大陆下地壳拆沉模式初探
A model of delamination of continental lower crust
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摘要: 下地壳拆沉是人们关注的问题,文中指出下地壳拆沉必须满足至少三个条件：（1）地壳加厚使其下部达到熘辉岩相是拆沉的前提.（2）大规模岩浆活动使大量低密度的中酸性物质移出下地壳,使下地壳密度增加直至超过下伏地幔.由于下地壳榴辉岩石部分熔融所形成的岩浆具有埃达克岩的地球化学特征,因此,大规模魂达克岩的熔出是下地壳拆沉的先决和必要条件.（3）岩石圈地幔转化为软流圈地幔,使下地壳能够进入地幔.陆壳下的岩石圈地幔原先是冷的、刚性的和不易流动的，如果有热和水的加入，可以被软化，使其变成热的、塑性的和易流动的软流圈地幔。因此，岩石圈了幔转化为软流圈地幔是下地壳拆沉的必要条件。作者认为，下地壳不大可能整体拆沉，而很可能是一块一块如飘雪花似地拆沉。如果下地壳的密度降低（低于下伏地幔），如果地幔停止热的供给，如果陆壳底部的软流圈地幔幔又恢复为岩石圈地幔，拆沉即终止。文中讨论了中国东部中生代下地壳拆沉的可能性，探讨了岩石圈减薄的机制，认为下地壳不需要也不可能与岩石圈地幔一道拆况。Abstract: It is the subject that people pay attention to continental lower crust delamination.The paper puts out that there are at least three conditions for lower crust delamination:(1)the thickened crust is the prerequisite that the delamination make the lower crust eclogite.(2)the extensive magma shifts out a large amount of low density acid magma,enables the crust density to be greater than the mantle density,Because the lower crust eclogitation and some magma melting are of the geochemistry characteristics of adakite,the lower crust delamination is the prerequisite and essential condition with the extensive adakite melted out.(3)that the lithosphere mantle transfers into the asthenoshpere mantle enables the lower crust into mantle.The mantle under continent crust is cold and rigidity originally,difficult to flow,and will be soften with heat and water joining,make into the asthenosphere heat,plasticity and easy flow.So that the transition between the lithosphere mantle and the asthenosphere mantle is the essential factor and precondition of the lower crust delamination.The author thinks that the crust will not delaminate as a wholly,otherwise,it will delaminate as the snow flake float probably.If the density of the lower crust is reduced (lower than the underlying mantle),and there is not heat supply in mantle,and the asthenosphere under continent crust turn into the lithosphere mantle again,delamination will stop promptly.In the article we discussed the possibility of lower crust delamination in the eastern China during Mesozoic Era,and the thinning mechanism of the lithosphere.We think that the lower crust is not necessary,is not possible to delaminate together with the lithosphere mantle as well.
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Key words:
- Contincntsl lower orust /
- Delamination /
- Adakite /
- Lithospheric mantlc /
- Ashoapherie mantle
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