大陆深部地壳脱水熔融与水致熔融的演化特征及其流变学意义

Evolution Characteristics and Rheological Significance of Dehydration Melting and Water-Fluxed Melting in Deep Continental Crust

深熔作用是链接地壳深部变质作用、构造变形以及岩浆活动的重要纽带,对大陆地壳演化及其流变学性质具有重要意义.根据是否有自由水的参与,深熔作用分为水致熔融和脱水熔融两种机制.脱水熔融主要通过云母、角闪石等含水矿物的分解来实现,其发生所需的温度一般大于650℃.在脱水熔融反应中会生成钾长石、石榴石等转熔矿物和部分熔体,该熔体呈水不饱和态,具有高Rb、高Rb/Sr比、高^(87)Sr/^(86)Sr,低Sr、Ba和Ca的特点.其产生的熔体含量主要受温压条件和全岩水含量的影响,在麻粒岩相条件下才有可能产生大量熔体.水致熔融是在含水流体参与下所发生的熔融反应,其最显著的特点就是所需温度较低,在角闪岩相条件下可以生成大量熔体.生成的熔体可以是水饱和或水不饱和,在高温条件下生成的水不饱和熔体具有向上迁移的能力.水致熔融生成熔体的地球化学特征与脱水熔融相反,Sr、Ca、Ba含量较高,Rb、Rb/Sr比较低.深熔作用不仅可以显著改变岩石的热力学和流变学性质,而且熔体的迁移可以促进地壳分异并形成广泛的淡色花岗岩,在陆壳的起源、改造和稳定中起着至关重要的作用.

Anatexis is significant for the evolution and rheological properties of the continental crust,linking metamorphism,tectonic deformation,and magmatic activities in deep crust.According to the presence or absence of free water,anatexis can be divided into water-fluxed melting and dehydration melting.Dehydration melting involves the breakdown of hydrous phases,such as muscovite,biotite or amphibole.Generally,dehydration melting reactions begin at~650℃and produce melt,peritectic garnet and K-feldspar.Melt resulting from dehydration melting is water unsaturated,characterized by high Rb,high Rb/Sr ratio,high ^(87)Sr/^(86)Sr ratio,and low Sr,Ba,Ca.Melt productivity of dehydration melting reaction is controlled by P-T condition and water content in rock,and it is only possible to produce a large amount of melt at granulite facies.Water-fluxed melting reactions involve the presence of free water.Its most significant feature is that the temperature required is relatively low,and can produce voluminous melting at amphibolite facies.The resulting melt is water-saturated or water unsaturated that can extract from the source.Melt derived from water-fluxed melting reactions has high Sr,Ba,Ca,low Rb/Sr ratio and low Rb.Anatexis has a profound impact on the thermodynamic and rheological properties of the rocks.At the same time,the movement of melt out of the lower continental crust promotes the chemical differentiation of the crust and forms a wide range of leucogranite,playing a crucial role in the origin,reworking,and stability of the continental crust.