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^(40)Ar/^(39)Ar and ^(18)O/^(16)O studies of the Chegem ash-flow caldera and the Eldjurta Granite: Cooling of two late Pliocene igneous bodies in the Greater Caucasus Mountains, Russia

Gazis, Carey A. and Lanphere, Marvin and Taylor, Hugh P., Jr. and Gurbanov, Anatoly (1995) ^(40)Ar/^(39)Ar and ^(18)O/^(16)O studies of the Chegem ash-flow caldera and the Eldjurta Granite: Cooling of two late Pliocene igneous bodies in the Greater Caucasus Mountains, Russia. Earth and Planetary Science Letters, 134 (3-4). pp. 377-391. ISSN 0012-821X. doi:10.1016/0012-821X(95)00141-X.

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Volcanic and intrusive rocks of the Chegem caldera and the nearby Eldjurta (Eldzhurtinskiy) Granite record a late Pliocene episode of silicic magmatism in the north-central Caucasus Mountains. Surface exposures, created by the recent rapid uplift and erosion of the Caucasus Mountains, span a 2 km vertical section of Chegem caldera fill and 1150 m of the Eldjurta Granite; cored mineral-exploration drillholes in the Eldjurta Granite extend the sampling to a depth of 4 km. The unique sampling range available in these two young igneous bodies affords an excellent opportunity to study their denudation and cooling histories, which we examine by means of ^(40)Ar/^(39)Ar and ^(18)O/^(16)O measurements on an extensive sample suite. Total-fusion biotite and sanidine ages from eight Chegem Tuff samples, both intracaldera and outflow, are analytically indistinguishable with a weighted mean of 2.82 ± 0.02 Ma. A cross-cutting granodiorite porphyry intrusion has a sanidine total fusion age of 2.84 ± 0.03 Ma, and whole-rock incremental heating of a post-caldera andesite flow, which caps the caldera fill, yields an age of 2.82 ± 0.02 Ma. Thus, caldera formation and post-caldera resurgence and volcanism all occurred within a very short time (< 50,000 yr). Biotite total-fusion ages of ten Eldjurta Granite samples, including seven from ∼ 500 m intervals in the 4 km deep drillhole, show a systematic linear decrease in age with depth from 1.90 Ma near the roof contact of the granite to 1.56 Ma at a depth of 3700 m. Assuming these ages were set at the same temperature, this age/depth gradient implies an isotherm migration rate of 13 mm/yr between 1.90 and 1.56 Ma. This migration rate is due to a combination of rapid denudation and downward relaxation of isotherms, with cooling rates between 200 and 500°C/Ma during this period. Oxygen isotopic compositions of quartz, K-feldspar, plagioclase and biotite from the drillhole samples below the 800 m depth are fairly uniform and record primary igneous δ^(18)O values with little evidence for subsolidus hydrothermal activity. However, in surface outcrop samples and in the shallowest drillhole sample, mineral δ^(18)O values have been lowered by up to 3‰ by interaction with an external (meteoric-hydrothermal?) fluid. The primary mineral δ^(18)O values of the Eldjurta Granite are distinctly higher than the corresponding phenocryst δ^(18)O values in the Chegem volcanic rocks, indicating that the two bodies evolved as separxate magma batches.

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Additional Information:© 1995 Elsevier Science B.V. Accepted 24 July 1995. We express our thanks to Peter Lipman and Ken Hon, who shared in discussions, fieldwork and sample collection. Reviews by Hans Lippolt, Denis Norton, John Obradovich, John Sutter and an anonymous reviewer greatly improved this paper. Logistical and field support provided by members of the Russian Academy of Sciences, especially Andrei Tsvetkov, Valentine Beus and Igor Nikolaev, was essential for this project and is greatly appreciated. We also thank Chris Lewis and Masha Bogina for sharing their unpublished microprobe data. This work was partially supported by NSF grant EAR90-19190 and forms Division of Geological and Planetary Sciences, California Institute of Technology contribution 5429. [MK]
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Caltech Division of Geological and Planetary Sciences5429
Issue or Number:3-4
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Official Citation:Carey A. Gazis, Marvin Lanphere, Hugh P. Taylor Jr., Anatoly Gurbanov, 40Ar/39Ar and 18O/16O studies of the Chegem ash-flow caldera and the Eldjurta Granite: Cooling of two late Pliocene igneous bodies in the Greater Caucasus Mountains, Russia, Earth and Planetary Science Letters, Volume 134, Issues 3–4, September 1995, Pages 377-391, ISSN 0012-821X, 10.1016/0012-821X(95)00141-X. (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:38358
Deposited By: Ruth Sustaita
Deposited On:08 May 2013 20:39
Last Modified:09 Nov 2021 23:37

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