Pumpellyit w zmienionych zasadowych skałach wulkanicznych z okolic Gorzowa Wielkopolskiego (północno-zachodnia część monokliny przedsudeckiej): przejawy metamorfizmu bardzo niskiego stopnia (VLGM)

Elżbieta Dubińska, Bogusław Bagiński, Grzegorz Kaproń, Paweł Bylina

Abstrakt


PUMPEILYITE IN ALTERED VOLCANIC BASIC ROCKS FROM THE REGION OF GORZÓW WIELKOPOLSKI (NORTH-WEST PART OF FORE SUDETIC MONOCLINE): EVIDENCES OF VERY LOW GRADE METAMORPHISM (VLGM)

Summary
Rotliegend metavolcanic rocks in the Gorzów Wielkopolski region (NW part of the Fore Sudetic Monocline) overlie the Paleozoic basement and are covered by the Zechstein evaporitic sequence as well as the Mesozoic sequence. The metavolcanic series provides the first documentation that very low grade metamorphism (VLGM) affected Rotliegend volcanic rocks from the Fore Sudetic Monocline. Up to the present assemblages including chlorite, celadonite, carbonates, quartz, albite, adularia, anhydrite, zeolites, barite, etc. were tentatively interpreted as products of a post-volcanic hydrothermal activity. The altered volcanic rocks typically contain no relict mineraIs; scarce relict clinopyroxene, ilmenite and Cr-spinel were identified only in vesiclepoor varieties. The very low grade metamorphic (<200°C and about 2 kbar) assemblage includes abundant pumpellyite, laumontite, quartz and chalcedony, corrensite-type mineral, albite, smectite, calcite, minor anhydrite, potassium feldspar, titanite, hematite, pyrite, and accessory prehnite and chalcopyrite. This assemblage occurs in amygdales, veinlets, and in a bulk rock. The phenocrysts of mafic mineraIs, both olivine and orthopyroxene (?) did not survive alteration and were completely obliterated by clay mineraIs and pumpellyite. Primary Ca-rich primary plagioclase is totally albitized and overprinted by Al-pumpellyite and K-feldspar. The groundmass contains irregularly distributed clay mineraIs, laumontite, pumpellyite, chalcedony, quartz, calcite, hematite, and titanite. Hydraulic fracturing commonly dismembered metavolcanic rocks into fragments of highly altered volcanic fragments successively cemented by younger anhydrite, chalcedony, quartz, and calcite, producing breccia-type species. The fan-shaped pumpellyite in amygdales is commonly zoned with the Fe-rich bas e and Al-rich apex; moreover, Fe-rich pumpellyite fragments are frequently trapped in amygdale filling and veiny quartz. The amygdales and veinlets can contain abundant albertite-type bitumens concomitant with Al-rich pumpellyite. Detailed petrographic study combined with extensive XRD and microprobe analysis reveal the folIowing mineral sequence: anhydrite+Fe-pumpellyite+laumontite+albite (older) Al-pumpellyite + layer silicates (younger). There were several episodes of Si02 group mineraIs and calcite formation. The VLG metamorphism of Rotliegend volcanic rocks can be tentatively attributed to the penetration of sea water through an evaporite sequence; however, albertite seems to be a product of upwelling fluids released during burial metamorphism of sedimentary rock series underlying extrusive Permian unit.

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