DEFINING THE NUCLEATION AND GROWTH DYNAMICS OF DIVERSE GLOMEROPORPHYRITIC CLOTS FROM BLUE ROCK, OREGON

Only limited petrological studies have been completed on the minor stratovolcanoes and peripheral flows found throughout the Oregon Cascades. Among these cones is Blue Rock, in Jackson County, Oregon. This basaltic-andesite has a diagnostic glomeroporphyritic texture, with clots ranging in size from two to seven millimeters in diameter. The mineralogy and growth patterns exhibited by these clots are variable. Plagioclase, pyroxene, and olivine are each found forming both cores and rims of these clots. Some clots also show a simultaneous rather than sequential growth of phases. Lavas erupted from Blue Rock also contain both ilmenite and titanomagnetite. The two Fe-Ti oxides permit pre-eruptive magmatic temperatures and oxidation states to be calculated (Ghiorso and Evans, 2008), which in turn allow the application of the plagioclase-liquid hygrometer (Lange et al., 2009). Applying the water concentrations derived from this model, the magma viscosity may be determined via the model of Hui and Zhang (2007). Magma chamber heterogeneities and residence times, along with potential ascent rates, may also be defined, based upon the petrographic characteristics of the glomeroporphyritic clots and their constituent minerals.