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喜马拉雅是地球上平均海拔最高的山脉(大于6000m),全球14座8000m以上的山峰均分布于印度-亚洲大陆边界的喜马拉雅山脉和喀喇昆仑山脉。它是地球上异常靓丽的风景线,吸引了无数攀登者、游客和地质学家。人们不禁会问,如此高耸的山脉是如何形成的呢?喜马拉雅山是通过印度-亚洲大陆从新生代初期以来的陆陆碰撞造山过程逐步形成的。研究喜马拉雅的碰撞造山过程,主要有两个层面的意义:① 喜马拉雅造山带是威尔逊旋回中陆陆碰撞造山过程的典型代表,现今正在活跃、过程相对清晰,从中揭示的造山过程和规律,可为探索地球从古至今的大陆碰撞造山演化所借鉴,如喜马拉雅造山带的下地壳部分熔融及隧道流(Nelson et al., 1996; Beaumont et al., 2001; Searle and Szulc, 2005)、地壳物质组成和结构(Gao Li'e et al., 2017)、反转变质带(Arita, 1983; Jain and Manickavasagam, 1993)、淡色花岗岩等(Le Fort, 1975; Searle et al., 1997; Zhang Hongfei et al., 2004a; Zeng Lingsen et al., 2011; 吴福元等, 2015);② 喜马拉雅山与青藏高原一起组成了世界第三极,其隆升开启或加强了亚洲季风环流(An Zhisheng et al., 2001; Guo Zhengtang et al., 2002; Clift et al., 2008; Ding Lin et al., 2017),从而改变了亚洲生物赖以生存的环境气候格局;使原本处于副热带高气压带的应为干旱贫瘠的我国东南地区,变成了如今湿润富饶的江南(李吉均,1998)。
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对喜马拉雅碰撞造山过程的研究,需要综合构造地质学、岩石学、沉积学、地球物理、地貌学等多种学科。本文从变质地质学视角出发,融入多学科研究成果,介绍了大陆碰撞造山机制存在的争论、喜马拉雅的基本构造单元,从空间(侧向生长—垂向演化)和时间(始新世—中新世)阐述近年来作者们在喜马拉雅碰撞造山过程研究中取得的进展。目的是以喜马拉雅为典型,认识大陆碰撞造山的过程、机制及其效应,为碰撞造山作用的研究提供借鉴和范例,并展望未来碰撞造山作用研究可能取得突破的研究方向。
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1 大陆碰撞造山机制
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认知大陆碰撞造山的核心机制,有助于我们理解大陆汇聚过程中山脉形成和高原生长的基本规律,更清晰地认识威尔逊旋回如何走向终结、完善板块构造理论,理解构造-气候相互作用的机理。喜马拉雅造山机制研究,是解析大陆碰撞造山核心机制的金钥匙,是固体地球科学研究的前沿科学问题。但是目前对于大陆碰撞造山的核心机制还存在较大争论,不同学者们曾提出过众多造山模型,但总体上以“临界楔”和“隧道流”两种端元模型为代表(图1)。
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1.1 大陆碰撞造山的端元模型:临界楔和隧道流
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临界楔( Critical Taper Wedge)多年以来一直被认为是造山带形成的核心机制(图1a),该机制由Davis et al.(1983)根据沙箱物理模拟实验和数理推导提出,最初被应用于解释北美科迪勒拉和台湾地区等增生造山带的扩展过程,后续被推广至解释碰撞造山带的形成机理,又被称为“推土机模型”。其核心依据为:造山带主体为刚性块体,在造山带俯冲碰撞前缘物质堆叠、发生挤压变形而形成哥伦布锥形楔(图1a),当应力积累到零界阈值时会产生破裂、逐渐向造山带前陆方向形成一系列叠瓦状逆冲推覆构造,从而主导堆叠的物质侧向被动挤出。在喜马拉雅,以该机制为基础又衍生出韧性挤出(Ductile extrusion;Grujic et al., 1996)、楔形挤出(Wedge extrusion;Robinson et al., 2003; Bollinger et al., 2006)、构造楔(Tectonic wedge; Webb et al., 2011)等详细模型。临界楔模型也被推广至解释全球众多碰撞造山带的构造-变质演化过程,如西阿尔卑斯造山带(如Engi et al., 2001)和加里东造山带(如Kontny et al., 2015)的褶皱冲断带。也被应用于解释与挤压造山过程相关的逃逸构造,如新生代以来亚洲东南部各个块体的走滑挤出(如Tapponnier et al., 1990; Zhang Bo et al., 2017a)。
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图1 大陆碰撞造山的两种核心机制:临界楔模型(a) (据Davis et al., 1983)和隧道流模型(b) (据Beaumont et al., 2001)
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Fig.1 Two core mechanisms of continental collision orogenesis: Critical Taper Wedge (a) (after Davis et al., 1983) and Channel flow (b) (after Beaumont et al., 2001)
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但是,随着隧道流(Channel flow)模型的提出(图1b),临界楔作为大陆碰撞造山核心机制的地位受到了挑战。该模型基于20世纪末“国际喜马拉雅和青藏高原深剖面及综合研究”(INDEPTH)计划的综合性成果而提出,发现藏南拉萨地块下方的中地壳存在“亮点构造”(剪切波低速带LVZ;Nelson et al., 1996; Brown et al., 1996; Unsworth et al., 2005)、被解释为部分熔融层。该模型强调加厚中下地壳的部分熔融物质具有低黏度、高浮力,能够提高岩石流动变形的能力(Hollister et al., 1986),数值模拟研究进一步演示了喜马拉雅造山带内广泛存在的部分熔融物质(混合岩)、在地形前锋剥蚀去载作用下(类似自来水龙头阀门),沿着管道像自来水一样流动而主动侧向折返(图1b;Beaumont et al., 2001),从而主导造山带的形成、维持其高地形,如喜马拉雅东构造结的“构造脉涨模型”(Zeitler et al., 2001)。由于该模型更全面地涵盖了喜马拉雅造山带内的构造-变质-岩浆现象、并对其内部高级变质岩的P-T-t轨迹给予了更全面的定量化约束,其适用性在近20年来受到了广泛关注和激烈讨论(如Jamieson et al., 2004, 2006; Searle and Szulc, 2005; Godin et al., 2006; Harris, 2007; King et al., 2007; Kohn, 2008; Leech, 2008; Gao Rui et al., 2016),并进一步被用于解释整个青藏高原的隆升过程(Clark and Royden, 2000)。该模型也逐渐被用于解释其他碰撞造山带的形成过程和高级变质岩的折返,如苏鲁大别造山带内发生部分熔融的超高压岩石的折返(Tsuboi et al., 2005)、巴西—非洲之间元古宙造山带的长期部分熔融(Cavalcante et al., 2018)。
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目前,对于大陆碰撞造山带形成的核心机制的争论正在激烈进行中。由于临界楔和隧道流模型在全球范围内的广泛应用,该争论严重影响了人们对于山脉和高原形成的深部过程的认识,造成上述模型在某些造山过程中被不合理借鉴。因此,有必要对这两种机制最主要的争论地区进行研究,定量区分究竟哪一种机制在何种条件下对造山带的形成起主导作用。
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1.2 临界楔与隧道流预测的P-T-t轨迹特征
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对大陆碰撞造山机制的不同认识,可通过反演碰撞造山带核部的高级变质岩的P-T-t-D轨迹来限定。P-T-t-D轨迹与岩石所经历的汇聚/埋藏速率、剥蚀速率、热传导过程等有关,因此可以被用来限定大陆碰撞造山的构造-热演化过程(England and Thompson, 1984; Ruppel and Hodges, 1994; 魏春景, 2011);将自然界观察所得的P-T-t-D轨迹与造山模型的预测进行对比,可以对高级变质岩折返过程中的核心机制进行区分(Kohn, 2008)。本文基于大陆碰撞带内岩石观测到的P-T-t轨迹和一维热模拟的结果(England and Thompson, 1984),对原始的临界楔模型预测的P-T轨迹进行了修正,受控于临界楔和隧道流模型而折返的碰撞造山带高级变质岩应分别具有以下P-T-t轨迹和变形特征(图2;据Beaumont et al., 2001; Kohn, 2008):
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(1)临界楔:造山带一级次序的岩石折返是顺序式向前陆盆地区域扩展的(图1a);因此,造山带内部的不同岩片从核部至前陆区域,压力峰期(P max)对应的温度和压力条件逐渐降低、压力峰期变质时代(t)逐渐变年轻,不同岩片之间具有P max阶段温压条件、峰期变质时代的间断,不同岩片间的构造边界可识别出强应变剪切带(D);在单个岩片中,中低级变质岩石通常具有发夹状的P-T轨迹,而高级变质岩石在到达压力峰期后只具有较小幅度升温、在折返过程中逐渐降温降压,单个岩石在部分熔融区间滞留的时间一般小于5~10Ma。
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图2 大陆碰撞造山带临界楔和隧道流模型预测的P-T-t轨迹(a~c)(据Beaumont et al.,2001; Kohn,2008)
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Fig.2 P-T-t paths predicted by the Critical Taper Wedge and Channel Flow models in continental collision orogens(a~c)(after Beaumont et al., 2001; Kohn, 2008)
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本文基于天然观测和一维热模拟结果(England and Thompson, 1984)对图2b中临界楔模型预测的P-T轨迹进行了修正
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In this paper, the P-T paths predicted by the Critical Taper Wedge model in Fig.2b are modified based on natural observations and results of 1D thermal simulation (England and Thompson, 1984)
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(2)隧道流:由于造山带核部的高级变质岩石主体是作为一个整体单一期次折返的(图1b),即使内部有多期折返、也是来自同一深度的部分熔融物质脉冲叠加的结果、是次级构造,因此高级变质岩的压力峰期(P max)对应的温度和压力条件以部分熔融的主隧道为中心逐渐向两边降低(图2),不同部位的压力峰期变质时代(t)无明显变化或是渐变过渡的,变形强度(D)是连续的、无明显强应变带;由于岩石浮力折返需要维持部分熔融,其中的高级变质岩石通常具有大幅度近等温降压的P-T轨迹,并且单个岩石部分熔融时间需要持续至少15Ma。
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1.3 喜马拉雅碰撞造山机制
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以上两种机制争论最激烈的为喜马拉雅和青藏高原地区,这里既是临界楔模型最重要的发展地(Grujic et al., 1996; Robinson et al., 2003; Bollinger et al., 2006; Webb et al., 2011)、也是隧道流模型的提出地(Nelson et al., 1996; Beaumont et al., 2001)。并且喜马拉雅造山带是最年轻的陆陆碰撞造山带、保存较为完整,是解决以上争议最重要的研究区(许志琴等, 2011; 张进江等, 2013; 曾令森和高利娥, 2017; 张泽明等, 2013, 2017)。在这其中,高喜马拉雅的折返过程研究,是揭示大陆碰撞造山核心机制的关键。
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在过去几十年的研究中,地质学家们普遍认为高喜马拉雅是由其顶部的藏南拆离系与其底部的主中央逆冲断层在早—中中新世同时代活动(26~14Ma)、作为一个整体单次折返的(Burchfiel et al., 1992; Hodges et al., 1992; Searle et al., 2009)。临界楔模型和隧道流模型都是依据这一基本构造格局、结合数值模拟的研究发展而来。但是,近20年来在喜马拉雅中部多个剖面的研究表明,高喜马拉雅内部存在两个或多个不同时期折返的变质岩片,其中有一条活动时代相似、空间层位相近的构造-变质不连续带,称为“高喜马拉雅逆冲断层”(High Himalayan Thrust, HHT;Kohn et al., 2004; Carosi et al., 2010, 2018; Imayama et al., 2012; Montomoli et al., 2013; Wang Jiamin et al., 2015a, 2016; Goscombe et al., 2018),对基于整体折返的传统造山模型及喜马拉雅的构造单元划分提出了挑战。
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针对新提出的“高喜马拉雅逆冲断层”对高喜马拉雅折返的贡献,目前有两种不同观点,分别对应于临界楔和隧道流两种造山机制,为区分究竟哪一种机制在造山过程起主导作用提供了机遇。观点一,认为高喜马拉雅的折返是顺序式(In-sequence)向南扩展的(即向前陆盆地方向;图1a, 图3),符合临界楔模型;高喜马拉雅的多个变质岩片及小喜马拉雅从北至南折返深度逐渐变浅、折返时代逐渐变年轻,构成一系列叠瓦状逆冲推覆岩片(Kohn et al., 2004; Carosi et al., 2018);此外,高喜马拉雅不连续带的活动时代相近、空间层位相似,可以构成一条区域规模的大型逆冲断层(Wang Jiamin et al., 2015a, 2016; Goscombe et al., 2018);因此,高喜马拉雅折返的主导机制为临界楔,而部分熔融物质的高浮力和低黏度只在岩石折返的早期阶段存在、贡献微弱。观点二,认为高喜马拉雅的折返符合隧道流模型;其作为部分熔融物质聚集的隧道、在高浮力和低黏度作用下整体折返,只不过后期受到来自同一深度的部分熔融物质脉冲叠加而形成了多期变质岩片(Pulsed Channel Flow; Hollister and Grujic, 2006; Larson et al., 2015; Mukherjee et al., 2012);因此高喜马拉雅的折返主体上应该是单一期次的,后期叠加改造时代具有多期性,而这种改造造成的折返过程应该是违序式扩展的(Out-of-sequence, 图3),其内部的高喜马拉雅不连续带为次级构造、在各个剖面中活动时代不一致、活动时间晚于主中央逆冲断层和藏南拆离系(Larson et al., 2013; Ambrose et al., 2015)。上述顺序式和违序式两种扩展模式下,高级变质岩片具有的核心区别是:顺序式模式下,变质年龄向南逐渐变年轻,即上盘变质年龄较老;而违序式模式下,部分上盘岩片折返时代更晚、变质年龄更年轻。
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2 喜马拉雅基本构造单元
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为理解喜马拉雅碰撞造山过程和机制研究中存在的问题和进展,以下首先对喜马拉雅的基本构造单元进行介绍。喜马拉雅造山带是由印度大陆和亚洲大陆在大约60~55Ma开始碰撞形成的(Hu Xiumian et al., 2015),位于印度大陆北缘的大型陆陆碰撞造山带(图4)。它北起雅鲁藏布江缝合带(以蛇绿混杂岩为标志),南至主前锋逆冲断层(MFT),东至缅甸东北部,西至西巴基斯坦褶皱带,为一呈狭长带状的强烈变形变质带, 东西绵延约2500km。喜马拉雅的基本构造单元,最早是由瑞士地质学家Augusto Gansser定义的(Gansser, 1964),后经法国地质学家Patrick Le Fort进一步修订(Le Fort, 1975),该格局一直被沿用至今,未有明显变化。
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图3 大陆碰撞造山带顺序式(左)和违序式(右)扩展方式下发育的逆冲断层时间和空间序列 (图中老、中、年轻代表造山带变质核的折返年龄)
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Fig.3 Time and space sequences of thrust faults developed under the in-sequence (left) and out-of-sequence (right) growth modes in continental collision orogens(the terms of “old, middle and young” in the figure represent the exhumation ages of orogenic metamorphic cores)
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由北至南,喜马拉雅被划分为(Gansser, 1964; Yin An, 2006;图4):① 特提斯喜马拉雅岩系(THS),原岩为奥陶系至始新统的海相复理石沉积夹中生代火山岩,变质级别主要为绿片岩相或未变质;② 高喜马拉雅结晶岩系(GHC),主要为角闪岩相至麻粒岩相变质、局部有榴辉岩透镜体,为喜马拉雅造山带核部的高级变质岩;③ 小喜马拉雅岩系(LHS),主要为绿片岩相至低角闪岩相变质的古元古代碎屑沉积;④ 次喜马拉雅岩系(SH),或称西瓦里克组(SG),为中新世前陆盆地沉积、岩性主要为粗—中粒砂岩(Najman, 2006),其中“西瓦”取自印度教神话主神湿婆Shiva。上述每一个构造单元,在地理格局上均对应于一座东西向绵延2000多千米的山脉。这些构造单元被四个大型构造边界所分隔,包括藏南拆离系(STD)、主中央逆冲断裂(MCT)、主边界逆冲断裂(MBT)、主前锋逆冲断层(MFT)。以藏南拆离系为界,藏南拆离系南部的构造单元构成了典型的背驮式逆冲推覆构造(Yin An, 2006);这些逆冲断层在深部的延伸被认为与主喜马拉雅逆冲断层(MHT)相连,是印度-亚洲碰撞带地震活动最发育的构造边界,以往许多7级以上大地震均发育于主喜马拉雅逆冲断层或其在浅表的分支断层(如2015年4月25日尼泊尔Gorkha大地震,矩震级7.8级;Galetzka et al., 2015; Panda et al., 2018)。藏南拆离系以北主要为伸展构造,包括藏南拆离系、北喜马拉雅片麻岩穹隆、南北向裂谷(Zhang Jinjiang and Guo Lei, 2007; Zhang Jinjiang et al., 2012)。本文接下来从变质地质学视角出发,对喜马拉雅研究中最受关注的核部高级变质岩(高喜马拉雅)及其边界构造做进一步简介。
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2.1 高喜马拉雅结晶岩系(GHC)
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高喜马拉雅结晶岩系(HHC)是喜马拉雅造山带核部的高级变质岩,在国际上,大喜马拉雅结晶岩系(Greater Himalayan Crystalline complex)作为地质概念被运用得更多(如Yin An, 2006)。本文遵从国内外学术界惯例,中文称之为高喜马拉雅(如许志琴等,2013),英文缩写沿用国际惯例GHC。需要注意的是,上述国内外学术界惯例用法差异造成本文中英文称谓并不对应。在地理上,它是喜马拉雅四条山脉中最高的一座,绝大多数7000m以上山峰的主体都是由高喜马拉雅岩石组成的(图4)。它主要为角闪岩相至麻粒岩相的混合岩化变质沉积岩组成(图5;Schelling, 1992; Groppo et al., 2009; Wang Jiamin et al., 2013, 2015a; Zhang Zeming et al., 2017b)、局部有榴辉岩透镜体(Lombardo and Rolfo, 2000; O'Brien et al., 2001; Groppo et al., 2007; O'Brien, 2019; Wang Jiamin et al., 2021),原岩主要为新元古界—寒武系的沉积岩。除此之外,高喜马拉雅中也夹杂着石英岩、钙质硅酸岩和变质火山岩夹层,并被早古生代的花岗质片麻岩侵位(Gehrels et al., 2006; Wang Xiaoxian et al., 2013)。在高喜马拉雅的顶部,常有大量淡色花岗岩侵位,多数成岩席状、少数为岩体,规模从几十至上千平方千米。岩性主要为含石榴子石花岗岩、含电气石花岗岩、二云母花岗岩等(图5),年龄主要为中新世(Le Fort, 1975; Harrison et al., 1995; Searle et al., 2003a; Cottle et al., 2007; Chambers et al., 2011; Wang Xianxian et al., 2013; 吴福元等, 2015; Wu Fuyuan et al., 2017, 2020;曾令森和高利娥,2017)。
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图4 喜马拉雅造山带地质简图及剖面示意图 (据Yin An, 2006: Wang Jiamin et al., 2016俢改)
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Fig.4 Schematic geologic map and diagrammatic cross-section of the Himalayan orogenic belt (modified after Yin An, 2006; Wang Jiamin et al., 2016)
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本图从西至东将喜马拉雅造山带按研究区细分为西构造结、藏斯卡喜马拉雅、库蒙喜马拉雅、尼泊尔喜马拉雅、锡金喜马拉雅、不丹喜马拉雅、东构造结7个地区; 高喜马拉雅内部的逆冲断层共同组成了高喜马拉雅逆冲断层 (HHT), 在每个剖面中具有当地称谓 (如JNF、BSZ、HHD、ST、RSZ、NT、HHT、SD、L-KT), 部分地区该断层位置尚末填图; 白色小三角除南迦巴瓦峰外均为8000m以上山峰
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From west to east, this map subdivides the Himalayan orogenic belt into seven regions: Western Himalayan syntaxis, Zanskar Himalaya, Kumaun Himalaya, Nepal Himalaya, Sikkim Himalaya, Bhutan Himalaya, and Eastern Himalayan syntaxis; The thrust faults in the Greater Himalayan Crystalline complex collectively form the High Himalayan Thrust (HHT), with has local names in each section (e.g., JNF, BSZ, HHD, ST, RSZ, NT, HHT, SD and L-KT); The location of the HHT has not yet been mapped in some areas; The small white triangles are all peaks above8000m except Namcha Barwa
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图5 高喜马拉雅代表性剖面:聂拉木地区岩石单元组成、变质带划分、变质温度条件及高喜马拉雅逆冲断层具体位置(剖面位置见图4中C—D)(据Wang Jiamin et al., 2013修改)
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Fig.5 Representative cross-section of the Greater Himalaya Crystalline complex: rock units, division of metamorphic zones, metamorphic temperature conditions, and specific locations of the High Himalayan Thrust in the Nyalam region (location of the cross-section is shown as C—D in Fig.4) (modified from Wang Jiamin et al.,2013)
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高喜马拉雅记录了印度-亚洲陆陆碰撞的关键信息,是喜马拉雅造山演化研究的焦点。涉及变质地质学的以往研究主要集中在以下几个科学问题:① 喜马拉雅的造山过程和机制,如著名的隧道流模型与临界楔模型的争论(Beaumont et al., 2001);② 跨越主中央逆冲断层的反转变质带的成因(Arita, 1983);③ 印度大陆地壳俯冲深度、方式(Guillot et al., 2008; Ding Huixia et al., 2016a);④ 淡色花岗岩成因与高喜马拉雅深熔作用的关系(Wu Fuyuan et al., 2017; Yang Lei et al., 2019)等等。
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2.2 藏南拆离系(STD)
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藏南拆离系位于高喜马拉雅顶部,是世界上规模最大的伸展构造之一(延伸2400km,图4),其将低级变质的特提斯喜马拉雅叠置于高级变质的高喜马拉雅之上(Burchfiel and Royden, 1985; Burchfiel et al., 1992)。藏南拆离系一般表现为几百米至几千米宽的剪切带,在运动学上表现为上盘向北的低角度下滑(＜30°)(Burchfiel et al., 1992; Hodges et al., 1992),剪切带发育于肉切村群内,主要包括黄带层和北坳组两部分(图6),其中黄带层由糜棱化大理岩、钙质硅酸岩和千枚岩组成,而肉切村群主要为糜棱化淡色花岗岩、长英质片岩、含角闪石片岩和少量钙质硅酸岩。它最早由我国学者常承法在第一次青藏高原综合科学考察时在聂拉木地区发现(常承法和郑锡澜,1973),当时认为该边界是逆断层;后经Burg et al.(1984)根据聂拉木地区的工作进一步厘定为正断层,其概念最终由Burchfiel et al.(1992)正式提出(图6a、b)。随后,Carosi et al.(1998)和Searle and Godin(2003a)根据珠峰地区的工作,进一步将藏南拆离系划分出两个拆离界面,上部为珠穆朗玛拆离断层、下部为洛子峰拆离断层(图6c)。
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图6 聂拉木和珠峰地区藏南拆离系野外特征和剖面图
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Fig.6 Field characteristics and cross-section and of the South Tibet Detachment in Nyalam and Mount Everest regions
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(a)—Burchfiel在聂拉木地区考察并于1992年首次提出藏南拆离系概念时依据的野外露头(注意黑色小箭头为STD位置);(b)—本文对聂拉木同一露头进行了岩石单元的详细划分和拆离界面的重新划分;(c) —珠峰北面山体藏南拆离系(据Carosi et al., 1998修改);本文依据1975年第二次珠峰综合科考采集的样品对岩性单元划分进行了修正(据尹集祥和郭师曾,1978);(d)—图5中上半段详细岩性-构造剖面图
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(a)—The field outcrop based on Burchfiel's investigation in the Nyalam area, who firstly proposed the concept of the South Tibetan Detachment in 1992 (note that the small black arrow is the STD position); (b)—the detailed division of rock units and the re-division of the detachment interface for the same outcrop in Nyalam; (c)—shows the South Tibetan Detachment in the north face of Mount Everest (modified after Carosi et al., 1998); in this paper, the division of lithologic units has been revised based on the samples collected during the1975Second Scientific Expedition of Mount Everest (modified after Yin Jixiang and Guo Shiceng, 1978);(d)—a detailed lithology-structural cross-section in the upper half of Fig.5
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藏南拆离系通常被认为是伸展构造(Searle et al., 2003b; Cottle et al., 2007; Leloup et al., 2010; Chambers et al., 2011),但是部分研究认为它是一条被动顶板断层,即在挤压应力作用下上下岩片相对运动产生,并不是大规模伸展的标志(如构造楔模型; Yin An, 2006; Webb et al., 2007, 2011)。其主要依据为研究者们报导的藏南拆离系中既有向北剪切的组构,也发现了向南剪切的组构(Carosi et al., 1998; Searle et al., 2003b; Zhang Jinjiang et al., 2012),认为是不同地区上下岩片运动速率的差异造成的。穿过经典的藏南拆离系剪切带、如聂拉木地区,变质级别从下部层位至上部层位骤降,从下盘高喜马拉雅的低角闪岩相骤变为特提斯喜马拉雅的低绿片岩相(图6d;Wang Jiamin et al., 2013a)。藏南拆离系的韧性剪切活动通常伴随着淡色花岗岩的侵位,侵位时间主要发生于中新世、少量为渐新世(Harrison et al., 1995; Searle et al., 2003b; Cottle et al., 2007; Yang Xiongying et al., 2009; Leloup et al., 2010; Chambers et al., 2011; Zhang Jinjiang et al., 2012)。大量研究基于藏南拆离系剪切带中淡色花岗岩的穿切关系、配合独居石、锆石、磷钇矿等的U-Pb定年或云母Ar-Ar年龄,限定藏南拆离系的活动时代主体为26~14Ma(图6d)。
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2.3 主中央逆冲断层(MCT)
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主中央逆冲断层是高喜马拉雅的下部边界,是喜马拉雅挤压构造中最显著的构造界线,它将高级变质的高喜马拉雅叠置于低级变质的小喜马拉雅之上(图4;Schelling, 1992; Pearson and DeCelles, 2005; Yin An, 2006; Kohn, 2008)。主中央逆冲断层主要基于构造地质学和变质岩石学的证据而命名,为一宽几百米至几千米的剪切带(Gansser, 1964; Le Fort, 1975),剪切带内岩石主要为强烈变形的古元古代正片麻岩和变泥质岩;跨越主中央逆冲断层,变质级别从南至北从绿片岩相迅速上升至角闪岩相,构成著名的反转变质带(Arita, 1983)。主中央断层带中缺乏新生代花岗岩的侵位,因而对其活动时代不容易确定。前人多根据其中发育的变泥质岩石中独居石和锆石U-Pb年龄,配合糜棱岩的云母Ar-Ar年龄,不同结果依然争议较大,在尼泊尔中部其大致确定活动时代为19~10Ma(Kohn et al., 2004; Searle et al., 2008; Braden et al., 2018)。
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主中央逆冲断层的位置争议是主中央逆冲断层研究中最热门的科学问题之一。其中研究最成熟的当属尼泊尔喜马拉雅地区,前人通过不同方法提出了多种位置划分方案,但是其位置在不同剖面中存在很大争议。在尼泊尔中西部,Arita(1983)将主中央逆冲断层识别为一条宽约2~3km的剪切带,其上边界命名为MCT-II(图4中的MCT或图7中的MCT-II),下边界命名为MCT-I或Munsiari逆冲断层(图4或图7)。① MCT-II位置的证据主要为:碎屑锆石U-Pb年龄间断、全岩Nd同位素过渡带或间断面(Imayama and Arita, 2008)、~100℃的峰期变质温度间断(Martin et al., 2005; Kohn, 2008; Imayama et al., 2010; Corrie and Kohn, 2011; Imayama, 2014; Wang Jiamin et al., 2015b)、以及约15~5Ma的独居石峰期变质年龄间断(Catlos et al, 2001; Kohn et al., 2004)。② MCT-I位置的证据:Searle(2008)认为上部的主中央逆冲断层位置缺乏强烈的应变带和构造证据,建议将主中央逆冲断层放置在主中央逆冲断层带下部的剪切带附近(MCT-I)。此位置在传统划分的小喜马拉雅内部,在首次出现石榴子石的等变质线附近(图7),可能比前述MCT-I位置更往南。此外,Searle(2008)还批判了碎屑锆石和Nd同位素方法在识别主中央逆冲断层位置中的作用,认为这些方法只能识别不同的原岩及地层单元,并不能作为断层位置的划分依据。这种观点与Yin An(2006)的观点一致,认为主中央逆冲断层的位置发育既可以沿着地层边界、也可以跨越地层单元,在印度喜马拉雅的许多地区,主中央逆冲断层在地层学定义的“小喜马拉雅”中出露,与地层学意义上的“小喜马拉雅”和“高喜马拉雅”边界是不一致的。在尼泊尔东部(Tumlingtar—Taplejung剖面),Goscombe et al.(2006)将从主中央逆冲断层下部至高喜马拉雅中部的广大地段都定义为主中央逆冲断层带(宽~10km),认为MCT-II位置缺乏足够的构造地质学证据、并将主中央逆冲断层置于18亿年的Ulleri-Phaplu眼球状片麻岩下部(MCT-I的位置),后续研究在上述两个位置均识别出了变质间断面(Groppo et al., 2009)。
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2.4 北喜马拉雅片麻岩穹隆(NHGD)
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北喜马拉雅片麻岩穹隆被认为是高喜马拉雅高级变质岩在特提斯喜马拉雅中的出露(图4)。穹隆核部为早古生代花岗质片麻岩(如康马穹隆)、古元古代花岗质片麻岩(如拉轨冈日穹隆)、或渐新世—中新世淡色花岗岩(如雅拉香波穹隆),片麻岩之上多为含蓝晶石、矽线石、十字石等的中压相系变泥质岩(如康马、麻布加、雅拉香波穹隆,Lee et al., 2000, 2004, 2006; Zhang Jinjiang et al., 2012;Ding Huixia et al., 2016a, 2016b; Wang Jiamin et al., 2018)、或者含红柱石和十字石的接触变质岩(如马拉山穹隆,Kawakami et al., 2007; Zhang Jinjiang et al., 2012)。穹隆边部多发生强烈糜棱岩化,多可划分出两条次级拆离断层,一般认为片麻岩与特提斯喜马拉雅之间的拆离断层和剪切带为藏南拆离系在北喜马拉雅的出露(Hodges, 2000; Aoya et al., 2005; Lee et al., 2006; Lee and Whitehouse, 2007; Zhang Jinjiang et al., 2012)。因此,北喜马拉雅片麻岩穹隆的形成演化对高喜马拉雅折返机制的研究具有重要意义。
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大部分片麻岩穹隆中均有淡色花岗岩侵位,岩性一般以二云母花岗岩-含石榴子石花岗岩-电气石花岗岩的组合为主,被认为是高喜马拉雅沉积岩部分熔融分离的熔体汇聚形成的原地—近原地侵位岩浆(Burg et al., 1984; Harrison et al., 1995; Hodges, 2000; Gao Li'e et al., 2014, 2017; 曾令森和高利娥,2017),近年来有学者提出淡色花岗岩是高分异型的异地深成侵入体(Liu Zhichao et al., 2014; Wu Fuyuan et al., 2017, 2020; 吴福元等,2015)。在北喜马拉雅片麻岩中存在始新世岩浆岩、其中打拉地区的二云母花岗岩具有埃达克质特征(Zeng Lingsen et al., 2011; Hou Zengqian et al., 2012),与大多数淡色花岗岩不同。
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图7 聂拉木剖面底部主中央逆冲断层剖面图(剖面位置见图4)(据Wang Jiamin et al., 2015b修改)
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Fig.7 Cross-section of the Main Central Thrust at the bottom of the Nyalam section (the location of the cross-section is shown in Fig.4)(modified after Wang Jiamin et al., 2015b)
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3 喜马拉雅地壳加厚和生长过程
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要探究喜马拉雅碰撞造山机制,必须从空间和时间上重建喜马拉雅的地壳加厚和生长过程。笔者近年来通过研究高喜马拉雅的折返过程,总结出喜马拉雅始新世至中新世(时间)的侧向生长和垂向演化过程(空间),为认识大陆碰撞造山机制提供了重要资料。具体地,作者运用岩性-构造剖面填图、温度-压力剖面、变质作用P-T-t轨迹、岩石年代学(Petrochronology)等手段,在西藏南部和尼泊尔的多个喜马拉雅山谷剖面(吉隆—Rasuwa、聂拉木、珠峰东坡、定结、加德满都)开展了系统研究。以下对其中部分研究进展进行介绍。
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3.1 始新世早期地壳加厚和山脉初始隆升
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若定义初始碰撞为两个大陆相互接触、大洋板片完全俯冲至上覆大陆板片之下,那么被广泛接受的沉积学证据显示初始碰撞的时代应在60~50Ma之间(如Hu Xiumian et al., 2015认为是59±1Ma)。但是,喜马拉雅高级变质岩中的变质变形记录,以渐新世—中新世年龄为主(30~15Ma;Carosi et al., 1998, 2010, 2018; Kohn et al., 2004; Imayama et al., 2012; Zhang Zeming et al., 2012, 2017; Kohn, 2014; Wang Jiamin et al., 2015a, 2017, 2021; 张泽明等,2017),早期碰撞造山相关的变质变形记录严重缺失,这阻碍了我们理解地壳加厚如何响应大陆初始碰撞、喜马拉雅山脉何时开始隆升。喜马拉雅的早期造山历史重建,是理解喜马拉雅地壳加厚和生长过程、厘定大陆碰撞造山机制不可或缺的部分。
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Khanal et al.(2021)对尼泊尔的加德满都飞来峰(图8中加德满都附近)进行了详细的野外构造单元、变质作用P-T轨迹和独居石岩石年代学研究,探索喜马拉雅前陆地区是否有早期碰撞造山的记录。研究表明,加德满都飞来峰中Galchi剪切带上盘(图8中GSZ)存在前人未揭示的始新世变质作用和深熔作用事件,岩石类型主要为混合岩化的正片麻岩和副片麻岩为代表(图9a~c),峰期变质时代为44~38Ma(图9d),峰期温压条件最高为730~760℃和1.05GPa(地温梯度~21℃/km,图9e中红色的P-T轨迹),为典型的中压相系巴罗式变质作用,代表始新世的地壳加厚事件产物。加德满都飞来峰中的混合岩(Bhimphedi Group)、可与高喜马拉雅顶部层位(Uppermost GHC)、北喜马拉雅片麻岩穹隆中的始新世变质记录对比,根据其独居石记录的退变质时代和浅色体的结晶时代、可知其折返时代为始新世晚期—渐新世(35~23Ma,图8),即GSZ的活动时代。加德满都飞来峰中的混合岩,是高喜马拉雅在造山早期逆冲推覆至喜马拉雅南坡、未被完全剥蚀的残留。该逆冲推覆活动发生于35~23Ma,在加德满都中表现为Galchi剪切带,我们将该未揭示的逆冲推覆构造命名为“原喜马拉雅逆冲断层”(Eo-Himalayan Thrust, 简称EHT;Khanal et al., 2021),后期藏南拆离系的拆离活动可能抹去了原喜马拉雅逆冲断层活动的痕迹(Yin An, 2006)。将喜马拉雅前陆地区飞来峰、高喜马拉雅主体的顶部、北喜马拉雅片麻岩穹隆中的相似记录进行总结(图9d、e),始新世的地壳加厚型(中压相系、巴罗式)变质和深熔作用沿着东西走向和南北不同构造单元,在近年来有越来越多的报道(代表性始新世变质记录见图8中紫色五角星,如麻布加穹隆、安娜普尔纳GHC、Karnali飞来峰)。Khanal et al.(2021)的发现是目前喜马拉雅南坡最古老的变质深熔记录,而前人研究的23~15Ma为后期构造改造事件,这对于理解印度-亚洲早期碰撞过程和喜马拉雅初始隆升至关重要。因该期中压相系变质作用与喜马拉雅西部榴辉岩变质时代相近,部分学者认为是印度大陆平板俯冲的产物(如雅拉香波穹隆;Ding Huixia et al., 2016a)。值得注意的是,喜马拉雅的始新世中压相系变质作用地温梯度在18~23℃/km之间,是岩石通过底冲(Underthrusting)、堆叠(Duplexing)或褶皱等方式埋藏至下地壳的产物,代表典型的地壳加厚事件,与大陆深俯冲具有的低地温梯度明显不同(冷俯冲5~10℃/km,热俯冲10~15℃/km;Brown, 2007)。在大陆碰撞的同一时期,深部大陆板片可发生较陡的深俯冲、造成低地温梯度,而浅部则发生地壳堆叠和加厚、产生中压相系巴罗式的中等地温梯度(15~25℃/km;如Soret et al., 2021),因而中压相系的变泥质岩并不能指示俯冲角度(图14a)。
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图8 喜马拉雅造山带中东部地区详细地质单元(a)及剖面图(b) (据Wang Jiamin et al., 2015a, 2016; Khanal et al., 2021)
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Fig.8 Detailed geologic map (a)and cross-section (b)of the Himalayan orogenic belt in central-eastern part (after Wang Jiamin et al., 2015a, 2016; Khanal et al., 2021)
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红色逆冲断层为高喜马拉雅逆冲断层(HHT)在各剖面中的具体位置,并标示了各个剖面中的具体简称和活动时代;Khanal et al.(2021)的工作显示,加德满都所在的高喜马拉雅具有始新世变质年龄,可对应穹隆中出露的高喜马拉雅顶部; GSZ—Galchi剪切带
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The red thrusts show the specific locations, abbreviation and active age of the High Himalayan Thrust (HHT) in each section.The work of Khanal et al.(2021) shows that the metamorphic age of the GHC in Kathmandu Klippe has Eocene metamorphic ages, which corresponds to the top of the GHC exposed in the North Himalayan Gneiss Dome; GSZ—Galchi shear zone
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图9 喜马拉雅始新世变质作用代表性记录(据Khanal et al.,2021修改)
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Fig.9 Representative records of Eocene metamorphism in the Himalaya (modified after Khanal et al.,2021)
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始新世深熔作用的岩相学特征(a)~(c);各构造单元记录的始新世中压型变质年龄与高喜马拉雅主体变质年龄对比(d);各构造单元记录的始新世中压型变质作用代表性P-T-t轨迹(e),中压型变质作用为造山带地壳加厚典型产物; Ky—蓝晶石; Sil—矽线石; Bt—黑云母; Qz—石英; Pl—斜长石
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The petrographic characteristics of the Himalayan Eocene anatexis (a)~(c); the metamorphic ages of the Eocene medium-pressure metamorphism recorded by each tectonic unit and comparison to the metamorphic age of the GHC (d); representative P-T-t trajectory of Eocene medium-pressure metamorphism, which is a typical product of crustal thickening of the collisional orogenic belt; Ky—kyanite; Sil—sillimanite; Bt—biotite; Qz—quartz; Pl—plagioclase
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总之,这些始新世的变质深熔事件表明(Khanal et al., 2021):① 喜马拉雅的上地壳在印度-亚洲初始碰撞后不久(20~10Ma)就加厚到了＞35km的下地壳、并产生了深熔作用,这一最早的地壳加厚事件与印度西北Subathu和尼泊尔Tansen等前陆次盆地记录的残留陆表海海退事件时代完全耦合(Najman et al., 2001; Najman, 2006; Ravikant et al., 2011),是喜马拉雅山脉初始隆升的诱因(图14a)。也就是说,喜马拉雅山是在大约44~38Ma通过地壳加厚的方式初始隆升至海平面以上的;地壳均衡计算表明,在40Ma时,其高度可能已经达到1000m。② 上述相似的始新世变质深熔记录,表明高喜马拉雅顶部存在一个始新世变质的岩片(图8a、b,图9e),其沿着原喜马拉雅逆冲断层的南向推覆于35~23Ma之间折返,目前只保存在喜马拉雅的南部飞来峰、北喜马拉雅片麻岩穹隆和部分地区的高喜马拉雅顶部,而高喜马拉雅主体的变质核中则大部分缺失了该记录。南部飞来峰中的始新世记录,为从喜马拉雅内陆地区推覆至此的早期造山记录残留。③ 古近纪—始新世的初始碰撞时代无法排除部分学者认为的多阶段碰撞模型(Aitchison et al., 2007; van Hinsbergen et al., 2012),要制约不同的碰撞模型(单次与多次),必须探索喜马拉雅从内陆至前陆的不同构造单元是否经历了统一的早期碰撞和地壳加厚过程。喜马拉雅从北至南各个不同构造单元(图8a、b,图9e)一致的始新世变质和深熔记录,表明它们经历了相同的早期大陆碰撞过程,支持印度-亚洲的单次碰撞模型(即大印度模型;Capitanio et al., 2010; Meng Jun et al., 2020)、而不支持多阶段碰撞模型。喜马拉雅在始新世构造格局单一,现今不均一性为中新世改造的结果。
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3.2 渐新世—中中新世“顺序式向南扩展”的侧向生长过程
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喜马拉雅的初始地壳加厚始于始新世,至渐新世—中新世,喜马拉雅的下地壳已经通过底垫作用(Underplating)堆叠了大量的酸性地壳物质。当喜马拉雅的加厚地壳堆叠到没有足够的容纳空间,便会向前陆盆地方向发生构造挤出,便形成了高喜马拉雅结晶岩系。但是,对高喜马拉雅以何种机制向前陆地区扩展还存在诸多争论。传统认为,高喜马拉雅为一个整体岩片、单次折返,内部不存在大型逆冲断层。近十多年,国内外地质学家们陆续在尼泊尔、藏南和印度西北部的多条剖面的高喜马拉雅内部发现了新的构造-变质不连续面(图4,图8a; Kohn et al.,2004; Goscombe et al., 2006; Carosi et al., 2010, 2018; Imayama et al., 2010; Corrie et al., 2011; Montomoli et al., 2013, 2015; Wang Jiamin et al., 2015a, 2016; Ambrose et al., 2015; Larson et al., 2015; Shrestha et al., 2017; Khanal et al., 2020; Benetti et al., 2021; Kawabata et al., 2021)。这些新的高喜马拉雅构造-变质不连续面究竟在高喜马拉雅折返过程中扮演了什么角色,是理解喜马拉雅渐新世—中中新世生长过程、从而认识大陆碰撞造山机制的突破口。关于高喜马拉雅构造-变质不连续面的研究,主要存在以下两个问题:① 高喜马拉雅内部的不同变质岩片折返是顺序式的(In-sequence)还是违序式的(Out-of-sequence)(图3)?② “高喜马拉雅不连续带”是仅在局部地区发育的构造还是造山带尺度的大型构造边界?基于此,作者对喜马拉雅中部聂拉木、吉隆—Rasuwa两个剖面的高喜马拉雅进行了详细研究。
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以聂拉木地区为例(图8中剖面1),高喜马拉雅可以被分为下部GHC、上部GHC两个岩片,两者之间为一宽约500m的剪切带、主要由糜棱岩化正片麻岩组成(1∶25万地质图中称为“康山桥剪切带”,图5中N18样品附近),上下岩片分别具有向南剪切和向北剪切的运动学指向,剪切带中剪切指向交替变化(图5)。进一步通过变质温度-压力剖面、P-T-t轨迹、独居石和锆石岩石年代学对上述两个岩片进行研究,取得了以下结果(图11a、b;Wang Jiamin et al., 2013, 2015a, 2015b, 2016):① 上部GHC岩片变质温度较高(750~800℃,图5),主体为高角闪岩相至麻粒岩相,部分熔融程度较高(15%~25%,白云母脱水熔融为主、少量黑云母脱水熔融, 图10a~c)、持续时间较长(7~12Ma),峰期变质年龄较老(~30Ma),具有近等温降压的P-T轨迹(图11b);② 下部岩片变质温度较低、主体为低角闪岩相(650~700℃,图5),部分熔融程度较低(0~10%,饱和水固相线熔融, 图10d~f)、持续时间较短(~3Ma),峰期变质年龄较年轻(~19Ma),具有小幅度降压、近似发夹状的P-T轨迹(图11b);③ 小喜马拉雅的变质温度更低、主体为绿片岩相至低角闪岩相(550~650℃)、未发生部分熔融(图10g~i),峰期变质年龄更年轻(~10Ma),具有发夹状的P-T轨迹(图11b)。聂拉木地区高喜马拉雅内部两个岩片间的变质年代穿时性、峰期温压条件的间断,表明上下两个岩片为两期折返的产物,中间存在构造-变质间断面,在构造上表现为一条剪切带。间断面上盘年龄较老、变质温度更高,下盘年龄较新、变质温度更低的特点表明,该间断面为一条顺序式向南扩展的逆冲断层,称为“聂拉木逆冲断层”(NT)。利用相似的方法在尼泊尔中部的吉隆-Rasuwa剖面进行研究(图8中剖面2),通过跨越高喜马拉雅的峰期温压条件剖面、地温梯度突变(高喜马拉雅下部20~25℃/km,高喜马拉雅上部＞35℃/km)、变质年龄不连续界面(图11c、d, 小喜马拉雅~13Ma,高喜马拉雅下部20~18Ma,高喜马拉雅上部29~23Ma),可以揭示出Rasuwa逆冲断层(RSZ)的存在、以及MCT在该剖面的具体位置(Khanal et al., 2020),从而将高喜马拉雅分隔为下部和上部两个岩片。
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传统认为高喜马拉雅是单次折返的,上述结果首次在聂拉木和吉隆-Rasuwa地区发现高喜马拉雅存在两期折返过程,这为喜马拉雅构造单元重新划分以及高喜马拉雅的折返机制提供了新的依据。聂拉木逆冲断层和Rasuwa逆冲断层与尼泊尔西部、尼泊尔东部、锡金及近年来在印度西北部等地区的逆冲断层活动时代一致(25~16Ma)、空间层位相似(均在高喜马拉雅中间层位、以强烈变形的眼球状片麻岩为标志)、变质级别均向上增加(上部温度高、下部温度低)。据Wang Jiamin et al.(2016)对这些逆冲断层或构造-变质不连续面的统计(图8a),结合近年来在印度西北部填图的新发现(Benetti et al., 2021; Kawabata et al., 2021),这些断层可以相连构成一条延伸达＞1400km的“高喜马拉雅逆冲断层”(High Himalayan Thrust,图4、图8),与藏南拆离系为同时代活动(26~16Ma)而先于主中央逆冲断层活动(19~10Ma)。高喜马拉雅逆冲断层可与主中央逆冲断层、主边界逆冲断层(10~4Ma)构成一系列背驮式逆冲推覆构造,顺序式向南活动(图8b)。即高喜马拉雅上部、高喜马拉雅下部、小喜马拉雅沿着高喜马拉雅逆冲断层、主中央逆冲断层、主边界逆冲断层依次向南挤出。喜马拉雅“顺序向南扩展”的侧向生长规律,与临界楔模型对碰撞造山过程的刻画及P-T-t轨迹的预测更为吻合(图11),特别是对已经冷却固结的岩石。但同时,高喜马拉雅内部的岩片都经历了初始较慢的冷却过程(35±8℃/Ma,10±5℃/Ma)和随后的快速冷却过程(120±40℃/Ma), 高喜马拉雅上部岩片部分熔融的持续时间(5~12Ma)长于临界楔模型的预测(~5Ma),这些特征暗示部分熔融物质的低黏度和高浮力在混合岩折返早期未完全固结时或起到部分作用。因此,大陆碰撞造山的侧向生长过程主要受控于临界楔模型,隧道流有可取之处、但只起次要作用。
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图10 聂拉木地区高喜马拉雅上部、高喜马拉雅下部和小喜马拉雅岩系代表性巴罗式变质带野外及岩相学特征 (据Wang Jiamin et al., 2013, 2015a, 2016修改)
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Fig.10 Field and petrographic characteristics of representative Barrow-type metamorphic belts across the upper GHC, lower GHC and LHS in the Nyalam region (modified after Wang Jiamin et al., 2013, 2015a, 2016)
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(a)~(c)—高喜马拉雅上部为高角闪岩相至麻粒岩相变质的混合岩化副片麻岩,其中浅色体较为发育;以矽线石、钾长石、堇青石发育为特征,矿物组合中白云母已经完全分解;(d)~(f)—高喜马拉雅下部为低角闪岩相变质的副片麻岩,其中浅色体含量较低、为原地毛孔状或串珠状;矿物组合中白云母尚且存在,蓝晶石较为常见;(g)~(i)—小喜马拉雅顶部代表性岩石,常见石英透镜体,其中浅色体不发育;变质级别从绿片岩相至低角闪岩相,主体为绿片岩,最高级别可至蓝晶石-十字石域; Kfs—钾长石; Crd—堇青石; Grt—石榴子石; Ms—白云母; Ser—绢云母
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(a)~(c)—The upper GHC is mainly composed of migmatized paragneiss of upper-amphibolite facies to granulite-facies, in which leucomorphs are extensively present.It is characterized by the development of sillimanite, K-feldspar and cordierite, and muscovite has been completely decomposed; (d)~(f)—the lower GHC is a migmatized paragneiss of lower-amphibolite facies, in which the proportions of leucosome is low, and occurs as in situ pores or beaded structure; muscovite is still present in the mineral assemblage, and kyanite is commonly seen; (g)~(i)—rock assembly at the top of the Lesser Himalayas; quartz lenses are common, of which leucosome are not present; Mineral assemblage varies from greenschist facies to lower-amphibole facies, within which it is mainly composed of greenschists and the highest grade can reach the kyanite-staurolite domain; Kfs-K—feldspar; Crd—cordierite; Grt—garnet; Ms—muscovite; Ser—sericite
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图11 聂拉木和吉隆地区高喜马拉雅变质沉积岩变质年龄及代表性P-T-t轨迹(据Wang Jiamin et al., 2015a, 2016修改)
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Fig.11 Representative metamorphic ages and P-T-t trajectories of Greater Himalayan metasedimentary rocks in the Nyalam and Gyirong regions (modified after Wang Jiamin et al., 2015a, 2016)
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3.3 中中新世加厚山根地壳和山根局部拆沉
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在大陆碰撞造山过程中,既有造山带的侧向生长,同时也发生着垂向演化,两者共同构成了造山带的生长过程。从古近纪—早始新世开始印度-亚洲初始碰撞,至渐新世—中新世成熟碰撞,喜马拉雅的山根地壳逐渐加厚,至中中新世,喜马拉雅的山根被加厚至60~70km,最终发生了山根局部拆沉。
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3.3.1 喜马拉雅两类榴辉岩的形成过程
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揭示上述过程最佳的岩石载体为榴辉岩。在喜马拉雅,出露有两种不同类型的榴辉岩,其中喜马拉雅中部的麻粒岩化榴辉岩,为揭示大陆碰撞造山带从俯冲至底垫到山根的深部垂向演化过程提供了绝佳机遇。喜马拉雅中部的榴辉岩出露于珠峰东坡、定结、阿鲁河谷、锡金、亚东、不丹等地区的高喜马拉雅、或日玛那杂岩(~1.8Ga花岗片麻岩)中,普遍经历了麻粒岩相叠加变质(Lombardo et al., 2000; Groppo et al., 2007; Corrie et al., 2010; Grujic et al., 2011; Warren et al., 2011),近年来在石榴子石包裹体或基质中发现了绿辉石残留,证明其确实经历了榴辉岩相变质(Wang Yuhua et al., 2017; Li Qinyun et al., 2019; Wang Jiamin et al., 2021),绿辉石中硬玉分子含量可达~29%(Wang Jiamin et al., 2021;图12a、b)。但是对喜马拉雅中部榴辉岩的叠加变质条件、榴辉岩相时代、叠加变质时代等尚存较大争议。相对而言,喜马拉雅西部的超高压变质榴辉岩研究较为成熟,其出露于印度Tso Morari和巴基斯坦Kaghan地区(Rehman et al., 2008; O'Brien, 2019),围岩为高喜马拉雅变质沉积岩或早古生代花岗岩。榴辉岩中发现了柯石英的存在,证明大陆深俯冲至超高压深度(O'Brien, 2001; Treloar et al., 2003);其峰期地温梯度7~8℃/km,峰期变质时代为51~46Ma (Parrish et al., 2006; Donaldson et al., 2013)。
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图12 喜马拉雅首次发现榴辉岩叠加超高温变质作用的部分证据(据Wang Jiamin et al., 2021修改)
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Fig.12 Evidences for eclogite overprinted by ultra-high temperature metamorphism discovered for the first time in the Himalayas (modified after Wang Jiamin et al., 2021)
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(a)~(d)—麻粒岩化榴辉岩及其围岩的岩石学和元素面扫描图像;(e)~(g)—尖晶石矿物成分和使用三元长石温度计、Grt-Opx温度计和Grt-Bt温度计计算的 P-T条件;(h)P-T-t轨迹和与其他古老碰撞造山带的比较;该经历超高温的榴辉岩及其围岩样品采集自珠峰东坡地区高喜马拉雅; Opx—斜方辉石; Cpx—单斜辉石; Omp—绿辉石; Amph—角闪石; Spl—尖晶石; Per—条纹长石; Mper—中条纹长石
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(a)~(d)—Petrography and element mapping of the granulitized eclogite and their country rocks; (e)~(g)—mineral compositions of spineland, calculated P-T using ternary feldspar thermometer, Grt-Opx thermometer, and Grt-Bt thermometer;(h)—P-T-t paths and comparison to other old collisional orogens;The ultra-high temperature eclogites and their surrounding rocks were collected from the GHC in the Mount Everest-east region; Opx—orthopyroxene; Cpx—clinopyroxene; Omp—omphacite; Amph—amphibole; Spl—spinel; Per—perthite; Mper—mesoperthite
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为了揭示喜马拉雅的深部演化过程,Wang Jiamin et al.(2021)对喜马拉雅中部珠峰东坡地区的麻粒岩化榴辉岩及其围岩(泥质麻粒岩)进行P-T轨迹和岩石年代学研究,并对喜马拉雅西部的冷俯冲榴辉岩特征进行总结,取得以下进展:① 发现喜马拉雅首个榴辉岩叠加超高温变质作用(定义为温度＞900℃,深度20~40km;Harley, 2008)的岩石学证据:通过变泥质岩中矿物结构(堇青石-石英-尖晶石平衡共生、低Zn低Cr尖晶石,黑云母中金红石出溶,三元长石出溶,图12c、d),确定岩石达到了高温或超高温变质条件;进一步通过变质相平衡模拟、三元长石温度计、石榴子石-斜方辉石Fe-Mg交换温度计、石榴子石-黑云母Fe-Mg交换温度计等多种方法(图12e~h),确定超高温变质条件为0.6~1.1Gpa和900~970℃(~40℃/km);② 配合锆石、独居石、榍石等多种U-Th-Pb岩石年代学方法,获得了榴辉岩相变质及超高温变质的时代,重建了其P-T-t轨迹(图12h),榴辉岩相峰期变质时间为~30Ma、温压条件为730~770℃和~2GPa(11℃/km),经历了降压升温的P-T轨迹(升温幅度~150℃),超高温变质作用时间为20~15Ma,超高温变质作用叠加改造使其基本失去了榴辉岩相应该具有的矿物成分和变质年龄。
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图13 喜马拉雅超高温变质作用可能的机制
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Fig.13 The possible mechanisms of ultra-high temperature metamorphism in the Himalaya
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(a)—山根局部拆沉模型。俯冲板片的脱流体作用交代了上覆加厚的地壳根部,导致榴辉岩化和重力不稳定。据Shi Danian et al.(2020)修改; (b) —软流圈对流减薄模型。对流使上覆板块的岩石圈地幔变薄,进一步引起钾质-超钾质岩浆作用。据Turner et al.(1996) and Wang Qiang et al.(2016)修改; (c)—俯冲板片撕裂模型。俯冲板块撕裂导致软流圈上涌,沿南北向裂谷形成超高温变质作用;据Chen Yun et al.(2015) 修改;(d)—南北向裂谷的张裂隙模型。印度-亚洲碰撞过程中的南北向挤压(σ1)导致东西向伸展形成张裂缝(σ3),切穿岩石圈地幔并引起超高温变质作用,据Kapp and Guynn (2004) 和Zhang Jinjiang et al.(2012)修改
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(a)—Localized delamination of the orogenic root model.The defluidization of the subducted slab metasomatized the overriding thickened crustal root, resulting in eclogitization and gravity instability.Modified after Shi Danian et al.(2020); (b)—asthenosphere convection thinning model.Convection causes thinning of the lithospheric mantle in the overriding plate, which further causes potassic-ultrapotassic magmatism.Modified after Turner et al.(1996) and Wang Qiang et al.(2016); (c)—the slab tearing model.The slab tearing causes the asthenosphere upwelling, and caused ultra-high temperature metamorphism along the North-South Trending Rifts; after Chen Yun et al.(2015); (d)—the tensile stress model for the North-South Trending Rifts.North-south compression (σ1) during the India-Asia collision leads to an east-west extension to form extensional fractures (σ3), which cut through the lithospheric mantle and cause ultra-high temperature metamorphism, modified after Kapp and Guynn (2004) and Zhang Jinjiang et al.(2012)
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进一步系统总结喜马拉雅西部的超高压榴辉岩和中部麻粒岩化榴辉岩在出露位置、折返速率、压力峰期变质时代、温度峰期滞留时间、P-T轨迹形态等的差异性(Wang Jiamin et al., 2021),可以提出大陆碰撞不同时期(初始时期与成熟时期)的垂向演化差异造成冷俯冲榴辉岩以及麻粒岩化榴辉岩折返的新模型(图14a、b;Wang Jiamin et al., 2021)。该模型指出喜马拉雅从大陆俯冲、山根加厚至岩石圈减薄的垂向演化过程,是大陆碰撞带冷俯冲榴辉岩与麻粒岩化榴辉岩形成的主控因素,而不需要前人提出的板片断离(Kohn et al., 2002)或俯冲角度变换(Guillot et al., 2008):① 在印度-亚洲碰撞初期(60~40Ma,图14a),地壳较薄(~30km),包裹榴辉岩的长英质岩片(密度约为2.8g/cm3)从深俯冲的印度大陆地壳拆离后,进入上伏地幔(密度约为3.3g/cm3),能够通过浮力作用(Buoyancy)而近垂直上升、快速折返(速率约为30mm/a),形成近等温降压或冷却降压的P-T轨迹,形成的榴辉岩大多沿缝合带分布、成穹隆状。② 在碰撞成熟期(40~16Ma,图14b),喜马拉雅山根堆积了大量从印度大陆拆离下来的未能发生深俯冲的酸性地壳岩石(60~70km、密度约为2.8g/cm3),包裹榴辉岩的长英质岩片从地壳拆离后(密度约为2.8g/cm3),被圈入加厚山根中,由于密度差不大,此时折返受控于底垫作用,比浮力折返要慢得多(速率2~3mm/a),并因缓慢抬升而遭受高温/超高温变质作用叠加改造。当被物质不断填充的地壳应变积累到产生破裂后,会沿着大型逆冲推覆构造(主中央逆冲断层)和被动顶板断层(藏南拆离系)形成侧向挤出。因此这类含榴辉岩的变质岩片大多数出露于造山带核部、规模较大,离缝合带较远。
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图14 喜马拉雅造山带三阶段演化过程
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Fig.14 Three-stage evolution model of the Himalayan orogenic belt
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(a)—软碰撞期(60~40Ma),发生早期地壳加厚直至约40km,对应喜马拉雅山脉的初始隆升至＞1000m;(b)—硬碰撞(40~16Ma),地壳加厚直至约70km,对应喜马拉雅山脉的主期隆升至≥5000m;逆冲断层发育的时间序列分别为:1—EHT, 35~23Ma;2—HHT, 25~16Ma;3—MCT, 19~10Ma;(c)—晚期碰撞(16~0Ma),山根榴辉岩化而发生局部拆沉,对应喜马拉雅山脉的进一步隆升至约6000m,达到并维持现今高度
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(a)—Soft collision period (60~40Ma), with early crustal thickening up to~40km, corresponding to the initial uplift of the Himalaya to ＞1000m; (b)—hard collision period (40~16Ma), crustal thickening up to 70km, corresponding to the main uplift of the Himalaya to ≥5000m; the time series of thrust fault development are: 1—EHT, 35~23Ma; 2—HHT, 25~16Ma; 3—MCT, 19~10Ma; (c)—late collision period (16~0Ma), the eclogitized root underwent local foundering, corresponding to the further uplift of the Himalaya to~6000m, which reached and maintained the present elevation
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上述两种不同榴辉岩的形成过程,也可为揭示古老碰撞造山带中榴辉岩的形成过程提供借鉴意义。例如,目前发现的最古老榴辉岩形成于20~18亿年前(坦桑尼亚, Möller et al., 1995;科拉半岛, Mints et al., 2010;加拿大Trans-Hudson, Weller and St-Onge, 2017;华北丰镇, Xu Cheng et al., 2018),记录的地热梯度为7.5~15℃/km、跨越了冷/热俯冲,除华北丰镇外均叠加了麻粒岩相变质。在大多数从古至今的碰撞造山带内,榴辉岩也常被叠加麻粒岩相变质、记录较高的地温梯度(10~22℃/km,如华力西,O'Brien et al., 1992; 华北中部带,Zhao Guochun et al., 2001)。Wang Jiamin et al.(2021)的模型表明,绝大多数碰撞造山带内榴辉岩,包括地球上最古老的榴辉岩之所以普遍被叠加麻粒岩相变质,是因为它们是古老碰撞造山带的山根地壳,在造山带山根中经历了相对缓慢的抬升和长时间的高温变质叠加改造。
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3.3.2 中中新世超高温变质作用的机制
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上述成果发现的喜马拉雅首个超高温变质作用(Wang Jiamin et al., 2021),对超高温变质作用产生的构造背景也具有重要的启示意义。虽然超高温变质岩石在前寒武纪及显生宙的大陆碰撞带中广泛分布(如南极东部超高温麻粒岩),其热源一直存在争议。由于喜马拉雅是正在活跃的、世界上最典型的陆陆碰撞造山带,在喜马拉雅首次发现超高温变质作用,将为理解大陆碰撞过程中超高温变质作用的热源提供极其重要的现代实例。前人通过一维热模拟中较高的放射性参数设置(3.5 μW/m3)、认为单纯的加厚地壳可以产生超高温变质,这明显不符合喜马拉雅等绝大多数大陆碰撞带的天然岩石放射性参数(1.5~2.5 μW/m3)。Wang Jiamin et al.(2021)通过改进一维热模拟的参数设置、使其更加接近喜马拉雅地壳加厚的时间尺度和厚度,发现在设置2 μW/m3的平均放射性参数下,单纯地壳加厚无法产生超高温变质作用,必须有来自软流圈物质的热贡献。因此,喜马拉雅超高温变质作用的产生,是由喜马拉雅的酸性地壳加厚至约60km后放射性生热(2 μW/m3),进一步通过岩石圈减薄至＜90km共同导致的,岩石圈减薄会造成软流圈物质的上涌、造成异常高的地热梯度(通常认为岩石圈底界即软流圈顶部温度为1300℃)。
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导致喜马拉雅超高温变质作用的可能机制有以下几种(图13)。① 山根拆沉模型(图13a),造山带山根加厚地壳受到俯冲板片的流体交代而发生榴辉岩化(密度大于3.4g/cm3)、会造成重力失稳而拆沉;在古老造山带中,山根拆沉通常被认为是造山带从挤压汇聚向伸展垮塌的标志(Nelson, 1992; Schott and Schmeling, 1998);喜马拉雅的不同之处在于现今印度-亚洲大陆的挤压汇聚还在强烈持续着,因而不可能是造山带垮塌而导致的岩石圈减薄,很可能是山根的局部拆沉;该机制与现今喜马拉雅局部地区Moho面附近异常热的地球物理观测相吻合(Shi Danian et al., 2020),不同的是,现今观测到的热异常是沿着南北向裂谷分布的,认为南北向裂谷提供了流体上升使造山带山根榴辉岩化的通道。② 软流圈对流减薄模型(图13b),该机制类似于现今藏北地区观测到的印度大陆俯冲导致上盘岩石圈地幔减薄(Turner et al., 1993);目前印度大陆板片已经俯冲至班公湖-怒江缝合带附近,大陆板片俯冲角度在此处变陡而导致上盘的岩石圈地幔对流减薄,从而形成大规模的钾质—超钾质岩浆作用(Turner et al., 1996; Wang Qiang et al., 2016);在中中新世时期,俯冲的印度大陆还滞留于藏南地区,此时在藏南地区发育了大量同时期的钾质—超钾质岩浆作用(Chung Sunlin et al., 2005, 2009)。③ 俯冲板片撕裂模型(图13c),该机制由现今的地球物理观测进一步解释所得,认为印度大陆板片差异俯冲会造成撕裂,进而在地表形成南北向裂谷(Chen Yun et al., 2015),软流圈物质沿着南北向裂谷进入地壳、造成超高温变质作用(Wu Chenguang et al., 2022);但是南北向裂谷的成因很可能是印度-亚洲汇聚南北向挤压导致东西向拉张(σ3,即机制4,图13d),是与主压应力(σ1)方向垂直的张应力分量(Kapp and Guynn, 2004; Zhang Jinjiang and Guo Lei, 2007; Kapp et al., 2008; Zhang Jinjiang et al., 2012),与伸展构造或板片撕裂无关;因此机制3作为解释喜马拉雅地区超高温变质作用的热源还需进一步探究。
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虽然机制1中并不排除南北向裂谷在超高温变质作用中的贡献,但是,南北向裂谷在喜马拉雅的发育时间始于13Ma(如申扎-定结裂谷;Zhang Jinjiang and Guo Lei, 2007),明显晚于超高温变质作用的时代(20~15Ma),这是否意味着南北向裂谷的深部起始活动时代是否早于现今地表观测到的变形时代呢?对喜马拉雅来说,上述模型中最为可能的模式为山根的局部拆沉(图14c),这与20~15Ma时喜马拉雅山根地壳已经加厚至60~70km,普遍发育榴辉岩,以及最新的地球物理观测资料相吻合(Shi Danian et al., 2020)。无论如何,在喜马拉雅发现超高温变质作用,将为理解大陆碰撞过程中超高温变质作用的热源(加厚地壳+岩石圈减薄)、时间尺度(单个岩片持续时间＜15Ma、初始碰撞后35~40Ma)等提供极其重要的实例。
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3.4 总结:喜马拉雅碰撞造山的三个演化阶段
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喜马拉雅的地壳加厚和生长过程,为理解大陆碰撞造山过程和机制提供了经典实例。进一步将该过程和机制进行凝练、归纳,能够更清晰地为其他大陆碰撞带的研究服务,提供借鉴意义。下文通过总结上述研究进展、结合前人研究成果,将喜马拉雅碰撞造山归纳为三个演化阶段,概括如下(图14)。
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阶段一:时间60~40Ma,软碰撞期(Soft-collision),早期地壳加厚至约40km,并发生小规模部分熔融,特提斯喜马拉雅抬升至海平面以上,喜马拉雅山初始形成(图14a)。碰撞早期,喜马拉雅地壳缓慢加厚,印度-亚洲汇聚速率迅速减缓、但整体较快(从17cm/a降至5cm/a),为软碰撞期。从俯冲大陆上剥离下来的地壳物质堆叠到两个板块边界的缝合带内,既有发生深俯冲的地壳物质,如喜马拉雅西部的超高压榴辉岩(图14a中浅蓝色P-T轨迹;St-Onge et al., 2013; O'Brien, 2019),也有只经历堆叠加厚埋藏的物质,如喜马拉雅中部地区广泛发育的中压相系巴罗式变质作用(图14a中深蓝色P-T轨迹;Khanal et al., 2021)。至始新世(44~38Ma),酸性地壳广泛加厚堆叠至约~35km(整个地壳厚度＞40km),发生了早期小规模部分熔融。这些早期地壳加厚记录大多已被剥蚀,零星保存于前陆飞来峰、北喜马拉雅片麻岩穹隆及个别剖面的高喜马拉雅顶部层位中。喜马拉雅山从海平面以下冉冉升起、直到抬升至＞1000m(Ding Lin et al., 2017),造成特提斯喜马拉雅出露、接受风化剥蚀而供给始新世前陆盆地物源(Najman et al., 2001; Ravikant et al., 2011)。
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阶段二:时间40~16Ma,硬碰撞期(Hard-collision),地壳加厚至60~70km,深部发生大规模高级变质和部分熔融,中上地壳形成大规模叠瓦状逆冲推覆构造,形成了现今喜马拉雅造山带的核部主体高级变质岩,喜马拉雅山隆升至约5000m,是喜马拉雅的主加厚期和主隆升期(图14b)。在该阶段,印度-亚洲汇聚速率较为稳定(5~3cm/a),为硬碰撞期。至中新世早中期(20~16Ma),喜马拉雅山根通过垂向底垫作用、广泛加厚至约60~70km,以酸性岩石为主(密度轻、难俯冲),发生大规模深熔作用,熔体汇聚进一步分异演化形成淡色花岗岩。由于岩石经历了较高的变质温度和压力,所发育的P-T轨迹以近等温降压或升温减压为主(图14b中粉色和红色P-T轨迹)。在造山带的中上地壳,堆叠的物质积累到无法再进一步容纳,从而在其内部产生破裂面,在南北向挤压作用下顺序式向南构造挤出。在这个过程中,造山带尺度的大型构造边界是“顺序式”向南发育的,而“违序式”发育的逆冲推覆构造则表现为次级构造。在该阶段早期,以原喜马拉雅逆冲断层(如GSZ剪切带)为代表的早期逆冲断层活动(35~23Ma),导致了特提斯喜马拉雅和部分高喜马拉雅顶部岩石被逆冲推覆至现今的喜马拉雅山前陆地区,但由于后期的叠加改造和风化剥蚀,只有部分记录被保留下来;在该阶段中晚期,高喜马拉雅逆冲断层和主中央逆冲断层的“顺序式”活动,导致高喜马拉雅上部和下部岩片向造山带前缘方向依次挤出。地壳物质的加厚堆叠,是喜马拉雅山脉在该时期快速隆升的主因,海拔隆升至接近现今高度(≥5000m;Gébelin et al., 2013),喜马拉雅山脉主体形成。
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阶段三:时间16~0Ma,晚碰撞期(Late-collision),山根榴辉岩化发生局部拆沉,汇聚造山持续进行而尚未发生垮塌,小喜马拉雅折返、前陆盆地形成,喜马拉雅山达到和维持现今平均高度~6000m(图14c)。在此阶段早期,巨厚的喜马拉雅造山带山根地壳发生榴辉岩化,同时受到俯冲大陆释放的流体交代,发生重力失稳而局部拆沉和岩石圈减薄,从而使山根地壳的残留岩石遭受了超高温变质作用(图14c中粉色P-T轨迹)。在中上地壳,高喜马拉雅下部和小喜马拉雅的岩石进一步通过构造挤出而在造山带前缘逐步形成叠瓦状构造,岩石记录的P-T轨迹以发夹状为主(图14c中绿色和浅蓝色P-T轨迹)。造山带尺度的逆冲推覆断层随后沿着主边界和主前锋逆冲断层活动,现今喜马拉雅构造格局最终形成。岩石圈减薄抹掉了随加厚地壳一起加厚的岩石圈地幔,导致浮在软流圈之上的岩石圈最终达到均衡状态(Molnar and England, 1990; Molnar et al., 1993),喜马拉雅山发生进一步隆升,最终达到现今高度(~6000m)。在该阶段,印度大陆越过雅江缝合带下方,逐步俯冲至现今的班公湖-怒江缝合带下方。但印度-亚洲汇聚的速率并没有减弱,这是由于印度-亚洲大陆碰撞直至今日都还在持续进行,造山带并没有因为山根局部拆沉而垮塌。但很显然,印度-亚洲的碰撞汇聚已经进入晚期阶段,山脉达到最大高度而不再增加(剥蚀与隆升达到动态平衡);在下一个阶段、喜马拉雅将进入造山带的垮塌,伴随着喜马拉雅山脉高程的降低。
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4 未来研究展望
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未来对喜马拉雅碰撞造山带的变质地质学研究,需要应对地球科学所面临的挑战,主要来自学科前沿转变、国家科技战略布局调整、国家自然科学基金委优先资助方向调整三个方面。首先,在学科前沿领域,原有的国际热点问题随着研究的不断深入,已经变成了成熟领域,到了瓶颈期、难以突破;其次,近年来我国科技战略布局调整,提出科技发展要面向“国家需求”,国家重点实验室在2021年开启重组;第三,近年来国家自然科学基金委改革和资助方向调整,地球科学领域优先资助地球系统科学、行星科学和海洋科学等方向(郭正堂, 2019;朱日祥等, 2021)。针对上述挑战、结合学科发展态势,本节列举喜马拉雅造山带变质地质学研究可能存在的突破口和机遇。但由于文章篇幅限制,不能一一举例。
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4.1 喜马拉雅极端变质作用与重大碰撞造山事件的关联
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4.1.1 喜马拉雅超高温变质作用的规模圈定和古今差异
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超高温变质作用的成因背景,是理解造山带热结构、造山带深熔作用、花岗岩成因等的重要支撑(Stevens et al., 2007; Harley, 2008, 2016; Clark et al., 2011; Kelsey and Hand, 2015; Cipar et al., 2020; Zheng Yongfei and Chen Renxu, 2021)。超高温变质作用大多形成于大陆碰撞过程中,如在前寒武纪的麻粒岩地体、华北孔慈岩带、东南极的冈瓦纳相关碰撞造山带(Brown, 2007; Kelsey et al., 2008; Harley, 2008; Kelsey and Hand, 2015; Jiao Shujuan and Guo Jinhui, 2020),但是在显生宙的碰撞造山带中却少得多。近年来超高温变质作用在新特提斯构造域中、特别是喜马拉雅有越来越多的报道(如Wang Jiamin et al., 2021; Chen Si et al., 2021; Wu Chenguang et al., 2022);此外,Wood(2019)报道了假蓝宝石的存在(单独的假蓝宝石并不是超高温变质的矿物学证据),但其计算的峰期变质温度(~800℃)并未到超高温变质条件。本文通过研究和总结,对喜马拉雅超高温变质作用的成因提出了新的见解,认为山根的局部拆沉可能是喜马拉雅超高温变质作用的形成机制。但目前喜马拉雅发现的超高温变质作用只有两处,喜马拉雅是否存在其他地区、不同时代、不同成因的超高温变质作用?目前还不清楚。喜马拉雅相对年轻、地质过程较为清楚,进一步厘定不同区域超高温变质作用的时代、条件和成因背景,对理解造山带深部组成和结构、地球从古至今构造体制变化等意义重大。
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未来对喜马拉雅的超高温变质作用研究,需要关注超高温变质作用与喜马拉雅重大碰撞造山事件的关联。特提斯域不同地区或时期的超高温变质作用,是否可以揭示印度-亚洲碰撞空间走向上的差异演化过程?喜马拉雅的超高温变质作用的成因过程,是否可以解释古老碰撞造山带发育的超高温变质作用?回答上述问题需要具体厘定以下两个方面:① 喜马拉雅超高温变质作用的空间规模和出露范围,与藏南拆离系、南北向裂谷等区域构造的空间关联;② 喜马拉雅超高温变质作用与东南极、华北孔兹岩带等经典超高温地区的P-T-t条件和成因过程对比研究。
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4.1.2 洋陆转换过程中喜马拉雅超高压榴辉岩的折返机制
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从20世纪80年代大陆地壳岩石中发现柯石英开始(Chopin, 1984; Smith, 1984),超高压变质岩的研究历史只有30多年,但它证明大陆地壳可以深俯冲到弧下地幔深度,导致了板块构造的第一次革命(Schreyer, 1995; Ye Kai et al., 2000; Chopin, 2003; Liou et al., 2004; Liu Fulai et al., 2006; Liu Fulai and Liou, 2011)。虽然超高压变质作用研究已经积累了大量研究资料,但是对深俯冲大陆地壳的折返机制研究仍然存在巨大的争论。地质实例和数值模拟研究提出了众多模型,如俯冲隧道、板片断离、构造挤出、逆冲推覆作用、构造去顶、底辟作用、浮力作用、底垫作用等等(Okay et al., 1992; Davies and von Blanckenburg et al., 1995; Ernst et al., 1997; Hacker et al., 2000; Warren et al., 2008; 郑永飞等, 2013; Wang Jiamin et al., 2021)。上述模型中,板片断离是被运用最多的模型,两篇原始文献已经被引用＞2000次(Davies and von Blanckenburg et al., 1995; von Blanckenburg and Davies et al., 1995),其机理是大洋岩石圈密度较大、大陆岩石圈密度较小,在初始碰撞过程中由于大陆物质俯冲受阻而容易在其连接处形成应力集中、最终造成大洋板片断离。在世界范围内造山带中,洋陆转换过程中常伴随着超高压变质岩的折返,绝大多数研究将该过程解释成与大洋板片断离有关(Maruyama et al., 1996; Kohn and Parkinson, 2002; Leech et al., 2005; von Hunen and Allen et al., 2011),其中的重要依据为该过程中超高压榴辉岩为单一期次,与初始碰撞时代相近。
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但是板片断离模型存在明显缺陷:① 首先,板块构造的一级驱动力为深俯冲的岩石圈板片拖拽(如Conrad and Lithgow-Bertelloni, 2004),而板片断离将会使大陆俯冲无法持续进行(Niu Yaoling, 2017),显然与现今GPS数据观测到的正在猛烈进行的印度-亚洲大陆碰撞现实不符(例如Zhang Peizhen et al., 2004; Meade, 2007),与板片断离模型的提出地、阿尔卑斯造山带的地球物理观测也不相符(Zhao Liang et al., 2015);② 其次,板块俯冲是整个岩石圈的运动,其中俯冲的岩石圈地幔厚达＞70km、远厚于地壳部分(7~33km),大洋与大陆岩石圈地幔的密度并无明显差异;大陆俯冲过程中,酸性地壳一般堆积在造山带中、与其下方的基性下地壳和岩石圈地幔发生解耦(Gao Rui et al., 2016),其连接处存在薄弱面的说法并不成立。因此,洋陆转换过程中超高压变质岩的折返并不一定是板片断离的结果,很可能只是发生地壳尺度的拆离而已(Crustal detachment;Wang Jiamin et al., 2021),而大洋与大陆岩石圈地幔则在洋陆转换过程中持续俯冲。
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本文通过喜马拉雅中部麻粒岩化榴辉岩的P-T-t轨迹和折返过程研究,表明这些榴辉岩的折返与板片断离无关,是底垫到加厚地壳山根中缓慢抬升的产物。随着造山过程推进的地壳加厚模式,也可合理解释喜马拉雅西部超高压榴辉岩的折返较快、未叠加高温变质等特点。因而榴辉岩的折返并不是板片断离的标志。但不可否认的是,要彻底否定板片断离与榴辉岩折返的关联性,还需更多的深入工作,特别是对洋陆转换过程中折返的超高压榴辉岩进行直接研究。在喜马拉雅西部,出露于印度Tso Morari和巴基斯坦Kaghan地区的的超高压榴辉岩峰期变质时代(52~47Ma)与印度-亚洲碰撞带的洋陆转换时间节点接近(60~50Ma),前人常用板片断离来解释。研究这些超高压变质岩的详细折返时代和P-T轨迹差异,是厘定该问题的潜在突破口。
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4.2 喜马拉雅接触变质作用与稀有金属成矿
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大陆碰撞带变质作用对矿产资源的制约效应,是未来变质地质学研究“面向国家需求”的重要组成部分。大陆碰撞带变质作用类型多样,变质流体与岩浆热液相互作用,可能会形成珍贵的矿产资源。以往对大陆碰撞带的变质地质学研究高度集中于碰撞造山演化过程的研究,变质作用相关的矿产资源研究尚处于起步阶段。造山带的温度结构、物质组成、氧逸度、流体活动等可能是众多金属矿床形成的重要因素,而厘定这些成矿条件则强烈依赖变质作用的研究。
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值得关注的是,近年来在喜马拉雅淡色花岗岩及伟晶岩中陆续发现了稀有金属成矿现象,有可能使喜马拉雅成为我国新的稀有金属矿产开发基地(Wang Rucheng et al., 2017;Wu Fuyuan et al., 2017, 2020;李光明等,2017)。其中,淡色花岗岩侵入特提斯喜马拉雅造成碳酸盐岩接触变质作用过程中,会形成矽卡岩化相关的W-Sn-Be-Li等稀有金属热液矿床。如喜马拉雅错那洞穹隆的矽卡岩型超大型W-Sn-Be矿床(李光明等,2017; Fu Jiangang et al., 2021);喜马拉雅穷嘉岗和热曲地区新发现的超大型锂矿(100万t),伟晶岩中全岩Li2O3含量可达1.5%~3%(秦克章等, 2021;刘小驰等, 2021)。当然,上述矿产的储量为科学预估,具体储量数值还需实业部门的进一步勘探厘定。在喜马拉雅的稀有金属成矿带中,围岩的矽卡岩中往往含有符山石、方柱石、锡石、富W块状硫化物等富含W-Sn-Be-Li元素的矿物,如错那洞穹隆(Fu Jiangang et al., 2021)和拉隆穹隆(据笔者初步考察和测试分析数据),是未来提取稀有金属元素资源的重要潜在对象。此外,矽卡岩中符山石F、Cl含量高、常见萤石,富F-Cl的变质流体无疑在稀有金属成矿过程中扮演了极其重要的角色。但是,对喜马拉雅造山带的稀有金属成矿的规模、空间展布、成矿时代、形成机制等研究还处于初步阶段(Wu Fuyuan et al., 2020),接触变质作用对稀有金属成矿机制和规模的贡献,尚需进一步厘定。
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4.3 变质脱碳作用与大陆碰撞带深部碳循环和通量研究
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地球系统科学是国家自然科学基金委改革所提出的未来优先资助方向,可能会形成继板块构造理论提出以来的第二次地球科学革命(郭正堂, 2019;朱日祥等, 2021)。以喜马拉雅为代表的大陆碰撞带是地球系统科学研究的国际主战场。大陆碰撞带深部变质作用与地壳加厚、山脉隆升和风化、深部碳循环、气候变化之间有着复杂的相互反馈关系(如Gaillardet and Galy, 2008; Scotese, 2021),可能是变质地质学走向地球系统科学研究的重要组成部分。在这其中,变质作用的P-T-t轨迹是反演地壳加厚程度和时间节点的重要内容,而P-T-X(CO2)是揭示变质脱碳作用和深部碳循环的重要手段。针对喜马拉雅的地球系统科学研究,未来可关注的其中一个重要方面为运用变质脱碳作用研究大陆碰撞造山带的深部碳循环和通量,揭示其在全球长周期碳循环过程中的贡献。
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具体地,印度-亚洲碰撞造成深部广泛的物质堆叠和地壳加厚、最终形成了地球上海拔最高的喜马拉雅山和青藏高原。第三极的形成,不仅形成了蔚为壮观的地貌景观,也深刻影响着亚洲甚至是全球气候,开启或加强了亚洲季风环流(An Zhisheng et al., 2001; Guo Zhengtang et al., 2002; Clift et al., 2008)。山脉隆升和季风加强,会造成硅酸盐岩风化剥蚀加强,随着河流搬运进入大洋,从大气中吸收CO2、形成碳酸盐岩沉积,从而带来巨大的碳汇,降低大气中的温室气体含量和气温(Zachos et al., 2001; Hilton and West, 2020; Märki et al., 2021)。如印度-亚洲碰撞造成喜马拉雅和青藏高原隆升被许多研究认为是新生代全球变冷的驱动引擎(Raymo and Ruddiman, 1992; Selverstone and Gutzler, 1993; Guo Zhengfu et al., 2021)。此外,在大陆汇聚碰撞过程中,原先被动大陆边缘斜坡上沉积的海相碳酸盐岩和含碳岩石的俯冲埋藏和区域变质脱碳(Groppo et al., 2017, 2021; Rapa et al., 2017; Rolfo et al., 2017; Guo Zhengfu et al., 2021),以及花岗岩岩浆侵位过程中造成围岩接触变质脱碳(Chu Xu et al., 2019; Wang Jiamin et al., under review),可能是被低估的地质历史时期的重要深部碳源,这已被喜马拉雅造山带温泉中的高CO2通量所证实(Becker et al., 2008; Evan et al., 2008)。上述大陆碰撞带的地质无机碳汇和碳源过程,是深部碳循环过程中不可或缺的重要环节,但对其具体过程、机制和通量,尚缺乏深入的研究。
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致谢:仅以此文纪念程裕淇先生诞辰110周年!本文第二作者在学生时代就与程裕淇先生有过接触,在随后的前寒武纪地质、花岗岩与混合岩及实验岩石学等研究工作过程中,多次得到他的指点与帮助。调任地质与地球物理研究所后,通过档案阅读发现,程裕淇先生曾任原中国科学院地质研究所副所长,为研究所的建立和新中国地质科技事业的发展倾注了大量心血。感谢郭敬辉研究员、魏春景教授、吴春明教授、张立飞教授、刘福来研究员、张泽明研究员、曾令森研究员、赵亮研究员、刘传周研究员、万博研究员等诸位老师在研究工作中给予的帮助和指导!感谢十多年来在喜马拉雅野外工作提供了大量帮助的藏族同胞们。董汉文博士和田作林博士对本文进行了认真审阅,并提出了宝贵建议。感谢万渝生研究员的撰稿邀请。由于篇幅有限,很多前人成果无法一一引用,特此致歉。
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摘要
本文从变质地质学视角出发,介绍了喜马拉雅造山带的研究意义、地质概况和近年来作者在喜马拉雅碰撞造山过程研究中的进展。喜马拉雅造山带是威尔逊旋回中陆陆碰撞造山带的典型代表,从中揭示的大陆碰撞造山过程、规律及效应,可为探索地球从古至今的碰撞造山带演化研究所借鉴。其中,大陆碰撞造山机制的研究是其核心内容。大陆碰撞造山机制存在临界楔和隧道流两种端元模型之争,其分别对造山带核部高级变质岩折返的P-T-t轨迹和时空演化序列进行了不同的预测。上述争议可通过研究喜马拉雅核部高级变质岩(高喜马拉雅)的P-T-t轨迹和折返过程来限定,据此可将喜马拉雅碰撞造山过程划分为三个演化阶段。阶段一:60~40 Ma,软碰撞期,造山带地壳加厚至约40 km并发生小规模部分熔融,这些早期地壳加厚记录大多已被剥蚀,零星保存于前陆飞来峰和北喜马拉雅片麻岩穹隆中;喜马拉雅山从海平面以下抬升至＞1000 m。阶段二:40~16 Ma,硬碰撞期,造山带地壳加厚至60~70 km,发生大规模高级变质和深熔作用,高喜马拉雅内部的三个次级岩片沿着“原喜马拉雅逆冲断层”、“高喜马拉雅逆冲断层”、“主中央逆冲断层”顺序式向南挤出,形成了现今喜马拉雅造山带的核部主体,地壳堆叠使喜马拉雅山快速隆升至≥5000 m。阶段三:16~0 Ma,晚碰撞期,造山带山根榴辉岩化发生局部拆沉,但大陆汇聚仍在持续、造山带尚未发生垮塌,小喜马拉雅折返、前陆盆地形成,喜马拉雅山达到和维持现今平均高度~6000 m。因此,喜马拉雅生长过程的一级次序是顺序式向南扩展的,受控于临界楔模型,而隧道流只起次级作用。山根深部热流过程对造山带的地壳结构和地表高程有巨大的改造作用。未来对喜马拉雅造山带的变质地质学研究可能存在以下几个关键科学问题:① 喜马拉雅极端变质作用与重大碰撞造山事件的关联;② 喜马拉雅稀有金属成矿与接触变质作用的关联;③ 喜马拉雅变质脱碳作用与大陆碰撞带深部碳循环和通量。
Abstract
From a metamorphic perspective, this paper introduces the research significance, a geological overview of the Himalaya, and the authors' research progress on the Himalayan collisional orogenic process in recent years. The Himalayan orogenic belt is a prototype of the continental collisional orogen in the Wilson cycle. The continental collisional process, rules and effects revealed from the Himalaya can be used as a reference for exploring the evolution of collision orogenic belts on Earth from ancient times to the present. Among them, the research on the orogenic mechanism of continental collision is its primary content. Specifically, the controversy of continental collision orogeny mechanisms lies in two endmember mechanisms: Critical Wedge and Channel Flow, which respectively predicted different P-T-t paths and exhumation spatio-temporal sequences of high-grade metamorphic rocks in the orogenic core. The above disputes can be constrained by studying the P-T-t paths and exhumation processes of the Himalayan Metamorphic Core (Greater Himalayan Crystalline complex). Consequently, the Himalayan collisional orogeny can be divided into three evolutionary stages. Stage one: 60~40 Ma, “soft” collision period; the crust was thickened to ~40 km and small-scale partial melting occurred; most of these early crustal thickening records have been denuded and are sporadically preserved in the foreland klippes and the northern Himalayan gneiss domes; the Himalaya has risen from below sea level to ＞1000 meters. Stage two: 40~16 Ma, “hard” collision period; the crust was thickened to 60~70 km, and abundant high-grade metamorphism and anatexis occurred; The three sub-slabs in the Greater Himalayan Crystalline complex were extruded southward sequentially along the "Eo-Himalayan Thrust", "High Himalayan Thrust" and "Main Central Thrust", forming the core of the Himalayan orogenic belt; the duplex caused uplift of the Himalaya to ≥5000 meters. Stage three: 16~0 Ma, Late collision period; the orogenic root underwent localized foundering due to eclogization; the orogenic belt has not collapsed because India-Asia convergent and collision is continuing today; the Lesser Himalaya was exhumed, and the Sub-Himalaya foreland basin was formed; the Himalayas reached and maintained the current average height of ~6000 meters. In a first-order, the growth of the Himalaya was dominated by in-sequence southward expansion, controlled by the Critical Taper model, while the Channel Flow only played a secondary role. The heat flow in the deep orogenic root has a significant transformation effect on the crustal architecture and topography of the orogenic belt. Future studies should pay attention to the following key scientific issues in the metamorphic research of the Himalayan orogen: ① the relationship between Himalayan extreme metamorphism and major collisional orogenic events; ② the relationship between rare metal mineralization and contact metamorphism in the Himalaya; ③ Himalayan metamorphic decarbonation, and carbon cycle and flux in collisional belts.
