Isotopic and petrologic investigation and model of genesis of large-volume high-silica rhyolites in arc environments: Karymshina Caldera, Kamchatka, Russia

Large-volume calderas are responsible for producing large deposits of rhyolite and high-silica rhyolite, but the mechanisms by which these deposits are produced are still poorly understood. The Kamchatka Peninsula of Russia contains several large calderas and is one of the most volcanically active areas on Earth. Karymshina Caldera, the largest (25 km x 15 km) caldera in Kamchatka, produced an estimated 800 km3 of high-silica rhyolitic ignimbrites and post-caldera extrusions, which erupted 1.78 and 0.5--0.8 Ma, respectively.;SiO2 content ranges from 66.27--71.89 wt% in the ignimbrites and 70.16--77.70 wt% in the post-caldera extrusions studied. Crystal content is primarily quartz and plagioclase, 0.5--2 mm in size, with other minerals. Values of delta18O, deltaD, 87 Sr/86Sr, and 144Nd/143Nd indicate little assimilation of crustal material, in contrast to modeling results. XRF analysis indicates a homogeneous magma. The rhyolite-MELTS program was used to model crystallization of a basaltic source with addition of amphibolite partial melt and hydrothermally-altered silicic rock to reproduce the observed compositions.;This thesis contains both previously published and co-authored material.