大陆地壳深熔作用中自由水的贡献——以高喜马拉雅淡色花岗岩为例

Contribution of free water in the anatexis of continental crust:Constraints from the High Himalayan leucogranites

高喜马拉雅淡色花岗岩是印度与亚洲板块碰撞过程中,地壳深熔作用的产物。尽管自由水对高喜马拉雅淡色花岗岩形成的重要性已被多次报导,但仍存在许多争议。本研究汇编了高喜马拉雅淡色花岗岩的全岩地球化学数据,证明高喜马拉雅淡色花岗岩可以由脱水熔融形成和水致熔融形成。进一步通过热力学模拟获得锆石结晶,并与锆石Ti温度计比较,限定了不同熔融反应中熔体的水含量。模拟结果显示锆石的结晶温度主要受控于初始熔体的水含量,可以依据初始熔体水含量的高低将高喜马拉雅淡色花岗岩分为两组:有自由水参与的初始熔体具有高含水量,约为5%;贫水条件下的初始熔体水含量不超过1%。因此本研究认为自由水在深熔作用中发挥了不可忽视的作用。自由水的存在促进了大陆地壳的壳内分异作用,增加了大陆地壳成熟度,并由此影响了大陆地壳演化。

The High Himalayan leucogranites are formed by the anatexis of continental crust during the collision of India with Asia. Although the importance of free water in the formation of High Himalayan leucogranites has often been cited, the debate is still ongoing. This study compiled available whole-rock geochemical data for the High Himalayan leucogranites, which demonstrated that both dehydration and water-fluxed melting reactions should have played a key role in formation of the High Himalayan leucogranites. This study further estimated water contents of melt formed by different melting reactions through combining the thermodynamic simulation of zircon crystallization temperature and Ti-in-zircon thermometry. The simulation results show that the crystallization of zircon is mainly controlled by the water content of the primary melt. Furthermore, the High Himalayan leucogranite can be divided into two groups: with the presence of free water, water contents of primary melts are high and commonly close to 5%, whereas the primary melt water contents of the other group do not exceed 1%. This study therefore concludes that free water is crucial to the anatexis. Free water could promote the intra-crustal differentiation and increase the maturity of the continental crust, and thus affect the evolution of the continental crust.