Petrogenesis of three East Fork Member rhyolites of the Jemez volcanic field, Valles caldera, New Mexico, USA

Abstract

The most recent volcanism in the Valles caldera is represented, in stratigraphic order, by the El Cajete Pyroclastic Beds (ECPB), Battleship Rock Ignimbrite (BRI), the VC-1 rhyolite (not studied herein), and Banco Bonito Flow (BBF), which are collectively known as the East Fork Member (EFM) of the Valles Rhyolite. The EFM was erupted approximately 74 to 68 ka after an approximate 460 ka lull in ring-fracture volcanism.

Crystal assemblages consist of plagioclase, biotite, clinopyroxene, orthopyroxene, amphibole, sanidine, quartz, and oxides, in order of abundance. Electron probe microanalysis and detailed petrography indicates that two distinct crystal populations are present in the ECPB, BRI, and BBF. Large (≥1 mm), typically resorbed or subhedral crystals represent one population, and small (≤0.5 mm), generally euhedral crystals represent the other. The large resorbed plagioclase crystals typically have oligoclase to andesine rim overgrowths. ⁴⁰Ar/³⁹Ar geochronology performed on euhedral biotite and groundmass glass from the BBF returned isochron ages of 478 ± 3 ka and 575 ± 2 ka and total gas ages of 125 ± 1 ka and 129 ± 1 ka, respectively. High molar Mg numbers of large euhedral biotite and ⁴⁰Ar/³⁹Ar ages older than the accepted age range indicate these crystals are xenocrystic. Radiogenic isotopes are consistent with mixing between the mantle and lower crustal reservoirs. General trends are evident between whole-rock major and trace elements with increasing SiO₂ and Nb, respectively. In general, incompatible trace elements increase and compatible trace elements decrease with increasing Nb. For the major elements, MgO, P₂O₅, Al₂O₃, FeO* show decreasing trends while K₂O and Na₂O show increasing trends with increasing SiO₂. Incompatible trace element ratios (Ta/Yb, Y/Yb, Th/Yb, Th/Nb) indicate the presence of a single magma batch.

The heterogeneity in crystal morphology and chemistry can be explained by a model in which partial melting of mid- to deep continental crust occurred due to an intrusion of an intermediate composition magma. Magma mixing and an exchange of crystals took place between the partial melt and the intruding magma. The hybrid magma rose to the upper crust, where it underwent fractional crystallization prior to eruption. The geochemical and isotopic data from this study are best explained by a modified version of the rapid production and eruption model put forth by Huppert and Sparks (1988).