汇聚板块边缘是大陆地壳形成、活化和分异的重要场所。碰撞造山作用是板块汇聚作用的主要作用之一,可产生足够的热量,引发深部岩石的高级变质和深熔作用(Brown, 2007, 2010, 2013)。碰撞造山带通常具有顺时针P-T-t变质作用轨迹,在相应的地壳温度和压力条件下,变沉积岩(变杂砂岩和变泥质岩)是最容易发生部分熔融的岩石。但在有水或在特殊构造环境下(如裂谷带或快速剥露的高级变质地体),易熔组分(如基性岩和富含云母的花岗质岩石)也可发生地壳深熔作用。这些不同类型岩石的部分熔融都可以通过两种主要深熔作用方式来实现——含水部分熔融(也叫水致部分熔融)和脱水部分熔融。导致含水部分熔融的水的来源包括:(1) 高压变质作用的脱水作用;(2) 岩石网状裂隙中含水流体的汇聚,和(3) 熔体迁移过程中流体循环和结晶作用出溶的水(Thompson, 2001; Brown, 2010, 2013; Weinberg and Hasalová, 2015)。但是,“干”地壳在更高的温度条件下,发生脱水熔融反应,特征熔融产物指示温度达到800℃以上(Brown, 2010, 2013)。
喜马拉雅碰撞造山带,世界上最年轻且仍在活动的陆-陆碰撞型造山带,较完整地记录了与大陆深俯冲-快速折返-伸展作用等地质作用相关的变形、变质和岩浆作用的重要信息(Hodges, 2000; Yin and Harrison, 2000; Nabelek and Liu, 2004; Yin, 2006; Nábělek et al., 2009),是研究碰撞造山带深部物质物理和化学过程的野外基地,是检验各种造山作用理论和造山带演化模型的天然实验室。喜马拉雅造山带经历了早期(ca.50~30Ma)的地壳增厚作用和进变质作用(Vance and Mahar, 1998; Vance and Harris, 1999; Searle et al., 1999; Vannay and Grasemann, 2001; Gao et al., 2012; Kohn, 2014)和后期的快速折返和减压作用(Kohn, 2014)。喜马拉雅造山带发育三种类型的构造:近东西向展布的南向逆冲推覆构造(MCT、MBT和MFT等)和北向藏南拆离系(STDS)和近南北向展布、东西向伸展的藏南裂谷系(STRZ)(图 1a)。自~60Ma碰撞以来,该造山带经历了广泛的地壳水平缩短和垂向加厚作用(Yin, 2006; Aikman et al., 2008; Diedesch et al., 2016),形成了数条延伸上千千米以上的构造带,如大型逆冲断层-主中央逆冲断层(MCT)、主边界逆冲断层(MBT)和主前锋逆冲断层(MFT),和大型伸展构造-藏南拆离系(STDS)、北喜马拉雅片麻岩穹窿(NHGD)和南北向裂谷系(STRZ)(Zhang et al., 2012)。已有大量研究来限定藏南裂谷系E-W向伸展作用的启动时限(Coleman and Hodges, 1995; Blisniuk et al., 2001; Williams et al., 2001; Jolivet et al., 2003; Jessup and Cottle, 2010; Zhang et al., 2012; Mitsuishi et al., 2012),最近的研究表明藏南裂谷系最早启动可能开始于19Ma(Zhang et al., 2012; Mitsuishi et al., 2012)。
在长度上千千米的喜马拉雅造山带中,广泛发育不同时代和不同地球化学性质的花岗岩类岩石(Le Fort et al., 1987; Harrison et al., 1997, 1999a, b; Prince et al., 2001; Zhang et al., 2004a, 2012; Ding et al., 2005; Aikman et al., 2008, 2012a, b; Searle et al., 2010; King et al., 2011; Zeng et al., 2011, 2012, 2015; Hou et al., 2012; Gao and Zeng, 2014; Wang et al., 2017; 王誉桦等, 2014; 吴福元等, 2015; 高利娥等, 2015, 2016)。花岗岩类岩石包括:(1) 原岩时代为古元古代(ca.1850~1800Ma)(Le Fort, 1981; Ahmad et al., 2000);(2) 原岩时代为新元古代(ca.850~800Ma)(DiPietro and Isachsen, 2001; Richards et al., 2006; Chu et al., 2011; Ahmad et al., 2013; Wang et al., 2017; 许志琴等, 2005);(3) 原岩时代为ca.520~480Ma的花岗岩(Schärer et al., 1986; Miller et al., 2001; Gehrels et al., 2006a, b; Quigley et al., 2008; Cawood et al., 2007; Wang et al., 2012; 许志琴等, 2005; 张泽明等, 2008; 高利娥等, 2012, 2015; 王誉桦等, 2014)和(4) 新生代( < 50Ma)淡色花岗岩。其中新生代淡色花岗岩的形成时间尺度从~45Ma一直到现今,且各主要时代的地球化学性质表现出较大的差异性。这些淡色花岗岩为了解中下地壳物质在碰撞造山过程的物理和化学行为提供了重要的岩石探针。厘定这些淡色花岗岩的产出状态、形成时限、岩石学、地球化学和地球物理性质及其成矿效应等是揭示碰撞造山带构造演化过程中,变质、岩浆和构造作用之间的互馈关系,浅表作用和深部地壳地质作用之间互动关系的关键。
在喜马拉雅造山带,按岩石的演化程度,淡色花岗岩包括二云母花岗岩、白云母花岗岩、含电气石或石榴子石淡色花岗岩和伟晶岩,多数高分异淡色花岗岩含电气石和石榴子石,少数含绿柱石、锡石和铌钽矿等(图 2)。自二十世纪八十年代以来,从不同侧面,大量学者开展了喜马拉雅淡色花岗岩的岩石学和地球化学(Vidal et al., 1982; Debon et al., 1986; Deniel et al., 1987; Harrison et al., 1997, 1999a, b)和地质年代学(Schärer et al., 1986; Harrison et al., 1995; Prince et al., 2001; Aoya et al., 2005; Lee and Whitehouse, 2007)的研究。
已有的结果表明,随着温压条件及原岩化学成分的变化,变泥质岩的部分熔融常常通过几个不同的部分熔融反应来实现,形成一系列地球化学特征不同的花岗岩熔体(Harris and Inger, 1992; Douce and Harris, 1998; Knesel and Davidson, 2002; Zeng et al., 2005a, b)。由于在部分熔融反应中,熔融相的化学计量上的差异,不同类型部分熔融反应形成的花岗质岩浆在元素和同位素地球化学特征上都可能表现出明显的差异性(Inger and Harris, 1993; Douce and Harris, 1998; Harrison et al., 1999a, b; Knesel and Davison, 2002; Zeng et al., 2005c; Gao and Zeng, 2014; Gao et al., 2017)。因此,在造山带构造演化过程中,随着P-T-X(温度-压力-含水量)条件的变化,地壳物质有可能发生不同类型的部分熔融反应,形成地球化学性质不同的花岗质岩浆,这些花岗质岩浆为探讨造山带深部物质如何响应造山过程提供了良好的岩石探针。
喜马拉雅造山带广泛发育淡色花岗岩,早期研究认为这些花岗岩主要形成于28~10Ma(Harris and Massey, 1994; Harris et al., 1995; Guillot and Le Fort; 1995; Harrison et al., 1997, 1999a, b; Simpson et al., 2000; Zhang et al., 2004b及其中的参考文献),是在喜马拉雅高级岩片快速折返过程中高级变泥质岩白云母脱水部分熔融的产物(Le Fort, 1981; Harrison et al., 1997; Douce and Harris, 1998)。但最近的研究揭示了在印度-欧亚大陆陆陆碰撞造山作用期间,喜马拉雅造山带经历了复杂多样的深熔作用,如角闪岩部分熔融作用(Zeng et al., 2009, 2011, 2015; Liu et al., 2014; 高利娥等, 2009)和变泥质岩水致白云母部分熔融作用(Prince et al., 2001; Zhang et al., 2004a; King et al., 2011; Guo and Wilson, 2012; Zeng et al., 2012)。同时,锆石U/Pb地质年代学研究也揭示上述深熔作用可追溯到始新世(~46Ma)(Ding et al., 2005; Aikman et al., 2008; Larson et al., 2010; Zeng et al., 2009, 2011, 2015; Hou et al., 2012; Liu et al., 2014, 2016; Carosi et al., 2014; Gao et al., 2016; 戚学祥等, 2008; 谢克家等, 2010; 胡古月等, 2011)。在本文中,综合近10年来有关喜马拉雅造山带新生代淡色花岗岩研究的进展,来刻画自50Ma以来,喜马拉雅造山带淡色花岗岩在不同时间段的元素和同位素(Sr和Nd)上的差异性,探讨(1) 淡色花岗岩源区的变化;(2) 地壳深熔作用的类型和(3) 与区域构造演化的关系。
2 主要地壳岩石部分熔融反应类型 2.1 变沉积岩部分熔融反应在地壳岩石中,变泥质岩和变花岗岩的部分熔融反应类型基本上相似,包括较低温度下的水致部分熔融和较高温度下的云母脱水部分熔融。沉积岩的部分熔融反应包括含水和脱水部分熔融,在水饱和条件下,主要有以下类型部分熔融反应:
(1) 石英+斜长石+流体=熔体
(2) 石英+斜长石+白云母+水=熔体
(3) 石英+斜长石+黑云母+水=石榴子石+堇青石+熔体
在无水条件下,沉积岩的部分熔融主要通过含水矿物(白云母或黑云母)的脱水部分熔融来实现(Vielzeuf and Holloway, 1988; Le Breton and Thompson, 1988; Douce and Johnston, 1991; Vielzeuf and Montel, 1994; Gardien et al., 1995; Douce and Beard, 1995, 1996; Singh and Johannes, 1996; Stevens et al., 1997; Douce and Harris, 1998)。常见的部分熔融反应有:
石英+斜长石+白云母±黑云母=铝硅酸盐±石榴子石±钾长石+熔体
石英+斜长石+黑云母+铝硅酸盐=石榴子石±堇青石±钾长石+流体
石英+斜长石+黑云母=斜方辉石+石榴子石+钾长石+熔体
在上述部分熔融反应中,其中白云母脱水部分熔融(MS1) 和水致白云母部分熔融(MS2) 反应可能是最常见的部分熔融反应,具体为
白云母脱水部分熔融:22Ms+7Pl+8Qtz=25Melt+5Kfs+5Sil+2Bt (MS1)
水致白云母部分熔融:9Ms+15Pl+7Qtz+xH2O=31Melt (MS2)
其中MS=白云母;Pl=斜长石;Qtz=石英;Kfs=钾长石;Sil=矽线石;Bt=黑云母;Melt=熔体。
值得指出的是,在高压(P≥10kbar)条件下,钠长石相对于钙长石稳定,形成熔体应较富集Ca、Sr、Eu及其与Ca相似性质的元素(Douce and Harris, 1998)。在变泥质岩部分熔融过程中,水的存在可以稳定含水相。在高压和低温条件下,由于水的存在,白云母和黑云母趋于稳定,当P=10kbar,T=700~775℃,富钾沉积岩可以通过水致熔融产生富Na2O和CaO的熔体(Douce and Harris, 1998)。在相似的温压条件下,脱水熔融产生的熔体具有明显相反的特征,富K2O,相对亏损CaO和Na2O(Conrad et al., 1988; Douce, 1996; Douce and Harris, 1998)。在南迦巴瓦地区,赋存于变泥质岩的~25Ma淡色体具有与苦堆穹窿~28Ma二云母花岗岩相似的特征(Zhang et al., 2004b; King et al., 2011)。与较年轻淡色花岗岩相比,这些花岗岩含蓝晶石,具有较高的CaO和较低的结晶温度,代表了高压条件下变泥质岩水致部分熔融的产物(Zhang et al., 2004a; King et al., 2011; Zeng et al., 2012)。
2.2 基性岩部分熔融反应在地壳温压条件下,基性岩石(玄武岩、辉长岩或角闪岩)的部分熔融通过角闪石脱水部分熔融来实现(Wolf and Wyllie, 1991, 1993, 1994; Wyllie and Wolf, 1993; Rapp et al., 1995; López and Castro, 2001),常见的部分熔融反应有:
角闪石+斜长石+石英+水=石榴子石+单斜辉石+榍石+熔体
角闪石+斜长石+石英=石榴子石+单斜辉石+熔体
上述两种反应都会形成由石榴子石和辉石组成的残留相,具有与榴辉岩类似的矿物组成和物理性质,密度比通常的上地幔岩石高0.2~0.3g/cm3。花岗岩岩基根部或增厚地壳的部分熔融主要是角闪岩脱水部分熔融,都可形成石榴辉石岩或榴辉岩(取决于角闪岩中角闪石和斜长石的组分)。富钙角闪岩部分熔融形成石榴辉石岩;富钠角闪岩部分熔融可能形成严格意义上的榴辉岩残留体。在高压和含水条件下,黑云母部分熔融可在温度低于800℃的条件下发生,形成石榴子石+角闪石+熔体的组合(Gardien et al., 2000), 也形成高密度的石榴角闪岩残留体;这种高密度残留相导致重力失稳,是引发造山带或大陆岛弧大规模拆沉作用的主要因素(Ducea and Saleeby, 1996; Saleeby et al., 2003; 曾令森等, 2006)。值得指出的是,地壳岩石常常含两种或两种以上的含水矿物,如角闪岩中,常出现角闪石和黑云母共存的情况,这种岩石的部分熔融行为较复杂,取决于温度、压力、是否有自由水及含水矿物的化学组成,有可能同时发生角闪石和黑云母脱水部分熔融的情况,形成具有混合特征的花岗质熔体。
3 地壳岩石部分熔融过程中矿物反应在地壳深熔作用中,与造岩矿物直接参与部分熔融不同,副矿物(锆石、磷灰石、独居石、榍石等)通常通过溶解作用,间接调整花岗质熔体的微量元素和同位素地球化学特征。要正确解译花岗质熔体的关键微量元素(如REE)和放射性同位素(Sr、Nd和Hf)的地球化学特征,需要了解不同副矿物在花岗质熔体中的溶解行为。
3.1 磷灰石在岩浆中的溶解度磷是磷灰石的必要结构组分之一,与磷灰石保持平衡关系的岩浆的磷的含量受磷灰石的溶解度的缓冲。实验结果表明了在熔体中,磷灰石的饱和度是熔体温度和成分的函数,在水含量为0~10%范围内,与熔体的含水量无关(Harrison and Watson, 1984; Pichavant et al., 1992; Wolf and London, 1994; London et al., 1999)。磷灰石的溶解度可以用磷灰石和熔体之间磷的配分系数来描述,磷的配分系数越小,越多的磷灰石要溶解到熔体中,直到熔体达到磷的饱和浓度。Harrison and Watson (1984)给出了下述公式来描述磷在熔体中的溶解度:
其中SiO2是熔体中SiO2的重量百分比,T是绝对温度。该方程适用于SiO2含量为45%到75%,含水量小于10%的熔体,及在地壳压力条件下。对过铝酸性岩浆,Pichavant et al. (1992)进一步的实验研究对Harrison-Watson模型进行了进一步的修正,认为:
其中P2O5HW按公式(1) 来计算,P2O5Per按下述公式来计算:
其中,A/CNK是Al/(2Ca+K+Na)的摩尔比值,PMR、HW和Per分别代表Pichavant-Montel-Richard、Harrison-Watson和过铝质。修正的模型可以解释过铝质花岗岩的较高的P2O5。
其中CApP=43%。
3.2 独居石溶解模型轻稀土元素(从La到Gd)及磷都是独居石基本组构组分(essential structural constituent)。实验研究表明:(1) 岩浆中的磷的浓度控制着含磷矿物的稳定性,如果磷足够多,独居石在控制岩浆的LREE的浓度中起重要作用;(2) 岩浆中的LREE饱和浓度是熔融体温度、成分及含水量的函数。在变泥质岩和高铝花岗岩中,独居石是轻稀土元素和生热元素U和Th的主要赋存矿物之一。独居石的稳定性影响着地壳深熔作用导致的地壳中包括轻稀土、U和Th在内的微量元素的重新分布。因此了解独居石在地壳深熔作用中的地球化学行为是认识某些花岗岩地球化学性质的关键。
在部分熔融过程中,独居石是否参与熔融作用(即独居石溶解到岩浆中)或在岩浆的演化后期是否结晶独居石取决于熔体的磷及LREE的浓度。独居石在贫钙的酸性岩浆中的溶解度可以通过以下公式来计算(Montel, 1993)
其中
在部分熔融稳定低于800℃的条件下,取决于源区的初始LREE的浓度,深熔作用产生过铝花岗质岩浆,相应熔融区残留部分则富集独居石。但是当熔融温度接近于850℃时,地壳深熔作用导致的下地壳富集独居石不太可能,因为在高温情况下产生的岩浆具有非常高的独居石饱和溶解度(Rapp and Watson, 1986)。在地壳深熔作用中,独居石的稳定程度是缓冲岩浆P2O5的浓度的主要因素之一。在磷灰石和独居石共存的情况下,根据Harrison-Waston的磷灰石溶解模型,磷灰石的溶解将抑制独居石在岩浆中的溶解程度,在致使岩浆的P2O5含量是否达到饱和上,独居石比磷灰石所起的作用弱些。
3.3 锆石溶解模型锆元素是锆石的必要结构组分。前人对不同成分岩石部分熔融所产生的熔体的锆溶解度实验结果表明在不同成分的岩浆(SiO2含量为54.2%~69.5%,M=1.01~2.09) 中,锆的饱和程度取决于岩浆温度和岩浆成分(Watson and Harrison, 1983; Harrison and Watson, 1984; Watson and Harrison, 1984)。锆石在岩浆中的溶解度满足下述方程:
其中DZrZircon/Melt是Zr在锆石和岩浆中浓度的比值,T绝对温度,和M是阳离子的含量比值,由公式(Na+K+2Ca)/(Al×Si)来计算。值得指出的是,在这些实验中,过碱性岩浆具有异常高的Zr溶解度(Watson, 1979; Watson and Harrison, 1983),如Watson and Harrison (1983)实验中的第21次实验,熔体具有9.8%的Na2O+K2O,为过碱性,Zr含量为860×10-6,熔体初始温度为860℃。这种现象的原因是,Zr4+与熔体中不与Al结合的自由碱离子(K+或Na+)形成络合物,Zr在过碱性岩浆中的溶解度增加。但对于非过碱性岩浆,Zr4+和碱离子不形成络合物。
最近的系统研究表明了上述公式可以用来估算岩浆的温度,即锆石饱和温度计(Miller et al., 2003)。他们的研究结果表明:如果岩浆中Zr不饱和,锆石饱和温度计所给出的锆石饱和温度(TZr)为初始岩浆温度的下限;而如果岩浆中Zr达到饱和状态,所给出的锆石饱和温度为初始岩浆温度的上限。如果已知某一岩浆的主要成分及锆的含量(Zrmelt),可以根据下述公式来计算锆石饱和温度(Miller et al., 2003):
Ryerson and Watson (1987)实验研究了压力为8~30kbar,温度在1000~1300℃,在含水-CO2饱和及不含水条件下,金红石在硅酸盐岩浆中的溶解度。硅酸盐岩浆的TiO2饱和溶解度由下述公式给出:
其中Cmelt和Crutile分别是熔体和金红石中TiO2的浓度;T为温度,单位为K;P为压力,单位为kbar;FM是岩浆的成分参数,由下述公式计算:
其中金属阳离子为摩尔数。压力的大小轻微影响TiO2在硅酸盐岩浆中的溶解度,但含水量对其几乎没有影响(Green and Pearson, 1987; Ryerson and Watson, 1987)。
4 喜马拉雅造山带地壳深熔作用的表现方式和产物地壳深熔作用表现在不同的尺度下,(1) 野外露头尺度(米-几十米级);(2) 手标本尺度(厘米级);(3) 显微尺度(亚毫米级)。高喜马拉雅结晶岩系由石榴黑云母片麻岩、眼球状片麻岩、变泥质岩(含变沙质岩块体或层)、石榴斜长角闪岩、含透辉石大理岩及石英岩组成。其中角闪岩、泥质岩和黑云母片麻岩都不同程度地混合岩化,包含厚度不等(亚毫米至几个厘米)的浅色体。浅色体类型包括:(1) 含石榴子石和角闪石的浅色花岗岩,以近平行于面理或呈梭形透镜体的形式,存在于斜长角闪岩中;(2) 含石榴子石淡色体;(3) 含电气石淡色体。同构造花岗岩脉呈两种产状,平行于面理和垂直/斜交面理。
喜马拉雅碰撞造山带发育多种典型的新生代S-型花岗岩。这些花岗岩以透镜体、岩脉或大型岩体的形式分布于北喜马拉雅片麻岩穹窿和高喜马拉雅结晶岩系中(图 1)。众多喜马拉雅淡色花岗岩体都存在多幕淡色花岗岩,在短短的几个百万年内,形成年龄和地球化学特征存在明显差异的淡色花岗岩。如玛纳斯鲁(Manaslu)岩体包括至少两阶段淡色花岗岩,早期形成于22~24Ma和后期形成于19.0~19.6Ma(Harrison et al., 1999b);佩古错岩体包含28.2±0.5Ma电气石淡色花岗岩和19.8±0.5Ma二云母花岗岩(Gao et al., 2013);在夏如穹隆中,除了原岩形成于古生代的淡色花岗质片麻岩外(Liu et al., 2016),还包括分别形成于~35Ma和~28Ma的含电气石淡色花岗岩(Liu et al., 2016; Gao et al., 2016)。因此,要深入了解各个淡色花岗岩体的形成过程和成因机理,需要细致确定它们的形成年代和全岩地球化学特征。
5 喜马拉雅造山带各主要阶段淡色花岗岩的地球化学特征在利用花岗岩全岩元素地球化学组成来反演地壳深熔作用的源区和部分熔融反应类型的过程中,需要甄别它们是否代表较原始的熔体。在花岗质熔体的形成、抽取、汇聚、融合和最终的侵位过程中,影响花岗岩最终组分的因素包括:源区特征、部分熔融反应类型、岩浆形成后的后期岩浆作用(包括分离结晶作用和同化混染作用)。在分析大量数据的基础上,考虑它们的矿物组合、元素和同位素(Sr和Nd)组成及其副矿物在花岗质熔体的溶解行为(Miller and Stoddard, 1981; Miller and Mittlefehldt, 1982, 1984; Mittlefehldt and Miller, 1983; Miller, 1985; Shearer and Papike, 1987; Watt and Harley, 1993; Bea, 1996; Sylvester, 1998; Zeng et al., 2005a, b; Claiborne et al., 2006; Clarke, 2007; Stevens et al., 2007; Ballouard et al., 2016; Liu et al., 2016; Gao et al., 2017; 吴福元等, 2015; 郭春丽等, 2017),可利用以下判别标志(表 1)来甄别较原始岩浆, 其中较原始的花岗质岩浆是指(1) 对于某类原岩,全岩花岗岩的矿物组成和主量元素地球化学特征较接近实验岩石学产生的熔体组分的那些花岗岩;或(2) 对某一岩体,全岩地球化学(主量元素和微量元素)组成演化程度最低的样品。从始新世到中新世,按其形成时代、元素和同位素地球化学特征,喜马拉雅淡色花岗岩可分为始新世(包括二云母花岗岩、淡色花岗岩和淡色花岗玢岩),渐新世淡色花岗岩和中新世(25~10Ma)淡色花岗岩。其中始新世淡色花岗岩包括二云母花岗岩、淡色花岗岩和淡色花岗玢岩(图 2a, b),渐新世主要为分异程度较高的淡色花岗岩,中新世包括A、B、C和F四类淡色花岗岩,其中A和B类代表较原始的岩浆。各类花岗岩的总体元素地球化学特征见表 2。
沿喜马拉雅造山带,始新世(ca.41~46Ma,锆石U-Pb年龄)地壳深熔作用表现为(1) 形成较原始的二云母花岗岩(TMG)(Zeng et al., 2011, 2015; Hou et al., 2012; Liu et al., 2014; 戚学祥等, 2008; 高利娥等, 2009; 谢克家等, 2010)及其演化程度较高的淡色花岗岩(LG)和淡色花岗玢岩(LGP)(Zeng et al., 2015; 胡古月等, 2011);(2) 石榴子石包裹的纳米花岗岩(Carosi et al., 2014)。另外除了雅拉香波-隆子地区(图 1b)和然巴穹窿外(图 1),在萨嘎的纽库地区也发育年龄相近的二云母花岗岩(Ding et al., 2005),但未见全岩地球化学数据。
5.1.1 始新世二云母花岗岩始新世二云母花岗岩具有以下特征:(1) 较高的SiO2(68.3%~72.5%)和Al2O3(15.1%~17.0%);(2) 较低的FeO( < 2.0%)和MgO( < 1.5%)(图 3);(3) Na2O/K2O(0.94~1.42) 和A/CNK(1.04~1.23) 比值都较高(图 4),表明它们是富钠过铝质花岗岩。
二云母花岗岩都富集LREE((La/Yb)N>20.0,高达56.8),亏损HREE且HREE(从Ho到Lu)较平直,(Ho/Yb)N比值为0.88到1.50,具弱或无Eu异常(Eu/Eu*=0.79~1.10)(图 5a)。在微量元素地球化学特征上,二云母花岗岩具有(1) 较高的Sr(252×10-6~416×10-6)但较低的Y( < 8.5×10-6),和较高的Sr/Y比(32.8~96.4)(图 6);(2) 与同期的淡色花岗岩和淡色花岗玢岩相比,在相似的Rb含量下,这些二云母花岗岩的Sr含量高2倍以上(图 7);和(3) 较低的Na/Ta比值( < 13.4) 和变化范围较大的Zr/Hf(28.6~36.5) 比值(图 8)。二云母花岗岩的87Sr/86Sr(t)和εNd(t)值都较低,为0.7112~0.7193和-14.9~-9.2(Zeng et al., 2011, 2015; Liu et al., 2014)(图 9)。
在主量元素组成上,该套淡色花岗岩的特征如下:(1) SiO2和Al2O3含量较高,分别为72.8%~75.5%和14.4%~17.0%;(2) TiO2( < 0.1%)、FeO( < 0.8%)、MgO( < 0.3%)和MnO( < 0.1%)含量较低(图 3);(3) 具有较高的Na/K比值(Na2O/K2O=1.10~2.29) 和A/CNK(1.05~1.63)(图 4),显示过铝质的特征,表明这些淡色花岗岩为过铝质富钠花岗岩。
在稀土元素上,该套花岗岩表现出较复杂的稀土配分模式,具有以下特征:(1) LREE总体上表现出较一致,但从MREE到HREE,含量变化较大;(2) 在11件样品中,2件样品具有强分馏的稀土元素分配模式,富集LREE,亏损HREE(图 5);(3) 与二云母花岗岩不同的是,除了具有强烈负Eu异常(Eu/Eu*=0.15~0.55) 的样品外,个别样品具有强烈的正Eu异常或无Eu异常(Eu/Eu*=1.03~1.46)(图 5b)。和二云母花岗岩相比,该套淡色花岗岩的Rb(92.5×10-6~206×10-6),Sr( < 172×10-6)、Ba( < 370×10-6)(图 6、图 7)和Zr( < 45×10-6)(图 8)都较低,Na/Ta(3.7~9.7) 和Zr/Hf(9.4~20.0) 比值较低(图 8),明显低于球粒陨石,但Rb/Sr明显较高(0.98~2.68)。淡色花岗岩的87Sr/86Sr(t)和εNd(t)值都较低,为0.7112~0.7153和-12.5~-9.8(Zeng et al., 2011, 2015),与二云母花岗岩的类似(图 9)。
5.1.3 始新世淡色花岗玢岩到目前为止,以花岗玢岩状态产出的淡色花岗质岩石仅在隆子县扎西岗-隆子县城一带出露(图 1b)。在主量元素组成上,该套岩石的特征如下:(1) SiO2(73.9%~77.3%)和Al2O3(13.8%~16.0%)含量较高;(2) TiO2( < 0.05%)、FeO( < 0.8%)、MgO( < 0.3%)和MnO( < 0.03%)含量较低(图 3);(3) 具有较高的Na/K比值(Na2O/K2O=1.09~2.12) 和A/CNK(1.14~1.71)(图 4),表明这些淡色花岗岩为过铝质富钠花岗岩。
在稀土元素上,与二云母和淡色花岗岩相比,该套花岗岩具有以下特征:(1) LREE含量较低,HREE相似;(2) 多数样品表现出四分组REE分配模式(胡古月等, 2011);(3) 明显的负Eu异常(Eu/Eu*=0.45~0.65)(图 5c),与高分异淡色花岗岩的特征类似。和二云母花岗岩相比,该套淡色花岗岩的Rb(134×10-6~275×10-6),Sr( < 92×10-6)、Zr( < 30×10-6)和Ba( < 135×10-6)都较低(图 6、图 7);(4) Na/Ta(4.8~7.0) 和Zr/Hf(14.9~18.6) 比值较低(图 8),明显低于球粒陨石,但Rb/Sr(0.98~2.68) 明显较高和Sr/Y(8.1~27.4) 比较低(图 6、图 7)。与同期的二云母花岗岩和淡色花岗岩相似,87Sr/86Sr(t)和εNd(t)值都较低,为0.7112~0.7185和-13.8~-13.2(图 9)(Zeng et al., 2011, 2015)。
5.2 渐新世淡色花岗岩在喜马拉雅造山带,已报道的具有年龄和地球化学限定的渐新世淡色花岗岩都是含石榴子石/电气石淡色花岗岩,包括夏如穹窿(Gao et al., 2016),苦堆穹窿(Zhang et al., 2004b; King et al., 2011)、康巴穹窿(Liu et al., 2016)和佩古错穹窿(Aoya et al., 2005; Gao et al., 2013, 2016; 张进江等, 2011; 王晓先等, 2015, 2016)。另外昌果穹窿也发育年龄~31Ma的淡色花岗岩(Larson et al., 2010),但未报道地球化学数据。
在主量元素组成上,该期淡色花岗岩表现为:(1) SiO2(72.3%~78.0%),Al2O3(12.9%~15.1%)和FeO(0.1%~2.1%)含量变化都较大;(2) TiO2( < 0.3%)、MgO( < 0.6%)和MnO( < 0.05%)含量较低(图 3);(3) Na/K比值(Na2O/K2O=0.37~0.91) 较低和A/CNK(1.00~1.14) 较高(图 4),为过铝质富钾淡色花岗岩。
在稀土元素上,该期花岗岩具有以下特征:(1) LREE含量较低,LREE和HREE含量相似,呈现出“海鸥状”的稀土分配模式;(3) 明显的负Eu异常(Eu/Eu*=0.08~0.81)(图 5d),与高分异花岗岩的特征相似(Gao et al., 2016; Liu et al., 2016; 吴福元等, 2015)。
该期花岗岩具有(1) 较高的Rb(281×10-6~712×10-6),但Sr(10.8×10-6~171×10-6),Ba(17.9×10-6~519×10-6)和Zr( < 19.9×10-6~133.0×10-6)变化较大,多数样品的Sr含量小于100×10-6,Zr含量小于80×10-6(图 6、图 7);(2) Nb/Ta(1.5~5.8) 和Zr/Hf(12.3~27.9) 比值较低,明显低于球粒陨石(Nb/Ta=19.9;Zr/Hf=36.0)(图 8);(3) 较高的Rb/Sr(1.64~45.19),多数样品的Rb/Sr比值大于6.0,但Sr/Y(0.7~17.4) 比较低(图 6、图 7)。
该期淡色花岗岩的87Sr/86Sr(t)较高且变化较大,为0.7537~1.0648,但εNd(t)值较低,和-13.8~-9.4(图 9)(Zhang et al., 2004a; King et al., 2011; Gao and Zeng, 2014; Gao et al., 2016)。虽然在Sr同位素组成上,与始新世花岗岩相比,该套淡色花岗岩明显不同,但两者具有相似的Nd同位素组成。值得指出的是,夏如渐新世淡色花岗岩具有异常高的初始Sr同位素组成(87Sr/86Sr(t)>0.8400)(Gao et al., 2016),可能反映了在喜马拉雅造山带折返早期,以黑云母脱水部分熔融为主的部分熔融作用。
在已经报道的渐新世淡色花岗岩中,除了个别样品保存较原始熔体特征外,多数样品都经历了不同程度的岩浆分异作用,表现为较低的Sr和Ba(图 6),较高的Rb和Rb/Sr比值(图 7),“海鸥状”的REE分配模式(图 5d)。
5.3 早中新世淡色花岗岩沿喜马拉雅造山带,无论是高喜马拉雅带还是特提斯喜马拉雅带,该期淡色花岗岩都广泛发育,是喜马拉雅新生代花岗岩的主体(Le Fort, 1981; Schärer et al., 1986; Debon et al., 1986; Harris and Massey, 1994; Harris et al., 1995; Harrison et al., 1997, 1999a, b; Searle et al., 1999, 2010; Simpson et al., 2000; Daniel et al., 2003; Zhang et al., 2004a; Aoya et al., 2005; King et al., 2011; Aikman et al., 2012a, b; Guo and Wilson, 2012; Zeng et al., 2012; Gao and Zeng, 2014; Gao et al., 2017; 于俊杰等, 2011; 吴福元等, 2015; 胡古月等, 2016; 王晓先等, 2016; 及其中的参考文献)。几乎在每一个淡色花岗岩岩体,都发育含黑云母淡色花岗岩,淡色花岗岩和含石榴子石或电气石的淡色花岗岩,个别岩体还发育含绿柱石粗粒淡色花岗岩和伟晶岩。如位于错那北部的拿日雍措穹隆,从西到东呈现为一向西倾斜的淡色花岗岩岩体,从西部的二云母花岗岩,过渡到淡色花岗岩,再到含石榴子石/电气石淡色花岗岩,最后到最东边的含绿柱石伟晶花岗岩,同时局部含堆晶作用形成的电气石团块(电气石岩),主体花岗岩形成于20.0±0.1Ma,侵入到十字石石墨片岩的淡色花岗岩脉形成于21.8±0.3Ma(锆石U-Pb年龄)(高利娥等, 2017)。其他淡色花岗岩体也表现出相似特征。为探讨在喜马拉雅造山带构造演化过程中,中下地壳岩石部分熔融的类型和作用方式,需要甄别较原始的淡色花岗质熔体。与基性岩相比,甄别较原始的花岗质熔体特征难度较大,至今还未有统一的标准。在本文中,较原始的花岗质岩浆是指(1) 对于某类原岩,全岩花岗岩的矿物组成和主量元素地球化学特征较接近实验岩石学产生的熔体组分的那些花岗岩;或(2) 对某一岩体,全岩地球化学(主量元素和微量元素)组成演化程度最低的样品。根据各类淡色花岗岩的矿物组合和结构特征,结合各自的元素和同位素地球化学特征,依据表 1的判别原则,把这些淡色花岗岩分为四类:A类、B类、C类和F类淡色花岗岩。其中A和B类都代表较原始的岩浆组分,C类代表经历明显堆晶作用的淡色花岗岩,在矿物组成上,斜长石或钾长石含量明显较高,同时在手标本尺度上,常常含富集长石的条带或团块,类似现象在淡色花岗岩体中普遍存在。F类为经历明显分离结晶作用的淡色花岗岩,表现为缺乏云母矿物,斜长石的钠长石组分明显升高甚至以钠长石为主。各类花岗岩的元素和同位素地球化学特征(表 2)分述如下。值得指出的是,即使是相同源区和相同的部分熔融反应,由于部分熔融程度的差异,形成的淡色花岗质熔体都可以在主量和微量元素地球化学特征上表现出一定程度的内在变化,但总体上,不同类型的部分熔融反应所形成的熔体在主量和微量元素地球化学特征应表现出明显的差别。
5.3.1 A类淡色花岗岩:水致白云母部分熔融在A类淡色花岗岩中,SiO2(69.5%~77.1%)、Al2O3(12.9%~16.2%)和CaO(0.6%~2.0%)都变化较大,但多数花岗岩的CaO大于1.0%;FeO( < 2.0%)、MgO( < 0.7%)、MnO( < 0.05%)和TiO2( < 0.31%)都较低(图 3)。除个别样品外,Na2O/K2O比值较高外,多数比值小于1.0,且A/CNK比值大于1.0,高达1.3(图 4),为过铝质花岗岩。
这些淡色花岗岩富集轻稀土(LREE),亏损重稀土(HREE)((La/Yb)N>12.5);显示较平直的重稀土分布特征((Gd/Yb)N=2.9~5.0) 和几乎无或弱负Eu异常(Eu/Eu*=0.7~0.9)(图 5e)。该类淡色花岗岩具有较高的Ba(>305×10-6)和Sr(>105×10-6),较低的Rb( < 270×10-6)和Rb/Sr比值( < 2.2)(图 6、图 7)。同时,随着Ba浓度的增加,Rb/Sr比值几乎保持不变(图 6),与变质岩水致白云母部分熔融的特征一致(Inger and Harris, 1993; Zeng et al., 2005b, c; Gao and Zeng, 2014; Gao et al., 2017)。Nb/Ta和Zr/Hf比值都小于球粒陨石,分别为5.3~10.2和24.1~40.6(图 8)。较均一的放射性同位素(Sr和Nd)组成(图 9),其中87Sr/86Sr(t)=0.7391~0.7523,εNd(t)=-14.4~-13.7(Gao and Zeng, 2014; Gao et al., 2017; 王晓先等, 2015)。
5.3.2 B类淡色花岗岩:白云母脱水部分熔融与A类淡色花岗岩相比,B类淡色花岗岩的SiO2(72.4%~74.2%)、Al2O3(14.2%~15.4%)都变化范围都较小,FeO、MgO、MnO和TiO2也都较低(图 3)。A/CNK比值同样大于1.0,高达1.28。除个别样品外,多数样品的Na2O/K2O比值小于1.0和CaO小于0.9%(图 3、图 4)。
与A类淡色花岗岩不同的是,此类花岗岩的稀土元素地球化学特征较一致(图 5f),表现为(1) 轻重稀土分馏明显,富集轻稀土(LREE),亏损重稀土(HREE),(La/Yb)N=4.56~19.8;(2) 较显著的负Eu异常(Eu/Eu*=0.37~0.71)(图 5f)。B类淡色花岗岩具有较高的Rb(>325×10-6)(图 6),但较低的Ba( < 265×10-6)和Sr( < 76.0×10-6)(图 7)和Rb/Sr比值较高(>4.6)。Ba和Rb/Sr比值呈负相关关系,随Rb/Sr比值的升高,Ba浓度降低(图 7),反映了变泥质岩白云母脱水部分熔融的特征。Nb/Ta和Zr/Hf比值都低于球粒陨石且变化较大(图 8),分别为3.2~10.3和17.0~33.5。较均一的放射性同位素(Sr和Nd)组成(图 9),其中87Sr/86Sr(t)=0.7441~0.7604,εNd(t)=-14.7~-12.3(Gao and Zeng, 2014; Gao et al., 2017)。
5.3.3 C类淡色花岗岩:堆晶淡色花岗岩除了与典型淡色花岗岩相似的低FeO、MgO和TiO2,高A/CNK等特征外(图 3、图 4),该类花岗岩最显著地特征是显著的正Eu异常(图 5g)和多数样品具有高CaO(>1.1%)(图 3)或较高的K2O(>5.0%),代表斜长石堆晶或部分熔融过程中捕获源区转熔型或残留钾长石的效应。无论在主量元素,还是REE或微量元素地球化学特征上,该类花岗岩都呈现高度变化特征,如(1) SiO2(70.8%~76.3%)、Al2O3(13.6%~16.4%)和CaO(0.5%~2.7%)(图 3);(2) 较复杂的REE分配模式,从轻重稀土高度分馏到不分馏((La/Yb)N=1.0~21.1)(图 5g);(3) 高度变化的Ba(14×10-6~2467×10-6)、Sr(22×10-6~439×10-6),Rb(43×10-6~375×10-6)和Rb/Sr比值(0.2~15.9)(图 6、图 7);(4) 高度变化的Nb/Ta(1.4~11.9) 和Zr/Hf(13.5~35.6) 比值(图 8)。
5.3.4 F类淡色花岗岩:高度分异淡色花岗岩与前三类淡色花岗岩相比,该类花岗岩含石榴子石、电气石或绿柱石(图 2),与渐新世淡色花岗岩相似。该类花岗岩除了较高的SiO2外,其它主量元素和其它淡色花岗岩相似(图 3)。虽然在Rb/Sr-Sr和Rb/Sr-Ba的系统关系上与白云母脱水部分熔融形成的熔体相似(图 7),这些花岗岩最显著地特征是具有“海鸥型”稀土分布模式和强烈的负Eu异常(Eu/Eu* < 0.5)(图 5h)。除了个别样品外,多数样品具有较低的Sr( < 80×10-6)和Ba( < 153×10-6),较高Rb(>150×10-6)和Rb/Sr比值(>3.0)(图 6、图 7),但较低的Nb/Ta( < 5.0) 和Zr/Hf( < 25.0) 比值(图 8)。
6 讨论 6.1 缩短增厚下地壳部分熔融在喜马拉雅造山带的构造演化过程中,变泥质岩和角闪岩是两种易发生部分熔融作用的地壳物质。变泥质岩具有高Rb,低Sr和Rb/Sr比值的特征,角闪岩正好相反。因此,变泥质岩和角闪岩部分熔融产生的熔体具有明显不同的Rb-Sr系统关系(Zeng et al., 2011)。高喜马拉雅和NHGD其他穹窿地区的淡色花岗岩一般有:(1) 高Rb、低Sr以及较高的Rb/Sr比(>1.0);(2) 高初始Sr(87Sr/86Sr(t)>0.7300) 和低初始Nd(εNd(t) < -12) 同位素组成(Vidal et al., 1982; Debon et al., 1986; Deniel et al., 1987; Ferrara et al., 1991; Searle et al., 1997; 杨晓松和金振民, 2001; Zhang et al., 2004a; King et al., 2011);(3) 形成于小于30Ma的年龄(Noble and Searle, 1995; Harrison et al., 1997; Zhang et al., 2004a, b; King et al., 2011)等特征。雅拉香波和然巴始新世二云母花岗岩具有较低的Sr同位素组成(87Sr/86Sr(t) < 0.7120)(图 9),较老的形成年龄(>42.0Ma),要求与年轻( < 30Ma)淡色花岗岩不同的源区和部分熔融反应。
与大多数喜马拉雅淡色花岗岩不同,始新世二云母花岗岩具有高Sr/Y和La/Yb比值的特征,Sr和LREE含量较高,Y( < 10×10-6)和Yb( < 1.0×10-6)都较低的典型特征(Zeng et al., 2011; Liu et al., 2014; 高利娥等, 2009; 谢克家等, 2010)。结合喜马拉雅造山带碰撞早期经历了明显的缩短变形等构造过程(Aikman et al., 2008; Zhang et al., 2012),元素和Sr-Nd同位素地球化学特征表明:要形成类似喜马拉雅二云母花岗岩的岩浆,需要以角闪岩脱水部分熔融为主的部分熔融过程,同时石榴子石或角闪石呈残留相,但斜长石为熔融相。形成该类花岗岩的最有利条件是增厚下地壳构造背景。在雅拉香波穹隆(图 1b),混合岩化石榴角闪岩的部分熔融时代为43.5±1.3Ma(锆石U-Pb年龄)(Zeng et al., 2011),与雅拉香波的二云母花岗岩的形成时间相似,同时混合岩化石榴角闪岩具有与二云母花岗岩类似的Sr和Nd同位素组成,也支持变基性岩在增厚地壳条件下部分熔融形成始新世高Sr/Y比二云母花岗岩的推论。
6.2 A和B类淡色花岗岩的成因机制实验岩石学(Douce and Harris, 1998; Knesel and Davison, 2002)、理论计算(Zeng et al., 2005a, b)和野外观测(Zhang et al., 2004a; King et al., 2011; Zeng et al., 2012)结果都表明:随着温压条件和含水量的变化,变泥质岩在无水或含水条件下发生从白云母到黑云母的递进部分熔融作用,形成性质各异、具有复杂Rb-Sr-Ba关系的花岗质岩浆(Harris and Inger, 1992; Knesel and Davison, 2002; Zeng et al., 2005b)。变泥质岩水致白云母熔融反应(MS2) 为7Qtz+15Pl+9Mus+xH2O→31M,白云母脱水部分熔融反应(MS1) 为:22Ms+7Pl+8Qtz=25Melt+5Kfs+5Sil+2Bt(Inger and Harris, 1993; Douce and Harris, 1998; Knesel and Davison, 2002)。与白云母脱水熔融形成的熔体相比,水致白云母熔融产生的熔体具有明显不同的Rb-Sr-Ba地球化学特征。在变泥质岩中,长石和云母分别是Sr和Rb的主要寄主矿物,与长石和全岩相比,云母具有较高的Rb/Sr比值和87Sr/86Sr同位素组成。Ca为斜长石的必要组成元素,Sr、Ba和Eu可以通过类质同象的方式替换Ca。因此,在变泥质岩水致白云母熔融过程中,与白云母脱水熔融相比,由于较多斜长石的加入,熔体具有较高的Sr、Ba和Ca,较低的Rb和Sr同位素比值(Inger and Harris, 1993; Knesel and Davidson, 2002; Zeng et al., 2005b)。白云母脱水部分熔融形成的熔体具有相反的特征。因此,花岗质岩浆的Rb-Sr-Ba关系是鉴定地壳岩石发生部分熔融作用类型的探针,可从地壳熔体的Rb-Sr-Ba系统关系来反推部分熔融类型和源岩性质(Inger and Harris, 1993; Knesel and Davidson, 2002; Zeng et al., 2005b, 2011)。
从Rb/Sr-Ba关系来看(图 7),A类花岗岩具有较高的Sr,较低的Rb和Rb/Sr比值。在Rb/Sr-Ba系统关系中,随着Ba浓度的增加,A类花岗岩Rb/Sr比值保持不变(图 7),与水致白云母部分熔融作用的特征一致(Inger and Harris, 1993),是变泥质岩水致白云母熔融的产物。如果年龄小于25Ma的喜马拉雅淡色花岗岩都源自相似的变泥质岩,B类淡色花岗岩具有较高的Rb(>325×10-6),但较低的Ba( < 265×10-6)和Sr( < 76.0×10-6)(图 7)和Rb/Sr比值较高(>4.6)。Ba和Rb/Sr比值呈负相关关系,随Rb/Sr比值的升高,Ba浓度降低(图 7),反应了变泥质岩白云母脱水部分熔融的特征。
上述两种部分熔融反应形成的熔体在主量、微量和同位素地球化学特征上都表现出明显的差异性。类似的现象在其它淡色花岗岩也有报道,如高喜马拉雅的玛纳斯鲁岩体。玛那斯鲁(Manaslu)淡色花岗岩体是喜马拉雅造山带中典型岩体之一,该岩体的淡色花岗岩存在两期,早期形成于22~24Ma,具有较高Rb(>300×10-6),较低的Sr(87×10-6~41×10-6)和较高的Rb/Sr(3.6~10.5) 和初始87Sr/86Sr(0.7523~0.7738) 比值;相反地,晚期淡色花岗岩形成于19.0~19.6Ma,具有较低的Rb( < 320×10-6),较高的Sr(83×10-6~114×10-6)和较低Rb/Sr(1.0~3.3) 和初始87Sr/86Sr(0.7446~0.7490) 比值(Vidal et al., 1982; Deniel et al., 1987; Guillot and Le Fort, 1995; Harrison et al., 1995, 1999a)。实验岩石学结果(Knesel and Davidson, 2002)和地球化学分析(Guillot and Le Fort, 1995)都表明:早期部分熔融为白云母脱水部分熔融,后期为含水白云母部分熔融的结果。
对比磷灰石和独居石在花岗质熔体的溶解模型(Harrison and Watson, 1984; Montel, 1993),影响这两种主要含磷和LREE载体矿物的溶解度的因素包括熔体温度,CaO活度和含水量。高温和干条件有利于磷灰石的溶解作用;湿条件有利于独居石的溶解作用(Zeng et al., 2005a, b)。与B类淡色花岗岩相比,A类淡色花岗岩具有较高的CaO,但较低的P2O5(图 4)和较低的Sm/Nd比值(图 9),反映了在含水部分熔融过程中,斜长石参与部分熔融比例较高,熔体的CaO含量和活度较高,抑制磷灰石溶解作用,但有利于独居石溶解,导致熔体的Sm/Nd和εNd值都较低(图 9)。
在获取了大量的地质年代学、全岩元素和同位素和矿物化学数据,甄别出两类地壳深熔作用—变泥质岩的白云母脱水熔融作用和水致白云母部分熔融作用。在大陆地壳的温度压力条件下,这两类地壳深熔作用是最常见、最普遍的深部地质过程,产生的熔体不仅在主量元素和微量元素,而且在同位素(Sr、Nd和Hf)和高场强元素(HFSE)上都表现出明显的差异性(Gao et al., 2017)。在这两类变泥质岩部分熔融作用中,主要造岩矿物和副矿物的溶解行为的差异性是调控熔体地球化学特征差异性的决定性因素。
7 未来研究方向和关键课题 7.1 花岗质片麻岩在地壳深熔作用中的作用喜马拉雅造山带发育大量花岗质片麻岩,原岩主要形成于寒武世-早奥陶世(Miller et al., 2001; Gehrels et al., 2006a, b; Cawood et al., 2007; Wang et al., 2012; 许志琴等, 2005; 张泽明等, 2008; 王誉桦等, 2014; 高利娥等, 2015)和新元古代(Singh et al., 2002; Ahmad et al., 2013; Wang et al., 2017)。许多花岗片麻岩经历了广泛的混合岩化作用,发育含石榴子石的淡色体(Pognante et al., 1990),同时还记录了20~18Ma的变质和部分熔融年龄(许志琴等, 2005),表明花岗质片麻岩也可能发生深熔作用,形成花岗质岩浆。但是,花岗质片麻岩在淡色花岗岩形成中的作用并不清楚。在日玛那穹窿和麻珈穹窿中,中新世淡色花岗岩具有特殊的Sr-Nd同位素系统特征(Zhang et al., 2004a; 于俊杰等, 2011; 胡古月等, 2016)。于俊杰等(2011)利用Sr-Nd同位素组成二元混合计算得出,花岗片麻岩参与部分熔融可以形成类似于淡色花岗岩成分的岩浆,定结淡色花岗岩的源岩可能是有花岗片麻岩和变泥质岩混合形成。虽然目前大多数学者认为高喜马拉雅变沉积岩是淡色花岗岩的主要源岩,但有限数据已揭示在特定阶段或构造背景下,花岗质片麻岩可能发生部分熔融,形成具有异常Sr和Nd同位素组成的淡色花岗岩(Zhang et al., 2004a; Gao et al., 2016; 于俊杰等, 2011)。但与变泥质岩相比,多数花岗片麻岩含云母和斜长石组分较少,较难熔融,只有在较高温或富流体的条件下,发生较高程度的部分熔融,形成淡色花岗质岩浆。
因此,有必要加强花岗质片麻岩的元素和同位素地球化学特征的研究,确定它们的变质和部分熔融反应的结构和时限,限定它们的Sr和Nd同位素组成,深入了解它们在喜马拉雅新生代地壳深熔作用中扮演的角色,甄别来自花岗质片麻岩的贡献程度,有可能在识别喜马拉雅造山带和其它碰撞造山带可能经历过的特殊构造过程提供重要的限定。
7.2 副矿物在地壳深熔作用中的作用和地幔部分熔融不同,地壳深熔作用往往通过一系列复杂的部分熔融反应来完成。富水原岩的部分熔融反应一般在较低温度下发生,熔融反应主要以石英+长石±白云母+水(水致部分熔融)进行。缺水或干燥原岩的熔融反应主要以含水矿物(如白云母、黑云母及角闪石)的脱水熔融反应进行。在变泥质岩、变杂砂岩和变基性岩中,上述两种反应是产生花岗岩质岩浆的重要机制。由于常见的副矿物(锆石、磷灰石、独居石等)是REE和关键放射性同位素(Nd、Hf)的主要赋存矿物,在这些反应中,副矿物是否参与部分熔融强烈影响着花岗岩质岩浆的关键微量元素和同位素(Sr、Nd、Hf和Pb)地球化学特征具有控制作用(Watson and Harrison, 1984; Ayres and Harris, 1997; Zeng et al., 2005a, b, 2012; Perini et al., 2009; Gao et al., 2017)。例如,在地壳深熔作用中,锆石影响花岗质熔体的Pb及Hf,石榴子石影响Hf,磷灰石及独居石影响Nd的同位素地球化学特征。通常条件下,由于不同副矿物的差异溶解行为,地壳物质部分熔融形成的熔体较源岩的LREE和HREE都低,但和源岩相比,熔体的LREE斜率((La/Gd)N)比源岩的陡, 导致残留体的(La/Gd)N小于源岩,但LREE含量增高。
野外、元素地球化学及放射性同位素地球化学观测揭示了许多具有独特地球化学特征的花岗岩岩体,如喜马拉雅的玛纳斯鲁(Malasulu)淡色花岗岩体、美国黑山(Black Hills)的Harney Peak花岗岩体(Shearer and Papike, 1987; Nabelek and Glasock, 1995; Harrison et al., 1999a, b)。相对于典型的大陆上地壳,这些花岗岩具有:(1) 强烈亏损轻稀土;(2) 高度的Sm-Nd的分馏;(3) 与邻近同时代花岗岩差别明显的初始143Nd/144Nd比值。这种怪异的地球化学特征往往归结于:岩浆后期副矿物分离结晶作用或在较深部部分熔融区,副矿物的差异溶解作用。甄别这些不同的岩浆作用,需要综合考虑副矿物的溶解动力学模型。
除了副矿物本身在深熔熔体中的溶解动力学行为外,副矿物在源岩中的赋存方式也是影响深熔熔体地球化学特征的因素之一。在变质岩中,主要造岩矿物(白云母、黑云母)通常都包含大量的副矿物(Watson et al., 1989; Watt and Harley, 1993),在部分熔融过程中,这些造岩矿物是否分解和参与部分熔融,释放包裹的副矿物并参与部分熔融是导致熔体较复杂的元素和同位素地球化学特征的原因。例如,在高级变质岩中,大部分独居石包裹在黑云母或石榴子石中。由于白云母脱水熔融反应温度普遍地低于黑云母脱水熔融反应,在白云母脱水部分熔融过程中,由于黑云母和石榴子石是非熔融相,多数独居石并不释放到熔体中,不参与部分熔融反应,导致岩浆与独居石的元素和同位素不平衡,产生的岩浆具有强烈亏损LREE的特征。相反地,在较高温情况下,黑云母的分解及熔融反应施放包含的独居石,独居石参与熔融反应,岩浆具有较高的LREE和Th(Nabelek and Glasock, 1995)。
在过去几十年来,已有一系列的实验研究了磷灰石、独居石、锆石及含Nb-Ta矿物在花岗质熔体的溶解行为(Watson and Harrison, 1983, 1984; Harrison and Watson, 1984; Rapp and Watson, 1986; Montel, 1993; Wolf and London, 1994, 1995; Linnen and Keppler, 1997, 2002; Linnen, 1998, 2005; Chevychelov et al., 2010; Bartels et al., 2010; Fiege et al., 2011; Linnen et al., 2012; Boehnke et al., 2013),为建立符合地质情况的地壳深熔作用的理论模型提供了关键的限定。如地壳深熔作用中,Pb、Nd和Hf同位素地球化学行为(Hogan and Sinha, 1991; Watson, 1996; Zeng et al., 2005a; Tang et al., 2014),但是至今还缺乏石榴子石和榍石在花岗质熔体的溶解行为的实验研究。
石榴子石是众多岩石类型中最常见的造岩矿物之一,在较宽的温度和压力条件下,都稳定存在。石榴子石不仅记录了生长时的时间和物理条件,而且即使在后期热扰动下,还常常保留了这些信息。和其它矿物相比,石榴子石具有以下特性:(1) 较高的Sm/Nd比值和Nd同位素比值;(2) 常具有化学成分环带,从核部到边部,无论是主量元素,还是微量元素(Sc、Y、HREE),甚至Nd和Hf放射性同位素常表现出环带特征;(3) Sm、Nd、Lu和Hf在石榴子石的扩散速率较低(Burton et al., 1995);在部分熔融中,无论是以固态残留相还是以熔融相存在,除了影响熔体中与石榴子石相容的元素(HREE、Y、Sc)的地球化学特征外,还可能导致Lu-Hf和Sm-Nd同位素系统的分馏和熔体的Hf和Nd同位素不平衡(Zeng et al., 2012)。在喜马拉雅造山带中,许多变质岩都含石榴子石,如果石榴子石形成时间远早于部分熔融时间,石榴子石将具有较高的Sm/Nd和143Nd/144Nd比值。在部分熔融过程中,如果石榴子石不参与反应,会降低熔体的143Nd/144Nd比值。如果熔体抽离,远离残留型石榴子石,那么熔体的Nd同位素就会出现同位素不平衡。但是,如果这些石榴子石被带入熔体,对淡色体的微量元素和放射性同位素产生影响。如南迦巴瓦~25Ma的高钙淡色花岗岩,与变泥质岩相比,含石榴子石的淡色花岗岩明显富集石榴子石相容元素(Sc、Y和HREE)和较高的εNd值,是变泥质岩部分熔融过程中,花岗质熔体携带源区石榴子石的结果(Zeng et al., 2012)。
除了石榴子石外,榍石是基性-花岗闪长质变质岩中的常见副矿物,富含Sr、REE、Nb和Ta。在这些岩石的部分熔融过程中,榍石的溶解行为及其如何影响花岗质熔体的Sr和Nd同位素组成及Nb-Ta的系统关系也需要野外实测和实验岩石学的研究。
7.3 高分异淡色花岗岩元素地球化学行为和成矿效应在花岗质岩浆从正常岩浆向高硅岩浆(通常SiO2>72.0%)演化过程中,岩浆不仅SiO2升高,同时挥发分(H2O、Cl或F)也随之升高,熔体结构发生实质性的变化,导致矿物组成和某些关键元素的地球化学行为发生变异,包括:(1) 主要造岩矿物相溶解度的改变(Ren et al., 2012);(2) 关键微量元素(Rb、Sr、Ba、Cs、REE等)分配系数的变化(Icenhower and London, 1995; Glazner et al., 2008; Ren et al., 2012);(3) 副矿物(如锆石、独居石、磷灰石、铪石等)溶解行为的变化(Ayers and Watson, 1991; Wolf and London, 1994, 1995; Bea, 1996; Linnen and Keppler, 1997, 2002);(4) 结晶石榴子石、电气石或绿柱石(Liu et al., 2016; 高利娥等, 2012)和(5) 金属元素(Nb、Ta、Sn等)的富集(Taylor and Wall, 1992)。
多数淡色花岗岩具有较低的Zr/Hf比值,导致高分异淡色花岗岩Zr/Hf比值降低的原因包括:(1) 锆石的分离结晶作用(Dostal and Chatterjee, 2000; Claiborne et al., 2006)或(2) 锆石(ZrSiO4)和铪石(HfSiO4)的溶解行为的变化(Linnen and Keppler, 2002)。Zr和Hf虽然具有相似的地球化学性质,但是随着分异作用的不断增强,花岗质岩浆的结构将逐渐发生变化,在达到熔体-流体相互作用阶段时,锆石(ZrSiO4)和铪石(HfSiO4)的溶解行为随之也会发生变化,虽然Zr和Hf在高分异花岗质岩浆中的溶解度都升高,熔体中的Zr和Hf的浓度都升高,但Hf的溶解度将明显高于Zr的溶解度(Linnen and Keppler, 2002),导致熔体的Zr/Hf比值逐渐变低。类似的情形也适用于Na和Ta孪生元素对(Linnen and Keppler, 1997; Bartels et al., 2010; Fiege et al., 2011)。但在确定高分异花岗岩的挥发分组成和熔体结构变化特征等方面的研究明显不足。
在上述高分异淡色花岗岩中,部分样品具有明显升高的B、Be、Sn,Nb和Ta含量(图 10),这些淡色花岗岩含电气石、绿柱石、锡石、钶铁矿和钽铁矿等,显示具有Be、B、Sn、Nb和Ta等元素富集和矿化特征,需要进一步的地质调查和研究,来确定喜马拉雅淡色花岗岩,尤其是高分异淡色花岗岩的关键元素,如B-Be、Nb-Ta和Zr-Hf等元素对的分异和富集行为及其控制因素,厘定它们的成矿潜力。
在众多碰撞造山带中,壳源花岗岩的岩石学和地球化学特征取决于温度-压力-成分(P-T-X),P-T条件与造山带物质所经历的构造相紧密相关。虽然在细节上可能存在差异,多数碰撞型造山带都经历了相似的顺时针P-T演化历史。早期为同碰撞缩短增厚作用,表现为高压-低温的P-T条件,在外来热的作用下,下地壳物质可在石榴子石或角闪石稳定域内发生部分熔融,形成高CaO,高Na/K和Sr/Y比的花岗岩;后期经历伸展垮塌作用,为低压-高温阶段,取决于含水条件,可能发生水致部分熔融和脱水部分熔融。与其他碰撞造山带相似,喜马拉雅造山带也经历了顺时针P-T-t构造演化和从早期缩短增厚向晚期伸展构造体制的转换,伴随相应的深熔作用,具体表现为(1) 在地壳缩短阶段(>35Ma),发生高角闪岩相-榴辉岩相的变质作用(55~46Ma, De Sigoyer et al., 2000; Kaneko et al., 2003; Parrish et al., 2006; Gao et al., 2012),增厚基性下地壳发生部分熔融作用, 形成高Na/K和Sr/Y比的过铝质花岗岩(46~35 Ma, Ding et al., 2005; Aikman et al., 2008; Zeng et al., 2011, 2015; Hou et al., 2012; Liu et al., 2014; 戚学祥等, 2008; 高利娥等, 2009, 2010);(2) 壳内伸展变形阶段,高喜马拉雅结晶岩系在快速折返过程中发生变泥质岩减压脱水部分熔融作用,形成高Rb/Sr、低Sr/Y比的过铝质花岗岩(28~10Ma; Le Fort, 1981; Deniel et al., 1987; Harris and Massey, 1994; Harris et al., 1995; Harrison et al., 1997, 1999a; Douce and Harris, 1998; Zhang et al., 2004b; Aoya et al., 2005; King et al., 2011; Aikman et al., 2012a, b);(3) 在部分地区,变泥质岩发生水致部分熔融作用(Harrison et al., 1999b; Prince et al., 2001; Zhang et al., 2004a; King et al., 2011; Zeng et al., 2012; Gao and Zeng, 2014; Gao et al., 2017),可能与藏南裂谷系的开启有关。始新世-早渐新世为碰撞缩短作用,中-下地壳物质发生部分熔融作用,这些熔体有效地改变了深部岩石的构造物理性质,促使地壳变形从缩短增厚向伸展减薄转换,诱发STDS的启动(Lee and Whitehouse, 2007; Yang et al., 2009; Zeng et al., 2011; Zhang et al., 2012)。伴随着STDS进一步伸展,深部地壳物质快速剥露,导致变泥质岩发生大规模减压部分熔融作用,形成了大量中新世典型的喜马拉雅淡色花岗岩(Schärer et al., 1986; Harrison et al., 1999a; Zhang et al., 2004a, 2012; Aoya et al., 2005; Lee and Whitehouse, 2007; King et al., 2011; Wang et al., 2012; Gao et al., 2013; Gao and Zeng, 2014; 于俊杰等, 2011)。由此可见,淡色花岗岩的形成与构造转换密切相关,地壳物质的深熔作用引起了构造变形的转换,同时构造环境控制了淡色花岗岩的地球化学特征。因此,准确厘定淡色花岗岩的地球化学特征和形成时代以及产出的构造背景是探讨喜马拉雅造山带及其他碰撞造山带构造演化过程的重要途径之一(Lee and Whitehouse, 2007; Zeng et al., 2011; Hou et al., 2012; Gao et al., 2017)。
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