THE COOLING HISTORY OF THE LATE PLIOCENE ELDZHURTINSKIY GRANITE (CAUCASUS, RUSSIA) AND THE THERMOCHRONOLOGICAL POTENTIAL OF GRAIN-SIZE AGE RELATIONSHIPS

Isotopic age investigations have been carried out on samples from the late Pliocene Eldzhurtinskiy granite (Great Caucasus, Russia) in order to elucidate the cooling history of this kilometere sized plutonic bo dy. The samples were taken from a horizontal profile (from the edge to the centre of the granite) and from a vertical profile (from the top to a depth of 4570 m). The Ar-40/Ar-39 results on biotites ranging fro m 2.5 to 1.2 Ma indicate a vertical age zoning of the granite caused b y regional uplift. An uplift rate of 4 mm/a is calculated from the age differences. Together with the results of fission track dating on apa tite and zircon and the present-day rock temperatures the biotite ages can be used to evaluate cooling rates below 400-degrees-C. The coolin g rates of about 180-degrees-C/Ma are relatively high and spatially an d temporarily fairly uniform throughout the granite. A geothermal grad ient of 45-degrees-C/km is derived from uplift and cooling rates; this is identical to the present-day borehole gradient. The Ar-40/Ar-39 ag es of distinct biotite grain fractions from one and the same sample sp ecimen are discordant. Generally, they show clear age/grain-size corre lations, as expected from Ar diffusion theory for cooling systems. Thu s, the biotite results support the additional detailed thermochronolog ical information. The cooling rates in the temperature range of biotit e closure, based on the grain-size/age correlations of the biotites, a re close to 180-degrees-C/Ma. They provide evidence that within a cert ain temperature interval the time-temperature path of a rock may be de termined exclusively by isotopic dating of different grain-size fracti ons of only one type of mineral.