花岗岩形成的热动力学过程数值模拟研究进展

摘要

形成过程的不可观察性是花岗岩成因长期争论的重要原因.数值模拟技术与超级计算机的结合,为花岗岩形成热动力学过程的数字重建提供了可能性.本文首先回顾了花岗岩形成过程数值模拟所需的物理化学参数的获取研究,其中一个重要进展是将上陆壳作为一个整体,重新厘定了岩石“平均强度”突降或流变学转换,即MCT、FMT、SLT对应的熔体比,为研究深熔岩浆的形成过程提供了重要的实验约束.在此基础上,介绍了基于岩浆侵入模型的物理和数值模拟研究进展.在岩浆侵入模型中,岩体与“源区”是分离的.各岩体与其相应“源区”之间地质条件的差别,使得现有的针对特定岩浆定位模式建立的数字模型,难以具有普适性.文章的最后部分展示了作者利用天河2号超级计算平台,在Chen and Grapes(2007)提出的“原地重熔”地质模型的基础上,对壳内大规模熔融和热对流的2-D数值模拟结果,初步重现了花岗岩和混合岩形成的热动力学过程.模拟结果揭示,热对流是壳内熔融能够形成大规模花岗岩浆的根本原因;岩浆“顶蚀”作用导致MI(SLT)界面向上移动和岩浆层增厚;壳内岩浆层发展的必要条件不是高的地壳温度,而是岩浆系统有持续的热供给,使系统能在较长时间内保持对流状态.%Unobservability of the rock-forming process of granite has resulted in a long-term controversy of granite genesis.Numerical techniques combined with super-computer create the possibility of digital reconstruction of the thermodynamic process of granite formation.Here we first look back the advance in study of physical-chemical parameter of rock-melting,introduce the concept of crustal ‘ average strength’ and re-determine the positions of the known rheological transitions,i.e.the MCT,FMT and SLT on the relationship of crustal average strength vs.melt fraction.Secondly,a review of advance in physical and numerical simulations of the various models of magma emplacement is given,and the un-universal characteristics of digital models constructed on different emplacement modes are discussed,which are mainly ascribed to the separation of source and room of magma in the intrusion model.Finally,we introduce the 2-D numerical simulation of large-scale crustal melting in the Tianhe-2 super-computer on the basis of in-situ melting model by Chen and Grapes (2007),preliminarily reconstructing the thermodynamic process of formation of granite and migmatite.The modeling result indicates that thermal convection within a crustal partial malting region is essential for formation of granite magma;Roof-stopping results in the upward motion of the MI (SLT) and thus thickening the convection magma layer;And prerequisite for development of a crustal magma layer is not a very high temperature,but a sustained energy input to maintain the convection state of the magma system.