The geotectonic typing of rapakivi granitoid formation

Kostenko N.V., Tolstoy M.I.

National T. Shevchenko University of Kyiv, Kyiv, Ukraine

Today, virtually no discussion on formation of rapakivi granitoids belonging to the A-type granites. Disagreements between authors are just about geodynamic position of the rocks and formations of this type in general.

For discriminating the tectonic setting of granitoid rocks are different classification charts, but the most suitable in our case is the diagram of J. Pearce et al. (1984) - Rb - (Y + Nb). In our opinion, it gives the results, which are most near to geological realities. Abundance of some trace elements of the 12 types of acidic rocks of the formation rapakivi from Ukrainian and Baltic shields plotted on this diagram (Fig.1). As we see, the points of most differentiated granites are in the contour of the field of the Within-plate granites (WPG), namely: lezniky type granite and Salmi granite. To the WPG group gravitate also Korsun-Novomirgorod rapakivi type granite and Viborg trachytoid granite. Figurative points of all other varieties of the investigated rocks (granite-porphyry, granites rapakivi Korsun-Novomirgorod, Salmi, Viborg, Iskrensky granite and aplitoid granite) are simultaneously and in a field of vagueness, and in a contour of a field Post-collision (post-COLG) granites. Does not it means, that at a part of granitoid rocks of this formation, by the way, the least differentiated, were saved the geochemical signs, inherent to Post-collision granites? In this connection it should be noted that, obviously, not by chance granitoids of rapakivi formation, which extend along outlying part of the East Europe Craton, characterized by the thickened crust (Sharkov E.V., 1999).

Figure 1 - Abundance of some trace elements of the acidic rocks from Ukrainian and Baltic shields plotted on the tectonic discrimination diagram of J. Pearce et al., (1984): fields for Within Plate(WPG), Syn Collision (Syn-Colg), Volcanic Arc (VAG), Ocean Ridge (ORG) and Post Collision (Post-COLG) granites are indicated. Symbols: 1, 4 – Divlin granite-porphyry, Lezniky type granite; 2, 5, 9, 11 - granites rapakivi Korosten, Korsun-Novomirgorod, Salmi, Viborg; 3, 6, 12 – granites rapakivi type Korosten, Korsun-Novomirgorod, Salmi; 7, 8 – Iskrensky granite and aplitoid granite (Korsun-Novomirgorod Pluton), 10 - Viborg trachytoid granite.

At the same time, considerable age-related break (for the rocks of the Ukrainian shield - 200 Ma), that fixed between these granites, which, according to a diagram, it is possible to equate with post-orogenic formations and orogenic, does not allow (Sylvester P.J., 1989) to add them neither to the first nor to second. According to this age’s criterion these rocks belong to the anorogenic granitoids. Contradiction will not be, if to assume, that granitoids appeared in the remote finale (as echoes) of collision processes at subduction of hypothetical plates (or microplates) under East Europe Craton (Fenoskandiya + Sarmatiya). In our opinion, in the places of their total-absorption by a mantle the local plumes could be formed in connection with the election change of geodynamic conditions of the mode of compression to tension. The plutonic massifs of anorthosite-rapakivi granite complexes were appeared above them.

Is it necessary to bind the introduction of these unique rocks to the subplatform stage of development of craton? A question remains open. Note, that in the field of intraplate granites are only figurative points of metasomatic altered rocks of rapakivi formation, and therefore their assignment to this geodynamic type, as follows from the diagram, may be incorrect. It gives some grounds the granitoids of rapakivi formation, in generally, to attribute to Post-collision creations, especially since the vast majority of rock varieties is within the contour of this type granites.

References

Pearce J.A., Harris N.B.W., Tindle A.G. 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of petrology, v.25, pp.956-983.

Sharkov E.V. The intraplate magmatic system of the Mezoproterozoic by the example of anorthosite-rapakivi granite complexes of the Baltic and Ukrainian shields // Russian Journal of Earth Sciences/ 1999. V.1. N 4. (in Rus). (URL:http://eos.wdcb.rssi.ru/rjes/)

Sylvester P.J. Post-collisional alkaline gtanites // J. Geology. 1989. V. 97. P. 261-280.