PETROLOGIC AND ISOTOPIC INVESTIGATION OF RHYOLITES FROM KARYMSHINA CALDERA, THE LARGEST “SUPER”CALDERA IN KAMCHATKA, RUSSIA

The 1.78 Ma Karymshina River large (25x15 km) silicic center in east-central Kamchatka is represented by ~1000m thick layer of intracaldera low and high-silica rhyolitic ignimbrite, and a dozen of post-caldera (1.7-0.5 Ma) extrusive domes of high to low- silica composition that surround this ignimbrite along the inferred ring fracture. The caldera size and the inferred ignimbrite volume of 400-800 km3 would define it as the largest caldera in Kamchatka. The presently uplifted and eroded caldera floor exposes only intracaldera ignimbrites, and the search for extracaldera ignimbrites is currently underway and we will present results of this search. Biotite crystals from both samples yielded identical ages of 1.78±0.02 Ma (MSWD=0.51), while zircon crystals from the top sample yielded a U-Pb crystallization age of 1.87±0.11 Ma (MSDW = 0.32, n=9), with no evidence of inheritance. It appears that the 1000 m thick sequence of intracaldera ignimbrites were formed in the course of a single eruption or, pending better dating and geological study, a rapidly emplaced sequence of compositionally and mineralogically similar ignimbrites, beyond analytical resolution of Ar-Ar geochronology. Oxygen isotopic investigation of Karymshina ignimbrite and postcaldera intrusions yielded nearly identical normal to slightly high-d¹⁸O values for all postcaldera extrusions (6.4±0.2‰, Fsp, 7.3±0.25‰ Qz), while ignimbrite display somewhat larger range (5.7-6.5‰, Fsp, 7.3-8.3‰ Qz). Sr and Nd isotopic values in 4 ignimbrite samples 0.703317 – 0.703328, and 0.513015-0.513065) also signify a geochemically homogeneous source without any significant involvement of hydrothermally-altered crust. Major and trace element geochemistry of these rhyolites indicate simple fractionation relationships, but crustal melting is required by O isotopic values. We present a model of formation of a homogeneous magma source.