Identification of discrete low-temperature thermal events in polymetamorphic basement rocks using high spatial resolution FE-SEM-EDX U-Th-Pb dating of uraninite microcrystals

Abstract

Low-temperature thermal events of Permian (ca. 265 Ma) and Triassic (ca. 215 Ma) age that predate medium-grade regional metamorphism were identified using high spatial resolution field emission-scanning electron microscopy-energy dispersive X-ray (FE-SEM-EDX) U-Th-Pb dating of uraninite microcrystals in basement rocks of the Tauern Window, Eastern Alps. Three novel points of generic geochronological importance are raised in this study. First, uraninite can be meaningfully dated with FE-SEM-EDX methods, with moderate precision. Second, uraninite is geochronologically robust, even at microcrystal scale, and can survive at least medium-grade metamorphic overprint without being reset. Third, uraninite microcrystals are powerful tools for identifying and dating discrete low-temperature thermal events in orogenic belts. Dating of uraninite microcrystals should be considered an important complementary geochronological method in the study of polymetamorphic rocks.