OLIVINE AND CHROMIAN SPINEL IN PRIMITIVE CALC-ALKALINE AND THOLEIITICLAVAS FROM THE SOUTHERNMOST CASCADE RANGE, CALIFORNIA - A REFLECTION OF RELATIVE FERTILITY OF THE SOURCE

Chromian spinel and coexisting olivine phenocrysts from a geochemicall y diverse suite of primitive tholeiitic and calc-alkaline basalts and magnesian andesites from the Lassen region, in the southernmost Cascad e Range, in California, show that the sub-are mantle is zoned. Deplete d calc-alkaline basalts and magnesian andesites erupt in the forearc r egion, and calc-alkaline basalts contain increasing abundances of inco mpatible elements toward the backarc. High-alumina olivine tholeiites erupt from the are and backarc areas. Olivine from all these lavas dis plays a limited compositional range, from Fo(86) to Fo(91), and crysta llized at high temperature, generally 1225-1275 degrees C. Chromian sp inel trapped in the olivine phenocrysts displays a large range of comp osition: Cr# values span the range 9-76. Excess Al in the spinel relat ive to that in l-atm spinel suggests that it crystallized at elevated pressure. The phenocrysts in these lavas are in equilibrium with their host liquids. The full range of Cr# of the spinel compositions cannot be explained by differentiation or variable pressure, variations in f (O-2), subsolidus equilibration or variations in degree of partial mel ting of a single peridotitic source. Rather, the systematic compositio nal differences among phenocrysts in these primitive lavas result from bulk chemical variability in their mantle sources. Correlations betwe en spinel and host-rock compositions support the assertion that the ge ochemical diversity of Lassen basalts reflects the relative fertility of their mantle sources.