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Gazis, C.A., 1995

An isotopic study of the fluid flow and thermal history of the 2.8 Ma Chegem ash-flow caldera and related intrusive rocks (Caucasus Mountains, Russia)

Bibliographic Reference

Gazis, C.A., 1995, An isotopic study of the fluid flow and thermal history of the 2.8 Ma Chegem ash-flow caldera and related intrusive rocks (Caucasus Mountains, Russia): Pasadena, California, California Institute of Technology, Ph.D. dissertation.


The 11 x 15 km Chegem ash-flow caldera (Caucasus Mountains, Russia) is only 2.8 Ma, but rapid uplift and erosion has exposed more than 2 km of caldera fill, consisting of densely welded tuff (rhyolitic to dacitic), overlain by andesite flows and cut by a granodiorite intrusion. The Eldjurta Granite, whose age and composition are similar to the Chegem rocks, is exposed 10 km away in an adjacent river valley. Major Mo-W deposits beside the granite have prompted mineral-exploration drillholes (to 4 km depth). An isotopic study of the Chegem caldera rocks and the Eldjurta Granite has been performed to examine their petrogenesis, fluid flow, and thermal histories. 40Ar/39Ar ages for both the Chegem Tuff and the granodiorite are analytically indistinguishable at 2.82 ± 0.02 Ma. 87Sr/86Sr ratios of unaltered Chegem igneous rocks (0.7044 to 0.7060) are significantly lower than values for surrounding country rock and correlate with compositional trends. Thus, the Chegem magmas were probably mantle-derived, and the magma chamber was both isotopically and compositionally zoned. In the Eldjurta Granite, a decrease in biotite ages from 1.90 ± 0.24 Ma near its roof to 0.83 ± 0.29 Ma at 3,970 m depth records an isotherm migration rate of 13 mm/yr, probably due to a combination of downward isotherm movement and regional uplift. The Eldjurta Granite has higher 87Sr/86Sr (0.7069), δ18O (quartz approximately 10.0), and δ18O (feldspar approximately 8.8) than the Chegem rocks, indicating that the two magmas evolved separately. Oxygen isotope studies of intracaldera Chegem Tuff reveal a caldera-wide stratigraphic horizon of δ18O-depleted rocks with extreme disequilibrium between phenocrysts and groundmass (sometimes still glassy). Quartz and feldspar phenocrysts have 'normal' igneous ?18O values of approximately 8.5 and approximately 7.0, respectively. Groundmass δ18O values are as low as -7.7. Pronounced disequilibrium between coexisting feldspar and groundmass/glass has never been observed before on this scale. The most likely δ18O-depletion process is a high-temperature (500-600 degrees C), short-lived (10–25 years) meteoric–hydrothermal event similar to that which occurred at the Valley of Ten Thousand Smokes, Alaska.

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