The Sunnhordland Batholith, W. Norway: Regional setting and internal structure, with emphasis on the granitoid plutons

The Caledonian Sunnhordland batholith is composite, ranging in composition from gabbro to granite. The batholith constitutes the main part of the Tysnes Nappe, but granitic rocks which are thought to belong to the same magmatic event also intrude rocks which are at the same tectono-stratigraphic level as the Major Bergen Are. The batholith was emplaced in middle/late Ordovician-early Silurian times as shown by intrusive relationships with the envelope, and the Rb-Sr whole-rock isochron age of the Håkre Monzogranite at 430 ± 10 Ma. The batholith comprises a number of major granitoid plutons which postdate the gabbroic rocks, the latter only superficially treated in the present study. The granitoids have been grouped in two units; the first represented by the Reksteren Granodiorite and the second by the Drøni and Håkre Monzogranites and the Rolvsnes Granodiorite. The Reksteren Granodiorite was emplaced forcefully, giving rise to an internal compositional banding and mineral fabric. The later granitoids were permitted intrusions, and are little deformed and recrystallized. Mineralogical and limited geochemical data show that the batholith is an I-type complex, and belongs to a cafemic or aluminocafemic association. It was emplaced through continental crust in a tectonic environment of oceancontinent convergence. Primary epidote in the granites suggests crystallization at a pressure of 6 kbars, and emplacement was associated with rapid uplift as debris from the granites is abundant in Silurian conglomerates in the area.