Amphiboles and their host rocks in the high-grade metamorphic Precambrin of Rogaland/Vest-Agder, Sw. Norway

Abstract

In the high-grade metamorphic Precambrian of the Sirdal-¢rsdal area, Rogaland/Vest-Agder,south-west Norway, the Ca-amphiboles show a change in pleochroic colours, not only with changes in metamorphic grade, but also to some extend in bulk composition. A regional study was performed on the amphiboles to study their response to changes in metamorphic grade, the influence of the host rock chemistry, and the relation between chemical and physical parameters of the amphiboles, with an emphasis on colour. Analyses have been performed on amphiboles and their host rocks, and calculation methods and analysis technics are discussed. Potassium-argon ages on 17 hornblendes indicate their cooling below 5500C about 950 Ma ago, after a high temperature, low pressure metamorphism (Abukuma-type). About the same time the Lopolith of Bjerkreim-Sokndal intruded. During the main metamorphic event, the pressure in the area was 3-5 kb, while water pressure and temperature varied, resulting in a metamorphic zoning from granulite facies to amphibolite facies. This metamorphic gradation formed a regional pattern of amphibole colours, which change from bluishgreen in the amphibolite facies terrain to brown in the granulite facies area. Titanium, and to a lesser degree ferric and ferrous iron, are the most important chemical determinants of the Ca-amphibole colours. The rock composition is less important for the colour, but very important (especially the An-content of the coexisting plagioclase) for some other chemical characteristics of the Ca-amphiboles. Ca-amphiboles in the granulite facies are richer in titanium and potassium (and possibly fluorine), and poorer in magnesia and aluminia (octahedral-site and total) than the Ca-amphiboles in the amphibolite facies, if host rocks of similar composition are compared. After the main metamorphic event the pressure increased while the temperature decreased, and secondary amphiboles formed (mainly actinolite, but also hornblende and ferro-magnesian amphiboles). The oxidation-ratio of the Ca-amphiboles of the main phase forms a clear regional pattern which is independent of the metamorphic zoning; it may be related to a later tectonic event. No important formation of amphiboles has taken place after 950 Ma ago. The area was lifted to its present position; the cooling rate in the amphibolite facies terrain during, at least, the period between 1000 and 870 Ma ago was about 30C/Ma, with an original geothermal gradient between 50 and 100oC/km. The cooling rate in the granulite facies area and in the lopolith has been much higher.