中亚造山带西段俯冲起始时限及机制探讨

Discussion on Time and Mechanism of Subduction Initiation in the Western Central Asian Orogenic Belt

中亚造山带是世界上最大的显生宙增生型造山带,是研究增生造山过程和大陆地壳生长的绝佳场所,其形成记录了新元古代—早中生代古亚洲洋发展演化历程,主要由一系列微陆块、岛弧、海山/大洋高原、增生杂岩及蛇绿混杂岩构成。中亚造山带西段蛇绿混杂岩物质组成基本一致,除了典型的蛇绿岩组分外,大多发育典型的海山岩石组合,即枕状玄武岩、火山角砾岩、礁灰岩、滑塌堆积岩、陆源碎屑岩。蛇绿混杂岩中基性岩在地球化学上可明显分为两类:一类为洋中脊玄武岩型,属于拉斑系列,显示俯冲带特征;另一类为洋岛玄武岩型,属于碱性玄武岩系列,形成于海山/大洋高原环境。中亚造山带西段蛇绿混杂岩和变质岩的时代、属性及空间分布,表明古亚洲洋俯冲起始时限不晚于新元古代早期,位置应在西伯利亚南缘,并逐渐向南发展。在古亚洲洋发育早期,地幔柱诱发俯冲起始占主导地位,后期可能由于海山/大洋高原阻塞俯冲通道,导致俯冲极性翻转和跃迁,从而发生俯冲起始。实际上,新俯冲带如何起始一直是地球科学领域的难点问题,主要原因是俯冲起始是一个极其短暂的过程,缺少直接的地质记录,并且现今地球上鲜有正在发生的俯冲起始案例。因此,需要地质学家共同努力去揭秘古亚洲洋的前世今生,破解板块俯冲起始的奥秘。

As one of the largest Phanerozoic orogens in the world,the Central Asian Orogenic Belt(CAOB)is an excellent place for studying accretionary orogenic process and continental crust growth.It is widely accepted that the CAOB formed by long-lived evolution of the Paleo-Asian Ocean(PAO)from Neoproterozoic to Early Mesozoic,comprises various allochthonous fragments,such as microcontinents,island arcs,seamounts/oceanic plateaus,accretionary prisms,and ophiolitic mélanges.The composition of ophiolitic mélange is almost same in the western CAOB.Except the typical ophiolite components,the ophiolitic mélanges contain pillow basalt,volcanic breccia,reef limestone,olistostrome,and terrestrial clastic rocks.Geochemically,the mafic rocks in ophiolitic mélange can be divided into two groups of MORB-and OIB-type.The MORB-type belongs to tholeiitic basalt series and shows subduction characteristics;the OIB-type belongs to alkaline basalt series,and is formed in seamount/oceanic plateau affinity.The age,property and distribution of ophiolitic mélanges and metamorphic rocks in the western CAOB indicate that the subduction initiation time of the PAO is no later than Early Neoproterozoic,and the location should be on the southern margin of Siberia plate and gradually develop southward.In the early development of the PAO,the mechanism of subduction initiation should be mantle plume-induced dominant.However,in the later period,the seamount/oceanic plateau may block the subduction channel,causing the subduction polarity reversal and transition,and the subduction initiation occurs.Initiation of new subduction zone is a difficult problem of Earth's plate tectonics regime.The reason is that the subduction initiation is an extremely short process,scarcity of geological record,and there are few cases of subduction initiation that are happening on Earth today.Therefore,it is necessary for geologists to work together to reveal the past and present lives of the PAO,and to solve the mystery of the subduction initiation of plate tectonics.