造山带中增生楔识别与地质意义

增生楔主要由海沟复理石、远洋—半远洋沉积和洋岛/海山等大洋板块地层岩石及蛇绿岩共同构成,是汇聚板块边缘古俯冲带构造—沉积作用的综合产物,以发育叠瓦状逆冲断层、双冲断层和紧闭—倒转褶皱,以及片理、碎裂构造、小型褶皱、膝折等不同尺度的构造为特征。增生楔组成包括连续单元和混杂岩两部分,严格受滑脱面位置控制。增生楔是增生型造山带最基本大地构造相,它与弧前盆地、岛弧/大陆边缘弧的时空配置关系可直接指示大洋板块俯冲极性、揭示洋盆演化与造山作用方式。准确识别增生楔,详细解剖其结构特征与组成特征,综合判别并恢复大洋板块地层,可为古板块边界与古大洋盆地消亡位置确定、造山带结构精细划分及造山作用过程重建提供基本依据。大比例尺地质填图是研究增生楔结构和组成、以及大洋板块地层重建的有效手段。以南祁连拉脊山石灰窑增生楔为例,地质填图结果表明它是由中寒武世—早奥陶世洋壳物质构成,自北而南依次可分为海沟—大洋盆地—海山OPS和六道沟SSZ型蛇绿岩,被南倾逆冲断层分割成多个岩片。海沟—大洋盆地OPS岩片基本保留完整地层序列,海山OPS岩片包括连续单元和混杂岩两部分。顶帽山增生楔主要由海山OPS构成,内部结构层序相对完好,是南祁连构造带中保留最为完整、规模最大的海山OPS岩石组合序列。上述不同类型OPS片段和六道沟SSZ型蛇绿岩组成的增生楔与其南侧岛弧带同时代。增生楔—岩浆弧的时空配置暗示寒武纪—早奥陶世时期原特提斯洋自北向南俯冲极性。这些认识为南祁连早古生代火山—岩浆—沉积构造演化以及是否存在古洋盆与古洋盆演化重建提供基本佐证。

蛇绿岩与蛇绿混杂带结构

蛇绿岩是刻画造山带洋—陆演化格局的标志性证据,由具有成因联系的超基性—基性岩和上覆远洋—深海沉积物组成,是一种理想剖面,非实际的野外露头表现。蛇绿岩分为MOR型和SSZ型两类,前者形成于洋中脊、洋盆宽度足够的弧前/弧后盆地扩张脊环境,后者形成于洋壳初始俯冲阶段的弧前环境。蛇绿岩的形成年龄可以通过古生物和同位素测年方法获得,同位素年龄与最老古生物化石(如放射虫、牙形刺等)年龄相近;洋壳运移过程中,有岩浆击穿洋壳而发育二元结构,由早期洋壳和叠覆的晚期岩浆岩构成;晚期岩浆岩比最老古生物年龄更为年轻。蛇绿岩通过俯冲铲刮、深部底垫和仰冲作用就位,发育褶皱和叠瓦状—双冲构造、及构造肢解为“基质夹岩块”结构成为蛇绿混杂带;蛇绿岩的形成年龄和就位年龄之间可从无年龄差到数十、甚至上百个百万年的时间跨度。蛇绿岩就位后继续经历俯冲带上盘和陆内活化等阶段的演化:蛇绿岩就位时的滑脱断层切穿俯冲带上盘,为后期岩浆提供通道,可侵入多期次的、非蛇绿岩套组份的中基性—酸性岩脉和岩体,包括SSZ型特征的岩浆岩,如新疆扎河坝—和布克赛尔蛇绿混杂带和美国Klamath山蛇绿混杂带。蛇绿混杂带在变质变形强烈的俯冲带上盘和活化的陆内环境中,受到构造改造,造成单一露头区不能够完整地出露有蛇绿岩套的岩石组份,而侵入在蛇绿混杂带中的岩浆岩被构造肢解、形成岩块并使得混杂带内岩石组份复杂化。现今露头的蛇绿混杂带是经历过多阶段演化的、一种现时状态的“四维时空”信息的综合体,即多个时期的物质叠置在同一个露头的三维空间内,一些岩石可能与蛇绿岩套并无成因联系。

白云鄂博赋矿白云岩的稀土地球化学及对稀土矿化的制约

东矿西缘的A-A´剖面延伸约500 m,从下盘白云岩一直到上盘板岩为连续剖面。在白云岩与板岩接触处出现一个过渡带,其矿物组成和结构不同于白云岩和其上覆的板岩,为富含稀土,铁和萤石的条带状矿石,与主东矿的条带状矿石相似,其间还有富铁的薄的夹层。A-A´剖面的白云岩与主东矿下盘的白云岩是相似的,除其含有稍高的铁之外。推测,该剖面的白云岩来自于含大量碱金属,REE和铁的碳酸岩浆,并有高含量的挥发分。在碳酸岩浆侵位和结晶分异的过程中,这些组分会富集在残余岩浆之中,最终聚集在白云岩的顶部与板岩接触处,从而形成了该剖面过渡带中富稀土、铁、铌和萤石的条带状矿石以及富铁的薄的夹层。这些矿石类型分割开了白云岩和上覆的板岩。但是,该剖面未出现主东矿强烈而广泛的钠交代现象,即未形成钠质霓长岩。但是,该剖面的板岩中则钾钡交代广泛发育,形成了富钾和钡的板岩,可定义为钾质霓长岩。通过对A-A´剖面的系统研究及与主东矿的对比表明,白云鄂博地区的稀土、铁和微量元素的高度富集,主要出现在白云鄂博碳酸岩与围岩的接触处。但是形成一定规模的矿体还是需要具备一定的条件。首先要有来自深部规模宏大可聚集大量挥发分、稀土、铁和其它微量元素的碳酸岩(白云鄂博矿即为白云岩),白云质碳酸岩的母岩浆分异结晶的晚期阶段在其顶部与H9板岩的接触处即在H9的底部,由于H9板岩的屏蔽作用,其下聚集了更加富集萤石,稀土和铁等的残余碳酸岩熔体形成条带状矿石,并对所接触板岩继续进行广泛而强烈的钠质交代作用即钠质霓长岩化,在这一过程中仍然发育了强烈的稀土和铁的矿化,这样即形成了超大规模的白云鄂博矿。但处于白云鄂博赋矿白云岩边缘且位置较浅的碳酸岩,钠交代作用甚微,在白云岩与板岩的接触带中虽有稀土和铁的明显富集,但不能形成具一定规模的矿体。表明,稀土的强烈矿化与钠质交代作用密切相关,而一旦钾质交代作用替代了钠质交代作用,稀土矿化明显减弱。

增生弧基本特征与地质意义

增生弧是发育在增生楔基底之上的岩浆弧,是由于海沟后撤、新的弧岩浆前缘向着洋的方向迁移至早期增生杂岩基底之上。增生弧的发育是Şengör(1992)提出的增生型(突厥型)造山带的核心动力学过程,然而对其识别与大地构造意义尚存诸多争议。日本岛弧是Şengör用以建立增生弧模式的范例,其白垩纪以来的岩浆弧发育在侏罗纪和更早的增生杂岩基底之上。日本岛弧的增生弧岩浆具有典型的岛弧型微量元素地球化学特征,包括富集的LREE和LILE、以及亏损的HFSE,但其Sr⁃Nd同位素随着海沟后撤和增生弧演化不断变亏损。地球物理剖面显示日本增生弧岩浆来自两个截然不同的岩浆源区,一个位于弧前俯冲大洋岩石圈莫霍面之上,一个位于弧岩浆轴部的莫霍面之下,这可能暗示了日本增生弧具有增生楔重熔和增生弧地幔楔部分熔融两种不同的岩浆形成机制,但增生弧的地幔楔如何形成这一问题尚未得到解决。中亚造山带是增生型造山带,其西段的西准噶尔地区出露早古生代成吉斯弧和晚古生代萨吾尔弧,均为增生弧。萨吾尔弧晚石炭世的弧岩浆岩以Ⅰ型花岗岩、花岗闪长岩—闪长岩岩墙、中⁃基性火山熔岩等为代表,发育在包括有蛇绿混杂岩、OPS混杂岩、连续单元浊积岩的增生杂岩基底之上这些侵入岩和喷出岩与增生楔物质分别呈侵入和不整合覆盖接触关系。成吉斯弧早泥盆世的弧岩浆岩以中⁃基性火山熔岩为代表,不整合覆盖在混杂带基底之上;其中混杂带的基质为晚志留世砂岩,夹持有晚奥陶世—晚志留世灰岩、硅质岩等岩块。以上案例展示了增生弧具有典型的二元结构:上盘为晚期增生弧弧岩浆、下盘是早期增生杂岩基底,二者被一个大型的不整合面分隔或构成岩浆侵入接触关系。萨吾尔增生弧岩浆岩具有典型的岛弧型微量元素地球化学特征,花岗岩端元Nd同位素亏损、Sr同位素富集,而玄武岩端元的Sr⁃Nd同位素均表现为亏损特征,表明基性岩端元直接源自新生的增生弧地幔楔的部分熔融,继承了亏损地幔的同位素特征;酸性岩端元则受到增生楔物质重熔的混染,如富集的Sr同位素,源自增生楔中大洋沉积物和蛇绿岩。增生弧的发育可能是增生型造山带大量年轻地幔物质加入和大规模地壳生长的原因。这一现象还可以进一步地探讨增生弧地幔楔的成因:即随着海沟后撤,原海沟位置的俯冲大洋岩石圈并不会发生断离下沉,而是保留在原位、形成新的增生弧地幔楔。

Anatomy of composition and nature of plate convergence： Insights for alternative thoughts for terminal India-Eurasia collision

The pattern and timing of collision between India and Eurasia have long been a major concern of the international community. However, no consensus has been reached hitherto. To explore and resolve the disagreements in the Himalayan study,in this paper we begin with the methodology and basic principles for the anatomy of composition and nature of convergent margins,then followed by an effort to conduct a similar anatomy for the India-Eurasia collision. One of the most common patterns of plate convergence involves a passive continental margin, an active continental margin and intra-oceanic basins together with accreted terranes in between. The ultimate configuration and location of the terminal suture zone are controlled by the basal surface of the accretionary wedge, which may show fairly complex morphology with Z-shape and fluctuant geometry. One plausible method to determine the terminal suture zone is to dissect the compositions and structures of active continental margins. It requires a focus on various tectonic elements belonging to the upper plate, such as accretionary wedges, high-pressure(HP)-ultra-high-pressure(UHP) metamorphic rocks, Barrovian-type metamorphic rocks and basement nappes, together with superimposed forearc basins.Such geological records can define the extreme limits and the intervening surface separating active margin from the passive one,thus offering a general sketch for the surface trace of the terminal suture zone often with a cryptic feature. Furthermore, the occurrence of the cryptic suture zone in depth may be constrained by geophysical data, which, in combination with outcrop studies of HP-UHP metamorphic rocks, enables us to outline the terminal suture zone. The southern part of the Himalayan orogen records complicated temporal and spatial features, which are hard to be fully explained by the classic "two-plate-one-ocean" template,therefore re-anatomy of the compositions and nature for this region is necessitated. Taking advantage of the methodology and basic principles of plate convergence anatomy and synthesizing previous studies together with our recent research, we may gain new insights into the evolution of the Himalayan orogeny.(1) The Yarlung-Zangbo ophiolite is composed of multiple tectonic units rather than a single terminal suture zone, and a group of different tectonic units were juxtaposed against each other in the backstop of the Gangdese forearc.(2) The Tethyan Himalayan Sequence(THS) contains mélanges with typical block-in-matrix structures, uniform southwards paleocurrents and age spectra of detrital zircons typical of Eurasia continent. All of these facts indicate that the THS belonged to Eurasia plate before the terminal collision, emplaced in the forearc of the Gangdese arc.(3) The Greater Himalayan Crystalline Complex(GHC) and Lesser Himalayan Sequence(LHS) comprise complex components including eclogites emplaced into the GHC and the upper part of the LHS. Judging from the fact that HP-UHP metamorphic rocks are exhumed and emplaced in the upper plate, the GHC and the upper part of the LHS where eclogite occur should be assigned to the upper plate, lying above the terminal subduction zone surface. It is the very surface along which the continuous subduction of the India subcontinent occurred, therefore acting as the terminal, cryptic suture. From the suture further southward, the bulk rock associations of the LHS and Sub-Himalayan Sequence(Siwalik) show little affinity of mélange, probably belonging to the foreland system of the India plate. By the anatomy of tectonic features of all the tectonic units in the Himalayan orogen as well as the ages of the subduction-accretion related deformation, we conclude that the terminal India-Eurasia collision occurred after 14 Ma, the timing of the metamorphism of the eclogites emplaced into the upper plate. The development of rifts stretching in N-S direction in Tibet and tectonic events with the transition from sinistral to dextral movements in shear zones, such as the Ailaoshan fault in East Tibet, can coordinately reflect the scale and geodynamic influence of the India-Eurasia convergence zone.By conducting a detailed anatomy of the southern Himalayas, we propose a new model for the final collision-accretion of the Himalayan orogeny. Our study indicates that the anatomy of structures, composition, and tectonic nature is the key to a better understanding of orogenic belts, which may apply to all the orogenic belts around the world. We also point out that several important issues regarding the detailed anatomy of the structures, compositions and tectonic nature of the Himalayan orogeny in future.