Origin of Quaternary basalts from the Black Rock Desert Region, Utah

Known relative age relations and element distributions in Quaternary high-alumina and tholeiitic basalts from the Black Rock Desert region in Utah are adequate to identify both long-term (~ 10⁶ yrs) and short-term (≲ 10³ yrs) geochemical trends. One major long-term trend is a decrease in Sr from the oldest to the youngest basalts. Sr distribution coefficients determined from plagioclase phenocryst-ground-mass pairs suggest an origin for this decrease involving removal of about 40 percent plagioclase by fractional crystallization. Progressive increases in K, Rb, and REE, and approximate constancy of most major and some trace elements over this range indicate in addition to removal of plagioclase, removal of small amounts (~ 7 percent) of olivine, orthopyroxene, and Fe-Ti oxides, and also contamination of the magmas with about 10 percent of Sr-poor granitic rock. More than enough heat is available from the latent heats of crystallization to melt this amount of contaminant. A model is proposed in which primary olivine tholeiite magmas are initially collected in deep (15 to 35 km) crustal chambers and receive small additions of mantle-derived magmas as they move upward in the crust and undergo extensive fractional crystallization and minor contamination over the last million years. Short-term, time-dependent geochemical trends occur in the flows of the youngest volcanic field (the Ice Spring field). These trends show increasing Fe, Ca, Zn, Cu, Ti, Mn, and Sr and decreasing Si, Ni, Co, Mg, Na, K, Rb, and Nb with age of eruption. To explain these trends, a model is proposed involving both progressive tapping of a zoned magma chamber and removal of 10 to 15 percent of Mg-rich orthopyroxene during fractional crystallization.