岩石学报  2019, Vol. 35 Issue (1): 1-15, doi: 10.18654/1000-0569/2019.01.01   PDF    
大洋岛弧的前世今生
吴福元1,2 , 王建刚1 , 刘传周1,2 , 刘通1 , 张畅1 , 纪伟强1     
1. 中国科学院地质与地球物理研究所岩石圈演化国家重点实验室, 北京 100029;
2. 中国科学院大学地球与行星科学学院, 北京 100049
摘要:根据板块构造理论,板块的边界是地质作用最为强烈的地区,因而它们是当今固体地球科学研究的重点。依据应力性质的不同,地球上板块的边界类型有扩张的洋中脊、汇聚的俯冲带和调节板块运动差异的转换断层三种。就汇聚型板块边界而言,它又可进一步划分为洋-洋俯冲的大洋或洋内岛弧带(Intra-oceanic arc)、洋-陆俯冲的安第斯型活动大陆边缘带和陆-陆接触的大陆碰撞带三种。相对而言,大洋岛弧的研究程度最低。传统认为最典型的大洋岛弧——日本诸岛,已不再被认为是洋-洋俯冲的产物,因为已有研究显示它是从亚洲大陆裂解的碎块。根据目前的调查,现今的大洋岛弧主要集中在西太平洋地区,以太平洋与菲律宾板块间的Izu-Bonin-Mariana弧和太平洋-澳大利亚间的西南太平洋岛弧为代表。大洋岛弧研究的最重要问题是,洋洋之间如何产生了俯冲。目前多倾向于认为:大洋中的转换断层可使不同时代的大洋岩石圈相互接触,在这种情况下,较老的岩石圈由于冷却时间较长而密度相对较大,因而可下沉而俯冲到较年轻的岩石圈之下。这一模型也被誉为蛇绿岩形成的初始俯冲定律(Subduction Initial Rule,简称SIR)。但存在的问题是,目前全球还没发现有转换断层转变为俯冲带的实例。更何况,全球大洋中发育如此众多的转换断层,但为何只在西太平洋发育大洋岛弧?本文通过对资料的总结还发现,这些大洋岛弧基本都是从亚洲或者澳大利亚大陆东部边缘裂解的碎块,只是后期的弧后扩张作用使裂解的碎块发生强烈的改造,形成具有类似大洋岩石圈的特点。目前提出的洋-洋自发形成俯冲带的模型并没有理论基础,也没有实际地质事实的支持。但在加勒比海、斯科舍海和阿留申地区,大洋岛弧的出现与洋底高原诱发的俯冲带跃迁或俯冲极性反转有关。因此,板块构造理论中的洋洋初始俯冲模式需要进一步资料的验证。
关键词: 大洋岛弧     洋-洋俯冲     洋-陆俯冲     弧后扩张     蛇绿岩    
Intra-oceanic arc: Its formation and evolution
WU FuYuan1,2, WANG JianGang1, LIU ChuanZhou1,2, LIU Tong1, ZHANG Chang1, JI WeiQiang1     
1. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
2. College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
Abstract: According to the theory of plate tectonics, plate boundary is the locus of the various geological processes shaping our Earth, and thus the focus of the modern geology. It is generally accepted that the plate boundaries can be divided into three types, i.e., divergent (mid-oceanic ridge), convergent (subduction and collision zones), and strike-slip (transform fault). As for the convergent boundary, it can be further subdivided into intra-oceanic arc by oceanic-oceanic subduction, Andean continental arc by oceanic-continental subduction, and collisional belt by continental-continental collision. The intra-oceanic arc is relatively less studied compared to other types of convergent boundary. Traditionally, the Japanese islands were thought as an intra-oceanic arc, but this point of view is not hold anymore since it was found that they are fragments from the Asian continental margin. According to the available investigations, intra-oceanic arcs are mostly located along the western Pacific Ocean, represented by the Izu-Bonin-Mariana arc between the Pacific and Philippine plates, and the southwestern (SW) Pacific arc between the Pacific and Australian plates. The most important topic for the study of the intra-oceanic arc is that how subduction was initiated between oceanic-oceanic plates. It is mostly thought that the transform fault can make contact of the different aged oceanic plates, and subsequently in this case that the older and dense lithosphere can underthrust beneath the younger and less-dense plate, which is also ascribed as "Subduction Initial Rule-SIR". However, no actual example was found to date on the Earth for the situation described above, and moreover, it is hard to understand that the intra-oceanic arcs are extensively developed in SW Pacific, although the transform faults occurs everywhere in the seafloor. Data compilation indicates that the above intra-oceanic arcs are in fact continental relics rifted from the Australian continent. The subduction of the old and dense Pacific plate resulted in formation of the continental arc and associated back-arc basin, and the spreading within the back-arc basin then rifted the arc and made it evolved toward oceanic lithosphere. Therefore, the oceanic-oceanic subduction is not valid theoretically, and not documented in the geological records either. However, in the Caribbean, Scotia and Aleutian areas, the intra-oceanic arcs were formed by subduction jump and polarity reverse due to subduction of oceanic plateau. Whatever, the spontaneous initial subduction model needs further geological data to be verified.
Key words: Intra-oceanic arc     Oceanic-oceanic subduction     Oceanic-continental subduction     Back-arc extension     Ophiolite    

1967年4月19日,在华盛顿举行的美国地球物理联合会(AGU)春季年度会议上,年仅32岁的普林斯顿大学教授W. Jason Morgan提出,地球由12个刚性板块组成,它们在软流层之上做独立的运动(Morgan, 1968)。当时30岁的法国人Xavier Le Pichon听完报告,大受启发,提出地球由太平洋、欧亚、印度洋、非洲、美洲和南极洲六大板块组成的思想(Le Pichon, 1968)。同年,McKenzie and Parker (1967)在Nature撰文,提出与Morgan和Le Pichon类似的思想。结合Wilson早先提出的转换断层和Wilson旋回的概念(Wilson, 1965a, b, 1966),大陆漂移、海底扩张到板块构造的地学革命三部曲至此全部完成,标志着板块构造已开始成为主导地球形成演化研究的重要理论假说。也正因为如此,板块构造与相对论、分子生物学和量子力学等一起被认为是20世纪自然科学研究的重大成就,从而开创了地质学研究的新纪元。

尽管半个世纪已经过去,但板块构造理论的核心内容并无明显变化。根据这一理论,地球的表层为刚性的岩石圈,它被划分为不同规模的块体,即板块。根据性质的不同,板块有大洋板块和大陆板块之分。这些板块在地球表面发生运动,由于运动方向的差异,板块边界处可发生拉张作用形成离散型板块边界(洋中脊)或发生挤压作用形成汇聚型板块边界(俯冲带和碰撞带)。同时,由于球面上刚性板块不同纬度间线速度的差异,产生特征的转换断层。这样,洋中脊、俯冲带、碰撞带和转换断层就构成不同类型板块间相互接触的最主要类型,成为板块构造理论框架的核心内容。

对于汇聚型板块边界,它又可进一步划分为俯冲带和碰撞带两种。其中碰撞带主要为两个大陆相互接触,并可发生一个大陆向另一个大陆下面俯冲,从而形成大陆俯冲带。但目前地球上还少见有大洋碰撞带的例子。就俯冲带而言,由于大洋板片的俯冲脱水作用,在上覆板块的前缘常形成大规模的岩浆岩带,称为岩浆弧(Magmatic arc; Hamilton, 1988)。根据上覆板块性质的不同,岩浆弧又可进一步划分为洋-洋俯冲的大洋或洋内岛弧(Intra-oceanic arc,又称Oceanic arc或Oceanic island arc,即洋弧)和洋-陆俯冲的活动大陆边缘弧(Active continental margin或continental arc,又可简称为陆弧)两类。目前全球俯冲带的总长度为55000km,其中大洋岛弧为22000km,占整个俯冲带长度的40%,足见洋内俯冲在地球演化过程中的重要地位(Leat and Larter, 2003; Stern, 2010; Gerya, 2011a, b)。

然而,我们目前对洋-洋俯冲带形成与演化的研究还非常薄弱(Leat and Larter, 2003)。其主要原因是,洋内岛弧大多远离大陆而使研究者难以抵达;另一方面,绝大部分洋内岛弧位于水下,研究难度大。本文从西南太平洋的具体实例出发,深入剖析了大洋岛弧的基底性质及洋-洋俯冲可能发生的机制。我们的研究发现,洋-洋俯冲既无实际地质资料支持,也无合理的理论基础,目前的大洋岛弧成因模式仍停留在假想阶段。西南太平洋的所谓岛弧,实际上是从西侧澳大利亚大陆裂解下来的大陆碎块,但由于后期的弧后拉张和长期反复的岛弧岩浆作用,这些块体不断向大洋岩石圈方向演化,最后导致目前“以假乱真”的局面。

1 大洋岛弧概述

在我们的传统认识中,日本是岛弧构造的代表。但后来的研究证实,日本列岛的主体是从东亚大陆裂解出来的地块,并非洋内俯冲形成的岛弧,反而应该属于一种活动大陆边缘弧(Taira and Ogawa, 1991; Jahn, 2010)。与安第斯型活动大陆边缘不同的是,日本岛弧之下的俯冲板片年龄老而比重大,从而俯冲角度较大,易于产生弧后扩张(Uyeda and Kanamori, 1979)。根据目前的调查,全球的大洋岛弧主要集中在西太平洋,其中尤以中部的Izu-Bonin-Mariana (简称IBM)岛弧和西南太平洋的Vanuatu-Tonga岛弧最为著名(图 1)。

图 1 全球汇聚型板块构造边界图(据Frisch et al., 2011) 示三种不同类型的汇聚板块边缘,即活动大陆边缘(红色)、硅铝质岛弧(橘黄色)和硅镁质岛弧(蓝色),其中后者即为洋内岛弧 Fig. 1 Global distribution of the convergent plate boundary (after Frisch et al., 2011)

大洋岛弧的地质剖面可以用Mariana弧来代表(图 2a)。当一个大洋板块俯冲到另一个大洋板块之下后,俯冲的大洋板块将随深度的增大,逐步发生角闪岩相-麻粒岩相乃至榴辉岩相的变质及脱水作用。脱水的流体向上运移和交代上覆的地幔楔,并使后者发生部分熔融,进而导致岛弧岩浆作用(Tatsumi and Eggins, 1995; Spandler and Pirard, 2013)。俯冲板块的不断下插,还会扰乱上覆板块下部的对流体系,使其产生弧后拉张而形成弧后盆地(Hamilton, 1988)。很显然,俯冲带是复杂的,其关键是俯冲角度的大小。在整个西太平洋地区,太平洋板块的俯冲角度要远大于东太平洋地区,这就是为什么西太平洋较多发育弧后盆地的重要原因。具体就Mariana弧而言,其俯冲角度明显大于北侧的日本诸岛,因而导致其弧岩浆作用的宽度要窄得多(图 2b)。初步估算显示,Mariana弧的宽度只有50km,而日本弧的宽度约为300km。同样,在Mariana弧,海沟到弧后扩张中心的距离约为200km,明显短于日本弧的~700km。总之,弧后扩张作用导致的弧后盆地是西太平洋地区俯冲带最大的地质特色。

图 2 沟-弧-盆体系地质剖面示意图(据Frisch et al., 2011) 示上盘地壳性质的不同,可划分为Mariana和Japan两种类型,前者即为大洋岛弧 Fig. 2 Simplified geological cross section of the trench-arc-basin system (after Frisch et al., 2011)

在俯冲作用过程中,俯冲带如果发生后撤或者俯冲角度增大,早先形成的岩浆弧将被弧后扩张所撕裂,太平洋西侧的菲律宾板块就是实例。该板块原为太平洋板块向西俯冲形成的岛弧地质体,54~32Ma期间的弧后扩张形成西部的弧后盆地,之后,由于板块俯冲的角度逐渐变大,弧后扩张中心不断向东转移,相继形成29~16Ma的东部弧后盆地和5~0Ma的Mariana弧后盆地,从而将原先形成的岛弧撕裂成性质不同的碎片(Dewey and Casey, 2011)。岛弧撕裂的另一个典型实例是Tonga岛弧和Lau弧后盆地。该盆地西侧的Lau Ridge和Tonga弧前地区,均发育始新世(52Ma)以来的岛弧岩石(Meffre et al., 2012),但原始的岛弧由于太平洋板块向东回撤发生撕裂,形成Lau弧后盆地。与此同时,新形成的岩浆弧向东迁移,宽度变窄(图 3)。实际上,西太平洋地区因弧后扩张作用导致的岛弧撕裂现象普遍存在。

图 3 菲律宾板块形成演化(a,据Dewey and Casey, 2011)及Tonga岛弧剖面及撕裂过程示意图(b,据Frisch et al., 2011) Fig. 3 Evolving history of the Philippine plate (a, after Dewey and Casey, 2011) and Tonga arc section and its back-arc tearing resulted from steep subduction (b, after Frisch et al., 2011)
2 大洋岛弧成因概述

尽管目前对大洋岛弧带发生的各类地质作用有了大致的了解,但目前研究的焦点是,上述洋-洋俯冲是如何发生的。遗憾的是,目前学术界对此问题研究甚少。实际上,这一问题涉及俯冲带如何形成, 即初始俯冲这一关键课题(Mueller and Phillips, 1991; Gurnis et al., 2004; Korenaga, 2013)。根据Stern (2004)的总结,初始俯冲可分为诱发式(Induced/forced)和自发式(或原发式,Spontaneous/self-sustaining)两类。诱发式俯冲,顾名思义,是指在外力作用下,板块边界发生重组或调整而形成的。比如,在大洋板块俯冲过程中,如果一个较大规模的洋底高原进入俯冲带,它的低密度将导致其卡位于俯冲带之中而不能被俯冲消亡,而持续的俯冲挤压将使俯冲带跃迁至洋底高原的靠海一侧,形成新的俯冲带(图 4aNiu et al., 2003)。俯冲带跃迁的典型实例是位于IBM与西南太平洋岛弧之间的Yap海沟,它原本是太平洋与菲律宾板块间的俯冲带,但太平洋中Caroline Ridge与菲律宾板块的碰撞,使得俯冲带从西侧的Yap海沟跳跃到Caroline Ridge东侧,形成Mussau俯冲带(Lee, 2004)。另一个实例是Solomon岛弧,太平洋中的Ontong Java洋底高原在中新世早期沿南北向Vitiaz俯冲带与Solomon岛弧碰撞,导致在Solomon岛弧的南侧产生向北的New Britain-San Cristobal俯冲带(Cooper and Taylor, 1985; Petterson et al., 1997; Mann and Taira, 2004)。

图 4 初始俯冲模式图(据Stern and Gerya, 2018) Fig. 4 Subduction initiation models (after Stern and Gerya, 2018)

自发式初始俯冲是指相邻板块由于密度差等方面的原因,形成一侧板块俯冲到另一侧板块之下的现象(图 4b)。目前研究认为,自发式初始俯冲可能在三种边界发生,分别为被动陆缘的洋陆过渡带(Cloetingh et al., 1982, 1989; Gurnis, 1992; Regenauer-Leib et al., 2001; Nikolaeva et al., 2010; Marques et al., 2014)、大洋中的断裂(包括转换断层、拆离断层或扩张脊; Uyeda and Ben-Avraham, 1972; Toth and Gurnis, 1998; Hilairet et al., 2007; Leng and Gurnis, 2011; Shervais and Choi, 2012; Maffione et al., 2015),以及大火成岩省周边(Burov and Cloetingh, 2010; Gerya et al., 2015; Lu et al., 2015; Baes et al., 2016)。其中转换断层模式最受青睐,其主要原因是:第一,根据计算结果,初始形成的大洋岩石圈的密度低于软流圈,从而漂浮在软流圈之上。但经过一定时间的冷却后,该大洋岩石圈的密度将大于软流圈,从而使该大洋板块有可能沉入下伏的软流圈。第二,在转换断层的两侧,早形成的岩石圈由于经历过较长时间的冷却而密度增大,从而诱发俯冲作用的发生。但需要注意的是,尽管转换断层两侧年龄相差越大,发生初始俯冲的可能性增大,但越老的板块,冷却导致的刚性程度也越大,板块弯曲难度增加。因此,并非两侧板块年龄相差越大,俯冲作用就越易发生(Mueller and Phillips, 1991)。

如果我们接受板块构造理论,那大洋就有演化轮替之说,而大洋的消亡只有通过俯冲作用才能实现。然而,到目前为止,我们还没有在地球上发现被动陆缘或者转换断层转化为俯冲带的确切例子(Gutscher et al., 2002; Kim et al., 2018 ),这究竟是何原因?大西洋中最老的洋壳至少达150Ma以上,可它所在的洋陆边界仍保持被动陆缘性质。即使转换断层能够转化为俯冲带,进而形成大洋岛弧,那为何太平洋中的洋内岛弧仅集中在西太平洋地区,而在其它地区并不发育?值得注意的是,整个西太平洋岛弧体系绵延数千千米,但岛弧岩浆作用的起始时代惊人地稳定在大约52Ma左右(Gurnis et al., 2004; Sutherland et al., 2017),与太平洋-Izanagi间洋脊向欧亚大陆下俯冲的时代基本一致(Seton et al., 2015; Müller et al., 2016),可能意味着二者之间存在密切成因联系。由于针对初始俯冲的研究目前还停留在理论探索阶段,对洋-洋俯冲的起始标志、地质记录和成因机制还有待更多探索,但这无疑是今后板块构造理论研究的前沿课题。

3 全球主要大洋岛弧及其基底性质 3.1 Izu-Bonin-Mariana岛弧

大洋岛弧主要发育在西太平洋地区,其中最具代表性的就是Izu-Bonin-Mariana弧,简称IBM (图 1)。该岛弧的东侧为太平洋板块,西侧为菲律宾板块(Deschamps and Lallemand, 2002, 图 5)。以Palau-Kyushu Ridge为界,菲律宾板块大致可划分为西部的盆地区和东部岛弧区。西部盆地区主要为NW走向的大洋盆地,为54~32Ma期间扩张作用的产物。东部岛弧区的主要构造线方向近南北向,其内部存在有西部的Parece Vela-Shikoku和Mariana Trough两个主要盆地,分别代表了29~16Ma和5~0Ma两次扩张形成的弧后盆地。值得指出的是,目前对西菲律宾海盆地是否存在更老的基底还存在一些争议。有研究提出,北部Amami高原的玄武岩和英云闪长岩Ar-Ar年龄约为115Ma (Hickey-Vargas, 2005),Daito Ridge安山岩Ar-Ar年龄为117~119Ma (Ishizuka et al., 2011b)。紧邻我国台湾的花东盆地(Huatung Basin),其形成年龄为119~131Ma (Deschamps et al., 2000)。

图 5 IBM主要地质单元分布及早期弧岩浆作用序列(据Ishizuka et al., 2011a) Fig. 5 Distribution of the main geological units of the IBM and its early sequence of arc magmatism (after Ishizuka et al., 2011a)

作为典型的大洋岛弧,人们目前对它早期的岩浆作用特征等进行了较为深入的研究,以探讨大洋岛弧的成因机制。按照目前的总结(Ishizuka et al., 2011a),IBM出露的岩石可划分为三套组合,包括底部的蛇绿岩基底、中部的初始俯冲岩石组合和上部的正常岛弧岩石组合(图 5)。基底岩石组合主要包括蛇纹石化橄榄岩、辉长岩及席状岩墙群。初始俯冲岩石组合主要包括弧前玄武岩(Forearc basalt,简称FAB)和玻安岩,其中弧前玄武岩为介于岛弧拉斑玄武岩和大洋拉斑玄武岩之间的一种过渡类型岩石,它位于玻安岩之下,偶尔与玻安岩互层产出,其绝对年龄介于52~48Ma之间,与下伏蛇绿岩基底中辉长岩的年龄一致。上部的玻安岩以高镁为特征,年龄为44~48Ma,较弧前玄武岩略显年轻,但与上部的高镁安山岩年龄一致。最上部的正常岛弧岩石组合为岛弧拉斑玄武岩和钙碱性火成岩,它们的出现表明至少在44Ma以来该地区存在板块俯冲作用。值得注意的是,上述岩石剖面中没有发现硅质岩。

目前,IBM岛弧被认为是研究洋-洋俯冲最理想的场所(Stern and Bloomer, 1992; Ishizuka et al., 2011a; Arculus et al., 2015),其岩石序列记录了太平洋板块向菲律宾板块下的初始俯冲。但对于IBM俯冲带形成的机制,目前观点众多,争论的核心是在俯冲发生之前该地区属何种构造背景,原始的构造是如何转变成俯冲带的。Uyeda and Ben-Avraham (1972)最早提出IBM俯冲带起始于太平洋和菲律宾板块之间转换断层的认识,并被众多的后来研究者所推崇(Stern and Bloomer, 1992)。这一方案的最重要证据是,菲律宾板块中残存的洋中脊方向与IBM的走向近于垂直。但是,后来研究发现,菲律宾板块的主体并不是中生代期间形成的,而且该板块在形成期间曾经历过~100°的逆时针旋转(图 3a),因而该模型受到部分研究者的质疑(Taylor and Goodliffe, 2004)。鉴于仰冲盘的应力性质,Hall et al. (2003)提出由于板块运动方向改变而导致的挤压作用,致使当时太平洋板块中的一些断裂转换为俯冲带。考虑到西菲律宾海盆北部的Amami Plateau、Daito Ridge和Oki-Daito Ridge可能为120~110Ma间形成的岩浆弧(Hickey-Vargas, 2005),以及Bonin岛还存在159Ma的玄武岩(Ishizuka et al., 2011b),因此有学者最近提出(Ishizuka et al., 2011b, 2018; Leng and Gurnis, 2015),太平洋板块与西侧岩浆弧的边界可能是初始俯冲带的位置所在。

我们不拟讨论上述模型的细节,只是总结IBM的特征作为下文讨论的基础。第一,早期蛇绿岩基底主要由蛇纹石化橄榄岩组成,但Parkinson et al. (1998)的研究显示,这些地幔橄榄岩具有元古代的Re亏损年龄(最老为12.3亿年),显示它并不是年轻的大洋地幔,而更可能是大陆岩石圈地幔的碎块。第二,弧前玄武岩、玻安岩与高镁安山岩显示弧岩浆作用逐渐增强的趋势,其年代也逐渐年轻。特别是弧前玄武岩与下伏辉长岩时代一致(约52Ma, Ishizuka et al., 2011a; Reagan et al., 2013),被认为是初始俯冲的标志。对这些玻安岩中尖晶石和磁铁矿的Re-Os同位素测定发现,它确实来源于古老地幔的部分熔融,而不是初始俯冲模式所暗示的年轻软流圈地幔的部分熔融(Senda et al., 2016),Sr-Nd-Hf同位素数据也支持这一认识(Li et al., 2013)。此外,最近在IBM北部Izu弧的西侧,发现有2.6Ma的花岗闪长岩(Tani et al., 2015)。与其它地区不同,该花岗岩的SiO2含量在70%左右,它要么来自年轻地壳的再循环,抑或来自古老大陆基底的部分熔融。古老继承锆石的发现,佐证这一认识(Schmitt et al., 2018)。第三,IBM弧东侧俯冲的太平洋板块的时代大约为127~167Ma的晚侏罗-早白垩世(Leat and Larter, 2003),西菲律宾板块最早的基底可能就是以Amami Plateau、Daito Ridge、Oki-Daito Ridge和花东盆地为代表的115~131Ma的岛弧及弧后盆地岩浆建造。在Bonin岛,还发育有159Ma的玄武岩(图 5Ishizuka et al., 2011a)。而目前确认的IBM最早的弧前岩浆作用发生在50~52Ma,此时间与西菲律宾海盆扩张时代及影响该洋盆的Oki-Daito地幔柱时代基本一致(Ishizuka et al., 2013)。综合这些资料,我们有理由相信,IBM是在侏罗-白垩纪岛弧或更古老大陆地壳基础上发展起来的新的岛弧,而以前认为的IBM俯冲带由转换断层演化而来的观点,需要进一步资料的检验。

3.2 西南太平洋岛弧

西南太平洋是大洋岛弧最发育的地区,是东侧太平洋板块向西俯冲到澳大利亚板块之下的结果。该岛弧自北而南依次有New Britain (新不列颠)、Solomon (所罗门)、Vanuatu (瓦努阿图,或New Hebrides-新赫布里底)、Tonga-Kermadec (汤加-克马德克)、New Zealand (新西兰)等。这些岛弧与澳大利亚板块之间构成复杂的弧盆系统(Baldwin et al., 2012; Matthews et al., 2015),北部的New Britain、Solomon和Vanuatu等岛弧表现为西侧弧后盆地向东俯冲到太平洋板块之下(Coleman and Kroenke, 1981; Cooper and Taylor, 1985; Mann and Taira, 2004; Patriat et al., 2015),而南部的Tonga-Kermadec和新西兰北岛岛弧表现为东侧太平洋板块向西俯冲到澳大利亚板块之下(图 1)。

对西南太平洋岛弧的组成及成因,目前有大量的研究成果,我们在此关心的只是它们的基底组成。Tapster et al. (2014)对北部Solomon大洋岛弧中晚渐新世-早中新世闪长岩-英云闪长岩进行测定发现,其中的很多锆石具有太古代-古生代的U-Pb年龄,表明Solomon岛弧含有大量的大陆地壳基底。Buys et al. (2014)对Vanuatu岛弧上始新世-中新世安山岩进行研究发现,这些岩石也存在大量捕获锆石,包含2.8~2.5Ga、2.0~1.8Ga、1.75~1.5Ga、0.85~0.70Ga、0.53~0.43Ga和0.33~0.22Ga的年龄峰值。对比研究发现,这些年龄在西南太平洋诸岛上均未出现,但却与澳大利亚大陆的基底年龄存在很大的相似性。因而原作者认为,Vanuatu很可能原先是澳大利亚大陆的一部分,只是在后来的扩张过程中被运移到目前的地理位置。另外,与Vanuatu隔海相望的New Caledonia (新喀里多尼亚)岛上,分布着大量的晚古生代-中生代沉积地层,其碎屑锆石年龄分布显示明显的澳大利亚亲缘性(Adams et al., 2009; Cluzel et al., 2012; Pirard and Spandler, 2017; Campbell et al., 2018)。

大洋岛弧区别于大陆岛弧的重要特征是它的洋内属性,这决定了它的基底组成应为年轻的大洋地壳。但从上述介绍的资料可以看出,西南太平洋大洋岛弧的基底与澳大利亚大陆存在很大的相似性。结合这些岛弧的地质发展历史,我们认为,这些岛弧的基底是从澳大利亚大陆裂解下来的碎块,其裂解原因可能就是太平洋板块俯冲而产生的弧后扩张作用。裂解的碎块在后期又经历过多次的俯冲-拉张作用改造,致使原始的岩石建造难以保留,但部分稳定大陆踪迹仍保留至今。

支持这一解释的重要证据来自近几年来对新西兰岛岩石圈地幔的研究(McCoy-West et al., 2013; Scott et al., 2014a, b; Liu et al., 2015)。Re-Os同位素测定发现,无论是北岛还是南岛,其岩石圈地幔均显示古元古代,甚至太古代的年龄特征,但同时代的地壳岩石在岛上从未有过报道。相反,古老的碎屑锆石却出现在岛上的白垩系地层中(Adams and Griffin, 2012)。结合New Caledonia地区的资料,我们推测,在西南太平洋大洋岛弧之下,可能存在古老的大陆岩石圈残留,值得今后甄别与研究(Wei et al., 2016)。更有甚者,最近就有学者专门撰文论述新西兰大陆(Zealandia)存在的可能性(Mortimer et al., 2017)。该文推测新西兰大陆面积达近500万平方千米,可与澳大利亚东部的拉克兰(Lachlan)造山带进行对比。由于晚中生代-始新世澳大利亚大陆东缘发生裂解,Tasman海打开,造成目前的大陆分布格局(Gaina et al., 1998; Matthews et al., 2015)。

3.3 Lesser Antilles岛弧

除上面我们介绍的菲律宾板块与太平洋板块间的IBM弧以及澳大利亚与太平洋间的西南太平洋岛弧之外,地球上其它地区大洋岩石圈相互接触的例子还有中美洲的Lesser Antilles (小安德烈斯)岛弧、南美洲南部的South Sandiwich或Scotia (南桑威奇或斯科舍)岛弧和太平洋北部的Aleutian (阿留申)岛弧。

在南、北美洲之间的加勒比海地区,存在着一个较为复杂的岛弧系统。它包括北面近东西向的Greater Antilles岛弧、南面近东西向的Leeward Antilles岛弧、东面近南北向的Lesser Antilles岛弧,及西部NW走向的中美洲岛弧,上述四个岛弧围限的区域即为加勒比板块(Caribbean plate)。实际上,在Lesser Antilles岛弧的西部,存在一个时代为90~55Ma的古岛弧(Aves Ridge),它以Grenada盆地与Lesser Antilles岛弧相接。根据目前的研究(Pindell and Kennan, 2009),加勒比海地区的岛弧发展可划分为两个阶段,90~55Ma期间,加勒比板块向北美和南美板块之下俯冲形成南北向的大规模大陆边缘弧(Greater Antilles- Aves Ridge-Leeward Antilles, 简称Great Arc, Burke, 1988)。随着西侧板块的持续向东挤压,加勒比板块沿南北美洲两大陆之间的转换断层楔入大西洋,并使上述岛弧发生弯曲,并在Aves Ridge岛弧与大西洋板块之间形成新的俯冲带。大约在12~15Ma之后,大西洋向Aves Ridge岛弧下的俯冲形成Lesser Antilles岛弧,并造成弧后扩张,从而将先前形成的岛弧撕裂。与其它岛弧不同的是(Hawkesworth et al., 1993),Lesser Antilles岛弧的岩浆岩具有非常富集的Sr-Nd同位素组成(Macdonald et al., 2000; Labanieh et al., 2010; Teng et al., 2016)。这一般归因于俯冲沉积物的部分熔融(White and Dupré, 1986; Labanieh et al., 2010),但大陆地壳基底的贡献并不能排除(Davidson and Wilson, 2011; Bezard et al., 2014, 2015)。

关于加勒比海板块本身,目前倾向于认为它可能是一个年龄约为90Ma的洋底高原(Kerr et al., 2003; Kerr and Tarney, 2005; Wright and Wyld, 2011; Loewen et al., 2013; Whattam and Stern, 2015)。它原本发育在Farallon板块之中,当其与南北美洲的大陆弧碰撞后,持续向东楔入,并多次发生俯冲带的跃迁和俯冲极性的反转。因此,如果这一模型正确的话,Lesser Antilles岛弧的形成与洋内俯冲并无实质性关联。

3.4 South Sandiwich岛弧

位于南美板块与南极板块之间的Scotia海及其东侧的South Sandiwich (或Scotia)岛弧具有相对复杂的演化历史(Barker, 2001; Dalziel et al., 2013)。South Sandwich弧以东的同名海沟,代表了南美板块向South Sandwich弧之下的俯冲,俯冲作用产生的弧后扩张形成东Scotia扩张脊,并将Scotia海分成西部的Scotia板块和东部的Sandwich板块。目前可以确定的是,东侧南美板块是晚白垩世-渐新世的大洋岩石圈,西侧Scotia板块的时代目前并不明确,但大多数研究支持它原为与太平洋相关的Phoenix大洋板块的一部分,只是后来由于构造作用而被夹持在南美和南极板块之间(Dalziel et al., 2013)。由于研究程度的限制,我们目前还难以对该岛弧的成因机制及基底性质进行评估。

3.5 阿留申(Aleutian)岛弧

在太平洋北部,阿留申岛弧(Aleutian arc)从西部的勘察加半岛到东部的阿拉斯加湾,长约3000km。该岛弧的南面是向西北俯冲的太平洋板块,北部为阿留申盆地。目前对阿留申盆地的成因存在多种不同的观点(DeLong et al., 1978; Steinberger and Gaina, 2007; Chekhovich et al., 2012; Wright et al., 2016),但多数研究倾向于认为(Marlow and Cooper, 1983),阿留申盆地原为Kula板块的一部分。大约在60Ma左右,Kula板块向北俯冲到白令海和阿拉斯加之下。56~42Ma左右,Kula板块中的大洋高原被卡位在俯冲带之中,从而使原来洋-陆的俯冲边界跳跃到目前位置,形成洋-洋俯冲带。

以上我们对世界上主要岛弧的基底情况进行了总结。这些资料显示,大洋岛弧的基底含有一定的古老大陆岩石圈成分。这种现象不仅体现在IBM和西南太平洋的诸多岛弧中,在世界其它地区及地质历史上不同时代的岛弧中多有见及(Smyth et al., 2007; Li et al., 2018),非常值得学术界今后注意。尽管这些岛弧目前主要表现为大洋岩石圈的特性,但这很可能是由于它们在形成后遭受到强烈的改造所致,它们的前身非常可能是大陆岩石圈,或者是与大陆岩石圈关系密切的大陆边缘弧。在西太平洋之外的其它地区,大洋岛弧的形成主要与由于大洋高原夹塞所导致的俯冲边界跳跃有关,目前在大洋中还没发现俯冲带可自发形成的实例。

4 Kohistan-Ladakh大洋岛弧成因

以上我们详细讨论了现今大洋中洋内岛弧的发育情况和成因问题,但实际上这一概念在古板块构造的恢复中也极为重要(van der Meer et al., 2012)。以当前备受学术界瞩目的喜马拉雅造山带为例,印度与亚洲板块的碰撞模式实质上与当时的新特提斯洋中是否发育大洋岛弧有关。在我国的藏南地区,传统的模式认为,印度与亚洲的碰撞发生在60~55Ma左右(Ding et al., 2005; Wu et al., 2014; Hu et al., 2015, 2016),为南北两大陆直接碰撞(图 6a)。但Aitchison et al. (2000)则提出,印度与亚洲之间的新特提斯洋中存在泽当洋内岛弧。该岛弧首先与南侧印度大陆碰撞,然后再一起与亚洲大陆碰撞(图 6b)。很显然,泽当岛弧的成立与否是区分上述不同模式的关键所在。我们不拟对这一问题进行全面的论述,只是提及,如果泽当弧成立的话,它与亚洲大陆之间应存在弧陆碰撞的缝合线。但到目前为止,我们在泽当弧与亚洲大陆之间并没有发现可以鉴别的碰撞边界(Zhang et al., 2014)。因此,泽当弧实际上是北部冈底斯岩浆弧的一部分,前人提出的弧-陆碰撞模式并没有地质事实的支持(Wu et al., 2014)。

图 6 印度-亚洲大陆间雅鲁藏布缝合带形成模式图(据Wu et al., 2014修改) Fig. 6 Model showing the formation of the Yarlung Zangbo suture between India and Asia (modified after Wu et al., 2014)

无独有偶,喜马拉雅造山带西段的印度-巴基斯坦地区,上述争论同样存在。与我国藏南地区不同的是,该区在印度和亚洲大陆之间发育有巨型的Kohistan-Ladakh岛弧,该岛弧以Shyok和Indus缝合线分别与北侧的亚洲大陆(喀喇昆仑)和南侧的印度大陆相接。传统观点认为(图 7a),Kohistan-Ladakh岛弧首先在70Ma左右与北侧亚洲大陆拼合,然后在55Ma左右与南侧印度大陆碰撞(Petterson and Windley, 1985, Rehman et al., 2011)。但最近也有学者提出(Khan et al., 2009; Bouilhol et al., 2013; Jagoutz et al., 2015),Kohistan-Ladakh岛弧首先在55Ma左右与印度大陆碰撞,然后该复合地体在50~40Ma左右与亚洲大陆拼合(图 7b)。我们不拟对上述不同模式的细节作全面的论述,只是指出下面两点供读者思考。首先,Shyok作为板块缝合线的地质证据明显不足。无论是在印度西北部的Ladakh还是巴基斯坦的Kohistan地区,沿Shyok缝合线并不发育指证板块俯冲-拼合的高压变质岩系。更为重要的是,沿线分布的少量镁铁质与超镁铁质岩体,也与我们通常定义的蛇绿岩相差甚远。因此,有学者认为,Shyok缝合线并不表征存在过一个消失的大洋,它至多记录了一个小规模的弧后盆地(Pudsey, 1986; Robertson and Collins, 2002)。

图 7 巴基斯坦北部大洋闭合模式图 Fig. 7 Proposed models showing the closure of the Neo-Tethyan ocean between Indian and Asian plates

其次,Kohistan-Ladakh被认为是目前世界上保留的最具代表性的大洋岛弧,它的成分演变被认为是大洋岛弧向大陆演化的范例(Jagoutz and Schmidt, 2012; Jagoutz, 2014)。在Kohistan弧内,在大量深成岩发育之前存在一套火山-沉积岩系,其自下而上分别被命名为Kamila斜长角闪岩、Jaglot火山沉积岩、Chalt火山岩和Yasin碎屑沉积岩(Burg, 2011; 图 8)。其中Jaglot和Yasin碎屑沉积岩成分成熟度高,显示古老大陆地壳长期风化的岩石建造特征。我们对Jaglot岩系副片麻岩样品的初步研究发现,该岩系中存在大量元古代的碎屑锆石,最老可达20亿年左右,其整体年代学特征与北侧的Karakorm地体极为类似。因此,Kohistan可能并不是一个洋-洋俯冲形成的洋内弧,它是在亚洲活动大陆边缘的一部分,只不过该活动大陆边缘在发育期间存在过弧后扩张,从而将早期形成的岩浆弧裂解成与现今西南太平洋类似的情形。

图 8 巴基斯坦北部Kohistan岛弧不同类型岩石发育关系示意图 Fig. 8 Sedimentary-igneous sequence of the Kohistan arc in northern Pakistan
5 洋-洋初始俯冲与蛇绿岩成因

蛇绿岩是一个重要的岩石学名词,它在古板块构造恢复中发挥着重要的作用。自从Gass (1968)首次提出蛇绿岩可能相当于大洋岩石圈开始,蛇绿岩就成为确定已消失大洋的重要地质证据。但Miyashiro (1973)创新性地提出塞浦路斯的Troodos蛇绿岩可能形成于俯冲有关的岛弧环境,从而使Alabaster et al. (1982)提出SSZ蛇绿岩的概念。随着资料的积累,人们发现世界上95%以上的蛇绿岩都发育弧岩浆作用的印迹,从而提出蛇绿岩更多的是代表小洋盆,而不代表浩瀚的大洋盆的认识(Pearce, 2003)。

既然是与岛弧有关,那蛇绿岩就可形成于弧前、弧后、弧间等不同部位,这种争论在塞浦路斯的Troodos蛇绿岩、阿曼的Semail蛇绿岩和美国西部的Coast Range蛇绿岩中表现最为清晰(Hopson et al., 2008)。这种争论的本质就是如何看待蛇绿岩中镁铁质岩石的岛弧地球化学指标和覆盖在蛇绿岩之上硅质岩的远洋沉积特征。在这一学术背景下,弧前初始俯冲模式成为当前蛇绿岩研究者关注的焦点(Pearce and Robinson, 2010; Reagan et al., 2010; Whattam and Stern, 2011; Leng et al., 2012; Stern et al., 2012; Zhou et al., 2018)。

图 9所示,当一侧板块相对另一侧板块发生下沉时,两者之间将产生一定的空当。很显然,该空当会被流动的软流圈所占据。而该软流圈在占据上述空当的过程中,势必发生上升以及与之相伴的减压部分熔融。这样,熔融的熔体形成蛇绿岩中的镁铁质岩系,而残留体形成蛇绿岩中的超镁铁质岩系。在这一模式中,由于早期仅表现为软流圈的部分熔融,所形成的熔体主要表现为洋中脊玄武岩(MORB)的特征。随着过程的进行,俯冲作用影响越来越显著,从而使后来产生的熔体中较多地携带俯冲作用的痕迹,甚至出现表征弧前岩浆作用的玻安岩。

图 9 初始俯冲与蛇绿岩形成示意图(据Stern, 2004) Fig. 9 Subduction initiation and the formation of ophiolite (after Stern, 2004)

上述模型显然从全新的角度,对蛇绿岩的特征和成因给予了重新解释。本文作者无意对上述模型进行全面的回顾与评述,只是指出以下几点供读者参考:第一,初始俯冲的引入尽管合理解释了蛇绿岩中部分以前难以解释的特征,但它仍没有涉及俯冲作用发生前的第一代大洋板块的形成机制,即上述俯冲或被俯冲的大洋板块如何产生;第二,在上述模型中,蛇绿岩代表初始俯冲,即蛇绿岩可被视为俯冲作用开始的标志(Stern et al., 2012)。那为何几乎所有的蛇绿岩,其上部都被远洋的硅质岩覆盖,且缺乏与岛弧形成相关的岩浆作用及沉积记录呢?第三,尽管初始俯冲模式预测镁铁质岩石可能会呈现从MORB向SSZ演变的趋势(Whattam and Stern, 2011),但绝大多数蛇绿岩并不表现出这种变化,甚至是相反的演化趋势;第四,玻安岩的岩石学特征要求其来源于难熔(refractory)地幔较大程度的部分熔融。而若发生这种程度部分熔融,该地幔应先发生过交代作用以降低它的熔点。但这种交代作用或许与初始俯冲有关,或许是与初始俯冲无关的更古老的地幔富集事件有关(Duncan and Green, 1987; Umino et al., 2015)。在塞浦路斯的Troodos蛇绿岩中,玻安岩大量侵入蛇绿岩的地幔橄榄岩之中,明显代表了蛇绿岩形成之后的另外一次岩浆事件。此外,玻安岩多被认为是弧前环境的标志岩石。但实际上,这种岩石在岛弧、裂谷和洋中脊等环境中同样存在(Hickey and Frey, 1982; Crawford et al., 1989; Nonnotte et al., 2005)。根据软流圈中存在古老地幔组分的事实(Liu et al., 2008),我们甚至怀疑,这些古老岩石圈地幔由于密度较轻和长期的交代作用改造,极易在洋中脊通过卸压而聚集。在减压或者地幔柱加热的情形下,它们完全可以部分熔融形成玻安岩(Duncan and Green, 1980, 1987; Golowin et al., 2017a, b)。或者说,洋中脊可能是玻安岩产出的较有利部位。第五,IBM被认为是初始俯冲弧前蛇绿岩的典型代表,但我们并无可靠资料证明,IBM在52~49Ma期间确实处于弧前位置。正如前面讨论过的那样,IBM作为菲律宾板块的一部分,经历了复杂的岛弧形成与撕裂历史(Dewey and Casey, 2011)。另一方面,从约50Ma至今,IBM发生过大约150km的俯冲侵蚀(Lallemand, 1995)。如果将这一距离恢复的话,IBM在始新世期间可能并不处于弧前位置,而更可能位于弧间甚至弧后位置。因此,IBM形成于弧前位置的说法需要进一步资料的验证。实际上,俯冲侵蚀目前已被证明在西南太平洋岛弧区普遍存在(Clift and Vannucchi, 2004; Stern, 2011),Tonga等地区的弧前认识等都需要进一步推敲。

最近,更有学者通过蛇绿岩和相伴生的变质底板(Metamorphic sole)的年代对比来进一步约束初始俯冲产生的自发与诱发机制(Guilmette et al., 2018)。这一学术思想实际上在早年已经提出(Wakabayashi and Dilek, 2003; Shervais and Choi, 2011; Agard et al., 2016),但一直未能发现可靠的证据予以约束。我们不拟讨论上述研究的细节,只是指出,变质底板石榴石的Lu-Hf等时线年龄与蛇绿岩锆石的U-Pb年龄不具可对比性,更何况该Lu-Hf年龄的可靠性有待进一步研究的验证。

因此,从目前资料看,蛇绿岩形成的弧前初始俯冲模型缺乏实际地质证据,它基本处于模型到模型的理论假设阶段,需要实证资料的进一步检验。在传统的岩石学理论框架中,弧前被认为是岩浆作用不发育的地区(Magmatic gap)。因为那里深度浅,俯冲的板片充其量只是发生少量的脱水,其地幔楔不具备发生熔融进而导致大量岩浆产生的条件(Abers et al., 2017; Perrin et al., 2018)。此外,即使上述初始俯冲模式成立,我们也不应该将其称之为fore-arc ophiolite,而应将其称之为Pre-arc ophiolite,因为它只是与岛弧或俯冲作用在发生的时间上存在先后关系。

但是,弧前是否可以由于其它机制导致拉张形成蛇绿岩,这是目前极受关注的重要问题(Butler and Beaumont, 2017)。

6 结论

显降低。对这些大洋岛弧仔细研究发现,它们基本上存在两种形成方式。在加勒比海、斯科舍海和阿留申等地区,当大洋中的洋底高原随板块运移而到达俯冲带时,它的低密度将引发洋底高原的夹塞,致使俯冲带跳跃到洋底高原的另一侧,从而开始洋-洋俯冲,继而形成大洋岛弧。然而,西太平洋的大洋岛弧却显示另一番不同的景象,它们的基底或多或少都显示古老大陆的特征,表明它们原本并非大洋板块的部分,而更可能是从周边大陆上裂解的碎块。在太平洋的西缘,大洋板块向大陆下的高角度俯冲,形成西太平洋的沟-弧-盆体系。而俯冲大洋板块的回返与后撤,致使早先形成的大陆岛弧发生裂解,并向大洋岩石圈方向演化。因此,从本质上看,这类大洋岛弧实际上是从大陆岛弧演化而来,目前盛行的洋-洋俯冲的大洋岛弧初始启动模式,既没有理论基础,也无实际资料的支持,因而需要进一步研究根据目前的调查,洋-洋俯冲及其所形成的大洋岛弧仅在西太平洋广泛存在,而在其它大洋以及太平洋的其它部位出现频率明的检验。但大陆边缘岛弧又是如何产生的,这又涉及到被动陆缘如何转化为俯冲带这一重要理论问题。尽管这一问题极为重要,但它已远远超出本文要讨论的范围。

致谢      本文第一作者在近年的工作过程中,多次得到叶大年先生的指点与教诲,特别就如何开展创新性科学研究,曾多次向他请教。恰逢他80华诞,我们撰写此文,表达对他的衷心祝愿;同时也以此文纪念板块构造理论提出50周年。本文的初步思想在2017年6月郑永飞教授主持的《板块俯冲带》会议上做过交流,文章撰写过程中与万博、赵亮、朱弟成、胡修棉、丁林等众多学者进行过多次不同形式的讨论。感谢王强研究员对本文认真而细致的评审,他的意见对本文质量的提高大有裨益。

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