GENESIS OF HIGH-ALUMINA BASALTS FROM KLYUCHEVSKOI VOLCANO

The mechanism and conditions of formation of the Klyuchevskoi high-alu mina basalts were determined as a result of a comprehensive examinatio n of 45 samples representing the rocks of 40 flank eruptions and a sub sequent computer simulation of the generation of high-alumina basalt m agmas from primary high-magnesia basalt melts. This petrological and g eochemical study included a petrochemical and geochemical classificati on of the Klyuchevskoi basalts and an analysis of the compositions of minerals that coexist as glomerocrysts and mutual solid phase inclusio ns, The results suggested that the observed spectrum of compositions ( from high-magnesia basalts to high-alumina varieties) represent a cont inuous igneous rock series that formed as a result of similar fraction ation of the primary magmas, which approximated the composition of the high-magnesia basalts of the Bulochka Bocca. Analysis of the cotectic relations of the coexisting minerals yielded the following crystalliz ation sequence: Ol(Fo(90.92)) + Aug(MGN(89.91)) +/- Sp(CRN(70.72)) --> Ol(Fo(87.88)) + Aug(MGN(86.87)) +/- Opx(MGN(88.89)) +/- Sp(CRN(65.70) ) --> Ol(Fo(75.77)) + Aug(MGN(79.80)) +/- Opx(MGN(78.79)) + Sp(CRN(20. 30)) + Pl(An(65.77)). A model of polybaric (decompression) basalt magm a fractionation under hydrous conditions was developed using the COMAG MAT petrologic programs designed to simulate and evaluate the conditio ns of fractionation. Using this model, 600 calculations were performed for the isobaric and decompression fractional crystallization of the Klyuchevskoi primary magma (average high-magnesia basalt) for a pressu re range of 1 atm-20 kbar and an initial H2O content of 0-2 wt %. The optimum model for the formation of this igneous rock series was determ ined to be polybaric primary magma fractionation at a pressure decline rate of 0.33 kbar per 1% crystallization with approximately 2 wt % H2 O in the initial melt. According to this model, magma crystallization starts with the separation of olivine and clinopyroxene (in the presen ce of spinel, according to natural data) at a pressure of approximatel y 19 kbar and temperature similar to 1350 degrees C. High-Mg orthopyro xene crystallizes at around 15 kbar and similar to 1260 degrees C. The accumulation of water in the melt results in a considerable delay of plagioclase crystallization and the formation of high-alumina differen tiation products containing more than 18 wt % Al2O3. The line of alumi na enrichment breaks as plagioclase appears at the liquidus at approxi mately 7 kbar, 1110 degrees C, and similar to 3 wt % H2O in the melt, By this time, the composition spectrum of the model liquidus correspon ds to all varieties of the Klyuchevskoi lavas. Further crystallization involves rapid saturation of the system with water and subsequent deg assing at about 1.5 kbar.