THE PETROGENESIS OF FELSIC CALC-ALKALINE MAGMAS FROM THE SOUTHERNMOSTCASCADES, CALIFORNIA - ORIGIN BY PARTIAL MELTING OF BASALTIC LOWER CRUST

The majority of felsic rocks from composite centers in the southernmos t Cascades have geochemical and Sr Nd and Pb isotopic ratios that sugg est derivation by partial melting of lower crust that is compositional ly similar to calc-alkaline basalts observed in the region. Only a few felsic rocks have delta(18)O and Pb isotopic compositions that indica te interaction with the upper crust. Mineralogical and geochemical dif ferences among the felsic magmas result primarily from melting under v ariable f(H2O) and temperature conditions. Partial melting under low f (H2O) and high temperature conditions leaves an amphibole-poor residuu m, and produces magmas that have orthopyroxene as the most abundant fe rromagnesian phenocryst. relatively low silica contents, and straight rare earth element patterns. Partial melting under higher f(H2O) and l ower temperature conditions leaves an amphibole-rich residuum, and pro duces magmas that have amphibole +/- biotite phenocrysts, relatively h igh silica contents, and pronounced middle rare earth element depletio ns. These conclusions are consistent with published thermal models tha t suggest that reasonable volumes of basaltic magma emplaced beneath l arge composite centers in the southernmost Cascades can serve as the h eat source for melting of the lower crust. Melting of the lower crust under variable f(H2O) conditions is likely to result from differences in the H2O contents of these basaltic magmas.