Petrogenesis of Trachyandesite from the McDermott Formation, San Juan Basin, Colorado and New Mexico

The McDermott Formation is a late Cretaceous volcaniclastic sedimentary unit formed mostly from mudflow deposits containing clasts of trachyandesite of a narrow range of whole rock composition. These rocks contain phenocrysts of plagioclase ± clinopyroxene ± amphibole set in a purplish groundmass. Most samples exhibit low grade phyllitic alteration. One sample contains xenocrystic potassic feldspar and quartz. Incompatible trace element diagrams show subduction signature of high Ba and low Nb. Ba/La vs. Nb/La ratio plots support a subduction-modified lithospheric source. The concentrations of these and other incompatible trace elements in lower crustal xenoliths of the nearby Navajo Volcanic Field are generally parallel to and lower than those of the McDermott samples, which may indicate a lower crustal source. The Proterozoic Yavapai accreted arc terrane underlies the region and is a potential source for this signature. The McDermott trachyandesite may have been produced by partial melting of lower crustal material. A second hypothesis is that these rocks were formed from a mantle derived melt that assimilated lower crustal material containing this subduction signature.

Major and trace element trends indicate a possible petrogenetic relationship between McDermott trachyandesite and nearby La Plata mountains intrusive rocks, as suggested by previous workers, although the La Plata rocks have a much wider range of compositions from basaltic trachyandesite to trachyandesite and andesite. The McDermott may have been derived from a higher of erosion than preserved in the La Plata mountains, where the intrusions represent subvolcanic magma bodies.