花岗岩浆形成定位机制的思考与研究进展

花岗岩(广义)是陆壳的标志,也是地球岩石圈区别于其它行星岩石圈的标志。文章介绍了行星探测和大洋调查等方面的成果对花岗岩形成的地质约束:行星从岩浆表壳向岩石表壳转换过程以及现代地幔过程,均没有产生有规模意义的花岗岩;花岗岩及其所标志的陆壳,应是星球出现水圈和沉积岩之后的产物;花岗岩在地球岩石圈二维空间上的平均生长速率,大约为485×10~3km~2/Myr;岩浆主要来自地壳岩石的部分熔融(深熔)。在此基础上,文章介绍了深熔作用方面的研究进展,讨论了部分熔融岩石的流变行为与其内熔体比的关系,并比较了岩浆侵入模型与岩浆对流模型在解释花岗岩形成定位机制方面的异同。侵入模型的困难之一来自岩体与源区分离。由于源区位于岩体下方且远离岩体,因而是不可观察的,除非岩体及其与源区之间的岩石因风化或构造被剥蚀殆尽。文章最后介绍了"深熔-对流"模型的研究进展。该模型认为"源区"与"定位区间"是统一的,当"源区"岩石的熔体比例超过流变学的临界熔体比,岩石转变为"脏"岩浆;"脏"岩浆层内的重力分异诱发热对流,后者引起"顶蚀作用",导致重熔界面(MI)或固-液转换界面(SLT)不断向上移动和岩浆层的逐渐增厚。基本认识是:熔区内的热对流是深熔作用能够形成大规模花岗岩浆的必要条件;没有对流,陆壳岩石的部分熔融只能产生混合岩,不能产生岩基规模的花岗岩。

Advance and consideration on the mechanism of formation and emplacement of granitic magma

Granite (sensu lato) that belongs to the continental crust is the rock of distinguishing the lithosphere of the earth from that of other planets. Geological evidence from both planetary exploration and ODP constrain the formation of granite: no granite was generated during transition from magmatic surface to rocky shell of the inner planets, and formation of granite and its related continental crust should initiate after the occurrence of sedimentary rocks on the earth''s surface. The average growth rate of granite in 2-D space of the lithosphere is about 485×10³km²/Myr and magma is considered mainly from the partial melting of crustal rocks (anatexis). On the basis, we introduce the progression in the study of crustal anatexis and the relationship between rheological behaviors and melt fraction of the partially melted rocks, and compare the similarities and differences of the both models, i.e. magma intrusion and the magma convection, on explaining the formation-emplacement mechanism of granitic magma. The magma source and its related granite body are separated in the magma intrusion model. One of the difficulties for the model is that the magma source is located beneath its related granite body and thus used to be unobservable unless the granite body and the rocks between the granite and the source have been moved out by erosion or structure. Finally, we brief the study advance of intra-crustal magma convection. In the convection model, the source and the emplacement place of magma are regarded as a whole. When the melt fraction of the rocks in the source region reaches the solid-liquid transition (SLT), the rocks change into "dirty" magma. Gravitational differentiation within the "dirty" magma layer initiates heat convection that results in moving up of the MI (SLT) and thickening of the crustal magma layer. It is concluded that thermal convection within a crustal melting layer is essential for formation of granite magma; without convection, partial melting generates migmatite, but not magma that forms granite batholiths.