花岗岩:大陆地质研究的突破口以及若干关键科学问题——“岩石学报”花岗岩专辑代序

Granites:Leading study issue for continental evolution

地球是宇宙中迄今唯一发现有花岗岩的星球。从物质组成而言,大洋的核心问题是玄武岩,大陆的核心问题是花岗岩。花岗岩从被发现的那一天起,围绕它的产状、成因和演化过程及其地质意义的争论就没有停止过。最早的陆壳由长英质片麻岩TTG组成,TTG不能从地幔中直接熔出,是地幔熔融的玄武岩再次熔融形成的。初始的陆壳富钠,太古宙以后的陆壳开始明显富钾。洋壳的寿命仅2~3亿年,陆壳的寿命可长达40~44亿年。花岗岩记录了大陆形成与演化的全过程,大陆演化对于认知地球具有无法替代的作用。因此,花岗岩在某种程度上可以作为陆壳的代用词。20世纪60年代兴起的板块构造理论是地球科学的一次革命,但是,板块构造理论并未解决大陆形成、演化和改造的根本问题。此外,花岗岩与大陆成矿作用关系也非常密切。花岗岩成因还存在诸多待解难题。对花岗岩的研究,只有跳出岩石学和岩石地球化学的范畴,才能真正理解大陆构造的问题。花岗岩所凝聚的科学问题是对板块构造理论的巨大挑战,也是固体地球科学理论创新的重大机遇。花岗岩研究从岩石地球化学层面跃升到陆壳演化层面,是研究思维和理论创新的突破,是21世纪花岗岩研究的重大转折。

The unique of the lithosphere in the earth is that it contains large amounts of granite, which does not occur elsewhere in the solar system. The continental crust of the earth is mainly composed of granite, whereas the oceanic crust is dominated by basalt. Unlike the short-lived oceanic crust which may have only existed in 200 to 300Myr on the earth, the continental crust can survive in 4000 to 4500Myr. The granite in the earth thus may record the history of the formation and evolution of the continental lithosphere of the Earth. The earliest continental crust is composed of Tonalite-Trondhjemite-Granodiorite （TTG） suite, however. TrG is neither a product of fraetionation of magma ocean, nor of partial melting of the mantle. The origin of TTG is thus crucial for understanding the origin of continental crust. Extensive continental growth and cratonization occurred in Phanerozoic, which was associated with massive granite production and mineralization. All these issues are difficult to be explained by the current theory of plate tectonics. The answers for the key issues on the granite may trigger the new theory of solid earth sciences. Reworking of continental crust is related to intensive tectonothermal events in the earth as proposed by Prof. Guoda Chen and his ＂platform activation＂ model. Mesozoic large-scaled intracontinental deformation and extensive magmatism in East China may provide an ideal natural laboratory to study the dynamic mechanism of the continental reactivation and to understand plate tectonics and cratonization. Presence of voluminous granite may change the internal structure of the earth and the stability of continental lithosphere, leading to the craton re-activation or destruction. However, it remains unclear how these processes worked in the history of Phanerozoie continental crust. New theory rather than classic plate tectonics is desired to establish to explain the granite issues. The change from routine granite geochemistry to continental growth will make a real breakthrough in the theory of solid earth sciences in the 21at century. The 540th Xiangshan Conference on the key issues of granite research is a milestone to prompt the research of granite and continental evolution, rheology and thermodynamics in the next five years in China.