OXYGEN ISOTOPIC AND GEOCHEMICAL EVIDENCE FOR A SHORT-LIVED, HIGH-TEMPERATURE HYDROTHERMAL EVENT IN THE CHEGEM CALDERA, CAUCASUS MOUNTAINS, RUSSIA

Within the 2.8 Ma Chegem ash-flow caldera(11 x 15 km), a single coolin g unit of rhyolitic to dacitic welded tuff more than 2 km thick is exp osed in deep valleys incised during recent rapid uplift of the Caucasu s Mountains. The intracaldera tuff is mineralogically fresh and unalte red, and is overlain by andesite lavas and cut by a resurgent granodio rite intrusion. Major- and trace-element compositions for a 1405-m str atigraphic section of intracaldera tuff display trends of upwardly inc reasing Na2O, CaO, Al2O3, total Fe, MgO, TiO2, Sr and Zr and decreasin g SiO2, K2O and Rb. This mafic-upward zoning (from 76.1 to 69.9 % SiO2 ) reflects an inverted view of the upper part of the source magma cham ber. Oxygen isotope studies of 35 samples from this 1405-m section def ine a striking profile with ''normal'' igneous delta(18)O values (+ 7. 0 to + 8.5) in the lower 600 m of tuff, much lower delta(18)O values ( - 4.0 to + 4.3) in a 700-m zone above that and a shift to high delta(1 8)O values (+ 4.4 to + 10.9) in the upper 100 m of caldera-fill exposu re. Data from two other partial stratigraphic sections indicate that t hese oxygen isotope systematics are probably a caldera-wide phenomenon . Quartz and feldspar phenocrysts everywhere have ''normal'' igneous d elta(18)O values of about + 8.5 and + 7.5, respectively, whereas groun dmass and glass delta(18)O values range from - 7.7 to + 12.3. Conseque ntly, the delta(18)O values of coexisting feldspar, groundmass and gla ss form a steep array in a plot of delta(feldspar) vs. delta(groundmas s/glass). Such pronounced disequilibrium between coexisting feldspar a nd groundmass or glass has never before been observed on this scale. I t requires a hydrothermal event involving large amounts of low-O-18 H2 O at sufficiently high temperatures and short enough time (tens of yea rs or less) that glass exchanges thoroughly but feldspar does not. The most likely process responsible for the O-18 depletions at Chegem is a very high temperature (500-600 degrees C), short-lived, vigorous met eoric-hydrothermal event that was focused within the upper 750 m of in tracaldera tuff. Mass balance calculations indicate fluid fluxes of ap proximate to 6 x 10(-6) mol cm(-6) s(-1). We believe that the closest historical analogue to this Chegem hydrothermal event is the situation observed in the Valley of Ten Thousand Smokes (Alaska, USA), where hu ndreds of steam fumaroles with measured temperatures as high as 645 de grees C persisted for 10 to 15 years in the much smaller welded ash-fl ow tuff sheet (approximate to 200 m thick) produced by the 1912 Katmai eruption.