Geology and petrochemistry of the Smøla-Hitra Batholith, Central Norway.

Abstract

The plutonic rocks which compose the Smøla-Hitra Batholith (SHB) cover an exposed area of c.1000 km\u00B2 and intrude folded, Arenig-Llanvirn, low-grade meta-sedimentary and bimodal metavolcanic rocks and higher-grade polydeformed gneisses of probable Proterozoic age. The SHB is unconformably overlain by Old Red Sandstone sediments of possible Late Silurian to definite Middle Devonian age. Field relations demonstrate that the oldest rock-types in the batholith comprise gabbro, monzodiorite and hornblende diorite. These are succeeded by tonalite, granodiorite and granite, which occur both as plutons and as abundant dykes and veins. Later members of the SHB include composite dykes, porphyritic microdiorite, appinitic pipe-like bodies, dolerite dykes and lastly, granophyre dykes. The hypabyssal rocks post-date a phase of weak, heterogeneous deformation manifested locally within the various plutons. Pressure estimates from hornblende compositions indicate a pressure of solidification of 0.26 +-0.1 GPa, corresponding to a depth of about 9 km. Preliminary isotopic dating has indicated an age range from c. 450 to c. 428 MA; latest Mid-Ordovician to Early Silurian. The rocks of the SHB show little sign of the Scandian deformation and metamorphism which are so common elsewhere in this part of Norway.