Petrogenesis of alkalic and calcalkalic volcanic rocks of Mormon Mountain Volcanic Field, Arizona

Abstract

The Cenozoic Mormon Mountain Volcanic Field (MMVF) of northern Arizona is situated in the transition zone between the Basin and Range and the Colorado Plateau. It is composed of alkalic to sub-alkalic basalts and calcalkalic andesites, dacites, and rhyodacites. Despite their spatial and temporal association, the basalts and the calcalkalic suite do not seem to be co-genetic. The petrogenesis of primitive MMVF basalts can be explained as the result of different degrees of partial melting of a relatively homogenous, incompatible element-enriched peridotitic source. The variety of evolved basalt types was the result of subsequent fractional crystallization of olivine, spinel, and clinopyroxene from the range of primitive basalts. Crustal contamination seems to have occurred, but affected only the highly incompatible element abundances. The formation of MMVF calcalkalic rocks is most readily explained by small to moderate amounts of partial melting of an amphibolitic lower crust. This source is LREE-enriched but depleted in Rb and relatively unradiogenic Sr ( 87 Sr/ 86 Sr ∼0.7040). Calcalkalic rhyodacites may also be derived from andesitic parents by fractional crystallization. The overall petrogenesis of the MMVF complex is the result of intra-plate volcanism where mantle-derived magmas intrude and pass through thick continental crust.