2020, Vol. 36
2. 中国地质科学院地质研究所, 自然资源部深地动力学重点实验室, 北京 100037;
3. 重庆市地质矿产勘查开发局川东南地质大队, 重庆 400038;
4. 中国地质大学(北京), 北京 100083
2. MNR Key Laboratory of Deep-Earth Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
3. Southeast Sichuan Geological Team, Chongqing Municipal Bureau of Geology and Mineral Exploration and Development, Chongqing 400038, China;
4. China University of Geosciences, Beijing 100083, China
作为岩浆演化过程晚期的结晶产物，高分异花岗岩往往经历了较为强烈的岩浆分离结晶作用，在矿物组合上以较低暗色矿物含量和较高的石英及长石含量为主要特征(Tao et al., 2013)，而岩浆演化过程中的分离结晶作用在岩浆W-Sn-Nb-Ta元素的富集中起了重要作用(Jiang et al., 2016)，因此高分异花岗岩具有其特殊的成矿专属性，并因此受到广泛关注(吴福元等, 2007a, 2015; Li et al., 2007; Liu et al., 2016)。现有研究表明，一些矿床如我国的华南、藏南以及欧洲的华力西造山带等地区分布的W-Sn-Nb-Ta、REE和Li-Be等矿床均被认为与高分异花岗岩具有成因上的关联(Yin et al., 1995; Marignac and Cuney, 1999; Zhu et al., 2001; Wu et al., 2017)。因此，高分异花岗岩的识别和成因研究可以为深化相关矿床研究提供直接的依据。
青藏高原自古生代以来经历了复杂的多地块拼贴历史，并被认为与不同时期的特提斯洋闭合及两侧地块的碰撞有关，漫长的地质演化历史为诸多矿床提供了良好的成矿条件(Yin and Harrison, 2000; Metcalfe, 2011, 2013; Xu et al., 2015)。班公湖-怒江结合带(后简称班-怒带)是青藏高原内一条重要缝合带，记录了班公湖-怒江特提斯洋(后简称班-怒洋)在中生代期间的打开及闭合事件，并因带内多龙矿集区、尕尔穷-嘎拉勒铜金矿床和舍索-雄梅铜矿化带的发现而受到广泛关注(宋扬等, 2014及其文献, 2019; Li et al., 2017a)。班-怒带两侧的南羌塘、北拉萨地块中发育了大量白垩纪花岗岩浆作用(潘桂棠等, 2006; 朱弟成等, 2006; 曲晓明等, 2009; Fan et al., 2016; Hao et al., 2016a, b; Li et al., 2013, 2014, 2016a, b, 2017a, b; Liu et al., 2012, 2014, 2017; Zhang et al., 2017; Zhu et al., 2011, 2016)，作者在区域地质调查工作中发现，部分岩体具有明显的高分异花岗岩特征。因此，班-怒带为寻找具有成矿潜力岩体并进行岩石成因方面的研究提供了良好条件。
本文将对北拉萨地块班戈地区的曲梅勒高分异花岗岩进行年代学、岩石地球化学和锆石Hf同位素方面的研究，讨论其岩石成因类型和岩浆演化过程，并对其含矿性进行评价，为今后找矿工作提供线索。
1 区域地质背景及岩相学青藏高原由多块体组成，自北向南依次为柴达木、松潘-甘孜、羌塘、拉萨和喜马拉雅等地块，并依次为阿尼玛卿-昆仑-慕士塔格、金沙江、班公湖-怒江和雅鲁藏布江等多个结合带所隔(Yin and Harrison, 2000; Zhu et al., 2013)(图 1a)。班-怒带分隔北侧的羌塘地块与南侧的拉萨地块，东西向延伸约2000km，主要由侏罗纪-白垩纪复理石、混杂岩和不连续分布的蛇绿岩套组成，代表了班-怒洋盆的残余。拉萨地块自北向南可分为北-中-南三个次级块体(Zhu et al., 2013)。其中北拉萨地块位于班-怒带和狮泉河-纳木错混杂带之间，主要出露中生代地层，如侏罗系砂岩夹火山岩夹层(接奴群)、复理石沉积建造(拉贡塘组)、灰岩(日拉组)，上覆早白垩世火山-沉积单元(去申拉和多尼组)、灰岩(朗山组)以及晚白垩世磨拉石建造(竟柱山组)(Leier et al., 2007)。此外，带内发育大量中-酸性岩浆岩，主要时代为白垩纪，被认为与班-怒洋的南向俯冲有关，典型代表为班戈岩基和阿翁错岩基(高顺宝等, 2011; Zhu et al., 2009a, 2011, 2013, 2016)。区域构造线以NW-SE向为主，断层多具逆冲性质，控制区内地层和岩体的展布方向。
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图 1 班戈地区区域地质简图 (b)据西藏自治区地质调查院, 2002①; 吉林省地质调查院, 2003②; 年龄数据高顺宝等, 2011; Haider et al., 2013; Volkmer et al., 2014; Zhu et al., 2016 Fig. 1 Distribution of Early Paleozoic granites and geological sketch map of Baingoin area Age data sources: Gao et al., 2011; Haider et al., 2013; Volkmer et al., 2014; Zhu et al., 2016 |
① 西藏自治区地质调查院. 2002. 1:25万班戈县幅区域地质图
② 吉林省地质调查院. 2003. 1:25万多巴区幅区域地质图
曲梅勒岩体位于班戈县城NNW方向5km左右，呈岩株状产出，出露面积较小(图 1b、图 2a)。1:25万多巴幅地质调查中，前人认为曲梅勒岩体侵入于SE方向的早白垩世班戈岩体，并且北部为古-始新统牛堡组砂岩不整合覆盖(吉林省地质调查院, 2003)。本次1:5万区域地质调查过程中，未见与班戈岩体石英闪长岩部分直接接触关系，根据锆石U-Pb定年结果，推测石英闪长岩时代晚于曲梅勒岩体。此外，曲梅勒岩体与北侧下白垩统多尼组(原古新统-始新统牛堡组)出露砂岩的接触界面见明显烘烤面及角岩化现象，推测北侧地层早于曲梅勒岩体。岩石外表中-轻度风化，节理较为发育，新鲜面为灰白色(图 2b)。岩性为二长花岗岩，少量钾长石和斜长石斑晶，其余为中-粗粒石英(30%)+斜长石(30%)+钾长石(35%)，以及少量黑云母和白云母，金属矿物少见(图 2c, d)。其中石英呈他形粒状充填在长石间隙中，波状消光普遍，裂隙发育；斜长石自形程度较高，聚片双晶发育，多绢云母化；钾长石多呈短柱状，主要为条纹长石和正长石，见简单双晶，普遍有较弱的泥化现象；黑云母半自形片状，解理发育，部分蚀变为白云母，见少量含铁质等成分沿解理缝析出；白云母多呈细小片状分布在斜长石等矿物颗粒中。此外，还可见锆石、榍石和磁铁矿等副矿物。
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图 2 班戈地区曲梅勒花岗岩野外(a、b)及显微镜下(c、d)照片 Kf-钾长石；Pl-斜长石；Qz-石英；Bt-黑云母；Ms-白云母 Fig. 2 Outcrops (a, b) and photomicrographs (c, d) of Qumeile granite in Baingoin area Kf-K-feldspar; Pl-plagioclase; Qz-quartz; Bt-biotite; Ms-muscovite |
锆石样品的分选、制靶和阴极发光(CL)图像在北京锆年领航科技有限公司实验室完成。样品破碎至80~120目，经淘洗粉尘、去除磁性矿物、重液分选等程序；在双目体视显微镜下随机挑选一定数量的锆石颗粒粘在双面胶上，然后以低温环氧树脂浇铸，然后在自动抛光机上对锆石表面进行抛光，并依次用酒精、洗涤剂和水在超声波中对靶表面进行清洗。制靶完成后先在偏光显微镜下按最宽视域依次进行透射光和反射光照相，然后对锆石靶进行镀金处理，CL图像是在配备了英国Gatan阴极荧光探头的日本电子JSM6510型电子显微镜采集的。
测试分析在中国地质科学院国家地质实验测试中心完成。锆石U-Pb定年工作所用的激光系统为美国New Wave公司生产的钇铝榴石系统，波长为213nm，束斑直径可调。质谱仪为Thermo Element XR。激光剥蚀采样过程以氦气作为载气，氦气携带样品气溶胶在进入ICP之前通过一个T型三通接头与氩气(载气、等离子体气和补偿气)混合。通过调节氦气和氩气气流大小，以获得NIST SRM 612(美国国家标准技术研究院研制的人工合成硅酸盐玻璃标准参考物质)最佳信号为条件实现测试系统最优化。优化条件主要为信号灵敏度最高、氧化物产率最低、双电荷干扰最小、气体空白最低和信号强度最稳定。在束斑直径为35μm、频率为10Hz、能量密度约为8J/cm2激光剥蚀条件下，线扫描方式剥蚀NIST SRM 612可获得：232Th灵敏度可达2×105cps/ppm；氧化物产率ThO/Th < 0.3%。每个样品同时测试202Hg、204Pb、206Pb、207Pb、208Pb、232Th、235U、238U等同位素，样品测试时采样方式为单点剥蚀、跳峰采集；单点采集时间模式为：20s气体空白+40s样品剥蚀+20s冲洗；每10个未知样品点插入一组标样。GJ-1作为外标进行数据校正，Plesovice用于数据质量监控，数据处理采用Glitter软件。
2.2 主、微量元素分析本项研究所选新鲜样品的岩石粉末碎样、化学全分析工作澳实分析检测(广州)有限公司完成。氧化物用X荧光光谱仪(XRF)测试，试样加入包含硝酸锂在内的助熔剂，充分混合后，高温熔融。熔融物倒入铂金模子形成扁平玻璃片后，再用X荧光光谱仪分析。同时称取另一份试样放入马弗炉中，于1000℃加热1小时。冷却后称重。样品加热前后的重量差即是烧失量。FeO经硫酸和氢氟酸消解后测定，试样倒入装有稀硫酸或磷酸、硼酸溶液的烧杯中，用重铬酸钾溶液滴定。稀土元素和微量元素试样称取两份，一份试样用高氯酸、硝酸、氢氟酸消解。蒸至近干后的样品用稀盐酸溶解定容，再用等离子体发射光谱与等离子体质谱进行分析。另一份试样加入到偏硼酸锂/四硼酸锂熔剂中，混合均匀，在1025℃以上的熔炉中熔化。熔液冷却后，用硝酸、盐酸和氢氟酸定容，再用等离子体质谱仪分析。根据样品的实际情况和消解效果，综合取值即是最后的检测结果。
2.3 LA-ICP-MS Hf同位素锆石Hf同位素的分析是在前述锆石U-Pb同位素基础上完成的，测试在中国地质科学院地质研究所Neptune Plus型多接收等离子质谱和GeoLasPro 193nm激光剥蚀系统(LA-MC-ICP-MS)上进行的，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44μm，测定时使用锆石国际标样GJ-1作为参考物质。相关仪器运行条件及详细分析流程见侯可军等(2007)。分析过程中锆石标准GJ-1的176Hf/177Hf测试加权平均值分别为0.282007±0.000025(2σ)。计算初始176Hf/177Hf时，Lu的衰变常数采用1.865×10-11y-1(Scherer et al., 2001)，εHf(t)值的计算时采用球粒陨石Hf同位素值176Lu/177Hf=0.0336，176Hf/177Hf=0.282785(Bouvier et al., 2008)。在Hf的地幔模式年龄计算中，亏损地幔176Hf/177Hf现在值采用0.28325，176Lu/177Hf采用0.0384(Griffin et al., 2000)，地壳模式年龄计算时采用平均地壳的176Lu/177Hf=0.015(Griffin et al., 2002)。
3 分析结果 3.1 LA-ICP-MS锆石U-Pb定年用于进行锆石U-Pb定年的样品号为17BGA31-3。阴极发光图像中，该样品中锆石多呈灰白色，主要为短柱状，少数呈长柱状，长度介于80~200μm之间，长宽比1.5:1~2:1；清晰岩浆振荡环带显示其可能为岩浆成因(图 3a)。此外，较亮的阴极发光图像表明锆石中U、Th和REE等微量元素含量不高(Hanchar and Miller, 1993; Rubatto and Gebauer, 2000)。测试结果显示，锆石中Th含量为26×10-6~1129×10-6，U含量为362×10-6~2084×10-6，Th/U比值平均为0.13(表 1)，可能为岩浆成因(吴元保和郑永飞, 2004)。通过锆石内部的206Pb/238U进行加权平均年龄计算，该样品加权平均年龄为128±2Ma(MSWD=1.3, n=22)(图 3b, c)。此外，测点8、12和14的U-Pb年龄分别为360Ma、921Ma和400Ma，结合阴极发光图像分析，上述年龄可能来自于锆石捕虏晶。
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图 3 班戈地区曲梅勒花岗岩锆石阴极发光图像(a)、U-Pb谐和曲线(b)和加权平均年龄图(c) 图 3a中红色实线和黄色虚线分别代表LA-ICP-MS U-Pb定年和Hf同位素分析测点 Fig. 3 Cathodoluminescence (CL) images (a), U-Pb concordia (b) and weighted average age plots (c) of zircon grains from Qumeile granite in Baingoin area Red solid and yellow dashed circles indicate the locations of LA-ICP-MS U-Pb and Hf analyses in Fig. 3a, respectively |
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表 1 班戈地区曲梅勒花岗岩锆石LA-ICP-MS U-Th-Pb数据 Table 1 LA-ICP-MS zircon U-Pb isotopic analyses for the samples of Qumeile granite in Baingoin area |
全岩地球化学数据结果见表 2。其SiO2含量为74.40%~76.67%，FeOT含量为0.61%~1.59%，Na2O含量为2.73%~2.89%，K2O含量为4.61%~4.83%，Na2O+K2O含量为7.49%~7.56%，Na2O/K2O比值为0.57~0.62(表 2)。根据SiO2-Na2O+K2O分类图(图 4a)，全部样品落入了花岗岩区域内，根据SiO2-K2O图(图 4b)，该花岗岩为高钾钙碱性系列。Al2O3含量为12.82%~13.74%，A/CNK为1.19~1.24，A/NK为1.31~1.41。在A/CNK-A/NK分类图(图 5)，该花岗岩样品落入了过铝质区域，属强过铝质。总体上，具有高Si、K，富全碱，低Mg、Fe、Ca的特征。此外，曲梅勒花岗岩与中国部分具高分异特征的花岗质岩石在主量元素特征方面具有一定的相似性(图 4、图 5)。
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表 2 班戈地区曲梅勒花岗岩主量(wt%)和微量(×10-6)元素含量 Table 2 Major (wt%) and trace (×10-6) elements compositions of Qumeile granite in Baingoin area |
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图 4 班戈地区曲梅勒花岗岩SiO2-(Na2O+K2O)图(a, 据Middlemost, 1994修改)和SiO2-K2O图(b, 据Martin et al., 2005修改) 引文数据来源：邱俭生等(2008); Jahn et al. (2001); Wu et al. (2003a); Zhu et al.(2009b, 2015); Tao et al. (2013).图 5、图 8、图 10、图 11、图 13、图 14中数据来源及图例同此图 Fig. 4 Whole-rock SiO2 vs. Na2O+K2O (a, after Middlemost, 1994) and SiO2 vs. K2O (b, after Martin et al., 2005) classification diagrams of the granite phases from Qumeile granite in Baingoin area Data sources: Qiu et al. (2008); Jahn et al. (2001); Wu et al. (2003a); Zhu et al.(2009b, 2015); Tao et al. (2013). Data sources and symbols in Fig. 5, Fig. 8, Fig. 10, Fig. 11, Fig. 13, Fig. 14 as in this figure |
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图 5 班戈地区曲梅勒花岗岩A/CNK-A/NK图(据Maniar and Piccoli, 1989) Fig. 5 A/CNK vs. A/NK diagram of Qumeile granite in Baingoin area (after Maniar and Piccoli, 1989) |
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图 8 班戈地区曲梅勒花岗岩成因类型判别图解(据Whalen et al., 1987; d, I/S分异型花岗岩范围据Wu et al., 2017) Fig. 8 Discrimination diagrams of genetic type for Qumeile granite in Baingoin area (after Whalen et al., 1987; d, I/S-type fractionated granite data after Wu et al., 2017) |
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图 10 班戈地区曲梅勒高分异花岗岩判别图解(a, b, 据Whalen et al., 1987; c, 据Sylvester, 1989; d, 据Wang et al., 2013) FG-高分异I/S/M型花岗岩; OGT-非高分异I/S/M型花岗岩 Fig. 10 Discrimination diagrams for Qumeile highly-fractionated granite in Baingoin area (a, b, after Whalen et al., 1987; c, after Sylvester, 1989; d, after Wang et al., 2013) FG-fractionated I-, S-, and M-type granite; OGT-unfractionated I-, S-, and M-type granite |
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图 11 班戈地区曲梅勒花岗岩Harker图解 HPFC-高压条件下分离结晶 Fig. 11 Harker diagrams for Qumeile granite in Baingoin area HPFC-high-pressure fractional crystallization |
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图 13 班戈地区曲梅勒花岗岩(La/Yb)N-Nb/La (a, 据Xiong et al., 2011)、La-(La/Yb)N (b, 据Li et al., 2017c)、Th-LREE (c, 据Li et al., 2017c)和Eu-Sr (d, 据Xiang et al., 2017)关系图 Fig. 13 Nb/La vs. (La/Yb)N (a, after Xiong et al., 2011), (La/Yb)N vs. La (b, after Li et al., 2017c), LREE vs. Th (c, after Li et al., 2017c) and Sr vs. Eu (d, after Xiang et al., 2017) diagrams for Qumeile granite in Baingoin area |
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图 14 班戈地区曲梅勒花岗岩TE1, 3-Nb/Ta (a)和Zr-Hf-Nb/Ta (b)关系图(据Ballouard et al., 2016; Sami et al., 2018) Fig. 14 Nb/Ta vs. TE1, 3 (a) and Nb/Ta vs. Zr-Hf (b) diagrams for Qumeile granite in Baingoin area (after Ballouard et al., 2016; Sami et al., 2018) |
该花岗岩REE总量较低，为57×10-6~67×10-6(表 2)，配分模式表现为轻稀土相对富集的右倾型(图 6a)，其中轻重稀土LREE/HREE比值为3.32~5.89，(La/Yb)N为3.07~5.84，表明轻重稀土之间分馏程度较低(表 2)。(La/Sm)N为2.63~3.51，(Gd/Lu)N为1.07~1.38，反映了LREE和HREE元素内部分馏均不明显。此外，该类花岗岩具有中等-强烈的负Eu异常，Eu/Eu*为0.24~0.62。微量元素蛛网图中，该花岗岩主要富集大离子亲石元素(LILEs)Rb、Th、U和K等，相对亏损高场强元素(HFSEs)，并且在Ba、Nb、Ta、P和Ti等元素上表现出了明显的负异常(图 6b)。
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图 6 班戈地区曲梅勒花岗岩全岩球粒陨石标准化稀土元素配分模式(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. 6 Chondrite-normalized REE patterns (a) and primitive-mantle normalized spiderdiagrams (b) for Qumeile granite in Baingoin area (normalization values after Sun and McDonough, 1989) |
此外，根据Miller et al. (2003)提出的锆元素饱和浓度温度计进行计算，该花岗岩的TZr为721~740℃，平均温度为731℃；根据Rapp and Watson (1986)和Montel (1993)提出的稀土元素饱和浓度温度计进行计算，该花岗岩的TREE为741~756℃，平均温度为751℃(表 2)。
3.3 Hf同位素特征对曲梅勒花岗岩中15颗已测U-Pb年龄的锆石进行原位Hf同位素分析。分析结果表明，176Hf/177Hf范围为0.282588~0.282781，对应的εHf(t)变化于-3.8~+2.0，根据锆石U-Pb年龄计算的二阶段模式年龄(tDM2)变化于997~1426Ma之间(表 3、图 7)，可能代表其壳幔分异作用时代(吴福元等, 2007b)。
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表 3 班戈地区曲梅勒花岗岩锆石Lu-Hf同位素数据 Table 3 LA-ICP-MS zircon Hf isotopic analyses for the samples of Qumeile granite in Baingoin area |
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图 7 班戈地区曲梅勒花岗岩锆石εHf(t)频率直方图(a)、tDM2频率直方图(b)和Hf同位素组成(c，据Zhu et al., 2011) Fig. 7 Histograms of εHf(t) (a) and tDM2 (b), and Hf isotopic compositions (c, after Zhu et al., 2011) of zircons from Qumeile granite in Baingoin area |
角闪石、堇青石和碱性暗色矿物是判断花岗岩类岩石成因类型的直接证据，曲梅勒花岗岩镜下未见上述标志性矿物，而在缺少矿物学证据的情况下，岩石地球化学便成了良好的指标(Pitcher, 1997)。花岗岩岩石类型判别图解(Ce、K2O/MgO、K2O+Na2O及Zr对104×Ga/Al)中(图 8)，曲梅勒花岗岩因其较高的Ga/Al值，样品落入了A型花岗岩和靠近A型花岗岩的区域内(Whalen et al., 1987)。但是，A型花岗岩与SiO2>72%的I-/S-型高分异花岗岩，由于矿物组合和化学成分都趋近于最低共熔点组分，往往在地球化学特征上表现出一定的相似性(Whalen et al., 1987; King et al., 1997; Wu et al., 2003a, b)。而曲梅勒花岗岩高SiO2和全碱含量，低Fe、Mg和Ca含量以及低K/Rb比值，富集Rb、Th、U和Pb，亏损Ba、P和Ti等特征(图 6、表 2)，均暗示其可能属于高分异花岗岩(Blevin and Chappell, 1992; Chappell and White, 1992; Sawka et al., 1990; Sylvester, 1998; Foley et al., 2000; Clemens, 2003; Xiong et al., 2005; Xiao et al., 2014; Qiu et al., 2017)。并且单阶段的部分熔融无法产生具低Sr(< 100×10-6)和高Rb含量的花岗岩，因此曲梅勒岩体很可能经历了强烈的分离结晶作用(Halliday et al., 1991; Sami et al., 2018)。
曲梅勒花岗岩具有较低的Ba、Zr、Y、Nb、La和Ce含量，相对较低的锆元素饱和浓度温度(Miller et al., 2003; 721~740℃; 表 2)，以及亏损高场强元素的特征，也与A型花岗岩的特点明显不同(Whalen et al., 1987; Bonin, 2007; Dall’Agnol and de Oliveira, 2007)，同时，Li、Rb和Cs含量的显著增高则表明该岩体更倾向于属于高分异岩石(Gelman et al., 2014; Lee and Morton, 2015)。K/Rb、Zr/Hf、Nb/Ta和Y/Ho等地球化学行为一致元素的比值在一般的岩浆体系中并不发生数值的变化(Green, 1995; Wu et al., 2017)，但当岩浆由于分异而发生性质上的明显改变时，这些比值都将显著变小(Bau, 1996; Dostal and Chatterjee, 2000; Linnen and Keppler, 2002; Claiborne et al., 2006; Dostal et al., 2015; Ballouard et al., 2016; Deering et al., 2016)。曲梅勒花岗岩较低的分异程度相关参数也明显低于球粒陨石，暗示其经历了较高程度的分异(图 9)。而在主量和微量元素方面，曲梅勒花岗岩也与高分异花岗岩表现出了较强的相似性(图 4、图 5、图 6)。结合高分异花岗岩判别图解以及较高的分异指数(DI=90.5~94.9，通过CIPW计算)综合判断(图 10)，曲梅勒花岗岩属于高分异花岗岩。
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图 9 班戈地区曲梅勒花岗岩关键元素比值 TE1, 3=(TE1×TE3)0.5，TE1=(Ce/Ce*×Pr/Pr*)0.5，TE3=(Tb/Tb*×Dy/Dy*)0.5(Irber, 1999).球粒陨石引自Sun and McDonough (1989)，岩浆岩范围、花岗岩范围和大陆地壳范围引自Jiang et al. (2016) Fig. 9 Variations in several key elemental ratios as indicators of differentiation from Qumeile granite in Baingoin area TE1, 3=(TE1×TE3)0.5, TE1=(Ce/Ce*×Pr/Pr*)0.5, TE3=(Tb/Tb*×Dy/Dy*)0.5 (Irber, 1999). Chondrite data from Sun and McDonough (1989), range data of magmatic rocks, granitoids and continental rocks from Jiang et al. (2016) |
过铝质花岗岩通常被认为是(变)沉积岩部分熔融的产物(S型花岗岩)(Chappell and White, 1974; Barbarin, 1996; Collins, 1998; Douce and Harris, 1998; Sylvester, 1998; Patiño Douce, 1999; Healy et al., 2004)，曲梅勒花岗岩具强过铝质特征(A/CNK=1.19~1.24)，并被认为是S型花岗岩(吉林省地质调查院, 2003)。但近年地球化学和实验岩石学研究均表明，I型花岗岩也可以具有过铝质甚至是强过铝质特征(Beard and Lofgren, 1991; Chappell, 1999; Sisson et al., 2005; Chappell et al., 2012)。近年来磷灰石溶解度实验表明，SiO2-P2O5相关关系可作为判断初始岩浆类型的重要指标，磷灰石在I型花岗岩中含量较低且与SiO2呈负相关关系，而在S型花岗岩中呈相反趋势(Chappell and White, 1992; Wolf and Wyllie, 1994; Chappell, 1999; Wu et al., 2003a, b)。由于不同包晶矿物的夹带作用，I型花岗岩被认为随着SiO2的增加而从准铝质向过铝质转变，而S型花岗岩的A/CNK值则维持常数或轻微降低(Clemens and Stevens, 2012; Gao et al., 2016)。曲梅勒花岗岩在上述相关特征方面均与I型花岗岩更为接近(图 11h)。此外，Rb-Th关系暗示该岩体具有I型花岗岩的地化特征(Chappell, 1999)(图 12)。因此，根据地球化学结果，曲梅勒花岗岩为高分异I型花岗岩。
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图 12 班戈地区曲梅勒花岗岩Rb-Th关系图(据Chappell, 1999) Fig. 12 Th vs. Rb diagrams for Qumeile granite in Baingoin area (after Chappell, 1999) |
高分异花岗岩的属性表明曲梅勒岩体在岩浆演化过程中可能发生了部分矿物的分离结晶作用，因此全岩主量和微量数据已无法反映初始岩浆特征，而同位素数据则受此影响较小，因此需要通过结合野外、镜下以及同位素数据来判断岩石成因。幔源岩浆分异成因的花岗岩往往需要其九倍体积的镁铁质岩浆，而岩体附近区域内缺少如此大面积出露的镁铁质岩浆活动证据(Turner et al., 1992; Winter, 2013)。此外，幔源岩浆通常在地壳中居留时间较短，因此锆石具有正的εHf(t)，而曲梅勒岩体中锆石εHf(t)变化范围为-3.8~+2.0(图 7)，负值εHf(t)也不支持其源于幔源岩浆分异的成因模式。壳幔岩浆发生混合经常导致镁铁质微粒包体的出现，并且会造成锆石εHf(t)变化范围较大，而曲梅勒花岗岩εHf(t)变化范围较为集中，反映了其岩浆来源较为均一，同时野外未见幔源暗色微粒包体，进一步排除了地幔物质的影响(Wang et al., 2018)。因此，野外及锆石Hf同位素共同表明，曲梅勒岩体来源于地壳物质的部分熔融，并且在岩浆演化过程中发生了较高程度的分离结晶。锆石二阶段模式年龄(tDM2)介于1.0~1.4Ga之间，表明曲梅勒花岗岩源岩时代可能是形成时代不晚于中元古代的(变)火成岩。
富Ti矿物中，只有金红石的残留会导致Nb/La的减小和(La/Yb)N的增加(Xiong et al., 2011)，(La/Yb)N-Nb/La图解表明金红石并非源区的残留矿物(图 13a)，因此，TiO2与SiO2的负相关关系以及Ti的亏损可能反映了富Ti矿物(如钛铁矿和金红石等)的分异(图 11a)。P2O5与SiO2的负相关以及P的亏损则可能是由于磷灰石的分异所致(图 11h)。而Zr元素随着岩浆演化而减少以及在微量元素蛛网图上表现出的亏损(图 6b、图 11j)，则可能是由于锆石的分异作用(King et al., 1997)。此外，曲梅勒花岗岩中FeOT、MgO和CaO均与SiO2表现出了明显的负相关性，表明岩浆演化过程中镁铁矿物的分离结晶作用较为明显(图 11c-e)。镁铁矿物角闪石是赋存中稀土元素的重要矿物(李承东等, 2004)，较为平坦的中稀土配分模式和大体相当的HoN和YbN值均暗示分离结晶过程中可能涉及了角闪石(图 5a)。此外，石榴石的分异通常会导致特殊的地球化学特征，如La与SiO2之间的负相关关系，而Dy/Yb和Sr/Y比值会随SiO2增加而增加(Castillo et al., 1999; Macpherson et al., 2006; Karsli et al., 2010)，Dy/Yb和Sr/Y比值显示岩浆演化过程中石榴石的分异并不明显(图 11k, l)，尽管La随着岩浆演化确实表现出了降低的趋势，这可能是由于褐帘石和独居石对稀土，特别是对轻稀土元素具有的较强控制作用所致，La-(La/Yb)N和Th-LREE的关系均反映了褐帘石和独居石的分异直接影响了曲梅勒岩体中LREE元素的含量(Bea, 1996; Li et al., 2017c)(图 13b, c)。同时，由于石榴石富集重稀土元素，而曲梅勒花岗岩稀土配分模式图中重稀土元素并未出现亏损，表明源区同样未发生石榴石的残留(Rapp and Watson, 1995)。Al2O3、Eu与SiO2表现出明显的相关性(图 11b, g)，同时Sr、Ba和Eu均表现出了明显的负异常(图 6b)，因此不能排除岩浆演化过程中发生过强烈的斜长石分离结晶作用(图 13d)，而斜长石的分异会导致残余岩浆富Rb并具有较高的Rb/Sr比值。Na2O和K2O均未与SiO2表现出明显的线性关系(图 4b、图 11f)，则表明碱性长石的分异作用并不强烈。而明显的稀土元素四分组效应(TE1-3=1.04~1.14)通常被认为是在岩浆发生高度分异后，熔体中出现大量流体，而流体与熔体之间的水-岩相互作用会改变岩浆中稀土元素的化学行为，致使呈现特殊的配分模式(赵振华等, 1992; Jahn et al., 2001; Monecke et al., 2011; Ballouard et al., 2016)。
总之，曲梅勒花岗岩在岩浆演化过程中可能发生了以部分镁铁矿物和斜长石为主的分异，同时伴有部分副矿物的分离结晶，而石榴石以及碱性长石未发生明显的分离结晶。
4.3 含矿性评价现有研究表明，诸多重要矿产与高分异花岗岩具有重要关联。如华南地区江西大湖塘W矿床，中亚造山带内蒙古东部的沙麦W矿床，泰国北部Khuntan岩基内的W-Sn矿床等(Jiang et al., 2016及相关文献)；华南地区灵山复式岩体和雅山414岩体内的Nb-Ta矿(Yin et al., 1995; Xiang et al., 2017)，南秦岭的梨园堂岩体内的Mo矿床等(Xiao et al., 2014)。前人研究表明，岩浆演化过程中的分离结晶作用在岩浆W-Sn-Nb-Ta元素的富集中起了重要作用(Jiang et al., 2016)。
Nb/Ta比值被视为区分无矿和矿化过铝质花岗岩的重要指标(Ballouard et al., 2016)，并在埃及东部Eastern Desert地区的Abu-Diab含稀有金属白云母花岗岩和含石榴石白云母花岗岩有较好的应用(Sami et al., 2018)。曲梅勒花岗岩的Nb/Ta多数小于5，与岩浆-水热系统下的矿化花岗岩具有一定的相似性(图 14)，与纯岩浆系统下的无矿花岗岩相比具有明显差别(Sami et al., 2018)。此外，曲梅勒花岗岩K/Rb含量比值小于100，Sr含量低于80×10-6，Ba含量同样低于80×10-6(除样品17BGA55-1外)，并且具有明显的负Eu异常和REE四分组效应，上述这些较为特殊的地球化学特征表明强烈的岩浆/流体分馏过程在该岩体的形成过程中起了重要作用(Farahat et al., 2011)。过铝质花岗岩的全岩组成可受到岩浆演化晚期岩浆流体的改造，而使岩体具有良好的成矿条件(Ballouard et al., 2016)。因此，今后可加大投入寻找与该高分异花岗岩有关的矿床。
5 结论(1) 根据锆石U-Pb定年结果，北拉萨地块曲梅勒花岗岩形成时代为早白垩世(128Ma)；地球化学数据表明其属于高钾钙碱性系列，并具强过铝质特征，具有明显的稀土元素四分组效应，且轻重稀土之间分馏程度较低；TZr和TREE平均温度分别为731℃和751℃；锆石Hf同位素显示其εHf(t)为-3.8~+2.0，tDM2变化于1.0~1.4Ga之间。
(2) 曲梅勒花岗岩属高分异I型花岗岩，其岩浆来源于地壳物质的部分熔融，并且在岩浆演化过程中可能发生了以部分镁铁矿物和斜长石为主的分异，同时伴有部分副矿物(磷灰石、锆石、独居石和富Ti矿物等)的分离结晶，而石榴石以及碱性长石未发生明显的分离结晶。
(3) 曲梅勒花岗岩较低的K/Rb和Nb/Ta比值、Sr和Ba含量等特征表明强烈的岩浆/流体分馏过程在该岩体的形成过程中起了重要作用，表明该岩体具有一定的成矿可能。
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