A-GRANITES: COMPOSITION, ORIGIN, SETTING

Makhlaev L.V. 

Institute of Geology of Komi Scientist Center UrB RAS, Syktyvkar, Russia, lev@geo.komisc.ru

Modern classification of granites which have been offered more that 30 years ago by B. Chappell (Chappell, White, 1974), it is widely used in petrology now. According to this scheme all granites are divided on 4 groups, determined by alphabetic symbols, namely "I-", S-, "M-" and A-granites. The A-group has been marked in this classification later, than others, and nature of corresponding granites remains less certain.

Letter's symbols of first three groups have the direct relation to an origin of corresponding rocks. In cases I-and S-granites their symbols show on the types of primary (graniteforming) substratas. I-granites have been generated by transformation igneous primary substrata (more precisely apoigneous). The symbol "S" has been generated in a similar way from the term "sedimentary" (aposedimentary). The symbol "M" is not connected to type of substrata, but also has essential genetic sense. It specifies a zone where the initial material has been made – on a mantle (White, 1979). This group unites the granitic differentiates which have been separated from not granite mantle magmas – basaltic or andesitic.

On the contrary, the letter "A" has no relation to a way of formation of corresponding rocks. It symbolizes the whole set of the certain special qualities inherent in these granite types. The letter «A» means both anhydrous, and alkaline, and anorogenic. Granites, possessing corresponding characteristics, for the first time have been revealed and described by M.Loisell. The "petrotypes" of these rocks was so-called «Red granites from Nigeria» (Loiselle, Wones, 1979). One of features of the last is their association with tholeitic basalts. In opinion of M.Loisell it could not be casual. He believed, that «Red granites» have been generated by fractiones these basalts or as a result of their selective melting. Accordingly, they could not make special genetic type. In a case acceptance of the first version of them it was necessary to examine as M-granites, whereas the model of the selective melting of basites demanded their reference to I-granites.

Having generalized M.Loisell papers and publications of other researchers studying the A-granites, B.Chappell has included, nevertheless, this type in the classification, paid special attention to the nature of an initial material as it was accepted for other divisions of his scheme B.Chappell has come to the conclusion, that A-granites are not derivated from basalts. Similarly to rocks of the first two groups they belong, in his opinion, to the crust anatertites, however as a substratum for these granites appears the rocks, whicn was alredy transformed by ultrametamorphism and granitization. It three main features A-granites marked above speak also. They appear waterless (anhydrous) just because their substratum "has been already drained" in a development previous regional metamorphism and ultrametamorphism. Increased alkaline contain and prevalence potassium above sodium, it is caused by that addition potassium is one of determining factors of granitization. At last, the A-granites are formed only in that blocks of lithosphere which have the powerful granitization zone, that is have a powerful continental crust, that inevitably determines their tectonic position as anorogenic or postorogenic (Whalen et al, 1987).

To identical conclusions there has come G.Ejbi who too believes, that A-granites are formed as a result of repeated melting of the basis of the granite layer of lithosphere. In his opinion, this melting was caused by a heat basaltic magma that from asthenosphere rising. It the assotiation A-granites with basites speaks. However the basites are in such associations only an energy source, but not of substance.

A-granites are widely submitted and in Russia. The author of the present report dealed with them on the Yenisey Ridge and Taymyr fold belt. During last 20 years I studied them in Polar and Subpolar Urals where this type of granites for the first time has been revealed and characterized by me (Махлаев, 1996).

So, A-granites possess quite precise set of diagnostic attributes, and also genetic characteristics inherent in them. It to the full proves their reference to special classification type. Nevertheless, last years there are publications which authors count allocation of this group inexpedient. Such reports were, in particular, and on last (The sixth) Hutton symposium "Origin of granites and related rocks", taken place in the Republic of South Africa in July, 2007. Opponents emphasize, that A-granites have no precise geodynamic position. Corresponding granitoids ("anorogenic, alkaline, anhydrous") are submitted in different tectonic conditions. In our opinion, it cannot be the basis for exception of this type. We shall note, as granitoids other groups of Chappell's scheme are not dated to strictly certain geodynamic position too. It is quite natural, as this sheme initially was based not on principles of geodynamics, and on the nature of an initial material. As a substratum for A-granites are, according to the author of this scheme, the granitizired complexes, blocks are especially favorable for their formation with powerful continental crust. Such blocks can be generation by within-plate continental rifting as it takes place in Nigeria, but it can be both zones of a collision, and a zone of active continental outskirts of Andian type where the largest A-granitas plutones in the World are described.

References

Махлаев Л.В. Гранитоиды севера Центрально-Уральского поднятия. Полярный и Приполярный Урал. Екатеринбург: изд-во УрО РАН, 1996. 150 с.

Chappell B.W., White A.J.R. Two contrasting granite types // Pacific Geologist, 1974. N 8. P.173-174.

Loiselle M.C., Wones D.R. Characteristics and origin of anorogenic granites // Geol. Soc. Am. Abstract Prog., 1979. V. 11. 468p.

Whalen J.B., Currie K.L., Chappell B.W. A-type granites: geochemical characteristics, discrimination and petrogenesis // Contribution Miner. and. Petr., 1987. V. 95. P.407-419.

White A.J.R. Source of granite magmas // Geol. Soc. Am. Abstracts, 1979. P.534.