Geothermometry and Geobarometry in Rogaland and Preliminary Results from the Bamble Area, S Norway

Abstract

Mineral textures in Rogaland and Bamble areas give evidence for polymetamorphism, beside the usual low-grade retrogradations in or below the greenschist facies. Due to the fact that geothermobarometry is based on the assumption of chemical equilibrium among minerals, the application has to be treated with an enormous amount of caution in such areas. Especially the presence (or absence) of index minerals must be used as a control upon the derived results. For example, in both areas involved the main Al-silicate is sill. The few occurrences of kya in SE Norway indicate high pressure retrogradation, while the few occurrences of anda in SW Norway presumably point at low pressure retrogradation. In fact most rocks in Rogaland are reset during the retrograde M3 metamorphism. Part of the chemical rock system, particularly biot, cord, oxide mineral, is reequilibrated in a temperature range of about 550-700 °C at about 3–5 kb total pressure. The conditions are regionally varying. These results are achieved by the use of the garn-cord, garn-biot, garn-opx-plag-qtz, cpx-opx and ilm-ulvø thermo-barometers. In order to establish the conditions of earlier metamorphisms a search for relic equilibrium domains is inevitable, which are not necessarily one restricted part of a thin section with contacting minerals. For the M2 metamorphism the calibrated conditions yielded 700-1050 °C grading to the lower temperatures towards ENE, at pressures of about 4 kb. These estimates are derived by the use of thermo- barometry on cpx-opx, ilm-ulvø, fay-qtz-fesil and some garn-opx assemblages. Equilibrium domains of mineral assemblages useful for geothermo-geobarometry upon the Ml metamorphism are not unambigous enough to establish PT-conditions. Based on experimentally derived mineral stability fields some estimates in the temperature range from 5–7 kb can be made. The regional gradients are not well understood yet. The main metamorphism in Bamble near Tvedestrand took place at temperatures of about 750 °C and about 7 kb. These results were achieved with garn-cord, garn-biot and garn-opx-plag-qtz thermometry. Only some local chemical resetting is established. The retrogradation was more or less isobaric down to about 600 °C passing into the kyanite stability field.