The Zirconometry and Thermochronology of Migmatized Gneisses of the Gondaray Metamorphic Complex (Greater Caucasus)

摘要

The results of the isotope dating of the crystalline basement of the Greater Caucasus (the Elbrus subzone of the Main Caucasus Range) within of the high-temperature gneiss-migmatite area of the Gondaray metamorphic complex is discussed. The relationships between regional metamorphism, migmatization, and heat flows in the lithosphere are considered. The measurements of the zircon isotope compositions were performed on a SHRIMP-II ion microprobe at the CIR VSEGEI (St. Petersburg). All zircon crystals from gneiss sample 526 have a zonal structure with ancient detrital cores. Almost all U-Pb isotope dating points lie on the concordant line and show a wide age range from 320-1000 Ma, partially inherited from detrital zircon grains from primary pelitic sediments. The youngest ages (320 Ma) were measured in the regeneration zones of zircon grains recrystallized during stage of the anatexis and migmatization. The other points of detrital zircons form an age cluster of 540-1000 Ma, which indicates their origin from different magmatic sources that existed during the formation of the protometamorphic protolith. Several detrital zircon grains yielded a Cambrian age; this is evidence for an Early Paleozoic age of the metamorphic protolith, which was traditionally suggested to be Precambrian or even Archean. The ages of the rims of recrystallized zircons (320 Ma) have a direct correlation with the postmetamorphic granitoid ages of the Greater Caucasus. The thermochronological modeling of the transformation of the Gondaray metamorphic complex during the retrogressive stage of metamorphism demonstrates that its cooling from the temperature of migmatite crystallization (650 degrees C) to the closure of the temperature of the K-Ar biotite isotope system (350 degrees C) was relatively rapid (the rate of cooling was 8-10 degrees C/Ma) under subisobaric conditions and during a time range approximately 30-40 Ma.