PETROLOGICAL AND GEOCHEMICAL CORRELATIONS BETWEEN RAPAKIVI GRANITES AND ACID VOLCANICS IN THE SOUTHERN FRINGING OF THE SIBERIAN PLATFORM

The granitic rocks of the Primorskii Complex, acid potassic volcanics of the Akitkan Group, and subalkaline migmatites of the Chuya Group ar e spatially associated on the southern framing of the Siberian Platfor m and have similar age. The rocks show a number of common and pervasiv ely occurring petrochemical features that are specific to both rapakiv i granite and modern within-plate granitic rocks. The rocks typically have a high alkalinity (with potassium clearly dominating over sodium) and a high iron mole fraction, are rich in such hydrolyzate elements as REE, Zr, Hf, and Nb (which are typical of alkaline rocks and metaso matites related to deep faults), and are simultaneously high in elemen ts specific to granites (Pb, Sn, Be, and F) and in characteristic trac e elements of basites (Co, Ni, Cr, and Sc) at low contents of Li, V, a nd Ta. These features led us to conclude that the rocks are the produc ts of a common magma-generating process but formed at different depths (from the depth of alkaline migmatization in the platform basement to those of intrusions and volcanic eruptions) and were brought to the p resent-day erosion surface by later tectonic processes. Comparative an alysis of the rocks indicates that this rock association is closest to associations arising on the rear portions of active continental margi ns in relation to evolving subduction zones. The wide compositional va riations of these rock associations, which are illustrated by a variet y of tectono-geochemical and geochemical plots, could be related to so me characteristic features of petrogenetic processes in the Precambria n, a time period when ovoid-ring tectonics was particularly active.