岩石学报  2020, Vol. 36 Issue (9): 2667-2700, doi: 10.18654/1000-0569/2020.09.05   PDF    
藏南冈底斯岩基晚白垩世含紫苏辉石侵入岩的地球化学特征及其成因探讨
高家昊1, 曾令森1, 高利娥1, 赵令浩1,2,3, 王亚莹1, 王亚飞1     
1. 自然资源部深部动力学重点实验室, 中国地质科学院地质研究所, 北京 100037;
2. 中国地质科学院国家地质实验测试中心, 北京 100037;
3. 北京大学地球与空间科学学院, 北京 100871
摘要: 冈底斯岩基南缘自西向东,从楠木林到米林广泛出露一系列含暗色细粒包体的含紫苏辉石侵入岩。这一系列含紫苏辉石的侵入岩具有斜长石以及紫苏辉石的巨晶(>5mm),呈现出堆晶结构。锆石U-Pb定年表明,这一系列含紫苏辉石侵入岩的结晶年龄为97~77Ma,并不随侵位位置具有显著的经度上的变化。含紫苏辉石的基性岩具有高的Al2O3(17.3%~18.2%)含量,较高的MgO(3.9%~4.1%)含量,FeOT含量在8.7%~9.0%之间;低的Cr(< 14.8×10-6)和Ni(< 15.0×10-6)含量,基本不具有Eu的异常,富集大离子亲石元素(LILE)和LREE,亏损高场强元素(HFSE)。含紫苏辉石的中-酸性岩具有高的Al2O3(14.9%~18.8%)含量,高的Mg#值(>39.7);变化较大的Cr(5.7×10-6~260×10-6)和Ni(10.2×10-6~78.2×10-6)含量,具有微弱-强烈的Eu的负异常,富集LILE和LREE,亏损HFSE。暗色细粒包体与含紫苏辉石基性岩相比具有相似的SiO2含量,FeOT(8.1%~9.0%)含量,稍高的MgO(4.7%~5.4%)含量,Al2O3(18.1%~19.4%)含量以及Mg#值(51.0~52.6);具有与含紫苏辉石基性岩相似的微量元素分布和稀土元素配分模式。这一系列含紫苏辉石的侵入岩具有较低的初始Sr同位素比值(87Sr/86Sr(t)=0.7037~0.7044),较高并变化较大的εNdt)值(+3.7~+9.4)和εHft)值(+9.9~+14.6)。这些特征共同说明,经流体+熔体交代的地幔楔中软流圈部分在俯冲流体存在的情况下发生部分熔融形成母岩浆,其母岩浆随后与俯冲板片熔体发生混合。在岩浆演化过程中经历了单斜辉石、斜方辉石以及斜长石的分离结晶并最终形成了冈底斯岩基南缘出露的含紫苏辉石侵入岩。暗色细粒包体可以代表母岩浆的早期堆晶,是岩浆淬火作用的产物。母岩浆中大量流体的存在,使其结晶顺序为单斜辉石-斜长石(紫苏辉石),随后的堆晶作用使得这一系列侵入岩得以赋存紫苏辉石。
关键词: 冈底斯岩基    紫苏辉石    堆晶作用    斜长石分离结晶    暗色细粒包体    
Geochemical characteristics and petrogenesis of Late Cretaceous hypersthene-bearing intrusive rocks in the Gangdese batholith, southern Tibet
GAO JiaHao1, ZENG LingSen1, GAO LiE1, ZHAO LingHao1,2,3, WANG YaYing1, WANG YaFei1     
1. Key Laboratory of Deep-Earth Dynamics, Ministry of Natural Resources, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
2. National Research Center for Geoanalysis, Chinese Academy of Geological Sciences, Beijing 100037, China;
3. School of Earth and Space Sciences, Peking University, Beijing 100871, China
Abstract: A series of hypersthene-bearing intrusive rocks occur along the southern margin of the Gangdese batholith from Nanmulin to Milin. These rocks show cumulate texture with plagioclase megacrysts and hypersthene megacrysts (>5mm). Zircon U-Pb dating indicates they were formed at a period of 97~77Ma with no change along with the longitudinal variation of the location. According to geochemical study, the hypersthene-bearing intrusive rocks include two types, the hypersthene-bearing mafic rocks and the hypersthene-bearing dioritic-granitic rocks, respectively. The former are characterized with: (1) high Al2O3 (17.3%~18.2%), MgO (3.9%~4.1%) contents, with the content of FeOT from 8.7% to 9.0%; (2) low Cr (< 14.8×10-6) and Ni (< 15.0×10-6) contents, with insignificant Eu anomaly, enrichment in LILE and LREE, depletion in HFSE. While the later are characterized with: (1) high Al2O3 (14.9%~18.8%) contents, Mg# (>39.7); (2) variable Cr (5.7×10-6~260×10-6) and Ni (10.2×10-6~78.2×10-6) contents, with slight-significant negative Eu anomaly, enrichment in LILE and LREE, depletion in HFSE. Furthermore, compared the mafic ones, the mafic microgranular enclaves contain similar amounts of SiO2 and FeOT, slightly higher amounts of MgO (4.7%~5.4%) and Al2O3 (18.1%~19.4%), and higher Mg# (51.0~52.6). The mafic microgranular enclaves show the similar distribution of the trace element and REE with the mafic ones. As for their isotopic compositions, the hypersthene-bearing intrusive rocks are characterized by low Sr (87Sr/86Sr < 0.7044) but high εNd(t) (>+3.7) and εHf(t) (>+9.9) isotope compositions. These features suggest that the source region of their parental magmas should be the mantle wedge metasomatized by the fluid and melt derived from the subducting slab. The parental magmas suffered a mixing of melt of the metasomatized asthenosphere and melt of the subducting slab with various degrees of fractional crystallization of clinopyroxene, hypersthene and plagioclase in the process of magma evolution. The mafic microgranular enclaves in these intrusive rocks that represented the cumulation at an early stage should be generated by quenching process in the parental magmas system. The sequence of mineral fractional crystallization (clinopyroxene-plagioclase and hypersthene) in response to large-volume fluids in the parental magmas combined with subsequent cumulation makes this suit of intrusive rocks bearing hypersthene.
Key words: Gandese batholith    Hypersthene    Cumulate    Fractional crystallization of plagioclase    Mafic microgranular enclaves    

冈底斯岩基作为世界上大规模的岩基之一,保存了大量岩浆活动的记录(Chung et al., 2003, 2009; Hou et al., 2004; Chu et al., 2006, 2011; Mo et al., 2007, 2008; Wen, 2007; Wen et al., 2008; Ji et al., 2009, 2014; Zhu et al., 2009, 2011, 2015, 2019; Zhang et al., 2010, 2011, 2014; Jiang et al. 2014; Wang et al., 2016; 张泽明等,2019),为研究岛弧岩浆作用和构造动力学过程提供了天然的野外实验室。苏长岩或含紫苏辉石岩浆岩作为一种“特殊类型”的岩浆岩,在世界各地的层状侵入杂岩体、岛弧杂岩体、蛇绿岩以及大洋洋壳中广泛发育(Hailwood, 1974; Fyfe, 1978; Hall and Hughes, 1987; Ross and Elthon, 1993; Python and Ceuleneer, 2003; Piccardo and Guarnieri, 2011; Gao and Zhou, 2013; 刘传周, 2015; Srivastava et al., 2016; Rajesh, 2019)。前人研究表明,苏长岩的母岩浆相较于MORB具有更高的含水量和氧逸度(Lachize et al., 1996),并且亏损Ti、Zr、Sr、Y以及REE(Ross and Elthon, 1993)。其母岩浆可能来自于有流体参与的大洋岩石圈地幔的部分熔融(Python and Ceuleneer, 2003)、大陆岩石圈地幔的部分熔融(Hall and Hughes, 1987)、软流圈的部分熔融(Fyfe, 1978; Olson, 1992; Ross and Elthon, 1993; Piccardo and Guarnieri, 2011)、软流圈与大陆岩石圈的相互作用(Ma et al., 2013a)以及亏损的MORB型熔体和大洋岩石圈部分熔融的混合产物(Nonnotte et al., 2005)。苏长岩特殊的矿物组成及矿物结晶顺序反映了母岩浆性质以及岩浆演化过程中岩浆性质的转变。厘清苏长岩的成因,对于揭露深部地幔、地幔-地壳岩浆作用的性质以及岩浆演化过程具有重要的意义。前人报道了冈底斯岩基米林地区出露的晚白垩世苏长岩,认为其母岩浆源自软流圈与受流体交代的岩石圈地幔的相互作用(Ma et al., 2013a)。同时,米林地区还发育晚白垩世高Sr/Y比含紫苏辉石花岗岩,Zhang et al.(2010, 2011)认为其是洋脊俯冲的产物,但Ma et al. (2013b)认为它们代表新特提斯俯冲板片回转导致俯冲洋壳部分熔融的产物。

野外地质考察和岩相学鉴定发现含紫苏辉石侵入岩并不局限在米林地区,而是沿冈底斯岩基南缘楠木林县-大竹卡村-曲水县-桑耶寺-里龙村一线广泛出露。本文通过开展元素地球化学、同位素地球化学和锆石U-Pb地质年代学测试,同时结合文献数据,来探讨含紫苏辉石侵入岩的成因,限定冈底斯岩基晚白垩世深部幔源岩浆的性质和岩浆演化过程。

1 地质背景和样品描述

拉萨地块东西向展布约2000km,南北以印度河-雅鲁藏布缝合带和班公湖-怒江缝合带为界,是青藏高原的重要组成部分(Yin and Harrison, 2000; 潘桂棠等, 2006)。以狮泉河-纳木错蛇绿岩混杂带以及洛巴堆-米拉山断裂为界限,可将拉萨地块分为北拉萨地块、中拉萨地块和南拉萨地块(Zhu et al., 2011)。冈底斯岩基侵位于拉萨地块南缘的早古生代及中生代地层中,东西延伸约1500km(Zhu et al., 2019)(图 1a)。在冈底斯岩基的形成与演化过程中,岩浆作用按活动时间可分为230~150Ma、100~80Ma、65~41Ma以及35~10Ma四个峰期(Chung et al., 2003, 2009; Hou et al., 2004; Chu et al., 2006, 2011; Mo et al., 2007, 2008; Wen, 2007; Wen et al., 2008; Ji et al., 2009, 2014; Zhu et al., 2009, 2011; 高家昊等, 2017; Wang et al., 2019; 张泽明等,2019; 王海涛等, 2019; 徐倩等, 2019a, b)。

图 1 拉萨地体南缘岩浆岩分布地质简图(a,据Chu et al., 2011修改)和采样位置简图(b,据Ma et al., 2013a修改) BNS-班公-怒江缝合带;ITS-印度河-雅鲁藏布江缝合带 Fig. 1 Simplified geologic maps of southern Tibet showing outcrops of the magmatic rocks (a, after Chu et al., 2011) and sampling locations (b, after Ma et al., 2013a) BNS-Bangong-Nujiang suture; ITS-Indus-Tsangpo suture

冈底斯岩基白垩纪岩浆作用形成了多种岩性的岩浆岩,包括火山岩,花岗岩、花岗闪长岩、闪长岩以及辉长岩等侵入岩。这些岩石都具有较低的Sr同位素初始比值(87Sr/86Sr(t) < 0.7044),但较高的εNd(t)(>0.9)和εHf(t)值(>+10),表现出强烈的亏损源区特征。研究认为这些岩浆作用均与新特提斯洋的北向俯冲作用相关(Wen et al., 2008; Ji et al., 2009; Zhu et al., 2009, 2011)。晚白垩世冈底斯岩基主要产出苏长岩、含紫苏辉石花岗岩和高Sr/Y比花岗岩类,岩基的形成主要受控于新特提斯洋的北向俯冲(Wen et al., 2008; 纪伟强等, 2009; Zhang et al., 2010, 2011; Ma et al., 2013a; 高家昊等, 2017徐倩等,2019a)。苏长岩的成因可能为软流圈与大陆岩石圈的相互作用(Ma et al., 2013a)。紫苏花岗岩成因则可能与洋中脊俯冲(张泽明等, 2009; Zhang et al., 2010, 2011)或与俯冲板片回转有关(Ma et al., 2013b)。

本文样品分别采自冈底斯岩基南缘的楠木林县(T0726-B)、大竹卡村(TDZK12、T0847-B)、曲水县(T1102)、桑耶寺西(T1099)以及里龙村(TLL12、T0578-A)附近(图 1b表 1)。在这五个采样点,含紫苏辉石的侵入岩均以岩体产出,包含大小不一的暗色细粒包体(图 2)。含紫苏辉石基性岩(样品T0847-B)为斑状结构,主要由斜长石(35%~40%)、紫苏辉石(15%~25%)、单斜辉石(5%~10%)、角闪石(20%~25%)、黑云母(10%~15%)以及钛铁矿(2%~3%)等组成(图 2)。斜长石主要为自形-半自形板状,粒度可达5mm以上;紫苏辉石和单斜辉石则呈半自形-他形,粒度也可达5mm以上;角闪石和黑云母多围绕在辉石周边产出,可能是紫苏辉石或单斜辉石与熔体反应的产物。其余样品为含紫苏辉石中-酸性岩,呈现斑状结构,主要由斜长石(35%~45%)、紫苏辉石(10%~15%)、单斜辉石(5%~10%)、角闪石(5%~10%)、黑云母(3%~5%)、石英(5%~15%)以及钛铁矿(2%~3%)等组成(图 2)。斜长石呈自形-半自形,粒度可达5mm以上;紫苏辉石和单斜辉石则呈半自形-他形,粒度也可达5mm以上;同样可见角闪石和黑云母多围绕在辉石周边产出。紫苏辉石和斜长石存在相互包裹的现象,说明紫苏辉石和斜长石的结晶时间相近。在里龙村附近岩体中采集到的包体(样品T0578-E)呈浑圆状,与寄主岩体呈截然接触,不存在冷凝边。暗色细粒包体呈似斑状结构,矿物组成与寄主岩石相似,为斜长石(35%~40%)、紫苏辉石(20%~25%)、单斜辉石(5%~10%)、角闪石(20%~25%)、黑云母(10%~15%)以及磁铁矿(2%~3%)等,但结晶颗粒粒度明显小于寄主岩石,暗色矿物(单斜辉石,紫苏辉石,角闪石以及黑云母)呈轻微定向分布(图 2)。

表 1 冈底斯岩基含紫苏辉石侵入岩样品汇总表 Table 1 Summary of the samples of the hypersthene-bearing intrusive rocks, Gangdese, southern Tibet

图 2 冈底斯岩基含紫苏辉石侵入岩野外地质照片及镜下照片 (a) T0726-B野外特征;(b) T1102-A野外特征;(c)含紫苏辉石侵入岩中的紫苏辉石、单斜辉石和斜长石;(d)含紫苏辉石侵入岩中的紫苏辉石包裹斜长石;(e)含紫苏辉石侵入岩中的紫苏辉石、斜长石、角闪石和磁铁矿,斜长石中包裹紫苏辉石;(f)含紫苏辉石侵入岩中的紫苏辉石、角闪石和石英;(g)暗色细粒包体中的紫苏辉石和斜长石;(h)暗色细粒包体中的紫苏辉石、单斜辉石、斜长石、角闪石和磁铁矿. Abbreviations: Opx-orthopyroxene; Cpx-clinopyroxene; Pl-plagioclase; Amp-amphibole; Mt-magnetite; Qtz-quartz Fig. 2 Photographs showing the occurrence textures and mineral assemblages of the hypersthene-bearing intrusive rocks (a) occurrence of T0726-B; (b) occurrence of T1102-A; (c) orthopyroxenes, linopyroxenes and plagioclase in the hyperthene-bearing intrusive rocks; (d) orthopyroxenes, clinopyroxenes and plagioclase in the hyperthene-bearing intrusive rocks; (e) orthopyroxenes, plagioclase, amphibole and magnetite in the hyperthene-bearing intrusive rocks; (f) orthopyroxenes, amphibole and quartz in the hyperthene-bearing intrusive rocks; (g) orthopyroxenes and plagioclase in the enclaves; (h) orthopyroxenes, clinopyroxenes, plagioclase, amphibole and magnetite in the enclaves. Abbreviations: Opx-orthopyroxene; Cpx-clinopyroxene; Pl-plagioclase; Amp-amphibole; Mt-magnetite; Qtz-quartz
2 测试方法 2.1 SIMS锆石U-Pb定年和SHRIMP锆石U-Pb定年

为了确定冈底斯岩基南缘这一系列含紫苏辉石侵入岩的形成时代,从样品T0726-B、T0578-A、T8047-B以及T1099中挑选锆石,经过手工挑选、制靶和抛光,然后进行阴极发光(CL)和扫描电镜背散射(BSE)成像观察,揭示锆石的内部结构。阴极发光成像在中国地质科学院地质研究所北京离子探针中心进行。在中国地质科学院地质研究所自然资源部深部动力学重点实验室进行了BSE图像和锆石内部包裹体的成分测试。在阴极发光和BSE图像的指导下,揭示锆石不同生长域的细微区别特征,选取锆石U-Pb测试点。样品T0726-B、T0578-A和T8047-B锆石U-Pb定年在中国科学院地质与地球物理研究所离子探针实验室进行,所用仪器为Cameca IMS-1280型二次离子质谱仪。测试方法参见高家昊等(2017)。用强度为10nA的一次O2-离子束通过-13kV加速电压轰击样品表面,束斑约为20μm×30μm。二次离子经过60eV能量窗过滤,质量分辨率为5400。为在高质量分辨率下获得较高的二次离子传输率,分析采用矩形透镜模式。二次离子的强度用电子倍增器跳峰模式顺序测量。单点分析7组数据,时间约为12分钟(Li et al., 2009, 2010)。锆石Pb/U比值用标准锆石TEMORA2(417Ma)的ln(206Pb/238U)与ln(238U16O2/238U)间的线性关系校正;U和Th的含量用标准锆石91500(Th=29×10-6;U=80×10-6)(Wiedenbeck et al., 1995)计算获得。普通Pb用实测的204Pb进行校正。由于多数锆石的普通Pb含量非常低,可认为其主要来源于制样过程中带入的表面Pb污染,用现代地壳的平均Pb同位素组成(Stacey and Kramers, 1975)作为普通Pb组成进行校正。除特殊说明的,单点分析的同位素比值及年龄误差为1σ,U-Pb平均年龄误差为2σ或为95%置信度。结果处理采用ISOPLOT软件(Ludwig, 2008)。样品T1099采用SHRIMP锆石U-Pb定年,SHRIMP锆石U-Pb同位素定年测试在北京离子探针中心进行,所用仪器为高分辨率高灵敏度离子探针SHRIMP Ⅱ,分析时所用标样为TEMORA锆石,每测定3个未知点,插入1次标样,以便及时校正,保障测试精度。U和Th含量以锆石标样M257为外标进行校正。

2.2 LA-MC-ICP-MS锆石U-Pb定年

为了确定其他含紫苏辉石侵入岩的形成年代,样品T1102-A、T1102-B1、T1102-B2、TLL12及TDZK12采用与样品T0726-B、T0578-A、T8047-B和T1099相同的前处理。锆石U-Pb同位素定年测试在中国地质科学院矿产资源研究所成矿作用与资源评价重点实验室进行。测试方法参见高家昊等(2017)。所用仪器为德国Finnigan公司生产的Neptune型激光多接收等离子体质谱(LA-MC-ICP-MS),并结合美国New Wave公司生产的UP213nm激光剥蚀系统,激光剥蚀所用斑束直径为25μm,频率为10Hz,能量密度约为2.5J/cm2,以He为载气。U和Th含量以锆石标样M127 (U: 923×10-6; Th: 439×10-6; Th/U: 0.475)为外标进行校正。在测试过程中,每测定10个样品点前后重复测量两次锆石标样GJ-1和一次锆石标样Plesovice。分析数据的离线处理(包括对样品和空白信号的选择、仪器灵敏度漂移校正、元素含量及U-Th-Pb同位素比值和年龄计算)采用软件ICPMSDataCal完成(Liu et al., 2010),锆石年龄谐和图用ISOPLOT获得。

2.3 全岩元素地球化学

为确定这一系列含紫苏辉石侵入岩以及暗色细粒包体的元素地球化学特征,在自然资源部国家地质实验测试中心对其主量及微量元素进行分析测试。测试方法参见高家昊等(2017)。主量元素通过XRF(X荧光光谱仪3080E)方法测试,分析精度为5%。微量元素和稀土元素(REE)通过等离子质谱仪(ICP-MS-Excell)分析,含量大于10×10-6的元素的测试精度为5%,而小于10×10-6的元素精度为10%。个别在样品中含量低的元素,测试误差大于10%。

2.4 锆石Hf同位素分析方法

在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对样品的锆石Hf同位素测试进行测试分析。测试方法参见高家昊等(2017)。锆石Hf同位素测试在Neptune多接收等离子质谱和Newwave UP213紫外激光剥蚀系统(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55μm或40μm,测定时使用锆石国际标样GJ1和Plesovice作为参考物质,分析点与U-Pb定年分析点为同一位置。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GJ1和Plesovice的176Hf/177Hf测试加权平均值分别为0.282007±0.000007 (2σ, n=36)和0.282476±0.000004 (2σ, n=27),与文献报道值(侯可军等,2007; Morel et al., 2008; Sláma et al., 2008)在误差范围内完全一致。

2.5 Sr-Nd同位素分析方法

选择具有代表性的32件样品进行了Sr和Nd同位素组成进行分析。Rb-Sr和Sm-Nd同位素的化学分离及同位素测试在中国科学技术大学中国科学院壳幔物质与环境重点实验室完成,详细的分析方法和流程参考文献(Chen et al., 2000, 2007)。首先称取100mg全岩粉末样品于Teflon溶样罐中,分别加入适量87Rb-84Sr和149Sm-150Nd的混合稀释剂以及约3mL纯化的HF和少量HClO4后,轻微摇晃溶样罐使样品和酸还有稀释剂混合均匀,加盖并拧紧并置于电热板上加热至约120℃,放置七天左右直至样品完全溶解。待样品溶解后,Rb-Sr和REE的化学分离采用装有5mL AG50W×12阳离子交换树脂(200~400目)的石英交换柱,而Sm和Nd的纯化和化学分离则用充填有1.7mL Teflon粉末为交换介质的石英柱完成。整个流程Sr和Nd的本低浓度分别小于200pg和30pg。同位素比值的测试分别采用德国Finnigan公司生产的MAT-262(中国科学技术大学)和升级版MAT-261(德国慕尼黑大学)型固体热电离质谱(Thermal Ionization MassSpectrometer,简称TIMS)上完成。Rb同位素比值测定采用双Ta-金属带形式,Sr同位素则用TaHf5发射剂和双Ta-金属带的方式进行分析,而Sm和Nd同位素比值测试采用双Re-金属带的形式进行。测定Sr-Nd同位素时所有样品的87Sr/86Sr和143Nd/144Nd比值分别采用86Sr/88Sr=0.1194和146Nd/144Nd=0.7219进行质量分馏矫正。Sr同位素测试过程中使用国际标样NBS987进行监测分别得到其87Sr/86Sr的实际测定值为0.710249±0.000012 (2SD, n=38;中国科学技术大学)和0.710243±0.000004 (2SD, n=9;德国慕尼黑大学)两者在误差范围内一致;在对Nd同位素分析过程中在中科大采用La Jolla作为监测标样,得到143Nd/144Nd=0.511869±0.000006 (2SD, n=25),而在德国慕尼黑大学采用国际标样JNDi-1作为监测,得到143Nd/144Nd=0.512100±0.000006 (2SD, n=8)。

2.6 电子探针分析方法

选取样品中干净并未发生反应的单斜辉石、斜方辉石和斜长石通过电子探针进行成分分析,实验在中国地质科学院矿产资源研究所电子探针实验室进行。所用仪器型号为JEOL-JXA8230,工作时的加速电压15kV,束流2×10-8A,束斑为5μm,ZAF修正,标准样品为天然矿物或国家标准合成金属,分析误差小于0.01%。

3 数据与结果 3.1 地质年代学

为了厘定冈底斯岩基南缘自里龙村-楠木林县出露的含紫苏辉石侵入岩的年龄,对其中4件样品进行了SIMS锆石U-Pb和SHRIMP锆石U-Pb定年测年分析,包括样品T0726-B、T0578-A、T8047-B及T1099。而对样品T1102-A、T1102-B1、T1102-B2、TLL12及TDZK12等5件样品开展了LA-ICP-MS锆石U-Pb测年分析。这9件样品的锆石具有相似的结构和形态,呈自形-半自形,粒度在100~400μm之间,长宽比在1:1~1:5之间。样品TLL12具有条带状环带,其他样品则表现出宽板状韵律环带(图 3)。

图 3 含紫苏辉石侵入岩中锆石阴极发光(CL)图像 Fig. 3 Cathodoluminescence (CL) images showing texture and analytical spot of the hypersthene-bearing intrusive rocks

出露于楠木林县的样品T0726-B的U和Th含量分别在324×10-6~762×10-6和242×10-6~828×10-6之间(除测试点T0726-B-7以外),Th/U比值在0.38~1.25之间(表 2)。10个测试点计算出的加权平均年龄值为88.7±0.6Ma(MSWD=1.8)(图 4a)。高Th/U比值和清晰的韵律环带表明了其岩浆成因,该年龄代表样品T0726-B的结晶年龄。

表 2 冈底斯岩基含紫苏辉石侵入岩SIMS锆石U-Th-Pb定年数据 Table 2 SIMS zircon U-Th-Pb isotopic data of the hypersthene-bearing intrusive rocks, Gangdese

图 4 含紫苏辉石侵入岩样品的锆石U-Pb谐和图 Fig. 4 U-Pb concordia diagrams for zircon U-Pb analytic results of samples from the hypersthene-bearing intrusive rocks

大竹卡地区的2件样品TDZK12和T0847-B的加权平均年龄分别为97.3±0.2Ma(N=21, MSWD=1.8)(图 4i)和93.9±1.4Ma(N=11, MSWD=2.9)(图 4c)。这2件样品具有变化较大的U(52.1×10-6~551×10-6)和Th(50×10-6~334×10-6)含量,Th/U比值在0.54~1.34之间。高Th/U比值和清晰的韵律环带表明了其岩浆成因,这组年龄数据代表大竹卡地区样品TDZK12和T0847-B的结晶年龄。

桑耶寺地区出露的样品T1099的U和Th含量分别在122×10-6~517×10-6和170×10-6~649×10-6之间,Th/U比值在0.80~2.34之间(表 3)。19个测试点计算出的加权平均年龄值为92.2±0.3Ma(MSWD=0.4)(图 4d)。高Th/U比值和清晰的韵律环带表明了其岩浆成因,该年龄代表样品T1099的结晶年龄。

表 3 冈底斯岩基含紫苏辉石侵入岩SHRIMP锆石U-Th-Pb定年数据 Table 3 SHRIMP zircon U-Th-Pb isotopic data of the hypersthene-bearing intrusive rocks, Gangdese

曲水地区出露的3件样品T1102-A、T1102-B1和T1102-B2的加权平均年龄变化较大,分别为84.7±0.5Ma(N=19, MSWD=1.0)(图 4e)、77.0±0.8Ma(N=14, MSWD=2.1)(图 4f)和83.5±0.6Ma(N=20, MSWD=1.6)(图 4g)。这3件样品具有变化较大的U(103×10-6~1454×10-6)和Th(57.3×10-6~1095×10-6)含量,Th/U比值在0.32~1.50之间(表 4)。高Th/U比值和清晰的韵律环带表明了其岩浆成因,这组年龄数据可以代表曲水地区出露的含紫苏辉石侵入岩的结晶年龄分别在~84Ma和~77Ma两个阶段。

表 4 冈底斯岩基含紫苏辉石侵入岩LA-MC-ICP-MS锆石U-Th-Pb定年数据 Table 4 LA-MC-ICP-MS zircon U-Th-Pb isotopic data of the hypersthene-bearing intrusive rocks, Gangdese

里龙村地区的2件样品TLL12和T0578-A的加权平均年龄分为81.7±0.3Ma(N=15, MSWD=4.6)(图 4h)和84.5±1.6Ma(N=6, MSWD=1.6)(图 4b)。这2件样品除测试点TLL12-9以外具有较为集中的的U(115×10-6~297×10-6)含量,Th含量在68×10-6~503×10-6,Th/U比值在0.52~1.70之间。高Th/U比值和清晰的韵律环带表明了其岩浆成因,这组年龄数据可以代表里龙村样品含紫苏辉石侵入岩的结晶年龄。与前人报道的里龙紫苏花岗岩结晶年龄90~86Ma(Zhang et al., 2010)和100~89Ma(Ma et al., 2013b)以及苏长岩的结晶年龄~93Ma(Ma et al., 2013a)相比,较为年轻。

上述测试结果表明,冈底斯岩基南缘自米林-日喀则地区出露的含紫苏辉石侵入岩的结晶年龄具有较大的跨度(100~77Ma)。这些样品的结晶年龄并不具有显著的经度上的变化。自西向东,楠木林-大竹卡-曲水-米林地区,最老都可以追溯到>~90Ma,结晶年龄的下限则出现在曲水和米林地区(~80Ma)。结合文献数据(Zhang et al., 2010; Ma et al., 2013a, b),上述数据表明:冈底斯岩基南缘含紫苏辉石侵入岩形成于三个阶段,分别为:>92Ma、~87Ma和~80Ma。

3.2 全岩地球化学特征

为了解冈底斯南缘含紫苏辉石侵入岩及细粒包体的元素地球化学特征,分析测试了48件含紫苏辉石侵入岩样品和8件细粒包体样品的全岩元素地球化学组成,结果见表 5

表 5 冈底斯岩基含紫苏辉石侵入岩元素地球化学组成(主量元素:wt%;稀土和微量元素:×10-6) Table 5 Geochemical compositions of the hypersthene-bearing intrusive rocks, Gangdese (major elements: wt%; trace elements: ×10-6)
3.2.1 主量元素地球化学特征

在主量元素地球化学特征上,含紫苏辉石基性岩的SiO2含量在51.9%~52.7%之间,具有高的Al2O3 (17.3%~18.2%),较高的MgO (3.9%~4.1%)和FeOT(8.7%~9.0%)(图 5c, e),较高的Mg#值(44.1~44.9)(图 5d);均落在辉长岩-辉长闪长岩区域范围内(图 5a);均具有中钾特征(图 5b)。含紫苏辉石的中-酸性侵入岩具有变化较大的SiO2(53.5%~67.1%),Al2O3 (14.9%~18.8%),FeOT和MgO含量变化较大分别在3.9%~7.8%和1.4%~5.3%之间(图 5c, e),较高的Mg#值(39.7~58.6)(图 5d);在TAS图解上,样品落在辉长闪长岩-二长闪长岩-闪长岩-二长岩-石英二长岩区域范围内(图 5a);具有中钾-高钾特征(图 5b);在A/CNK-A/NK图解上,大部分样品落在了准铝质范围内(图 5g)。细粒包体具有较均一且较低的SiO2含量(51.2%~52.7%),高Al2O3 (18.1%~19.4%)、FeOT(8.1%~9.0%)、MgO (4.7%~5.4%)以及Mg#值(51.0~52.6)(图 5c, e, d);在TAS图解上,样品落在辉长闪长岩区域范围内(图 5a);具有低钾-中钾特征(图 5b);在A/CNK-A/NK图解上,样品则落在了准铝质范围内(图 5g)。

图 5 含紫苏辉石侵入岩和细粒包体地球化学图解 SiO2对K2O+Na2O (a, 据Middlemost, 1994)、K2O (b, 据Peccerillo and Taylor, 1976)、MgO (c)、Mg# (d, 据Rapp et al., 1999)、FeOT (e)及La/Yb (f);(g) A/CNK-A/NK (据Maniar and Piccoli, 1989);(h) Y-Sr/Y (据Defant and Drummond, 1990Castillo et al., 1999);(i) YbN-(La/Yb)N (据Martin, 1986)协变关系.文献数据为紫苏花岗岩引自Zhang et al. (2010)Ma et al. (2013b)图 6 Fig. 5 Co-variation diagrams for the hypersthene-bearing intrusive rocks and the enclaves SiO2 against (K2O+Na2O) (a, after Middlemost, 1994), K2O (b, after Peccerillo and Taylor, 1976), MgO (c), Mg# (d, after Rapp et al., 1999), FeOT (e) and La/Yb (f); (g) A/CNK vs. A/NK (after Maniar and Piccoli, 1989); (h) Y vs. Sr/Y (after Defant and Drummond, 1990; Castillo et al., 1999); (i) YbN vs. (La/Yb)N (after Martin, 1986). Literature data of the charnockites from Zhang et al. (2010) and Ma et al. (2013b), also in Fig. 6
3.2.2 微量元素地球化学特征

在微量元素地球化学特征上,冈底斯岩基南缘含紫苏辉石基性岩特征如下:(1)具有较低的Cr和Ni含量,分别为3.1×10-6~14.8×10-6和8.7×10-6~15.0×10-6;(2)富集Cs、Rb、K、Ba、Pb等大离子亲石元素,并且显示Nb、Ta、Ti的显著负异常以及较弱的Zr和Hf负异常(图 6b);(3)具有高的Sr(707×10-6~836×10-6)含量,低的Y含量(19.0×10-6~20.3×10-6)以及较高的Sr/Y比值(36.5~43.5)(图 5h)。含紫苏辉石中-酸性岩的微量元素特征如下:(1)具有变化较大的Cr(5.7×10-6~260×10-6)和Ni(10.2×10-6~78.2×10-6)含量;(2)富集Cs、Rb、K等大离子亲石元素,大部分样品显示Nb、Ta、Ti的显著负异常以及较弱的Zr和Hf负异常(图 6d);(3)具有高的Sr(223×10-6~860×10-6)含量,低的Y含量(11.3×10-6~25.7×10-6)以及较高的Sr/Y比值(14.9~59.7)(图 5h)。与寄主岩石相比,细粒包体在微量元素特征上:(1)Cr和Ni含量较为集中,分别为32.9×10-6~87.9×10-6和28.3×10-6~37.2×10-6;(2)具有富集大离子亲石元素亏损高场强元素的特征(图 6f);(3)同样具有高的Sr(618×10-6~730×10-6)含量,低的Y含量(15.3×10-6~21.4×10-6)以及较高的Sr/Y比值(34.1~44.1)(图 5h)。这一系列含紫苏辉石侵入岩和细粒包体都显示出富集大离子亲石元素亏损高场强元素Nb、Ta、Zr、Hf和Ti的典型岛弧岩浆岩特征。

图 6 含紫苏辉石基性岩(a、b)、含紫苏辉石中-酸性岩(c、d)以及细粒包体(e、f)的稀土元素及微量元素地球化学特征(标准化值据Sun and McDonough, 1989) Fig. 6 Rare earth element and trace element distribution diagrams for the the hypersthene-bearing mafic rocks (a, b), the hypersthene-bearing dioritic-granitic rocks (c, d) and the enclaves (e, f) (normalization values after Sun and McDonough, 1989)
3.2.3 稀土元素地球化学特征

在稀土元素地球化学组成上,冈底斯岩基南缘含紫苏辉石基性岩的ΣREE在93.4×10-6~107.0×10-6之间,轻、重稀土元素分馏显著,(La/Yb)N=5.69~6.61(图 5i),(Gd/Yb)N=2.95~3.23,La/Yb比值在8.09~9.40之间(图 5f)。样品几乎不具有Eu的异常(Eu/Eu*=0.95~1.05),在球粒陨石标准化图解上,HREE呈平坦分布(图 6a)。含紫苏辉石中-酸性侵入岩的ΣREE在65.1×10-6~168.5×10-6之间,轻、重稀土元素分馏显著,(La/Yb)N=4.96~16.98,(Gd/Yb)N=1.45~2.83,La/Yb比值在7.05~24.15之间。样品具有微弱-强烈的Eu的负异常(Eu/Eu*=0.50~0.99),在球粒陨石标准化图解上,HREE呈平坦分布(图 6c)。细粒包体的ΣREE在59.3×10-6~88.6×10-6之间,轻重稀土分馏较寄主岩石稍弱,(La/Yb)N=3.93~5.51,(Gd/Yb)N=1.58~1.93,La/Yb比值在5.59~7.83之间。基本不具有Eu的异常(Eu/Eu*=0.85~1.12),在球粒陨石标准化图解上,HREE呈平坦分布(图 6e)。相对于细粒包体,寄主岩石更加富集LREE而亏损HREE,显示了更加强的轻、重稀土元素分馏,并且La/Yb比值随着SiO2含量的增加而增加(图 5f),显示了更高的岩浆演化程度。

3.3 放射性同位素地球化学特征

对冈底斯岩基南缘含紫苏辉石的侵入岩,我们选取楠木林县、大竹卡地区以及里龙村地区的4件样品(T0726-B、TDZK12、T0847-B和TLL12)分别进行了锆石的Hf同位素组成分析,分析结果见表 6

表 6 冈底斯岩基含紫苏辉石侵入岩中锆石Hf同位素数据 Table 6 Zircon Hf isotopic data of the hypersthene-bearing intrusive rocks, Gangdese

含紫苏辉石基性岩样品T0847-B锆石的176Yb/177Hf比值在0.010821~0.031602之间,均小于0.2,176Lu/177Hf比值在0.000461~0.001288之间,εHf(t)值在+11.9~+14.6之间(图 7a),并具有年轻的Hf同位素一阶段地幔模式年龄(tDM=173~283Ma),这些特征显示它们亏损源区特征。含紫苏辉石中-酸性岩样品(T0726-B、TDZK12和TLL12)的锆石Hf同位素组成相似,这些样品锆石的176Yb/177Hf比值在0.009386~0.039755之间,均小于0.2,176Lu/177Hf比值在0.000383~ 0.001587之间,εHf(t)值在+9.8~+14.3之间(图 7a),与Ma et al.(2013b)报道的米林紫苏花岗岩的结果相近,并具有年轻的Hf同位素一阶段地幔模式年龄(tDM=181~370Ma),这些特征也指示了亏损源区特征。

图 7 含紫苏辉石侵入岩锆石Hf同位素随时间分布图(a)和εNd(t)-87Sr/86Sr(t)关系图解(b) 文献Hf同位素数据引自Chung et al. (2009), Chu et al.(2006, 2011), Ji et al.(2009, 2014), Jiang et al. (2014), Guo et al.(2013, 2019), Shu et al. (2018), Chen et al. (2019).雅鲁藏布江蛇绿岩数据引自Miller et al. (2003), Xu and Castillo (2004), Zhang et al. (2005), 牛晓露等(2006).新生下地壳数据引自Wen (2007), Wen et al. (2008), Jiang et al. (2014).岩石圈地幔数据高家昊等(2017) Fig. 7 Hf isotopic compositions vs. ages (a) and εNd(t) vs. 87Sr/86Sr(t) (b) diagrams of the hypersthene-bearing intrusive rocks Literature data of Hf isotope literature data from Chung et al. (2009), Chu et al.(2006, 2010), Ji et al.(2009, 2014), Jiang et al. (2014), Shu et al. (2018), Guo et al.(2013, 2019), Chen et al. (2019); Yarlung Zangbo ophiolite after Miller et al. (2003), Xu and Castillo (2004), Zhang et al. (2005), Niu et al. (2006); Newly underplated lower crust after Wen (2007), Wen et al. (2008), Jiang et al. (2014); lithospheric mantle after Gao et al. (2017)

含紫苏辉石基性岩样品T0847-B的Rb-Sr和Sm-Nd同位素组成特征(表 7):(1)较低的Rb含量(39.9×10-6~78.8×10-6),较高的Sr含量(707×10-6~836×10-6),Rb/Sr比值在0.05~0.11之间;(2)较低的Sm(4.35×10-6~4.74×10-6)含量和Nd(20.3×10-6~23.1×10-6)含量,Sm/Nd比值在0.20~0.21之间;(3)较低的初始Sr同位素比值(87Sr/86Sr(t)=0.703710~0.703858),较高的143Nd/144Nd(t)值(0.512740~0.512894),εNd(t)值为+3.7~+6.6之间(图 7b)。含紫苏辉石中-酸性侵入岩显示了与含紫苏辉石基性岩相似的Rb-Sr和Sm-Nd同位素组成特征:(1)Rb含量较低,但Sr含量较高,分别为10.7×10-6~139×10-6和268×10-6~860×10-6,Rb/Sr比值在0.02~0.32之间;(2)较低的Sm(2.77×10-6~7.01×10-6)和Nd(13.3×10-6~31.4×10-6)含量,Sm/Nd比值小于0.23;(3)初始Sr同位素比值(87Sr/86Sr(t)=0.703734~0.704364)较低,但143Nd/144Nd(t)值(0.512711~0.512992)较高,εNd(t)值在+3.7~+9.4之间(表 7)。初始Sr同位素比值与Ma et al.(2013a)报道的数据相近,但具有更高及变化范围更大的εNd(t)值。这些样品的Rb-Sr和Sm-Nd同位素组成特征显示了明确的亏损源区特征,并且εNd(t)变化较大,指示了混合源区特征。

表 7 冈底斯岩基含紫苏辉石侵入岩的Sr和Nd同位素组成 Table 7 Sr and Nd isotope compositions of the hypersthene-bearing intrusive rocks, Gangdese

总体上,冈底斯南缘含紫苏辉石的侵入岩具有相似的Sr、Nd和Hf同位素组成特征,较低的Sr但较高的Nd和Hf同位素组成都指示这些岩浆岩主要来源于亏损源区。

3.4 矿物主量成分

测试样品中的单斜辉石属于普通辉石-透辉石,SiO2含量在51.15%~52.86%之间;TiO2含量在0.02%~0.19%之间;MgO含量在12.47%~13.66%之间;FeO含量在8.55%~11.24%之间;Mg#值在66.8~74.0之间;En=36.46~39.84,平均为37.73;Fs=13.99~18.44,平均为16.94;Wo=44.16~46.61,平均为45.33(表 8)。

表 8 冈底斯岩基含紫苏辉石侵入岩中单斜辉石的主量元素地球化学(wt%) Table 8 Chemical compositions (wt%) of clinopyroxene in the hypersthene-bearing intrusive rocks, Gangdese

测试样品中的斜方辉石均为紫苏辉石,SiO2含量在50.86%~52.73%之间;TiO2含量在0.03%~0.31%之间;MgO含量在17.04%~22.00%之间;FeO含量在23.75%~28.63%之间;Mg#值在51.7~62.3之间;En=50.67~61.44,平均为54.95;Fs=37.20~47.30,平均为43.32;Wo=1.00~3.57,平均为1.73(表 9)。

表 9 冈底斯岩基含紫苏辉石侵入岩中斜方辉石的主量元素地球化学(wt%) Table 9 Chemical compositions (wt%) of orthopyroxene in the hypersthene-bearing intrusive rocks, Gangdese

测试样品中的斜长石为中长石,An在27.3~45.8之间,Ab在52.8~70.9之间,Or在0.8~2.2之间;SiO2含量在56.44%~61.45%之间;Al2O3含量在23.89%~27.51%之间;CaO含量在5.76%~9.53%之间;Na2O含量在6.07%~8.89%之间;K2O含量在0.14%~0.42%之间(表 10)。

表 10 冈底斯岩基含紫苏辉石侵入岩中斜长石的主量元素地球化学(wt%) Table 10 Chemical compositions (wt%) of plagioclase in the hypersthene-bearing intrusive rocks, Gangdese
4 讨论 4.1 母岩浆性质及其源区

堆晶作用是形成苏长岩的重要机制,其母岩浆在上升侵位过程中可能经受了岩浆同化混染作用以及分离结晶作用并最终形成苏长岩。现有岩相的地球化学特征仅能代表分离结晶之后的地球化学特征或者堆晶相的地球化学特征亦或者是二者混合体的地球化学特征(Li and and Ripley, 2011)。所以,反演其母岩浆的性质,对于揭示苏长岩的岩浆演化过程和岩石成因十分重要。利用单斜辉石的成分可以很好的反应母岩浆的成分特点(Le Bas, 1962; Seyler and Bonatti, 1994; 邱家骧和廖群安, 1996)。冈底斯岩基南缘出露的含紫苏辉石侵入岩中的单斜辉石具有高SiO2、低TiO2和Na2O的特点,表明其母岩浆为拉斑玄武质岩浆。在单斜辉石Al2O3-SiO2协变图解上,所有样品点均落入亚碱性系列区域内(图 8a),而在单斜辉石(Ca+Na)-Ti协变图解上,指示其母岩浆属于拉斑玄武质岩浆(图 8b)。

图 8 含紫苏辉石侵入岩中单斜辉石Al2O3与SiO2 (a)和(Ca+Na)与Ti (b)协变关系(底图据Irvine and Baragar, 1971) Fig. 8 Co-variation diagrams of Al2O3 vs. SiO2 (a) and (Ca+Na) vs. Ti (b) for clinopyroxene in the hypersthene-bearing intrusive rocks (base map after Irvine and Baragar, 1971)

冈底斯岩基南缘含紫苏辉石基性岩具有正的εNd(t)值(+3.7~+6.6),较低的初始Sr同位素比值(87Sr/86Sr(t)=0.703710~0.703858),εHf(t)值在+11.9~+14.6之间(图 7a)。而含紫苏辉石的中-酸性岩的εNd(t)值在+3.7~+9.4之间,较低的初始Sr同位素比值在0.703734~0.704364之间,εHf(t)值则在+9.8~+14.3之间(图 7b)。含紫苏辉石的基性岩和中-酸性岩具有相似的Sr-Nd-Hf同位素组成,并且由于含紫苏辉石的中-酸性岩与含紫苏辉石的基性岩在Harker图解上具有连续演化的趋势,并且具有相似的稀土元素配分模式以及微量元素的地球化学特征,我们有理由相信冈底斯岩基南缘这一系列的含紫苏辉石的侵入岩具有相同或相近的源区。这一系列含紫苏辉石侵入岩的Sr-Nd-Hf同位素组成共同表明其源区是具有亏损性质的混合源区。其Sr-Nd-Hf同位素组成与雅鲁藏布江蛇绿岩的Sr-Nd-Hf同位素组成相近(Miller et al., 2003; Xu and Castillo, 2004; Zhang et al., 2005, 2016; 牛晓露等, 2006)(图 7b),表明具有亏损性质的软流圈和俯冲洋壳是其可能的源区。同时,在图 9a上可以看到含紫苏辉石侵入岩的Sr-Nd和Hf同位素发生了解耦。在流体参与的俯冲过程中,由于流体携带LREE和大离子亲石元素的能力强于高场强元素,因此,被流体交代的地幔楔往往包含更多的非放射性成因的Nd而非放射性成因的Hf则较少,从而导致Sr-Nd和Hf同位素发生解耦(Pearce et al., 1999)。Sr-Nd和Hf同位素的解耦表明其源区可能经受过俯冲板片流体交代作用。在岩浆分离结晶过程中,稀土元素的地球化学行为主要受到磷灰石、榍石和磷钇矿的控制。在冈底斯岩基含紫辉石基性岩样品中并未发现相关矿物,表明HREE和LREE在母岩浆的演化过程中均显示不相容元素特征,即可以用样品的HREE和LREE的比值近似代表其母岩浆的HREE和LREE的比值。冈底斯岩基含紫苏辉石基性岩的(La/Yb)N在5.68~6.61之间而(La/Gd)N在2.9~3.23之间,其(La/Yb)N和(La/Gd)N均稍高于E-MORB的平均值并低于OIB的平均值(Sun and McDonough, 1989)。Ma et al.(2013a)对米林地区苏长岩母岩浆成分的计算表明,其成分更接近于E-MORB,表明其源区明显具有软流圈组分特征。冈底斯岩基含紫苏辉石的侵入岩样品中均未发现橄榄石,Ma et al. (2013a)也仅在一个样品中发现存在蛇纹石化反应边的小颗粒橄榄石,并且除了部分样品外,绝大多数样品均具有较低的Ni和Cr含量,表明其母岩浆几乎并没有与橄榄石发生平衡或橄榄石作为残余相出现。同时,尝试使用单斜辉石电子探针数据计算母岩浆Mg#值(计算公式为:Mg#=1/([1/Fo-1]/kd+1),Kd=0.24(Langmuir et al., 1992; Niu et al. 2002)),母岩浆的Mg#值在56.9~66.7之间,也小于与橄榄岩平衡的原生岩浆的Mg#值范围(65~75之间)(Green, 1975; Frey et al., 1978),表明含紫苏辉石侵入岩的母岩浆几乎并未与橄榄岩发生平衡。通常超镁铁质地幔经流体蚀变产生蛇纹石化橄榄岩和绿泥石化橄榄岩, 而熔体交代形成贫橄榄石的角闪石岩和辉石岩。贫橄榄石的辉石岩的部分熔融形成的母岩浆则可以具有较低的Ni和Cr含量,以及Mg#值。上述特征表明,含紫苏辉石侵入岩的母岩浆可能来自于经流体+熔体交代地幔楔中软流圈部分形成的贫橄榄石的辉石岩在俯冲流体存在的情况下发生部分熔融。含紫苏辉石侵入岩具有跨度较大SiO2含量(51.9%~67.1%),在板片俯冲环境下并不具有岩浆长期分离结晶演化的环境。而铁镁质岩浆和长英质岩浆的混合则可以形成一系列SiO2含量从低到高的岩石组合(Beard and Day, 1988; Wiebe, 1993; Xu et al., 1999; Waight et al., 2001; Patwardhan and Marsh, 2011)。在Th-Th/Nd和Rb-Rb/Nd协变图上可以看出(图 9b, c),含紫苏辉石侵入岩并不具有分离结晶的趋势。同时冈底斯岩基在晚白垩世发生大规模的俯冲洋壳的部分熔融(Ji et al., 2009, 2014; Jiang et al., 2012; 徐倩等, 2019a)。后期俯冲洋壳部分熔融产生熔体的混合则可以解释含紫苏辉石侵入岩样中角闪石和黑云母多围绕在辉石周边产出的特征。综上,经流体+熔体交代地幔楔中软流圈部分形成的贫橄榄石的辉石岩在俯冲流体存在的情况下发生部分熔融形成含紫苏辉石侵入岩的母岩浆,母岩浆在演化过程中与俯冲板片熔体发生混合,使其性质发生改变。

图 9 含紫苏辉石侵入岩εNd(t)与εHf(t) (a)、Th与Th/Nd (b)、Rb与Rb/Nd (c) (底图据Gao and Zhou, 2013)、SiO2εNd(t) (d)、Nb/U与εNd(t) (e)和Ce/Pb与εNd(t) (f)协变关系 文献数据为紫苏花岗岩引自Ma et al. (2013b) Fig. 9 Co-variation diagrams of εNd(t) vs. εHf(t) (a), Th vs. Th/Nd (b), Rb vs. Rb/Nd (c) (after Gao and Zhou, 2013), SiO2 vs.εNd(t) (d), Nb/U vs.εNd(t) (e) and Ce/Pb vs.εNd(t) (f) for the hypersthene-bearing intrusive rocks Literature data of the charnockites from Ma et al. (2013b)

幔源岩浆在上升侵位过程中通常会受到地壳物质的混染。由于岩浆分离结晶作用的影响,部分微量元素的比值会发生改变,并不能体现其母岩浆的性质。由于Ce、Pb、Nb和U具有相同的总分配系数,在部分熔融和分离结晶过程中,其比值不会发生改变。所以可用其比值来限定其母岩浆性质。冈底斯岩基南缘出露的这一系列含紫苏辉石侵入岩的Nb/U比值在1.45~12.36之间,Ce/Pb比值在1.70~7.18之间,与大陆地壳的Nb/U=4、Ce/Pb=9±3(Taylor and McLennan, 1995; Rudnick and Gao, 2014)相近,而与典型地幔的Nb/U比值和Ce/Pb比值有很大区别,指示其存在地壳物质的混染。同时,不相容元素Nb和La的比值较高(Nb/La=0.23~0.55)同样指示了壳源物质对岩浆的混染(Lassiter and DePaolo, 1997)。如果源自地幔的岩浆在侵位过程中受到上覆地壳物质的混染,则会导致其εNd(t)值降低,SiO2含量的升高,Nb/La比值的升高,Nb/U比值和Ce/Pb比值的下降。但含紫苏辉石侵入岩样品的εNd(t)值相对稳定,并未与SiO2含量、Nb/U比值和Ce/Pb比值显示线性相关(图 9d-f),说明壳源组分来自上覆地壳的可能性不大,而地壳混染的特征很可能来自下伏洋壳板片产生的流体和熔体的交代作用或直接由俯冲板片熔体的混合作用带来。并且以雅鲁藏布江蛇绿岩和印度洋深海沉积物为两个端源(雅鲁藏布江蛇绿岩Sr=131×10-687Sr/86Sr(t)=0.703039,Nd=9.46×10-6εNd(t)=+9.6;印度洋深海沉积物Sr=119×10-687Sr/86Sr(t)=0.71682,Nd=23.05×10-6εNd(t)=-9.3)进行两端源混合模拟计算(Othman et al., 1989; 牛晓露等, 2006),最多需要约16%的深海沉积物混入亏损地幔才可以表现出含紫苏辉石侵入岩的Sr-Nd同位素组成特征。大量的壳源物质混入,将会导致熔体微量元素组成的变化,例如La/Yb比值的降低,但这一些列含紫苏辉石的侵入岩却具有较高的La/Yb比值(7.0~24.1)。俯冲板片部分熔融产生的熔体可以携带Nb等高场强元素,通过与上覆地幔楔的交代作用以及与母岩浆的混合,使得含紫苏辉石侵入岩表现出上述壳源物质混染的特征。综上,冈底斯岩基南缘含紫苏辉石侵入岩的母岩浆在上升侵位过程中并没有受到壳源组分的混染。

4.2 含紫苏辉石侵入岩及暗色细粒包体成因探讨

在基性岩浆的演化过程中,分离结晶作用对岩浆性质起到重要的控制作用。冈底斯岩基含紫苏辉石侵入岩中包含的斜长石、单斜辉石以及紫苏辉石巨晶,其粒度可达5mm以上,粒间填充了其它矿物,显示出堆晶结构的特征,指示其母岩浆经历了斜长石、单斜辉石以及紫苏辉石的分离结晶作用。单斜辉石的分离结晶过程是Al六次配位逐渐增加的过程(Irvine and Baragar, 1971; Robinson et al., 2000; Späth et al., 2001),含紫苏辉石侵入岩单斜辉石的Al与Si原子数之间呈现明显的正相关关系(图 10a),说明其母岩浆在演化过程中存在单斜辉石的分离结晶。通过Cr-Ni和Cr-V的协变关系图,可以看出其母岩浆在演化过程中发生了斜方辉石的分离结晶(图 10b, c)。含紫苏辉石中-酸性岩Eu的负异常则指示了斜长石的分离结晶。显微镜下观察显示,紫苏辉石和斜长石具有相互包裹的产出状况,说明斜长石的结晶与紫苏辉石相近。实验岩石学研究表明,在玄武质岩浆源区中大量流体的存在,使得单斜辉石的结晶早于斜长石(Stolper, 1980; Grove et al., 1992; Berndt et al., 2004; Feig et al., 2006)。水的存在有利于Fe-Ti氧化物从岩浆中晶出,导致SiO2活度增大,致使斜方辉石结晶(Grove and Juster, 1989)。含紫苏辉石侵入岩中含水矿物相(角闪石,黑云母等)的出现,以及较为中等的斜长石An28-46都指示其母岩浆含水,使其结晶顺序为单斜辉石-斜长石(紫苏辉石)。

图 10 含紫苏辉石侵入岩中单斜辉石的Si与Al (a)及含紫苏辉石侵入岩和细粒包体的Cr与Ni (b, 底图据Chen et al., 2014)、Cr与V (c, 底图据Chen et al., 2014)、SiO2与Gd/Yb (d)、Eu/Eu*与Sr/Y (e)和SiO2与Sr/Y (f)协变关系 文献数据为紫苏花岗岩引自Zhang et al. (2010)Ma et al. (2013b)图 11 Fig. 10 Variation diagram of Si vs. Al for the clinopyroxene in hypersthene-bearing intrusive rocks (a) and co-variation diagrams of Cr vs. Ni (b, after Chen et al., 2014), Cr vs. V (c, after Chen et al., 2014), SiO2 vs. Gd/Yb (d), Eu/Eu* vs. Sr/Y (e) and SiO2 vs. Sr/Y (f) for the hypersthene-bearing intrusive rocks and the enclaves Literature data of the charnockites from Zhang et al. (2010) and Ma et al. (2013b), also in Fig. 11

冈底斯岩基南缘出露的这一系列含紫苏辉石侵入岩样品中,除样品TLL12以及DZK12-4和DZK12-5以外,均具有高的Sr含量(>400×10-6)。除T0847-B5以外的全部含紫苏辉石基性岩样品以及含紫苏辉石中-酸性岩样品T1099和T0726B均具有较高的Sr/Y比值(>40)。同时,Ma et al. (2013b)报道的米林地区的紫苏花岗岩样品全部落入埃达克岩区域内。这些样品具有的高Sr/Y比值特征很可能是俯冲板片熔体混合作用的产物,而其余低Sr/Y比值的含紫苏辉石中-酸性岩样品则受到了分离结晶作用的影响。在岩浆体系中石榴石,角闪石和斜长石通常被认为是控制Sr/Y比值的重要因素。通过前文论证,含紫苏辉石侵入岩的母岩浆不来自石榴石为残留相的源区而且在岩浆演化过程中并未发生石榴石的分离结晶。角闪石中Sr的分配系数远小于1(Ewart and Griffin, 1994),随着角闪石的分离结晶,岩浆中Sr含量增加而Sr/Y比值上升。由于角闪石对MREE显示强的相容性(Brenan et al., 1995; LaTourrette et al., 1995),角闪石的分离结晶会导致Gd/Yb值的增加。在图 10d中,含紫苏辉石侵入岩的Gd/Yb值并未随着SiO2含量增加而增加。并且在稀土元素标准化图解上,并未显示强烈的MREE亏损,说明在岩浆演化过程中并未发生角闪石的分离结晶。斜长石对Sr显示出强相容性(KD(Sr)=5.28)(Ewart and Griffin, 1994),所以斜长石的堆晶也会造成岩浆中Sr含量的增加和Sr/Y比值的上升。通常,斜长石的堆晶和分离结晶可以用Eu的正、负异常来表征。含紫苏辉石的中-酸性岩样品的具有强烈-微弱Eu的负异常(Eu/Eu*=0.50~0.99),指示了斜长石的分离结晶作用。而含紫苏辉石的基性岩样品则基本不具有Eu的异常(Eu/Eu*=0.90~1.05),并不显示斜长石的堆晶作用。但是,在图 10e上,含紫苏辉石侵入岩样品的Eu/Eu*比值与Sr/Y比值之间呈现正相关,表明岩浆体系中Sr/Y比值与斜长石活动的相关性。研究表明,在岩浆体系较为氧化的环境下,斜长石/熔体DEu会降低,而基性矿物/熔体DEu则会升高,斜长石堆晶产生的Eu的正异常会因为基性矿物(橄榄石、辉石等)的分离结晶得到补偿(Drake and Weill, 1975; Drake, 1976)。在岩浆演化的早期,由于单斜辉石早于斜长石晶出并发生分离结晶,使得熔体中的Eu相对富集,但由于补偿作用,并不显示出显著的Eu的正异常,但使得随俯冲板片熔体带来的高Sr/Y比值的特征记录在岩石中。后期斜长石开始发生分离结晶,使得熔体中Eu/Eu*比值和Sr/Y比值下降。受到不同程度斜长石分离结晶的影响,使得含紫苏辉石中-酸性岩分为高Sr/Y比值和低Sr/Y比值两组(图 10f)。

冈底斯岩基南缘出露的这一系列含紫苏辉石的侵入岩均具有较高的Al2O3含量(14.9%~18.8%),含紫苏辉石的基性岩Al2O3含量在17.3%以上,与高铝玄武岩的Al2O3含量相当(Kuno, 1950; Sisson and Grove, 1993)。在其母岩浆系统中,Al主要以铝硅酸盐的形式赋存在单斜辉石、斜方辉石以及斜长石中。由于其母岩浆在演化过程中发生分离结晶作用,全岩的Al2O3含量则受控于其赋存的矿物。在图 11ab中,全岩Al2O3含量与CaO含量以及CaO/Al2O3比值之间存在微弱的正相关关系,表明单斜辉石的分离结晶可能对全岩Al2O3的含量起到控制作用。但在Cr-Al2O3协变图解上,Al2O3含量并不显示随Cr含量的变化而发生变化的趋势(图 11c),说明在岩浆体系中,辉石类并不对全岩的Al2O3含量其决定性控制作用。在Eu*-Al2O3协变图解上(图 11d),可以看到Al2O3含量与Eu*显示明确的正相关关系,表明斜长石对岩浆体系中的Al含量起到主要的控制作用。斜长石具有钠长石和钙长石两个端源,其化学式分别为NaAlSi3O8和CaAl2Si2O8,从钠长石到钙长石组分的转变为Ca2+替换Na1+,并且由一个Al3+替换一个Si4+,使得斜长石组分从Na质转变为Ca质,同时也赋存有更多的Al。通过电子探针测得含紫苏辉石侵入岩中斜长石的成分属于中长石,属于钠长石和钙长石两个端源混合的产物。在含水的玄武质岩浆分离结晶过程中,由于水的存在,抑制了斜长石的结晶,使得Al更加富集在熔体中而不是随着早期晶出的铝硅酸盐分离结晶脱离熔体相,并且晶出具有较高An值的钙长石。岩浆演化过程中斜长石的堆晶作用则使得岩石中的Al更加富集,从而显示出高Al2O3特征。

图 11 含紫苏辉石侵入岩和细粒包体Al2O3对CaO (a)、CaO/Al2O3 (b)、Cr (c)和Eu/Eu* (d)协变关系 Fig. 11 Co-variation diagrams of Al2O3 vs. CaO (a), CaO/Al2O3 (b), Cr (c) and Eu/Eu* (d) for the hypersthene-bearing intrusive rocks and the enclaves

暗色细粒包体具有与含紫苏辉石基性岩相似的矿物组成,暗色细粒包体的结晶粒度明显小于含紫苏辉石基性岩,暗示其快速冷凝而导致其结晶粒度较小。与含紫苏辉石基性岩相比,暗色细粒包具有相似的SiO2含量、FeOT含量,稍高的MgO含量、Al2O3含量以及Mg#值;同样富集LILE,亏损HFSE,较高的Cr和Ni含量;相似的稀土元素配分模式。地球化学特征的相似以及在Harker图解上与含紫苏辉石侵入岩显示出的连续演化趋势,可以推测暗色细粒包体与含紫苏辉石侵入岩同源并且成因相似,暗色细粒包体可能是早期堆晶的产物。在母岩浆上升侵位至围岩温度低于岩浆体系的液相线温度时,发生岩浆淬火作用。早期结晶的矿物会迅速晶出并加速成核,导致众多晶核相互争夺生长空间,都难以长大,因此形成暗色细粒包体(Chen et al., 2015)。这些早期形成的晶核聚集体随着后续岩浆侵位的扰动而均匀的分布在岩体中,并最终形成暗色细粒包体。

5 结论

(1) 冈底斯岩基南缘楠木林县-大竹卡村-桑耶寺-曲水县-里龙村广泛出露一些列年龄为97~77Ma的含紫苏辉石侵入岩,其岩浆侵位年龄可以分为>92Ma、~87Ma和~80Ma三个阶段。

(2) 冈底斯岩基南缘出露的这一系列晚白垩世含紫苏辉石侵入岩及其内包裹的暗色细粒包体是经流体+熔体交代地幔楔中软流圈部分在俯冲流体存在的情况下发生部分熔融,并且随后与俯冲板片熔体发生混合后经过分离结晶作用形成的。

(3) 含紫苏辉石侵入岩具有高的Sr含量、Sr/Y比值特征来自于俯冲板片熔体的混合作用,并受斜长石分离结晶作用的控制,高的Al2O3含量是由于斜长石的堆晶造成的。

(4) 母岩浆中大量流体的存在,使其结晶顺序为单斜辉石-斜长石(紫苏辉石),随后的堆晶作用使得这一系列侵入岩得以赋存紫苏辉石。

致谢      感谢中国科学技术大学肖萍老师在Sr和Nd同位素测试中的帮助;感谢中国地质科学院地质研究所张泽明研究员和戚学祥研究员的细致审稿,提出诸多建设性修改意见。

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