刘建强 ,任钟元

(1.中国科学院 广州地球化学研究所,同位素地球化学国家重点实验室,广东 广州 510640;2.中国科学院大学,北京 100049)

0 前 言

在过去的几十年时间里,大量的地球化学研究表明地球的地幔在微量元素和同位素组成上是高度不均一的(Hofmann and White,1982;Zindler and Hart,1986;Weaver,1991;Hauri,1996;Lassiter and Hauri,1998;Hofmann,1997,2003;Ren et al.,2004,2005,2006,2009)。作为板块构造的主要驱动力,地幔对流作用能将深部的地幔物质向上运输直至发生减压熔融,形成的熔体最终上升至地表并喷发形成玄武岩,因此玄武质熔岩能够提供许多有关地幔源区的信息(Hofmann,1997)。然而,玄武岩最终的化学组成取决于很多因素,如源区母岩的物质组成,地幔源区的温压条件及部分熔融程度,岩浆传输系统中的结晶分异及岩浆混合,以及受陆壳物质的同化混染,岩浆期后蚀变作用的影响等。本文主要探讨地幔源区岩性的差异对玄武质熔岩的化学组成所造成的影响。玄武岩源区岩性的识别对原生岩浆组成的估计、地幔潜能温度的估算以及地幔岩石学和矿物学复杂性的探讨具有十分重要的意义(Herzberg,2006,2011;Herzberg and Asimow,2008)。本文总结了近几十年来有关玄武岩源区母岩多样性及其识别特征的研究,并利用这些识别特征来判别海南岛玄武岩源区母岩的岩性。对海南岛玄武岩的研究结果表明干的地幔橄榄岩,橄榄岩+CO2或地幔交代的角闪石岩脉作为海南岛玄武岩的源区母岩都不能很好地解释其独特的地球化学特征,而辉石岩作为海南岛玄武岩的源区母岩则可以很好地解释其独特的地球化学特征。

1 玄武岩源区母岩的多样性及识别特征

地球的上地幔是主要由橄榄岩组成的(Washington,1925;Sobolev et al.,2005)。多年来人们通常认为玄武质岩浆都起源于地幔橄榄岩的熔融(Yoder and Tilley,1962;Green and Ringwood,1963;O’Hara and Yoder Jr,1967;Walter,1998)。然而,对洋岛玄武岩(OIB)和洋中脊玄武岩(MORB)进行大量的微量元素和同位素研究表明地幔的组成是不均一的(Hofmann and White,1982;Zindler and Hart,1986;Weaver,1991;Hauri,1996;Lassiter and Hauri,1998;Hofmann,1997,2003;Ren et al.,2004,2005,2006,2009)。镁铁质的地壳组分(例如,俯冲的洋壳)可以再循环进入地幔(Hofmann and White,1982;Hauri,1996;Hofmann,1997;Sobolev et al.,2000;Ren et al.,2005),与地幔橄榄岩发生反应形成二阶段辉石岩(或再循环的洋壳物质部分熔融体与橄榄岩反应,或再循环的洋壳物质在下地幔深处固态状态下与橄榄岩反应形成)(Sobolev et al.,2005,2007,2008,2009;Herzberg,2006,2011;Ren et al.,2006,2009)成为玄武岩的源区母岩。碳酸盐熔体对岩石圈地幔的交代作用可以形成碳酸盐化的橄榄岩,而硅酸盐熔体的交代作用则可以形成少量的角闪石岩交代脉存在于地幔中(Hirose,1997;Pilet et al.,2004,2005,2008;Dasgupta et al.,2007)。大量的实验岩石学研究表明传统的干的橄榄岩部分熔融形成的熔体无法匹配许多板内玄武岩的地球化学特征,因此,橄榄岩+CO2(Brey and Green,1975,1976,1977;Wyllie and Huang,1976;Brey,1978;Wendlandt and Mysen,1980;Hirose,1997;Dasgupta et al.,2007;Zeng et al.,2010),辉石岩(Hauri,1996;Hirschmann et al.,2003;Kogiso et al.,2003,2004;Kogiso and Hirschmann 2006;Ren et al.,2004,2006,2009;Sovolev et al.,2000,2005,2007;Herzberg,2006,2011;Jackson and Dasgupta,2008)以及角闪石岩(Pilet et al.,2004,2005,2008)分别被提出可以作为板内玄武岩的源区母岩。下面我们主要讨论当这几种超镁铁质岩石作为玄武岩的源区母岩时,其部分熔融形成的熔体分别具有什么样的地球化学特征:

(1) 传统的地幔橄榄岩:橄榄岩是指主要由橄榄石和辉石组成,橄榄石含量为40%～90%的超镁铁岩。橄榄岩作为玄武质熔岩的源区母岩是早期地学界最为接受的一种观点。Washington (1925)首次提出上地幔是由橄榄岩组成的,以及 Bowen (1928)就开始探讨橄榄岩的减压熔融产生玄武质熔体的可能性。Yoder and Tilley (1962)和Green and Ringwood(1963)进行的实验岩石学研究证实了橄榄岩作为玄武岩源区母岩的可能性,以及 McKenzie and O’Nions (1991)进一步指出拉斑玄武岩和碱性玄武岩可以分别由地幔橄榄岩经过不同程度的熔融形成。Hirose and Kushiro (1993),Takahashi et al.(1993),Kushiro (1996)以及Falloon and Danyushevsky (2000)等的橄榄岩的部分熔融实验结果都成功地证明大洋中脊玄武岩来自地幔橄榄岩的部分熔融。时至今日,橄榄岩作为MORB的源区母岩的观点已经被人们广泛地接受(Putirka,2005;Workman and Hart,2005;Putirka et al.,2007,2011;Niu and O’Hara,2008)。然而越来越多的研究表明橄榄岩部分熔融形成的熔体并不能匹配板内玄武岩尤其是OIB的地球化学特征(Hauri,1996;Ren et al.,2004,2005;2006,2009;Sobolev et al.,2000;2005,2007;Herzberg,2006,2011)。Herzberg (2011)计算了地幔橄榄岩部分熔融形成的原生岩浆及其结晶的橄榄石斑晶所具有的主量元素特征,并指出现今的洋中脊玄武岩和太古代的科马提岩均起源于正常的橄榄岩源区,而 Hawaii和Canary的洋岛玄武岩则起源于辉石岩的源区母岩。

(2) 橄榄岩+CO2:CO2是地球上主要的挥发性组分之一,对碳酸盐化橄榄岩进行大量的熔融实验结果表明CO2对地幔的熔融过程具有十分重大的影响。Brey and Green (1975,1976,1977)和Brey (1978)对橄榄石黄长岩在 CO2+H2O出现的情况下进行大量的逆实验(inverse experiments)的研究结果表明,CO2可以提高强碱性熔体液相线上斜方辉石的稳定性,因此首次提出贫硅的碱性玄武岩可以由碳酸盐化的橄榄岩部分熔融形成。Hirose (1997)和Dasgupta et al.(2007)对天然的碳酸盐化橄榄岩(KLB-1+ 2.5%CO2和KLB-1+1% CO2)进行的高压(3 GPa)部分熔融实验研究结果显示,低温条件下碳酸盐化橄榄岩部分熔融形成的熔体组分是碳酸岩质的,随着温度的升高熔体的组分逐渐向黄长岩-霞石岩-碧玄岩转变。因此Dasgupta et al.(2007)指出碳酸盐化的橄榄岩低熔融程度(1%～5%)形成的熔体可匹配许多高碱OIB的地球化学特征。此外,碳酸盐岩是地球上含量最多,分布范围最广的沉积岩,对海相沉积碳酸盐岩的研究发现其具有强烈富集不相容元素,亏损Zr、Hf、K及Ti的特征(Hoernle et al.,2002;Bizimis et al.,2003)。碳酸盐沉积物与俯冲的洋壳物质一起进入地幔的过程中会发生脱水熔融,形成的碳酸盐熔体可以交代上覆的地幔楔物质从而形成碳酸盐化的地幔橄榄岩。原始地幔标准化的蛛网图上碳酸盐岩呈现REE富集,Zr、Hf、Ti及K的负异常特征,碳酸盐熔体交代的地幔橄榄岩也将继承这些异常特征(Hirose,1997;Dasgupta et al.,2007;Zeng et al.,2010)。

(3) 辉石岩:广义的辉石岩是指主要由辉石和石榴石组成,橄榄石含量小于 40%的超镁铁岩。Hauri (1996) 将夏威夷熔岩的组成与橄榄岩部分熔融产生的熔体相比,发现夏威夷玄武岩具有十分高的 SiO2含量,因此首次提出榴辉岩的组分可能参与了玄武岩的源区。Ren et al.(2004) 注意到在相同MgO含量下,与众多的橄榄岩实验部分熔融所产生的熔体相比,夏威夷Haleakala玄武岩的CaO,Al2O3含量较低,而TiO2含量较高。Ren et al.(2004,2006,2009) 根据夏威夷玄武岩的主量元素组成,结合微量元素和同位素地球化学提出了夏威夷火山的源区母岩中有辉石岩的存在。Hirschmann et al.(2003) 和Kogiso et al.(2003) 对石榴石辉石岩进行的高压部分熔融实验结果表明,石榴石辉石岩(相比于干的橄榄岩)具有更低的固相线温度,并且石榴石辉石岩部分熔融形成的熔体能够很好地匹配洋岛玄武岩的组成特征。Kogiso et al.(2004) 进一步指出碱性洋岛玄武岩可以起源于硅不饱和(Silica-deficient)的辉石岩源区,而拉斑质的洋岛玄武岩可以起源于硅饱和(Silica-excess)的辉石岩源区。Sobolev et al.(2005,2007) 发现夏威夷盾状玄武岩中的橄榄石斑晶具有低的MnO、CaO以及高的NiO含量,指出无橄榄石的辉石岩可作为夏威夷玄武岩的源区母岩。Herzberg(2006)和Herzberg and Asimow (2008) 认为夏威夷玄武岩的原生岩浆中低CaO的特征同样需要辉石岩的源区母岩。Herzberg (2011)进一步指出,与橄榄岩部分熔融形成的熔体结晶的橄榄石相比,辉石岩熔体结晶的橄榄石具有较低的Mn、Ca以及较高的Ni和Fe/Mn比值。Le Roux et al.(2010) 根据各种矿物与熔体之间的分配系数,使用不同的方法估算得到因此指出当橄榄石和斜方辉石为主要的结晶或残留相时,残留或形成熔体的Zn/Fe,Zn/Mn以及Fe/Mn比值几乎不发生分馏。然而,当单斜辉石或石榴石是主要的结晶或残留相时,这些比值将会发生很大的分馏。当单斜辉石或石榴石为主要的残留相矿物时,部分熔融形成的熔体具有更高的 Zn/Fe,Zn/Mn及Fe/Mn比值。依据La和Nb在橄榄岩和辉石岩体系中具有截然相反的分配行为,即在辉石岩中 La比Nb更相容,而在橄榄岩中La具有比Nb更不相容的特征,因此Stracke and Bourdon (2009)指出辉石岩熔体的La/Nb<<1,而橄榄岩熔体的La/Nb>>1。

(4) 角闪石岩:角闪石岩是指主要由角闪石组成,橄榄石含量小于 40%的超镁铁质岩石。最近的研究结果表明富含角闪石脉的交代岩石圈地幔也可以作为板内碱性玄武岩的源区母岩(Sun and Hanson,1975;Niu and O’Hara,2003;Pilet et al.,2004,2005,2008)。Pilet et al.(2008) 对角闪石岩,单斜辉石角闪石岩以及角闪石岩和橄榄岩的混合物的高压熔融实验结果表明,角闪石岩交代脉部分熔融形成的熔体可以很好地匹配许多碱性玄武岩的主量元素组成。并且,原始地幔标准化的蛛网图上角闪石岩部分熔融形成的熔体具有显著的 Nb-Ta,Ti的正异常以及Zr-Hf,Pb,K的负异常特征(Pilet et al.,2008;Zeng et al.,2010),这可以很好地解释许多洋岛玄武岩的不相容元素分布型式。

2 海南岛玄武岩的源区母岩的岩性

2.1 地质背景及研究现状

海南岛北部是中国东南部玄武岩出露面积最大的地区,玄武岩的总面积多达 4000 km2(黄振国等,1993;樊祺诚等,2004)。根据玄武岩玻璃的K-Ar和Ar-Ar定年、地层接触关系以及岩石学特征,海南岛新生代玄武岩可分为五个喷发期次,从上到下依次为全新世石山组,晚更新世道堂组,中更新世晚期多文组上段,早更新世晚期-中更新世早期多文组下段以及中-上新世的石马村组和石门沟组(樊祺诚等,2004;龙文国等,2006a,2006b;Wang et al.,2012)。主量元素的标准矿物分子计算表明海南岛玄武岩的变化范畴可以从石英拉斑玄武岩到橄榄拉斑玄武岩,再到碱性橄榄玄武岩和碧玄岩(Ho et al.,2000;Wang et al.,2012)。玄武岩的微量元素组成具有类似于OIB的REE配分模式及不相容元素分布型式,同时也指示了大陆板内玄武岩的构造环境(Ho et al.,2000;樊祺诚等,2004;Zou and Fan,2010;Wang et al.,2012)。玄武岩的同位素组成指示了Dupal铅同位素的组成特征以及DMM和EMⅡ的混合(Tu et al.,1991;Flower et al.,1992;Zhang et al.,1996;Zou et al.,2000;韩江伟等,2009)。尽管前人对该地区的玄武岩进行了广泛的岩石学和地球化学研究,积累了大量的、丰硕的研究资料,然而有关海南岛玄武岩的源区物质组成还是存在很大的争议。Fan and Hooper (1991)根据中国东部(包括海南岛)新生代玄武岩的地球化学特征,认为尖晶石二辉橄榄岩和石榴石二辉橄榄岩不同程度的熔融可以分别形成中国东部的拉斑玄武岩和碱性玄武岩。Hoang et al.(1996) 和Hoang and Flower (1998) 则认为拉斑和碱性玄武岩分别起源于岩石圈地幔和软流圈地幔。Wang et al.(2012) 根据海南岛玄武岩的岩石地球化学特征认为地幔橄榄岩和再循环洋壳的混合可作为拉斑玄武岩的源区物质组成,而地幔橄榄岩和含钾榴辉岩(或石榴石辉石岩)的混合可作为碱性玄武岩的源区物质组成。鉴于不同的镁铁质岩石部分熔融产生的熔体具有截然不同的地球化学特征,本文采用橄榄石化学组成结合全岩主、微量元素地球化学来重新审视海南岛玄武岩的源区物质组成。

2.2 岩相学特征与化学分析方法

本次研究我们共采取了海南岛北部地区 14个岩石样品,其中 7个拉斑玄武岩,7个碱性玄武岩,海南岛玄武岩的分布和采样位置如图1。拉斑和碱性玄武岩都具有斑状结构,斑晶矿物主要为半自形-自形粒状的橄榄石,含量在 3%～10%之间,有的含有极少量的单斜辉石斑晶(含量<1%)。橄榄石斑晶的粒度在 0.2～0.5 mm,有的橄榄石斑晶发生了蚀变,形成富铁的皂石。基质主要由橄榄石、单斜辉石和斜长石微晶以及玻璃和不透明矿物组成,块状构造及少量的样品为气孔状构造。

图1 海南岛北部晚新生代玄武岩的分布及采样位置图Fig.1 Distribution and sampling locations of the Late Cenozoic basalts in North Hainan Island

先用切样机将岩石样品切成厚0.5～1 cm的薄片状块体,然后用锤将岩石敲成几毫米大小的碎块,选取中间新鲜的岩石碎块用做全岩化学分析。在超声波清洗槽中,将用做全岩分析的岩石碎块分别用纯净水超声2遍和Milli-Q水超声1遍,每次超声20分钟,待洗好烘干后用德国 Retch振动盘式粉碎仪RS200的铬钢钵磨成约200目大小的粉末。全岩的主量元素分析是在中国科学院广州地球化学研究所同位素地球化学国家重点实验室的 Rigaku 100e型X-射线荧光分光仪上进行分析的,分析方法见 Goto and Tatsumi(1996)。SiO2,Al2O3,Fe2O3,MgO,CaO,Na2O及K2O的分析精度好于3%,TiO2,MnO及P2O5的分析精度好于5%。全岩的微量元素分析是在中国科学院广州地球化学研究所同位素地球化学国家重点实验室的 PE ELan 600电感耦合等离子体质谱(ICP-MS)上完成的。称取～40 mg的粉末样品于清洗干净的Bomb溶样器中,将样品在HF-HNO3-HClO4的溶液中置于150 ℃的条件下进行溶解。溶液样品中加入4 g 10 ng/g的Rh内标溶液来监视信号的漂移,使用 BHVO-2,AGV-2,W-2,GSR-1,GSR-3,GSP-2等标样来标定样品的元素含量,绝大多数微量元素的分析精度好于 5%(刘颖等,1996)。橄榄石的化学成分分析是在中国科学院广州地球化学研究所同位素地球化学国家重点实验室的 JEOL JXA-8100型电子探针上完成的。橄榄石的测试条件与Sobolev et al.(2007)方法相似,具体条件为:电压20 kv,电流 3.0E-07A,束斑直径 2 µm,Si、Fe、Mg元素的计数时间为90 s,Mn、Ca为120 s,Ni为150 s,每一次换样之后都对橄榄石内标进行测定来监测仪器的运行状态。橄榄石内标10次重复测量的结果表明,SiO2,FeO,MgO的分析精度好于0.3%,MnO的分析精度好于 2%,NiO的分析精度好于 4%,以及CaO的分析精度好于7%。

2.3 结果

2.3.1 全岩主、微量元素组成

海南岛玄武岩的主、微量元素分析结果及相关参数列于表1中。根据Le Bas et al.(1986)的TAS投图结果(图2),本次研究的样品中有7个位于拉斑玄武岩区域,另外 7个位于碱性玄武岩区域。海南岛玄武岩的 LOI的变化范围为-0.008%～0.01%,平均值为-0.003%,指示了海南岛玄武岩几乎未受岩浆期后蚀变作用的影响。在球粒陨石标准化的稀土元素配分模式图上(图3a),海南拉斑和碱性玄武岩均呈现LREE富集和HREE亏损的特征,与OIB相类似。在原始地幔标准化的不相容元素蛛网图上(图3b),海南拉斑和碱性玄武岩均呈现 Nb-Ta,Rb,Ba,Sr的正异常以及Th,U,Pb的负异常特征。此外,海南拉斑和碱性玄武岩均未呈现显著的Zr-Hf,K的异常。碱性玄武岩呈现显著的Ti的负异常,然而拉斑玄武岩却未呈现明显的Ti的异常特征。拉斑玄武岩的La/Nb为0.741～0.916,以及碱性玄武岩的La/Nb为0.677～0.737,均小于1。

2.3.2 橄榄石的化学组成

图2 (Na2O+K2O)-SiO2图解(据Le Bas et al.,1986)Fig.2 (Na2O+K2O) vs SiO2 diagram for the basalts

图3 海南岛玄武岩稀土元素配分模式图和微量元素蛛网图Fig.3 REE patterns and trace element spider diagrams of the Hainan basalts

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表2 海南岛玄武岩中橄榄石斑晶的主量(%)和微量元素(µg/g)组成Table2 Major(%) and trace element(µg/g) compositions of the olivine phenocrysts in the Hainan basalts

续表

图4 海南岛玄武岩中橄榄石的CaO与Fo之间相关关系图(来自Wang et al.(2012)中的橄榄石同样投在图中。投图结果显示橄榄石的 CaO>0.1%,指示了这些橄榄石为岩浆结晶成因的)Fig.4 Diagram of CaO contents against Fo values for the olivines from the Hainan basalts

通过电子探针,我们共测定了80颗橄榄石的化学组成,其中包括3个拉斑玄武岩的40颗橄榄石和3个碱性玄武岩的40颗橄榄石,结果列于表2中。拉斑和碱性玄武岩中的橄榄石具有较低的 Fo(拉斑玄武岩:74.7～81.9和碱性玄武岩:66.9～82.9)以及较高的CaO含量(>0.1%,图4),并且岩石薄片显微镜下观察表明橄榄石晶体均为自形晶,上述证据指示了所分析的橄榄石为岩浆体系中结晶的橄榄石斑晶,而不是岩浆捕获的地幔捕掳晶。拉斑和碱性玄武岩中橄榄石的Mn与Fo之间呈现较好的负相关(图5b),而Ni与Fo之间呈现较好的正相关(图5c)。拉斑和碱性玄武岩中的橄榄石具有较高的Fe/Mn比值(>80),然而Fe/Mn与Fo之间无明显的相关性(图5d)。与橄榄岩部分熔融形成的熔体结晶的橄榄石相比,海南玄武岩中的橄榄石斑晶具有较低的Ca和Mn,较高的Ni和Fe/Mn比值(图5)。

2.4 海南岛玄武岩源区岩性探讨

图5 海南岛玄武岩中橄榄石的Ca,Mn,Ni及Fe/Mn与Fo之间的相关关系图 (同样投在图上的有来自Koolau,Loihi,MORB及Komaiites中的橄榄石(Sobolev et al.,2007)。投图结果表明,与橄榄岩部分熔融形成的熔体结晶出的橄榄石相比,海南岛玄武岩中的橄榄石斑晶具有较低的Ca和Mn,以及较高的Ni和Fe/Mn)Fig.5 Plots of Ca,Mn,Ni and Fe/Mn against Fo for the olivines from the Hainan basalts

玄武岩源区岩性的识别是进行原生岩浆组成的估计,地幔潜能温度的估算以及源区物质部分熔融程度反演的前提。前人对海南岛北部地区出露的新生代玄武岩进行了大量的全岩主、微量及同位素研究,认为海南岛玄武岩源区母岩的组成为地幔橄榄岩,例如,Fan and Hooper (1991)根据中国东部(包括海南岛)新生代玄武岩的主、微量元素组成,认为拉斑和碱性玄武岩分别起源于尖晶石二辉橄榄岩和石榴石二辉橄榄岩的部分熔融;Tu et al.(1991) 根据海南岛玄武岩的同位素组成,认为海南岛玄武岩起源于大陆之下岩石圈地幔的减压熔融。Wang et al.(2012) 对只受橄榄石分离结晶影响的海南岛玄武岩进行加橄榄石的方法来估算其原始岩浆的组成,并利用此原始岩浆的组成对海南岛的地幔潜能温度进行了计算。然而,Herzberg (2011)的研究表明通过对熔体组分加橄榄石直到其与某一设定的高镁橄榄石(如 Fo=90)相平衡的方法来计算玄武岩母岩浆的组成及其地幔潜能温度的计算只适用于源区母岩为橄榄岩的前提。因此,如果海南岛玄武岩的源区母岩的岩性不是橄榄岩的话,那么他们的计算结果需要重新审视。

海南岛玄武岩的地球化学特征表明干的地幔橄榄岩并不能够作为其源区母岩的岩性。首先,在海南岛玄武岩与各种超镁铁质岩石高压实验部分熔融体之间的比较图中(图6),干的地幔橄榄岩熔体不能匹配海南岛玄武岩高的TiO2和低的Al2O3含量。其次,与橄榄岩部分熔融形成的熔体相比,海南岛玄武岩具有较低的CaO含量以及较高的Fe/Mn,Zn/Mn以及Zn/Fe比值(图7)。最后,与地幔橄榄岩部分熔融的熔体结晶出的橄榄石相比,海南岛玄武岩(包括拉斑和碱性玄武岩)中的橄榄石斑晶具有更低的 Ca和Mn含量,以及更高的Ni和Fe/Mn比值(图5),指示了单纯的地幔橄榄岩并不能够作为海南岛玄武岩的源区母岩(Sobolev et al.,2005,2007;Herzberg,2011)。海南岛玄武岩的上述地球化学特征指示了其地幔源区存在非橄榄岩组分的贡献。

图6 海南岛玄武岩与各种超镁铁质岩石高压实验产生的部分熔融体之间的比较Fig.6 Comparison of the Hainan basalts with the high-pressure experimental partial melts of various ultramafic rocks

Dasgupta et al.(2007)和Pilet et al.(2008)的实验岩石学研究分别指出橄榄岩+CO2以及角闪石岩交代脉可以作为板内玄武岩的源区母岩,然而这也不适用于海南玄武岩。首先,橄榄岩+CO2以及角闪石岩交代脉部分熔融形成的熔体只能匹配碱性OIB的组成特征,而海南岛地区则呈现拉斑玄武岩和碱性玄武岩共存的特征。其次,在海南岛玄武岩与各种超镁铁岩高压实验部分熔融体之间的比较图中(图6),橄榄岩+CO2以及角闪石岩熔融形成的熔体不能匹配海南岛玄武岩各种主量元素的组成特征,包括低的MgO以及高的SiO2含量。最后,在原始地幔标准化的不相容元素蛛网图上,碳酸盐化的橄榄岩部分熔融形成的熔体呈现 Zr、Hf、K、Ti的负异常特征(Hirose,1997;Dasgupta et al.,2007;Zeng et al.,2010),以及角闪石岩部分熔融形成的熔体呈现Zr、Hf、K的负异常及 Ti的正异常特征(Pilet et al.,2008)。然而,海南岛拉斑和碱性玄武岩的微量元素蛛网图上均未呈现明显的Zr、Hf、K的异常特征(拉斑玄武岩 Hf/Hf*=0.96±0.19,碱性玄武岩 Hf/Hf*=0.9±0.09)(图3b)。尽管海南岛碱性玄武岩呈现显著的Ti的负异常,然而拉斑玄武岩却未呈现明显的Ti的异常特征。因此橄榄岩+CO2以及角闪石岩交代脉也不能够作为海南岛玄武岩的源区母岩。

图7 海南岛玄武岩的CaO,Fe/Mn,Zn/Mn及Zn/Fe与MgO之间的相关关系图Fig.7 Plots of CaO,Fe/Mn,Zn/Mn and (Zn/Fe)×104 against MgO for the Hainan basalts

我们认为辉石岩作为海南岛玄武岩的源区母岩可以很好地解释其独特的地球化学特征。首先,在海南岛玄武岩与各种超镁铁岩高压实验部分熔融体之间的比较图中,海南岛玄武岩各种主量元素的含量很好地落在辉石岩部分熔融形成的熔体区域(图6)。其次,作为橄榄岩的最主要造岩矿物,橄榄石(DCaOl=0.025,Leeman and Scheidegger,1977)对Ca是极不相容的;而作为辉石岩的最主要矿物组成,单斜辉石(DCaCpx=1.82～1.95,Pertermann and Hirshmann,2002)对Ca是相容的。因此,辉石岩源区部分熔融形成的熔体比橄榄岩源区形成的熔体具有较低的CaO含量(Herzberg,2006,2011;Herzberg and Asimow,2008),这可以很好地解释海南岛玄武岩中低CaO含量的特征(图7a)。类似于夏威夷玄武岩,海南岛玄武岩的橄榄石斑晶中低Ca、Mn以及高Ni、Fe/Mn的特征也指示了其源区有辉石岩的贡献(Herzberg,2011)。再次,海南岛拉斑玄武岩的La/Nb为 0.741～0.916,以及碱性玄武岩的 La/Nb为 0.677～0.737,也指示了海南岛玄武岩源区母岩的岩性为辉石岩(Stracke and Bourdon,2009)。最后,岩相学特征显示只受橄榄石分离结晶的海南岛玄武岩具有高的Zn/Fe,Zn/Mn,Fe/Mn比值(图7b,c,d)。Humayun et al.(2004) 和Qin and Humayun (2008) 将夏威夷玄武岩高的Fe/Mn比值的特征归因于地核物质的贡献:地核中具有高含量的铁,地核-地幔相互作用为下地幔提供大量的铁,因此起源于核幔边界的地幔柱形成的熔岩中具有高的Fe/Mn。然而地核中的Zn含量(～30 µg/g)(Corgne et al.,2008),远小于熔岩所具有的Zn(～120 µg/g),并且核幔交代作用引起的Fe富集会降低 Zn/Fe的比值,因此核幔边界处的地核-地幔交代作用并不是高Zn/Fe,Zn/Mn,Fe/Mn比值的起因(Le Roux et al.,2010)。因此,我们将玄武岩中高的Zn/Fe,Zn/Mn,Fe/Mn比值解释为辉石岩源区部分熔融的产物(Sobolev et al.,2007)。

综上所述,我们认为干的地幔橄榄岩,橄榄岩+CO2以及角闪石岩都不能作为海南岛玄武岩的源区母岩,而辉石岩作为海南岛玄武岩的源区母岩则能很好地解释其独特的地球化学特征,因此Wang et al.(2012)利用在熔体中加橄榄石的方法计算得到的原始岩浆组成及地幔潜能温度需要重新审视。原始地幔标准化的不相容元素蛛网图上,海南岛玄武岩呈现Rb,Ba,Sr,Nb-Ta的正异常以及Th,U的负异常特征,指示了在源区辉石岩的形成过程中有再循环洋壳物质的参与(Hofmann and Jochum,1996)。

3 结 论

海南岛北部广泛分布的新生代玄武岩是由拉斑玄武岩和碱性玄武岩组成的,岩相学观察表明海南岛玄武岩在形成过程中只发生了橄榄石的结晶分异作用。我们分析海南岛14个玄武岩样品的主、微量元素以及80颗橄榄石斑晶的化学组分,结合前人的数据,探讨了海南岛玄武岩源区母岩的岩性。与典型地幔橄榄岩部分熔融形成的熔体相比,海南岛玄武岩具有较低的 CaO含量和较高的 Fe/Mn,Zn/Mn及 Zn/Fe比值;与此同时海南岛玄武岩中的橄榄石斑晶具有较低的Ca、Mn及较高的Ni,Fe/Mn;此外,海南岛拉斑玄武岩的La/Nb为0.741～0.916,以及碱性玄武岩的La/Nb为0.677～0.737,均小于1,指示了海南岛玄武岩源区母岩的岩性为辉石岩。原始地幔标准化的蛛网图上呈现的Nb-Ta,Rb,Ba,Sr的正异常以及 Th-U的负异常特征指示了海南岛玄武岩的源区有再循环洋壳物质的参与。

致谢:中国科学院广州地球化学研究所的熊小林研究员和另一位匿名审稿专家对本文提出了建设性的意见和建议,我们在此表示衷心的感谢。此外,中国科学院广州地球化学研究所的宋茂双研究员,张艳和钱生平同学以及吴蕾女士在样品采集等野外工作中给予了很大的帮助,以及全岩样品主、微量以及电子探针分析得到中国科学院广州地球化学研究所刘颖、胡光黔、涂湘林以及陈林丽老师和吴蕾女士的帮助,在此我们一并表示衷心的感谢。

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