UMA CLASSIFICAÇÃO QUÍMICO-MINERALÓGICA DAS ROCHAS PLUTÔNICAS COMUNS E SUAS ASSOCIAÇÕES, MÉTODO E APLICAÇÕES

FRANÇOIS DEBON, PATRICK LE FORT, PIERRE SABATÉ

Resumo


A classification has been proposed (Debon & Le Fort 1983), based mainly on major element analytical data plotted in a coherent set of three simple chemical-mineralogical diagrams: Q = Si/3 - (K + Na + 2Ca/3) vs. P = K - (Na + Ca) ("nomenclature" diagram, adapted from La Roche 1964, 1966; parameters as gram-atoms x 103 or millicátions in 100 g of rock); A = Al - (K+ Na+2Ca) vs. B = Fe + Mg + Ti ("characteristic minerals" diagram; parameters of Shand 1927, and La Roche 1964, again expressed as gram-atoms x 103 in 100 g of rock); quartz-dark minerals-feldspars+ muscovite ("Q-B-F" triangular diagram; parameters in weight percent, directly calculated from chemical analyses; La Roche 1964, Moine 1974). The procedure follows two complementary steps at two different levels, each using the three diagrams. The first is concerned with the individual sample; the sample is given a name (e.g, granite. granodiorite, gabbro,...) and its chemical and mineralogical characteristics are determined (e.g, its more or less aluminous character). The second one, more important, aims at defining the type of magmatic association to which the studied sample or group of sample belongs. Three main types of associations are distinguished: cafemic (approximately equivalent to the I-type of Chappell & White 1974), aluminous (similar to the S-type), and alumino-cafemic (intermediate between the other two types). Subtypes are then distinguished among the cafemic and alumino-cafemic associations: tholeiitic (or gabbroic-trondhjemitic), calc-alkaline (or granodioritic), subalkaline (or monzonitic), alkaline (and peralkaline), etc. In the same way, numerous subtypes and variants are also distinguished among the aluminous associations using a set of complementary criteria such as quartz content, colour index, alkali ratio, quartz-dark minerals-alkalies relationships and alumina index. Although involving a new approach using partly new criteria, this classification is consistent with most of the divisions used in previous typologies. It has been put to use and tested on numerous examples and has shown to be easy to implement, coherent, comprehensive, and very discriminant. Based on chemical data, it may also be used in the typology of the volcanic rocks. As an example, the typology is applied to three plutonic belts of the Himalaya, South Tibet.

Texto completo:

PDF

Apontamentos

  • Não há apontamentos.