岩石学报  2019, Vol. 35 Issue (10): 3130-3140, doi: 10.18654/1000-0569/2019.10.11   PDF    
柴北缘超高压地体折返过程中地壳深熔的岩石学研究
于胜尧1,2, 张建新3, 李三忠1,2, 彭银彪1, 李云帅4, 吕沛1, 姚勇1, 李卓凡1     
1. 中国海洋大学海洋地球科学学院, 海洋高等研究院, 海底科学与探测技术教育部重点实验室, 青岛 266100;
2. 青岛海洋科学与技术国家实验室, 海洋地质过程与环境功能实验室, 青岛 266037;
3. 中国地质科学院地质研究所, 北京 100037;
4. 天津大学表层地球系统科学研究院, 天津 300072
摘要: 宏观、微观岩石学、地球化学和年代学研究表明,柴北缘锡铁山和绿梁山单元富含斜长石的浅色体和富含钾长石的浅色体是超高压地体折返过程中榴辉岩和片麻岩部分熔融的产物。阴极发光图像显示富含斜长石的浅色体中锆石具有明显的核-边双层结构,锆石核部无明显分带特征,并呈现出重稀土平坦和无Eu异常的稀土配分模式,~450Ma的年龄结果与区域上榴辉岩峰期变质时代一致;发光较弱的锆石边部具不明显的环带结构和较低的Th/U比值,~426Ma年龄结果代表了熔体的结晶时代。富含钾长石的浅色体中的锆石U-Pb定年结果记录的~910Ma、~450Ma和~426Ma三组年龄分别代表了片麻岩原岩结晶时代、高压-超高压变质作用时代和熔体结晶时代。富含斜长石的浅色体具有高SiO2、Al2O3、CaO、Na2O、Sr和LREE,而低MgO、FeOT、K2O、Y、Yb和HREE的英云闪长岩-奥长花岗岩的地球化学特征;而富含钾长石的浅色体具有高的SiO2、Al2O3和K2O+Na2O,而较低的CaO、MgO、REE的花岗岩地球化学特征。黝帘石和少量的多硅白云母的脱水分解是触发超高压榴辉岩发生部分熔融形成富含斜长石的浅色体的主要机制;而多硅白云母的脱水分解则是触发超高压片麻岩部分熔融形成富含钾长石浅色体的主要机制。这些浅色体显著的促进了柴北缘超高压地体的快速折返,并对大陆俯冲隧道中的元素迁移和壳-幔作用具有重要的影响。
关键词: 柴北缘    地壳深熔    折返过程    岩石学    
Petrological study on anatexis during exhumation of North Qaidam UHP terrane
YU ShengYao1,2, ZHANG JianXin3, LI SanZhong1,2, PENG YinBiao1, LI YunShuai4, Lü Pei1, YAO Yong1, LI ZhuoFan1     
1. MOE Key Lab of Submarine Geosciences and Prospecting Techniques, Institute for Advanced Ocean Study, College of Marine Geosciences, Ocean University of China, Qingdao 266100, China;
2. Laboratory for Marine Geology, Pilot National Laboratory for Marine Science and Technology, Qingdao 266037, China;
3. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
4. Institute of Surface-Earth System Science, Tianjin University, Tianjin 300072, China
Abstract: Field, microscopic petrological, geochemical and geochronological observation indicate that the plagioclase-rich and K-feldspar-rich leucosomes were derived from partial melting of eclogite and gneiss during exhumation of the North Qaidam deeply subducted continental slab. Cathodoluminescence images show that zircons from the plagioclase-rich leucosomes have core-rim texture. The zircon cores exhibit weak zoning and flat heavy rare earth elements (HREE), without obvious Eu anomaly, and the age of~450Ma agrees well with the timing of eclogite-facies metamorphism in this area. The zircon rims show weak zoning and low Th/U ratios, and the age of~426Ma represents timing of melt crystallization. The K-feldspar-rich leucosomes record three clusters of age including~910Ma, ~450Ma and~426Ma, which represent the timing of protolith, HP-UHP metamorphism and anatexis, respectively. The plagioclase-rich leucosomes are characterized by high in SiO2, Al2O3, CaO, Na2O, Sr and light rare earth elements (LREE), but low in MgO, FeOT, K2O, Y, Yb and HREE, similar to tonalite and trondhjemite. In contrast, the K-feldspar-rich leucosomes are characterized by enrichment of SiO2, Al2O3 and K2O+Na2O, but poor in CaO, MgO and REE, similar to granite. Partial melting of UHP eclogite and felsic gneiss during the initial retrogression stage with plagioclase-rich and K-feldspar-rich leucosome formation was triggered by dehydration melting involving predominant zoisite and muscovite. The melt evolution from the leucosomes produced at the early exhumation stage to syn-collisional granitoids produced at the late exhumation might contribute greatly to the exhumation of the North Qaidam UHP metamorphic belt, which will also have important implication on element migratization and crust-mantle interaction in continental subduction channel.
Key words: North Qaidam Mountain    Anatexis    Exhumation    Petrology    

大陆俯冲带超高压变质岩石的部分熔融是近年来超高压变质作用的重要进展之一,它不仅会影响深俯冲岩片流变学特征和促进深俯冲岩片快速折返,同时也对研究大陆俯冲隧道中矿物反应和元素迁移行为具有重要意义(Labrousse et al., 2011; Zheng et al., 2011; Liu et al., 2012; Chen et al., 2013; Chen and Zheng, 2013; Wang et al., 2014)。在大陆碰撞过程中,超高压变质岩在俯冲和折返过程中通常经历了进变质、峰期变质和多期退变质等复杂的变质演化阶段,有效识别深熔作用发生的时间和温压条件对于揭示超高压岩石的深熔作用与深俯冲岩片折返之间的联系具有重要意义。但围绕该问题的研究还存在较多争议,一些学者通过对苏鲁造山带、挪威西部片麻岩单元、格陵兰加里东造山带深熔作用的野外观察、年代学、地球化学,以及实验和模拟研究认为部分熔融作用主要发生在超高压变质条件下(Hermann and Green, 2001; Wallis et al., 2005; Lang and Gilotti, 2007; Labrousse et al., 2011; Chen and Zheng, 2013)。相反地,也有一些学者认为深熔作用可能发生在热的麻粒岩相退变质阶段(the hot exhumation stage)(Zong et al., 2010; Liu et al., 2012)或晚期角闪岩相退变质阶段(Banno et al., 2000; Gordon et al., 2012)。值得注意的是,近年来的研究显示一些典型的大陆碰撞造山带(如喜马拉雅、挪威西部片麻岩单元、新几内亚超高压-高压变质地体和阿尔卑斯造山带)可能记录了多期深熔事件(Monteleone et al., 2007; Rubatto et al., 2009; Imayama et al., 2012; Gordon et al., 2012)。因此,揭示大陆碰撞造山带深熔作用的时限、规模、程度、P-T条件和流体机制等不仅对于深入理解深俯冲岩片的流变学特征、大陆碰撞带的构造热演化过程和折返动力学机制具有重要意义,同时也为对大陆俯冲隧道中元素迁移和壳-幔相互作用的研究提供了重要依据。

柴北缘是近十余年所厘定的一条主要由榴辉岩、石榴橄榄岩及相关片麻岩组成的大陆碰撞造山带(图 1Yang et al., 1998, 2006; Yang and Powell, 2008; Song et al., 2003, 2004, 2005, 2006, 2007, 2014a; Zhang et al., 2005, 2008a, b, 2009a, b, 2010, 2016, 2017; Mattinson et al., 2006, 2007; Yu et al., 2013; Chen et al., 2017, 2018)。前人已从岩石学、年代学和地球化学等方面对柴北缘地区高压/超高压榴辉岩、石榴橄榄岩及围岩片麻岩开展了大量工作,并取得了一系列重要研究成果。近年来,与高压-超高压变质岩石形成和折返有关的深熔作用也开始引起了一些学者重视,但目前关于柴北缘地区深熔作用的成因机制还存在洋壳折返熔融、陆壳折返熔融和上盘增厚陆壳熔融等争议(Chen et al., 2012; Yu et al., 2012, 2014, 2015a, b, 2018, 2019a, b; Liu et al., 2014; Song et al., 2014b; Zhang et al., 2015; Cao et al., 2017)。有鉴于此,本文选择柴北缘绿梁山和锡铁山单元的混合岩为主要研究对象,在详细的宏观和微观岩石学研究的基础上,综合已有的年代学和地球化学结果,探讨超高压折返过程的深熔作用机制及其与同碰撞花岗岩的成因联系。

图 1 柴北缘UHP变质带区域地质简图(a)及锡铁山单元(b)和绿梁山单元(c) Fig. 1 Simplified geological map of the North Qaidam UHP metamorphic belt (a), Xitieshan unit (b) and Lvliangshan unit (c)
1 区域地质背景

位于青藏高原北缘的柴北缘超高压变质带是典型的早古生代大陆深俯冲变质带(图 1a),以发育花岗片麻岩(>80%)、副片麻岩/片岩,以及少量呈透镜体(或块体)形式存在的榴辉岩、基性麻粒岩、石榴橄榄岩等为特征(Zhang et al., 2016, 2017)。泥质片麻岩锆石中的柯石英包体,榴辉岩锆石、石榴石和绿辉石等矿物中的柯石英包体(Zhang et al., 2009a, b, 2010),以及石榴橄榄岩锆石中的金刚石包体指示大陆板片俯冲深度可能超过200km(Song et al., 2005)。根据岩石组合、变质演化历史等特征的明显差异,柴北缘超高压变质带由东向西可划分出四个次级构造单元(Zhang et al., 2008b):(1)都兰榴辉岩-片麻岩单元(DLT);(2)锡铁山榴辉岩-片麻岩单元(XTT);(3)绿梁山石榴橄榄岩-片麻岩单元(LLT);(4)鱼卡榴辉岩-片麻岩单元(YKT)。本文所研究的混合岩主要位于柴北缘的锡铁山和绿梁山单元。

锡铁山榴辉岩-片麻岩单元主要由石榴蓝晶(夕线)黑云二长片麻岩/片岩、花岗片麻岩和少量以透镜体形式存在的榴辉岩、超基性岩组成,并被~428Ma花岗岩岩体侵入(图 1b孟繁聪等, 2005)。定向的夕线石和黑云母所组成的SSE-NNW向面理(S1)和近水平拉伸线理指示区域高温变形(Zhang et al., 2008b)。锡铁山单元发育的榴辉岩以石香肠状、透镜状抑或是夹层的形式存在于花岗片麻岩和副片麻岩中。与其它超高压单元相比,锡铁山单元很少存在新鲜的榴辉岩,其榴辉岩大多已退变为麻粒岩和石榴角闪岩。榴辉岩峰期矿物组合为石榴子石+绿辉石+多硅白云母(极少)+金红石,其峰期变质条件为T=750~790℃和P=27~32kbar(Zhang et al., 2011)。研究表明,锡铁山榴辉岩和围岩石榴蓝晶(夕线)黑云片岩/片麻岩经历了相同的变质P-T演化轨迹,即在430~450Ma经历了陆壳深俯冲超高压变质,随后在422~425Ma经历了折返麻粒岩相退变质改造(Zhang et al., 2008b; Chen et al., 2013)。

绿梁山石榴橄榄岩-片麻岩单元分布在大柴旦南部约20km处,主要由含有蓝晶石/夕线石的副片麻岩/片岩、花岗片麻岩、基性麻粒岩和超镁铁质杂岩组成,并被志留纪花岗岩岩体侵入(图 1cZhang et al., 2008b)。超镁铁质岩以石榴二辉橄榄岩为主,夹有少量的纯橄岩和石榴辉石岩,主要呈不规则透镜体出露在片麻岩中(Yang and Deng, 1994; Song et al., 2004, 2005; Zhang et al., 2008a)。石榴子石中的金红石+辉石+角闪石和橄榄石中的钛铁矿+铝铬铁矿的出溶结构,以及锆石中存在的金刚石包体,表明该石榴橄榄岩俯冲深度超过200km。石榴橄榄岩的峰期变质条件为T=980~1130℃和P=45~65kbar(Song et al., 2004, 2005)。基性麻粒岩主要由石榴子石、单斜辉石、斜方辉石、斜长石、角闪石和石英组成,含少量金红石、钛铁矿和锆石。岩石学观察和P-T计算结果表明其主要经历了高压麻粒岩相阶段(T=720~830℃和P=10~14kbar)和随后的中压麻粒岩相退变质阶段(T=730~850℃和P=6.2~8.5kbar)(Zhang et al., 2008b)。最近, 绿梁山单元中也报道有少量新鲜榴辉岩的出露(Cao et al., 2017; Chen et al., 2018),推测其峰值变质条件为T=647~768℃和P>30kbar(Chen et al., 2018),但仍缺乏可靠温压计算结果和超高压标志矿物支持。

2 岩石学特征

在野外露头上,富含斜长石的浅色体呈层状、斑块状或脉状分布在退变榴辉岩中(图 2a),这些浅色体通常与寄主退变榴辉岩同时变形,且浅色体没有直接与片麻岩接触(Yu et al., 2019a)。由石英+斜长石±钾长石组成的粗粒斑块通常包裹中-粗粒石榴子石(图 2b),被解释为花岗质浅色体和转熔相组成的原位新成体(In situ)。在薄片尺度上,退变榴辉岩(残留相)主要由石榴子石、角闪石、以及单斜辉石+斜长石组成的后成合晶等矿物组成(图 3a),石榴子石内部保存较少的绿辉石包体指示早期经历了峰期榴辉岩相变质作用(Cao et al., 2017; Chen et al., 2018)。石榴子石内部发育的云母+石英+长石组成的多相固体包裹体或纳米花岗岩通常被解释为石榴子石在生长过程中捕获的花岗质熔体(Cesare et al., 2009; Yu et al., 2015a)。单斜辉石和/或石榴子石之间或沿着石榴子石的颗粒边界,发育的不规则拉长的斜长石+石英可能代表了早期熔体中的结晶(Cao et al., 2017)。沿着白云母-石榴子石(或单斜辉石)的颗粒边界存在不规则、拉长的尖锐长石和石英颗粒同样结晶自先前的熔体(图 3b)。由石英和长石组成的长英质显微脉体代表了早期熔体通道,表明熔体的微尺度迁移可能限于毫米级别(图 3c)(Sawyer, 2008; Holness et al., 2011)。

图 2 锡铁山和绿梁山地区混合岩的野外产物特征 (a)退变榴辉岩中不规则的花岗质浅色体,脉体与围岩不连通;(b)浅色体中见转熔的粗粒石榴子石;(c、d)薄层浅色体平行于片麻岩的片麻理或面理;(e)浅色体发生褶皱变形;(f)浅色体发生近距离迁移和汇聚;(g)星云状全熔岩中半自形-自形石榴子石;(h、i)浅色体发生大规模汇聚和远距离迁移形成长英质岩席或岩墙 Fig. 2 Field characteristics of migmatites in the Xitieshan and Lvliangshan units (a) field occurrences of granitic leucosomes in the retrograde, euhedral-subhedral garnet grains within the leucosomes are larger than those in the melanosome; (b) peritectic garnets within the leucosomes are commonly larger than those within the melanosome; (c, d) thin layers of leucosomes are aligned subparallel to the gneissosity or foliation; (e) folding deformation leads to the migration and aggregation of felsic leucosome in migmatite; (f) the granitic leucosome merges into the felsic sheet with petrographic continuity; (g) euhedral-subhedral garnet in the nebulous diatexite migmatite; (h, i) array of interconnected leucosomes coalesce in felsic sheet or dyke

图 3 锡铁山和绿梁山地区混合岩的显微结构特征 (a)残留相主要由石榴子石、角闪石,以及单斜辉石+斜长石后成合晶组成;(b)多硅白云母-辉石颗粒边界发育的高度尖锐长石颗粒;(c)石榴子石-辉石颗粒边界发育的显微长英质脉体;(d)混合片麻岩中浅色体和暗色体交替出现;(e、f)长石-石英颗粒边界高度尖锐钾长石;(g)多硅白云母分解为黑云母,钾长石和石英;(h)长石颗粒边界发育的钾长石+斜长石+石英楔形矿物集合体;(i)长石颗粒发育的拉长的显微脉体,主要由钾长石、斜长石和石英等矿物组成. Grt-石榴子石;Cpx-单斜辉石;Amp-角闪石;Pl-斜长石;Kfs-钾长石;Fsp-长石;Qtz-石英;Bt-黑云母;Ms-白云母 Fig. 3 Microtextures of migmatites in the Xitieshan and Lvliangshan units (a) the residuum is composed of garnet, amphibole, and clinopyroxene+plagioclase symplectite; (b) irregularly shaped films of feldspar grains along boundaries of phengite-phengite and phengite-clinopyroxene; (c) elongated veinlets between garnet and clinopyroxene; (d) leucosome and redidum within migmatite, the plagioclase and quartz grains in the leucosome layers are much coarser than those in the melanosome layers; (e, f) elongated K-feldspar grains occur in the form of veinlets along the feldspar-quartz and feldspar-feldspar grain boundaries, exhibiting low feldspar-feldspar-quartz dihedral angles; (g) phengite is rimmed by biotite, K-feldspar and quartz; (h) patch of plagioclase, K-feldspar, quartz occurs as pocket between boundaries of plagioclase-quartz; (i) elongated, veinlet comprising of K-feldspar + plagioclase + quartz between plagioclase boundaries. Grt-garnet; Cpx-clinopyroxene; Amp-amphibole; Pl-plagioclase; Kfs-K-feldspar; Fsp-feldspar; Qtz-quartz; Bt-biotite; Ms-muscovite

从野外关系上看,富含钾长石的浅色体主要呈层状、似脉状或网脉状分布在灰色、灰白色的长英质片麻岩中,并显示出混合岩化的特征,与典型原地深熔作用的野外特征基本一致(图 2c, d)。浅色体宽度可从几毫米变化到十几厘米,在浅色体中偶尔可见暗色矿物(角闪石或云母)组成的条带。在变形较强区域,浅色体和暗色体一起发生了褶皱变形(图 2e),也可见暗色的长英质片麻岩在浅色体中成布丁状分布(Yu et al., 2019a)。一些浅色体发生近距离迁移和局部汇聚形成源区浅色体(In-source)(图 2f)。在全熔混合岩中(Diatexite),转熔中-粗粒石榴子石的粒度可达1~2cm(图 2g)。浅色体发生大规模、远距离的迁移可形成长英质岩墙或岩脉(图 2h, i)。从显微结构来看,不规则、细长且高度尖锐的钾长石颗粒出现在基质中长石-石英边界之间,可解释为先前熔体的结晶(图 3e, f)。白云母颗粒边界发育尖锐的、不规则的微斜长石颗粒,而且白云母边界溶蚀明显,形成锯齿状不规则的边界,指示深熔作用可能与白云母的分解密切相关(图 3g)。在粗粒石英和斜长石颗粒边界发育的楔形斑或囊状体主要由钾长石+斜长石+石英±云母等矿物组成,可能结晶自先前的花岗质熔体(图 3h)(Sawyer, 2008; Holness et al., 2011)。由钾长石+石英+斜长石组成的长英质显微脉体沿斜长石的颗粒边界分布,也代表了早期熔体迁移的通道(图 3i)。

3 年代学和地球化学特征

阴极发光图像显示,锡铁山和绿梁山地区退变榴辉岩中富含斜长石的浅色体中的锆石具有明显的核-边双层结构(图 4)。锆石核部无明显分带特征,属典型变质成因锆石,拉曼光谱分析结果显示锆石内部发育石榴子石和单斜辉石等矿物包裹体,并呈现出重稀土平坦和无Eu异常的稀土配分模式,反映锆石可能形成在榴辉岩相变质阶段,且~450Ma的年龄结果也与区域上榴辉岩的峰期变质时代一致。发光较弱的锆石边部具不明显的环带结构和较低的Th/U比值,~426Ma的年龄结果代表了熔体的结晶时代(图 5Yu et al., 2015a, 2019a)。长英质片麻岩中发育的富含钾长石的浅色体中的锆石U-Pb定年结果主要分为三组:~910Ma、~450Ma和~426Ma。锆石的继承性核部发育典型岩浆环带,而且~910Ma的年龄结果与区域上花岗质片麻岩原岩结晶时代基本一致。锆石幔部发光较弱,无明显分带特征,属典型变质成因锆石,而~450Ma的年龄结果与区域上榴辉岩相变质作用时代基本一致,指示锆石弱发光的变质幔部可能形成在高压/超高压榴辉岩相变质阶段。锆石边部具有低的Th/U比值,并发育不明显环带,与大别-苏鲁、阿尔卑斯等地混合岩中的深熔锆石特征较为相似,~426Ma的年龄结果代表了富含钾长石浅色体的结晶时代(Yu et al., 2015b, 2019a)。

图 4 锡铁山和绿梁山地区浅色体中锆石的阴极发光特征 (a-c)榴辉岩中浅色体;(d-f)长英质片麻岩中浅色体;年龄数据来自于Yu et al., 2015a, b Fig. 4 Cathodoluminescence images of zircons from leucosome in the Xitieshan and Luliangshan units (a-c) leucosome within eclogite; (d-f) leucosome within felsic gneiss. Some age data from Yu et al., 2015a, b

图 5 锡铁山和绿梁山地区浅色体中锆石U-Pb年龄结果 年龄数据来自于Chen et al., 2012; Yu et al., 2015a, b, 2019a; Zhang et al., 2015; Cao et al., 2017; 图 6图 7 Fig. 5 Zircon U-Pb ages of leucosomes in the Xitieshan and Lvliangshan units Some age data from Chen et al., 2012; Yu et al., 2015a, b, 2019a; Zhang et al., 2015; Cao et al., 2017; also in the Fig. 6 and Fig. 7

图 6 锡铁山和绿梁山地区浅色体和同碰撞花岗岩的K2O-Na2O (a)和A/NK-A/CNK (b)图解 Fig. 6 Plots of K2O vs. Na2O (a) and A/NK vs. A/CNK (b) for leucosomes and plutons in the Xitieshan and Lvliangshan units

图 7 锡铁山和绿梁山地区浅色体和同碰撞花岗岩的An-Or-Ab分类图解 Fig. 7 Plots of An-Or-Ab for leucosomes, felsic sheets and plutons in the Xitieshan and Lvliangshan units

总体上讲,退变榴辉岩中富含斜长石的浅色体具有较高的SiO2、Al2O3、Na2O和CaO含量, 而相对较低的K2O、TiO2、MgO和MnO含量,其A/CNK值主要介于1.0和1.1之间,显示出弱过铝质的特征(图 6bChen et al., 2012; Yu et al., 2015a, 2019a)。值得注意的是,富含斜长石的浅色体Na2O/K2O比值明显大于2.0。在An-Ab-Or的分类判断别图解中(图 7),浅色体主要落入英云闪长岩和奥长花岗岩区域,与变玄武岩或榴辉岩部分熔融实验的熔体地化特征较为相近。此外,浅色体具有高Sr和La,低Y和Yb含量,以及相对应的高Sr/Y和La/Yb比值,与典型的埃达克质岩石类似。根据稀土元素特征,富含斜长石的浅色体可以进一步划分为两种类型:类型Ⅰ具有相对较低的稀土含量,以及明显的正Eu、Sr异常,可能代表了早期的斜长石堆晶;类型Ⅱ具有更高的稀土含量,且无明显的Eu异常(Yu et al., 2015a; Cao et al., 2017)。片麻岩中发育的富含钾长石的浅色体具有高的SiO2、Al2O3和K2O+Na2O含量,以及较低的CaO、MgO和FeOT含量,且Na2O/K2O比值大多都明显小于1.0(图 6a)。浅色体的A/CNK值主要介于1.0和1.1之间,属弱过铝质(图 6b)。在An-Ab-Or的分类判别图解中,这些富含钾长石的浅色体主要落入花岗岩区域,与变杂砂岩、云母片岩或英云闪长岩部分熔融实验的熔体地化特征较为相近。稀土元素球粒陨石标准化配分图整体表现为右倾型,即轻稀土富集,而重稀土相对亏损,部分样品具有明显的正Eu异常(Yu et al., 2015b, 2019a; Zhang et al., 2015)。

4 形成机制讨论 4.1 超高压榴辉岩的部分熔融机制

实验岩石学研究表明,在流体存在的条件下,变基性岩和变泥质岩在650~700℃时即可发生部分熔融作用形成长英质熔体(Sawyer, 2010; Zheng et al., 2011; Brown, 2013)。然而,在没有流体加入的情况下,变基性岩脱水熔融反应所需的温度要明显高于变泥质岩和花岗质岩石(Clemens, 2006)。在柴北缘绿梁山和锡铁山地区,宏观和微观的岩石学观察结果显示富含斜长石的长英质浅色体是来源于寄主退变榴辉岩的原地部分熔融,而非外来熔体注入(图 2图 3)。另外,富含斜长石的浅色体和退变榴辉岩具有近乎一致的Sr和Nd同位素组成,进一步证明它们具有亲缘性(Yu et al., 2015a)。多硅白云母和黝帘石是超高压榴辉岩中最重要的含水矿物,这些含水矿物的分解在大陆碰撞过程中超高压岩石的部分熔融作用中至关重要(Lang and Gilotti, 2007; Zheng et al., 2011; Chen and Zheng, 2013)。微观结构观察表明,榴辉岩的部分熔融可能与多硅白云母的分解有关,例如,不规则细长的长石或石英颗粒沿着多硅白云母的颗粒边界生长(图 3b)。在变基性岩中,多硅白云母的脱水熔融通常会产生富K的熔体(Hermann et al., 2006; Schmidt et al., 2004)。然而,地球化学特征表明,退变榴辉岩中发育的浅色体含有少量的钾长石矿物且K2O含量低,这排除了榴辉岩的深熔作用是以多硅白云母分解为主的可能性。此外,锡铁山和绿梁山地区大多数的榴辉岩是具有非常低的K2O含量的双矿物组合(绿辉石和石榴子石)(孟繁聪等, 2003; 杨经绥等, 2003),通常几乎没有多硅白云母。也就是说,较少的多硅白云母不足以产生如此多的熔体。锡铁山和绿梁山地区富含斜长石的浅色体具有高Sr、Sr/Y值和富集LREE的地球化学特征,与黝帘石分解实验产生的熔体性质相似(Skjerlie and Patiño Douce, 2002)。实验结果表明:在P≤2.5GPa条件下,且温度足够高时,黝帘石和多硅白云母可以或多或少地同时分解。两种含水矿物共存时的熔融起始温度比只有一种矿物的熔融起始温度低约100~200℃(Vielzeuf and Schmidt, 2001; Zheng et al., 2011)。锡铁山和绿梁山榴辉岩的P-T轨迹也与多硅白云母和黝帘石脱水熔融实验结果吻合。因此,锡铁山和绿梁山单元的榴辉岩在折返过程中部分熔融可能主要受大部分的黝帘石和少量的多硅白云母的脱水熔融反应控制(如:黝帘石+白云母+石英→石榴子石+蓝晶石+熔体)(Skjerlie and Patiño Douce, 2002)。基于浅色体较高的Ba/Rb和Na/K比值,以及较低的Rb/Sr比值,Cao et al. (2017)认为钠质角闪石脱水熔融也可以解释柴北缘地区富含斜长石浅色体的形成。

4.2 超高压长英质片麻岩的部分熔融机制

野外关系和岩相学特征指示锡铁山和绿梁山地区富含钾长石的浅色体可能主要来自于寄主超高压长英质片麻岩的原地深熔作用。富含钾长石的浅色体和寄主片麻岩的锆石中记录的三组U-Pb年龄结果进一步证明了这一推断:例如~910Ma的继承岩浆核、~450Ma的榴辉岩相变质幔和~426Ma的深熔边。富含钾长石的浅色体和超高压长英质片麻岩之间的亲缘关系也得到类似的Nd和Hf同位素证实(图 8)(Yu et al., 2015b; Zhang et al., 2015)。实验和微观研究表明,多硅白云母是超高压变泥质岩和变花岗岩中最常见的含水矿物之一,而多硅白云母的分解则通常会导致深俯冲板片发生部分熔融(Hermann, 2002; Skjerlie and Patiño Douce, 2002; Auzanneau et al., 2006)。在锡铁山和绿梁山单元,混合片麻岩中粗粒多硅白云母的颗粒边界发生明显的溶蚀,且边缘局部发育有黑云母、石英和不规则微斜长石等矿物,指示了多硅白云母的分解(图 3g)。在石榴子石内部,也可见到多硅白云母颗粒边界被钾长石和石英等矿物取代,同样证实了白云母的脱水熔融反应(Yu et al., 2015b)。实验研究表明,多硅白云母分解引起的超高压片麻岩熔融通常会形成富钾的花岗质熔体,这与研究区的富含钾长石浅色体组分一致(Huang and Wyllie, 1981, 1986; Stern and Wyllie, 1981; Auzanneau et al., 2006)。此外,超高压长英片质麻岩的P-T轨迹与实验数据约束下的白云母脱水熔融曲线相交(Yu et al., 2015b)。因此,在锡铁山和绿梁山单元,白云母参与的脱水反应是触发超高压长英质片麻岩部分熔融的主要机制:白云母+斜长石+石英→黑云母+蓝晶石/夕线石+钾长石+熔体(Vielzeuf and Holloway, 1988)。

图 8 锡铁山和绿梁山地区浅色体和同碰撞花岗岩的Sr-Nd同位素特征 数据来自于Yu et al., 2015a, b, 2019a Fig. 8 Sr-Nd isotope for leucosomes and syn-granitoids in the Xitieshan and Lvliangshan units Some data from Yu et al., 2015a, b, 2019a
5 超高压片麻岩的部分熔融与同碰撞花岗岩的成因关系

地壳深熔作用能产生规模不一的熔体,如纳米花岗岩(Nanogranite)、原位(In-situ)和源区浅色体(In-source),长英质岩席和深成岩体等。然而,同碰撞花岗质岩浆作用与深俯冲板片部分熔融之间的关系还不清楚,主要受限于以下几个因素:(1)后造山伸展作用叠加;(2)精确约束部分熔融、变质作用和深成岩体的侵位时间存在困难;(3)变质作用,岩浆作用和部分熔融作用的多期性(Costa and Rey, 1995; Foster and Fanning, 1997; Vanderhaeghe and Teyssier, 1997)。因而,野外观察、岩石学、地球化学和年代学的综合研究有助于揭示同碰撞花岗质岩浆作用与深俯冲板片部分熔融之间的成因关系。

在柴北缘超高压变质带的锡铁山和绿梁山地区分别出露~60km2、~15km2的同碰撞花岗岩。野外观察研究表明这些花岗岩与周围的长英质片麻岩呈侵入接触关系。年代学研究揭示同碰撞花岗岩在430~415Ma期间侵入到锡铁山和绿梁山地体,与深俯冲板片部分熔融的时间重叠,这为我们建立超高压变质岩的部分熔融与同碰撞岩浆作用之间联系提供了重要契机。一些研究认为这些同碰撞花岗岩可能与超高压榴辉岩或未俯冲的长英质片麻岩的部分熔融有关。然而,部分同碰撞花岗岩中的继承锆石记录了约450Ma的变质年龄,与之前报道的锡铁山和绿梁山榴辉岩以及长英质片麻岩的超高压变质作用时代一致,这表明同碰撞花岗岩更可能来源于深俯冲大陆板片的部分熔融,而非未俯冲的长英质片麻岩的部分熔融。地球化学结果表明这些富钾的同碰撞花岗岩和超高压片麻岩中富含钾长石的浅色体具有相似的主微量元素特征,明显不同于退变榴辉岩中富集斜长石的浅色体(Yu et al., 2019a)。相似的高87Sr/86Sr比值和低εNd(t)值同样证明超高压片麻岩和同碰撞花岗岩之间具有一定的亲缘性。然而,我们最新的研究发现柴北缘超高压片麻岩在后期折返阶段也经历了黑云母分解所引起的部分熔融作用。因此,柴北缘同碰撞花岗岩究竟是来源于超高压片麻岩中白云母抑或是黑云母脱水熔融作用仍需进一步工作验证。

6 深熔作用与大陆深俯冲板片的快速折返

柴北缘锡铁山和绿梁山单元超高压片麻岩和退变榴辉岩保存有完好的指示部分熔融的宏观和微观结构(Chen et al., 2012; Yu et al., 2015a, b; Zhang et al., 2015; Cao et al., 2017)。尽管一些学者认为深俯冲板片的部分熔融可能发生在超高压变质阶段(如深熔锆石中的柯石英包体)(Chen and Zheng, 2013),但越来越多的研究显示大规模的部分熔融可能主要发生在热的折返阶段或者后期角闪岩相阶段(Zong et al., 2010; Liu et al., 2012)。大量的研究表明柴北缘榴辉岩和片麻岩中长英质浅色体的变质锆石记录了榴辉岩相变质作用时代约为445~450Ma,而新生锆石或者锆石增生边部则记录的熔体结晶时限约为422~433Ma,明显早于晚期角闪岩相退变质作用时代(约410Ma,Li and Yu, unpublished)。因此,柴北缘超高压变质岩在大陆碰撞的早期折返阶段经历了由白云母+黝帘石或者角闪石脱水熔融诱发的初始深熔作用。实验结果表明少量的熔体可以急剧的改变深俯冲板片的流变学性质从而促进超高压板片的快速折返(Hermann and Green, 2001; Labrousse et al., 2002; Chopin, 2003; Zheng et al., 2011)。数值模拟结果证实含熔体的超高压板片折返到地壳深度的平均速率可达2~4.5cm/yr(Ellis et al., 2011; Sizova et al., 2012),这一速率与许多超高压地体中超高压岩石折返到地壳深度的最快折返速度类似。在巴布亚新几内亚地区,含熔体的超高压板片折返到地壳深度的平均速率是1.5~2cm/yr(Monteleone et al., 2007; Gordon et al., 2012)。在柴北缘地区,含熔体的超高压板片从地幔折返到地壳深度的折返速率为1~2cm/yr(Yu et al., 2015b),这一速率可以与其它超高压地体中含熔体岩石的折返速率以及数值模拟的数据结果相比较。因此,来源于早期折返阶段产生的浅色体显著的促进了柴北缘超高压地体从地幔深度折返到下地壳深度。岩石学、地质年代学和地球化学的观察结果表明深熔熔体经历了不同规模的运移和聚集,并以长英质岩脉、岩席的形式迁移到深俯冲大陆板片的外侧。熔体的迁移导致了上覆地幔楔富集LILE、LREE和放射性同位素,这种熔体交代作用对于大陆俯冲隧道中的元素迁移和壳-幔作用具有重要影响(Zheng et al., 2011)。

致谢      感谢宋述光教授、刘平华副研究员和匿名审稿人对本文提出的宝贵修改意见。

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