近代花岗岩研究的回顾

近代花岗岩实验和地球化学的研究使花岗岩研究走向一个最终成因和在地构造环境变迁结合的动力学方向。

A GENERAL REVIEW ON THE RECENT STUDIES OF GRANITE

Inspired by the experimental studies and overall geochemical investigations in granites in association with the detailed works in some granite batholiths, the recent studies of granite have been advancing toward the elucidation of the ultimate reasoning of the origin of the granite in connection with the geological context through dynamic aspect. The experimental and thermal constraints on some granitic magmas of crustal sources and on the deep-seated parentage related to subduction revealed that: H2O-saturated liquid of granitic composition is a normal consequence of regional metamorphism; a wide range of granitoid magmas (including tonalite) can be generated by combinations of granitic liquid with suspended crystals of the refractory residual group; batholiths largely of granite and granodiorite could be produced by crustal anatexis, whereas those dominated by tonalites the heat and material transported from mantle in addition to crustal materials;and the granitoids are not primary magmas from mantle or subducted oceanic crust. The source and amount of H2O available have a direct effect on the initial melting temperature, the subsequent chemical composition of the magma and its crystallization history. The replacement textures.which play an important role in the formation of granitic melt, are directly related to the H2O available in the melt. They can be distinguished as the migmatic and magmatic replacement textural series corresponding to the initial stage of the granitic melt through ultrametamorphism and the sub-solidus crystallization in the late magmatic stage, respectively. Owing to the advancement of the geochemical works in studying granites, there has been controversy between the restite and mixing hypotheses-a recurrent theme as anatexis held by migmatists versus syntexis devoted by magmatists. The restite hypothesis rests on the geochemical and mineralogical characteristics of granitoids, which appear as the products of ultrametamorphism in melts residuum. The nature of the source material can be deduced in first approximation as I- and S-type granites from igneous and sedimentary sources, respectively. Based on type localities such as synplutonic dykes granite batholith and some mafic microgranular enclaves within granites, the mixing hypothesis argued that the mixing processes induced by the injection of coeval basaltic magma into the pre-existing granitic magma, which has been melted with the aid of the rising heat of basaltic magma itself, are important in categorizing the granite suites. It seems, however, from field occurrence, that the mixing may represent but secondary processes, despite their importance in some areas. Granite types, imaging their sources, also have various processes during their formation. Their ensemble is directly related to the overall geological Context, a manifestation of the thermal perturbation initiating the granitic melt. Under this heading, Pitcher’s classification named as granite typology is cited and discussed.