Localized occurrences of granulite: P–T modeling, U–Pb geochronology and distribution of early-Sveconorwegian high-grade metamorphism in Bamble, South Norway

Abstract

New localities of granulite in the Bamble lithotectonic domain of south Norway illustrate localized distribution of granulite facies rocks in an amphibolite facies gneiss terrain. U–Pb zircon SIMS data constrain the magmatic age of the protoliths to be both 1545±7 to 1542±5Ma and 1149±8Ma. They provide a robust date of 1139±11Ma for metamorphic zircon, supporting available estimates for early-Sveconorwegian metamorphism in Bamble. P–T modeling using Grt+Opx±Cpx+Pl+Bt+Qz-assemblages yields pressures up to 1.15GPa and T>850°C. Clinopyroxene-free granulite assemblages are preserved in low-strain lenses, while high-strain granulite shows crystallization of clinopyroxene. The replacement of orthopyroxene by amphibole illustrates hydration and reequilibration at amphibolite facies. The new findings in Bamble show that granulite-facies rocks occur unevenly distributed in high-grade metamorphic terrains and that relationship between granulite and amphibolite facies is complex and irregular. It suggests that factors as fluid availability, deformation and whole-rock composition are important for metamorphic evolution in addition to pressure and temperature gradients.