Seventh Annual V. M. Goldschmidt Conference 2260.Pdf

Seventh Annual V. M. Goldschmidt Conference 2260.pdf

GEOCHEMICAL AND ISOTOP EVOLUTION OF HYDROTHERMAL- MAGMATIC SYSTEMS OF THE UKSICHAN VOLCANO (KAMCHATKA ISLAND ARC SYSTEM). A. B. Perepelov, V. Yu. Prokofiev and S. I. Dril'. Vinogradov Institute of Geochemistry, Russian Academy Sciences ( Favorsky Str., 1A, 664033, Irkutsk , Russia; re- [email protected]).

Main types of fresh and hydrothermally altered rocks Petrochemical and PT studies showed that high-K and of the Uksichan volcano, one largest(50 km in diameter) subalkalic shoshonite-latite rocks of the volcano were Pliocene-Quaternary volcanic center of the Kamchatka produced by fractional crystallization of high-K aluminous island arc system, were studied in order to identify com- basaltic primary magmas. Crystallization point for plagio- positions of the primary melts, to ana-lyze magma differ- clase was found out to decrease from basalts (1200°C) to entiation from data on melt inclusi-ons, and to investigate rhyodacites (960°C) in the basement and from 1240°C in features of hydrothermal pro-cesses in the caldera on the Uksichan basalts to 920°C in dacites of its extrusions, the basis of data on pressure, temperatures, geochemistry and latter exhibiting elevated water contents. The fluid pres- isotopy. sure in the dacite melt from the central extrusion was es- 56 The Uksichan volcano is timated to be within 2.5-1.9 kbar under 780°C and its Pacific located in the back vol-canic water content to range from 4.3 to 1.3 wt%. The presence Ocean belt of Kamchatka, in the of water is possibly due to accumulation of volatiles dur- Sredinny Range (Fig.1). The ing crystallization. geological structure of the Studies of inclusions in hydrothermal quartz and cal- 156 volcano comprises the Middle cite showed that they proceeded from chloride-magnesian- 2 Uksichan v. Miocene (?)-Pliocene ( N1 - sodium water solutions, usually involved into heterogeni- N2 ) volcanogenic basement, zation (boiling) processes, with salt concentrations of 3.5 Okhotsk the Upper Pliocene- Middle- to 0.2 wt%, in NaCl equivalent, under temperature ranges Sea 3 Upper Pleistocene (N2 - of 275 to 150°C and pres-sures between 55 and 8 bar. The -volcanic belts Q2(Q3?)) edifice of the vol- Au-Ag mineralizati-on of the basement originated from cano proper, and an Upper solutions with salt concentrations of 3.5 to 1.4 wt%, in Pleistocene-Holocene (Q3-Q4) assemblage of small shield NaCl equivalent and under temperatures from 275 to volcanoes and cinder-lava cones. The basement rocks are 175°C and pressures from 55 to 9 bar, whereas the same strongly affected by hydrothermal alteration; hydrotherms parameters for caldera metasomatites show to be consid- are widely distributed at pre-sent near basement expo- erably lower: salt concentrations of 2.1 to 0.2 wt%, in sures. The volcano has gone through three stages of evolu- 3 NaCl equivalent, temperatures between 220 and 150 °C tion. The first, stratovolcano stage (N2 -Q1), involved for- and pressures within 20 to 8 bar. mation of high-K basalt lavas, high-K basaltic andesites Markedly higher 87Sr/86Sr ratios in the altered rocks and andesites. The second stage, that of shield volcano with respect to the fresh Uksichan volcanics (0.70325 - (Q1-Q2), was associated with voluminous shoshonite and 0.70767 against 0.7032 -0.70343) attest to the fact that the latite lavas and their ignimbrites. The third, caldera sea water may have played a meaningful role at the be- stage(Q2-Q3?), was marked by eruptions of quartz latites ginning of evolution of the hydrothermal system. The pre- and their ignimbrites, accompanied by collapse of the sent day mineral waters in the Uksichan caldera and hot central block of the edifice on a system of concentric springs around the volcano have formed uniquely due to faults; thereby a complex of high-K andesites, latites, low mineralized meteoric water. high-K dacites, trachydacites and trachyrhyodacites came The studies were supported by the Russian Scence to existence in the caldera center. The final sta-ge of the Foundation, grant N 95-05-14358. magmatic activity was followed by hydrothermal rework of the caldera rocks with zeolitization, alunitization, sul- phidization, propylitization and quartzitization on high-K andesites and latites. Chloride-sulfate 7 to 65°C hot springs in the Uksichan river valley and the associated epithermal Au - Ag mineralization of the basement showed no direct geochemical relationship with the magmatic system of the volcano whereas the mineral springs in the caldera were related to the evolution of the volcanic center. Inclusions of siliceous melts in plagioclase from basement, volcano itself and caldera magmatics were studied separately by air-hardening with subsequent X-ray micro- probing “Superprob-733” for compositi-ons of the yielded glass of the largest homogenous inclusions. In the quartz and calcite of quartz-carbo-nate veins, investigated were hydrotherm-derived in-clusions.