岩石学报  2020, Vol. 36 Issue (9): 2714-2728, doi: 10.18654/1000-0569/2020.09.07   PDF    
变基性岩部分熔融过程中榍石的微量元素效应:以南迦巴瓦混合岩为例
赵令浩1,2,3, 曾令森1, 高利娥1, 高家昊1, 王亚飞1, 张立飞2     
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. 北京大学地球与空间科学学院, 北京 100871;
3. 国家地质实验测试中心, 北京 100037
摘要: 南迦巴瓦地区广泛出露的中下地壳变基性岩部分熔融形成的层状混合岩和淡色花岗岩,为研究部分熔融过程中榍石的地球化学行为对熔体的微量元素组成的影响提供了良好的机会。相对于源岩或熔融残留体,淡色体亏损Ti、V、REE、Y、Nb、Ta、U等元素,与混合岩中榍石的微量元素特征互补。混合岩、淡色体和榍石微量元素特征表明南迦巴瓦角闪岩部分熔融形成的淡色体的微量元素特征主要受控于榍石的地球化学行为。角闪岩脱水部分熔融过程中,由于长英质熔体的低Ti溶解度,榍石以未熔残留体形式存在于暗色体中,导致熔体亏损Ti、REE、Nb、Ta、V、U等元素和Sr/Y比值相对升高。关键元素在榍石和熔体之间的配分系数受熔体成分影响明显。角闪岩中变质榍石DNb/Ta < 1,因此变质榍石残留导致熔体Nb/Ta相对于源岩升高;而高Si-Al花岗质熔体中榍石DNb/Ta>1,因此与高Si-Al熔体平衡的榍石的分离(转熔或结晶分异)将导致熔体Nb/Ta比值相对源岩降低。榍石在部分熔融过程中的微量元素效应为理解变基性岩部分熔融产生熔体的地球化学特征提供新的认识。
关键词: 南迦巴瓦变角闪岩    部分熔融    富钠过铝质熔体    榍石    微量元素地球化学    
Role of titanite in the redistribution of key trace elements during partial melting of meta-mafic rocks: An example from Namche Barwa migmatite
ZHAO LingHao1,2,3, ZENG LingSen1, GAO LiE1, GAO JiaHao1, WANG YaFei1, ZHANG LiFei2     
1. Key Laboratory of Deep-Earth Dynamics, Ministry of Natural Resources, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
2. School of Earth and Space Sciences, Peking University, Beijing 100871, China;
3. National Research Center for Geoanalysis, Beijing 100037, China
Abstract: Layered migmatite formed by partial melting of amphibolites from mid to lower crust levels is widespread in the Namche Barwa. Titanite, as a key metamorphic residual and magmatic phase, occurs in the migmatite as well as in the leucosome, which provides a good opportunity to investigate the role of titanite in regulating the trace element compositions in the amphibolite-derived melts. Compared with migmatites, the leucosomes are depleted in elements such as Ti, V, REE, Y, Nb, Ta and U, which are enriched in titanite. Such characteristics indicate that the trace element compositions in the anatectic melts are mainly controlled by the geochemical behavior of titanite. During partial melting of amphibolite, due to low solubility of TiO2 in felsic melts, titanite grains commonly retain in the residue and in turn result in the depletion of those key elements compatible in titanite structure. The trace element partition coefficients between titanite and silicic melts are significantly affected by the melt's compositions. Residual metamorphic titanite with DNb/Ta < 1 will result in higher Nb/Ta in melt than that in the protolith. However, when equilibrating with high Si-Al melts, magmatic titanite preferred Nb over Ta with DNb/Ta>1. Separation of such titanite grains will decrease the Nb/Ta ratio in the residual melts. Data and results presented in this study demonstrate that titanite, similar to other accessory phases, could strongly affect the geochemical characteristics of melts produced by melting mafic rocks.
Key words: Namche Barwa amphibolite    Partial melting    Na-rich peraluminous leucosome    Titanite    Trace element geochemistry    

在岩浆岩地球化学研究中,常应用放射成因同位素(Nd、Sr、Pb、Hf)和微量元素的组成或比值来反演岩浆或部分熔融的源区、限定构造背景及形成演化过程。但由于相对较低的熔融温度、丰富的流体相加入和强烈的构造变形,地壳部分熔融产生的熔体常具有复杂的微量元素和放射成因同位素地球化学特征(Hammouda et al., 1996; Davies and Tommasini, 2000; Knesel and Davidson, 2002; Zeng et al., 2005a, b, 2012; McLeod et al., 2012; Gao et al., 2017),严重地影响了对花岗岩的成因和岩浆过程的认识。野外和实验研究已揭示深熔熔体的微量元素和同位素地球化学特征主要取决于深熔过程中参与熔融反应的矿物相,而非全岩成分(Tommasini and Davies, 1997; Knesel and Davidson, 2002; Zeng et al., 2005c)。与造岩矿物相比,副矿物(如锆石、磷灰石、独居石、榍石等)在岩石中所占的比例常常不到1%,但它们赋存的REE、HFSE、U、Th等元素可占全岩含量的90%以上,因此了解副矿物在部分熔融过程中地球化学行为是评估熔体微量元素和放射成因同位素特征形成机制的关键(Watson and Harrison, 1984)。

Ayres and Harris (1997)Bea and Montero (1999)Zeng et al.(2005a, b, c)建立了地壳变泥质岩部分熔融过程中主要副矿物(磷灰石、独居石、锆石和磷钇矿)对于熔体微量元素和Nd同位素组成影响的概念模型;Gao et al. (2017)揭示了喜马拉雅造山带中新世存在的两种类型较原始的淡色花岗岩在全岩主量和微量元素及放射成因同位素(Sr和Hf)组成上都表现出显著差异,它们分别对应于变沉积岩白云母脱水部分熔融和水致白云母熔融反应的产物(Gao et al., 2017; 曾令森和高利娥, 2017)。地壳岩石经历的压力、温度和挥发分(P-T-X)条件决定了部分熔融过程中可发生不同类型的熔融反应,导致部分熔融反应中造岩矿物与副矿物溶解行为的差异,形成地球化学性质各异的熔体。上述研究为深入解译地壳熔体复杂的微量元素和同位素特征提供了理论基础。

基性下地壳的部分熔融是形成大型花岗岩岩基、TTG和高Sr/Y比值岩浆岩的主要机制(Wolf and Wyllie, 1991; Atherton and Petford, 1993; Rapp and Watson, 1995; Ducea and Saleeby, 1996, 1998; Foley et al., 2002; Klemme et al., 2002; Rapp et al., 2003; Saleeby et al., 2003; Zeng et al., 2011; Wang et al., 2018)。基性下地壳主要由角闪岩相-榴辉岩相变基性岩组成,榍石是这些岩石中主要副矿物之一。在基性岩石部分熔融过程中,榍石如何影响熔体的微量元素组成特征是值得深入研究的课题之一。

榍石(CaTi1-xAlxSiO4(O, OHx, Fx))在基性变质岩和中酸性钙碱性岩浆岩中广泛存在,是稀土元素和高场强元素的重要载体。榍石中Ti是主要结构元素,因此榍石在硅酸盐熔体中的溶解受Ti溶解度的影响。实验研究表明TiO2在熔体中的溶解度与压力、氧逸度负相关,与温度正相关,并且熔体成分对于TiO2溶解度影响明显:高Si、Al含量会降低TiO2溶解度,而在高Ca、Na等阳离子含量的熔体中TiO2溶解度升高(Green and Pearson, 1986a; Xiong et al., 2009),熔体中F、Cl含量升高会大幅度提高TiO2溶解度(Rapp et al., 2010)。因此,基性岩高TiO2含量导致在部分熔融过程中,含Ti矿物相(如金红石、榍石和钛铁矿)可能以反应残留矿物或转融矿物形式在残留体和淡色体中广泛存在,进而控制着全岩的Ti、V、Nb、Ta、REE、U、Th等元素配分(Hellman and Green, 1979; Rushmer, 1991; Storkey et al., 2005)。如全岩“U”形REE配分模式常被认为与榍石的部分熔融残留或结晶分异作用相关(Hellman and Green, 1979; Green and Pearson, 1986b; Glazner et al., 2008)。

榍石微量元素配分系数受熔体或全岩成分影响明显(Prowatke and Klemme, 2005),尤其是在高Si、Al花岗质熔体中,榍石配分系数急剧增大(作者待发表数据),因此,了解榍石在地壳部分熔融过程中的行为及其对于熔体微量元素的调控效应对于认识花岗岩的微量元素特征及其形成过程具有重要意义。

南迦巴瓦高级变质地块中广泛出露中下地壳变基性岩部分熔融形成的层状混合岩和淡色花岗岩。多数淡色体代表不同熔融程度形成的花岗质熔体,少数淡色体可能裹挟源区组分(如未熔钾长石、锆石或榍石)。这些基性混合岩中发育不同成因的榍石,记录了从部分熔融到熔体分离过程的信息,为深入了解中下地壳变基性岩部分熔融过程及榍石的地球化学行为对于熔体的微量元素组成(尤其是REE和HFSE)的影响提供了良好机会。本文结合基性混合岩、淡色花岗质熔体及不同成因榍石的微量元素数据,探讨榍石在部分熔融过程中的地球化学行为及其对熔体微量元素的影响。

1 地质概况及样品描述

喜马拉雅东构造结位于拉萨地块东部,由冈底斯岩基、雅鲁藏布缝合带和高喜马拉雅结晶岩系三个单元组成(图 1),其中高喜马拉雅地体俯冲到冈底斯岩基之下。南迦巴瓦地块位于东构造结核部,隶属于高喜马拉雅结晶岩系,被冈底斯岩体包围,由经历高级变质作用的片麻岩、角闪岩、石榴辉石岩、变泥质岩、钙硅酸盐和各种成分的混合岩组成(Zhong and Ding, 1996; Liu and Zhong, 1997; Booth et al., 2004, 2009; Geng et al., 2006; Liu et al., 2007; 戚学祥等, 2010; Shen et al., 2010; Zhang et al., 2010b, 2012; Guilmette et al., 2011; Su et al., 2012)。研究表明新生代以来(>40Ma),南迦巴瓦地块经历了多期变质作用,最高可达麻粒岩相变质作用(14~18kbar,>800℃;Liu and Zhong, 1997; Ding and Zhong 1999; Ding et al., 2001; Geng et al., 2006; Liu et al., 2007; Guilmette et al., 2011; Su et al., 2012; Xu et al., 2012; Peng et al., 2018),同时伴随着部分熔融的发生(Burg et al., 1998; Booth et al., 2009; Zhang et al., 2010a, b; Zeng et al., 2012),形成不同类型的混合岩和淡色花岗岩。尽管对于区域岩石经历高压麻粒岩相变质作用时间仍存在争议(Ding et al., 2001; Liu et al., 2007; Zhang et al., 2010b; Su et al., 2012; Peng et al., 2018),但越来越多的锆石和独居石U-Pb年代学研究结果表明南迦巴瓦地块经历的高压麻粒岩相变质作用发生于~24Ma,在随后的折返过程中经历了退变质作用的影响,例如~18Ma经历了角闪岩相变质作用(Xu et al., 2012; Su et al., 2012)。岩石学和U-Pb定年研究表明变泥质岩、石榴辉石岩和石榴角闪岩在30~10Ma期间处于高压条件(>10kbar)(Liu et al., 2007; Booth et al., 2009; Zhang et al., 2010b; Zeitler et al., 2014),变基性岩和变泥质岩经历了同期的高压麻粒岩相变质作用,并发生了部分熔融,产生了大量富Na和富K的花岗质熔体(Burg et al., 1998; Ding et al., 2001; Booth et al., 2004; Zeng et al., 2008, 2012)。热年代学研究结果表明南迦巴瓦岩体在20~10Ma处于缓慢抬升阶段,抬升速率不足以明显影响地温梯度;从10~8Ma开始快速剥露和抬升过程,抬升速度可达5~10mm/yr,对应的地表剥蚀厚度达30~60km;5Ma后再次进入缓慢抬升阶段(Zeitler et al., 2014)。快速而强烈的地表剥蚀导致相对较热的地壳岩石快速抬升至地壳浅部(Zeitler et al., 2001; Finnegan et al., 2008),由于压力的快速降低,岩石发生大范围脱水部分熔融,部分地区由于浅表水的加入发生水致部分熔融(Burg et al., 1998; Ding et al., 2001; Booth et al., 2004, 2009)。野外观察表明角闪岩和变泥质岩都经历了高角闪岩相到麻粒岩相的变质作用,同时发生部分熔融,形成层状混合岩及顺层或斜切面理的淡色花岗岩(Burg et al., 1998; Liu et al., 2007; Booth et al., 2009; Zhang et al., 2010b; Zeng et al., 2012)。两种混合岩中的淡色体都具有过铝花岗质成分特征,富K的淡色体主要发育在变泥质岩中,富Na的淡色体主要包裹在角闪岩中(Zeng et al., 2012)。

图 1 南迦巴瓦地质简图和采样点(据Zeng et al., 2012) Fig. 1 Simplified geological map of the Namche Barwa area, showing the sample locations (after Zeng et al., 2012)

南迦巴瓦发育多种类型的角闪岩,包括斜长角闪岩(LZ06-22)、含石榴子石角闪岩(ZB06-04)和含黑云母石榴角闪岩(ZB06-03),这些角闪岩都经历了部分熔融,形成条带状混合岩。

ZB06-03为层状混合岩化含黑云母石榴角闪岩,由薄层淡色体(~1cm)和厚层基性暗色体构成(图 2a)。暗色体主要由角闪石、石榴子石、黑云母组成,副矿物包括磷灰石、锆石和榍石,大量副矿物包裹在黑云母或角闪石颗粒中(图 2b);淡色体由斜长石、石英、钾长石、三元长石、角闪石、绿帘石、榍石、磷灰石、锆石和铁氧化物组成。该混合岩中主要发育两种类型的榍石:暗色体中的榍石包裹在角闪石和黑云母颗粒中,呈梭形,长轴定向排布,粒径10~50μm,部分颗粒发生明显变形反映了构造作用的影响;淡色体中的榍石较自形,粒径100~200μm(图 2c)。

图 2 南迦巴瓦变基性混合岩标本及显微照片 (a-c)含黑云母石榴角闪岩ZB06-03;(d)斜长角闪岩LZ06-22M,金红石是主要的含Ti副矿物,包裹于角闪石中;(e、f)石榴角闪岩ZB06-04和淡色体,角闪石中含有大量半自形变质榍石定向排列,淡色体中含有少量自形岩浆榍石颗粒. Amp-角闪石;Grt-石榴子石;Pl-斜长石;Bt-黑云母;Ttn-榍石;Rt-金红石 Fig. 2 Photograph and microphotographs of migmatitic amphibolites and their leucosomes from the Namche Barwa (a-c) migmatitic biotite-bearing garnet amphibolite ZB06-03; (d) amphibolite LZ06-22M with rutile as main Ti-bearing mineral, where rutile grains are enclosed in amphibole; melansome (e) and leucosome (f) from migmatitic garnet amphibolite ZB06-04, respectively. Amp-amphibole; Grt-garnet; Pl-plagioclase; Bt-biotite; Ttn-titanite; Rt-rutile

ZB06-04为混合岩化石榴角闪岩,由角闪石、斜长石、榍石、金红石、磷灰石和锆石组成。榍石呈半自形-他形结构分布在基质中或包裹于其他矿物中(图 2e)。

样品ZB06-04L1和ZB06-04L2是混合岩中分选出的相对较厚的包裹在角闪岩中的淡色体。2件淡色体都由斜长石、石英、钾长石和石榴子石组成,副矿物为锆石、磷灰石和榍石,其总体含量较低(图 2f)。

LZ06-22为混合岩化斜长角闪岩。暗色体(LZ06-22M)由角闪石、斜长石、石榴子石和石英组成,副矿物包括少量金红石、锆石、磷灰石(图 2d)。部分角闪石颗粒中包裹钛铁矿和定向分布的金红石。淡色体(LZ06-22L)由石英、斜长石、钾长石、石榴子石、角闪石组成,副矿物包括绿帘石、白云母、榍石、锆石和磷灰石。石榴子石颗粒中包裹斜长石、白云母、黑云母、绿泥石、黄铁矿和钛铁矿。

成分不同的角闪岩发生不同程度部分熔融,形成花岗质淡色体。整体上,混合岩暗色体矿物主要包括角闪石、石榴子石、榍石和黑云母;而淡色体主要由石英、斜长石、钾长石组成,少数淡色体包含源区或转熔型矿物,如石榴子石、角闪石、钾长石、黑云母、榍石、锆石等,被花岗质熔体从原岩分离时裹挟。这些混合岩为研究部分熔融的过程及副矿物(尤其是榍石)对熔体微量元素及同位素地球化学特征的影响提供了良好的实验对象。

2 测试方法 2.1 全岩地球化学分析

选择混合岩中相对较厚的向两侧尖灭的淡色体(~1cm)及混合岩样品进行全岩地球化学分析。全岩主、微量元素分析在国家地质实验测试中心完成。主量元素含量采用X射线荧光光谱仪(XRF-PW4400)测定,测试精度为5%;微量元素和稀土元素含量采用等离子质谱仪(ICP-MS-Excell)测定,含量大于1×10-6的元素的测试精度为5%,小于1×10-6的元素测试精度为10%,少数含量较低的元素,测试误差大于10%。

2.2 榍石微量元素分析

榍石微区原位微量元素分析在国家地质实验测试中心完成,使用New Wave 193nm ArF准分子激光器及Finnigan ELEMENT 2高分辨电感耦合等离子体质谱仪。激光剥蚀采用30μm斑束,频率10Hz。ICP-MS分析采用低分辨模式。详细分析方法和流程同赵令浩等(2017)。数据处理采用Excel完成,利用多外标结合内标基体归一定量技术(胡明月等, 2008),选择Ca为内标元素。利用该分析方法以NIST610为标准样品(Pearce et al., 1997),计算监控样品KL2-G(Jochum et al., 2006)主量元素含量误差小于5%,微量元素和稀土元素含量误差小于10%。

3 数据结果

南迦巴瓦变基性混合岩和淡色体全岩主、微量元素数据及榍石微量元素数据分别列于表 1表 2。混合岩样品中淡色体向周围尖灭或包裹在基性岩中,未发生明显熔体丢失,其成分可以代表或接近于原始熔体特征。

表 1 南迦巴瓦变基性混合岩及淡色体地球化学组成(主量元素:wt%;稀土和微量元素:×10-6) Table 1 Whole-rock geochemical compositions of the migmatites and the leucosomes from the Namche Barwa(major oxides: wt%; trace element: ×10-6)

表 2 南迦巴瓦混合岩(样品ZB06-03)中变质榍石和岩浆榍石微量元素含量(×10-6) Table 2 Trace element contents of metamorphic and magmatic titanites in migmatite (Sample ZB06-03) from the Namche Barwa (×10-6)
3.1 全岩地球化学特征

淡色体具有以下地球化学特征:较高的SiO2(66.64%~79.03%)、Al2O3(10.99%~18.20%)、CaO(0.59%~5.20%)、Na2O(2.11%~6.12%)、TiO2(0.03%~0.30%)含量和A/CNK(0.97~1.30)、Na2O/K2O(0.35~38.0)比值。样品ZB06-03L与源岩分离过程中裹挟了钾长石和黑云母导致Na2O/K2O <1。变角闪岩中包裹的淡色体主量元素显示富钠过铝质特征(图 3)。

图 3 南迦巴瓦变基性混合岩和淡色体主量元素地球化学特征 Fig. 3 Covariation diagrams of the selected major oxides of Al2O3 (a), TiO2 (b), CaO (c) and FeO+MgO (d) vs. SiO2 for the mafic migmatites and leucosomes from the Namche Barwa area

在微量元素组成上,淡色体样品总体亏损Ti、Nb、Ta和HREE(图 4)。(La/Yb)N=1.30~57.2,(Ho/Yb)N=0.85~1.76,相对富集LREE,HREE分异不明显。所有淡色体具有Eu负异常特征,Eu/Eu*=0.19~0.85。与混合岩相比较,淡色体中的REE、Sc、V、Y、Zr、Hf、Nb、Ta、U含量较低。淡色体ZB06-04L和LZ06-22L具有高Sr/Y比值特征(Sr/Y=106~237);ZB06-03L的Sr含量接近于混合岩,Sr/Y比值低(<3.28)。淡色体LZ06-22L和ZB06-03L2的Nb/Ta比值略高于混合岩,其他的淡色体Nb/Ta比值低于混合岩。

图 4 南迦巴瓦混合岩和淡色体原始地幔标准化微量元素蛛网图(a)和球粒陨石标准化稀土元素配分图(b)(标准化值据Sun and McDonough, 1989) Fig. 4 Primitive mantle-normalized trace elements diagrams (a) and chondrite-normalized rare earth element distribution diagrams (b) for the mafic migmatites and the leucosomes from the Namche Barwa (normalization values from Sun and McDonough, 1989)

与淡色体相比较,混合岩具有以下地球化学特征:较低的SiO2(49.93%~63.99%)和Al2O3(12.60%~13.82%)含量,高FeO、MgO、CaO含量;Na2O含量1.80%~4.06%,K2O含量较低0.43%~4.14%,Na2O/K2O比值0.16~1.50,A/CNK比值0.54~0.78;混合岩TiO2含量为0.94~3.77,明显高于淡色体(图 3)。混合岩ZB06-03相对于其他样品SiO2含量偏高可能是由于分析的全岩样品中淡色体比例偏高造成。

混合岩的稀土元素总量变化较大,ΣREE=38.1×10-6~412)×10-6,REE配分呈缓右倾模式,LREE弱富集,(La/Yb)N=1.11~8.59,HREE配分曲线平坦,(Ho/Yb)N=0.97~1.13,Eu负异常(Eu/Eu*=0.37~0.94)(图 4)。

混合岩和淡色体样品TiO2含量与SiO2含量呈负相关关系,与全岩V、Nb、Ta、ΣREE含量及Nb/Ta比值呈正相关关系(图 5),全岩P2O5含量与关键微量元素无相关性。

图 5 南迦巴瓦变基性混合岩及淡色体微量元素关系图 关系图反映了含Ti副矿物不同程度部分熔融对于淡色体关键微量元素含量的控制 Fig. 5 Covariation diagrams of TiO2 (a), ΣREE (b), Nb (c) and Nb/Ta ratios (d) vs. V for the mafic migmatites and the leucosomes from the Namche Barwa The diagrams show the role of Ti-bearing phase in regulating the trace element compositions in the amphibolite-derived melts
3.2 榍石地球化学特征

南迦巴瓦混合岩中含有两种榍石:(1)岩浆榍石,部分熔融过程中从熔体中结晶的榍石;(2)变质榍石,混合岩中残留的榍石,分布在基质中和包裹在角闪石和黑云母中。这两种类型的榍石在变基性岩部分熔融及岩浆演化过程中广泛存在。为探讨榍石对熔体微量元素特征的影响,采用LA-ICP-MS分析样品ZB06-03中两类榍石的微量元素组成(表 2)。

岩浆榍石:(1)富集MREE,稀土配分模式呈“上凸”状(图 6),(La/Yb)N=0.31~3.94,(Gd/Yb)N=1.83~15.0,Eu负异常明显(0.27~0.43);(2)高Nb、Ta含量,具有超球粒陨石的Nb/Ta比值特征(Nb/Ta=19.2~44.8);(3)高U低Th、Pb含量,U/Th比值1.30~6.48。

图 6 混合岩ZB06-03中变质榍石和岩浆榍石微量元素图 (a) REE配分模式;(b) Ta-Nb;(c) Sr-Eu/Eu*;(d) U/Th-Nb/Ta Fig. 6 Plots of trace elements in magmatic titanite and metamorphic titanite in migmatite ZB06-03 (a) chondrite-normalized REE distribution diagram; (b) Ta vs. Nb; (c) Sr vs. Eu/Eu*; (d) U/Th vs. Nb/Ta

变质榍石:(1)富集LREE(图 6),(La/Yb)N=0.63~4.17;(Gd/Yb)N=0.76~1.47,MREE和HREE未发生明显分异,Eu弱负异常至无异常(Eu/Eu*=0.74~1.00);(2)Nb、Ta含量及Nb/Ta比值低于岩浆榍石,Nb/Ta比值低于球粒陨石;(3)U含量低于岩浆榍石,Th含量较高,U/Th比值0.21~1.59。

总体上,混合岩ZB06-03中两种榍石均富集REE和HFSE,但是二者的REE配分模式、Nb/Ta和U/Th比值具有明显差异,指示了两种榍石不同的成因和形成环境。

4 讨论 4.1 南迦巴瓦变基性岩部分熔融过程

南迦巴瓦混合岩化角闪岩包裹的淡色体具有相似的主量元素特征,即富Na和Ca过铝花岗质。实验岩石学和野外研究表明,富Na过铝花岗质熔体形成机制有两种:变泥质岩在高压条件下(>10kbar)的水致部分熔融(Patiño Douce and Harris, 1998; Zeng et al., 2012)或基性岩(如角闪岩)部分熔融(Beard and Lofgren, 1989; Wolf and Wyllie, 1991, 1994; Rapp and Watson, 1995; López and Castro, 2001; Zeng et al., 2011)。高利娥等(2009)Zeng et al. (2011)分别在雅拉香波地区发现~35Ma淡色花岗岩和~43Ma二云母花岗岩具有高SiO2、Na2O、Sr/Y比值特征,并认为该富Na过铝质花岗岩是角闪岩脱水部分熔融形成;同时Zeng et al. (2012)报道了南迦巴瓦地区21~25Ma变泥质岩水致部分熔融形成的富Na和Ca淡色花岗岩。

与上述数据相比较,除ZB06-03L外,南迦巴瓦变基性岩中的淡色体与雅拉香波二云母花岗岩表现出相似的成分和地球化学特征,高SiO2、CaO、Na/K和Sr/Y比值,亏损Ti、Nb、Ta、REE等元素,具有类埃达克质特征(Zeng et al., 2011)。

ZB06-03L具有高K2O、Rb、Ba、Zr和Hf含量的特征,与混合岩及淡色体中含有黑云母和钾长石现象一致。在角闪岩脱水部分部分熔融的温压条件下,取决于黑云母的成分,尤其是具有高Fe/Mg比的黑云母,也可发生不同程度的部分熔融作用(Patiño Douce and Beard, 1995; Skjerlie and Patiño Douce, 1995; Gardien et al., 2000),同时黑云母部分熔融产生的钾长石被裹挟进入淡色体,使淡色体K2O、Rb、Ba含量相对于其他角闪岩熔融体系明显升高。另外,本文中变基性岩和淡色体Sr同位素组成(作者待发表数据)明显低于南迦巴瓦变泥质岩(Zeng et al., 2011)。野外及地球化学数据表明南迦巴瓦变基性岩包裹的淡色体是由角闪岩部分熔融形成,同时伴随着不同程度的黑云母脱水部分熔融反应:

(1)
(2)

混合岩ZB06-03锆石SHRIMP U-Pb年龄结果表明该基性混合岩原岩形成于元古代(作者,待发表数据),角闪石脱水部分熔融时间为7.1±0.5Ma,形成的温压条件为950±25℃和7.5±0.2kbar(曾令森等, 2009)。其原岩年龄与Zhang et al. (2012)报道的南迦巴瓦岩体角闪岩原岩年龄一致,表明在南迦巴瓦地区,发生过深俯冲的喜马拉雅岩片在快速折返-减压过程中于7.1~7.4Ma经历了高温变质作用,变角闪岩发生脱水部分熔融(Booth et al., 2004)。

角闪岩部分熔融实验表明:随熔融程度升高,熔体中SiO2含量逐渐降低,而铁镁质成分含量逐渐升高(Sen and Dunn, 1994; Wolf and Wyllie, 1994)。南迦巴瓦变基性岩包裹的淡色体SiO2含量与Al2O3、TiO2、FeO、MgO等元素含量呈负相关系(图 3)表明该角闪岩在抬升折返过程中经历了不同程度部分熔融,其中LZ06-22部分熔融程度最高,ZB06-04L为低度部分熔融的产物。

相对于主量元素的一致性,南迦巴瓦不同类型角闪岩部分熔融产生的淡色体的微量元素特征具有较大差别。地壳岩石部分熔融过程中,不同熔融反应导致的造岩矿物与副矿物的溶解行为差异是造成熔体的微量元素和放射性同位素差异的主要原因之一(Watson and Harrison, 1984; Ayres and Harris, 1997; Bea et al., 1999; Zeng et al., 2005a, b, c; Gao et al., 2017)。

榍石作为南迦巴瓦变角闪岩样品中主要的副矿物,在部分熔融过程中,花岗质熔体低Ti溶解度使榍石以变质残余矿物或转熔矿物形式残留在固态相中,导致淡色体相对于源岩或混合岩亏损Ti、REE、V、Y、Nb、Ta、U等榍石富集元素(图 4);淡色体中TiO2含量与SiO2含量呈负相关关系,与全岩V、Nb、Ta、ΣREE含量及Nb/Ta比值呈正相关关系(图 5)。尽管基性岩中含有的磷灰石和金红石也可能对熔体微量元素造成影响,但本文中熔体的P2O5含量与关键微量元素无明显相关性;LZ06-22样品中金红石为主要含Ti副矿物,其淡色体微量元素变化趋势与其余样品明显不同(图 5)。因此角闪岩样品ZB06-03和ZB06-04中,榍石的行为影响并控制着熔体的关键微量元素特征。

4.2 榍石微量元素配分特征

实验和野外样品研究表明,榍石与熔体微量元素配分系数受温度、压力、氧逸度、晶体和熔体成分等因素影响(Green and Pearson, 1986b; Tiepolo et al., 2002; Prowatke and Klemme, 2005; Schmidt et al., 2006; Olin and Wolff, 2012),其中以熔体成分和结构的影响最为明显(Prowatke and Klemme, 2005; Schmidt et al., 2006)。

样品ZB06-03混合岩和淡色体中发育两种榍石,分别计算岩浆榍石/淡色体和变质榍石/混合岩全岩配分系数,结果如图 7。计算结果落入Prowatke and Klemme (2005)实验测定的榍石-熔体配分系数范围内,具有一致的变化趋势。榍石富集REE、V、Y、Nb、Ta、Zr、Hf、Th、U等元素,而Rb、Cs、Sr、Pb、Ba等大离子亲石元素在榍石中为不相容元素。

图 7 榍石-全岩微量元素配分系数(阴影区域数据据Prowatke and Klemme, 2005) Fig. 7 Titanite-whole rock partition coefficients (shaded area data from Prowatke and Klemme, 2005)

相对于LREE,榍石更富集MREE和HREE,因此榍石的残留或分异导致熔体REE含量降低的同时,REE配分呈现MREE亏损的特征。角闪岩ZB06-04熔融程度较低,榍石残留导致淡色体REE配分模式略呈“U”形(图 4)。

Nb和Ta在变质榍石和岩浆榍石中都为强相容元素。变质榍石DNb/Ta=0.85,DNb<DTa,与Prowatke and Klemme (2005)实验测定榍石Nb-Ta配分特征一致;岩浆榍石的Nb和Ta配分系数明显高于合成实验数据,DNb/Ta=1.72,Nb配分系数略高于Ta。Nb和Ta进入含Ti副矿物的替换机制包括:(1)Al3++(Ta, Nb)5+=2Ti4+(Horng and Hess, 2000); 和(2)Na++(Ta, Nb)5+=Ca2++Ti4+(Cerny et al., 1995; Tiepolo et al., 2002)。因此高Al-Na熔体中,榍石Nb-Ta配分系数增大,这反映了熔体成分和结构对于榍石配分系数的影响。尽管目前已报道的合成实验法榍石-熔体Nb和Ta配分系数特征不支持DNb>DTa的推论(如DNb/Ta=0.32~0.44,Green and Pearson, 1987; DNb/Ta=0.07~0.28, Prowatke and Klemme, 2005),但上述合成实验采用的熔体多为中基性熔体(SiO2<62%),与南迦巴瓦变基性岩形成的高Si-Al花岗质熔体成分差异较大。上述差异可能反映了随着熔体聚合程度的升高,熔体成分及其结构的变化可严重影响关键元素在榍石和熔体间的配分行为特征。

Schmidt et al. (2004)综合文献中金红石-熔体的Nb和Ta配分系数结果表明,尽管DNb<DTa,但随着熔体中SiO2含量增高,DNb/Ta比值呈增大趋势;Linnen and Keppler (1997)根据Nb和Ta在偏铝质和过铝质熔体中的溶解度实验推算800℃、2kbar、流体饱和条件下,过铝花岗质熔体中金红石DNb>DTa。因此本文测定的岩浆榍石Nb-Ta配分系数可能代表了高Si-Al花岗质熔体中榍石的Nb-Ta配分特征。南迦巴瓦变基性岩部分熔融过程中,与淡色熔体平衡的转熔榍石残留或岩浆榍石结晶分异是导致淡色体Nb/Ta比值低于混合岩的重要原因。岩浆榍石Nb/Ta比值高于淡色体,说明淡色体中可能有富Ta矿物相存在。

U和Th在岩浆榍石和变质榍石中均表现为强相容特征,岩浆榍石DTh/U=0.023~0.028,变质榍石DTh/U=0.14~0.57,二者差异截然(图 6d),因此与锆石类似(Yakymchuk et al., 2018),Th/U比值可能成为区分岩浆成因榍石和变质成因榍石的重要指示参数。

Sr在榍石中为不相容元素,而Y为强相容元素,因此榍石具有较低Sr/Y比值。榍石在部分熔融过程中的残留将导致熔体的Sr/Y比值升高。但在变基性岩中榍石含量远低于石榴子石和角闪石,因此熔体的Sr/Y比值主要受控于石榴子石和角闪石(曾令森等, 2019)。上述淡色体中,Sr/Y比值与熔体V含量成负相关关系,表明在变基性岩部分熔融中,榍石的残留可升高熔体的Sr/Y比值。

总体上,榍石富集Ti、V、REE、Y、Nb、Ta、U等元素,相对亏损Sr、Ba、Pb等元素,因此在变基性岩部分熔融或花岗质岩浆演化过程中,榍石与熔体的分离将导致熔体亏损Ti、V、Y、REE、Nb、Ta、U等关键元素,Sr/Y比值相对升高。变质榍石残留导致熔体Nb/Ta比值相对于源岩升高,而与高Si-Al熔体平衡的榍石(转熔榍石或岩浆结晶榍石)的分离将导致熔体Nb/Ta比值降低。

为定量反应高Si-Al花岗质熔体中榍石的行为对于熔体微量元素(尤其是REE配分模式)的影响,根据岩浆榍石与高Si-Al淡色体微量元素配分系数进行模拟计算。假定榍石从熔体分离服从瑞利分馏模型:

CL是残留熔体中的微量元素浓度,C0为熔体中原始微量元素浓度,F为熔体发生分异的比例,D是岩浆榍石与花岗质熔体之间的配分系数。经历0.5%~2.0%榍石结晶分离对于熔体微量元素特征的影响如图 8。榍石发生结晶分异或者转熔残留,1.0%的榍石分离就会导致熔体明显亏损MREE和HREE,并造成轻微的Eu正异常特征,但对LREE含量影响不明显。高Si-Al熔体中榍石分离将导致熔体的Nb和Ta含量及Nb/Ta比值显著降低。

图 8 高Si-Al熔体中榍石结晶分异模拟 Fig. 8 Fractional crystallization modeling showing the consequences of removal of 0.5%, 1.0%, and 2.0% titanite on the trace element characteristics of the leucosome
5 结论

(1) 南迦巴瓦中下地壳变基性岩在10Ma以来发生广泛的不同程度的部分熔融作用,形成地球化学特征各异的淡色体或淡色花岗岩。相对于源岩或熔融残留体,淡色体亏损Ti、V、REE、Y、Nb、Ta、U等元素,与混合岩中榍石微量元素特征互补,且淡色体中TiO2含量与这些元素含量呈正相关关系。

(2) 南迦巴瓦角闪岩熔融形成的淡色体的微量元素特征主要受控于榍石的地球化学行为。在角闪岩脱水部分熔融过程中,由于长英质熔体低Ti溶解度,榍石以未熔残留体形式存在于暗色体中,导致熔体亏损Ti、V、REE、Y、Nb、Ta、U等元素,且Sr/Y比值相对升高。

(3) 角闪岩中变质榍石DNb/Ta<1,因此变质榍石残留导致熔体Nb/Ta相对于源岩升高;高Si-Al花岗质熔体中榍石DNb/Ta>1,因此与高Si-Al熔体平衡的榍石(转熔榍石或岩浆结晶榍石)的分离将导致熔体Nb/Ta比值降低。

(4) 关键元素在榍石和熔体之间的配分系数受熔体成分影响明显,尤其是高Si-Al花岗质熔体中榍石的微量元素配分行为相对于在中基性成分熔体中可能发生明显转变,对于认识花岗岩的微量元素特征及其形成过程具有重要意义,亟待更多野外及实验研究。

致谢      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审阅本文并提出了宝贵的修改意见。

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