洋底凸起地质体及其对造山带中蛇绿岩组分的贡献

Buoyant units on oceanic crust and their contributions to components of ophiolitic mélanges in orogenic belts

蛇绿岩作为造山带中保存的曾经存在过的大洋或与大洋有关地质体的残留,除了部分的露头出露有完整的岩石组分之外,研究发现许多露头只出露部分的岩石组分,如只出露第1层的深海/远洋沉积物和第2层的玄武岩或绿岩,且其基性岩以MORB和OIB的地球化学特征为主,而不是SSZ型岩浆的地球化学特征。审视以前的蛇绿岩就位模型,学者们认为俯冲洋壳的洋底是平坦的;该假设与现状大洋洋底地形探测资料不吻合。大洋洋底测绘资料显示洋底可以在洋中脊处和海山/洋底高原的位置发育凸起地貌。本文根据岩石破裂的力学性质,分析了它们的破裂过程;结合中国新疆西准噶尔玛依勒地区、日本造山带和美国加州海岸山脉增生楔和蛇绿岩中岩石组合和构造属性,以及它们的就位时代和形成时代年龄差可达几十个百万年到上百个百万年,认为洋底表面的凸起地质体上段部位的岩石容易被滑脱断层错断并卷入到造山带中,而且它们比构造叠置的围岩-海沟沉积物更老。

Ophiolitic mélange presents the relict of oceanic crust or other ocean-related tectonic settings. Recent studies show that many exposures onland only crop out part of ophiolitic suite,such as pelagic sediment（ Layer 1） and basalt or greenstone（ Layer 2）,which dominantly bears MORB and OIB geochemical characteristics,not SSZ features. Bathymetric survey shows that there widely develops convex topography of buoyant units in the middle oceanic ridge and seamount/oceanic plateaus on the oceanic crust,which is contrast with previous models suggesting that the surface of oceanic crust is flat and smooth. Based on mechanical behavior of rocks,we analysis the fracture processes of buoyant units with convex topography underthrusting into subduction channel,and suggest that their upper parts are easier to be detached and juxtaposed into orogenic belts,which can be proved by outcropping of components of basalt and chert from Mayile accretionary complex,Western Junggar（ NW China） and accretionary complexes in Japan and California Coast Ranges,U. S. A. The formation and emplacement ages of accreted basalt and chert（ ophiolites） can be different as tens or more than one hundred million years. These relict oceanic materials generally older than their host trench sediments.