Spilitization of Early-Permian Volcanics from Głuszyca Górna (the Intra-Sudetic Basin, Poland)—Constraints from Chlorite Thermometry Coupled with Apatite Fission-Track Dating (AFT) ^†

The Intra-Sudetic Basin, a Late-Paleozoic intramontane trough located on the NE flank of the Bohemian Massif, is comprised of numerous outcrops of continental (extension-related) Early-Permian volcanogenic rocks that are commonly altered to spilites. In this contribution, we provide insights into the formation of spilitized (albite- and chlorite-rich) trachyandesites from the Głuszyca quarry (Lower Silesia, Poland), based on mineralogical and micro-textural investigations supported by apatite fission-track dating (AFT). Our results indicate that the trachyandesites, emplaced as a shallow-level laccolith-type body, have been strongly affected by chloritization of both aegirine and augite, combined with an occasional celadonitization of volcanic glass. Furthermore, chlortitization of sodic pyroxenes must have released notable amounts of Na⁺, which could be involved during later pervasive albitzation of primary andesine-labradorite. According to various chemical and semi-empirical thermometers, the replacive chlorites formed in the range of 124–170 °C. Trachyandesites from Głuszyca contain abundant fluorapatites, marked by the occurrence of swallow-type terminations, which are indicative of rapid-cooling formation conditions. Central AFT ages of the samples vary between 161–182 Ma and correspond to the Middle-Jurassic period. Meanwhile, these ages are significantly younger than the emplacement of igneous rocks during the Middle-Rotliegendes period (~299–271 Ma). The discrepancy between the stratigraphic age of the rocks and the AFT results cannot be, however, explained by, for example, slow cooling rates of magmatic body, compositional variations of apatite, or burial under Late-Mesozoic sediments. Hence, it may be assumed that the obtained AFT ages (161–182 Ma) reflect the timing of spilitization and associated partial reheating of volcanic rocks from the Intra-Sudetic Basin above the apatite partial annealing zone (70–110 °C).