Permian granitoid intrusives caused extensive contact metamorphism of Lower Palaeozoic sediments in the Grua area, located in the central Oslo Rift. Phase relations within metamorphic shales and carbonates have been examined by scanning electron microscopy for constraining the peak metamorphic conditions during contact metamorphism in this area. Metamorphic effects on the primary sedimentary mineralogy are observed more than 2400 m from the intrusive contact. Stability calculations accounting for relevant solid solutions have been used to construct a temperature-distance profile across the contact aureole. Peak metamorphic temperatures near the contact with the Mylla syenite are estimated to ca. 560°C (at 1 kbar), based on the coexistence of olivine, clinopyroxene, amphibole, dolomite and calcite in limestones, and the presence of cordierite, biotite, feldspar and quartz in shales. Phase-petrological data are consistent with extemal controls on the metamorphic fluid composition in most layered shale-bearing lithologies. Massive limestones appear to have buffered the pore fluid composition during progressive metamorphism, even dose to the intrusive contact.
H. Svensen & B. Jamtveit, Department of Geology, University of Oslo, PO Box 1047 Blindern, N-0316 Oslo, Norway.