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

During the Vendian-Cambrian, the geodynamic setting in Gorny Altai corresponded to the West-Pacific type of complicated active continental margin. A back-arc basin setting with deposits of isolated troughs and volcanic uplifts is reconstructed in the Cheposh zone of the Katun’ Anticlinorium. Since the second half of the Early Cambrian and especially in the Middle Cambrian, basalt lavas saturated with crystals of Cr-bearing diopside effused over a large area of the Cheposh zone. According to the chemical composition, several groups are recognized among the diopside-porphyry basalts of the Cheposh zone: high-magnesium, low-alumina, aluminous, titaniferous, potassium, and alumina-low-silica. Analysis of the behavior of petrogenic and incongenetic elements shows that the formation of these groups of rocks was governed mainly by conditions of depth fractionation; so, the trend for low-alumina rocks corresponds to the fractional crystallization in an open system. As compared with them, the high-alumina series corresponds to a higher degree of fractionation. The potassic rocks are derivatives of the same initial magma but under the conditions of a closed system. The characteristic features of the composition of the titaniferous group of basalt show that their initial melt differed from the above-specified groups, and the conditions of its fractionation corresponded to a closed system. High-magnesium diopside-porphyry basalts are differentiates of a low-Ti initial magma. Their compositions are controlled by the enrichment in cumulative phenocrysts of diopside. As compared with them, the low-silica-alumina group of compositions consists of peculiar residual melts that appear after removal of pyroxene from the system.