THE REGIONAL AMPHIBOLITE TO GRANULITE-FACIES TRANSITION AT ARENDAL, NORWAY - EVIDENCE FOR A THERMAL DOME

A refinement of the metamorphic zoneography of the Proterozoic amphibo lite - granulite facies transition zone at Arendal in the Bamble Secto r of the Baltic Shield is presented together with the first set of P - T data covering the entire transition zone. The regional amphibolite to granulite facies transition zone at Arendal is characterized by the following south facing isograd sequence: -Ms Muscovite-out in metapel itic rocks, +Crd Cordierite-in in metapelitic rocks, + Opx(1) Orthopyr oxene-in in amphibolites, +Opx(2) Orthopyroxene-in in acidic gneisses, -All Allanite-out in gneisses, metapelites and amphibolites, -Ttn Tit anite-out in gneisses, metapelites and amphibolites. These isograds ar e cross cut by a possible south facing -Ky Kyanite-in isograd in metap elites and cordierite-orthoamphibole rocks. Cordierite and orthopyroxe ne-in isograds should be located several kilometres to the northwest o f the currently existing ones. The isograds are accompanied by increas ing recrystallization of sillimanite and an overall increase in the ti tanium content of hornblende in amphibolites. Temperature estimates ar e 752 +/- 34-degrees-C (range 710 - 780-degrees-C) for the amphibolite facies zone at 7.1 +/- 0.4 kb (range 6.5 - 7.6 kb). Granulites of zon e B + C show an average temperature of 836-degrees +/- 49-degrees-C at 7.7 +/- 0.3 kb; surprisingly, the peak thermal area is situated on th e mainland, and temperatures start to decrease towards the Skagerrak: those in zone D yielded slightly lower P-T estimates: 7970 +/- 15-degr ees-C at 7.2 +/- 0.5 kb. All granulite facies stamples together yield 822-degrees +/- 40-degrees-C (range 770 - 890-degrees-C) at 7.4 +/- 0. 5 kb (range 6.6 - 8.1 kb). Based on the structural trend, isograd patt ern and temperature distribution it is tentatively suggested that the metamorphic complex represents a thermal dome. Increased heat flow in the centre of the dome caused a temperature gradient. At the same time , P(H2O) was significantly lowered in the centre of the dome, while P( Tot) remained roughly constant. Different cooling paths are establishe d for the area along the Porsgrunn-Kristiansand Fault and the remainin g part of the investigated area.