PLAGIOCLASE STRATIGRAPHY AS A GUIDE TO SEPARATING LAVA FLOWS OF THE TIETON ANDESITE, AND MAGMA EVOLUTION

The Tieton andesite lava flow erupted in two phases from the NE flank of Goat Rocks Volcano located in the central Washington Cascades and terminated ~2 km NW of Yakima, WA. The older ~74 km long flow has 61%-63% SiO₂, is <60 m thick, and has an ⁴⁰Ar/³⁹Ar date of 1.64 +- 0.07 my. The second flow (52 km in length) has 59%-61% SiO₂, and an ⁴⁰Ar/³⁹Ar of 1.39 +- 0.10 my. The two large flows provide an opportunity to evaluate what factors contributed to the length of the flows. In addition, plagioclase zoning provides a detailed record of the conditions leading up to an andesitic eruption. Details of plagioclase zoning and chemistry were compared and used for correlation of the composite Tieton andesite (TA) lava flow. Initial plagioclase micro-textures from the TA appear to be grouped according to eruptive events, in which the textures record equilibrium at crystal-melt interface as a result of fluctuations in TºC, P, PH2O, or composition. Morphological textures record influences in dynamic behavior of the magma such as, turbulence, convection, and degassing. Compositional profiles across plagioclase show little variation in An content because of oscillatory zoning. However, cores are richer in An than rims. The eruptive phases appear to be divisible by their plagioclase micro-textures. The Naches Heights plagioclase have spongy cores with diffuse oscillatory rims and spongy to coarse sieve textures near the rims and patchy cores. While spongy borders with a thin clear rim overgrowth characterize the Pinegrass Ridge, patchy-cores with a border of mineral inclusions and a rim of diffuse oscillatory zoning. The magma has undergone extensive fractional crystallization at depth followed by ascent to a shallow chamber where mixing occurs between crystal-rich melts and possible crystal mushes. Repeated recharge events and recycling processes produce the various plagioclase crystal populations with different micro-textural stratigraphy. The use of plagioclase zoning stratigraphy has allowed for the separation of lava flows and can be used to constrain the conditions and eruptive events of andesitic lava flows.