New Mexico Geological Society Annual Spring Meeting — Abstracts

The Valles Caldera is located near the center of the Jemez Mountains Volcanic Field in north-central New Mexico. The caldera was formed at 1.12 +/-0.03 Ma upon eruption of the Tshirege Member of the Bandelier Tuff (upper Bandelier Tuff) and subsequent emptying of the upper part of a zoned rhyolitic magma chamber. These events were followed by resurgence, the formation of a central structural dome, and eruption within the caldera of rhyolitic pyroclastic rocks, lave flows, and domes of the Valles Rhyolite Formation. The Valle Grande Member, a group of high-silica rhyolite domes, compisite domes, and flows erupted on or near ring fracture vents encircle the caldera and volumetrically dominate the Valles Rhyolite Formation. These rocks have K/Ar ages ranging from 1.18 +/-0.03 to 0.45 +/-0.02 Ma and show progressive changes in both petrography and chemistry with time. Phenocryst assemblages include Qtz + San + Plag + Bio + Mag-llm + Zir +/-HbI +/-All +/-Ap +/-Cpx. In general phenocryst percentages range from < 2 to ~35 whiIe plagioclase and ferromagnesian minerals increase in abundance when going from older to younger domes. Changes of Na, Si, As, Rb, Y, Nb, Mo, Sb, Cs, HREE's, Ta, Pb, Th, and depletions Mg, AI, P, K, Ca, Ti, Mn, Fe, Sr, Zr, Ba, LREE's, Eu, and Hf with time. Models proposed to acccunt for these changes include progressive partial melting of a lower crustal source, roof rock contamination, silicate liquid immiscibility, thermal (Soret) fractionation, and crystal-liquid fractionation involving side wall crystallization. The latter is the favored model here. Mechanisms proposed to account for reversals in differentiation trends at ~0.73 Ma and ~0.51 Ma are eruption of sufficient volumes of magma to disrupt compositional zonation or magma chamber replenishment.