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?engör （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?engör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。

The basic characteristics of accretion arcs and its geological implications

The term “accretion arc” refers to the island arc that develops on top of an accretionary complex basement, which was formed by ocean?ward migration of arc magmatic front triggered by the retreat of the trench. The accretion arc was suggested by ?engör （1992） as the major tectonic process responsible for characteristics of the accretionary type （Turkic-type） orogenic belt, however, the diagnostics and tectonic implication of accretion arcs remains controversial. The Japan arc, which represents Creteaous arc magmatism developing on the Jurassic and earlier accretionary complex basement, was used by ?engör （1992） to build the ideal accretion arc model. The accretion arc magmatism of the Japan arc carries out typical island?arc-type trace element characteristics, including enriched LREE, LILE, and depleted HFSE, but the depletion of the Sr-Nd isotope is deepened while the treach is retreating and the accretion arc is developing. The geophysics profile suggests two distinguish magmatic sources of the Japan accretion arc：above the MOHO of the fore-arc and below the MOHO of the magmatic axis of the arc. It implies that the Japan accretion arc can generate magma through the melting of the accretionary complex basement or partial melting of the newly generated mantle wedge, but the generation of the accretion arc mantle wedge is still unclear. The West Junngar area of the western CAOB outcropped Early Palaeozoic Chingiz arc and Late Palaeozoic Saur arc, both of which were considered potential accretion arc. The Late Carboniferous Saur arc represents I-type granite, granodiorite-diorite dike, intermediate-mafic lava, etc, which develop on a typical accretionary complex basement including ophiolitic mélange, OPS mélange, and coherent turbidite. This case suggests that the accretion arc magmatism can from large pluton, dike, and volcanic lava when developing on an accretionary complex basement, forming invading and unconformable contact. The Early Devonian Chingiz arc magmatism is represented by intermediate-mafic lava, covering the mélange basement through an unconformity. The age of the matrix of this mélange belt is Late Silurian, and Late Ordovician to Late Silurian limestone, chert blocks are hosted in the matrix. This case shows the typical dual structure of an accretion arc：the later hanging wall accretion arc magmatism and the earlier footwall accretionary complex basement dividing by a large unconformity or magmatic invading contact. The Saur accretion arc magma carries out a typical island-arc-type trace element feature. The granite endmembers represent depleted Nd isotope and enriched Sr isotope, while the basalt endmembers show depletion on both Nd and Sr isotope. It suggests that the mafic endmember is extracted directly from the newly generated accretionar arc mantle wedge through partial melting, while the granitic endmember is contaminated by the accretionary complex and inherits the enriched Sr isotope from the pelagic sediments and ophiolite of the accretionary complex. It provides a possible model for the generation of the accretion arc mantle wedge, that when the trench retreats, the subducted oceanic lithosphere is kept as the new accretion arc mantle wedge instead of break-off and sink into the mantle. The recognition and anatomy of accretion arcs play an important role in the understanding of the accretionary type orogenic belt because the development of accretion arcs might be the reason for large persentage of juvenal material and large volumn of crust growth of the accretionary type orogen.