰石(0.04%~0.09%)。

图 8 磷灰石与全岩的(La/Sm)N-(Yb/Sm)N(a)、Sr-Ap-Sr-WR(b)以及Cl-SO3(c)图解 Fig. 8 Diagrams of the whole rock and apatite (La/Sm)N vs. (Yb/Sm)N (a), the apatite Sr content (Sr-Ap) vs. whole-rock Sr content (Sr-WR) (b) and the apatite Cl content vs. SO3 content (c)

磷灰石中的Sr与其主岩的分馏程度、铝饱和指数等有关(Chu et al., 2009)。Belousova et al.(2001)也认为磷灰石中的Sr与主岩中SiO2、Al2O3、FeO、K2O、Rb/Sr呈线性关系。磷灰石中的Sr含量与其主岩中的Sr含量的关系可被用于指示岩浆混合以及岩浆熔体源区的均一性(Chu et al., 2009；Xie et al., 2018a, b)。角闪辉长岩中磷灰石的Sr含量分布在4665×10-6~666×10-6(平均值581×10-6)之间，低于主岩中Sr含量(724×10-6~787×10-6)，其Sr-Ap/Sr-WR比值分布在0.61~0.87之间；花岗斑岩中磷灰石的Sr含量分布在325×10-6~603×10-6(平均值436)之间，大部分磷灰石颗粒中的Sr含量高于其主岩中Sr含量(334×10-6~479×10-6)，其Sr-Ap/Sr-WR比值分布在0.78~1.45之间。在Sr-Ap-Sr-WR图解中(图 8b)，角闪辉长岩中磷灰石落在Sr-Ap:Sr-WR=1:1线之上，而大部分花岗斑岩中磷灰石落在Sr-Ap:Sr-WR=1:1线之下。Chu et al.(2009)认为源区均一正常演化的岩浆岩，其磷灰石中的的Sr含量应该低于主要中的Sr含量。角闪辉长岩中磷灰石中的Sr含量低于主岩，与拉萨地块南缘S型花岗岩、I型花岗岩、碰撞后的埃达克岩类似(Chu et al., 2009)，但花岗斑岩中大部分磷灰石的Sr含量均低于主岩，与雄村铜金矿集区Ⅰ号矿床含矿斑岩(Xie et al., 2018a)以及拉萨地块南缘侏罗纪白嘎花岗斑岩、山巴花岗闪长岩和大竹卡二长岩类似(Xie et al., 2018b)。具有更高Sr含量的镁铁质岩浆存在于岩浆房或岩浆源区会造成磷灰石的Sr含量高于主岩(Chu et al., 2009)。Sha and Chappell (1999)也认为镁铁质的I型花岗岩中的磷灰石的Sr含量高于长英质的I型或S型花岗岩。因此，我们认为花岗斑岩的岩浆源区是不均一的，加入了更多镁铁质的岩浆熔体，导致其早期结晶的磷灰石的Sr含量高于主岩。花岗斑岩较高的而且范围较大的锆石Ti结晶温度也证实花岗斑岩的岩石成因受到多期岩浆灌入的影响。

磷灰石中的Mn、As、Fe、S、Eu、Ce等元素对氧化还原条件十分敏感，这是因为这些元素在不同的氧化还原条件下表现为不同价态，例如S具有S2-、S4+、S6+共三种价态(Koneck et al., 2017)，As具有As3+、As5+两种价态(Shannon, 1976)，Fe具有Fe2+、Fe3+两种价态(Pan and Fleet, 2002)，Mn具有Mn2+、Mn3+、Mn4+、Mn5+四种价态(Sha and Chappell, 1999; Pan and Fleet, 2002)，Eu具有Eu2+、Eu3+两种价态(Prowatke and Klemme, 2006)，Ce具有Ce3+、Ce4+两种价态(Colombini et al., 2011)。由于磷灰石与共存熔体中As、Fe、Ce、Eu元素的分配实验较少，角闪辉长岩和花岗斑岩中磷灰石的Mn含量也较低，我们选取磷灰石中的S来评价两套岩体的氧逸度。前人研究表明磷灰石中的SO3含量可以用来评价共存熔体的氧逸度(Streck and Dilles, 1998; Imai, 2002; Parat and Holtz, 2004)以及硅酸盐熔体中的S含量(Peng et al., 1997; Parat et al., 2011)。Koneck et al. (2017)研究了磷灰石中S的不同价态与氧逸度的关系，认为氧逸度从FMQ(铁橄榄石-磁铁矿-石英氧逸度缓冲线，Huebner, 1971)到FMQ+1.2再到FMQ+3，S的价态从S2-为主到S6+>S4+，再到S6+>>S4+。磷灰石中SO3含量从氧逸度为FMQ时的0.04%增加到氧逸度为MH(磁铁矿-赤铁矿氧逸度缓冲线，Chou, 1978)时的1%~2.6%(Peng et al., 1997)。当硅酸盐熔体中硬石膏饱和时，磷灰石中SO3>0.5%(Parat and Holtz, 2005)。在磷灰石的Cl-SO3图解中(图 8c)，花岗斑岩中磷灰石SO3(平均值0.12%)较高，部分磷灰石中SO3含量高于NNO(Parat and Holtz, 2005)，说明花岗斑岩岩浆具有较高氧逸度；角闪辉长岩中磷灰石SO3较低(平均值0.07%)，说明角闪辉长岩岩浆氧逸度较低。这与花岗斑岩和角闪辉长岩的锆石Ce4+/Ce3+得出的结论一致。

4.3 榍石微量元素地球化学

榍石中的Zr对温度和压力极为敏感，可被用于岩浆结晶温度和压力的计算(Hayden et al., 2008)。我们根据角闪辉长岩中的角闪石压力计获得了角闪辉长岩的结晶压力为0.175GPa，侵位深度为5.3~7.0km (Xie et al., 2018c)，根据公式：T=(7708+960×P)/(10.52-log10(Zr榍石))(P为压力，单位为GPa，T为绝对温度，Hayden et al., 2008)获得了角闪辉长岩中榍石的结晶温度为602~778℃(平均值714℃)。该温度表明部分榍石颗粒结晶温度大于锆石的Ti结晶温度，说明存在少量榍石先于锆石结晶。角闪辉长岩中的锆石微量元素特征也表明随着温度的降低，部分锆石颗粒的结晶受到了榍石结晶的影响。

榍石是许多微量元素的重要储库(Tiepolo et al., 2002)，共存熔体的组分以及矿物相的结晶是引起榍石微量元素差异的主要原因(Watson, 1976; Tiepolo et al., 2002; Prowatke and Klemme, 2006; Smith et al., 2009; Anand and Balakrishnan, 2011; Olin and Wolff, 2012)，但共存熔体组分的影响大于其他矿物相结晶(Xu et al., 2015)。角闪辉长岩中榍石富集LREE，亏损HREE，(La/Yb)N分布在7.53~50.7之间，具有正铕异常(δEu=1.01~1.89)。榍石是富LREE的矿物，常表现为负铕异常(Pan et al., 1993)，角闪辉长岩中榍石的正铕异常可能与其岩浆氧逸度有关。这是因为Eu3+(i.r., 0.1066Å)与Ca2+(i.r., 0.112Å)具有相似的离子半径(Pan et al., 1993; Tiepolo et al., 2002)，在还原条件下Eu3+还原为Eu2+替代榍石中的Ca2+，造成榍石的正铕异常。Micko(2010)也认为正铕异常反应了氧化性的流体与还原性岩石发生了反应。因此，角闪辉长岩中榍石具有正铕异常反映其岩浆氧逸度较低，与拉萨地块南缘的大竹卡二长岩相似(Xie et al., 2018b)。

4.4 对晚白垩世岩浆成岩成矿作用的指示

西藏是我国重要的铜金属资源储备和开发基地，随着甲玛、雄村、铁格隆南、拿若等矿床的相继发现(Lang et al., 2014; Tang et al., 2015; Lin et al., 2017, 2019; Song et al., 2018)，西藏已经初步查明的铜资源量大于6000万吨(唐菊兴等，2017)。位于拉萨地块南缘的冈底斯成矿带不仅分布着与新特提斯洋俯冲有关的弧岩浆系统形成的斑岩型铜金矿床(Lang et al., 2014; Tang et al., 2015)，还存在众多后碰撞环境下形成的斑岩-矽卡岩型铜(钼)多金属矿床，其铜金属资源量已超过30Mt。目前在冈底斯成矿带上已发现的成矿作用主要分为三期：1)侏罗纪斑岩型Cu-Au矿化；2)古新世-始新世的的斑岩型Cu-Mo矿化、矽卡岩型Pb-Zn ± Cu-Mo-Ag-Au矿化；3)中新世斑岩型Cu-Mo或斑岩-矽卡岩型铜多金属矿化。但白垩纪的成矿作用仅发现94.5Ma桑布加拉矽卡岩型铜矿床(赵珍等，2012)和与~90Ma的埃达克岩有关的克鲁铜金矿床(Jiang et al., 2012)。斑岩型矿床成矿的关键因素包括岩浆富水、高氧逸度、富金属以及富S(Richards, 2003; Cooke et al., 2005; Sillitoe, 2010)。为了评价拉萨地块南缘白垩纪岩浆作用的成矿潜力，我们分别对角闪辉长岩和花岗斑岩的岩浆氧逸度和S含量两个成矿关键因素进行了研究。

磷灰石中的SO3随着岩浆的温度、氧逸度和硫逸度而变化(Peng et al., 1997; Parat and Holtz, 2005; Parat et al., 2011)，通过磷灰石中的SO3无法准确测定岩浆中S含量，但是通过磷灰石/熔体的分配实验可以半定量的获取岩浆中S的含量(Peng et al., 1997; Parat et al., 2011)。我们利用Parat et al.(2011)提出的方法获得角闪辉长岩岩浆中S含量分布在0.0010%~0.0013%之间，平均值0.0012%；花岗斑岩岩浆中S含量分布在0.0009%~0.0043%之间，平均值0.0018%。通过Peng et al.(1997)的方法获得角闪辉长岩岩浆中S含量分布在0.0002%~0.0004%之间，平均值0.0003%；花岗斑岩岩浆中S含量分布在0.0012%~0.0102%之间，平均值0.0044%。可见不论用哪种方法，花岗斑岩岩浆中S含量均高于角闪辉长岩，甚至高于十倍以上。花岗斑岩中锆石具有较高的Ce4+/Ce3+、磷灰石具有较高的磷灰石的SO3，指示花岗斑岩具有较高的氧逸度。通过Myers and Eugster (1983)对FMQ上氧逸度与温度的关系：log fO2=-24441.9/T (K) + 8.290 (±0.167)(T为磷灰石饱和温度)，计算得到角闪辉长岩log fO2分布在-17.10~-14.13之间，而花岗斑岩log fO2分布在-13.51~-12.71之间，同样说明花岗斑岩氧逸度较高，角闪辉长岩的氧逸度较低。

上述结果表明花岗斑岩具有较高的S含量和氧逸度，指示更好的成矿潜力。花岗斑岩与Ji et al.(2014)报道的拉萨地块南缘68~60Ma的基性-酸性的侵入岩具有相似的地球化学性质(Xie et al., 2018c)。区域上，同时代的火山岩为林子宗群典中组，形成时限为69~60Ma(Zhou et al., 2004; He et al., 2007; 李皓揚等, 2007)，与拉萨地块南缘68~60Ma的侵入岩具有相似的地球化学特征和成因机制(Mo et al., 2008; Ji et al., 2014)。已发现的同时代矿床包括产于林子宗群典中组中的纳入松多隐爆角砾岩型铅锌矿床，成矿时代~58Ma(纪现华等，2014)以及新发现的斯弄多低硫化浅成低温热液型银铅锌矿床(唐菊兴等，2016)，成矿时代为63~61Ma(Li et al., 2019)。林子宗群典中组火山岩及同时代侵入岩在拉萨地块南缘分布广泛，随着纳入松多、斯弄多等矿床的发现以及同时代侵入岩具有较好的成矿潜力，相信拉萨地块南缘晚白垩世火山-侵入岩覆盖区必将取得新的找矿突破！

5 结论

通过对拉萨地块南缘~100Ma角闪辉长岩和~68Ma花岗斑岩中锆石、磷灰石、榍石主、微量元素地球化学特征的研究，我们得出以下结论：

(1) 结合锆石饱和温度、Ti结晶温度、Hf、Th/U、Ce/Sm、Yb/Gd，磷灰石饱和温度、REE、Sr，榍石结晶温度、REE等可以有效指示岩浆结晶历史、结晶条件和源区性质。角闪辉长岩早期高温阶段锆石的结晶受到磷灰石结晶影响，随着温度降低，受到少量榍石结晶的影响；而花岗斑岩中的锆石从高温到低温阶段都受到磷灰石和榍石的共同结晶影响，磷灰石的结晶受到斜长石影响。

(2) 花岗斑岩的岩浆源区是不均一，受到多期岩浆熔体的脉冲式灌入，加入了更多镁铁质的岩浆熔体，其熔体具有富F/Cl的特征；而角闪辉长岩源区均一，熔体具有富Cl特征。

(3) 花岗斑岩具有较高的S含量和氧逸度，指示较好的成矿潜力。

(4) 结合岩浆岩中锆石、磷灰石、榍石微量元素特征可有效指示岩浆源区组成、结晶条件、结晶历史以及成矿潜力。

致谢      实验过程得到中国地质大学(北京)相鹏、天津地质调查中心郭虎、中国科学院地球化学研究所戴智慧的帮助；匿名评审专家提出了诸多宝贵意见；在此一并表示衷心感谢！

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