Influences of pluton growth on magma crystallinity, aureole rheology, chamber stability and metamorphic fluid flow : numerical modeling of the growth of Papoose Flat pluton, White-Inyo Range, California

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Magma is a molten rock that rises in the Earth's crust because it is hotter and less dense than the surrounding rocks. If the magma finds a way to the surface, a volcanic eruption occurs. When the magma cannot find a path upwards, it pools into a magma chamber. This process also deforms the cold country rocks (contact aureole) around it by pushing them and warming them up. Metamorphic reactions, especially fluid generation reactions, occur in the country rocks during the process. Magma chambers cool down at depth to form different types of igneous rocks (plutons), such as granite. Today, we still have many questions about what goes on during magma emplacement. For example, how do magma chambers grow? Are they emplaced effectively instantaneously by injection of large volumes of magma, or incrementally with magma injections accumulating over thousands to millions of years? Unlike studies of lava flows on the surface, we have had to rely on methods to "see" deep into the Earth. Numerical computer modeling is one of the methods. The Papoose Flat pluton in the White-Inyo Range, California, is one of the best examples of forcefully emplaced plutons within the crust, having country rocks surrounding it that deformed plastically without fracture (ductile deformation). I used the computer language Fortran to build pluton growth models and explore evolution of the pluton and its aureole. Instantaneous intrusion models and incremental intrusion models (continuous and episodic) were constructed. I explored how the frequency of magma input to the bottom of the pluton affects ductile width of the contact aureole, crystal distribution within the pluton, eruptibility of the magma chamber, and expulsion of the metamorphic fluid from the ductile aureole. The modeling results show that the ductile region above the Papoose Flat pluton is related to thermal weakening. The ductile region in the incremental emplacement models is ~110 m thick, matching the observed thickness. It is 10 times thinner than in the instantaneous growth model. The pluton remains hot and only partially crystalline at the bottom throughout incremental growth. When a pluton grows by lowfrequency but high-flux injections, the chamber overpressure can be as high as 105 MPa. Dike propagation will occur under this condition. Conversely, the growth of magma chambers is favored by high-frequency injections of small melt batches. Metamorphic fluid is likely to be expelled from ductile aureoles by waves of self-propagating, fluid-filled porosity. However, when the growth rate of a pluton is high, fluids can be squeezed out because of compaction imposed by the growing pluton from below.