A new method for the estimation of cooling and denudation rates using paramagnetic centers in quartz: a case study on the Eldzhurtinskiy Granite, Caucasus

We present a new method for the assessment of the most recent cooling and denudation rates using paramagnetic centers in quartz measured by electron spin resonance (ESR) spectroscopy. These centers have a relatively low thermal stability. For cooling rates of 40° and 1000°C Myr−1, effective closure temperatures vary between 55° and 82°C (Ti center) and 49° and 64°C (Al center), respectively. Samples were collected from two cores that were drilled into the Eldzhurtinskiy Granite, which has an emplacement age of ∼2 Ma as measured by U/Pb analyses of zircons. One 1500 m core was taken from a drill hole into the dome of the granite, a second core of 4000 m from a drill hole at the base of the Baksan Valley. Our results yield cooling rates of between 160 and 250°C Myr−1 for the upper core and between 570° and 600°C Myr−1 for the lower core; the corresponding denudation rates are ∼2.5 (upper core) and 5.5mm a−1 (lower core). The shape of the temperature profile of the lower core indicates recent erosion. When fitting the temperature data with a two-dimensional heat-transfer model, we obtain a net denudation rate of ∼10 mm a−1 and cooling rates in the range of 500°C Myr−1, thus confirming the cooling rates estimated by ESR. However, the ESR denudation rates underestimate the erosion rate of the Baksan Valley because the geothermal gradient is not equilibrated between the surface and the depth of the annihilation temperatures, 950 and 1800 m for the Al and Ti centers, respectively. We conclude that ESR measurements of paramagnetic centers in quartz will allow the reconstruction of landscape dynamics for the past 10–1000 kyr and that in conjunction with U/Pb, fission track, and Ar/Ar analyses it will be possible to develop dynamic models for Quaternary tectonic movements

^(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

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

Elbrusite-(Zr)-A new uranian garnet from the Upper Chegem caldera, Kabardino-Balkaria, Northern Caucasus, Russia

. Elbrusite-(Zr) is radioactive and nearly completely metamict. The calculated cumulative dose (α-decay events/mg) of the studied garnets varies from 2.50 × 10 14 [is equivalent to 0.04 displacement per atom (dpa)] for uranian kimzeyite (3.36 wt% UO 3 ), up to 2.05 × 10 15 (0.40 dpa) for elbrusite-(Zr) with 27.09 wt% UO 3