INSIGHTS INTO THE ASCENT RATES OF MAGMAS BENEATH CONTINENTAL ARC SYSTEMS

Amphiboles within lava-hosted hornblendites (>90% hornblende) provide a window into the magmatic processes occurring beneath volcanic centers. Specifically, hornblende reaction rim widths within hornblendites have the potential to provide insights into the timescales of magmatic ascent. The sampled hornblendites in this study are hosted in andesitic-dacitic lavas from the Quillacas Plio-Pleistocene monogenetic volcanic center on the Bolivian Altiplano, Central Andes. They are characterized by ~90-95% hornblende, ~5-10% plagioclase feldspar, minor titanomagnetite, and trace apatite. Ubiquitously associated with the hornblende crystal population are gabbroic reaction rims which are composed of plagioclase feldspar, pyroxene and oxides. These reaction rims are interpreted to be re-equilibration textures associated with dehydration which formed in response to changes in pressure and temperature during their entrainment (magmatic ascent). The amphibole reaction rims were imaged and measured using a Zeiss Supra 35 VP Scanning Electron Microscope (SEM) at the Center for Advanced Microscopy and Imaging (CAMI), Miami University. This approach permitted detailed mineralogical and textural observations to be made at the (sub) micron scale. Based on 80 individual rim width measurements, widths ranged from 11.13μm to 84.9μm. The average recorded width was 27.25μm (±26.71μm) with ~⅔ of measured rims between 15-35μm in size. When this 27.25μm average is compared to the experimental results of Rutherford and Hill (1993), which modelled the ascent rates of magmas associated with the 1980 Mount St. Helens eruption, the ascent time of the lava-hosted Quillacas hornblendites is estimated to be between 5 and 15 days. The rim width measurements reported by this study will next be compared to hornblende rim widths from the host andesitic-dacitic lavas at Quillacas. The results of this study will also be used to compare pre-eruptive processes and magmatic ascent rates (as informed by amphibole reaction rims) to other continental arc magmatic systems.