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)

• Main Author: Gazis, Carey Alice
• Format: Others
• Language: en; en; en; en
• Published: 1995
• Online Access: https://thesis.library.caltech.edu/4375/1/Gazis_ca_1995.pdf; https://thesis.library.caltech.edu/4375/2/Gazis_ca_1995_plate_1.PDF; https://thesis.library.caltech.edu/4375/3/Gazis_ca_1995_plate_2.PDF; https://thesis.library.caltech.edu/4375/4/Gazis_ca_1995_plate_3.PDF; Gazis, Carey Alice (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). Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/mjb1-6r66. https://resolver.caltech.edu/CaltechETD:etd-11022007-110943 <https://resolver.caltech.edu/CaltechETD:etd-11022007-110943>

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The 2.8 Ma Chegem caldera, an 11x15 km ash-flow caldera located in the Caucasus Mountains, presents a unique opportunity to study silicic magma systems because of its combination of youth, exposure and simplicity. Rapid uplift and erosion in the region has exposed over 2 km of flatlying caldera fill, consisting of densely welded tuff (rhyolitic to dacitic), overlain by glacial deposits and andesite flows and cut by a granodiorite porphyry intrusion. The Eldjurta Granite, whose age and composition are similar to the Chegem volcanics, is exposed in an adjacent river valley 10 km to the northeast. Major Mo-W deposits located in nearby skarns have prompted mineral-exploration drillholes (to 4 km depth) in the granite. An isotopic study of the rocks of the Chegem caldera and the Eldjurta Granite has been performed to examine their petrogenesis, fluid flow and thermal histories. Biotite and sanidine [...] ages for 8 Chegem Tuff samples and the granodiorite intrusion are analytically indistinguishable at 2.82 ± 0.02 Ma. Thus, the Chegem Tuff was erupted, cooled and intruded by the granodiorite within < 50,000 yrs. In the nearby Eldjurta Granite, biotite and K-feldspar [...] ages for 11 samples, including 8 from the deep drillhole (to 3970m depth) yield ages between 0.83 ± 0.29 Ma and 2.78 ± 0.09 Ma. A decrease in biotite ages from 1.90 ± 0.24 Ma near the roof of the granite to 0.83 ± 0.29 Ma at 3970m depth, apparently records the uplift and cooling history of this pluton. The ages of the upper 10 samples imply an isotherm migration rate of 13 mm/yr, probably due to a combination of downward migration of isotherms and regional uplift. Oxygen isotope studies of the intracaldera tuff, including 38 samples from a continuous 1405m-stratigraphic section, reveal a striking caldera-wide stratigraphic horizon of [...]-depleted rocks in which there is extreme disequilibrium between phenocrysts and groundmass (sometimes still glassy). All quartz and feldspar phenocrysts have "normal" igneous [...] values of [...]8.5 and [...]7.0, respectively. Whole-rock and groundmass [...] values are as low as -4.0 and -7.7, respectively. Infrared spectroscopic analyses of glassy pumices reveal that they contain 3.3 to 4.8 wt% water. The [...] and water speciation of these glasses reflects low-temperature hydration by meteoric water, whereas some of their [...] values require higher temperature water-glass interaction. Pronounced disequilibrium between coexisting feldspar and groundmass or glass has never been observed before on this scale. It requires a hydrothermal event involving large amounts of low-[...] H2O at sufficiently high temperatures and short enough time that glass exchanges thoroughly but feldspar does not. The most likely process responsible for the [...] depletions at Chegem is a high-temperature (500-600°C), short-lived (10-25 years), vigorous meteoric-hydrothermal event similar to that which occurred at the Valley of Ten Thousand Smokes, Alaska. Mass balance calculations indicate fluid fluxes of [...] mol/[...]-sec over that time period. Sr isotopic studies reveal that this hydrothermal event caused increases in whole-rock [...], possibly because the hydrothermal waters attained radiogenic Sr from crystalline rocks which were incorporated in the caldera fill during caldera collapse. [...] ratios of unaltered Chegem volcanic and intrusive rocks range from 0.7044 to 0.7060, significantly lower than values for surrounding country rock (0.7070 to 0.7319). Thus, the Chegem magmas were probably derived from the mantle or lower crust. The [...] ratios correlate with major- and trace-element trends, indicating that the Chegem magma chamber was both isotopically and compositionally zoned. Compared to the Chegem rocks, the Eldjurta Granite has higher [...] (0.7069), [...] and [...], and so must have evolved separately at some time.