| Abstract |
A greater understanding of the evolution of shallow silicic magma systems may help interpret the thermal structure of young systems associated with geothermal resources. The Twin Peaks volcanic center of West-Central Utah is a small scale shallow silicic magma system which produced four cubic kilometers of exposed volcanics. Chemical modeling of the system's evolution indicates that two pulses of magma gave rise to two separate differentiation sequences, spanning periods of 2.7 to 2.5 m.y. and 2.43 to 2.35 m.y. A sustained heat influx from below is required to maintain the system above its solidus for periods exceeding 50,000 years. The resulting thermal gradient in the magma chamber rives rise to liquid state differentiation which produces increasingly pronounced chemical gradients in a stagnant roof zone above the convecting bulk of the magma. A distinct pattern of chemical change, characterized by increases in SiO2, heavy rare earths, H2O, Na2O, and highly charged cations, and decreases in FeO, MgO, CaO, K2O, and light rare earth elements, results. The degree and rate of liquid state differentiation may indicated the relative magnitude of heat flux from below, and whether heat input is continuing. Chemical changes over the first sequence, spanning about 200,000 years, was dominated by liquid state differentiation, indicating sustained heat input. The second sequence spanned less that 100,000 years and liquid state differentiation was considerably less pronounced, suggesting a diminished heat influx. |