板块俯冲起始与大陆地壳演化

The onset of plate subduction and the evolution of continental crust

组成大陆地壳的物质主要来自两个地质过程:地幔柱活动和板块俯冲。目前大多数研究认为板块俯冲起始于30多亿年前。在板块俯冲起始之前,基性的初始地壳物质受热重熔是大陆地壳生长的主要方式,其中,地幔柱活动是关键。地幔柱不仅向地壳输送玄武质岩浆,同时导致已有玄武质岩石和沉积岩通过部分熔融向中酸性岩石转化。当原始岩石圈强度足够大时,地幔柱会导致岩石圈倾斜、破裂,产生下滑力,诱发板块俯冲。板块俯冲引发岩浆活动,产生大量的岩浆岩,如岛弧安山岩、弧后盆玄武岩等。这些岩浆岩通过喷发、侵位,再经由块体拼贴、增生等过程加入到大陆地壳,是大陆地壳生长的主要途径。同时,板内岩浆活动乃至地幔柱活动等也与板块俯冲有直接或者间接的联系。俯冲再循环物质促进地幔柱发育,也为大陆地壳的生长提供物源和热能。与此同时,大陆地壳不断风化剥蚀,其中一部分沉积物随俯冲板块再循环到地幔,而板块俯冲过程也通过俯冲剥蚀等过程,将仰冲盘岩石圈物质刮削带入地幔。这些是大陆地壳消减的主要途径。目前大陆地壳增生和消减基本处于动态平衡。

The continental crust is mainly formed through two geological processes:mantle plume activity and plate subduction.It is generally accepted that plate subduction has operated since~3 billion years.Before the onset of plate subduction,the melting of proto-crustal material was the main way for continental crust growth,which was controlled by the mantle plume activity.The mantle plume not only transported basaltic magma to the continental crust but also altered the existing basaltic rocks and sedimentary rocks,transforming them into intermediate-acidic rocks through partial melting.When the lithosphere was strong enough,it could be tilted and broken by mantle plumes,which then caused the slab sliding and powered plate subduction.Plate subduction could result in the eruption of magmatism and produce a large quantity of igneous rocks(e.g.,arc andesite and back-arc basin basalts).These igneous rocks were added to the continental crust through eruption,emplacement,accretion and collisions,which are the main ways of continental crust growth.In addition,intraplate magmatism,mantle plumes in particular,are also related to plate subduction.Recycling of subducted slabs promotes the development of mantle plume and also provides materials and heat energy for the continental crust growth.Meanwhile,the continental crust is reduced by weathering and erosion and recycled into the mantle via subduction.In this case,the growth and reduction of continental crust are basically in dynamic equilibrium in the modern earth.