CONTAMINATION OF BASALTIC MAGMA BY MAFIC CRUST AT AMBOY AND PISGAH CRATERS, MOJAVE DESERT, CALIFORNIA

Quaternary alkali basalts from Pisgah Crater and Amboy Crater in southern California exhibit unusual chemical and isotopic variations which probably result from assimilation of mafic crust. Although lavas from both volcanoes are alkali basalts and hawaiites with isotopic and chemical characteristics that are similar to ocean island basalts (OIB) (e.g., epsilon-Nd = 2.6 - 5.9, Sr-87/Sr-86 = 0.7038 - 0.7049, Hf/Ba = 0.014 - 0.017), they display highly correlated and unusual variations in their chemical and isotopic compositions. At each volcano, MgO decreased during the eruptive sequence from almost-equal-to 8.5 wt % to almost-equal-to 4.5 wt %. Incompatible elements are positively correlated with MgO and therefore also decreased during the eruptive sequence. Nd, Sr, and Pb isotope ratios correlate strongly with MgO. Compositional and isotopic data cannot be explained by any combination of closed-system fractionation, partial melting of the mantle, or silicic contamination. These data indicate that the basalts represent mixing between a high-MgO, high-epsilon-Nd component, common to both Pisgah and Amboy craters, with lower-MgO and -epsilon-Nd components that are unique to each center. The high-MgO component most likely is a primitive mantle-derived magma, based in part on its similarity to nearby xenolith-bearing Quaternary basalts. The low-MgO components are interpreted to be partial melts of mafic crust. If our model is correct, then both volcanoes evolved from eruption of nearly pure mantle melts early in their history to eruption of nearly pure remelted mafic crust late in their history. The crustal source could not have been underplated Mesozoic or younger oceanic crust, but its age is otherwise unconstrained; nearby Mesozoic gabbros and Proterozoic diabases have appropriate isotopic compositions. The basalt data provide no evidence that ancient enriched lithospheric mantle currently underlies the Mojave Desert. If such mantle was present at any time beneath this region, it must have been removed during one or more of the many tectonic events that affected the Mojave Desert during the Phanerozoic. Regional variability of isotope ratios in basalts is commonly interpreted to reflect variability of the underlying mantle. Data from this study raise the possibility that some of this variability may result from cryptic contamination of OIB-like basalts by mafic crust.