Fayalite rhyolites and a zoned magma chamber of the Paleocene Yakutinskaya volcanic depression in Primorye, Russia

Abstract

During the Late Paleocene, at least five ignimbrite units were emplaced from the Yakutinskaya caldera complex in Primorye, Russia. The erupted ignimbrites show two distinct chemical cycles, believed to represent the “high”-silica and “low”-silica parts of the compositionally zoned magma chamber. Two petrographically distinctive types of rhyolites are distinguishable in each chemical cycle, based on their phenocryst chemistry and silica content: (1) “low”-silica rhyolites with mineral assemblages of quartz, sanidine, plagioclase, ferrohypersthene, ferroaugite (Ca₄₁Mg₂₁Fe₃₈), biotite, and hornblende, and (2) “high”-silica rhyolites with a similar mineral assemblage to “low”-silica, but containing more Fe-rich clinopyroxene (Ca₄₄Mg₂Fe₅₄) and biotite, and a with lower phenocryst abundance. This difference is related to the variation in chemical composition and temperature of the magma in the zoned magma chamber for each eruption cycle. Rb-Sr mineral-rock isochron ages show that the ignimbrites erupted between 59.7 ± 1.6 and 54.8 ± 2.6 (2σ) Ma (Late Paleocene), and initial ⁸⁷Sr/⁸⁶Sr ratios are distinct in the different ignimbrite units. The “high”-silica rhyolites show the highest ⁸⁷Sr/⁸⁶Sr_I ratios (0.70810-0.70738), whereas “low”-silica rhyolites show lower ⁸⁷Sr/⁸⁶Sr_I ratios (0.70659-0.70724). The compositional zoning of the single magma chamber can be explained by the large-scale mass transport in the liquid phase due to roofward migration and concentration of volatile species.