南京大学学报(自然科学), 2022, 58(6): 1087-1106 doi: 10.13232/j.cnki.jnju.2022.06.016

喜马拉雅夏如淡色花岗岩⁃伟晶岩的矿物学特征

程飞越1, 谢磊,1,2, 陈小明1

1.内生金属矿床成矿机制研究国家重点实验室,南京大学地球科学与工程学院,南京,210023

2.关键地球物质循环前沿科学中心,南京大学,南京,210023

Mineralogical characteristics of the Xiaru leucogranite⁃pegmatite in the Himalayan oregen

Cheng Feiyue1, Xie Lei,1,2, Chen Xiaoming1

1.State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing, 210023, China

2.Frontiers Science Center for Critical Earth Material Cycling, Nanjing University, Nanjing,210023, China

通讯作者: E⁃mail:xielei@nju.edu.cn

收稿日期: 2022-08-08  

基金资助: 第二次青藏高原综合科学考察研究.  2019QZKK0802
国家自然科学基金.  91855209.  42222202
关键地球物质循环前沿科学中心“科技人才团队”项目和中央高校基本科研业务费专项资金.  2022300192

Received: 2022-08-08  

摘要

喜马拉雅夏如岩体出露大量淡色花岗岩⁃伟晶岩,主要包括(含电气石⁃石榴子石)白云母花岗岩和花岗质伟晶岩,局部岩石与铌钽钨等稀有金属成矿作用相关,包含富含铌钽钨的氧化物,如铌铁矿族矿物、骑田岭矿、黑钨矿、铌铁金红石等.为了查明夏如稀有金属矿化花岗岩⁃伟晶岩的岩石学、矿物学特征,更好地了解夏如岩体稀有金属成矿特征,对夏如岩体中出现矿化痕迹的样品进行研究,包括含黑钨矿的白云母花岗岩、含有铌钽钨氧化物的白云母花岗岩和含有铌钽氧化物的伟晶岩作为研究对象,同时还选取典型的未出现稀有金属矿物的白云母花岗岩(简称未矿化白云母花岗岩)进行对比研究.研究主要通过全岩地球化学主量和微量元素成分,造岩矿物和副矿物的产状、主量和微量元素成分特征进行对比.随着样品中铌钽氧化物矿物含量的出现/增加,电气石和石榴子石的含量也明显增加.这些花岗岩⁃伟晶岩富集SiO2(72.29~75.53 wt.%),富Al2O3(13.17~15.94 wt.%),富碱(K2O+Na2O 8.0~9.8 wt.%),贫CaO,MgO和FeO,它们微量元素含量接近,含铌钽伟晶岩具有较高的Nb和Ta含量.花岗岩⁃伟晶岩的稀土配分曲线相似,具有明显的Eu负异常,但三种具有矿化痕迹的花岗质岩石中的稀土总量(平均21×10-6)明显低于未矿化白云母花岗岩的稀土总量(平均98×10-6),且具有更明显的四分组效应.矿物学研究工作显示,未矿化白云母花岗岩与含钨白云母花岗岩中的主要组成矿物成分相似,含铌钽钨白云母花岗岩与含铌钽伟晶岩中的主要组成矿物成分相似,其中白云母的Al2O3和FeO含量,石榴子石的Mn/(Mn+Fe)比值和Eu负异常、电气石的Mg/(Mg+Fe)值都显示了含铌钽钨白云母花岗岩和含铌钽伟晶岩比未矿化白云母花岗岩和含钨白云母花岗岩具有更高的岩浆演化程度.出现铌钽氧化物的花岗岩⁃伟晶岩中锆石的结构指示了岩浆晚期流体活动,岩浆演化和流体活动的共同影响导致Zr/Hf值的降低.因此,本次研究表明花岗岩⁃伟晶岩全岩成分指示流体活动更有效,而白云母、电气石和石榴子石的成分特征指示岩浆演化过程和演化程度更有效.

关键词: 淡色花岗岩 ; 造岩矿物 ; 花岗岩演化 ; 矿物学指标

Abstract

A large number of leucogranites and pegmatites occurred in the Xiaru pluton,Himalayan oregen,mainly including (tourmaline⁃garnet) muscovite granite and granitic pegmatite.Granite and pegmatite which are related to Nb⁃Ta⁃W rare⁃metal mineralization locally,contain Nb⁃Ta⁃W oxide minerals,such as columbite⁃group minerals,qitianlingite,wolframite,and ilmenorutile. In order to investigate the petrological and mineralogical characteristics of the Xiaru granite⁃pegmatite related rare⁃metal mineralization,and gain insight of the rare⁃metal mineralization metallogenisis of the Xiaru pluton,the samples with rare⁃metal mineralization are selected,including muscovite granite with the occurrence of wolframite,muscovite granite with Nb⁃Ta⁃W oxide minerals and pegmatite with the occurrence of columbite⁃group minerals. Meanwhile,the typical muscovite granite without any rare⁃metal minerals (barren muscovite granite hereafter) is selected for comparison. Major and trace elements of whole rock geochemistry,the occurrence,major and trace elements of rock forming minerals and accessory minerals are comparable. The abundances of tourmaline and garnet increase significantly with the occurrence/increased abundance of Nb⁃Ta oxide minerals in the samples. The Xiaru granite⁃pegmatite are rich in SiO2 (72.29~75.53 wt.%),Al2O3 (13.17~15.94 wt.%),total alkali (K2O+Na2O 8.0~9.8 wt.%),but poor in CaO,MgO and FeO. The trace elements concentrations of the granite⁃pegmatite are similar,and the Nb⁃Ta⁃bearing pegmatite is rich in Nb and Ta. The shapes of normalized REE patterns are similar,with obvious negative Eu anomalies. However,the mineralized granite⁃pegmatite have significantly lower total REE concentrations (~21×10-6 on average) than the barren muscovite granite (98×10-6 on average),and they have more obvious REE tetrad effect. Mineralogical works show that the mineral compositions of barren muscovite granite are similar to that of W⁃bearing muscovite granite,while the mineral compositions of Nb⁃Ta⁃W⁃bearing muscovite granite are similar to that of Nb⁃Ta⁃bearing pegmatite. The Al2O3 and FeO contents of muscovite,Mn/(Mn+Fe) ratio and Eu negative anomaly of garnet,the Mg/(Mg+Fe) ratio of tourmaline show a progressive trend of magma evolution from barren muscovite granite and W⁃bearing muscovite granite to Nb⁃Ta⁃W⁃bearing muscovite granite and Nb⁃Ta⁃bearing pegmatite. The occurrences and Zr/Hf ratios of zircons in the Xiaru granite⁃pegmatite with Nb⁃Ta oxide minerals indicate strong fluid activity in the later magma. Therefore,it is suggested that whole⁃rock composition is indicative for the fluid activity,while the composition characteristics of muscovite,tourmaline and garnet are indicative for the process and degree of the magma evolution.

Keywords: leucogranite ; rock⁃forming minerals ; magma evolution ; mineral indicators

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程飞越, 谢磊, 陈小明. 喜马拉雅夏如淡色花岗岩⁃伟晶岩的矿物学特征. 南京大学学报(自然科学)[J], 2022, 58(6): 1087-1106 doi:10.13232/j.cnki.jnju.2022.06.016

Cheng Feiyue, Xie Lei, Chen Xiaoming. Mineralogical characteristics of the Xiaru leucogranite⁃pegmatite in the Himalayan oregen. Journal of nanjing University[J], 2022, 58(6): 1087-1106 doi:10.13232/j.cnki.jnju.2022.06.016

淡色花岗岩特指暗色矿物含量较低(<5 vol.%)的花岗岩.喜马拉雅造山带由新生代以来印度与亚洲碰撞形成,带中发育沿东西向展布、规模巨大的淡色花岗岩带1,包括二云母花岗岩、白云母花岗岩、钠长花岗岩、伟晶岩(细晶岩)等多种岩石类型2,它们具有高分异花岗岩的特征,与稀有金属成矿作用密切相关.在错那洞、普士拉、珠峰、告乌等多个岩体中发现Li,Be,Nb⁃Ta,Sn等稀有金属矿物大量赋存3-7,而W成矿作用相关的报道较少,目前仅在喜马拉雅东段错那洞岩体和拉隆岩体中发现与矽卡岩和蚀变带相关的W成矿作用,出现了黑钨矿和白钨矿58.而近期研究在特提斯喜马拉雅淡色花岗岩带中部的夏如岩体中发现了与铌钽钨成矿作用相关的淡色花岗岩与花岗质伟晶岩,包含大量铌铁矿⁃铌锰矿、骑田岭矿、黑钨矿和白钨矿39,铌钽钨成矿作用的出现表明夏如岩体在喜马拉雅诸多淡色花岗岩中具有特殊性.该稀有金属成矿作用也是目前喜马拉雅造山带内发现的最早稀有金属成矿事件(约34~33 Ma).在前人研究的基础上,为了全面了解夏如淡色花岗岩和伟晶岩的岩石学、矿物学特征,剖析成岩成矿的过程,本次研究选取了含钨白云母花岗岩、含铌钽钨白云母花岗岩和含铌钽伟晶岩等含有不同铌钽钨矿物的花岗岩⁃伟晶岩,并选取了未出现稀有金属矿物的白云母花岗岩(简称未矿化白云母花岗岩)进行了对比研究,通过全岩地球化学成分、矿物(白云母、石榴子石、电气石、锆石)EMPA(电子探针)成分和LA⁃ICP⁃MS(激光剥蚀等离子质谱仪)微量元素成分特征,从矿物学角度揭示夏如淡色花岗岩⁃伟晶岩的演化和稀有金属成矿作用的关系.

1 地质背景

喜马拉雅淡色花岗岩主要形成于48~7 Ma,大致分为三个阶段,包括始喜马拉雅阶段(44~26 Ma)、新喜马拉雅阶段(26~13 Ma)和后喜马拉雅阶段(13~7 Ma),较早的淡色花岗岩和喜马拉雅造山带的碰撞缩短阶段有关,而后两阶段的淡色花岗岩都与造山作用晚期伸展减薄作用相关110-13.喜马拉雅淡色花岗岩根据所在的构造单元分为北部的特提斯喜马拉雅带和南部的高喜马拉雅带,特提斯喜马拉雅淡色花岗岩带分布在特提斯喜马拉雅构造单元内部,高喜马拉雅淡色花岗岩沿着藏南拆离系分布或以规模不等的岩席形式侵入高喜马拉雅单元内部1图1a).夏如穹隆位于特提斯喜马拉雅带中部,雅鲁藏布缝合带以南30 km处,出露面积约200 km2,被近东西走向的雅鲁藏布江贯穿1214.穹隆核部为古生代花岗片麻岩(约478~429 Ma)和渐新世淡色花岗岩(约36~34 Ma),淡色花岗岩侵入到花岗岩片麻岩和中级变沉积岩(图1b).变沉积岩的变质程度从底部的角闪岩相变质作用到顶部的低级变质作用不等14-16.夏如淡色花岗岩主体为含有不等量电气石⁃石榴子石的白云母花岗岩,野外观察到淡色花岗岩露头(图1c),密集分布的花岗质伟晶岩脉切穿白云母花岗岩(图1d),部分侵入至花岗片麻岩中.本次研究以夏如出露的白云母花岗岩和伟晶岩为研究对象,根据富铌钽钨矿物(铌铁矿族矿物(化学式为(Fe,Mn)(Nb,Ta)2O6)、骑田岭矿((Fe,Mn)2(Nb,Ta)2WO10)、黑钨矿((Mn,Fe)WO4)、白钨矿(CaWO4)的出现与否,分为未矿化白云母花岗岩、含钨白云母花岗岩、含铌钽钨白云母花岗岩和含铌钽伟晶岩,后三者也统称为矿化花岗岩⁃伟晶岩.

图1

图1   (a)喜马拉雅淡色花岗岩带地质简图(据文献[1]修改);(b)夏如岩体地质简图(据文献[14]修改);

(c)夏如淡色花岗岩;(d)伟晶岩脉切穿白云母花岗岩

Fig.1   Simplified geological maps of (a) Himalayan leucogranite belt (modified after ref.[1]) and (b) the Xiaru leucogranite (modified after ref.[14]),(c) the outcrop of the Xiaru leucogranite,

(d) the pegmatite dike cut through the muscovite granite


2 分析方法

全岩主量数据在核工业230研究所分析测试中心完成,采用湿化学法进行分析,具体标准参考GB/T 14506.14⁃2010和DZG93⁃05.除F元素采用离子活度计外,其他主量元素采用X射线荧光光谱仪测定,氧化物含量的相对误差低于±5%.全岩微量元素测定在南京聚谱检测科技有限公司完成,采用Agilent 7700x型ICP⁃MS测定微量元素,以USGS标样(玄武岩BHVO⁃2、安山岩AGV⁃2、花岗闪长岩GSP⁃2)作为质控盲样,微量元素的偏离范围不超过±10%17.

矿物主量元素分析在南京大学内生金属矿床成矿机制研究国家重点实验室完成,使用仪器为JEOL JXA8230型电子探针,工作条件为加速电压15 kV,束流20 nA.分析矿物包括白云母、电气石、石榴子石、锆石.不同矿物束斑直径不同,白云母为5 μm,其余矿物的束斑直径为1 μm.主要元素的峰位测定时间是10 s,次要元素的峰位测定时间是20 s,背景测定时间为峰位时间的一半.标样包括天然矿物(角闪石、磷灰石、磷氯钡石、黄玉、钠长石、蔷薇辉石、锆石、锡石、橄榄石)和合成化合物(TiO2,SrSO4),数据由ZAF校正程序进行统一校正18.

矿物微量元素分析在南京大学内生金属矿床成矿机制研究国家重点实验室完成,实验仪器为RESOlution⁃155(193 nm ArF准分子激光剥蚀器)和Thermo Fisher Scientific iCAP Q等离子质谱仪.矿物的激光剥蚀频率为4.0 Hz,束斑直径为43 µm,白云母、电气石和石榴子石的能量密度分别为4.4,4.6,5.33 J·cm-2.采用硅酸盐玻璃 NIST SRM 610,NIST SRM 612和玄武岩玻璃USGS BCR⁃2G,USGS GSE⁃1G作为外标,同时采用探针主量数据中Si作为内标校正微量元素分析时仪器漂移.分析结果使用ICP MS Data Cal 9.9程序进行离线数据处理19-20,含量相对偏差优于±10%.

3 岩相学与岩石地球化学特征

未矿化白云母花岗岩粒径在0.1~2 mm,主要矿物组成为石英(~35 vol.%)、钾长石(~30 vol.%)、钠长石(~23 vol.%)、白云母(~10 vol.%)(图2a),电气石(图2b)(1~2 vol.%)和石榴子石(图2c)(不高于0.5 vol.%)的含量较低且分布不均匀,样品部分区域未观察到石榴子石和电气石,副矿物主要有磷灰石、锆石、独居石、磷钇矿(图2d)、金红石等.

图2

图2   夏如淡色花岗岩⁃伟晶岩的显微照片和背散射电子(BSE)图像

未矿化白云母花岗岩的(a、b)显微照片和(c、d)背散射电子图像,未矿化白云母花岗岩含有石英、钾长石、钠长石、白云母、电气石等矿物和石榴子石、磷钇矿等副矿物;含钨白云母花岗岩(e)的显微照片和(f)背散射电子图像;含铌钽钨白云母花岗岩的(g)显微照片和(h,i)背散射电子图像,含有石榴子石和铌钽钨氧化物;含铌钽伟晶岩的显微照片(j,k,l),包含石英、钾长石、钠长石、白云母,石榴子石、电气石等,(m,n,o)石榴子石、铌铁矿族和磷钇矿的背散射电子图像.矿物缩写:Kfs⁃钾长石,Ab⁃钠长石,Qtz⁃石英,Ms⁃白云母,Tur⁃电气石,Grt⁃石榴子石,Xtm⁃磷钇矿,Wlf⁃黑钨矿,Nb⁃Ta⁃W oxides⁃铌钽钨氧化物(包括铌铁矿族矿物、骑田岭矿和黑钨矿),Clm⁃铌铁矿族矿物,Zrn⁃锆石

Fig.2   Microphotographs and backscattered electron (BSE) images of the Xiaru leucogranite⁃pegmatite


含钨白云母花岗岩粒径在0.1~2 mm,主要矿物组成为钾长石(~33 vol.%)、石英(~30 vol.%)、钠长石(~26 vol.%)、白云母(~10 vol.%)和黑云母(~0.5 vol.%)(图2e),副矿物主要有磷灰石、锆石、独居石、金红石、黑钨矿(图2f)、白钨矿等.

含铌钽钨白云母花岗岩粒径在0.1~4 mm,主要矿物组成为钾长石(~35 vol.%)、钠长石(~30 vol.%)、石英(~30 vol.%)、白云母(~5 vol.%)、电气石(~2 vol.%)和石榴子石(~1 vol.%)(图2g和图2h),副矿物主要有铌铁矿族矿物、骑田岭矿、黑钨矿(图2i)、白钨矿、萤石、金红石、磷灰石、锆石、独居石、磷钇矿等.

含铌钽伟晶岩中发育粗粒⁃巨晶矿物,粒径可达15 mm,主要矿物组成为钠长石(~40 vol.%)、石英(~30 vol.%)、钾长石(~25 vol.%)、白云母(~2 vol.%)、电气石(~2 vol.%)和石榴子石(~1 vol.%)(图2j~m),副矿物主要有铌铁矿族矿物(图2n)、铌铁金红石、磷灰石、锆石、独居石、磷钇矿(图2o)等.

表1列出了本次研究中代表性样品的全岩主量元素与微量元素分析结果9.夏如淡色花岗岩⁃伟晶岩总体特征:富SiO2(72.29~75.53 wt.%)和Al2O3(13.17~15.94 wt.%),富碱(Na2O+K2O 8.0~9.8 wt.%),较低CaO(0.36~0.75 wt.%),MgO(0.06~0.15 wt.%)和FeO(0.07~0.16 wt.%).略有差异的是未矿化白云母花岗岩中和含钨白云母花岗岩的TiO2(0.08~0.12 wt.%),Fe2O3(0.83~1.08 wt.%),MgO(0.09~0.15 wt.%)含量较高,而含铌钽钨白云母花岗岩和含铌钽伟晶岩的TiO2(0.01~0.04 wt.%),Fe2O3(0.08~0.70 wt.%),MgO(0.06~0.07 wt.%)含量较低.

表1   夏如淡色花岗岩⁃伟晶岩的全岩主量(wt.%)和微量成分(×10-6)分析结果

Table 1  Whole⁃rock major (wt.%) and trace (×10-6) elements of the Xiaru leucogranite⁃pegmatite

岩性未矿化白云母花岗岩含钨白云母花岗岩含铌钽钨白云母花岗岩含铌钽伟晶岩岩性未矿化白云母花岗岩含钨白云母花岗岩含铌钽钨白云母花岗岩含铌钽伟晶岩
样号20XZ0520XZ34A19XZ12419XZ122🞷19XZ123🞷20XZ34B20XZ36样号20XZ0520XZ34A19XZ12419XZ122🞷19XZ123🞷20XZ34B20XZ36
SiO2(wt.%)72.6172.6675.5275.5372.8772.2974.86Sr3727105.4101912
TiO20.120.090.080.040.040.030.01Y6038124.48.0147.6
Al2O315.0515.1413.1713.5215.4315.9414.02Zr936743206.73028
FeO0.110.160.070.110.140.150.12Nb36363258397256
Fe2O30.831.081.070.700.550.370.08Mo0.150.270.170.100.550.700.27
MnO0.030.060.050.020.020.220.06Sn2428259.1249.11.9
MgO0.090.130.150.060.070.070.07Cs611085030418622
CaO0.380.750.560.480.490.360.69Ba9578244.415163.4
Na2O3.203.443.184.634.266.025.82La21114.91.21.94.33.2
K2O5.295.004.843.985.103.743.41Ce4425143.65.09.67.3
P2O50.040.210.240.240.260.040.02Pr4.72.91.20.360.611.10.83
H2O0.490.350.270.230.280.340.34Nd16114.21.12.13.42.8
LOI0.890.790.780.460.390.320.64Sm4.23.41.30.561.01.41.4
总量99.1499.8599.9899.9899.9199.90100.14Eu0.260.190.060.020.060.030.04
F0.140.150.130.080.080.040.02Gd4.33.71.30.541.01.31.2
K2O+Na2O8.498.448.028.619.359.779.23Tb1.10.840.300.140.260.350.24
A/CNK1.291.221.151.061.151.090.97Dy8.25.82.10.921.52.11.3
A/NK1.371.371.261.131.231.141.06Ho1.91.20.410.150.220.320.19
Er5.93.41.20.430.550.840.46
Li(×10-614612414931632824Tm1.10.590.210.100.100.150.08
Be7.3265.34.66.51740Yb7.13.91.40.880.651.00.51
B197376593780229618.1Lu1.00.560.200.120.080.120.07
Sc5.94.81.22.39.91.40.41Hf3.42.62.01.50.292.31.4
Ti75166146722025819150Ta8.67.89.5176.01918
V2.72.82.50.310.421.80.29W56193667153.42.7
Cr3.35.40.770.607.68.34.1Pb30362119263541
Mn2145123421241461750389Th25156.44.31.96.83.3
Co0.680.910.580.190.390.150.19U8.86.72.42.12.0114.1
Ni1.62.40.550.233.43.62.0ΣREE120.6074.4532.4210.1615.1325.9919.62
Cu3.92.60.670.380.856.21.8δEu0.180.160.130.120.170.070.08
Zn22302012192612Tzr759.90729.07694.65635.80572.49659.52648.34
Ga22221317263221Nb/Ta4.144.693.413.426.573.703.08
As1.20.741.00.161.01.11.7Zr/Hf27.3826.3321.6913.6922.9312.9819.57
Se0.720.530.200.130.300.150.23Rb/Sr21.0939.9678.87101.2975.2035.1741.54
Rb7711085790543758675486TE1,31.081.111.211.401.351.331.29

“🞷”的全岩主量和微量数据引自谢磊等[14];δEu=Eu/Eu*;TE1,3=(TE1×TE3)0.5,TE1=(Ce/Ce*×Pr/Pr*0.5,TE3=(Tb/Tb*×Dy/Dy*0.5 [29]

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夏如淡色花岗岩⁃伟晶岩中Nb,Ta,W含量分别为(33~72)×10-6,(6.0~19)×10-6,(2.7~67)×10-6.相较未矿化白云母花岗岩和含钨白云母花岗岩的Nb((32~36)×10-6)和Ta((7.8~9.5)×10-6),含铌钽钨白云母花岗岩和含铌钽伟晶岩中Nb,Ta含量较高,分别为(39~72)×10-6,(6.0~19)×10-6;出现钨矿物的含钨白云母花岗岩和含铌钽钨白云母花岗岩中W含量较高,为(36~67)×10-6,未矿化白云母花岗岩中W含量为(19~56)×10-6,而含铌钽伟晶岩中W含量也较低,为(2.7~15)×10-6.从未矿化白云母花岗岩(Y (38~60)×10-6,Th (15~25)×10-6)到出现矿化痕迹的花岗岩⁃伟晶岩(Y (4.4~14)×10-6,Th (1.9~6.8)×10-6),Y和Th出现明显降低.淡色花岗岩⁃伟晶岩的Nb/Ta值为3.1~6.6、Zr/Hf值为13~27、K/Rb值为38~61、Y/Ho值为29~44,尤其未矿化白云母花岗岩的Zr/Hf值(约为26~27),含钨白云母花岗岩的Zr/Hf值(约22),含铌钽钨白云母花岗岩和含铌钽伟晶岩的Zr/Hf值(13~23)呈逐渐降低的趋势.

夏如淡色花岗岩⁃伟晶岩的ΣREE总体较低,未矿化白云母花岗岩中ΣREE为(74~121)×10-6,含钨白云母花岗岩中ΣREE(32×10-6)略低于未矿化白云母花岗岩,而含铌钽钨白云母花岗岩和含铌钽伟晶岩中ΣREE较低((10~26)×10-6).稀土元素球粒陨石标准化配分曲线属于“海鸥型”,具有明显的Eu负异常(δEu为0.07~0.18)(图3),且相较未矿化白云母花岗岩(δEu为0.16~0.18),含矿花岗岩⁃伟晶岩具有更明显的Eu负异常(δEu为0.08~0.17).从未矿化白云母花岗岩(TE1,3=1.08~1.11)到含钨白云母花岗岩(TE1,3=1.21),再到含铌钽钨白云母花岗岩和含铌钽伟晶岩(TE1,3=1.29~1.40),四分组效应逐渐增强.

图3

图3   夏如淡色花岗岩⁃伟晶岩的稀土元素球粒陨石标准化配分曲线(标准化数据来自文献[21])

Fig.3   Chondrite⁃normalized REE patterns of the Xiaru leucogranite⁃pegmatite (normalized data are from ref. [21])


4 矿物学特征

4.1 云母族矿物

4.1.1 白云母

夏如淡色花岗岩⁃伟晶岩中白云母呈片状,粒径在0.1~1.5 mm(图2),背散射电子图像中未见明显环带.白云母主量元素成分显示所有白云母的SiO2(44.64~46.90 wt.%)和K2O(9.83~10.53 wt.%)含量变化范围小,贫CaO,Na2O,Rb2O,Cs2O,F,Cl,含量分别不高于0.11 wt.%,0.50 wt.%,0.48 wt.%,0.07 wt.%,0.67 wt.%,0.01 wt.%,而其他一些元素含量有差异(表2,见1099页).未矿化白云母花岗岩和含钨白云母花岗岩中白云母的成分接近,含有一定的Al2O3(29.09~32.37 wt.%),TiO2(0.43~0.87 wt.%),MnO(0.14~0.26 wt.%),MgO(0.48~1.05 wt.%),FeO(5.29~6.59 wt.%)和Li2O(0.33~0.88 wt.%);含铌钽钨白云母花岗岩和含铌钽伟晶岩中的白云母成分相似,与前两者样品中白云母相比,它们有更高的Al2O3含量(31.84~37.22 wt.%),但TiO2(0.01~0.58 wt.%),MnO(0.07~0.19 wt.%),MgO(0.05~0.55 wt.%),FeO(2.06~5.06 wt.%),Li2O(0.04~0.44 wt.%)含量下降(图4b~d,表2).

表2   夏如淡色花岗岩⁃伟晶岩中白云母化学成分

Table 2  Chemical compositions of micas from the Xiaru leucogranite⁃pegmatite

未矿化白云母花岗岩平均含钨白云母花岗岩平均含铌钽钨白云母花岗岩平均含铌钽伟晶岩平均
123n=1045n=7678n=149101112n=19
EPMA (wt.%)
SiO246.2646.0646.6445.8746.6445.4345.7745.2845.6746.6845.5845.2145.9346.0346.3645.70
TiO20.590.550.870.610.660.430.630.580.560.420.450.360.180.140.310.25
Al2O329.6830.4832.3730.3530.2031.5830.5933.6633.7632.3333.5335.2335.4135.8633.1934.62
FeO6.375.936.596.045.635.295.734.414.275.054.473.442.392.623.983.36
MnO0.190.260.260.210.140.180.170.110.140.140.120.090.070.190.100.11
MgO1.050.721.050.830.730.480.640.180.210.260.190.350.510.130.430.32
CaO0.02-0.050.020.01-0.01-0.01-0.00-0.050.020.010.02
Na2O0.180.190.400.250.400.290.290.320.420.340.370.320.200.150.290.32
K2O10.4510.3510.4810.3010.0310.4010.1910.3310.3210.1810.2810.5310.5310.4310.0210.25
F0.590.180.670.340.500.230.410.370.380.500.370.130.12-0.040.10
Cl0.00-0.010.000.01-0.000.000.000.000.00-0.000.000.000.00
H2O*4.114.294.404.224.164.294.194.264.284.224.264.424.444.514.444.41
O=F,Cl0.250.080.280.140.210.100.170.160.160.210.160.050.05-0.020.04
LA⁃ICP⁃MS (wt.%)
Li2O0.650.330.880.560.430.870.650.300.350.340.330.240.240.140.430.24
Rb2O0.290.330.390.320.370.370.350.280.280.360.300.280.270.430.220.30
Cs2O0.030.030.040.030.020.030.020.010.010.020.020.010.010.020.000.02
Total100.2099.62100.2099.7499.7199.7699.4699.94100.51100.64100.12100.56100.30100.6799.8099.98
以22个O原子计算
Si6.3146.3086.4056.2786.3596.1926.2676.1276.1396.2806.1556.0526.1216.1206.2376.138
Al iv1.6861.6921.8931.7221.6411.8081.7331.8731.8611.7201.8451.9481.8791.8801.7631.862
Al vi3.0883.2283.3273.1693.2123.2653.2043.4953.4883.4073.4923.6123.6833.7413.5003.618
Ti0.0600.0570.0900.0630.0680.0440.0650.0590.0570.0420.0460.0360.0180.0140.0310.025
Fe0.7270.6790.7560.6910.6410.6030.6570.4990.4800.5690.5050.3850.2660.2910.4480.378
Mn0.0220.0300.0300.0250.0170.0210.0190.0130.0160.0160.0140.0110.0080.0220.0110.013
Mg0.2140.1460.2140.1700.1490.0980.1300.0370.0410.0530.0380.0710.1020.0260.0860.063
Li0.3580.1820.4830.3080.2350.4750.3590.1650.1880.1870.1770.1300.1270.0750.2340.129
Ca0.0040.0070.0030.0010.0010.0010.0010.0070.0020.0010.003
Na0.0480.0510.1060.0670.1060.0760.0770.0840.1090.0890.0960.0840.0510.0390.0750.084
K1.8181.8071.8391.7991.7441.8071.7801.7821.7701.7471.7701.7981.7911.7691.7201.756
Rb0.0250.0290.0340.0280.0320.0320.0310.0250.0240.0310.0260.0240.0230.0360.0190.026
Cs0.0020.0020.0020.0020.0010.0020.0010.0010.0010.0010.0010.0010.0010.0010.0000.001
F0.2550.0780.2910.1480.2170.0970.1760.1590.1630.2130.1580.0540.0500.0170.044
Cl0.0010.0030.0010.0030.0010.0000.0000.0010.0010.0000.0000.0000.001
OH*3.7443.9224.0003.8523.7803.9033.8233.8413.8373.7863.8413.9463.9504.0003.9833.955
LA⁃ICP⁃MS (×10-6)
Be34394735222324392735322632462631
B717218910012516812214413910416698170348124174
Sc614761494943501141071311169663113157
V232125201414152.02.31.92.04.10.400.99185.5
Cr41251107.55113--1.42.51.5-2.32.72.3
Co2.63.43.52.92.83.02.60.32-1.31.01.60.63--0.62
Ni2.10.932.81.62.32.61.4-0.66-0.220.630.67--0.20
Cu-1.12.10.660.680.480.450.380.720.050.481.1-0.15-1.2
Zn901091159711411310477787278124166275135143
Ga1029811694104101107150147167157151135212187167
Ge2.13.76.34.13.62.63.22.84.46.85.16.44.93.33.95.3
Sr1.11.62.21.41.32.21.40.680.700.790.680.740.820.140.870.93
123n=1045n=7678n=149101112n=19
Zr0.491.21.20.840.771.00.900.910.930.680.800.940.671.40.150.68
Nb1962093412332473142983433542953299017324517168
Mo-0.080.260.09--0.040.10-1.60.28----0.11
Ag0.13-0.530.150.030.100.180.03--0.020.10-0.570.120.11
Cd-0.370.670.200.07-0.140.100.090.100.030.09-0.370.200.06
In1.21.41.61.11.21.11.42.02.02.31.81.71.60.671.21.3
Sn23525428921421021823723325332024423620485259200
Sb0.15-0.600.26-0.060.10---0.08-0.210.32-0.09
Ba786696786561496.13.40.914.85.17.23.42.17.4
Hf0.040.080.130.080.060.130.090.140.120.150.120.130.150.17-0.15
Ta27339141396970485337501938191.141
W233231276183143222221181209136183100138181690
Au0.110.260.440.210.220.400.340.290.330.140.250.030.220.180.020.21
Tl8.61113101111118.28.38.48.37.68.0157.09.6
Pb3.33.54.43.01.52.12.53.43.33.33.14.03.14.26.14.4
Th0.00-0.040.010.040.030.010.00-0.010.01---0.000.15
U-0.010.030.010.030.010.010.01-0.010.010.010.02-0.010.24

“-”表示含量低于检测限

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图4

图4   夏如淡色花岗岩⁃伟晶岩的(a)云母分类图(据文献[22]修改)和(b~e)白云母成分特征

Fig.4   (a) Classification diagram (modified after ref. [22]) and (b~e) variable chemical composition diagrams of muscovites from the Xiaru leucogranite⁃pegmatite


白云母的V,Co从未矿化白云母花岗岩和含钨白云母花岗岩(V (11~25)×10-6,Co (1.5~3.5)×10-6)到含铌钽钨白云母花岗岩和含铌钽伟晶岩(V (0.17~25)×10-6,Co (0~1.6)×10-6)明显降低(图4e;表2).含铌钽伟晶岩中白云母具有最高的Nb(459×10-6),Ta(117×10-6),W(302×10-6),Sn(508×10-6)含量(表2).

4.2 电气石

夏如淡色花岗岩⁃伟晶岩中电气石粒径从1~3 mm不等,在单偏光镜下呈蓝黑色⁃黑色(图2b和图2i),均为黑电气石(图5a).未矿化白云母花岗岩中电气石含量为1~2 vol.%,样品部分区域未观察到电气石,且含钨白云母花岗岩中未观察到电气石,而含铌钽钨白云母花岗岩和含铌钽伟晶岩中电气石含量为2 vol.%.电气石具有相近的SiO2(32.98~35.34 wt.%),Al2O3(29.66~34.43 wt.%),B2O3(9.38~10.90 wt.%),Na2O(1.54~2.39 wt.%)含量,均贫TiO2,CaO,MnO,K2O,F和Cl,含量分别不高于1.21 wt.%,0.74 wt.%,0.28 wt.%,0.11 wt.%,0.02 wt.%和0.02 wt.%(表3,见1100页).而不同岩石类型中的电气石的Mg,Fe含量有差异,在未矿化白云母花岗岩的电气石中MgO含量为1.43~2.38 wt.%,FeO含量为13.48~16.56 wt.%,Mg/(Mg+Fe)值介于0.15~0.24,而含铌钽钨白云母花岗岩和含铌钽伟晶岩中电气石MgO含量略低(0.23~1.82 wt.%),FeO含量略高,为13.57~17.11 wt.%,Mg/(Mg+Fe)值(0.02~0.19)较低(图5b和图5c,表3).

图5

图5   夏如淡色花岗岩⁃伟晶岩的(a)电气石分类图(据文献[23]修改)和(b~d)成分特征

Fig.5   (a) Classification diagram (modified after ref.[23]) and (b~d) composition diagrams of tourmalines from the Xiaru leucogranite⁃pegmatite


表3   夏如淡色花岗岩⁃伟晶岩中代表性电气石的主量(wt.%)和微量成分(×10-6)分析结果

Table 3  Major (wt.%) and trace (×10-6) elements of representative tourmalines from the Xiaru leucogranite⁃pegmatite

未矿化白云母花岗岩含铌钽钨白云母花岗岩含铌钽伟晶岩
123456789101112
EPMA (wt.%)
SiO234.0134.5234.5234.9534.5633.8634.5934.6033.9634.8135.1133.87
TiO20.350.200.330.320.270.430.310.200.620.240.461.21
Al2O332.3133.6932.8833.5434.2032.2733.6534.2432.8032.4533.2732.42
Cr2O31.640.04-0.040.01----0.01--
FeO13.6614.2114.2513.7515.1116.7615.4615.0115.6815.8115.5715.90
MgO2.131.432.332.320.760.650.660.540.970.230.530.78
CaO0.570.200.430.390.110.280.140.100.340.260.150.39
MnO0.030.120.100.130.100.110.100.120.140.250.280.23
Na2O1.922.012.161.911.842.161.851.721.981.962.122.11
K2O0.080.050.050.040.040.060.030.030.070.040.030.02
Cl0.020.010.020.01-0.01----0.010.01
LA⁃ICP⁃MS (wt.%)
B2O310.209.769.609.7310.3310.4110.519.779.789.689.689.55
H2O*3.543.533.523.563.573.513.573.533.503.493.543.49
Total*100.4599.76100.19100.69100.9100.52100.8699.8699.8499.23100.7599.99
以31个阴离子(O2-,OH-,F-)计算
T: Si5.7575.8675.8655.8785.8065.7755.8165.8795.8145.9865.9455.814
Al0.2430.1330.1350.1220.1940.2250.1840.1210.1860.0140.0550.186
B2.9802.8622.8152.8242.9953.0663.0502.8652.8912.8742.8282.830
Z: Al6.0006.0006.0006.0006.0006.0006.0006.0006.0006.0006.0006.000
Y: Al0.2030.6140.4500.5260.5780.2610.4840.7350.4320.5620.5840.373
Ti0.0450.0260.0420.0400.0340.0550.0390.0260.0800.0310.0590.156
Cr0.2190.0050.0050.0010.001
Mg0.5380.3620.5900.5820.1900.1650.1650.1370.2480.0590.1340.200
Mn0.0040.0170.0140.0190.0140.0160.0140.0170.0200.0360.0400.033
123456789101112
Fe2+1.9342.0202.0251.9342.1232.3902.1742.1332.2452.2742.2052.282
∑Y2.9943.0873.1543.1602.9732.9372.9353.0833.1003.0443.0913.117
X: Ca0.1030.0360.0780.0700.0200.0510.0250.0180.0620.0480.0270.072
Na0.6300.6620.7120.6230.5990.7140.6030.5670.6570.6530.6960.702
K0.0170.0110.0110.0090.0090.0130.0060.0070.0150.0090.0060.004
X□0.2490.2900.1990.2980.3720.2220.3650.4090.2650.2900.2700.222
OH3.9943.9973.9943.9974.0003.9974.0004.0004.0004.0003.9973.997
Cl0.0060.0030.0060.0030.0030.0030.003
Mg/(Mg+Fe)0.220.150.230.230.080.060.070.060.100.030.060.08
LA⁃ICP⁃MS (×10-6)
Li350289223372225332404247509544471487
Be2.8111.06.48.20.926.19.21011139.5
V722936233.72.52.12.318242523
Co201317188.24.79.1102.83.12.83.1
Ni155.18.65.80.550.280.410.480.57-0.120.14
Cu-0.35-0.69-0.15--2.31.6--
Zn2201941932512692574144081294123212071234
Ga91117116631231328790144147134119
Ge1.75.92.33.32.55.14.35.75.15.13.52.1
Rb-0.060.060.04-----0.390.22-
Sr9.34.47.3210.710.164.52.24.04.74.13.8
Y0.100.010.020.510.03-0.02-0.100.430.010.01
Zr0.160.110.180.020.120.100.020.090.100.030.08-
Nb1.51.41.30.351.50.581.10.510.921.40.991.1
Mo0.39-0.23--0.080.07--0.070.08-
Ag0.02--0.250.120.430.090.05---0.20
Cd-0.06--0.15---0.070.540.28-
In0.190.220.200.080.170.180.100.090.140.190.130.22
Sn263644112832151416211712
Sb--0.150.420.030.100.04-0.04---
Cs-0.020.04---0.020.050.090.160.020.02
Ba---0.06-0.07------
Ta0.491.20.440.140.630.280.620.260.430.710.840.63
W0.07-0.10-0.07-0.06-0.030.020.030.04
Bi0.020.010.000.03-0.02-0.020.030.040.04-
Pb2.51.52.25.60.810.336.62.38.110107.2
Th0.01-0.010.030.000.000.00-0.01-0.000.01
U---------0.010.00-

*计算所得;X□表示X位置的空位;“-”表示含量低于检测限

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未矿化白云母花岗岩中的电气石Sr((4.4~21)×10-6),V((20~79)×10-6),Co((13~20)×10-6)含量高于矿化花岗岩⁃伟晶岩中电气石的Sr((0.10~6.4)×10-6),V((0.63~32)×10-6)、Co((1.6~14)×10-6)含量(图5c和图5d).含铌钽伟晶岩的电气石具有所有样品中最高的Zn含量(高达1851×10-6)(表3).所有样品中电气石均贫Zr,Nb,Ta,含量分别不高于0.45×10-6,6.6×10-6,2.0×10-6.

4.3 石榴子石

夏如淡色花岗岩⁃伟晶岩中石榴子石粒径为0.1~1 mm,常包裹石英等造岩矿物,未见明显环带(图2c,h,m).未矿化白云母花岗岩中石榴子石含量不高于0.5 vol.%,且含钨白云母花岗岩中未观察到石榴子石,而含铌钽钨白云母花岗岩和含铌钽伟晶岩中石榴子石可达1 vol.%.夏如石榴子石都富集FeO(9.04~24.84 wt.%)和MnO(16.01~31.86 wt.%),CaO和MgO含量总体较低(分别低于2.49 wt.%和0.13 wt.%),属于铁铝榴石⁃锰铝榴石中间成分(图6a).未矿化白云母花岗岩中石榴子石CaO含量约为2.24~2.49 wt.%,而矿化花岗岩⁃伟晶岩的石榴子石CaO含量更低,为0.88~1.81 wt.%(图6b).石榴子石中MnO含量从未矿化白云母花岗岩(16.72~17.05 wt.%)、含铌钽钨白云母花岗岩(16.01~21.07 wt.%)到含铌钽伟晶岩(17.79~31.86 wt.%)出现明显上升(表4,见1101页),Mn/(Mn+Fe)值亦随之逐渐升高,未矿化白云母花岗岩、含铌钽钨白云母花岗岩和含铌钽伟晶岩的Mn/(Mn+Fe)值分别为0.42~0.43,0.39~0.52和0.44~0.78(图6).

图6

图6   夏如淡色花岗岩⁃伟晶岩中石榴子石(a)分类图解和(b~i)成分特征

Fig.6   (a) Classification diagram and (b~i) compositional diagrams of garnets from the Xiaru leucogranite⁃pegmatite


表4   夏如淡色花岗岩⁃伟晶岩中代表性石榴子石的主量(wt.%)和微量成分(×10-6)分析结果

Table 4  Major (wt.%) and trace (×10-6) elements of representative garnets from the Xiaru leucogranite⁃pegmatite

未矿化白云母花岗岩含铌钽钨白云母花岗岩含铌钽伟晶岩
123456789101112
EPMA (wt.%)
SiO236.1236.3635.8435.8935.6635.8336.6437.1236.5936.3236.3136.37
TiO20.000.020.140.130.180.150.070.110.110.060.140.12
Al2O321.2521.2421.1421.1421.3321.2321.2720.4421.5521.3621.0821.10
FeO22.2722.5424.5824.8422.5919.0222.7021.5216.9115.6412.439.53
123456789101112
MnO16.8217.0516.2516.0118.4921.0717.7920.5523.2324.9528.9331.86
MgO0.100.120.080.130.090.080.050.040.040.03--
CaO2.492.321.331.581.311.631.540.991.491.531.231.00
Total99.0599.6599.3699.7299.6599.01100.06100.7999.9299.89100.1299.98
锰铝榴石39.439.738.337.743.849.741.146.853.758.167.374.0
铁铝榴石52.853.057.457.152.045.254.250.241.837.329.123.0
钙铝榴石7.57.04.04.73.94.94.72.94.64.63.63.0
镁铝榴石0.40.50.30.50.40.30.20.20.20.10.00.0
Mn/(Mn+Fe)0.430.430.400.390.450.520.440.490.580.610.700.77
LA⁃ICP⁃MS (×10-6)
Li17222130418837434423632835736913577
Be-0.140.11-0.340.67-0.150.370.12--
B8.07.3113.211134.3128.7141915
Sc53511161191661506148142403822
V5.97.80.960.751.30.220.360.641.61.20.11-
Cr5074.9-2.0-1.40.98-2.32.5-2.1
Co2.42.31.51.81.61.30.720.630.390.71-0.18
Ni-0.260.320.110.100.21-0.380.230.13-0.52
Cu0.51--0.010.650.06-3.2-3.41.40.18
Zn121055565247639197879781
Ga151632233838374240523737
Ge506262556257648370896343
Rb0.72160.210.270.500.28-0.430.250.140.07-
Sr0.280.470.130.250.360.200.010.060.190.160.01-
Y18012161292016783916361223263224361843931109556
Zr0.490.273.31.87.78.56.58.214252822
Nb0.041.50.420.489.4151.57.64.4215269
Mo2.11.41.91.71.22.02.01.63.02.63.12.6
Ag--0.20--0.170.150.340.200.060.340.02
Cd1.72.35.04.75.65.55.35.96.27.1109.1
In0.030.050.130.050.240.170.380.340.310.560.250.36
Sn2.21.5179.242507975621293834
Sb-1.70.070.08-0.08-0.400.10---
Cs1.41.40.020.87-0.040.03---0.000.00
Ba0.060.33----------
Hf0.070.120.320.190.600.940.600.820.952.21.91.7
Ta0.110.550.960.393.54.52.2113.3241415
W0.59140.460.360.730.380.060.130.360.400.240.38
Au----0.030.02-0.04-0.110.060.05
Tl0.100.160.000.010.00-----0.01-
Bi-0.200.000.02--0.00--0.02-0.01
Pb0.09-0.070.05--0.06-0.00-0.050.03
Th0.020.060.03-------0.020.00
U0.020.120.070.060.220.330.090.210.260.941.82.0
La0.090.01----0.01----0.01
Ce0.270.01-0.020.030.040.030.020.040.040.070.13
Pr0.040.00-0.010.030.030.030.010.060.070.070.11
Nd0.410.040.190.060.690.990.710.721.11.21.52.4
Sm0.660.604.32.39.1108.48.213161521
Eu0.100.060.050.010.090.070.070.050.060.010.040.05
123456789101112
Gd7.37.0442275847094851198574
Tb5.67.230164346385447703422
Dy122157355197495492344464482638213110
Ho5982865412611160771041112211
Er3424773022164743681611993182893919
Tm94132584810272303459525.42.8
Yb97013914794699756162182664874003319
Lu148251637014281243261462.71.9
LREE1.60.724.52.410.0119.39.014171624
HREE174825041417109324331871947122016441726434260
ΣREE174925051422109524431882956122916581743451284
δEu0.140.080.010.010.010.010.660.711.330.893.664.51
k(HREE)0.310.340.130.180.150.110.040.030.090.04-0.11-0.12

“-”表示含量低于检测限;k(HREE)表示石榴子石稀土配分曲线中重稀土(Gd⁃Lu)回归直线的斜率]

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未矿化白云母花岗岩中的石榴子石贫Nb,Ta,Zr,Hf等高场强元素,含量分别不高于1.5×10-6,0.55×10-6,0.49×10-6,0.12×10-6,Nb/Ta值为0.33~3.13.含铌钽钨白云母花岗岩和含铌钽伟晶岩中的石榴子石略富Nb(最高69×10-6)、Ta(最高38×10-6)、Zr(最高28×10-6)、Hf(最高3.0×10-6)等高场强元素,Nb/Ta值可达5.39(图6c~e,表4).随着Mn/(Mn+Fe)值增加,石榴子石的Y含量从未矿化白云母花岗岩((1722~2161)×10-6)到含铌钽钨白云母花岗岩((1678~4064)×10-6)和含铌钽伟晶岩((1369~4392)×10-6)出现明显上升,含铌钽伟晶岩内部出现Y的降低((439~1109)×10-6)(图6f).含铌钽钨白云母花岗岩和含铌钽伟晶岩的石榴子石Zn((45~116)×10-6)和Ga((19~52)×10-6)含量高于未矿化白云母花岗岩的石榴子石中Zn((10~22)×10-6)和Ga含量((15~17)×10-6)(图6g).含铌钽钨白云母花岗岩和含铌钽伟晶岩中石榴子石存在一定程度的Sn富集(最高为156×10-6)(表4).

未矿化白云母花岗岩中的石榴子石轻稀土含量极低((0.68~5.0)×10-6),重稀土含量较高((1748~2504)×10-6)(图6h~i),具有较弱的Eu负异常(δEu=0.05~0.14)(图7a,表4).含铌钽钨白云母花岗岩和含铌钽伟晶岩中的石榴子石中轻稀土((1.1~24)×10-6)含量略微上升,含铌钽伟晶岩中石榴子石重稀土含量(HREE=(198~1841)×10-6)整体低于含铌钽钨白云母花岗岩中石榴子石的重稀土(HREE=(1093~3060)×10-6),且重稀土含量随着MnO含量上升出现从较高(HREE=(407~1841)×10-6)到较低(HREE=(52~539)×10-6)的变化(图6i),两种含铌钽矿化痕迹的花岗岩⁃伟晶岩具有相近的稀土元素配分曲线型式和强烈的Eu负异常(δEu<0.02)(图7b,表4).

图7

图7   夏如淡色花岗岩⁃伟晶岩中石榴子石稀土元素球粒陨石标准化配分图解(标准化数据引自文献[21])

Fig.7   Chondrite⁃normalized REE patterns of garnets from the Xiaru leucogranite⁃pegmatite (the data of chrondrite are from ref.[21])


4.4 锆石

夏如未矿化白云母花岗岩和含钨白云母花岗岩中锆石呈自形⁃半自形颗粒(图8a和图8b),而含铌钽钨白云母花岗岩和含铌钽伟晶岩中的锆石多呈半自形⁃他形颗粒,内部常含有晶质铀矿且孔洞较多(图8c和图8d).锆石粒径为10~40 μm,SiO2含量为32.55~34.82 wt.%,ZrO2含量为54.53~64.67 wt.%(表5,见1103页).未矿化白云母花岗岩和含钨白云母花岗岩中锆石HfO2含量分别为1.66~3.74 wt.%和1.50~2.51 wt.%,Zr/Hf值分别为14~32和22~37;含铌钽钨白云母花岗岩HfO2含量较高,为2.67~5.37 wt.%,Zr/Hf值为9.7~20;含铌钽伟晶岩中的锆石HfO2含量最高,为3.92~10.79 wt.%,Zr/Hf值为4.4~13(图9表5).锆石均贫P2O5,ThO2,Y2O3含量分别不高于1.14 wt.%,0.13 wt.%,1.01 wt.%(表5).

图8

图8   夏如淡色花岗岩⁃伟晶岩中锆石的背散射电子(BSE)图像

(a)未矿化白云母花岗岩中;(b)含钨白云母花岗岩中,自形程度较高;(c)含铌钽钨白云母花岗岩中的锆石,内部孔隙较多,与晶质铀矿和磷灰石共生;(d)含铌钽伟晶岩中的锆石,斑状,有孔洞.矿物缩写:Qtz⁃石英,Ab⁃钠长石,Kfs⁃钾长石,Ms⁃白云母,Zrn⁃锆石,Ap⁃磷灰石,Urn⁃晶质铀矿

Fig.8   Backscattered electron (BSE) images of zircons from the Xiaru leucogranite⁃pegmatite


表5   夏如淡色花岗岩⁃伟晶岩中代表性锆石的主量元素数据(wt.%)

Table 5  Major element composition (wt.%) of representative zircons from the Xiaru leucogranite⁃pegmatite

未矿化白云母花岗岩n=10含钨白云母花岗岩n=6含铌钽钨白云母花岗岩n=14含铌钽伟晶岩n=23
123平均456平均789平均101112平均
P2O50.950.140.340.240.400.330.100.250.060.070.210.330.170.270.490.19
SiO233.3933.5334.0833.9434.4633.8934.1434.1734.2134.1934.5534.1233.5134.2233.4833.78
ZrO261.5062.8162.2961.7862.6162.7262.1062.6661.8359.7760.7461.1054.5360.7857.4159.46
HfO21.662.442.422.342.331.792.512.083.475.374.033.9810.793.948.435.73
ThO20.13-0.05---0.040.030.020.050.010.01--0.050.02
UO21.270.560.650.840.720.420.250.440.140.220.420.421.390.460.510.73
Y2O30.55-0.12--0.02-0.00---0.020.030.04-0.11
Total99.4599.4899.9499.14100.5399.1799.1399.6497.7299.6999.9599.97100.4299.70100.37100.01
Zr/Hf32.3322.4923.3423.0523.4530.6821.6427.3615.569.7213.1713.874.4113.465.9410.15

“-”表示含量低于检测限

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图9

图9   夏如淡色花岗岩⁃伟晶岩中锆石HfO2与Zr/Hf图解

Fig.9   The diagram of HfO2 content and Zr/Hf ratio of zircons from the Xiaru leucogranite⁃pegmatite


5 讨论

5.1 全岩地球化学成分对淡色花岗岩⁃伟晶岩演化的指示

野外地质考察发现夏如淡色花岗岩被密集分布的花岗质伟晶岩脉切穿,地球化学研究表明夏如淡色花岗岩经历了高度结晶分异14,淡色花岗岩⁃伟晶岩以过铝质为主,稀土元素含量低且Eu负异常明显.夏如淡色花岗岩⁃伟晶岩的Nb/Ta,Zr/Hf,K/Rb和Y/Ho值均表现出偏离大陆平均地壳值(Nb/Ta=11.4,Zr/Hf=35.7,K/Rb=349,Y/Ho=24.7)的特征24,表明夏如淡色花岗岩⁃伟晶岩的微量元素存在不受价态和离子半径控制的行为25-26,指示夏如淡色花岗岩⁃伟晶岩在成岩过程中存在明显的流体相出溶.随着花岗质岩浆结晶分异程度增强,分离结晶作用促进残余熔体中H2O等挥发份和碱金属富集,饱和后可能导致液相分离和富H2O等挥发份流体相的出现27.岩浆⁃热液共存体系中稀土元素在熔体⁃流体之间的分配系数不同导致M型四分组效应的形成28,夏如含矿花岗岩⁃伟晶岩的稀土配分曲线表现出明显的四分组效应特征,从未矿化白云母花岗岩到含钨白云母花岗岩,再到含铌钽钨白云母花岗岩和含铌钽伟晶岩,TE1,3值出现明显上升达到1.1以上,TE1,3>1.1时为典型的四分组效应29,指示了夏如含矿花岗岩⁃伟晶岩流体活动的增强.

相较夏如未矿化白云母花岗岩和含钨白云母花岗岩,含铌钽钨白云母花岗岩和含铌钽伟晶岩含有较高的Na2O,较低的ΣREE,Zr,Th,Y,略低的FeO+MgO+TiO2含量和Zr/Hf,Th/U值,指示了含铌钽钨白云母花岗岩和含铌钽伟晶岩具有较高的结晶分异演化程度.根据锆石饱和温度计算公式30,未矿化白云母花岗岩(TZr=729~760 ℃)和含钨白云母花岗岩(TZr=695 ℃)的锆石饱和温度接近,而含铌钽钨白云母花岗岩(TZr=636 ℃)和含铌钽伟晶岩(TZr=572~660 ℃)的锆石饱和温度较低.因此推断含钨白云母花岗岩与未矿化白云母花岗岩的初始结晶温度相似,仅后期受到了微弱的含W流体作用,而随着演化程度升高,结晶温度降低,岩浆发生了铌钽成矿作用形成了含有铌钽氧化物的白云母花岗岩和伟晶岩.

5.2 矿物成分特征对淡色花岗岩⁃伟晶岩岩浆演化的指示

矿物的种类和成分可以指示花岗岩演化31,夏如岩体中未矿化花岗岩与矿化花岗岩⁃伟晶岩中出现的矿物种类和成分显示了它们的演化程度的差异,同时也可以反映演化过程中元素的富集程度变化32-35.本次研究重点选择了白云母、电气石和石榴子石进行讨论,白云母是造岩矿物,它贯穿于岩浆演化阶段形成的不同岩相中,而特征副矿物电气石和石榴子石的体积含量明显在铌钽成矿作用相关的花岗岩⁃伟晶岩中有所升高,并具有特征的元素组成来显示岩浆的演化过程.

岩浆演化过程中,随着黑云母、钛铁矿、金红石、石榴子石等矿物的结晶分异,熔体的Fe,Mg,Ti,Mn含量随之降低,随着演化形成于不同岩相中的白云母所含的相关元素含量也会逐渐降低36-37.在夏如未矿化白云母花岗岩和含钨白云母花岗岩中白云母的FeO,MgO,TiO2,MnO含量接近,而到含铌钽钨白云母花岗岩,再到含铌钽伟晶岩,白云母的这些元素含量呈逐渐降低而Al2O3含量上升的趋势(图4b~d),反映了未矿化白云母花岗岩与含钨白云母花岗岩形成母岩浆中镁铁质组分相近,而与铌钽矿化相关的白云母花岗岩与伟晶岩中铁镁组分降低,演化程度逐渐升高.这两种演化程度较高的岩石中的白云母还呈现出降低V和Co的趋势,体现了早期熔体中与之相关的矿物的分异结晶作用.花岗伟晶岩也是锂矿床相关的重要岩石类型,锂的工业品位可达到0.4%~0.6%38,锂还通常赋存在锂云母中.夏如淡色花岗岩⁃伟晶岩中白云母的Li2O含量较低,且全岩Li含量不高于149×10-6,虽然夏如岩体南部普士拉、珠峰地区有明显富锂区域39-41,但夏如淡色花岗岩⁃伟晶岩全岩和白云母的特征都未显示出该地区存在Li成矿潜力.

夏如淡色花岗岩⁃伟晶岩中电气石均属于黑电气石,从未矿化白云母花岗岩到含铌钽钨白云母花岗岩和含铌钽伟晶岩,电气石体积含量明显增加.电气石中V含量可以区分电气石的成因,岩浆成因的电气石中V含量通常小于100×10-6,热液或变质成因的电气石中V含量较高且通常具有较大的变化范围42-43.夏如电气石大都具有自形柱状晶型和弱环带结构,结合V含量最高为79×10-6,判别这些电气石属岩浆成因.未矿化白云母花岗岩和铌钽成矿的花岗岩⁃伟晶岩的电气石在Sr,V,Co等微量元素含量上也有差别,呈降低趋势(图5c和图5d),与白云母中这些微量元素的变化趋势相一致.岩浆电气石中Mg/(Mg+Fe)值通常可以用来指示岩浆演化44,从未矿化白云母花岗岩到含铌钽钨白云母花岗岩和含铌钽伟晶岩,电气石的Mg/(Mg+Fe)值出现明显降低,与其他特征指示的演化趋势相同(图5b).

岩浆成因的石榴子石以无明显环带,高MnO和FeO,低CaO和MgO,富集重稀土,亏损轻稀土为特征1245,其Mn/(Mn+Fe)值可以指示结晶分异演化程度46-47.夏如的石榴子石成分介于锰铝榴石⁃铁铝榴石之间,属于岩浆成因,从未矿化白云母花岗岩到演化程度更高的含铌钽钨白云母花岗岩和含铌钽伟晶岩,石榴子石的体积含量增加且Mn/(Mn+Fe)值上升.不相容元素在结晶分异过程中趋向于在晚期熔体中逐步富集,石榴子石中Nb,Ta从未矿化白云母花岗岩到含铌钽钨白云母花岗岩,再到含铌钽伟晶岩出现明显的递增(图6c和图6d),Zn,Ga等微量元素也呈现富集趋势(图6g).在夏如淡色花岗岩⁃伟晶岩演化过程中,早期岩浆结晶了锆石、磷灰石、磷钇矿等富含稀土元素的矿物,导致残余熔体中稀土含量降低,相对晚期形成的铁铝榴石⁃锰铝榴石中的稀土含量随着MnO含量上升而降低,尤其以重稀土更明显,同时含铌钽伟晶岩中石榴子石的Y含量也出现明显降低(图6f和图6i).石榴子石稀土配分曲线的重稀土回归直线斜率也是指示岩浆演化程度的重要指标35,从未矿化白云母花岗岩中石榴子石重稀土曲线斜率(0.31~0.34)为正(图7a),到含铌钽钨白云母花岗岩中重稀土曲线斜率为正(0.11~0.19)但略小于未矿化白云母花岗岩,再到含铌钽伟晶岩中重稀土曲线斜率小于未矿化白云母花岗岩且部分为负(图7b).斜长石的分离结晶会导致其他共生矿物出现明显的Eu负异常45,未矿化白云母花岗岩中石榴子石的稀土配分曲线表现出明显的Eu负异常(δEu=0.05~0.12),而演化程度更高的含铌钽钨白云母花岗岩和含铌钽伟晶岩中石榴子石Eu负异常更明显(δEu<0.02).

5.3 矿物锆石结构和成分特征对淡色花岗岩⁃伟晶岩岩浆演化和流体富集的指示

不同于夏如淡色花岗岩⁃伟晶岩中白云母、电气石和石榴子石较少受到流体活动的影响,锆石受到岩浆演化和岩浆晚期流体活动共同影响.岩浆锆石内部结构均匀,少见包裹体,热液锆石包括热液中结晶的锆石或热液改造的岩浆锆石,常以多孔状为特征,并且存在矿物包裹体48-49.未矿化白云母花岗岩和含钨白云母花岗岩中的锆石自形程度较高,HfO2含量均较低,内部少见孔洞和包裹体,常与磷灰石等矿物共生,认为其主要为岩浆锆石,锆石的结构和成分特征表明含W流体并未对锆石产生明显的影响.而含铌钽钨白云母花岗岩和含铌钽伟晶岩中不仅存在自形程度较高的岩浆锆石,亦存在内部孔洞较多,自形程度较差的热液锆石,并发现岩浆锆石与热液锆石共生(图8d),指示演化程度更高的含铌钽钨白云母花岗岩和含铌钽伟晶岩存在岩浆后期流体活动.过铝质花岗岩熔体中锆石的分离结晶会降低残余熔体的Zr/Hf值,增加锆石中HfO2的丰度50-52,如雅山铌钽花岗岩体53、加拿大Tanco伟晶岩54等.Hf在锆石中的富集还与岩浆晚期至岩浆后热液阶段的热液活动有关,热液活动导致残余熔体温度降低,Hf在锆石/熔体中的分配系数随温度降低而增加55,热液锆石从残余熔体⁃富水流体系统中结晶,其HfO2含量显著高于岩浆锆石56,从未矿化白云母花岗岩和含钨白云母花岗岩到演化程度更高的含铌钽钨白云母花岗岩和含铌钽伟晶岩演化的过程中,锆石的HfO2含量上升,Zr/Hf值下降(图9),可能是结晶分异和热液流体共同作用的结果.

6 结论

(1)夏如岩体出露中的花岗质岩石主要为白云母花岗岩和伟晶岩,部分出现了矿化痕迹,含有铌⁃钽⁃钨矿物(铌铁矿族矿物、骑田岭矿、黑钨矿、白钨矿),全岩微量元素数据表明铌⁃钽⁃钨矿化的花岗质岩石流体活动更强.

(2)白云母、电气石和石榴子石的矿物学特征显示了它们是指示岩浆演化程度的标志性矿物.相较夏如未矿化白云母花岗岩与含钨白云母花岗岩,含铌钽花岗岩⁃伟晶岩中含有更高含量的电气石和石榴子石,且白云母更高的Al2O3含量和更低的FeO含量,电气石更低的Mg/(Mg+Fe)值,石榴子石更高的Mn/(Mn+Fe)值、更低的稀土总量和更明显的Eu负异常等这些特征有效地显示了花岗质岩浆的演化过程和含铌钽花岗岩⁃伟晶岩更高的演化程度.

(3)含铌钽钨白云母花岗岩和含铌钽伟晶岩中热液锆石的出现,指示了岩浆晚期的流体活动.相较未矿化白云母花岗岩和含钨白云母花岗岩中锆石较低的HfO2含量和较高的Zr/Hf值,岩浆演化和热液流体的共同作用导致含铌钽钨白云母花岗岩和含铌钽伟晶岩中锆石HfO2含量升高和Zr/Hf值降低.

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