A Porphyroblastic Granite with Preserved Bedding

Abstract

A regular banding is preserved in many parts of the potash granite at Nagu in the archipelago of southwestern Finland. The principal differences between the bands involve their grain size and their content of porphyroblasts of micro-cline and garnet, for some bands lack both kinds of porphyroblasts. The micro-cline porphyroblasts lie subparallel to the banding or to the axial plane in those parts of the granite where the banding has been destroyed. The banding is interpreted as a relict sedimentary bedding. It forms concentric folds conformable with the folds in the supracrustal amphibolites within the granite. These folds seem to be survivals from the first folding phase, when the sediments were transformed into gneisses. Later, the gneisses were granitized without appreciable orogenic movements. The chemical composition is unexpectedly constant in all parts of the granite. The formation of garnets in the granite, as well as that of hypersthene in gabbroic layers in the granite, points to an expulsion of water in connection with the granitization.