MAGMA MIXING IN ALKALINE MAGMAS - AN EXAMPLE FROM TENERIFE, CANARY-ISLANDS

The central region of Tenerife (Canary Islands) offers a wide variety of alkaline rocks of intermediate composition between basalts and more differentiated magmas. A recent (less than 0.1 Ma old) eruption on th e southwestern slopes of Teide volcano produced separate felsic (SiO2 approximately 59%) and mafic (SiO2 approximately 47%) rocks as well as abundant hybrid inclusions (SiO2: 50-57%) that show a wide range of g eochemical and mineralogical variation between the end members. Minera logical evidence (reverse-zoning, skeletal and patchy feldspars, incom patible assemblages, mineral disequilibrium) and geochemical data (cha racteristic trends of major and trace elements, REE ratios, major and trace mass-balance calculations) support the generation of a mixing pr ocess. We present a model in which basaltic magma entered a trachyte m agma chamber, which resulted in a stratified chamber of trachytic abov e more mafic (hawaiitic) melts. The formation of hybrid magmas between these two end-members is believed to involve some chemical diffusion across the interface between the two magmas before mechanical mixing. A lowering of temperature due to the cooling effect of the trachytic m agma and, consequently, a decrease in density by crystallization of pl agioclase permitted the basaltic magma to penetrate through the trachy te, which caused the eruption. This natural example is in good agreeme nt with the results obtained in experimental work on interdiffusion of silicate melts.