从地壳上地幔构造看大陆碰撞带岩石圈的克拉通化

Cratonization Processes of Continental Collision Belts: Reveled by Worldwide Comparison of Crust and Upper Mantle Structures

本篇讨论超级大陆汇聚后逐渐变为克拉通或扩大克拉通的作用过程,即指经及大陆碰撞地体汇聚后新形成的大陆块逐渐转变为刚性克拉通的作用过程。增生大陆岩石圈的克拉通化的作用后果,包括大陆地壳密度的增加,岩石圈地幔的增厚和大地热流值的下降,使大陆岩石圈逐渐刚性强化。大陆碰撞后形成的大陆块必须经过克拉通化的过程,才能逐渐成为刚性克拉通。作用过程主要包括:①上地壳沉积碎屑岩石结晶岩化和中地壳岩石角闪岩化;②下地壳岩石基性化;③大陆碰撞带下凹莫霍面的磨平;④岩石圈地幔底侵加厚形成陆根。从大陆碰撞带转变为克拉通的过程也是岩石圈地幔不断增厚而地壳缓慢变硬变冷的过程。这个过程包含以下作用:区域变质作用,交代作用和岩石圈幔源岩浆的底侵。这个过程时间尺度比碰撞造山作用大一个级次。长期的底侵作用使地壳岩石密度和强度不断加大,改变岩层的矿物成分和局部结构。当大陆岩石圈克拉通化到一定程度之后,由于下方软流圈的热能供应逐渐减缓,使岩石圈地温梯度缓慢下降,最终结果会形成大陆根。由于显生宙大陆碰撞带岩石圈强度弱,大陆碰撞时更容易造成岩石圈变形,因此大陆碰撞的板内效应主要发生在大陆内的显生宙碰撞带。显生宙大陆碰撞带如果再次受到大陆碰撞板内效应的作用,其克拉通化的过程必然会推迟。

This paper continues to review continental accretion processes,focusing on cratonization of the lithosphere of Phanerozoic continental collision zones.Based on geophysical investigations,cratonization of the lithosphere results in increasing crustal density,thickening the uppermost mantle and strengthening the lithosphere.The prosesses include following types：（1） metamorphism of upper crust sedimemnts and amphibolitization of middle crust rocks; （2） granulitization of lower crust rocks; （3） wearing out lowest crustal roots of the collisional zones; and （4） cooling the upper mantle and increasing the thickness of the lithsperic mantle.The cratonization involves geological processes such as metamorphism,metasomatism,magma underplating,during a long time period about billion years.After the lithosphere approaches a typical craton,cooling of the lithosphere would follow as cratonization consumes a lot of heat energy that reduces temperature beneath the lithosphere,resulting in creation of a continental root.Because Phanerozoic collision zones are week under compressional forces,they are easier to be deformed during later-on continental collision,which causes the so-called intra-plate effects.The intra-plate effects in turn might delay the original cratonization processes.