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 magmas [Mg/(Mg + Fe*) > 0.7, MgO > 8 wt% and Ni > 150 ppm] ranging in composition from high-alumina basalt to andesite and these record variable extents of melting in their mantle source. Trace and major element chemical variations, petrologic evidence and the results of phase equilibrium studies are consistent with variations in H2O content in the mantle source as the primary control on the differences in extent of melting. High-SiO2, high-MgO (SiO2 = 52% and MgO = 11 wt%) basaltic andesites 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-alumina basalts (SiO2 = 48.5% and Al2O3 = 17 wt%) represent nearly anhydrous low degree partial melts (from 6 to 10% depletion) of a mantle source that has been only slightly enriched by a fluid component derived from the subducted slab. The temperatures and pressures oflast equilibration with upper mantle are 1200 degrees C and 1300 degrees C for the basaItic andesite and basaltic magmas, respectively. A model is developed that satisfies the petrologic temperature constraints and involves magma generation whereby a heterogeneous distribution of H2O in the mantle results in the production of a spectrum of mantle melts ranging from wet (calc-alkaline) to dry (tholeiitic).