Evolution of the quaternary magmatic system, Mineral Mountains, Utah: Interpretations from chemical and experimental modeling

Abstract

The evolution of silicic magmas in the upper crust is characterized by the establishment of chemical and thermal gradients in the upper portion of magma chambers. The chemical changes observed in rhyolite magmas erupted over a period of 300,000 years in the Mineral Mountains are similar to those recorded at Twin Peaks, Utah, and in the spatially zoned Bishop Tuff from Long Valley, California. Chemical and fluid dynamic models indicate that cooling of a silicic magma body from the top and sides can result in the formation of a roof zone above a convecting region which is chemically and thermally stratified, as well as highly fractionated and water rich. Crystallization experiments have been performed with sodium carbonate solutions as an analog to crystallization in magmatic systems. Top and side cooling of a homogeneous sodium carbonate solution results in crystallization along the top and sides and upward convection of sodium carbonate-depleted fluid. A stably stratified roof zone, which is increasingly water rich and cooler upwards, develops over a thermally and chemically homogeneous convecting region. Crystallization at the top ultimately ceases, and continued upward convection of water-rich fluid causes a slight undersaturation adjacent to the roof despite cooler temperatures. By analogy, crystallizationmore » at the margins of a magma chamber and buoyant rise of the fractionated boundary layer into the roof zone can account for the chemical evolution of the magma system at the Mineral Mountains. To produce compositionally stratified silicic magmas requires thermal input to a silicic system via mafic magmas. The small volume, phenocryst-poor rhyolite magma which persisted for at least 300,000 years in the Mineral Mountains requires the presence of a continued thermal input from a mafic magma source. The presence of silicic lavas signifies that there is a substantial thermal anomaly both in the crust and upper mantle. The production of silicic lavas requires (1) the heating of the lower crust to near the solidus for silicic melts, (2) partial fusion by the additional convective transfer of heat from the mantle by injection of the basaltic magma, (3) continued input of heat in excess of the conductive and convective heat loss to allow the crustal melt to grow to some critical size so that it can rise buoyantly into the upper crust. In the Mineral Mountains there has been an inadequate prolonged thermal flux to produce caldera-forming eruptions. Moreover, the distributed extension in the Basin and Range allows for the propagation of small volumes of magma upward probably in dike-like bodies parallel to the direction of maximum horizontal compressive stress. The erupted lavas represent a highly differentiated and presumably small fraction of the total volume of silicic magma which is contained at considerable depth.« less