GARNET PROVENANCE IN MIXED FIRST-CYCLE AND POLY-CYCLE HEAVY-MINERAL ASSEMBLAGES OF THE ROPIANKA AND MENILITE FORMATIONS (SKOLE NAPPE, POLISH FLYSCH CARPATHIANS): CONSTRAINTS FROM CHEMICAL COMPOSITION AND GRAIN MORPHOLOGY

摘要

Garnet in heavy-mineral assemblages, occurring in sandstones of the Campanian–Maastrichtian part of the Ropianka (Late Cretaceous–Palaeocene) and Menilite (Oligocene) formations of the Skole Nappe, is present as first-cycle and poly-cycle grains, derived from a proximal source, remote areas and/or from sedimentary rocks of the Skole Basin foreland. The garnets in the formations are compositionally similar, suggesting an origin from the same source rocks. Relatively large amounts of garnet, represented by euhedral or slightly rounded, weakly etched or unetched almandine and spessartine-almandine garnet, and minor pyrope-enriched almandine, were derived directly from a source close to the Skole Basin. These garnets are from sediments, metamorphosed at low- to medium-grade conditions (such as mica-schists, gneisses) and perhaps also granitic bodies. Rounded and variously etched garnets, especially high pyrope-almandine and pyrope-almandine-grossular varieties, but also partly almandine-dominated varieties, are suggested to have been derived from distant sources, such as sedimentary rocks of the Upper Silesian and Ma?opolska blocks. Rocks, forming uplifted parts of the crystalline basement of Brunovistulicum and/or crystalline domains of the Bohemian Massif, could have been protoliths for part of the almandine-dominated garnet population, whereas pyrope-grossular-almandine garnets may originate from the granulitic, eclogitic or metabasic rocks of the Bohemian Massif. The study shows that analyses of garnet composition, combined with observations on grain textural features and data on the lithology of clasts and pebbles, can permit the determination of sources for different garnet varieties in mixed-provenance populations.