PRIMITIVE BASALTS AND ANDESITES FROM THE MT SHASTA REGION, N CALIFORNIA - PRODUCTS OF VARYING MELT FRACTION AND WATER-CONTENT

Quaternary volcanism in the Mt. Shasta region has produced primitive m agmas [Mg/(Mg + Fe) > 0.7, MgO > 8 wt% and Ni > 150 ppm] ranging in c omposition from high-alumina basalt to andesite and these record varia ble extents of melting in their mantle source. Trace and major element chemical variations, petrologic evidence and the results of phase equ ilibrium studies are consistent with variations in H2O content in the mantle source as the primary control on the differences in extent of m elting. High-SiO2, high-MgO (SiO2 = 52% and MgO = 11 wt%) basaltic and esites resemble hydrous melts (H2O = 3 to 5 wt%) in equilibrium with a depleted harzburgite residue. These magmas represent depletion of the mantle source by 20 to 30 wt% melting. High-SiO2, high-MgO (SiO2 = 58 % and MgO = 9 wt%) andesites are produced by higher degrees of melting and contain evidence for higher H,O contents (H2O = 6 wt%). High-alum ina basalts (SiO2 = 48.5% and Al2O3 = 17 wt%) represent nearly anhydro us low degree partial melts (from 6 to 10% depletion) of a mantle sour ce that has been only slightly enriched by a fluid component derived f rom the subducted slab. The temperatures and pressures oflast equilibr ation with upper mantle are 1200 degrees C and 1300 degrees C for the basaItic andesite and basaltic magmas, respectively. A model is develo ped that satisfies the petrologic temperature constraints and involves magma generation whereby a heterogeneous distribution of H2O in the m antle results in the production of a spectrum of mantle melts ranging from wet (calc-alkaline) to dry (tholeiitic).