2. 中国科学院大学, 北京 100039;
3. 内蒙古有色地质矿业有限责任公司, 呼和浩特 010010
2. University of Chinese Academy of Sciences, Beijing 100039, China;
3. Inner Mongolia Nonferrous Geological Mining Limited Liability Company, Hohhot 010010, China
富闪深成岩系列是一套以角闪石为标志矿物的侵入杂岩,多以包体、岩瘤、岩株等小侵入体形态与花岗岩基一道产出。一方面,富闪杂岩独特的产出状态和宽泛的岩性组合记录了岩浆从产生、运移、侵位至冷却的完整细节,使其成为反演岩浆演化机理的重要工具(Murphy,2013)。另一方面,富闪杂岩呈现“俯冲型”地球化学亲缘性,与板片断离、洋脊俯冲等具有重要的热力学联系,是示踪会聚板块边缘熔/流体-地幔相互作用,地球动力学演变的重要窗口(Murphy et al.,1990,1999; Atherton and Ghani,2002; Pe-piper et al.,2010; Zhang et al.,2012a,b)。因此,富闪深成岩研究在近一个世纪以来,获得了持续而广泛的关注。本文简要回顾了富闪深成岩的研究历史,在概述其岩相学、地球化学特征的基础上介绍了富闪深成岩的成因、地球动力学意义等,并对目前研究中存在的问题及将来的研究方向进行了探讨。
2 富闪深成岩研究历史简要回顾富闪深成岩(appinite)得名于其发现地苏格兰阿平地区,起初特指与浅成相富角闪石煌斑岩相对应的基性-超基性深成侵入岩(Bailey and Maufe,1916),仅代表一种具体岩性,即“阿平岩”。类似的术语还有“肯特伦岩”(kentallenite,橄榄二长岩,源自苏格兰阿尔盖郡肯特伦杂岩,Hill and Kynaston,1900; Bowes,1962)、“科特兰岩”(cortlandtite,角闪橄榄岩,源自阿巴拉契亚造山带纽约州科特兰杂岩,Williams,1886)、“都尔巴赫岩”(Durbachite,暗云正长岩,源自中欧海西造山带波西米亚都尔巴赫杂岩,Bowes and Košler,1993)等。二十世纪九十年代以前,有关富闪深成岩的研究主要集中于加里东造山带的苏格兰、爱尔兰西北地区(French,1966; Pitcher and Berger,1972; Bowes and McArthur,1976)。随着研究的持续深入,人们逐渐达成共识——富闪深成岩并非拘泥于某一特殊的岩石类型,而是一套复杂的岩石组合。上述具有标志性矿物组成的岩浆岩往往密切伴生,同时发育典型的富流体岩石组构(Bowes et al.,1964; Bowes and Wright,1967; Bowes and McArthur,1976; Bowes,1989),并经常出现在近同时代或略晚的花岗质岩基周缘(Wright and Bowes,1979; Hammidulah and Bowes,1987)。近年来富闪深成岩的研究陆续扩展到其它不同时代的造山带地区(图 1),例如苏必利尔克拉通绿岩地体中的新太古代(约2.7Ga)富闪深成岩(Stern et al.,1989; Wilkin and Bornhorst,1993)、阿巴拉契亚造山带阿瓦隆尼亚地体中的新元古代(622~607Ma)富闪深成岩(Murphy et al.,1997a,b; Pe-pier and Piper,2003; Pe-Piper et al.,2010)、华南华夏造山带奥陶纪(473~452Ma)武功山杂岩(Zhong et al.,2014)、伊比利亚地块中的石炭纪(345~310Ma)富闪深成岩(Bea et al.,1999; Castro et al.,2003)、华北克拉通辽西阜新晚二叠世(257~252Ma)富闪深成岩-花岗岩杂岩(Zhang et al.,2012b)、巴布亚新几内亚中新世(6Ma)富闪杂岩(Richards et al.,1990; Richards and McDougall,1990)。富闪深成岩的概念也随之得以更新与完善。Neuendorf et al.(2005)指出富闪深成岩是一组富含角闪石的暗色侵入岩。然而Murphy(2013)认为这一简练但模糊的定义无法限定富闪深成岩的确切化学组成,也缺乏将岩石系列内部不同端元有机联系起来的内敛性,进而建议采用“富闪深成岩套”(appinite suite)来统称化学成分上可能涵盖超基性到酸性整个谱系的一系列同时代侵入杂岩,标志矿物角闪石既可以作为斑晶产出,也可以是基质的组成部分。
富闪深成岩多以岩株、岩瘤、岩筒、岩墙等小侵入体形态伴生于花岗质岩基周缘(如苏格兰高地志留纪阿许杂岩,Fowler and Henney,1996),或作为复合岩基的有机组成部分(如挪威晚奥陶世霍尔塔维尔岩浆杂岩,Barnes et al.,2003),或呈基性包体散布在花岗质岩基内部(如西昆仑造山带布雅岩体与苏格兰罗加特花岗岩基中的富闪杂岩,Ye et al.,2008; Fowler et al.,2001)。就侵位深度而言,富闪深成岩可固结于从深成岩相(Bea et al.,1999; Barnes and Prestvik,2000; Castro et al.,2003; Pe-Piper et al.,2010)至浅成次火山岩相(Bowes and Wright,1967; Richards et al.,1990; Fowler and Henney,1996; Fowler et al.,2001)等不同地壳层次。这主要依据熔体原位结晶产物的矿物晶出顺序(Murphy et al.,2012)和伴随侵位发生的变质作用(包括混合岩化作用)来判断(Dallmeyer et al.,1997; Barnes and Prestvik,2000; Castro et al.,2003)。
大量实例指示富闪深成岩套可呈现复杂的岩性组合(表 1),并以角闪石(±单斜辉石±黑云母)暗色闪长岩最具特征,其它的岩石类型包括(辉石±)角闪石岩、角闪橄榄岩、金云母苦橄岩、角闪辉长岩,以及少量淡色闪长岩、花岗闪长岩和黑云母花岗岩(Fortey et al.,1994; Fowler and Henney,1996; Bea et al.,1999; Castro et al.,2003; Hamidullah,2007; Zhang et al.,2012b; Zhong et al.,2014)。此外,从角闪石伟晶岩到花岗伟晶岩等成分复杂的伟晶岩透镜体也是许多富闪深成岩系列的重要构成,它们可能代表不同阶段从岩浆中分离出来的富流体不混熔相(Fowler et al.,2001; Pe-Piper et al.,2010; Murphy et al.,2012; Murphy,2013),与“爆破角砾岩筒”和“侵入细粒火山碎屑岩”一道指示富闪深成岩浆的富水特性(Bowes et al.,1964; Bowes and Wright,1967; Pitcher,1997; Hamidullah,2007)。
角闪石是富闪深成岩套中最具标志性的镁铁质矿物,在具堆晶结构的角闪石岩中含量可达90%以上,在角闪辉长岩中也可达80%~90%(Castro et al.,2003; Pe-Piper et al.,2010; Zhong et al.,2014)。角闪石可发育环带或核幔结构(图 2a-c)(Molina et al.,2009; Scarrow et al.,2009),例如伊比利亚阿维拉杂岩基性-超基性端元发育从晶间残留熔体沉淀出来的角闪石钛成分环带(钛闪石→镁铁闪石)(Molina et al.,2009);苏格兰阿盖尔富闪深成岩中普通角闪石外围发育阳起石和透闪石组成的韵律生长边,其形成与流体压力周期性增大及爆破释放有关(Bowes et al.,1964)。除作为独立矿物晶体之外,角闪石还可以和橄榄石、辉石、斜长石(钙质斜长石)形成反应结构(图 2d,e)(Pitcher,1997; Bea et al.,1999; Hamidullah,2007; Scarrow et al.,2009; Pe-Piper et al.,2010)。比较罕见的现象是基性岩浆中长英质捕虏体周围所发育的“角闪石晕”(Castro et al.,2003)。另外,相对次要的黑云母、磷灰石等含水矿物可呈柱状、骸晶状斑晶产出(Pitcher,1997; Pe-Piper et al.,2010; Murphy et al.,2012);同时含水矿物之间也可形成反应结构,如在钾质岩浆中钛角闪石可替代金云母(图 2f)(Molina et al.,2009)。
与庞杂的岩石类型相对应,富闪深成岩杂岩总体上具有非常宽泛的化学组成。就酸度而言,尽管基性端元的岩石在数量上占有明显优势,但多数岩石仍倾向于双峰组合特征,典型的实例如苏格兰地区阿平杂岩(Hall,1967)、阿瓦隆尼亚地体新斯科舍格林戴尔杂岩(Murphy et al.,1997a,2012)(图 3a)、华北克拉通辽西阜新铁匠各冷杂岩(Zhang et al.,2012b)(图 3a)、西班牙海西造山带比韦罗杂岩(Galán et al.,1996)、华南福建平潭-岱前山侵入杂岩(Xu et al.,1999)等。富闪深成岩碱度的变化一般限于亚碱性系列(Pe-piper et al.,2010)(图 3a,b)。譬如,阿瓦隆尼亚格林戴尔杂岩呈低钾拉斑质-钙碱性(Murphy et al.,1997a);伊比利亚阿维拉杂岩为钙碱性-高钾钙碱性(Bea et al.,1999; Scarrow et al.,2009);萨拉布里亚岩浆杂岩为高钾钙碱性-钾玄质(Castro et al.,2003);苏格兰高地富闪深成岩具有钾玄质特征(Fowler and Henney,1996; Fowler et al.,2001)。此外,相对普通辉长质-闪长质岩石,富闪深成岩系列中-基性端元普遍高Mg、Ni、Cr等过渡金属元素(Fortey et al.,1994; Fowler and Henney,1996; Bea et al.,1999; Zhang et al.,2012a; Zhong et al.,2014)。
在微量元素组成方面,富闪深成岩套总体以富集大离子亲石元素(LILEs,如Ba、Sr、Th、U、K、Pb)和轻稀土元素(LREEs),亏损高场强元素(HFSEs,如Nb、Ta、Ti、Zr、Hf)和重稀土元素(HREEs)为特征(图 4),但不同的岩石系列和杂岩体内部不同岩性之间存在明显差异。例如,基性岩的(La/Yb)N比值可以从拉斑质系列的~5(如格林戴尔杂岩,Murphy et al.,1997a)变化到钾玄质系列的>50(如罗加特岩体,Fowler et al.,2001)(图 4b);在伊比利亚萨拉布里亚杂岩中,(La/Yb)N比值可以从基性岩的~10变化到中酸性岩的~100(Castro et al.,2003)(图 4c-e)。
类似地,同位素组成特征也呈现显著的系列间差异和系列内变化。例如,华北克拉通-内蒙三道沟杂岩(全岩εNd=-14~-9,锆石εHf=-18~-9; Zhang et al.,2012a)与阜新铁匠各冷杂岩(全岩εNd=-13~-8,锆石εHf=-19~-10; Zhang et al.,2012b)以及华夏造山带-武功山杂岩(全岩εNd=-8~-3,锆石εHf=-9~-3; Zhong et al.,2014)具有明显富集的全岩Nd和锆石Hf同位素组成;而在英国加里东造山带诸多杂岩体中,全岩εNd变化于-6~+4(Fowler et al.,2008);阿瓦隆尼亚格林戴尔杂岩具有高度亏损的全岩Nd同位素特征,其基性端元εNd=+3~+8,酸性端元εNd=+2~+4(Murphy et al.,1997a; Murphy,2013)。与富闪深成岩稳定同位素有关的研究相对缺乏,目前的分析资料主要来自苏格兰加里东造山带(Fowler and Henney,1996; Fowler et al.,2001,2008)。全岩O同位素分析揭示,富闪深成岩一般具有高于正常地幔的δ18O(>5.3‰,Valley,2003)。
4 富闪深成岩系列的岩石成因4.1 富闪岩的岩浆起源富闪深成岩系列中基性岩石主导的事实决定了地幔源区在其形成过程中的核心地位。首先,一个系列中富集过渡金属元素的基性端元可能更接近初始岩浆,其放射性同位素(Sr-Nd-Hf)组成可以示踪地幔源区。例如,伊比利亚萨拉布里亚杂岩基性端元εNd低于同时代亏损地幔,反映其主要来源于岩石圈富集地幔(SCLM)(Castro et al.,2003);阿瓦隆尼亚格林戴尔杂岩中部分镁铁质岩石εNd(Murphy et al.,1997a)超出SCLM范围(Murphy and Dostal,2007; Murphy et al.,2008),表明其源区既有岩石圈地幔组分,又有软流圈地幔贡献;巴布亚新几内亚波尔盖拉基性杂岩具有亏损的Sr-Nd同位素组成((87Sr/86Sr)i=~0.7035,εNd=~+6),其源区主要为软流圈地幔(Richards et al.,1990)。其次,富闪深成岩系列标志性的“岛弧型”元素地球化学行为(钙碱性,富LILE-LREE)及高δ18O特性暗示其母岩浆起源于交代地幔。鉴于不同来源的交代介质持有各自独特的元素地球化学印记(Plank and Langmuir,1998; Kessel et al.,2005; Portnyagin et al.,2007),利用一些特征的微量元素比值可以进一步约束地幔源区的微观属性及其形成过程。例如,Ba赋存于沉积物中,同时也能被含水流体搬运(McCulloch and Gamble,1991);Sr是高度可溶元素,在榴辉岩相或角闪岩相部分熔融时是不相容元素(Barth et al.,2002),可来自板片流体或熔体;Th来自俯冲沉积物,但在流体中为不活动元素(Johnson and Plank,1999; Turner et al.,2012)。因此,华北克拉通北缘辽西富闪杂岩具有高Ba/La,高Sr/Th比值,反映地幔交代过程主要受板片脱水流体控制;个别岩石样品具有较高的Th/Ce比值,暗示其源区在一定程度上也受沉积物硅质熔体影响(图 5)(Zhang et al.,2012b)。
富闪深成岩系列复杂的岩石组成和宽泛的成分谱系指示其形成涉及复杂的壳幔相互作用(Murphy,2013)。这主要体现在基性母岩浆分离结晶与同化-混染(AFC)以不同的强度比率联合作用(Platten,1991; Bea et al.,1999; Fowler and Henney,1996; Fowler et al.,2001; Castro et al.,2003)以及壳幔岩浆混合作用(Murphy et al.,1997a)。苏格兰阿许-罗加特富闪杂岩由与之伴生的钙碱性-钾玄质煌斑岩经钙质单斜辉石、黑云母、磷灰石、榍石分离结晶及轻微的陆壳混染而形成(Fowler,1988; Fowler and Henney,1996; Fowler et al.,2001)。支持该岩浆过程的地球化学证据为各端元岩石具有近平行的微量元素和稀土元素配分模式(图 6)(Fowler and Henney,1996; Fowler et al.,2008)。而环境相对开放的伊比利亚阿维拉与萨拉布里亚岩浆体系则主要受控于陆壳同化-混染作用。前者的早期结晶相(橄榄石、辉石、钛角闪石)俘获了大量来自过铝质花岗岩和变泥质岩的副矿物(独居石、钍石、磷钇矿、毒砂等)(Bea et al.,1999);后者各岩石端元呈同轴环状产出,暗示核部基性岩浆冷凝过程所释放的流体与周缘陆壳物质相互作用衍生出了幔部中性端元(图 7)(Castro et al.,2003)。除了普遍的AFC过程外,不同阶段补注的基性岩浆与各种性质迥异的壳源熔体或酸性残余岩浆之间的混合作用也同样控制着富闪深成岩系列的岩性组成。主要的岩石学证据包括不同岩性之间高度复杂的接触关系(图 8a,b),混合端元中出现多种异源捕虏晶(图 8c),针状磷灰石包裹体在各种结晶相内频繁出现(图 8d),角闪石、斜长石等发育生长环带或筛状结构(图 8e,f)(Xu et al.,1999; Molina et al.,2012)。同时,利用高精度放射性同位素示踪可在杂岩体中识别出独立的异源岩浆,典型的应用如对阿瓦隆尼亚格林戴尔杂岩及华南福建平潭-岱前山杂岩的研究,杂岩体中比例甚微的闪长岩-花岗闪长岩形成于岛弧基底深熔析出的酸性岩浆与富集地幔析出的基性岩浆之间的混合作用(Murphy et al.,1997a; Xu et al.,1999)。
作为角闪石稳定性及其结构特征的唯一要求,熔体富水体现了富闪岩浆体系的根本特征,也是驱动其演化的决定性因素。大量实验岩石学研究表明,熔体中水含量增加可以促使角闪石稳定域向辉石、橄榄石、斜长石区拓展(图 9)(Moore and Carmichael,1998; Müntener et al.,2001)。一方面,水主导的挥发分体系可以抑制斜长石(贫水相)结晶而促进角闪石(含水相)沉淀(Sisson and Grove,1993; Nekvasil et al.,2004);另一方面,岩浆富水利于熔体解聚(Mysen et al.,1982; Mysen,1988),降低粘度,促进元素离子向矿物生长部位运移,从而加速角闪石成核生长。该岩浆过程反映到具体的岩相学上,即表现为角闪石与橄榄石、辉石形成各种反应结构(Pitcher,1997; Bea et al.,1999; Pe-Piper et al.,2010)或者在含量上显示此消彼长的关系(Fowler et al.,2001; Ye et al.,2008),表明角闪石的生长依赖于富水岩浆与橄榄石、辉石之间的反应。然而随着岩浆不断分异演化,斜长石终会沉淀,并影响与它一道结晶的角闪石的化学成分。因此我们可在富闪深成岩中观察到不同世代的角闪石,并能通过微量元素配分模式等手段加以区分(图 10)(Murphy et al.,2012)。
岩浆富水的另一个要义在于它保证了富闪深成岩的钙碱性演化趋势。部分基性-超基性岩石中钛闪石的出现暗示富闪岩母岩浆可能偏碱性(Richards et al.,1990; Molina et al.,2009),但我们所观察到的绝大多数富闪岩却呈钙碱性。相对于富闪岩中性端元,早期易于析出的钙质斜长石(SiO2=46.2%~47.7%,Na2O≤3.1%)与含水矿物,如钛金云母(SiO2=36.2%~39%)与钛角闪石(SiO2=42.4%~43.1%,Na2O≤2.8%),具有相似的Na2O含量,但SiO2却相对贫乏。它们的分离使得岩浆中Na2O在与SiO2、K2O的增长博弈中居于弱势,导致岩浆向着硅饱和,亚碱性方向演化(Kaszuba and Wendlandt,2000; Barclay and Carmichael,2004; Molina et al.,2009)。一个例外的情形是挪威加里东造山带霍尔塔维尔杂岩的碱度随着岩浆分异而升高,Barnes et al.(2003)将其归结于开放体系下熔体对碳酸盐岩的持续同化所引起的单斜辉石过度分离,并将该过程归纳为如下反应:橄榄石+方解石+熔体Ⅰ→单斜辉石+熔体Ⅱ+CO2↑,即碳酸盐的加入改变了熔体挥发分的组成,也改变了岩浆演化的途径。
5 富闪深成岩的形成环境及其地球动力学意义5.1 富闪岩的形成环境富闪深成岩系列通常产于板块会聚终末期的洋脊-海沟交互场景和后碰撞伸展环境,分别对应板片窗作用(Thorkelson,1996; Thorkelson and Breitsprecher,2005)和板片断离(或岩石圈拆沉),二者均可诱发软流圈上涌,加热含水的大陆岩石圈地幔,产生富闪深成岩母岩浆。与此同时,富闪岩岩浆作用也受控于区域构造作用,多侵位于主剪切带(主要为走滑断裂,包括转换断层)附近(Hutton,1988; Murphy and Hynes,1990; Rogers and Dunning,1991; Galán et al.,1996; Zhang et al.,2012b)。薄弱的断裂带既可以作为岩浆快速转移上侵的通道,也可以作为不同地壳层次来源的岩浆汇聚、混合、均一的绝佳场所(Brown and Solar,1998; Pe-piper et al.,2010; Murphy,2013)。加之岩浆自身富水,结晶过程中不断向岩浆房顶部释放挥发分,加速断层活化与传播(Downey and Lentz,2006),从而确保富闪岩岩浆通道体系作为岩浆补给与运输的干线(Millward et al.,1978; Pitcher,1997; Barnes et al.,2003; Pe-Piper et al.,2010)。
洋脊俯冲往往伴随转换断层、陆缘弧裂解效应。新斯科舍安蒂戈尼什格林戴尔杂岩形成于火山弧岩浆作用衰退期,转换断层作用早期(Murphy et al.,1999),它出露于两条断裂带之间被低绿片岩相火山-沉积岩系充填的拉分盆地内,岩体就位受控于边界断层不定期右旋剪切作用(Murphy and Hynes,1990)。杂岩体中超基性、基性、酸性岩在元素地球化学及Nd同位素组成上分别与寄主岩中大陆拉斑玄武岩、钙碱性玄武质安山岩和流纹质英安岩相当(Murphy et al.,1997a)。富闪深成岩(607±2Ma,Murphy et al.,1997b)与火山岩(≤613±5Ma,Keppie et al.,1990)在时代上的耦合,以及岩性组合与地球化学的相似性表明它们形成于深部长期活跃的同一岩浆弧体系(Murphy et al.,1997a; Murphy,2013),是俯冲晚期(原)冈瓦纳古陆北缘(阿瓦隆尼亚地体)弧后裂谷作用的结果(Murphy et al.,1990)。来自邻区科贝奎高地的青蛙湖岩体也具有同样的产出背景与岩石学特征(Pe-Piper et al.,2010; Muphy,2013)。
后碰撞阶段指主碰撞构造事件(陆-陆对接)之后到板内非造山阶段开始(裂谷化)之前这段复杂地质时期,可发生大规模横推剪切带活动、板片断离、岩石圈拆沉等地质事件,由此引发的强烈壳-幔作用为富闪深成岩的形成创造了条件。苏格兰高地与爱尔兰多尼戈尔富闪岩主要形成于波罗的古陆、阿瓦隆尼亚古陆和劳伦古陆碰撞之后的伸展背景(Atherton and Ghali,2002; Neilson et al.,2009; Conliffe et al.,2010),并受到同时期左旋走滑断层作用控制(Hutton,1988; Rogers and Dunning,1991; Stewart et al.,2001; Dewey and Strachan,2003)。伊比利亚阿维拉、托莱多富闪杂岩形成于冈瓦纳古陆和劳伦古陆碰撞之后的伸展跨塌阶段(Bea et al.,1999,2006; Montero et al.,2004; Scarrow et al.,2009)。西昆仑造山带布雅富闪深成岩-花岗岩岩基是对塔里木克拉通南缘早古生代地体拼贴后俯冲板片断离的响应(Ye et al.,2008)。
个别地区富闪深成岩也可能形成于活动陆缘俯冲板片回撤和板内伸展阶段,前者如挪威晚奥陶世霍尔塔维尔杂岩(Barnes et al.,2003),后者如华南奥陶纪武功山杂岩(Zhong et al.,2014)。此外,伊比利亚萨纳布里亚杂岩形成于板块碰撞缝合过程中局部伸展环境(Castro et al.,2003; Murphy,2013),巴布亚新几内亚波尔盖拉杂岩形成于微板块双边俯冲环境下陆缘弧后区因缺乏洋脊推力而出现的短暂松弛期(Richards et al.,1990)。
5.2 富闪岩的地球动力学意义富闪深成岩母岩浆的形成涉及橄榄岩地幔的交代作用与随后的部分熔融作用。前者与俯冲板片(玄武质洋壳与俯冲沉积物)对上覆地幔楔的改造有关,因而借助富闪杂岩可了解俯冲带元素的迁移机制。尽管早期的地幔楔水化作用有助于降低橄榄岩的固相线温度(Ulmer,2001),但放射性同位素示踪表明许多富闪杂岩都含有新生幔源组分,暗示软流圈上涌带来的热异常才是诱发熔融作用的关键因素(Pe-Piper et al.,2009),进而将富闪深成岩与板片窗作用、岩石圈拆沉等地幔动力学过程联系起来,使其成为俯冲末期脊-沟交互、后碰撞环境的重要时-空标尺。
起源于交代地幔楔部分熔融的富闪深成岩几乎无一例外地产出在会聚板块边缘(或者沿造山带分布)。晚太古代(约2.7Ga)富闪深成岩(Stern et al.,1989; Sutcliffe et al.,1989,1990; Wilkin and Bornhorst,1993)的发现使得这一特殊岩石家族卷入“现代板块构造运动何时开始”这一激烈的争论之中,而它们与赞岐岩类的时-空伴生更有“推波助澜”之功效。作为太古代年轻大陆地壳重要组成的TTG(英云闪长岩-奥长花岗岩-花岗闪长岩)系列,因缺乏与上覆地幔楔反应的直接证据,其能否代表俯冲环境下洋壳消减的证据仍充满争议(Smithies and Champion,2000; Rapp et al.,2003; Condie,2005)。与此不同的是,高Mg、Ni、Cr,富集LILE-LREE的晚太古代富闪深成岩与赞岐岩类对地球早期构造体制演化具有明确的里程碑意义。二者均为滞后于TTG的俯冲末期或俯冲后岩浆作用产物(Wilkin and Bornhorst,1993; Heilimo et al.,2010),支持两阶段成因模式(Smithies and Champion,2000),这意味着地幔楔源区在发生部分熔融之前需要经历广泛而持久的与板片俯冲相关的交代作用。因此,晚太古代-早元古代之交的富闪杂岩无疑为探索现代板块构造的起源提供了新的线索。
尽管富闪深成岩在世界范围内分布非常有限,并且出露尺度极小,但它们与大规模高Ba-Sr花岗岩基的时-空耦合关系及内在成因联系为揭示大陆地壳的增生与演化开辟了新的视角。目前关于高Ba-Sr花岗岩的成因存在两类观点,一部分学者认为高Ba-Sr花岗岩由富闪岩浆经AFC(同化-分离结晶)过程演化而成(Fowler and Henney,1996; Fowler et al.,2001; Qian et al.,2003; Jiang et al.,2012; Peng et al.,2013),另一部分则支持高Ba-Sr花岗岩形成于富闪杂岩基性端元组成的下地壳重熔(Atherton and Ghani,2002; Ghani and Atherton,2006; Zhang et al.,2012b)。显然,上述成因模式提供了两种截然不同的陆壳演化途径,而对于富闪杂岩来说,无论高Ba-Sr花岗岩基形成于岩浆分异过程还是重熔作用,都暗示着它在深部的岩浆作用规模远比我们在地表所观察到的范围要大。更为重要的是,因在岩相学与地球化学上的相似性,具有幔源成因的显生宙高Ba-Sr花岗岩很可能是晚太古代赞岐岩类在显生宙的类似物(Fowler and Rollinson,2012)。因此,富闪深成岩系列在地球分异演化过程中所扮演的角色不容小觑。
6 富闪深成岩研究展望归纳起来,富闪深成岩主要具有以下三个重要特征:1)(角闪石)暗色闪长岩在岩性组合中占据主导地位;2)角闪石异常发育;3)化学组成以高Mg、Ni、Cr,富集LILE-LREE为特征。因此,形成富闪深成岩的首要条件是存在一个富水的交代地幔源区,其次还需要合适的动力学环境与空间条件,以控制岩浆的诱发与侵位。然而纵观富闪深成岩在整个地质演化历史上的分布,它们的出现似乎并没有明显的规律可循。大约在中-晚太古代之交(3.0~2.8Ga)许多绿岩带基性-超基性杂岩中开始出现以角闪石岩或角闪辉长岩为代表的含水单元(Ivanic et al.,2010; Polat et al.,2012; Berger et al.,2013),在晚太古代(约2.7Ga)则出现了具有加里东造山带富闪深成岩典型特征的岩石系列,如环绕苏必利尔湖区出露于加拿大安大略省西南部(Stern et al.,1989; Sutcliffe et al.,1989)、美国密歇根州西北部(Wilkin and Bornhorst,1993)和明尼苏达州东北部(McCall et al.,1990)的富闪杂岩。而事实上也只有晚太古代与晚加里东期(早古生代晚期)的富闪岩系列最相近,其它地质时期的富闪杂岩除了富水这一基本特征外,在侵位层次、共生岩石组合(不一定有煌斑岩、高Ba-Sr花岗岩伴生)、元素化学(不止有钙碱性/钾玄质系列)等方面均有所变化(见表 1)。若晚太古代已经存在岛弧环境下的交代地幔(Smithies and Champion,2000; Heilimo et al.,2010,2013),那么该时期相对于显生宙所具有的特殊性可能就在于它的高热流环境。正如Fowler and Rollinson(2012)所总结的那样,加里东富闪深成岩的“爆发”或许仅代表晚太古代热体制的偶然重现。同时,富闪深成岩与俯冲作用的关系似乎并没有我们所期望的那么紧密,中-新生代是板块构造的活跃期,但我们所发现的富闪深成岩在数量上并不匹配。已报道的巴布亚新几内亚晚中新世波尔盖拉杂岩(Richards et al.,1990; Richards and McDougall,1990)虽然含有大量富流体相,但缺乏Nb、Ta异常,是否为富闪深成岩家族的成员仍存争议(Richards et al.,1991)。因此,不同地质历史时期零星出现的富闪深成岩并没有一成不变,预示着与之相关的俯冲作用或板块构造体制也发生着微妙的变化。在将来的富闪深成岩研究中,除了寻找不同时期的“类似物”以外,还有必要以时间轴为线索对其进行综合审视,以窥探它们所蕴藏的“连续的”动力学变化信息。
另外,富闪深成岩研究虽然已经持续了将近一个世纪,但在全球各造山带地区的研究资料积累仍旧极不均衡(图 1)。相对于国外岩石学家围绕富闪深成岩所做的研究工作,中国学者开展的相关工作还相当有限。近年来有关华北克拉通北缘古生代岩浆作用的研究揭示,该地区亦发育大量富闪杂岩。除文中引用的在研究中明确指出的内蒙古集宁三道沟石炭纪富闪深成岩岩株(Zhang et al.,2012a)和辽西阜新晚二叠世铁匠各冷杂岩(Zhang et al.,2012b)之外,内蒙乌拉特地区的一些石炭纪角闪闪长岩岩株(Wang et al.,2015)、冀北承德高寺台晚古生代基性-超基性环状杂岩(马旭等,2012)、辽北法库的晚二叠世-早三叠世基性杂岩(Zhang et al.,2009,2010),均符合富闪深成岩的野外产出和矿物学特征。华北克拉通北缘位于古老克拉通与显生宙造山带之间的过渡区域,与古亚洲洋的多岛洋体制(Windley et al.,2007; Xiao et al.,2009)相呼应,在洋盆消减-闭合过程中该地区经历了多次“洋壳俯冲-弧陆碰撞-碰撞后伸展”造山旋回,为富闪深成岩的发育创造了地质条件,具有成为世界上又一个发育典型富闪深成岩系列代表性地区的潜在可能性。因此,我们建议将华北克拉通北缘晚古生代富闪深成岩作为一个区域案例开展系统深入的年代学与岩石地球化学研究,并从空间产出、岩相学与矿物学、地球化学亲缘性等方面与世界上其它地区的富闪深成岩进行对比研究。一方面,借助富闪深成岩系列在揭示会聚大陆边缘动力学、壳-幔相互作用过程等方面的独特优势为解析华北克拉通北缘晚古生代地质演化另辟蹊径;另一方面,不同于以往报道的显生宙富闪岩系列,这些晚古生代富闪深成岩直接出露于克拉通内部,局部地区甚至出现晚太古代与古生代类似岩性共存的现象(如旧庙基性杂岩),这不仅是对全球富闪深成岩研究资料的更新、完善,也为直接对比晚太古代与古生代地幔性质提供了新的契机。
如引言所述,富闪深成岩因其独特的岩相学与矿物学特征而成为探讨复杂岩浆演化机理的天然实验室。新世纪以来,微量元素原位测试技术的发展使得这一研究不再局限于定性的观察描述与粗略的全岩地球化学模拟,如角闪石、磷灰石等微区元素化学分析可以为岩浆分异过程获取更直接的信息(Murphy et al.,2012; Bruand et al.,2014)。然而,受早期富闪深成岩研究模式的影响,或碍于富闪杂岩过于鲜明的产出与组成,岩石学家在致力于构建一个具有普适性的岩石成因模型时似乎过多地倾向于解释母岩浆的分凝过程,而对岩浆起源的发掘却略显薄弱。尽管现有的研究一致认为交代地幔楔代表其岩浆源区,相似岩石(钾玄岩、高镁安山岩等)的类比研究似乎也证实了这一点(Grove et al.,2002,2005; ConceiÇão and Green,2004; Tatsumi,2006),但这在很大程度上是依据暗含多解性的全岩微量元素分析得出的结论,而岩石样品微量元素组成无法避免来自源区部分熔融以及母岩浆分离结晶的干扰。因此,对富闪深成岩源区形成过程的解剖还远不够精确,以致对岩浆中必不可少的水的最终来源也尚难定论。在岩石地球化学研究方法的技术革新浪潮中,不同分析手段联合应用与高分辨率示踪工具相结合为解开富闪深成岩系列的终极奥妙提供了可取之道。角闪石在富闪深成岩中属于遍在性矿物,同时含有H、O元素和丰度足够高的微量元素(Hofmann,2003),并且不易受晚期蚀变作用影响,是示踪地幔源区富集过程的理想工具(Powell et al.,2004; Demény et al.,2012; Bartoli et al.,2013)。O同位素在识别陆壳物质直接添加(Hoefs,2010)与年轻表壳物质快速循环(Lackey et al.,2005)方面有显著优势,交代介质与地幔楔之间O同位素的交换程度取决于水/岩(流体/岩石)比率(Halama et al.,2011),而H元素主要存在于含水流体中,其同位素体系对流体引起的交代作用非常敏感(Demény et al.,2012)。此外,岩浆岩中常见副矿物锆石微区原位U-Pb年龄与Hf-O同位素组成在岩石成因解析中的应用已经相当成熟。在现有的地球化分析技术中,激光剥蚀多接收器电感耦合等离子体质谱技术是锆石Hf同位素和矿物微量元素分析的常用方法,离子探针是锆石U-Pb同位素定年和氧同位素分析的常规方法,纳米离子探针则成为H同位素和水含量原位分析的基本工具。这些新手段有望为富闪深成岩的成因研究带来质的突破。
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