Late Jurassic High-Pressure Metamorphism of Variscan I-Type Granitoids in the Northern Part of the Pelagonian Unit (Republic of North Macedonia).

This article focuses on the high-pressure/high-temperature (high-P/T) metamorphism of Variscan I-type granitoids within the Pelagonian Unit (PU) in the Republic of North Macedonia (NMK). The PU comprises a Variscan basement intruded by I-type granitoids, a metamorphosed sedimentary sequence from the Permian to Lower Triassic, and carbonate marbles from the Late Triassic to Middle Jurassic, all subjected to Late Jurassic high-P/T metamorphism at pressures of 1.3–1.5 GPa and temperatures of 560–590 °C. Detailed petrographic, mineralogical, and geochemical analyses reveal that the original igneous mineral assemblages were extensively transformed during metamorphism, including plagioclase converting to Na-rich varieties with epidote and phengite, alkali feldspar altering to orthoclase or microcline, and biotite reacting with plagioclase to form garnet, titanite, and other metamorphic minerals. Equilibrium phase diagram calculations indicate that these metamorphic assemblages are stable under H2O-undersaturated conditions. The study suggests that the northern PU underwent subduction beneath the eastern Vardar Ocean during the Late Jurassic, with subsequent uplift and tectonic processes influencing regional geology, contrasting with younger metamorphic events observed further south in Greece.