Low-temperature thermal evolution of the Azov Massif (Ukrainian Shield-Ukraine) - Implications for interpreting (U-Th)/He and fission track ages from cratons

摘要

The low-temperature thermal evolution of the Azov Massif (eastern part of the Ukrainian Shield, Ukraine) is investigated by combined zircon fission track (ZFT), apatite fission track (AFT) and apatite (U-Th)/He (AHe) thermochronology. The data help to better understand the geodynamic evolution of the Azov Massif and the adjacent intro-cratonic rift basin (Dniepr-Donets Basin) as follows: ZFT data reveal that the Precambrian crystalline basement of the Azov Massif was heated to temperatures close to similar to 240 degrees C during the Late Palaeozoic. The heating event is interpreted in terms of burial of the basement beneath a several kilometres thick pile of Devonian and Carboniferous sedimentary deposits of the adjacent Dniepr-Donets Basin. During Permo-Triassic times, large parts of the basement were affected by a thermal event related to mantle upwelling, associated magmatic activity and increased heat flow in the adjacent rift. The major part of the basement cooled to near-surface conditions in the Early to Middle Triassic and since then was thermally stable as suggested by AFT and AHe data. Further, AFT data confirm Late Triassic magmatic activity in the Azov Massif, which, however, did not influence regional thermal pattern. The northern part of the basement and its sedimentary cover record a cooling event in the Jurassic, which was probably related to erosion. However, although Ar-Ar data of Jurassic magmatic activity in the Donbas Foldbelt are about 20 My younger than the AFT data, thermal relaxation after elevated heat flow associated with this magmatic event cannot be completely ruled out. Our results reveal apparent inconsistencies between AFT and AHe data: the AHe ages corrected for alpha ejection according to the standard procedure [Farley, KA., Wolf. RA, Silver, LT., 1996. The effect of long alpha-stopping distances on (U-Th)/He ages. Geochim. Cosmochim. Acta 60(21),4223-4229.; Farley, K.A., 2002. (U-Th)/He dating: Techniques, calibrations, and applications. Mineral. Soc. Am. Rev. Mineral. Geochem. 47, 819-8441 are older than corresponding AFT ages. In order to test the relevance of alpha ejection correction, mechanical abrasion of several apatite grains was applied. We found that in case of relatively slow cooling, alpha ejection correction of raw (U-Th)/He ages leads to erroneously high ages. This shows that alpha ejection correction should be used with caution. We propose that this correction should only be applied to samples with a fast cooling history, whereas it should not be applied to slowly cooled samples and to samples whose thermal history is not constrained by other means.