Quaternary volcanism in northwestern Colorado: Implications for the roles of asthenosphere and lithosphere in the genesis of continental basalts

Abstract

Quaternary volcanic rocks were erupted at four locations in NW Colorado; Dotsero (4150 y.B.P.), Willow Peak (undated), McCoy (0.64 m.y. B.P.), and Triangle Peak (1.98-1.87 m.y.B.P.). At Triangle Peak, there are at least eleven lava flows, but eruptions at the other locations were monogenetic. Dotsero was the only hydrovolcanic eruption. The volcanic rocks are alkali basalts, containing the phenocryst assemblage: olivine, Fe-Ti oxide, ± clinopyroxene, ± plagioclase. The basalts are chemically similar to OIB, as would be expected from their intraplate crustal setting. Nevertheless, they have La/Ta, K/Ta, Ba/Ta and K/La ratios which are significantly higher than those of oceanic OIB. These differences cannot be explained by contamination during uprise of OIB-like basalt by continental crust of reasonable composition. It is, therefore, logical to assume that the Quaternary magmas contained a component of partial melt of subcontinental lithospheric mantle. This conclusion is in accord with the low 143Nd/144Nd ratios of the basalts. The geochemistry of the Quaternary basalts can be explained by mixing between three separate mafic magma end-member groups that were erupted in the same area during the Miocene. Group 1 magmas were OIB, representing partial melts of OIB-source asthenosphere. Group 2 magmas were minettes, with low 143Nd/144Nd ratios, regarded as partial melts of sub-Colorado lithospheric mantle. Group 3 magmas had high La/Ta ratios, and, generally, low LIL/HFS ratios. During the Miocene, the latter group of magmas are interpreted to have been derived by partial melting of asthenosphere that had been modified by subduction of oceanic lithosphere below the North America plate. The presence of a component of Group 3 magma in the Quaternary basalts indicates that the mantle source of this group was trapped for 8 m.y. in an uppermost asthenospheric layer which experienced very sluggish flow. We propose that this layer is equivalent to the thermal boundary layer — situated between the rigid part of the lithospheric mantle (above), and the convecting asthenosphere (below) — originally identified by calculations of the thermal histories of lithospheric plates.