Petrology and geothermobarometry of granulite facies metapelites from the Hisøy‐Torungen area, south Norway: new data on the Sveconorvegian P–T–t path of the Bamble sector

The high‐grade rocks (metapelite, quartzite, metagabbro) of the Hisøy‐Torungen area represent the south‐westernmost exposures of granulites in the Proterozoic Bamble sector, south Norway. The area is isoclinally folded and a metamorphic P–T–t path through four successive stages (M1‐M4) is recognized. Petrological evidence for a prograde metamorphic event (M1) is obtained from relict staurolite + chlorite + albite, staurolite + hercynite + ilmenite, cordierite + sillimanite, fine‐grained felsic material + quartz and hercynite + biotite ± sillimanite within metapelitic garnet. The phase relations are consistent with a pressure of 3.6 ± 0.5 kbar and temperatures up to 750–850°C. M1 is connected to the thermal effect of the gabbroic intrusions prior to the main (M2) Sveconorwegian granulite facies metamorphism. The main M2 granulite facies mineral assemblages (quartz+ plagioclase + K‐feldspar + garnet + biotite ± sillimanite) are best preserved in the several‐metre‐wide Al‐rich metapelites, which represent conditions of 5.9–9.1 kbar and 790–884°C. These P–T conditions are consistent with a temperature increase of 80–100°C relative to the adjacent amphibolite facies terranes. No accompanying pressure variations are recorded. Up to 1‐mm‐wide fine‐grained felsic veinlets appear in several units and represent remnants of a former melt formed by the reaction: Bt + Sil + Qtz→Grt + lq. This dehydration reaction, together with the absence of large‐scale migmatites in the area, suggests a very reduced water activity in the rocks and XH 2 O = 0.25 in the C–O–H fluid system was calculated for a metapelitic unit. A low but variable water activity can best explain the presence or absence of fine‐grained felsic material representing a former melt in the different granulitic metapelites. The strongly peraluminous composition of the felsic veinlets is due to the reaction: Grt +former melt ± Sil→Crd + Bt ± Qtz + H 2 O, which has given poorly crystalline cordierite aggregates intergrown with well‐crystalline biotite. The cordierite‐ and biotite‐producing reaction constrains a steep first‐stage retrograde (relative to M2) uplift path. Decimetre‐ to metre‐wide, strongly banded metapelites (quartz + plagioclase + biotite + garnet ± sillimanite) inter‐layered with quartzites are retrograded to (M3) amphibolite facies assemblages. A P–T estimate of 1.7–5.6 kbar, 516–581°C is obtained from geothermobarometry based on rim‐rim analyses of garnet–biotite–plagioclase–sillimanite–quartz assemblages, and can be related to the isoclinal folding of the rocks. M4 greenschist facies conditions are most extensively developed in millimetre‐wide chlorite‐rich, calcite‐bearing veins cutting the foliation.