Field relations and geochemical evolution of the Gothian rocks in the Kvamsøy area, southern Western Gneiss Complex, Norway

Abstract

The igneous rocks of the Kvamsøy area in the southern parts of the Western Gneiss Complex (WGC) of Norway comprise a wide variety of granitic, syenitic. granodioritic and gabbroic compositions and formed in the interval 1640 to 1622 Ma during the Gothian orogeny. Strongly deformed gneissic equivalents of the igneous rocks form the granodioritic and granitic basement gneisses, which are the dominant lithologies in the Kvamsøy area as well as elsewhere in the southern parts of the WGC. The gneisses show variable amounts of partial melting during the Sveconorwegian migmatisation event dated at 965 +- 12 Ma.\rQuartzites in the Kvamsøy are represent remnants of a pre-Gothian basement through which the Gothian rocks were emplaced. All the Gothian igneous rocks are predominantly metaluminous, and the granodioritic rocks have calc-alkaline to high-K alkaline compositions, indicating that they had igneous sources. Sm-Nd and RB-Sr isotope data indicate that the sources for the rocks had either a substantial component of juvenile melts or a short crustal residence time. The contribution of pre-Gothian basement was probably minor. The granodioritic rocks were most likely generated by partial melting of underplated mantle-derived basaltic rocks, while alkali-calcic granitic and syenitic rocks may alternatively have been generated by partial melting of more evolved rocks such as tonalites.The geochemistry of the igneous rocks is consistent with generation within a tectonic setting at an active continental margin, by remobilisation of young magamatic rocks at a continental margin during a collision or post-collision setting.