SEPARATION OF GRANITOIDS IN THE NORTHERN MASSIF OF CHUKOTKA ON THE BASE OF STEREOMETRIC AND EVOLUTIONARY CRYSTALLOMORPHOLOGICAL ANALYSES

摘要

There are two groups of rare-metal granites distinguished within the Northern massif: biotitic granites and zinnwalditic leucogranites, attributed to different intrusive complexes. Biotitic granites of the earlier complex were subjected to strong action of the fluid saturated melts of the later complex and, in this way, have lost their primary appearance. The most significant physiographic peculiarity of these rocks is the remained irregularly grained porphyritic structure characterizing them as the A granites (Beskin e.a., 1996). From the other side, zinnwalditic leucogranites of the later complex have structural features relating them to the C type granites. This factor allow to consider them as typical representatives of the F-Li subtype of the subalkaline-leucogranitic formation. The later complex influence stipulated an appearance of similar features in the biotitic granites also. To resolve question of the earlier complex formational belonging, the method of accessory zircon ECA was used (Pupin, 1980), which formerly had been successfully applied for the same purpose (Brodskaya, 1990; Brodskaya e.a., 1986, 1987, 1989). Character of zircon distribution allows to prove the possible belonging of the earlier complex rocks to the granite-leucogranitic formation with a subsequent transformation of the A granites in rocks with higher structural composition making them similar to granites of C type.