The Petrogenesis of Felsic Calc-alkaline Magmas from the Southernmost Cascades, California: Origin by Partial Melting of Basaltic Lower Crust

Abstract

The majority of felsic rocks from composite centers in the southernmost INTRODUCTION Cascades have geochemical and Sr, Nd and Pb isotopic ratios Despite the importance of felsic magmas in calc-alkaline that suggest derivation by partial melting of lower crust that is rock suites, as both the end of the compositional spectrum compositionally similar to calc-alkaline basalts observed in the region. of lavas and as a potential assimilant or mixing endOnly a few felsic rocks have d 18 O and Pb isotopic compositions member, most petrogenetic studies have focused on bathat indicate interaction with the upper crust. Mineralogical and saltic magmas. This stems from the fact that sparsely geochemical diVerences among the felsic magmas result primarily crystalline basaltic lavas are usually the most primitive from melting under variable f(H2O) and temperature conditions. lavas within a given suite of rocks, and therefore have Partial melting under low f(H2O) and high temperature conditions compositions that are most indicative of magma sources. leaves an amphibole-poor residuum, and produces magmas that In contrast, felsic rocks have high incompatible element have orthopyroxene as the most abundant ferromagnesian phenocryst, abundances and are often highly crystalline, and may relatively low silica contents, and straight rare earth element patterns. therefore have been extensively modified by fractional Partial melting under higher f(H2O) and lower temperature con- crystallization and assimilation fractional crystallization ditions leaves an amphibole-rich residuum, and produces magmas (AFC). This study focuses on sparsely crystalline felsic that have amphibole ‐ biotite phenocrysts, relatively high silica (SiO2 > 68 wt %) volcanic rocks erupted in the southcontents, and pronounced middle rare earth element depletions. These ernmost Cascades that demonstrate evidence for only a conclusions are consistent with published thermal models that suggest minor amount of diVerentiation. We use mineralogy, that reasonable volumes of basaltic magma emplaced beneath large major and trace element geochemistry, and isotopic compositions of felsic rocks erupted from composite cencomposite centers in the southernmost Cascades can serve as the ters to constrain their petrogenesis. These felsic rocks heat source for melting of the lower crust. Melting of the lower provide new insights into petrogenetic processes that crust under variable f(H2O) conditions is likely to result from generate felsic arc magmas.