Investigations in the Västervik area, southeastern Sweden - 1. Structural geology and genesis of the "younger" granites

Abstract

A geological map, short description and general petrological discussion is given of a small part of Svecofennian in SE-Sweden. The problems concern a relatively small sedimentary complex and surrounding granite gneisses, hybide diorites, migmatites and associated "younger" granites. Structural analysis has revealed that deformation has taken place in at least two phases. The first phase is characterized by isoclinal folding of the bedding plane with horizontal axial plane and generation of axial plane schistosity of newly formed mica. The second and dominant deformation phase is characterized by parallel, often plastic, folding with vertical axial plane. The metamorphism linked with this second phase is characterized by the crystallization of andalusite and sillimanite in sediments of appropriate composition. Moreover, this deformation is accompanied by extensive migmatization and the intrusion of granite dikes, the so-called "younger" granites, A mushroom-like interference pattern of superposed folds from both phases is extensively discussed. A new geochemical test is applied to establish the depth of intrusion of the "younger" granites. This test is based on the variation of the eutectic composition of a granitic melt at varying pressures, as determined experimentally by von Platen and Höller (1966). The results suggest a rather shallow depth of intrusion, which corresponds to the type of the broadly contemporaneous metamorphism of the associated sediments. Results of the analysis of trace elements of the "younger" granites, especially the low K/Rb-ratio, indicate an anatectic origin of these granites.