FLUID INCLUSIONS IN CHARNOCKITES FROM THE BJERKREIM-SOKNDAL MASSIF (ROGALAND, SOUTHWESTERN NORWAY) - FLUID ORIGIN AND INSITU EVOLUTION

Fluid inclusions and mineral associations were studied in late-stage charnockitic granites from the Bjerkreim-Sokndal lopolith (Rogaland anorthosite province). Because the magmatic and tectonic evolutions of this complex appear to be relatively simple, these rocks are a suitable case for investigation of the origin and evolution of "granulitic fluids". Fluid inclusions, primarily contained in quartz, can be divided into four types: carbonic (type I), N2-bearing (type II), CO2+ H2O (type III) and aqueous inclusions (type IV). For each type, the role of leakage and fluid mixing are discussed from microthermometric and Raman spectrometric data. The most striking features of CO2-rich inclusions (the predominant fluid) is the presence of graphite in numerous, trail-bound inclusions (Ib) and its absence in a few isolated, very dense (d = 1.16), pure CO2 inclusions (Ia) and in the late carbonic inclusions (Ic). Fluid chronology and mineral assemblages suggest that carbonic Ia inclusions represent the first fluid (pure CO2) trapped at or close to magmatic conditions (T = 780-830-degrees-C, fO2 = 10(-15) atm and P = 7.4 +/- 1 kb), outside the graphite stability field. In contrast, type Ib inclusions enclosed graphite particles from a channelized fluid during retrograde rock evolution (P = 3-4 kb and T = 600-degrees-C). Decreases in T-fO2 could explain a progressive evolution from a CO2-rich fluid to an H2O-rich fluid in a closed C-O-H system. However, graphite destabilization observed in type Ic inclusions implies some late introduction of external water during the last stage of retrogression. The main results of this study are the following: (1) a carbonic fluid was present in an early stage of rock evolution (probably in the charnockitic magma) and (2) this granulite occurrence offers good evidence of crossing the graphite stability field during post-magmatic evolution.