介绍一个产生玄武岩的模型

地幔柱存在的一个主要证据是大规模高熔玄武岩省的出现,而且多认为玄武岩的来源依赖于地幔柱从下地幔输送。Michele Lustrino研究了造山时下地壳和岩石圈地幔的拆沉和拆离作用,提出了产生玄武岩的一个新模型。该模型认为即使地幔柱不存在,拆沉到地幔的下地壳物质再循环同样可以解释小规模的板内(大洋岛弧和大陆内部)火山岩和大洋、大陆溢流玄武岩及洋中脊玄武岩的生成及其常见的几种地球化学特征。在陆-陆碰撞过程中,下地壳中的变质反应生成石榴石,导致岩石的密度增大,致使过厚岩石圈底部(下地壳和岩石圈地幔)和上地壳分离并沉入上地幔。下地壳发生部分熔融形成富SiO2的熔体,和上涌的软流圈地幔(充填在下沉的岩石圈地幔和下地壳的空间)发生变质交代反应,导致具有强烈的地壳特点的富含斜方辉石层的形成。这个变质交代地幔体可以在拆沉后保持不变长达几个百万年。这种源的部分熔体可以保有下地壳的明显特征,产生类似富集地幔1型玄武岩浆作用。因此,该模型是提供了玄武岩浆来源的一个新选择。

Introducing a Model of the Basalts formation

Based on the study papers about the plume,the author mainly introduce Michele Lustrino's model about the basalts formation.This model proposes that the lower crust and lithospheric mantle can be pushed to the upper mantle through the delamination and detachment during continental-continental collision.The metasomatic reactions between SiO_2-rich lower crust partial melts and the uprising asthenospheric mantle(replacing the volume formerly occupied by the sunken lithospheric mantle and the lower crust) lead to formation of orthopyroxene-rich layers with strong crustal signatures.Such metasomatized mantle volumes may remain untapped for several Ma before being reactivated by geological processes.Partial melts of such sources would bear strong lower crustal signatures giving rise to Enriched Mantle type 1(EMI)-like basaltic magmatism.High-grade metamorphic reactions(granulite to eclogite facies) force crustal material to partially melt creating a low Rb/Sr and U/Pb restite,showing nearly unchanged high Sm/Nd.Time-integrated decay within these isotopic systems(showing low U/Pb,low Sm/Nd and relatively low Rb/Sr) therefore produce low ~(206)Pb/~(204)Pb,low ~(143)Nd/~(144)Nd and mildly radiogenic only ~(87)Sr/~(86)Sr ratios.