岩石学报  2018, Vol. 34 Issue (8): 2441-2480   PDF    
东昆仑东段三叠纪岩浆混合作用:以香加南山花岗岩基为例
陈国超1,2 , 裴先治1 , 李瑞保1 , 李佐臣1 , 裴磊1 , 刘成军1 , 陈有炘1 , 李小兵1,3     
1. 长安大学地球科学与资源学院, 西部矿产资源与地质工程教育部重点实验室, 国土资源部岩浆作用与找矿重点实验室, 西安 710054;
2. 南阳理工学院土木工程学院, 南阳 473000;
3. 山西师范大学地理科学学院, 临汾 041000
摘要:香加南山花岗岩基位于东昆仑造山带东段,岩基主要岩石类型为花岗闪长岩。千瓦大桥-加鲁河一带花岗岩体为香加南山岩基的重要组成部分。香加南山花岗岩基含大量暗色微粒包体,包体中捕掳晶丰富。千瓦大桥-加鲁河一带花岗岩体寄主岩中斜长石和暗色微粒包体中捕掳晶斜长石具正常环带,An值震荡变化,角闪石和黑云母MgO含量和Mg#值较低,具壳源特征;暗色微粒包体中基质斜长石具核边结构,核部和边部An值存在间断,角闪石和黑云母MgO含量和Mg#值较高,具幔源特征。LA-ICP-MS锆石U-Pb同位素定年结果显示千瓦大桥花岗闪长岩、暗色微粒包体和加鲁河辉长岩的结晶年龄分别为251.0±1.9Ma、252.8±3.0Ma和221.4±3.3Ma。千瓦大桥花岗闪长岩和加鲁河花岗闪长岩富集轻稀土元素(LREE)和大离子亲石元素(LILE),亏损高场强元素(HFSE),具较低的Mg#和Nb/Ta比值;从千瓦大桥到加鲁河花岗闪长岩呈现出由准铝质中钾钙碱性系列向准铝-弱过铝质中钾-高钾钙碱性系列演化;暗色微粒包体和加鲁河辉长岩轻重稀土元素分异程度相对较低,具较高的Mg#和Nb/Ta比值。千瓦大桥花岗闪长岩和加鲁河花岗闪长岩分别为古特提斯演化俯冲阶段和后碰撞阶段幔源岩浆底侵新生地壳使其部分熔融产物。镁铁质岩浆注入长英质岩浆的混合作用形成了暗色微粒包体。岩浆混合过程中,如果岩浆不完全混合,混合岩浆中混入物质除了长英质岩浆的残留岩浆和捕掳晶,还应该有镁铁质岩浆与长英质岩浆之间的元素梯度差导致的物质扩散;如果岩浆为近完全混合,混合岩浆近似为镁铁质岩浆和长英质岩浆以一定比例二元混合。东昆仑东段晚古生代-早中生代幔源岩浆对花岗质岩浆的影响是一个持续的过程,从俯冲阶段早期流体交代地幔熔融,到俯冲阶段后期板片断离,然后同碰撞阶段板片断离的持续影响,再到后碰撞阶段加厚地壳的拆沉作用,由于地球动力学体制不同,导致幔源岩浆影响的大小和特征不同。
关键词: 东昆仑造山带     香加南山花岗岩基     暗色微粒包体     岩浆混合     三叠纪    
Triassic magma mixing and mingling at the the eastern section of Eastern Kunlun: A case study from Xiangjiananshan granitic batholith
CHEN GuoChao1,2, PEI XianZhi1, LI RuiBao1, LI ZuoChen1, PEI Lei1, LIU ChengJun1, CHEN YouXin1, LI XiaoBing1,3     
1. MOE Key Laboratory of Western China's Mineral Resources and Geological Engineering, MLR Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits, Faculty of Earth Science and Resources, Chang'an University, Xi'an 710054, China;
2. School of Civil Engineeringy, Nanyang Institutte of Technolog, Nanyang 473000, China;
3. College of Geographical Sciences, Shanxi Normal University, Linfen 041000, China
Abstract: Xiangjiananshan granitic batholith located in the eastern part of the Eastern Kunlun Orogenic Belt (EKOB) is mainly composed of granodiorit. Granite pluton in Qianwadaqiao-Jialuhe area is an important of Xiangjiananshan granitic batholith and contains a large number of mafic microgranular enclaves (MMEs), MMEs rich in xenocrysts. Plagioclase in the host rock and xenocrystals plagioclase in MMEs has osicillatory zonings; the contents of MgO and Mg# values of amphibole and biotite are low, with the features of the crust source; plagioclases in matrix of MMEs have core-rim structures with an An value discontinuity; the contents of MgO and Mg# values of amphibole and biotite are high, and they are characterized by mantle source. LA-ICP-MS zircon U-Pb dating suggests that the granodiorite, MMEs in Qianwadaqiao area and gabbro in Jialuhe area crystallized 251.0±1.9Ma, 252.8±3.0Ma and 221.4±3.3Ma ago, respectively. Granodiorite in Qianwadaqiao-Jialuhe area is geochemically featured by LILE enrichment and HFSE depletion, low Mg# value and Nb/Ta ratio. The calc alkaline series granodiorite evolved gradually from metaluminous middle-K to weakly peraluminous middle-to high-K in geochemistry in the place from Qianwadaqiao area to Jialuhe area. The MMEs and the Jialuhe gabbro have unconspicuous differentiation between HREE and LREE, and high Mg# value and Nb/Ta ratio. Inferred by the geochronological and geochemical data, it is believed that the granodiorites in Qianwadaqiao area and Jialuhe were the products of partial melting, triggered by underplating from mafic magma into juvenile crust in the EKOB, occurred in subduction and post-collision period of paleo-Tethyan respectively. And the MMEs are results of magma mixing and mingling between mafic and felsic magma. During the magma mixing and mingling, if the magma is mixing, the mixture should be material diffusion between mafic and felsic magma as a result of element difference as well as the residual felsic magma and xenocrysts; if the magma is mingling, the mingling magma would be composed of mafic and felsic magma in a certain volume proportion. The influence to the granitoid melts by the Late Palozoic-Early Mesozoic mantle-derived magma of Eastern Kunlun is a continuous process, which was from the mantle melting with fluid metasomatism in early subduction stage to the large-scale plate break-off in late subduction atage, from the continuous influence of plate break-off in syn-collision stage to delamination of thicken crust in post-collision stage. The influence degrees and features of mantle-derived magma were different in the different geodynamic system of the process.
Key words: Eastern Kunlun Orogenic Belt (EKOB)     Xingjiananshan granitic batholith     Mafic microgranular enclaves (MMEs)     Magma mixing and Mingling     Triassic    

岩浆混合作用作为形成花岗质岩石的一种重要机制,造成火成岩的复杂性和多样性,长期以来受到地质学家的广泛关注(Didier and Barbarin, 1991; Bonin, 2004, 2007; Barbarin, 2005; 王德滋和谢磊, 2008; Brown, 2013; Castro, 2013; Clemens and Stevens, 2016; 翟明国, 2017),其已经成为研究花岗岩岩石成因和大陆地壳物质演化的一个重要窗口(Pitcher, 1997; Kemp et al., 2007; Clemens and Stevens, 2012)。东昆仑造山带作为复合型大陆造山带,是我国中央造山系的重要组成部分(姜春发等, 1992, 2000; 许志琴等, 2013; Dong et al., 2017)。在长期的地质演化过程中,经历了复杂多样的构造岩浆作用,广泛出露不同时代和不同成因的花岗岩类,其中晚古生代晚期-早中生代花岗岩类构成东昆仑岩浆岩带的主体(罗照华等, 1999; 莫宣学, 2011; 陈有炘等, 2015; Li et al., 2015a, 2018; Chen et al., 2017e, f)。

香加南山花岗岩基为东昆北弧岩浆岩带的重要组成部分(图 1),早期研究者对其进行了较详细的研究工作。结果显示,香加南山花岗岩基的形成时代跨度较大,从早三叠世到晚三叠世(刘成东等, 2004; 陈国超, 2014; 罗明非等, 2014)。但对其成因,却有不同的认识,大部分学者通过年代学和岩石地球化学研究,包体和寄主岩有近似的形成时代和相似的岩石地球化学特征,认为该岩基中暗色微粒包体为岩浆混合作用产物(刘成东等, 2002, 2003; 谌宏伟等, 2005; 陈国超等, 2016);但部分学者通过同位素化学研究,寄主岩和其中暗色微粒包体具有近似的同位素特征,认为暗色微粒包体为寄主岩早期结晶相,岩基中寄主岩和暗色微粒包体为同一岩浆房演化结果(Huang et al., 2014)。但研究显示,东昆仑地区晚古生代-早中生代的幔源岩浆和壳源岩浆具有近似的同位素特征(马昌前等, 2013),所以岩基的成因还存在较大争议。由于早期研究相对缺乏系统性,特别是针对岩浆混合作用中矿物化学特征的研究较少。因此,本文通过野外地质、岩石学、矿物化学、年代学、岩石地球化学和Lu-Hf同位素化学等方法,对香加南山花岗岩基及其暗色微粒包体的成因进行研究,进而探讨东昆仑造山带东段三叠纪壳幔岩浆相互作用和地球动力学背景。

图 1 东昆仑地区晚古生代-早中生代花岗岩分布图 Fig. 1 Distribution map of granite in Late Paleozoic to Early Mesozoic from Eastern Kunlun
1 区域地质背景及岩体地质特征 1.1 区域地质背景

东昆仑造山带位于中央造山系西段,青藏高原东北缘,由北到南东昆仑造山带可划分为东昆北构造带、东昆中蛇绿构造混杂岩带、东昆南构造带和布青山-阿尼玛卿蛇绿构造混杂岩带(殷鸿福和张克信, 1997; 许志琴等, 2006; Meng et al., 2013; 裴先治等, 2015)。研究区位于东昆北构造带东部,该带位于柴达木南缘断裂以南,东昆中蛇绿构造混杂岩带以北,向西延伸进入新疆境内,向东被NW向瓦洪山断裂终止,呈近东西向展布。该构造带出露大面积前寒武纪中深变质岩系,以及少量泥盆纪、石炭纪和三叠纪沉积地层。结晶基底主要包括古元古界白沙河岩组(Pt1b)和中元古界小庙岩组(Pt2x)。该带一个明显的特征是出露巨量花岗岩类,以华力西晚期-印支期为主,显示出弧岩浆岩特征,因此也有东昆北弧岩浆岩带之称(郭正府等, 1998; 罗照华等, 1999, 2002; 袁万明等, 2000; 杨经绥等, 2005; 莫宣学等, 2007; 马昌前等, 2013, 2015)。东昆仑华力西晚期-印支期花岗岩类岩性变化较大,从英云闪长岩、花岗闪长岩、二长花岗岩和正长花岗岩皆有出露,但以花岗闪长岩和二长花岗岩为主;大部分花岗岩含有暗色微粒包体,另有少量镁铁质小岩体在空间上与花岗岩体相伴生。

1.2 岩基地质特征 1.2.1 寄主岩地质特征

香加南山花岗岩基位于东昆仑造山带东段可可沙-香日德一带,千瓦大桥以北,香日德以南,出露面积约1200km2,呈不规则状,长轴近EW向展布。香加南山花岗岩基主体侵位于古元古界白沙河岩组和中元古界小庙岩组,南侧局部与下古生界纳赤台岩群呈断层接触关系,北侧被第四纪覆盖(图 2)。早期研究者认为香加南山岩基形成于早中三叠世,近年来研究显示,香加南山花岗岩基为一复式岩基,由早三叠世到晚三叠世不同时代的花岗岩体组成,例如香日德花岗闪长岩(220Ma, 罗明非等, 2014)、加鲁河黑云母花岗闪长岩(217Ma, 刘成东, 2008)和千瓦大桥花岗闪长岩(251Ma, 本文)等。香加南山花岗岩基主要岩石类型包括中粗粒花岗闪长岩、中-中粗粒二长花岗岩、中-中粗粒似斑状二长花岗岩和中粗-粗粒正长花岗岩。本文重点研究的千瓦大桥-加鲁河一带的花岗岩体,为香加南山岩基的重要组成部分。香加南山花岗岩基北段加鲁河一带岩性主体为花岗闪长岩,其中含有暗色微粒包体,岩体中部被辉长岩体侵入(图 3a),在两者的接触带,可以见到暗色微粒包体成群分布(图 3b),局部可见花岗闪长岩和辉长岩相互侵入穿插(图 3a);在辉长岩边部,岩性过渡为石英闪长岩。南段千瓦大桥一带岩性主体也为花岗闪长岩,岩体中也含有暗色微粒包体(图 3c, d),其包体密度大于加鲁河花岗闪长岩。为讨论岩浆混合作用,下文称千瓦大桥一带花岗闪长岩和加鲁河一带花岗闪长岩为寄主岩。

图 2 东昆仑东段香加南山花岗岩基分布示意图 1-第四系;2-上三叠统鄂拉山组;3-下三叠统洪水川组;4-下古生界纳赤台岩群;5-中元古界长城纪小庙岩组;6-古元古界白沙河岩组;7-正长花岗岩;8-似斑状二长花岗岩;9-二长花岗岩;10-花岗闪长岩;11-哈拉尕吐花岗岩基;12-中基性岩体;13-脆性断层/韧性断层;14-角度不整合界面;15-矿物化学样品采样点;16-同位素年龄样品采样点.年龄数据来源:(1)Chen et al., 2015a;(2)罗明非等, 2014;(3)刘成东, 2008;(4)Huang et al., 2014;(5)刘成东等, 2004;(6)本次研究 Fig. 2 Distribution map of the Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun

图 3 东昆仑东段香加南山花岗岩基野外地质特征 (a)岩基北段加鲁河花岗闪长岩和辉长岩相互穿插;(b)加鲁河花岗闪长岩和辉长岩接触带附近包体成群分布;(c)岩基南段千瓦大桥花岗闪长岩暗色微粒包体中的反向脉;(d)千瓦大桥花岗闪长岩暗色微粒包体中的冷凝边.加鲁河花岗闪长岩形成时代217~225Ma(刘成东, 2008),加鲁河辉长岩形成时代221.4Ma;千瓦大桥花岗闪长岩形成时代251.0Ma,千瓦大桥花岗闪长岩暗色微粒包体形成时代为252.8Ma Fig. 3 Outcrop photos of Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun
1.2.2 暗色微粒包体地质特征

千瓦大桥花岗闪长岩中的包体大小不一,短轴长度主体介于10~30cm。暗色微粒包体形状变化较大,以椭圆状居多(图 3c)。暗色微粒包体颜色和粒度变化范围较大,颜色一般介于浅灰色-黑色,明显深于寄主岩;粒度小于寄主岩,主体为细粒。在一些暗色微粒包体中有浅色脉体灌入(反向脉)(图 3c),部分暗色微粒包体边部可见粒度较细冷凝边(图 3d)。暗色微粒包体中含有各种类型捕掳晶,包括斜长石(图 4a)、钾长石(图 4b)、角闪石(图 4c)和石英(图 4d)等,偶可见捕掳晶横跨包体和寄主岩(图 4f)。

图 4 东昆仑东段香加南山花岗岩基暗色微粒包体中不同类型的捕掳晶 (a-d)千瓦大桥花岗闪长岩暗色微粒包体中不同类型的捕掳晶;(e、f)加鲁河辉长岩中的暗色环边石英 Fig. 4 Xenocrysts developed in the mafic microgranular enclaves of the Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun

加鲁河花岗闪长岩中也含有暗色微粒包体,并且在加鲁河花岗闪长岩和辉长岩接触带附近可见暗色环边石英(图 4e, f)。

2 样品概况与分析方法 2.1 样品概况

样品采集于东昆仑造山带东段千瓦大桥-香日德一带(图 2),共采集锆石U-Pb定年样品3件,样品编号为JLH03(N35°43.567′、E98°6.432′;千瓦大桥花岗闪长岩)、JLH04(N35°57.367′、E97°54.577′;千瓦大桥花岗闪长岩中暗色微粒包体)和XRD018/6(N35°48.460′、E98°09.272′;加鲁河辉长岩)。岩石地球化学样品17件,样品新鲜无蚀变。

本文选择寄主岩和暗色微粒包体中主要造岩矿物(斜长石、角闪石和黑云母)进行详细的矿物化学研究。斜长石类型包括:(1)寄主岩斜长石(013-5-1);(2)暗色微粒包体中基质斜长石(013-9a-3);(3)暗色微粒包体中斜长石捕掳晶(013-9b-2、019-4c-3)。角闪石类型包括:(1)寄主岩角闪石(013-5-2);(2)暗色微粒包体基质角闪石(013-9b-3);(3)暗色微粒包体中角闪石捕掳晶(013-9b-1);(4)辉长岩中环石英捕掳晶角闪石(019-4c-1)。黑云母类型包括:(1)寄主岩黑云母(013-5-3);(2)暗色微粒包体中斜长石捕掳晶嵌晶黑云母(016-3c-1);(3)暗色微粒包体中基质黑云母(013-9a-1、019-4c-4)。其中,矿物编号013和016来自千瓦大桥花岗闪长岩及其暗色微粒包体,019来自加鲁河辉长岩和花岗闪长岩及其暗色微粒包体。

2.2 电子探针分析

电子探针片磨制在陕西省区域地质矿产研究院完成。矿物主量元素在长安大学西部矿产资源与地质工程教育部重点实验室采用JXI-8100型电子探针分析,加速电压15kV,电流1.0×10-8A,束斑1μm。分析结果见表 1表 2表 3,主要物理特征见表 4

表 1 东昆仑东段香加南山花岗岩基中斜长石电子探针成分测试结果(wt%) Table 1 Electron microprobe analyses of plagioclase in Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun (wt%)

表 2 东昆仑东段香加南山花岗岩基中角闪石电子探针成分测试结果(wt%) Table 2 Electron microprobe analyses of amphiboles in Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun (wt%)

表 3 东昆仑东段香加南山花岗岩基中黑云母电子探针成分测试结果(wt%) Table 3 Electron microprobe analyses of biotites in Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun (wt%)

表 4 东昆仑东段香加南山花岗岩基中黑云母和角闪石的组成和物理特征对比 Table 4 Correlation of biltites and hornblendes compositons and physical characteristics in Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun
2.3 锆石U-Pb定年

样品破碎和锆石挑选由河北廊坊区域地质矿产调查研究所采用常规方法进行粉碎,并用常规浮选方法进行分选出锆石后,再用双目镜挑选出晶形和透明度较好的锆石颗粒作为测定对象。将锆石颗粒粘在双面胶上,经环氧树脂固定-环氧树脂固化-表面抛光工序后,进行锆石显微照相和阴极发光照相。锆石的反射光和透射光显微照相及阴极发光(CL)显微照相在北京锆年领航科技有限公司完成。

锆石U-Pb同位素组成分析在西北大学大陆动力学国家重点实验室激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)仪上完成。分析仪器为配备有193nmARf-excimer激光器的Geo-Las200M型(Microlas Gottingen Germany)激光剥蚀系统和Elan6100 DRC型四极杆质谱仪。分析采用激光剥蚀孔径30μm,剥蚀深度20~40μm,激光脉冲为10Hz,能量为32~36mJ。测试中用人工合成的硅酸盐玻璃标准参考物质NIST610进行仪器最佳化。锆石年龄计算采用国际标准锆石91500作为外标校正。在所测锆石样品分析前后各测一次NIST610,同时以29Si作为内标测定锆石的U、Th、Pb含量。详细分析步骤和数据处理方法见袁洪林等(2003)。样品的同位素比值和元素含量数据处理采用GLITTER(ver4.0, Macquarie University)程序,并采用Andersen软件对测试数据进行普通铅校正,年龄计算及谐和图绘制采用ISOPLOT(2.49版)软件完成。所有数据点年龄值的误差均为1σ,采用206Pb/238U年龄,其加权平均值具95%的置信度(Andersen, 2002; Ludwig, 2003),分析结果见表 5表 6

表 5 东昆仑东段香加南山花岗岩基LA-ICP-MS锆石U-Pb同位素分析结果 Table 5 LA-ICP-MS zircon U-Pb isotope analysis results for Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun

表 6 东昆仑东段香加南山花岗岩基中锆石微量及稀土元素(×10-6)分析结果 Table 6 Zircon trace and REE (×10-6) analysis results for the Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun
2.4 岩石地球化学分析

样品碎样工作在河北省廊坊区域地质矿产调查研究所实验室完成,岩石样品首先粗碎至2~4cm,然后用3%~5%的稀盐酸经超声波清除表面杂质,再研磨至200目。岩石地球化学测试分析实验在中国科学院地质与地球物理研究所岩石圈演化国家重点实验室完成。主量元素使用X-射线荧光光谱仪(XRF-1500)测试,采用0.6g样品与6g四硼酸锂制成的玻璃片在ShimadzuXRF-1500上测定氧化物的含量,精度优于2%~3%。微量元素及稀土元素利用酸溶法制备样品,再使用ICP-MS(Element Ⅱ)测试,分析精度为:按照GSR-1和GSR-2国家标准,当元素含量大于10×10-6时,其精度优于5%,当含量小于10×10-6时,其精度优于10%。化学分析测试流程参考Chen et al. (2002)介绍的方法。分析结果见表 7

表 7 东昆仑东段香加南山花岗岩基主量元素(wt%)和微量元素(×10-6)分析结果 Table 7 Major (wt%) and trace element (×10-6) analysis results for the Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun
2.5 锆石Lu-Hf同位素分析

原位锆石Lu-Hf同位素分析在西北大学大陆动力学国家重点实验室完成,所用仪器为Nu plasma型MC-ICP-MS,激光为193nm ArF准分子激光,激光束斑直径为44μm,所用激光脉冲速率为为10Hz。详细分析方法与参数同Yuan et al. (2008)。分析结果见表 8

表 8 东昆仑东段香加南山花岗岩基千瓦大桥花岗闪长岩及其暗色微粒包体锆石Lu-Hf同位素组成 Table 8 Zircon Lu-Hf isotopic compositions from Qianwadaqiao granodiorite and their MME in Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun
3 岩相学和矿物化学特征 3.1 千瓦大桥花岗闪长岩

花岗闪长岩为千瓦大桥一带岩体主要岩石类型,呈浅灰色-灰白色,中粗粒花岗结构,块状构造。主要矿物组成为斜长石(50%±)、石英(20%±)、钾长石(15%±)、角闪石(10%±)和黑云母(3%±),副矿物为锆石、榍石及磷灰石等。斜长石呈自形-半自形柱状、板柱状;样品(013-5-1)从核部到边部An值呈震荡变化(39~48)(图 5a)。角闪石呈长柱状、短柱状(图 6a),在部分角闪石中包裹有斜长石、黑云母和锆石等矿物;样品(013-5-2)Mg#为45~55,主要为铁角闪石(图 7)(Leake, 1978)。黑云母呈黄褐色,不规则片状(图 8a),样品(013-5-3)Mg#为41~44,为铁质黑云母(图 9)。

图 5 东昆仑东段香加南山花岗岩基中斜长石正交偏光镜下特征及成分剖面 (a)千瓦大桥花岗闪长岩中斜长石;(b)千瓦大桥暗色微粒包体中斜长石,具核-幔-边结构;(c)千瓦大桥暗色微粒包体中斜长石捕掳晶;(d)加鲁河暗色微粒包体中斜长石捕掳晶;图中数字为斜长石An值 Fig. 5 Petrographic features and compositional profiles of plagioclase in Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun

图 6 东昆仑东段香加南山花岗岩基中角闪石正交偏光镜下特征 (a)千瓦大桥花岗闪长岩中角闪石;(b)千瓦大桥暗色微粒包体中基质角闪石;(c)千瓦大桥暗色微粒包体中角闪石捕掳晶;(d)加鲁河辉长岩中石英捕掳晶暗色环边角闪石;图中数字为探针测定点号 Fig. 6 Petrographic features of amphibole in Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun

图 7 角闪石分类图(底图据Leake, 1978) 1-千瓦大桥花岗闪长岩中角闪石;2-千瓦大桥暗色微粒包体中基质角闪石;3-千瓦大桥暗色微粒包体中角闪石捕掳晶;4-加鲁河辉长岩中角闪石;5-加鲁河暗色微粒包体中角闪石;6-加鲁河花岗闪长岩中角闪石;7-加鲁河辉长岩中环石英捕掳晶角闪石.4、5、6数据据史洪峰等, 2016); 图 17b图 18b图例同此图 Fig. 7 Classification of the hornblendes (after Leake, 1978)

图 8 东昆仑东段香加南山花岗岩基中黑云母正交偏光镜下特征 (a)千瓦大桥花岗闪长岩中黑云母;(b)千瓦大桥暗色微粒包体中基质黑云母;(c)加鲁河暗色微粒包体中基质黑云母;(d)千瓦大桥暗色微粒包体中斜长石捕掳晶嵌晶黑云母;图中数字为探针测定点号 Fig. 8 Petrographic features of biotite in Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun

图 9 黑云母Mg-(Al+Fe3++Ti)-(Fe2++Mn)分类图解(据Foster, 1960) 1-千瓦大桥花岗闪长岩中黑云母;2-千瓦大桥暗色微粒包体中黑云母;3-加鲁河暗色微粒包体中黑云母;4-千瓦大桥暗色微粒包体中斜长石嵌晶黑云母; 图 17a图 18a图例同此图 Fig. 9 Mg-(Al+Fe3++Ti)-(Fe2++Mn) classification diagram of biotites (after Foster, 1960)
3.2 加鲁河辉长岩

辉长岩呈灰黑色,细粒辉长结构,块状构造,主要组成矿物为斜长石(45%±)、角闪石(32%±)和黑云母(18%±),另外有少量钾长石和石英(图 10a, d)。岩体大部分样品SiO2含量小于52%,在基性岩范围,但未见辉石,这可能由于俯冲环境富水,导致结晶的暗色矿物为角闪石和黑云母。

图 10 东昆仑东段香加南山花岗岩基加鲁河一带岩体野外地质(a-c)和正交偏光镜下(d-f)特征 Fig. 10 Field (a-c) and microscope (d-f) features of the Jialuhe pluton in Jialuhe area of Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun
3.3 加鲁河石英闪长岩

石英闪长岩呈深灰色,中细粒结构,块状构造(图 10b),主要矿物组成为斜长石(55%±)、角闪石(20%±)、黑云母(10%±)和石英(10%±);另有少量锆石、磷灰石和磁铁矿等副矿物。斜长石呈自形-半自形柱状、板状,部分具弱环带(图 10e)。角闪石呈半自形短柱状,黄褐色;黑云母呈半自形-他形深黄褐色,可见部分角闪石和黑云母边部被熔蚀成港湾状(图 10e)。

3.4 加鲁河花岗闪长岩

加鲁河花岗闪长岩与千瓦大桥花岗闪长岩相似,具中粗粒花岗结构,块状构造,主要矿物组成为斜长石(40%±)、石英(25%±)、钾长石(15%±)、黑云母(10%±)和角闪石(5%±),副矿物为锆石、榍石及磷灰石等(图 10c, f)。

3.5 暗色微粒包体

暗色微粒包体同寄主岩的矿物组成相似,但含量不同,暗色矿物含量较高,主体呈闪长质。暗色微粒包体具中细粒结构,块状构造。主要矿物组成为斜长石(50%±)、角闪石(30%±)、黑云母(10%±)和石英(5%±)。斜长石分为2类:(1)基质斜长石,粒度较细,呈半自形-他形柱状、粒状,环带不发育,粒径约0.2×0.4mm~0.5×1.1mm,样品(013-9a-3)核部呈卵形,未见环带,在核部外围有一层较窄的幔部,边部最宽,具明暗相间的环带(图 5b),核部具有最高An值(71),幔部(49)和边部(46~55)An值明显低于核部;(2)斜长石捕掳晶,呈自形,粒径约2.1×4.0mm~3.2×8.0mm,部分斜长石的环带明显,An值变化范围较小(013-9b-2:41~49),呈震荡变化(图 5c);另有斜长石捕掳晶环带较差,An具变化范围较大(019-4c-3:27~78)(图 5d)。角闪石分为3类:(1)基质角闪石,呈浅黄褐色,半自形-他形柱状、粒状(图 6b),约0.1×0.3mm~0.3×0.7mm,样品(013-9b-3)Mg#为60~62,为镁角闪石(图 7);(2)角闪石捕掳晶,呈浅褐绿色,自形长柱状(图 6c),粒度约0.5×3.5mm~1.2×7.6mm,样品(013-9b-1)也为镁角闪石,Mg#(59~61)稍低于包体基质角闪石(图 7);(3)环石英捕掳晶角闪石,呈深褐色(图 5d),他形粒状,具有最高的Mg#(77),为阳起石(图 7)。黑云母分为2类:(1)基质黑云母,多色性较强,颜色变化较大,呈绿-浅黄绿色,半自形片状(图 8b, c),不同样品Mg#(013-9a-1:49,019-4c-4:54~55)具有一定差别,为镁质黑云母(图 9);(2)斜长石捕掳晶中嵌晶黑云母,呈深褐色,半自形片状(图 8d),样品(016-3c-1)Mg#为47~50,为镁质黑云母(图 9)。石英可分为3类:(1)具暗色环边的石英捕掳晶,颗粒巨大,半径约4~8mm,肉眼下可见石英捕掳晶的边部具暗色矿物环带(图 4e, f),显微镜下观察大部分为角闪石,黑云母含量较少(图 5d);(2)以细小的填隙矿物出现;(3)以嵌晶包裹在斜长石或角闪石中。

4 分析结果 4.1 锆石U-Pb年代学

千瓦大桥花岗闪长岩及其暗色微粒包体和加鲁河辉长岩的锆石都呈黄色-无色透明,具震荡环带,Th/U比值大于0.30(表 5表 6),锆石的球粒陨石标准化稀土元素配分图显示,锆石轻、重稀土元素分馏明显,具Ce的正异常和Eu的负异常,整体为左倾斜模式的轻稀土亏损型(图 11),以上指示它们都具有岩浆成因锆石特征(吴元保和郑永飞, 2004)。

图 11 东昆仑东段香加南山花岗岩基锆石球粒陨石标准化稀土元素配分图解(标准化值据Boynton, 1984) Fig. 11 Chondrite-normalized REE distribution pattern of of zircon for Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun (normalization values after Boynton, 1984)

千瓦大桥花岗闪长岩锆石(样品号JLH03)多呈长柱状和短柱状(图 12a),长约120~240μm,宽约45~150μm。共测试了24个点,测点的206Pb/238U和207Pb/235U谐和性较好(图 12b),其206Pb/238U年龄为241±4Ma~257±4Ma,206Pb/238U加权平均年龄为251.0±1.9Ma(MSWD=0.79)。因此,千瓦大桥花岗闪长岩的结晶时代为251.0±1.9Ma。

图 12 东昆仑东段香加南山花岗岩基代表性单颗粒锆石阴极发光(CL)图像及U-Pb年龄(实线圆圈)和锆石Hf分析结果(虚线圆圈)(a、c、e)及其LA-ICP-MS锆石U-Pb年龄谐和图和直方图(b、d、f) Fig. 12 Cathodoluminescence photos (CL) of typical single-crystal zircons, with marked U-Pb ages and Hf isotopic compositions (a, c, e) and LA-ICP-MS zircon U-Pb concordant age diagram and weighted histogram (b, d, f) for Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun

千瓦大桥花岗闪长岩中暗色微粒包体的锆石(样品号JLH04)呈较自形长柱状(图 12c),长约110~200μm,宽约40~110μm。共测试了24个点,其中有6个测点(JLH04-03、-04、-09、-11、-12、-23)谐和性较差,测点偏离于一致线,表明受后期构造运动影响,锆石存在Pb丢失,不参与年龄计算。剩余测点207Pb/235U和206Pb/238U谐和性较好(图 12d),206Pb/238U加权平均年龄为252.8±3.0Ma(MSWD=0.10)。因此,本文将千瓦大桥暗色微粒包体的结晶时代为252.8±3.0Ma。

加鲁河辉长岩(样品号XRD018/6)锆石呈长柱状和短柱状(图 12e),长度约100~140μm,宽约60~120μm。锆石环带较宽,同镁铁质岩浆岩锆石相似(Corfu et al., 2003; Jian et al., 2012)。共测试了25个点,测点206Pb/238U和207Pb/235U谐和性较好(图 12f)。锆石年龄可以明显地分为2组:第一组年龄有12个测点,206Pb/238U加权平均年龄为248.4±4.8Ma(MSWD=0.10);第二组年龄有13个测点,206Pb/238U加权平均年龄为221.4±3.3Ma(MSWD=0.30)。结合前人研究成果,加鲁河辉长岩锆石U-Pb年龄可分为251~252Ma、239Ma和220Ma三组(刘成东等, 2004),三组锆石都具有岩浆岩锆石特征(图 12)。251~252Ma和239Ma两组年龄为可能捕获锆石年龄。所以,加鲁河辉长岩的形成时代应为221.4±3.3Ma。

4.2 主量元素

千瓦大桥花岗闪长岩比暗色微粒包体偏酸性,具有较高的SiO2、Na2O和K2O含量,两者主体都呈准铝质钙碱性系列,在TAS图中寄主岩样品落入花岗闪长岩范围,暗色微粒包体岩性变化较大,落入辉长岩、二长岩和闪长岩范围(图 13a)。岩石Harker图解中,花岗闪长岩与暗色微粒包体存在间断,花岗闪长岩样品的MgO、CaO、P2O5、TiO2、Al2O3和FeOT随SiO2的增大呈现出减少的趋势,而N2O随着SiO2的含量增大显示出升高的趋势(图 14)。

图 13 东昆仑东段香加南山花岗岩基的SiO2-ALK分类图解(a, 据Wilson, 1989)、SiO2-K2O图解(b, 据Rollinson, 1993)和A/CNK-A/NK图解(c, 据Maniar and Piccoli, 1989) 数据来源:白日其利岩脉(熊富浩等, 2011);香日德花岗岩(罗明非等, 2014);俯冲阶段花岗岩范围(刘成东等, 2004; 杨经绥等, 2005; 陈宣华等, 2011; 丰成友等, 2012; 李碧乐等, 2012; Xiong et al., 2012, 2014; Zhang et al., 2012; 罗明非等, 2015; Xia et al., 2015b);后碰撞阶段花岗岩范围(陈国超等, 2013b; 李佐臣等, 2013; 罗明非等, 2014; Xia et al., 2014, 2015a; Li et al., 2015b).图 14-图 16图 20图 23图例同此图 Fig. 13 SiO2 vs. ALK classifing diagram (a, after Wilson, 1989), SiO2 vs. K2O diagram (b, after Rollinson, 1993) and A/CNK vs. A/NK diagram (c, after Maniar and Piccoli, 1989) for Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun

图 14 东昆仑东段香加南山花岗岩基Harker图解 Fig. 14 Harker diagrams for Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun

加鲁河辉长岩的SiO2、TiO2、K2O和Na2O含量较低,但MgO、Al2O3含量较高,花岗闪长岩正好与其相反,石英闪长岩主量元素含量主体介于辉长岩和花岗闪长岩之间。加鲁河大部分样品介于准铝质中钾钙碱性系列和高钾钙碱性之间(图 13b, c)。在TAS图中(图 13a),加鲁河辉长岩、石英闪长岩和花岗闪长岩样品分别落入辉长岩、闪长岩和花岗岩范围。

4.3 稀土元素和微量元素

千瓦大桥一带和加鲁河一带岩体各岩石类型都富集轻稀土元素(LREE),亏损重稀土元素(HREE),重稀土元素配分模式较平坦(图 15);并且都富集Rb、Th、Ba、Cs等大离子亲石元素(LILE),亏损Nb、Ta、Ti等高场强元素(HFSE)(图 16)。但各岩石类型的稀土元素总量、轻重稀土及微量元素的富集和亏损程度具有一定差别。

图 15 东昆仑东段香加南山花岗岩基的球粒陨石标准化稀土元素配分图(标准化值据Boynton, 1984) Fig. 15 Chondrite-normalized REE distribution patterns (normalization values after Boynton, 1984) for Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun

图 16 东昆仑东段香加南山花岗岩基的微量元素原始地幔标准化蛛网图(标准化值据Sun and McDonough, 1989) Fig. 16 Primitive mantle-normalized trace element spider diagrams (normalization values after Sun and McDonough, 1989) for Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun

千瓦大桥暗色微粒包体稀土元素总量(REE)高于花岗闪长岩,轻重稀土和重稀土分异程度小于花岗闪长岩,(La/Yb)N平均值分别为2.44和6.71;Nb/Ta平均值分别为15.1和12.2,样品都具Eu负异常和正异常。

加鲁河花岗闪长岩的稀土元素总量(REE)高于辉长岩,石英闪长岩的稀土元素总量(REE)介于两者之间。加鲁河花岗闪长岩(La/Yb)N平均为13.2,轻重稀土元素分馏较强,辉长岩的(La/Yb)N较低,平均为3.70,石英闪长岩的(La/Yb)N介于两者之间,平均为4.52。Nb/Ta和δEu具有相似的特征,石英闪长岩介于辉长岩和花岗闪长岩之间。

4.4 锆石Hf同位素

对千瓦大桥花岗闪长岩及其暗色微粒包体的锆石进行了Lu-Hf同位素组成测定,测定结果及计算的相关参数见表 8。花岗闪长岩及其暗色微粒包体的176Yb/177Hf和176Lu/177Hf比值范围分别在0.010554~0.115789和0.000505~0.004807,表明锆石在形成后具有极地的放射成因Hf的累积,测定的176Hf/177Hf比值可以代表其形成时体系的Hf同位素组成(吴福元等, 2007)。花岗闪长岩测定14个测点,176Hf/177Hf比值介于0.282532~0.282622之间,εHf(t)为-3.2~0,平均为-1.4,二阶段Hf模式年龄(tDM2)为1078~1240Ma。暗色微粒包体测定10个测点,176Hf/177Hf比值介于0.282369~0.282633之间,εHf(t)为-9.1~+0.5,平均为-3.5,二阶段Hf模式年龄(tMD2)为1050~1539Ma。

5 岩浆结晶环境 5.1 压力 5.1.1 黑云母

利用Uchida et al. (2007)的黑云母全铝压力计经验公式:

(1)

计算得到千瓦大桥寄主岩-包体基质-暗色微粒包体中斜长石的嵌晶黑云母压力分别为1.68~1.71kbar(平均为1.70kbar)(013-5-3)、1.76~1.77kbar(平均为1.77kbar)(013-9a-1)和1.77~1.86kbar(平均为1.82kbar)(016-3c-1)。

计算得到加鲁河花岗闪长岩中包体基质黑云母压力为1.95~2.10kbar(平均为2.04kbar)(019-4c-4)。

侵位深度的计算公式为P=ρgh,一般情况下,重力加速度g取9.8m/s2,上覆岩层的密度ρ近似取2700kg/m3,计算得千瓦大桥寄主岩-包体基质-暗色微粒包体中斜长石的嵌晶黑云母的形成深度分别为6.36~6.47km(平均为6.41km)(013-5-3)、6.66~6.71km(平均为6.68km)(013-9a-1)和6.70~7.03km(平均为6.87km)(016-3c-1)。

加鲁河花岗闪长岩中包体基质黑云母的形成深度为7.38~7.95km(平均为7.70km)(019-4c-4)。

5.1.2 角闪石

Schmidt (1992)校正过的压力计算公式:

(2)

计算得到千瓦大桥寄主岩-包体基质-包体捕掳晶角闪石压力分别为3.42~4.75kbar(平均为4.26kbar)(013-5-2)、2.36~2.78kbar(平均为2.56kbar)(013-9b-3)和2.86~3.18kbar(平均为3.05kbar)(013-9b-1);侵位深度分别为12.94~17.96km(平均为16.11km)、8.91~10.52km(平均为9.66km)和10.79~12.01km(平均为11.53km)。

计算得到加鲁河辉长岩-寄主岩-包体基质-环石英捕掳晶角闪石压力分别为4.42~4.94kbar(平均为4.68kbar)(dl06-4)、3.37~4.27kbar(平均为3.67kbar)(dl07-1和dl01-1)、4.08~4.80kbar(平均为4.40kbar)(dl02-8)和0.12~0.51kbar(平均为0.31kbar)(019-4c-1);侵位深度分别为16.69~18.67km(平均为17.68km)、12.73~16.15km(平均为13.85km)、15.43~18.13km(平均为16.63km)和0.45~1.93km(平均为1.19km)。

以上计算结果显示,千瓦大桥不同类型黑云母的结晶压力较为近似,但低于角闪石形成压力;千瓦大桥寄主岩和暗色微粒包体捕掳晶角闪石的结晶压力高于暗色微粒包体基质角闪石;千瓦大桥寄主岩角闪石稍高于暗色微粒包体中角闪石捕掳晶。加鲁河镁铁质岩浆角闪石(辉长岩和暗色微粒包体)结晶压力大于寄主岩;环石英捕掳晶角闪石具有最低的结晶压力。

5.2 温度 5.2.1 角闪石

通过Ridolfi et al. (2010)的角闪石温度计算公式:

(3)

计算得到千瓦大桥寄主岩-包体基质-包体捕掳晶角闪石结晶温度分别为767~809℃(平均为794℃)(013-5-2)、756~773℃(平均为762℃)(013-9b-3)和760~770℃(平均为766℃)(013-9b-1)。

计算得到加鲁河辉长岩-寄主岩-包体基质-环石英捕掳晶角闪石结晶温度分别为820~844℃(平均为832℃)(dl06-4)、818~840℃(平均为827℃)(dl07-1和dl01-1)、828~842℃(平均为835℃)(dl02-8)和694~705℃(平均为699℃)(019-4c-1)。该公式误差为±22℃,最大误差为±57℃。

5.2.2 角闪石-斜长石温度计

通过Holland and Blundy (1994)提出的角闪石-斜长石温度计计算公式:

(4)

计算获得千瓦大桥寄主岩-包体基质-包体捕掳晶角闪石温度分别为700~733℃(平均为721℃)(013-5-2)、716~734℃(平均为723℃)(013-9b-3)和693~698℃(平均为697℃)(013-9b-1)。

计算获得加鲁河辉长岩-寄主岩-包体基质-环石英捕掳晶角闪石温度分别为781~798℃(平均为790℃)(dl06-4)、741~765℃(平均为749℃)(dl07-1和dl01-1)、790~819℃(平均为798℃)(dl02-8)和618~623℃(平均为641℃)(019-4c-1)。

不同计算方法得出的温度有所差别,但整体趋势具有一定相似性。千瓦大桥寄主岩和暗色微粒包体角闪石的结晶温度较为接近,高于暗色微粒包体中捕掳晶角闪石结晶温度。加鲁河镁铁质岩浆的结晶温度较为接近(辉长岩和暗色微粒包体),高于寄主岩。加鲁河寄主岩和镁铁质岩浆的角闪石结晶温度高于千瓦大桥岩体,与显微镜下特征相符(加鲁河花岗闪长岩暗色微粒包体的中斜长石熔蚀程度明显高于千瓦大桥花岗闪长岩暗色微粒包体中斜长石)(图 5)。

5.3 氧逸度 5.3.1 黑云母

岩浆物理条件变化对黑云母的化学成分有一定影响,不同氧逸度下生成的黑云母具有不同的Fe3+、Fe2+和Mg2+值(Wones and Eugster, 1965)。实验证明,在相同温度下从Fe2O3-Fe3O4缓冲线经Ni-NiO缓冲线到Fe2SiO4-SiO2-Fe3O4缓冲线氧逸度系统降低。在Fe3+-Fe2+-Mg2+图解上(图 17a),千瓦大桥寄主岩-包体基质-暗色微粒包体中斜长石的嵌晶黑云母和加鲁河花岗闪长岩中包体基质黑云母都投到Ni-NiO缓冲线附近,说明黑云母的岩浆-热液体系中氧逸度较高(Carmichael, 1991)。

图 17 黑云母Fe3+-Fe2+-Mg图解(a, 据Wones and Eugster, 1965)和角闪石Al-(Fe2+/(Fe2++Mg))图解(b, 据Anderson and Smith, 1995) Fig. 17 Fe3+-Fe2+-Mg diagram of biotites (a, after Wones and Eugster, 1965) and Al vs. (Fe2+/(Fe2++Mg)) diagram of amphiboles (b, after Anderson and Smith, 1995)
5.3.2 角闪石

根据Ridolfi et al. (2010)提出的用角闪石分子式计算结晶时的氧逸度公式:

(5)

根据公式(5)计算出角闪石结晶时的氧逸度,千瓦大桥寄主岩-包体基质-包体捕掳晶角闪石相对氧逸度为△NNO分别为-0.09~0.63、0.81~0.88和0.75~0.89;Logf(O2)分别为-13.93~-13.52、-13.97~-13.63和-13.85~-13.71。加鲁河辉长岩-寄主岩-包体基质-环石英捕掳晶角闪石相对氧逸度为△NNO分别为0.44~0.80、-0.24~-0.06、0.09~0.41和1.62~1.94;Logf(O2)分别为-12.60~-12.48、-13.61~-13.26、-13.02~-12.70和-14.49~-14.44。

研究表明,在特定温度下,镁铁质硅酸盐矿物的Fe/(Fe+Mg)比值可以为氧逸度指数,镁铁质硅酸盐矿物具有低的Fe/(Fe+Mg)比值和较低的Al值,其氧逸度较高,反之较低(Anderson and Smith, 1995)。千瓦大桥寄主岩-包体基质-包体捕掳晶的角闪石Fe/(Fe+Mg)比值和Al值分别为0.45~0.55、0.38~0.40、0.39~0.41和0.93~1.16、0.80~0.88、0.86~0.89。加鲁河辉长岩-寄主岩-包体基质-环石英捕掳晶角闪石Fe/(Fe+Mg)比值和Al值分别为0.46、0.54~0.61、0.45~0.48、0.23和1.29~1.40、1.33~1.50、1.28~1.39、0.44~0.47。在Fe/(Fe+Mg)-Al图(图 17b)中显示与Logf(O2)相似的趋势,镁铁质岩浆氧逸度较高,寄主岩氧逸度较低;环石英捕掳晶角闪石具有最高氧逸度,可能与侵位深度较浅有关。

6 讨论 6.1 香加南山花岗岩基岩石成因

香加南山花岗岩基出露范围广泛,形成时代跨度较大,所以岩基不可能为一次岩浆作用的产物,应该有着特殊成因,以下以千瓦大桥花岗闪长岩和加鲁河花岗闪长岩为对象探讨岩基的成因。

6.1.1 千瓦大桥花岗闪长岩

千瓦大桥花岗闪长岩发育大量暗色微粒包体,所以镁铁质岩浆对千瓦大桥岩体的形成具有一定贡献,可能为代表暗色微粒包体的镁铁质岩浆结晶分离的结果。但东昆仑东段岩浆分布特征显示,晚古生代晚期-早中生代岩浆岩中花岗质岩浆岩占主体,只有少量镁铁质岩浆相伴生,所以镁铁质岩浆结晶分离不能解释千瓦大桥花岗闪长岩成因。

千瓦大桥花岗闪长岩含大量角闪石,具有较高的SiO2和Al2O3含量,主体为准铝质中钾钙碱性系列,在SiO2-P2O5图中呈负相关趋势(图 14),这些特征显示岩体具I型花岗岩特征(Li et al., 2007; 吴福元等, 2007)。千瓦大桥花岗闪长岩矿物化学特征显示,寄主岩黑云母具岩浆成因黑云母特征(图 13a),MgO含量较低(8.43%~8.94%),与壳源岩浆黑云母相似(丁孝石, 1988);在FeO/(MgO+FeO)-MgO图中(图 18a),寄主岩黑云母落入壳源范围。寄主岩角闪石(Ca+Al)较高,属岩浆成因角闪石(Giret et al., 1980);寄主岩角闪石具有较高FeO含量(19.02%~20.83%)和较低的Mg#值(平均49),与壳源角闪石相似(壳源角闪石FeO>20%, Mg#小于50; Leake, 1978; 谢应雯和张玉泉, 1990);在Ca-(Fe2++Fe3+)-Mg图解中(图 18b),寄主岩角闪石主体落入壳型范围。千瓦大桥花岗闪长岩富集Rb、Th、Ba、Cs等大离子亲石元素(LILE),亏损Nb、Ta、Ti等高场强元素(HFSE),具弧岩浆岩特征;岩体具有较低的Nb/Ta比值(平均12.2),低于原始地幔(Nb/Ta=17.5)(McDonough and Sun, 1995; Weyer et al., 2003),接近大陆地壳(Nb/Ta=13.4)(Rudnick and Gao, 2003);岩体的Mg#平均值为34,低于幔源部分熔融形成的岩浆,与壳源岩浆岩相似(Rapp and Watson, 1995; Rapp et al., 1999)。千万大桥花岗闪长岩的εHf(t)为-3.2~0,平均为-1.4,二阶段Hf模式年龄(tDM2)为1078~1240Ma,远大于岩体的形成年龄。千瓦大桥花岗闪长岩的εNd(t)和ISr具有相似的特征(分别为-5.3~-2.1和0.70814~0.71164)(Huang et al., 2014),与东昆仑古特提斯大部分花岗岩和镁铁质岩相似(部分镁铁质岩稍微高于花岗岩)(图 19)。研究显示,这一时期的镁铁质岩浆岩εHf(t)和εNd(t)值主体为负值,与亏损地幔形成的洋中脊玄武岩不同(图 19a; 郭安林等, 2007),为受俯冲流体交代的地幔部分熔融结果(熊富浩等, 2011; Zheng et al., 2011; Xiong et al., 2013, 2014, 2016; Liu et al., 2014, 2017; Hu et al., 2016)。花岗岩的εHf(t)和εNd(t)值明显高于东昆仑地区老的结晶基底。所以,底侵的镁铁质岩浆与古老下地壳部分熔融形成的长英质岩浆的混合不大可能形成具有较高εHf(t)和εNd(t)值的花岗岩,更可能为受俯冲流体交代的新生镁铁质地壳(早期俯冲作用产生的底侵到下地壳的镁铁质岩石)部分熔融的结果,岩浆源区可能有一定古老结晶基底混入。

图 18 黑云母MgO-FeO/(FeO+MgO)图解(a, 据周作侠, 1986)和角闪石Ca-(Fe2++Fe3+)-Mg图解(b, 据谢应雯和张玉泉, 1990) Fig. 18 MgO vs. FeO/(FeO+MgO) diagram of biotites (a, after Zhou, 1986) and Ca-(Fe2++Fe3+)-Mg diagram of amphiboles (b, after Xie and Zhang, 1990)

图 19 东昆仑277~210Ma岩浆岩的ISr-εNd(t)图(a)、εHf(t)-锆石U-Pb年龄图(b)、εNd(t)-年龄直方图(c)和εHf(t)-年龄直方图(d) 数据来源:东昆仑洋中脊玄武岩和海山玄武岩据郭安林等, 2006, 2007; 东昆仑基底岩石据余能等, 2005; 东昆仑277~210Ma岩浆岩据丁烁等, 2011; 吴祥珂等, 2011; 高永宝等, 2012, 2015; 熊富浩等, 2011; Xiong et al., 2012, 2013, 2014; Zhang et al., 2012, 2017; Ding et al., 2014, 2015; 甘彩虹, 2014; Huang et al., 2014; Liu et al., 2014, 2017; 罗明非等, 2014, 2015; Xia et al., 2014, 2015a, b, 2017; Chen et al., 2015b, 2017g; Li et al., 2015a, b; Ren et al., 2016; Yu et al., 2015; Hu et al., 2016; Shao et al., 2017; Wang et al., 2018; Yin et al., 2017 Fig. 19 Diagrams of ISr vs. εNd(t) (a), εHf(t) vs. zircon U-Pb age (b), the statistical εNd(t) vs. age histogram (c) and the statistical εHf(t) vs. age histogram (d) for 277~210Ma magmatite in Eastern Kunlun area

千万大桥花岗闪长岩寄主岩角闪石具有最大的形成压力(平均4.26kbar),暗色微粒包体中角闪石捕掳晶形成压力稍低(平均3.05kbar);角闪石的形成压力都高于黑云母(平均1.70kbar);寄主岩斜长石和暗色微粒包体中斜长石捕掳晶有演化的An值。以上显示,随着岩浆演化,存在角闪石、斜长石和黑云母的结晶,与主量元素相似,花岗闪长岩样品的MgO、CaO、P2O5、TiO2、Al2O3和FeOT随SiO2的增大呈现出减少的趋势,而Na2O随着SiO2的含量增大显示出升高的趋势。

6.1.2 加鲁河花岗闪长岩

加鲁河花岗闪长岩同千瓦大桥花岗闪长岩具有相似的矿物学和岩石地球化学特征,例如:岩体含有角闪石(图 10c)、ISrεNd(t)值(分别为0.70356~0.71148和-6.4~-3.8,刘成东等, 2004; Huang et al., 2014; 罗明非等, 2014)(图 19a),SiO2与P2O5呈负相关(图 14)、Nb/Ta比值(平均12.2)与大陆地壳近似和较低的Mg#值(平均39)等。这些特征显示,加鲁河花岗闪长岩同千瓦大桥花岗闪长岩相似,主体为地壳部分熔融的产物。但两者又有一定区别,加鲁河花岗闪长岩的角闪石含量低于千瓦大桥花岗闪长岩,黑云母和钾长石含量高于千瓦大桥花岗闪长岩,加鲁河花岗闪长岩具有较低的Na2O/K2O比值、主体为弱过铝质-过铝质中钾-高钾钙碱性系列,而千瓦大桥花岗闪长岩Na2O/K2O比值较高、主体为准铝质中钾钙碱性系列(图 13b, c)。以上说明加鲁河花岗闪长岩主体也为新生地壳部分熔融结果,源区混入物质可能具有较高的成熟度。

6.2 岩浆混合作用

岩浆岩中暗色微粒包体的成因主要有以下三种观点:(1)花岗岩源区熔融的残留或围岩的捕掳体(Chappell et al., 2012);(2)寄主岩早期的堆晶矿物(Noyes et al., 1983; Clemens and Wall, 1988; Shellnutt et al., 2010);(3)镁铁质岩浆注入长英质岩浆混合的结果(Vernon, 1984, 2014; Kumar and Rino, 2006; Barbarin, 2005; Clemens and Stevens, 2012)。香加南山花岗岩基富含暗色微粒包体,并且在岩基中部有加鲁河辉长岩侵入(图 3a),这些镁铁质岩浆对香加南山花岗岩基的形成具有重要作用。以下从香加南山花岗岩基的野外地质、岩石学、矿物化学、年代学、岩石地球化学和同位素化学等多方面综合探讨暗色微粒包体成因及岩浆混合作用。

6.2.1 野外地质证据

千瓦大桥花岗闪长岩中大部分暗色微粒包体呈椭圆状,包体的边部未见烘烤现象,这些形态特征明显不同于围岩捕虏体,为镁铁质岩浆注入长英质岩浆塑性状态下的液滴(Wiebe et al., 1997, 2007; Barbarin, 2005)。千瓦大桥花岗闪长岩中暗色微粒包体中捕掳晶丰富(图 4),这些不同种类的捕掳晶是岩浆混合作用的产物,是从寄主岩捕获而来(Troll and Schmincke, 2002; Chen et al., 2009)。偶可见捕掳晶横跨包体和寄主岩(图 4c),这是两种岩浆呈塑态混合很好的证据(Didier and Barbarin, 1991; Weinberg, 2006)。部分暗色微粒包体可见反向脉和冷凝边(图 3c, d),为镁铁质岩浆注入长英质岩浆快速冷却标志(Wiebe et al., 1997; Kumar et al., 2004)。以上野外地质特征显示千瓦大桥花岗闪长岩中暗色微粒包体为岩浆混合作用结果。

加鲁河花岗闪长岩与千瓦大桥花岗闪长岩相似,岩体含有大量暗色微粒包体(图 3b),并且包体中捕掳晶丰富,说明加鲁河花岗闪长岩中暗色微粒包体也为岩浆混合作用结果。

6.2.2 岩石学证据

千瓦大桥花岗闪长岩和加鲁河花岗闪长岩中暗色微粒包体都具有典型的岩浆岩结构和构造(图 5图 6图 8),并且暗色微粒包体矿物粒度明显小于寄主岩,说明其不是源区的残留体(Chappell and White, 1992; White et al., 1999)。千瓦大桥花岗闪长岩中暗色微粒包体含有复杂环带斜长石(图 5),为寄主岩矿物进入高温镁铁质岩浆快速冷却结晶的结果(Baxter and Feely, 2002)。

显微镜下显示,加鲁河石英闪长岩中少量角闪石和黑云母边部被熔蚀(图 10b)。在加鲁河辉长岩和暗色微粒包体中还发现具暗色环边的石英捕掳晶(图 3b, e),这些暗色矿物晚于石英结晶,是一种异常的结晶顺序。石英捕掳晶在高温的镁铁质岩浆中(包体)受到熔蚀,熔蚀过程中的吸热效应使石英的边部快速冷却,其边部的岩浆快速结晶,使暗色矿物围绕石英晶出(Troll and Schmincke, 2002; 陈国超等, 2016, 2017b)。

6.2.3 矿物化学证据

千瓦大桥花岗闪长岩暗色微粒包体中斜长石捕掳晶(013-9b-2,016-3a-2)的An值范围稳定,呈震荡变化,明显低于暗色微粒包体中基质斜长石(013-9a-3),同寄主岩中斜长石(013-5-1)近似(表 1),说明这些斜长石捕掳晶来自寄主岩(Wiebe, 1968; Tsuchiyama, 1985; Mortimer et al., 2008; 陈国超等, 2017c)。千瓦大桥包体中角闪石捕掳晶(013-9b-1)的主量元素含量、Mg#值、温度、压力和氧逸度等与暗色微粒包体基质角闪石不同(013-9-3),接近寄主岩角闪石(013-5-2)(表 2表 4)。千瓦大桥暗色微粒包体角闪石的形成压力(平均2.56kbar)低于寄主岩和暗色微粒包体中角闪石捕掳晶的形成压力(平均值分别为4.26kbar和3.05kbar),显示暗色微粒包体角闪石结晶时间稍晚,说明镁铁质岩浆在注入寄主岩前,寄主岩已经有部分矿物结晶。寄主岩和暗色微粒包体中黑云母具有近似的形成压力(平均值分别为1.70kbar,1.77kbar),并且都明显低于角闪石,说明不同类型的黑云母结晶时间稍晚,近同时结晶。暗色微粒包体中斜长石捕掳晶嵌晶黑云母形成压力(平均值1.82kbar)与寄主岩和暗色微粒包体的黑云母形成压力近似,从包裹关系显示,斜长石形成时间靠后,应该有更低的形成压力,这与显微镜下特征和计算深度不符,这些黑云母可能为斜长石捕掳晶进入暗色微粒包体后,斜长石表面受到熔蚀后再次结晶形成的。

加鲁河花岗闪长岩与千瓦大桥花岗闪长岩相似,暗色微粒包体中斜长石捕掳晶(019-4c-3)的An值稳定,明显低于暗色微粒包体中基质斜长石(dl02-8),同寄主岩中斜长石(dl01-1和dl07-1)近似(表 1),说明这些斜长石捕掳晶来自寄主岩。加鲁河辉长岩和暗色微粒包体角闪石的形成压力(平均值为4.68kbar、4.40kbar)高于花岗闪长岩角闪石(平均值为3.67kbar),显示镁铁质岩浆在注入花岗闪长岩时已经有部分镁铁质岩浆结晶。辉长岩中环石英捕掳晶角闪石具有最低的形成深度(平均值为0.31kbar),明显小于辉长岩、暗色微粒包体和花岗闪长岩,较为接近暗色微粒包体中黑云母的结晶压力(2.04kbar)。花岗闪长岩中石英为岩浆晚期结晶晶体,其进入辉长岩后由于温差使石英旁边的岩浆快速冷凝形成暗色矿物环边,这些暗色矿物由于最晚结晶,具有最低的结晶压力和温度。

以上结果显示,暗色微粒包体中早期形成矿物可以在镁铁质岩浆注入寄主岩之前结晶(千瓦大桥),也可以在之后结晶(加鲁河),更加说明岩浆混合过程的复杂性。

6.2.4 年代学证据

锆石U-Pb年代学研究显示,千瓦大桥花岗闪长岩的形成时代为242~251Ma,花岗闪长岩中暗色微粒包体的形成时代为248~252Ma(Huang et al., 2014; 本文);千瓦大桥花岗闪长岩中暗色微粒包体同寄主岩有着相近的形成时代,这与残留模式明显不符,说明了暗色微粒包体不是变质岩和沉积岩熔融的残留体,为镁铁质岩浆注入长英质岩浆近同时结晶的结果(White et al., 1999; Donaire et al., 2005)。

加鲁河辉长岩形成时代为221.4Ma(本文),花岗闪长岩形成时代为217~225Ma(刘成东, 2008),石英闪长岩的形成时代为220.7±4.4Ma(陈国超等, 2017a),说明三者近同时结晶,石英闪长岩为岩浆混合作用产物。

6.2.5 岩石地球化学证据

通过前人研究成果和本文数据,以白日其利岩脉(熊富浩等, 2011)代表东昆仑早三叠世岩浆混合作用的镁铁质端元,以千瓦大桥花岗闪长岩代表长英质端元,千瓦大桥花岗闪长岩中暗色微粒包体为岩浆混合的产物。在哈克图解中(图 14),镁铁质端元和长英质端元存在明显的间断,暗色微粒包体主体介于两者之间,这较好地说明了暗色微粒包体为两种端元岩浆混合的产物。在同分母比值Al2O3/FeOT-Na2O/FeOT和FeOT/Al2O3-Na2O/CaO图解中分别呈线性趋势和双曲线趋势(图 20),也显示其具岩浆混合特征(Langmuir et al., 1978)。

图 20 东昆仑东段香加南山花岗岩基岩浆混合作用图解 Fig. 20 Diagram of the magma mixing for Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun

相似的,东昆仑晚三叠世镁铁质端元、长英质端元和混合单元分别为加鲁河辉长岩、花岗闪长岩和石英闪长岩,辉长岩和花岗闪长岩间存在成分间断,同分母比值图解呈线性趋势,说明加鲁河石英闪长岩为岩浆混合作用产物。

6.2.6 同位素地球化学证据

香加南山花岗岩基不论是千瓦大桥一带岩体还是加鲁河一带岩体,寄主岩及其暗色微粒包体具有近似的εHf(t)、εNd(t)和ISr值(表 9图 19a, b),与东昆仑古特提斯构造演化俯冲阶段和后碰撞阶段大部分花岗岩及其暗色微粒包体的同位素分布特征相似,显示这一时期东昆仑地区大部分花岗岩的寄主岩及其暗色微粒包体可能为同一岩浆房演化的结果。但部分镁铁质岩浆和暗色微粒包体的具有比花岗岩更低的εHf(t)和εNd(t)值,并且东昆仑地区古特提斯岩浆岩分布特征显示,花岗岩占绝对主体,只有少量的镁铁质岩浆出露,显然同源岩浆演化不能解释包体的成因,也说明东昆仑地区同位素特征较难判别岩浆混合作用。镁铁质岩浆和暗色微粒包体的同位素特征更多的是体现俯冲流体对幔源的交代程度。

表 9 东昆仑东段香加南山花岗岩基及同时代镁铁质岩浆的Sr-Nd-Hf同位素组成 Table 9 Sr-Nd isotopic data in Xiangjiananshan granitic batholith and simultaneous mafic pluton in eastern part of Eastern Kunlun
6.3 岩浆混合过程中物质交换

千瓦大桥花岗闪长岩和加鲁河花岗闪长岩的暗色微粒包体中含有不同类型捕掳晶,这些捕掳晶来自寄主岩(前述),说明岩浆混合过程中存在物质成分的交换。为研究岩浆混合过程中物质交换的详细过程,以长英质端元(千瓦大桥花岗闪长岩,加鲁河花岗闪长岩)和镁铁质端元(白日其利镁铁质岩脉,加鲁河辉长岩)不同比例进行简单二元岩浆混合模拟。混合分为两类,一类是不完全混合,寄主岩中含有暗色微粒包体,这里以千瓦大桥花岗闪长岩为长英质端元,因岩体附近没有相伴生的镁铁质岩浆,我们用同时代的白日其利镁铁质岩脉作为镁铁质端元(熊富浩等, 2011),暗色微粒包体为混合单元;另一类为完全混合,形成长英质端元和镁铁质端元的过渡型岩浆,这里我们用加鲁河辉长岩作为镁铁质端元,加鲁河花岗闪长岩为长英质端元,加鲁河石英闪长岩为混合单元。

以千瓦大桥花岗闪长岩和白日其利镁铁质岩脉不同比例进行简单二元岩浆混合模拟,混合比例根据暗色微粒包体的野外地质特征,例如包体的颜色和矿物组成。以80%的花岗闪长岩+20%的镁铁质岩脉、70%的花岗闪长岩+30%的镁铁质岩脉和60%的花岗闪长岩+40%的镁铁质岩脉进行混合。结果显示,模拟结果很难与混合单元相吻合(暗色微粒包体的平均含量)(表 10图 21a, b)。所以,简单的镁铁质岩浆和长英质岩浆二元混合模式不能很好解释暗色微粒包体的岩石地球化学组成。

表 10 东昆仑东段香加南山花岗岩基和模拟结果主要元素特征对比 Table 10 Correlation of maior element characteristics in Xiangjiananshan granitic batholith and simple calculations in eastern part of Eastern Kunlun

图 21 千瓦大桥岩体和加鲁河岩体的地壳平均值标准化主量元素二元岩浆混合模拟图(a、c)及球粒陨石标准化稀土元素二元岩浆混合模拟图(b、d) 1-80%花岗闪长岩+20%镁铁质岩脉;2-70%花岗闪长岩+30%镁铁质岩脉;3-60%花岗闪长岩+40%镁铁质岩脉;4-暗色微粒包体平均值;5-71%镁铁质岩浆+29%注入成分(残留岩浆(注入成分的50%)+角闪石(注入成分的20%)+斜长石(注入成分的30%));6-镁铁质岩浆;7-花岗闪长岩;8-70%辉长岩+30%花岗闪长岩;9-60%辉长岩+40%花岗闪长岩;10-50%辉长岩+50%花岗闪长岩;11-石英闪长岩;12-辉长岩;13-花岗闪长岩;图例1-7为千瓦大桥一带岩体,图例8-13为加鲁河一带岩体; 数据来源:地壳平均标准化值(Rudnick and Gao, 2003), 球粒陨石标准化值(Boynton, 1984), 千瓦大桥岩体镁铁质岩浆(熊富浩等, 2011) Fig. 21 Simple modeling diagrams of major element (a, c) and simple modeling diagrams of rare earth elemen (b, d) for magma mixing of pluton in Qianwadaqiao area and of pluton in Jialuhe area

根据暗色微粒包体的野外地质特征(图 4),千瓦大桥一带岩体暗色微粒包体中的捕掳晶主要为角闪石和斜长石,假设镁铁质岩浆中只混入捕掳晶,暗色微粒包体的地球化学特征也不符合岩浆混合结果,所以镁铁质岩浆中除了捕掳晶,应该还有寄主岩的残留岩浆混入。

显微镜下特征显示千瓦大桥花岗闪长岩中的角闪石和斜长石的结晶世代比较宽。所以,假设镁铁质岩浆注入花岗闪长岩时,花岗闪长岩已有部分角闪石和斜长石结晶。以一定比例的花岗闪长岩残留岩浆和矿物(角闪石和斜长石)进入暗色微粒包体模拟岩浆混合。假设岩浆混合之前,花岗闪长岩中斜长石和角闪石已结晶50%,来计算残留岩浆主量元素含量进行岩浆模拟。当镁铁质岩浆占71%,混入岩浆占29%(混入岩浆包括三部分,其中花岗闪长岩残留岩浆占50%,角闪石占20%,斜长石占30%)时,SiO2、Al2O3和MgO与暗色微粒包体的含量近似,但CaO、Na2O和K2O有较大偏差(由于钛铁矿和磁铁矿结晶较早,TiO2、FeO和MnO不参与岩浆模拟)。CaO、Na2O和K2O主要含于角闪石(Ca)、斜长石(Na)和钾长石(K)中。如果增加或者减少注入的残留岩浆以及角闪石和斜长石捕掳晶的含量,都不能得到较好的模拟结果。假如CaO、K2O和Na2O要接近暗色微粒包体的平均值,镁铁质岩浆中混入的成分要占包体成分比例的80%左右,其中残留岩浆的含量要占混入比例的50%以上,这与野外观察结果明显不符,并且模拟结果中SiO2含量也明显高于暗色微粒包体平均值。

所以混入镁铁质岩浆的成分除了花岗闪长岩残留岩浆和捕掳晶外,应该还有别的物质成分。元素的化学梯度和流体扩散是比较合理解释。研究表明,Na、K、P、Zr、Rb等元素在岩浆混合过程中,容易从寄主岩中扩散到暗色微粒包体(Watson and Jurewicz, 1984; Bussy, 1991; Orsini et al., 1991; Donaire et al., 2005)。花岗闪长岩残留岩浆中Na2O和K2O含量高于镁铁质岩浆,而镁铁质岩浆中的CaO高于寄主岩,两者化学梯度差导致的成分扩散,使暗色微粒包体具有现在的岩石地球化学特征。

以上混合结果在千瓦大桥一带岩体的稀土元素特征上得到较好体现。千瓦大桥花岗闪长岩、暗色微粒包体及同时代的镁铁质岩浆(白日其利镁铁质岩脉)REE含量(平均值分别为107.1、140.4、95.87)以及(La/Yb)N(平均值分别为6.17、2.06、3.59)和(Gd/Yb)N(平均值分别为1.09、0.92、1.44)比值与简单二元混合模式不符。研究显示,角闪石-熔体分配特征,轻稀土元素(LREE)的分配系数较小DLa=0.11,而重稀土元素(HREE)的分配系数较大DYb=1.23(Hanson, 1980; Blundy and Sparks, 1992)。暗色微粒包体中重稀土元素的增高比例高于轻稀土元素,说明千万大桥暗色微粒包体较高的REE含量与角闪石的混入具有一定关系,这与暗色微粒包体的野外地质特征相符,包体中含有角闪石捕掳晶(图 4c)。但暗色微粒包体的REE含量明显高于镁铁质岩浆,如果要达到这么高的含量需要大量角闪石,这与野外地质特征又有一定差别。所以除了角闪石的混入,残留岩浆和镁铁质岩浆的元素浓度差也是重要因素(Farner et al., 2014)。

加鲁河辉长岩和花岗闪长岩间存在岩浆混合作用,石英闪长岩为岩浆混合作用的产物(前述)。这里以70%的辉长岩+30%的花岗闪长岩、60%的辉长岩+40%的花岗闪长岩和50%的辉长岩+50%的花岗闪长岩进行二元模拟混合。结果显示,当辉长岩占到60%,花岗闪长岩占到40%时,模拟的结果与石英闪长岩的含量较吻合(图 21c, d)。

以上显示,岩浆混合过程中镁铁质端元和长英质端元存在物质成分的交换,但岩浆混合的程度以及混合前两种端元结晶程度不同,对岩浆混合过程具有较大影响。如果岩浆混合比较充分,混合结果接近简单二元岩浆混合结果(加鲁河石英闪长岩);如果不完全混合,混合岩浆的成分(千瓦大桥暗色微粒包体)除了要受控于镁铁质和长英质岩浆的成分,还同混入的捕掳晶、残留岩浆和长英质岩浆与镁铁质岩浆之间的化学梯度差有一定关系。

6.4 构造意义

东昆仑晚古生代-早中生代岩浆岩年龄直方图显示(图 22),根据岩浆活动强弱程度,东昆仑地区晚古生代-早中生代可划分为三个阶段:俯冲阶段(277~240Ma)、同碰撞阶段(240~230Ma)和后碰撞阶段(230~205Ma)。相应的,东昆仑壳幔岩浆活动也与此相对应。这三个阶段壳幔岩浆的特征和表现形式明显不同,为东昆仑古特提斯洋构造演化过程不同阶段的产物。

图 22 东昆仑277~200Ma岩浆岩年龄直方图 Fig. 22 The statistical histogram for 277~200Ma magmatite in Eastern Kunlun

俯冲阶段的壳幔岩浆活动最为剧烈,花岗质岩体含有大量暗色微粒包体(例如香加南山花岗岩基千瓦大桥一带岩体),其密度高于同碰撞和后碰撞阶段;并且在这一阶段,出露有与包体同时代的镁铁质岩浆,如大量的镁铁质岩脉、一些小的镁铁质岩体。这一时期的花岗质岩浆岩主体为准铝质中钾钙碱性系列,具有I型花岗岩特征(姜春发等, 1992, 2000; 郭正府等, 1998; 袁万明等, 2000; 孙雨等, 2009; 李碧乐等, 2012; Chen et al., 2017e)(图 13b, c)。在构造环境判别图中,千瓦大桥花岗闪长岩落入俯冲阶段相关的弧岩浆岩区域(图 23)。镁铁质岩浆也显示俯冲环境特征,例如富集大离子亲石元素,亏损高场强元素,主体呈中钾钙碱性系列(熊富浩等, 2011)。在沉积方面,东昆仑下三叠统洪水川组为布青山-阿尼玛卿古特提斯洋向北俯冲的沉积反映(李瑞保等, 2012, 2015)。

图 23 东昆仑东段香加南山花岗岩基构造环境判别图解(a, 底图据Pearce et al., 1984; b, 底图据Batchelor and Bowden, 1985) Fig. 23 Diagrams of the tectonic setting for Xiangjiananshan granitic batholith in eastern part of Eastern Kunlun (a, after Pearce et al., 1984; b, after Batchelor and Bowden, 1985)

同碰撞阶段壳幔岩浆相互作用程度很弱。这一时期侵入岩出露稀少,具有同碰撞花岗岩特征(Zhang et al., 2012; 夏锐等, 2014; Xiong et al., 2014),并且东昆仑地区还未见到同时期的镁铁质岩体出露,但少量岩体中含有暗色微粒包体(可日岩体, 陈国超等, 2018),包体的密度明显小于俯冲和后碰撞阶段。这可能是因为东昆仑处于同碰撞挤压环境,不利于岩浆活动产生(柴耀楚等, 1984; Zhang et al., 2012)。这一时期的壳幔岩浆相互作用可能为俯冲阶段板片断离后持续影响。在沉积上,中三叠统希里克特组在东昆仑地区只有零星出露,具海陆相交互相沉积特征,代表了巴颜喀拉地块与东昆仑地块的碰撞和布青山-阿尼玛卿洋的消失,印证了当时东昆仑处于抬升阶段,使局部受到沉积(李瑞保等, 2012)。

后碰撞阶段,有少量的镁铁质岩体和岩脉报道(罗照华等, 2002; 马昌前等, 2013; 奥琮等, 2015; Hu et al., 2016),花岗质岩体中包体的密度(香加南山花岗岩基北段、和勒冈西里可特岩体,陈国超等, 2013b, c)大于同碰撞阶段,但小于俯冲阶段。这一时期的岩浆岩向着过铝质高钾-钾玄岩钙碱性系列靠近(加鲁河一带)(图 13b, c),具有后碰撞岩浆岩特征(Condie, 1976; Liégeois, 1998)。在构造环境判别图中,加鲁河花岗闪长岩向着板内环境靠近(图 23)。这一时期部分岩浆岩具A型花岗岩和埃达克质岩浆岩特征也反映了东昆仑地区已处于加厚下地壳拆沉导致的伸展构造背景(丁烁等, 2011; 陈国超等, 2013a, b, c, 2017a; 罗明非等, 2014; Xia et al., 2014; Xiong et al., 2014; 奥琮等, 2015; Li et al., 2015a; Hu et al., 2016)。沉积上,上三叠统八宝山组为陆相碎屑岩沉积组合(李瑞保等, 2012; 陈国超等, 2017d),印证东昆仑地区在晚三叠世已经完全进入到后碰撞陆内环境。

香加南山花岗岩基为东昆仑古特提斯洋不同构造演化阶段岩浆作用形成的复式岩体,早三叠世千瓦大桥一带岩体为俯冲阶段幔源岩浆底侵新生地壳使其部分熔融产物,晚三叠世加鲁河一带岩体为后碰撞阶段拆沉作用引起拉张背景下幔源岩浆底侵新生地壳的结果(图 24)。

图 24 东昆仑造山带晚古生代-早中生代构造演化模式图 Fig. 24 Late Paleozoic-Early Mesozoic tectonic evolution model of the EKOB
7 结论

(1) 锆石U-Pb定年结果显示香加南山花岗岩基千瓦大桥花岗闪长岩及暗色微粒包体的结晶年龄分别为251.0±1.9Ma和252.8±3.0Ma,香加南山花岗岩基加鲁河辉长岩的结晶年龄为221.4±3.3Ma。

(2) 香加南山花岗岩基千瓦大桥花岗闪长岩为布青山-阿尼玛卿洋俯冲阶段镁铁质岩浆底侵新生地壳使其部分熔融的产物;加鲁河花岗闪长岩为东昆仑古特提斯演化的后碰撞阶段加厚地壳导致的拆沉作用使新生地壳部分熔融的结果。

(3) 香加南山花岗岩基暗色微粒包体为镁铁质岩浆注入长英质岩浆混合的产物。

(4) 镁铁质岩浆和长英质岩浆不完全混合,混合岩浆的物质成分主要受控于混入的捕掳晶、长英质岩浆的残留岩浆和元素的化学浓度差;如果完全混合,混合结果近似为两种端元岩浆以一定比例的混合。

致谢      感谢王盟博士在写作过程中提出的问题和修改意见;特别感谢两位匿名评审老师和编辑部俞良军老师认真指导使本文更加完善。

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