Petrogenesis of Valle Grande Member rhyolites, Valles Caldera, New Mexico: Implications for evolution of the Jemez Mountains Mgmatic System

Abstract

The Valles Caldera, located near the center of the Jemez Mountains Volcanic Field in north‐central New Mexico, was formed at ∼1.12 Ma upon eruption of the upper Bandelier Tuff. The Valle Grande Member (VGM) consists of 8 rhyolite domes or dome complexes erupted from vents which define the ring fracture within the caldera. They range in age from the time of caldera formation to ∼0.45 Ma and are volumetrically and temporally the dominant post‐caldera eruptives. VGM rhyolites are high‐silica (>75% SiO2 anhydrous), high‐K, are characterized by low abundances of Fe, Mg, Mn, Ti, and Ca, and are metaluminous to weakly peraluminous. Phenocryst assemblages include sanidine (Or44–61) + quartz + plagioclase (An7–20) + biotite + hornblende + Fe‐Ti oxides ± zircon ± allanite ± apatite ± clinopyroxene ± orthopyroxene. Many phenocryst phases show progressive changes in composition, size, and abundance with decreasing age. Whole rock chemistry, phenocryst chemistry, and isotopic dating indicate that VGM rhyolites can be divided into 3 groups: group 1 ranging in age from 1.18 to 0.71 Ma, group 2 from 0.55 to 0.51 Ma, and group 3 at 0.45 Ma. Groups 1 and 2 define differentiation trends in which trace elements such as Rb, Cs, Y, Nb, HREE, Ta, Th, and U increase with decreasing age, whereas Sr, Ba, Zr, LREE, and Eu decrease. Abundances of trace elements may increase or decrease 2–3 fold within eruptive groups; major elements remain relatively constant. The observed differentiation trends can be accounted for by crystal‐liquid fractionation at a minimum or eutectic using modal phenocryst phases. Fractionation may have occurred by roof and side wall crystallization with the collection of bouyant differentiated liquids at the roof of the magma chamber. Two‐feldspar geothermometry, geobarometry using hornblende rim Al contents, and plots of normative compositions on the Q‐Ab‐Or diagram indicate that fractionation took place at depths of 2.5–7.5 km with pre‐eruptive temperatures ranging from ∼720–810°C. Rhyolites erupted in the Jemez Mountains over the last 2–3 m.y. appear to be the products of many separate magma batches and were not derived from one large long‐lived magma chamber. VGM rhyolites represent 3 separate magma batches (groups 1, 2, and 3), two of which remained closed systems for sufficient periods of time to generate highly evolved differentiates (groups 1 and 2).