Evolution of a mafic volcanic field in the central Great Basin, south central Nevada

Abstract

Evolution of a mafic volcanic field is investigated through a study of Pliocene age rocks in the Reveille Range in south central Nevada. Pliocene activity began with the eruption of relatively abundant hawaiite (episode 1, 5–6 Ma), which was followed by trachytic volcanism (4.3 Ma) and by a second episode of lower-volume hawaiite and basanite (episode 2, 3.0–4.7 Ma). Incompatible elements indicate an asthenospheric source. Isotopically, episode 2 basalts cluster around 87Sr/86Sr = 0.7035 and εNd = +4.2, but episode 1 samples vary to high 87Si/86Sr (up to 0.7060) over a narrow range of εNd (+0.8 to +4.5). Trachytic rocks (MgO ∼ 0.5%) are isotopically akin to the episode 1 basalts. Geochemical variation requires the addition of a crustal component (high 87Sr/86Sr, Sr/Nd, Pb/La, low εNd) to the episode 1 hawaiites and trachytic samples, probably by assimilation of carbonate-rich sedimentary wall rock. The volcanic field developed in at least two eruptive cycles of approximately equal duration. Basanites (deeper and lower percentage melts) appear only in the younger episode. Eruptive episodes were apparently linked to separate melting events in the mantle. Through time, basalts were produced in diminishing volumes by lower percentage melting, magma generation and storage was at greater depths, and magma ascent was at higher velocities. Spatially, the melting anomalies were large in the Pliocene but progressively diminished in size so that by Pleistocene time, volcanism was restricted to a small area near the northern end of the initial outbreak.