Geochemistry and mineralogy of Rotliegend metavolcanic mafic rocks from Poland: pervasive low-grade metamorphism versus parent rock signature

Abstract

AbstractPermian volcanic rocks from the Gorzów Wielkopolski region (NW Poland), although pervasively altered by low-grade metamorphism, still preserve the geochemical characteristics of continental mafic volcanic rocks formed by partial melting of an enriched mantle source. The metamorphic assemblage comprises corrensite, pumpellyite, laumontite, quartz and chalcedony, albite, calcite and solid bitumen (major components). Petrological studies combined with microthermometric determinations indicate a low-pressure zeolite-greenschist-facies transitional zone metamorphic grade with a clockwise (pressure-temperature) P-T path: earliest event 140–210 °C and 630–760 bar; metamorphic peak 220–300 °C and 950 bar; youngest episode: T ⩾ 130 °C and 630–760 bar.The metamorphism of the Rotliegend volcanic rocks is generally ascribed to penetration of upwelling fluids released from clastic rocks underlying the extrusive Permian unit. However, the ubiquitous occurrence of anhydrite in the altered volcanic rocks suggests an influence of pore water from the overlying Zechstein evaporite sequence. The source of metamorphic heat can be tentatively assigned to abnormal heat flow and/or exothermic reactions during magmatic mineral alteration processes. Dating of metamorphism in neighbouring areas suggests an Upper Jurassic thermal event related to the upwelling of a mantle diapir during the initiation and early evolution of the North Atlantic rift.