MELT-ENHANCED DEFORMATION DURING EMPLACEMENT OF GABBRO AND GRANODIORITE IN THE SUNNHORDLAND BATHOLITH, WEST NORWAY

The Caledonian Sunnhordland Batholith comprises calc-alkaline plutons that have been assigned to three units according to their relative age and composition: a gabbro-diorite unit, a granodiorite unit and a later granodiorite-granite unit. The batholith was emplaced into an envelope including ophiolite and island-arc complexes, sediments and volcanites of early Ordovician age that were developed in a zone of plate convergence. Continued convergence resulted in the formation of a mature magmatic arc and a thickened crust; the late granitoids (unit 3), which commenced their crystallization at pressures around 6 to 7 kb, rose as permitted diapiric intrusions. The ingress and ascent of the magmas in this setting is considered to have been facilitated by the presence of major shear zones developed in relation to plate convergence. In this model, plastic instabilities were formed in an otherwise elastic middle and upper crust. Non-coaxial deformation was accelerated by the emplacement of magmas and the formation of abundant partial melts in water-rich sediments of the envelope. The deformation, which was accelerated by magma and melt lubrication in aureoles, controlled both the shape and internal structure in the gabbro and granodiorite plutons.