Low-grade metamorphism of permian mafic rocks from the Gorzów Wielkopolski block (Fore Sudetic Monocline, NW Poland): age and mechanism

Themetavolcanic rocks in the Gorzów Wielkopolski area (NWPoland) are andesite-basaltsand andesites derived from contaminated within-plate subalkaline basalt melts. K-Ar dating of primaryK-feldspar yields an extrusion age of 285±5 Ma. This new date suggests that these rocksmay be the youngestdated Permo-Carboniferous volcanic rocks within the eastern margin of theNorth German Basin.Petrological and micro-thermometric data indicate that the volcanic rocks were altered by a sub--greenschist facies, metamorphic process that proceeded within temperatures and pressures ranging from195–290°C and 63–96 MPa, respectively. The major metamorphic phases, corrensite, pumpellyite, laumontiteand prehnite, formed due to interaction between the original volcanic rock and low-saline Na2SO4-richhydrothermal fluids.Numericalmodelling indicates fluid flow in fractures as themost probable explanationfor the low-grade hydrothermal metamorphism. Model calculations suggest that the process was of shortduration, ca 140–250 years. K-Ar dating reveals three Mesozoic episodes at 121±2 Ma, 188–190 Ma and149±4 Ma that can be ascribed to the metamorphism. These ages are similar to previously determinedMesozoic ages from altered volcanic- and clastic rocks in the North German Basin and in the Polish Basin.