地球科学  2018, Vol. 43 Issue (4): 1038-1050.   PDF    
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蛇绿岩中铬铁岩母岩浆的富Ca特征:矿物包裹体证据
刘霞1,2, 苏本勋2,3, 白洋2,3, 陈晨2,3, 肖燕4, 梁子3,4, 杨赛红4, 彭青山5, 苏本灿6, 刘斌6     
1. 成都理工大学地球科学学院, 四川成都 610059;
2. 中国科学院地质与地球物理研究所, 中国科学院矿产资源研究重点实验室, 北京 100029;
3. 中国科学院大学, 北京 100049;
4. 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, 北京 100029;
5. 长春工程学院勘查与测绘工程学院, 吉林长春 130012;
6. 中国石油天然气股份有限公司长庆油田分公司第八采油厂, 陕西西安 710000
摘要:铬铁矿作为蛇绿岩中的重要矿产,其成矿母岩浆性质及演化一直存在较大争议.铬铁矿的矿物包裹体同时或先于铬铁矿结晶,其成分和类别能很好地记录成矿母岩浆性质和演化过程.土耳其Pozantı-Karsantı蛇绿岩不同类型铬铁岩的铬铁矿中发现了多种类型包裹体:不含水硅酸盐矿物(如橄榄石和单斜辉石)、含水硅酸盐矿物(如角闪石和金云母)、复合型矿物包裹体(如蛇纹石、硅灰石和单斜辉石的复合型包裹体)和不常见矿物(如磷灰石、铂族元素硫化物).含水矿物包裹体的出现以及矿物的高Mg#特征(如橄榄石Fo=95.4~97.1;单斜辉石Mg#=92.0~99.9;角闪石Mg#=88.9~99.8)表明结晶铬铁矿的母岩浆具有富水、富Mg的特征.同时,除钙铬榴石和磷灰石的包裹体外,在铬铁矿中首次发现富Ca矿物方解石和硅灰石,其中方解石和菱镁矿以复合型包裹体形式产出,硅灰石则分布于蛇纹石矿物包裹体中.这些富Ca矿物的出现以及硅酸盐矿物的高CaO含量均揭示了铬铁岩母岩浆的富Ca特征.母岩浆中的Ca组分可能来源于俯冲板块中富Ca岩石/矿物的部分熔融,Ca离子的大量出现使得Cr3+在熔体中更加稳定,同时富Ca矿物的结晶促进了岩浆中Cr的进一步富集而利于铬铁矿的大量结晶沉淀.
关键词蛇绿岩    铬铁矿    包裹体    母岩浆    矿物学    岩石学    
Ca-Enrichment Characteristics of Parental Magmas of Chromitite in Ophiolite: Inference from Mineral Inclusions
Liu Xia1,2 , Su Benxun2,3 , Bai Yang2,3 , Chen Chen2,3 , Xiao Yan4 , Liang Zi3,4 , Yang Saihong4 , Peng Qingshan5 , Su Bencan6 , Liu Bin6     
1. College of Earth Sciences, Chengdu University of Technology, Chengdu 610059, China;
2. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
3. University of Chinese Academy of Sciences, Beijing 100049, China;
4. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
5. Faculty of Prospecting and Survey Engineering, Changchun Institute of Technology, Changchun 130012, China;
6. Eighth Oil Production Plant, Changqing Oilfield Company, PetroChina Co. Ltd., Xi'an 710000, China
Abstract: The origin and mechanisms involved in the formation of chromitite deposit in ophiolites remain a controversial subject of continuous debate. One of the important ways to address this issue is to investigate the nature and composition of parental magmas of chromitite, which may be revealed by mineral inclusions in chromite interpreted to have crystallized contemporaneously with or earlier than the host chromite. Various types of inclusions have been found in chromite ores with different textures in the Pozantı-Karsantı ophiolite in Turkey, which include (1) anhydrous silicate type such as olivine and clinopyroxene, (2) hydrous silicate type such as amphibole and phlogopite, (3) composite type such as the association of serpentine, wollastonite and clinopyroxene, (4) and uncommon mineral type such as apatite and platinum group element sulfide. The occurrence of hydrous mineral and the high Mg# of some minerals (e.g., olivine Mg#=95.4-97.1, clinopyroxene Mg#=92.0-99.0, amphibole Mg#=88.9-99.8) suggest that parental magmas of the chromite are rich in Mg and water contents. Besides the inclusions of apatite and uvarovite, it is reported, for the first time, that calcite and wollastonite inclusions in chromite, which, together with high-CaO features in silicate minerals, indicate Ca-enrichment. The elevated Ca contents in melts are favorable in stabilizing Cr3+ in silicate melt, while crystallization of Ca-bearing minerals could result in Cr enrichment in the melts. The Ca-rich component was probably derived from Ca-enriched rocks in subducting slab.
Key Words: ophiolite    chromite    inclusion    parental magma    mineralogy    petrology    

0 引言

铬铁矿作为蛇绿岩中的主要矿产之一,多呈豆荚状赋存于地幔橄榄岩中,偶见似层状产出于莫霍面之上(Paktunc, 1990).同时,铬铁矿对蛇绿岩的形成、演化、侵位及铬铁矿成矿过程具有重要指示意义.然而,国内外学者对蛇绿岩中豆荚状铬铁矿床的成因尚未达成共识,目前主流的成因机制有以下几种:(1)熔-岩反应:主张铬铁矿为熔体和岩石反应形成,强调不同性质的熔体和地幔橄榄岩反应生成不同类型的铬铁矿(Pearce et al., 1984; Zhou et al., 1996, 2005; Melcher et al., 1997; Rollinson, 2005; Tamura, 2005; Uysal et al., 2007),其中橄榄岩和幔源岩浆反应形成的富Si熔体后期侵入岩浆的过程对形成豆荚状铬铁矿尤为重要(Arai, 1994; Zhou et al., 1994);(2)地幔柱假说:铬铁矿中的透辉石出溶条纹及柯石英、金刚石、碳硅石等超高压超还原矿物的发现表明铬铁矿可能结晶于地幔过渡带(>380 km,Yamamoto et al., 2009),随后被地幔柱带入浅部(Yang et al., 2014, 2015a, 2015b; Xiong et al., 2015);(3)部分熔融:豆荚状铬铁矿为原始地幔岩高度部分熔融的产物,其成分差异指示地幔部分熔融程度,从而制约铬铁矿床的产出背景(王希斌和鲍佩声, 1987王希斌等,1992鲍佩声和王希斌, 1997鲍佩声,2009).

由于铬铁矿及伴生矿物(主要为橄榄石)的成分较为简单,缺乏常见并有效的微量元素和同位素示踪体系,研究方法较为有限.目前铬铁矿成矿作用的研究主要是建立在铬铁矿主量元素的对比上(Kamenetsky, 2001; Pagé and Barnes, 2009),该方法忽略了与铬铁矿共生的橄榄石的成分差异,故有学者尝试利用Li、Fe、Mg等非传统稳定同位素对蛇绿岩中的铬铁矿和橄榄石进行研究(Chen et al., 2015; Su et al., 2015, 2016, 2018; Xiao et al., 2016),力求从新的角度对铬铁矿成因和成矿作用机制提供新的制约.同时,原生包裹体作为先于或与寄主矿物同时结晶的矿物相,其存在对指示寄主矿物的成因及成矿母岩浆的成分具有重要意义(Melcher et al., 1997; Arai et al., 2006, 2010; Zhou et al., 2014).而铬铁矿作为岩浆早期结晶的矿物相之一,其中发现的各类包裹体一直备受关注.目前,阿曼、哈萨克斯坦Kempirsai、土耳其Pozantı-Karsantı、俄罗斯Ray-Iz、中国罗布莎和萨尔托海等多处蛇绿岩铬铁矿中都被报道有不同类型的矿物包裹体(Melcher et al., 1997; 白文吉等, 2004; Borisova et al., 2012; Saka et al., 2014; Zhou et al., 2014; Rollinson and Adetunji, 2015; 黄竺等, 2015; 田亚洲等, 2015; Robinson et al., 2015; Yang et al., 2015a, 2015b; 郭国林等, 2016; Avcı et al., 2017).

本文总结了土耳其Pozantı-Karsantı蛇绿岩中不同类型铬铁岩中的矿物包裹体,并结合其成分、产出特征及寄主铬铁矿的成分,以期厘定其母岩浆的成分和性质,并对铬铁矿成因机制提供新的制约.

1 Pozantı-Karsantı蛇绿岩地质背景及样品描述

土耳其境内分布有大量的中生代蛇绿岩,自北向南分为北部、中部和南部3个蛇绿岩带(图 1a),其中北部蛇绿岩带被认为是古特提斯洋残余,而南部蛇绿岩带为新特提斯洋的岩石圈残片(Dilek et al., 1990).Pozantı-Karsantı蛇绿岩东段呈北东-南西走向,长约85 km、宽20 km,出露面积达1 300 km2(图 1b).该蛇绿岩西部为左行走滑断层区域的渐新世和晚第三纪沉积物及第三纪安山岩,北部和东部为Tauride带的古生代灰岩,南部为Adana盆地的晚第三纪沉积物(Polat and Casey, 1995).

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图 1 土耳其蛇绿岩分布图(a),Pozantı-Karsantı蛇绿岩地质简图(b)和Pozantı-Karsantı蛇绿岩柱状剖面图(c) Fig. 1 Distribution of ophiolites in Turkey (a), simplified geologic map (b) and columnar section of the Pozantı-Karsantı ophiolite (c) 图a据Robertson(2002)修改;图b据Su et al.(2018)修改;图c据Saka et al.(2014)修改

Pozantı-Karsantı蛇绿岩多被认为是SSZ(俯冲带之上)构造背景的产物(Saka et al., 2014; Lian et al., 2017a; Su et al., 2018),位于晚泥盆世-早白垩世的碳酸盐岩台地之上(Dilek et al., 1990; Robertson, 2002),其底部与Alada蛇绿混杂岩和变质底板呈断层接触关系(Parlak et al., 2002, 2009).该蛇绿岩各岩石单元保存较为完整,地幔部分主要为方辉橄榄岩、纯橄岩,含少量二辉橄榄岩.地壳部分自下而上分别为超镁铁质堆晶岩(包括堆晶纯橄岩、异剥橄榄岩、橄榄单斜辉石岩、单斜辉石岩)、镁铁质堆晶岩(包括辉长岩、辉长苏长岩)、均质辉长岩及上覆大洋沉积物(图 1c).最近对辉长岩的锆石U-Pb定年结果为87 Ma,被认定为是Pozantı-Karsantı蛇绿岩的形成年龄(Lian et al., 2017a).

Pozantı-Karsantı蛇绿岩中赋存较大规模的铬铁矿床,包括地幔层位的豆荚状铬铁矿床和壳幔过渡带/地壳层位的似层状铬铁矿床(图 1c),目前均在开采中.豆荚状铬铁矿床位于莫霍面之下,矿石结构多样(图 2a),包括反豆状、豆状、块状、半块状、稠密浸染和稀疏浸染状(图 2b~2f; Avcı et al., 2017).这些豆荚状铬铁矿体常被纯橄岩薄壳包裹,围岩为方辉橄榄岩,其侧向延伸有限,以断续出露的豆荚状为特征(Su et al., 2018).似层状(图 2g)产于莫霍面之上的超镁铁质堆晶岩(特别是堆晶纯橄岩)中,多为条带状构造,其与堆晶纯橄岩互层产出(图 2g; Avcı et al., 2017; Lian et al., 2017b; Su et al., 2018),无明显界限.这些铬铁矿多呈自形-半自形,粒度变化较大(几μm至1~2 cm),颗粒内部裂隙发育,基本无蚀变特征,仅部分铬铁矿裂隙附近和边缘可见蚀变.铬铁矿主要与橄榄石伴生,个别样品可见他形单斜辉石充填于铬铁矿颗粒之间.此外,铬铁矿中的矿物包裹体较为发育,类型复杂,形态多样(Avcı et al., 2017; Lian et al., 2017b).

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图 2 土耳其Pozantı-Karsantı蛇绿岩不同类型铬铁矿石野外及扫描薄片照片 Fig. 2 Field and scanned photos showing different textures and structures of chromitite in the Pozantı-Karsantı ophiolite in Turkey a.多种类型矿石;b.反豆状结构矿石;c.豆状矿石;d.块状矿石;e.半块状/豆状矿石;f.稀疏浸染状矿石;g.似层状铬铁矿石
2 分析方法

铬铁矿包裹体的主量元素分析是在中国科学院地质与地球物理研究所的JEOL JXA8100电子探针(EPMA)上完成的,仪器的分析状态为5 μm直径的束斑、10 nA的束流和15 kV的加速电压;采用天然矿物硬玉[NaAlSi2O6]作为元素Na、Al和Si的标样,蔷薇辉石[MnSiO3]作为Mn的标样,透长石[KAlSi3O8]作为K的标样,石榴子石[Fe3Al2Si3O12]作为Fe的标样,铬透辉石[(Mg, Cr)CaSi2O6]作为Ca的标样,橄榄石[(Mg, Fe)2SiO4]作为Mg的标样;人工合成金红石(TiO2);99.7%的Cr2O3和Ni2Si分别作为Ti、Cr和Ni的标样;基体校正采用ZAF程序.JEOL JXA8100的主量元素的检测限为180×10-6,分析精度优于1.5%(1SD).铬铁矿相对其他硅酸盐矿物性质稳定,不易受后期流体交代和风化作用的影响,同时为避免矿物蚀变导致分析误差,选取靠近铬铁矿的中心位置进行化学成分分析.

矿物的背散射图和元素面分布图是在中国科学院地质与地球物理研究所配备有英国牛津X-MAXN80 X射线能谱仪的FEI Nova NanoSEM 450场发射扫描电镜上获取的.高分辨率(1 536像素×1 103像素)的背散射图像在15 kV加速电压、5.5 μm束斑、6 mm工作距离(WD)的仪器状态下获得,每像素点的驻留时间为30 μs.矿物元素面分布分析采用自动获取能量范围及通道数量,图像分辨率为1 024,每点像素驻留时间为100 μs,采集总时间约10 min.

3 分析结果 3.1 寄主矿物特征及化学组成

土耳其Pozantı-Karsantı蛇绿岩铬铁矿(为表达方便起见,本文铬铁矿和铬尖晶石均表述为铬铁矿)的成分变化较大,Cr2O3含量为46.4%~60.0%,Al2O3含量为8.90%~18.5%,TiO2含量较低(0.10%~0.40%;附表 1).铬铁矿的Cr#=[100×Cr/(Cr+Al)]为62.8~81.6,Mg#=[100×Mg/(Mg+Fe2+)]为50.2~72.7,为高Cr型铬铁矿.在铬铁矿Al2O3与TiO2的成分投图中,所有样品均落入SSZ范围内,多数显示ARC(岛弧)成分特征,显示其成矿母岩浆可能具弧岩浆性质.铬铁矿Cr#与TiO2的成分相关性图解(图 3b)中,大部分样品落入玻安岩成分区域内,与Al2O3与TiO2的成分投图结果一致.与铬铁矿伴生的主要硅酸盐矿物为橄榄石和单斜辉石,橄榄石发生不同程度的蛇纹石化,可见新鲜颗粒,其Fo值(即Mg#值)为93.1~95.0(附表 1).

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图 3 土耳其Pozantı-Karsantı蛇绿岩铬铁矿成分相关性图解 Fig. 3 Correlation diagrams of compositions of chromite in chromitite in the Pozantı-Karsantı ophiolite in Turkey a.铬铁矿Al2O3和TiO2的成分相关性图解,据Kamenetsky et al.(2001);b.铬铁矿Cr#和TiO2的成分相关性图解,据Pagé and Barnes(2009).MOR.大洋中脊;MORB.大洋中脊玄武岩;LIP.大火成岩省玄武岩;OIB.洋岛玄武岩;ARC.岛弧火山岩;SSZ.俯冲带之上
3.2 矿物包裹体形态特征

寄主铬铁矿中的包裹体具多种形态(如自形柱状、长板状及半自形粒状),且大小不一,在块状、浸染状和条带状铬铁岩中均有出露,但浸染状和条带状铬铁岩中的包裹体种类相对较多.按照包裹体是否常见,可将包裹体分为硅酸盐矿物和不常见矿物;硅酸盐矿物根据其是否含水,可分为不含水硅酸盐矿物及含水硅酸盐矿物.除单个矿物包裹体外,复合型包裹体也常见于不同类型的铬铁矿中.

3.2.1 不含水硅酸盐矿物包裹体

铬铁矿中发现的不含水硅酸盐矿物包裹体主要是橄榄石和单斜辉石,Avcı et al.(2017)还发现有斜方辉石的矿物包裹体.橄榄石包裹体粒径为10~50 μm,自形、半自形和他形均有出露,可呈边界平直的四边形(图 4a~4b)、半自形(图 4c~4d)、六边形(图 4e~4f)和浑圆状(图 4g~4h)等多种形态,表面干净或发育裂隙,见于各种类型的铬铁岩中.单斜辉石作为最常见的硅酸盐矿物包裹体,多与含水硅酸盐矿物角闪石共生,呈近八边形短柱状、半自形粒状及长条状等不同形态(图 4i~4l).大多数单斜辉石的粒径为8~40 μm,部分样品(如PK14-41)小于1 μm.Lian et al.(2017b)在Pozantı-Karsantı蛇绿岩铬铁矿中也发现了相同的不含水硅酸盐矿物包裹体.

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图 4 土耳其Pozantı-Karsantı蛇绿岩铬铁矿中不同类型的硅酸盐矿物包裹体 Fig. 4 BSE images showing various types of silicate inclusions with different shapes and sizes in chromite in the Pozantı-Karsantı ophiolite in Turkey a, b.自形橄榄石(Ol);c, d.半自形橄榄石;e, f.自形六边形橄榄石;g, h.他形浑圆状橄榄石;i.单斜辉石(Cpx)矿物集合体;j.近八边形自形单斜辉石;k.半自形单斜辉石;l.他形长条状单斜辉石;m.角闪石(Amp)矿物集合体;n.近菱形自形角闪石;o.半自形长柱状角闪石;p.他形浑圆状角闪石;q.港湾状角闪石;r.金云母(Phl)矿物集合体;s.近五边形自形金云母;t.七边形自形金云母
3.2.2 含水硅酸盐矿物包裹体

含水硅酸盐矿物包裹体主要为角闪石,偶见云母及蛇纹石组成.角闪石常以集合体形式产出(图 4m),呈自形菱形(图 4n)、半自形长柱状(图 4o)、他形浑圆状(图 4p)和港湾状(图 4q)等不同形态,粒度变化于1~25 μm,可与金云母、单斜辉石等硅酸盐矿物共生.金云母呈片状,偶见集合体形式(图 4r),可呈近五边形形态(图 4s),一组极完全解理清晰可见,粒度为1~20 μm.单独产出的云母包裹体较为少见(图 4t),多同角闪石、单斜辉石等硅酸盐矿物共生.此外,云母常见于浸染状、条带状矿石中,块状和半块状矿石中少见.

3.2.3 复合型包裹体

不同样品中发现的复合型包裹体主要有4种类型:(1)金云母和单斜辉石;(2)单斜辉石、硅灰石和蛇纹石;(3)单斜辉石、绿泥石和菱镁矿;(4)金云母和蛇纹石(图 5).金云母和单斜辉石的复合型包裹体中,金云母大小为5 μm×12 μm,单斜辉石约30 μm×40 μm.第2类复合型包裹体大小约为30 μm×30 μm,其中单斜辉石、硅灰石和蛇纹石大小分别为15 μm×20 μm、10 μm×12 μm、10 μm×13 μm.单斜辉石作为第3类复合型包裹体中的主要矿物,其大小为60 μm×100 μm,绿泥石和菱镁矿的大小分别为5 μm×30 μm、20 μm×30 μm.最后一类包裹体中金云母呈半自形片状,可见应力弯曲的解理,并与他形的蛇纹石组成大小约为65 μm×70 μm的椭圆状集合体.

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图 5 土耳其Pozantı-Karsantı蛇绿岩铬铁矿中不同类型复合型包裹体的背散射图像 Fig. 5 BSE images showing different types of composite inclusions in chromite in the Pozantı-Karsantı ophiolite in Turkey a.单斜辉石和金云母的复合型包裹体;b.单斜辉石、硅灰石(Wo)和蛇纹石(Srp)的复合型包裹体;c.单斜辉石、绿泥石(Chl)和菱镁矿(Mgs)的复合型包裹体;d.金云母和蛇纹石的复合型包裹体
3.2.4 不常见矿物包裹体

铬铁矿中含有多种类型的富Ca矿物包裹体和其他不常见矿物,包括方解石(图 6a)、硅灰石(图 6b)、磷灰石(图 6c)、钙铬榴石(图 6d6e)、基性斜长石(图 6f)、铂族元素硫化物(图 6g~6i)等,其形态、大小不一,常和其他矿物共生形成复合型包裹体.富Ca的硅灰石可与蚀变矿物绿泥石、蛇纹石共生.三角状磷灰石包裹于单斜辉石包裹体中,粒径小于5 μm.钙铬榴石存在两种包裹形式:单斜辉石裂隙中填隙状的他形钙铬榴石和半自形粒状的钙铬榴石.基性斜长石包裹体“内嵌”于蛇纹石中,大小约为3 μm×10 μm.铂族元素以单质或硫化物形式产出,可与角闪石组成复合型包裹体.硫钌矿、硫锇矿、IPGE合金、贱金属硫化物等包裹体在铬铁矿中也有出现(Avcı et al., 2017).此外,铬铁矿石中还存在超高压矿物金刚石、超还原矿物碳硅石和锆石、金红石和独居石等壳源矿物(Lian et al., 2017b).

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图 6 土耳其Pozantı-Karsantı蛇绿岩铬铁矿中不常见的矿物包裹体 Fig. 6 BSE images of uncommon mineral inclusions in chromite in the Pozantı-Karsantı ophiolite in Turkey a.方解石(Cal)和绿泥石的复合型包裹体;b.硅灰石、绿泥石、蛇纹石的复合型包裹体;c.磷灰石(Ap)和单斜辉石的复合型包裹体;d.钙铬榴石(Uv)和单斜辉石的复合型包裹体;e.钙铬榴石;f.斜长石(Pl)和蛇纹石的复合型包裹体;g.半自形角闪石和铂族元素硫化物的复合型包裹体;h.铂族元素硫化物和他形角闪石的复合型包裹体;i.柱状铂族元素硫化物
3.3 包裹体成分特征

矿物包裹体的元素面分布图无明显环带,表明其成分较为均一(图 7).包裹体成分整体上具富Mg的特征,其中橄榄石包裹体Fo值(Mg#)为95.4~97.1,相对粒间橄榄石(93.1~95.0)更富Mg(附表 1;图 8a).微量元素上,橄榄石MnO含量很低,仅为0.04%~0.09%,而NiO(0.46%~1.01%)的含量较高.单斜辉石Mg#和CaO的成分相关性图解(图 8b)显示二者呈明显的正相关,其成分变化较大,Mg#变化范围为92.0~99.9,CaO含量为20.1%~24.6%,整体均高于大洋中脊玄武岩,少数与玻安岩重合,表现高Mg和富Ca的特征.单斜辉石TiO2含量较低,多小于0.10%(附表 1),相比玻安岩和大洋中脊玄武岩中的单斜辉石贫Ti(图 8b, 8c).

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图 7 土耳其Pozantı-Karsantı蛇绿岩中铬铁矿的矿物包裹体背散射图和元素分布 Fig. 7 BSE images and elemental distribution mapping of mineral inclusions in chromite in the Pozantı-Karsantı ophiolite in Turkey a.橄榄石;b.单斜辉石;c.单斜辉石和金云母的复合型包裹体
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图 8 土耳其Pozantı-Karsantı蛇绿岩铬铁矿中橄榄石(a)、单斜辉石(b, c)和角闪石(d)包裹体的成分相关性图解 Fig. 8 Compositional correlation diagrams for olivine (a), clinopyroxene (b, c) and amphibole (d) inclusions in chromite in the Pozantı-Karsantı ophiolite in Turkey 玻安岩和大洋中脊玄武岩数据来自GEOROC,http://georoc.mpch-mainz.gwdg.de/georoc/Start.asp

含水硅酸盐矿物角闪石的Cr2O3和Na2O含量较高,分别为2.96%~11.3%和2.07%~3.67%(附表 2).其高Mg#(88.9~99.8)和高CaO含量(10.4%~12.0%)表明角闪石成分上为钙镁闪石(除PK14-60)(图 8d),依据国际矿物协会的分类标准为韭闪石.金云母的K2O和Cr2O3含量分别为3.38%~6.82%和2.16%~2.35%,其CaO(0.11%~1.01%)含量较高,TiO2较低(<0.55%),表现低Ti特点(附表 2).方解石、硅灰石和钙铬榴石的CaO分别为51.2%、31.5%和34.4%(附表 2),均表现富Ca的特征.

4 讨论 4.1 矿物包裹体所揭示的铬铁矿母岩浆成分特征

蛇绿岩从形成到就位的过程十分复杂,不同期次的岩浆活动、变质作用和流体作用导致蛇绿岩中普遍存在矿物蚀变,如橄榄石蚀变为蛇纹石、辉石蚀变为角闪石.土耳其Pozantı-Karsantı蛇绿岩铬铁矿的矿物包裹体靠近裂隙的地方多发生蚀变,远离裂隙的中心位置比较新鲜且与寄主铬铁矿之间的界线分明(图 9a~9d).本次研究以铬铁矿中心未蚀变的矿物包裹体为研究对象,排除后期蚀变可能,保证矿物包裹体为母岩浆结晶的产物.

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图 9 土耳其Pozantı-Karsantı蛇绿岩中铬铁矿的矿物包裹体特征 Fig. 9 Occurrence of inclusions in chromite in the Pozantı-Karsantı ophiolite in Turkey a.新鲜的单斜辉石和蚀变的蛇纹石;b.新鲜的橄榄石和蚀变蛇纹石;c.单斜辉石和磷灰石的复合型包裹体;d.新鲜的橄榄石包裹体

目前,诸多研究者在世界各地的蛇绿岩(如罗布莎、Ray-Iz、Masquard、Wadi Hilti、Kempirsai、萨尔托海、Semial、印度-缅甸蛇绿岩带)铬铁矿中发现了多种矿物包裹体,最常见的为角闪石、单斜辉石、橄榄石、云母(包括钠云母和钾云母)等硅酸盐矿物包裹体(Borisova et al., 2012; 田亚洲等, 2015; Arai and Miura, 2016; Maibam et al., 2017),各种铂族元素矿物(Melcher et al., 1997; 白文吉等, 2004; Arai and Miura, 2016; 郭国林等, 2016)、贱金属硫化物(如黄铁矿、针镍矿、希兹硫镍矿,Melcher et al., 1997; Borisova et al., 2012; Maibam et al., 2017)和壳源矿物(锆石、长石、金红石、榍石等)等矿物包裹体也较常出现(Melcher et al., 1997; Robison et al., 2015; Yang et al., 2015a, 2015b; Maibam et al., 2017),部分蛇绿岩(如Kempirsai蛇绿岩)铬铁矿中还发现流体包裹体(Melcher et al., 1997).学者还在多地蛇绿岩中发现有超高压和超还原矿物,如金刚石、碳硅石和柯石英等(Yang et al., 2014; Robinson et al., 2015; Lian et al., 2017b),但这些异常矿物多为大量矿石分选得出而并非原位的矿物包裹体,故不能准确反映铬铁矿的母岩浆性质和结晶时的物理化学条件.然而,原位包裹体同时或先于寄主矿物结晶,故其能很好地记录铬铁矿结晶时的物理化学条件,并能够反映母岩浆的性质(Melcher et al., 1997).俯冲的新鲜或蚀变洋壳脱水产生了富水的母岩浆(Schiano et al., 1997; Ahmed et al., 2006),从而结晶角闪石、云母等含水矿物包裹体,同时水的加入导致亏损的方辉橄榄岩进一步部分熔融产生富Mg的熔体,进而结晶出高Mg的橄榄石、单斜辉石和角闪石.此外,在高氧逸度环境中,岩浆体系中的水分有利于结晶形成大量铬铁矿(Ford et al., 1972; Matveev and Ballhaus, 2002; Feig et al., 2006; Borisova et al., 2012).土耳其Pozantı-Karsantı蛇绿岩铬铁矿中高Fo(95.4~97.1)橄榄石、高Mg#(92.0~99.9)单斜辉石、菱镁矿和含水硅酸盐矿物金云母、角闪石等包裹体的发现,表明铬铁矿的母岩浆具有“高Mg、富水、氧化”的性质.尽管与铬铁矿的Fe-Mg交换也会使硅酸盐矿物的Mg#升高(Arai, 1980; Melcher et al., 1997; Bai et al., 2017),但笔者近期的模拟工作(Xiao et al., 2016)表明要达到如附表 2中高的Mg#,那硅酸盐矿物最初的Mg#应高于正常玄武质岩浆结晶矿物的Mg#.

值得注意的是,土耳其Pozantı-Karsantı蛇绿岩铬铁矿中发现方解石、硅灰石、钙铬榴石、磷灰石等富Ca矿物包裹体,其中,方解石和硅灰石的发现在铬铁矿研究中尚属首例,磷灰石和钙铬榴石在哈萨克斯坦Kempirsai蛇绿岩、乌拉尔Ray-Iz蛇绿岩和罗布莎蛇绿岩中也有发现(Melcher et al., 1997; Robinson et al., 2015).富Ca矿物包裹体多与硅酸盐矿物共生(图 6),产出位置远离铬铁矿表面的裂隙,可排除后期蚀变矿物的可能性,揭示结晶铬铁矿的母岩浆具明显的富Ca特征.此外,单斜辉石包裹体的成分相关性图解显示其相对玻安岩和大洋中脊玄武岩的单斜辉石明显富Ca(图 8b~8c),角闪石包裹体的钙镁闪石成分特征(图 8d)也从侧面反映了铬铁矿母岩浆的富Ca特征.除土耳其Pozantı-Karsantı铬铁矿母岩浆显示富Ca特征外,阿曼蛇绿岩高Cr铬铁矿的母岩浆也表现高Ca特征(Rollinson, 2008).

4.2 富Ca母岩浆中Ca的可能来源及其在铬铁矿成矿中的作用 4.2.1 Ca的来源

土耳其Pozantı-Karsantı蛇绿岩铬铁矿中高Ca矿物包裹体的发现和其他硅酸盐矿物包裹体的高CaO含量,均揭示了结晶铬铁矿的母岩浆具有富Ca特征,其可能的来源有:(1)洋内俯冲的构造环境中,随着板块的持续俯冲,在120~160 km处岩石发生变质作用(主要是榴辉岩相),使得俯冲板块密度增加导致板块断裂,板块断裂处软流圈地幔上涌发生减压熔融,并有俯冲物质参与而形成钙碱性岩浆(Zhou et al., 2014; Lian et al., 2017a, 2017b);(2)初始俯冲阶段,俯冲板块的撕裂为软流圈地幔上涌提供了通道,使得软流圈地幔在拉张的构造环境中发生减压重融并产生钙碱性岩浆(Robinson et al., 2015);(3)洋内俯冲起始,板块的断裂导致软流圈地幔上涌,其产生的热量使得俯冲板块发生部分熔融.新鲜或蚀变洋壳、海沟处刮擦带入的远洋沉积物(如高岭土等黏土矿物)和俯冲板块中台地碳酸盐岩的部分熔融产生富水的钙碱性熔体(Su et al., 2018).

对土耳其Pozantı-Karsantı蛇绿岩铬铁矿Cr#和TiO2的成分投图(图 3b)显示,只有部分铬铁矿落入玻安岩区域内,不同岩性(方辉橄榄岩、纯橄岩、异剥橄榄岩、铬铁岩)的橄榄石Li同位素研究结果表明熔体的性质主要在OIB、不成熟岛弧和成熟岛弧之间变化(Su et al., 2018),这些特征均表明富Ca的铬铁矿母岩浆更可能与初始俯冲的部分熔融有关.Pozantı-Karsantı蛇绿岩北部和东部大量分布的同时期灰岩可能有部分参与了俯冲过程,或者直接加入岩浆源区而被结晶的铬铁矿所包裹而出现方解石和菱镁矿的包裹体,或者发生熔融以熔体形式加入岩浆源区,从而为铬铁矿母岩浆直接提供了富Ca物质.另一方面,地幔橄榄岩或辉石岩中的单斜辉石也很有可能是Ca的来源之一.对这一问题的厘定尚需进一步的同位素研究工作.

4.2.2 Ca在铬铁矿成矿中的作用

结晶铬铁矿的高Mg、富Si、以及含水的高氧逸度镁铁质熔体中Ca的加入对铬铁矿成矿有重要作用,主要体现在以下几个方面:(1)硅酸盐熔体中CaO含量的升高利于Cr3+在熔体中更加稳定(Berry et al., 2006),从而保证母岩浆中富Ca矿物(如方解石、硅灰石)结晶时Cr3+仍能稳定存在.Berry et al.(2006)通过研究不同熔体成分对Cr2+/Cr3+比值的影响发现,熔体CaO的含量与logγ#Cr3+O1.5(Cr3+O1.5活度系数的对数)之间呈明显的负相关,随着CaO含量的增加,logγ#Cr3+O1.5逐渐降低,即Cr3+在熔体中的稳定性增加.然而,由于熔体中高MgO的影响,二者并未呈现线性关系.同时,Berry et al.(2006)的实验还表明熔体中MgO含量的增加也有利于Cr3+在熔体中以八次配位的形式存在,该结论与铬铁矿母岩浆的高Mg特征相一致;(2)Ca在硅酸盐熔体中主要以八次配位的形式存在,而在富Ca矿物中配位数则较低(如方解石中为六次配位),根据鲍林原则,在离子电价一定的情况下,其配位数越高键强越弱,因此铬铁矿母岩浆中Ca的离子键容易遭到破坏,从而进入富Ca矿物中以更稳定的低次配位存在.同时,随着富Ca矿物的结晶,熔体中Cr元素含量升高而稳定性降低,有利于大量铬铁矿的结晶和富集.

5 结论

土耳其Pozantı-Karsantı蛇绿岩中的铬铁矿含有多种类型的矿物包裹体,包括不含水硅酸盐矿物包裹体、含水硅酸盐矿物包裹体、复合型矿物包裹体和不常见矿物包裹体.包裹体大小不一、形态各异,可呈自形-半自形短柱状、包裹体粒状、他形港湾状等多种形态.其中方解石和硅灰石是蛇绿岩铬铁矿中的首次报道.这些富Ca矿物组合以及其成分特征指示铬铁矿母岩浆具有富Mg、富H2O和富Ca的性质.结合橄榄石的Li同位素研究结果(Su et al., 2018)和蛇绿岩的产出特征,笔者认为该蛇绿岩形成于初始俯冲的构造环境.俯冲板片中的灰岩可能为铬铁矿母岩浆中Ca的来源.硅酸盐熔体中的高CaO含量有利于熔体中Cr的富集及铬铁矿的大量结晶.

致谢 土耳其黑海技术大学Ibrahim Uysal对野外工作给予了大力支持,三位评审人对本文提出了非常有建设性的建议,在此一并表示感谢!

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