The never-ending pursuit of a definitive chemical classification system for granites

[摘要] Chemical classifications of granites sensu lato have been developed and revisited over decades, but no classificationscheme has been universally accepted yet. The more or less known coupled reasons for this apparently impossible taskare reviewed here. The main problem is that different granitoids do not fall in distinct categories with sharp boundaries,but comprise a continuous spectrum of rock types both in their chemical and modal compositions. The unifying factoris the minimum-melt nature of the granites sensu stricto, as primary and evolved melts can have a granitic composition.This minimum-melt nature has two consequences, which are the main reasons for the absence of sharp boundaries inevery compositional classification system, either modal or chemical. Firstly, the chemistry of granites spreads from theminimum melt to non-minimum compositions, and thus some granites represent a rock series formed by a continuousmagmatic evolution, not by discrete steps; secondly, granite series, which are generated from different sources and byseveral petrogenetic processes, eventually converge at the most silica-rich compositions. There is a relationship betweenthe tectonic scenarios of formation of granites and the chemical overlap that contributes to the absence of a satisfactorychemical classification: the protracted evolution of the tectonic settings following the Wilson cycle and more complicated scenarios change the chemical and modal composition of the granite sources. The overlap in the most silica-richcompositions of the granites s.l. due to the minimum melt nature may extend to more mafic members in a granite series:the closer the sources are in their composition, the greater is the overlap, becoming a second contribution to the lack ofsharp boundaries between granite types.The huge efforts to create a satisfactory chemical compositional classification system have actually led to a significantcontribution to granite petrology: the discovery of the main chemical differences between granite types, the main chemicalparameters (silica content, alkalinity, aluminosity, maficity or FeOt+MgO content, and the Fe/Mg and Na/K ratios) andthe petrogenetic processes that cause the change in these parameters. Therefore, despite the lack of agreement over the‘perfect’ classification system, the investigations have not been fruitless: they have led to the realization that non-geneticclassifications are preferable to name the individual rock samples; chemical classification schemes should be left todistinguish magmatic suites and to unravel their prospective petrogenesis and geotectonic setting.

[学科分类] 自动化工程

[关键词] granites;chemical classifications;tectonic settings;magmatic evolution;petrogenesis