Petrogenesis of the East Fork Member Rhyolites, Valles Caldera, New Mexico, USA

摘要

The most recent volcanism in the Valles caldera is represented by the El Cajete Pyroclastic Beds (ECPB), Battleship Rock Ignimbrite (BRI), and Banco Bonito Flow (BBF) as well as the VC-1 rhyolite, which are collectively known as the East Fork Member (EFM) of the Valles Rhyolite. The EFM was erupted at approximately 55 ka and 40 ka after an approximate 460 ka lull in volcanism. Previous studies suggested a mafic intrusion at depth triggered the eruptions. This thesis represents the first detailed study of the EFM.Crystal assemblages consist of plagioclase, biotite, clinopyroxene, orthopyroxene, amphibole, sanidine, quartz, and oxides. 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 rim overgrowths. Both normal and reverse zonation is present. 40Ar/39Ar geochronology performed on euhedral biotite and groundmass glass from the BBF returned isochron ages of 478 ± 27 ka and 575 ± 15 ka and total gas ages of 125.01 ± 1 ka and 129.82 ± 0.80 ka, respectively. High Mg numbers of large euhderal 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. In general, incompatible trace elements increase and compatible trace elements decrease. Incompatible trace element ratios 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. Trends in the trace element data indicates fractional crystallization was the last process to take 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).