花岗岩体的累积生长与高结晶度岩浆的分异

花岗岩成因研究是认识大陆地壳形成和分异的有效方式。野外地质和地球物理观测、岩石学和地质年代学研究以及热演化模拟证明,很多花岗岩体是在数百万年甚至更长的时间跨度内、由多次岩浆累积添加侵位而成的。地壳内可能不存在岩基尺度的大岩浆房,具有流动能力的岩浆体一般规模很小(宽度1000m)。1000m宽的岩浆体冷凝到固相线只需要数千年时间。复式岩体的形成一般要经历三个阶段,即源区岩浆沿岩墙的上升、在脆-韧性地层界面处岩墙转化为岩床以及无数的岩床的垂向堆垛导致侵入体长大。存在于上地壳的岩浆储库,特别是多次先后侵位产生的岩浆体,主体上是晶粥体,其晶体含量高,粘度大,活动性弱,不利于发生对流、分异和混合。当幔源镁铁质岩浆大规模注入到地壳时,使粘稠的晶粥状岩浆受到加热,熔体含量增大,岩浆的粘度降低,引起岩浆体内部的成分分异和不同成分的岩浆之间的混合;当逐渐加厚的熔体层产生了足够大的浮力后,特别是有挥发份加入后,就会快速上升,甚至穿透上部的晶粥体,触发大规模的火山喷发。幔源岩浆的通量越大,地壳岩浆的活动性也越强,大规模的长英质岩浆聚集就可能发生大喷发,形成超级火山。本文提出,只有将侵入岩与火山岩相结合、长英质岩石与镁铁质岩石相结合,重点从侵入体形成的时间长短、岩浆相互作用的规模和频率、岩浆通量的演变、高结晶度的岩浆分异机理、侵入岩与火山岩的关系、地幔热和物质的贡献、挥发份在岩浆分异和火山喷发中的作用等方面入手,开展野外地质、岩石学、地球化学、同位素年代学及岩浆动力学的综合研究,才能深入认识花岗岩的成因机制,深化对大陆地壳形成和演化过程的理解。

Incremental growth of granitoid plutons and highly crystalline magmatic differentiation

Understanding the genesis of granites is fundamental to understanding the formation and differentiation of continental crust. Geological, geophysical, geochronological and field studies, combined with modeling of thermal evolution of plutons, indicate that many granitic bodies emplaced in the upper crust result from the amalgamation of several, discrete magma pulses over several million years or even a longer timescale. Hypothesized batholith-scale magma chambers may not exist in the crust, and magma bodies with the capacity for flow of melts are generally small (<1000m). A magma body 1000m wide would cool down to solidus on a timescale of thousands of years. The formation of composite intrusions generally has three stages: source magma upwelling along dikes, transformation from dykes to sill-like intrusions at the brittle-ductile transition of the crust, and growth of the magma body by the vertical stacking of numerous sill-like magma bodies. Magma chambers in the crust, especially those successively-intruded magma bodies, are mainly composed of crystal mush. The crystal mush is adverse to convection, differentiation or mixing owning to the high crystal content, high viscosity and weak activity. However, the viscous mushy magma can be heated, becoming more highly melted and less viscous when the mantle-derived mafic magmas intrude into the crust. This leads to differentiation inside one magma body and mixing between magmas with distinct compositions. Finally, when the buoyancy of the bottom highly molten magma is high enough, or with an injection of volatiles, it will rise rapidly, penetrate the upper mushy magma and trigger large-scale volcanic eruptions. The activity of crustal magmatism is enhancing when there is an increase in the flux of mantle-derived magma. Thus, large-scale felsic magma may form a super volcano. It is proposed in this paper that understanding relationships between plutonism-volcanism and felsic-mafic rocks is fundamental for a better understanding of the genesis of granites. Moreover, we must pay close attention to multiple factors, such as time- and spatial-scale of the intrusions, evolution of the magma fluxes, differentiation mechanism of highly crystalline rocks, contribution of mantle heat and materials, and the role of the volatiles during magma differentiation and volcanic eruption. These factors should be combined with a comprehensive study of field observation, petrology, geochemistry, isotopic chronology and magma dynamics to achieve a more complete understanding of the formation and evolution of continental crust.