Fluid Regime in Southern Norway: The Record of Fluid Inclusions

Abstract

Fluid inclusions have been studied in representative rocks from Southern Norway, notably in the Bamble granulites. On the basis of the earliest fluid inclusions trapped in rock-forming minerals (mainly quartz), five major types of fluid distribution have been recognized: 2-phase aqueous (H20 dominant, without solid), carbonic (mostly pure CO2, possiDle occurrence of N2 and/or CH4), mixed 1 (aqueous and carbonic inclusions in comparable amounts, but in separate cavities), mixed 2 (aqueous and carbonic fluids in the same cavity, trapped in the miscible state of the H20-C02 system), brines (H20 + solids, NaC1 dominant). Only brines show a relation between a dominant inclusion type and a given protolith; these are especially abundant in 3 well-defined environments: Al-rich metasediments (meta-pelites), skarns and acid volcanics. The distribution of other types is more related to metamorphic grade: high- density carbonic inclusions are typical for the granulite- -facies domain, early 2-phase aqueous inclusions occur almost exclusively in the north-western part of the Bamble and in the Telemark gneiss-granites, mixed (1 and 2) inclusions characterize the complicated transition zone between the amphibolite- and granulite-facies domains north of the orthopyroxene-in isograd. P-T estimates from fluid inclusions are apparently very different for Bamble (maximum C02 density during peak metamorphism) and Rogaland (maximum CO2 density after the peak of metamorphism). Most of the C02 originates from the breakdown of carbonate melts (carbonatites) emplaced as immiscible droplets in deep-seated synmetamorphic intrusives. Southern Norway is a classical example of amphibolite — granulite facies transition in a Proterozoic terrain. Although the age of metamorphism remains controversial (see various entries in this volume, notably by R.H. Verschure, D. Demaiffe and J. Michot. D. Field et al.), the pressure and temperature conditions start to be relatively well understood (Jansen et al., this volume). All recent studies emphazise the key importance of fluids, notably H2O, the fugacity of which decreases suddenly at the granulite — facies boundary. Much information has been derived from the analysis, both experimental and theoretical, of characteristic mineral assemblages; however, since the first discovery of specific, high density CO2 inclusions in many granulites (Touret, 1971), it has become evident that the direct observation of fluids trapped in rock-forming minerals (notably quartz, plagioclase, pyroxene etc.) can provide a great deal of information. CO2-rich fluids, sometimes mixed with other species (N2, CH4) have later been observed in virtually all granulites in the world (Hollister and Crawford 1981, Roedder 1984), leading to the notion of “carbonic metamorphism”, which has provided a new insight into the geology of the lower continental crust (e.g Newton et al. 1980, Newton, this volume). Various studies dealing with Southern Norway have been issued in a number of publications (see e.g. review in Roedder 1984), but the rapid development of fluid-inclusion techniques, notably the possibility of in-situ, non-destructive analysis by micro-Raman spectroscopy, necessitates a critical reevaluation of earlier data. In this paper, a review of all studies performed in Soutern Norway during the last 15 years will be attempted. It will refine earlier syntheses (notably Touret 1981, 1984) and address three fundamental aspects of fluid investigations in high-grade metamorphic rocks: i) The distribution of fluid inclusions in amphibolite- and granulite-facies rocks and the relative importance of lithological composition (protolith) and metamorphic grade. ii) The pressure and temperature estimates derived from fluid inclusions and their comparison with data from solid mineral assemblages. iii) The origin of the CO2 fluids.