CAMBRIAN DIOPSIDE-BEARING BASALTS OF THE CHEPOSH ZONE IN GORNY-ALTAI - A MODEL FOR FRACTIONATION OF HYBRID MAGMAS IN INTERMEDIATE MAGMATIC CHAMBERS

During the Vendian-Cambrian time the geodynamic setting in Gorny Altai corresponded to the West-Pacific type complicated active continental margin. The back-are basin setting with deposits of isolated troughs a nd volcanic uplifts is reconstructed in the Cheposh zone of the Katun' Anticlinorium. Since the second half of the Early Cambrian and, espec ially, in the Middle Cambrian, basalt lavas saturated with crystals of Cr-bearing diopside effused over a large area of the Cheposh zone. Ac cording to chemistry, several groups are recognized among the diopside -porphyraceous basalts of the Cheposh zone: high-magnesian, low-alumin a and aluminous, titaniferous, potassium, and alumina-low-siliceous. A nalysis of the behavior of petrogenic and incongenetic elements shows that the formation of these groups of rocks was mainly governed by con ditions of depth fractionation, so the trend for low-alumina rocks cor responds to the fractional crystallization in an open system. As compa red with them, the high-aluminous series corresponds to a higher degre e of fractionation. The potassium rocks are derivatives of the same in itial magma but under the conditions of a closed system. The character istic features of the composition of the titaniferous group of basalt show that their initial melt was distinguished from the above-specifie d groups, and the conditions of its fractionation corresponded to a cl osed system. High-magnesian diopside-porphyraceous basalts are differe ntiated of a low-Ti initial magma. Their compositions are controlled b y the enrichment by cumulative phenocrysts of diopside. As compared wi th them, the low-silica-aluminous group of compositions consists of pe culiar residual melts that appear upon removing pyroxene from the syst em.