FLUID INCLUSIONS IN SCOURIAN GRANULITES FROM THE LEWISIAN COMPLEX OF NW SCOTLAND - EVIDENCE FOR CO2-RICH FLUID IN LATE ARCHEAN HIGH-GRADE METAMORPHISM

In this paper the first fluid-inclusion data are presented from Late A rchaean Scourian granulites of the Lewisian complex of mainland northw est Scotland. Pure CO2 or CO2-dominated fluid inclusions are moderatel y abundant in pristine granulites. These inclusions show homogenizatio n temperatures ranging from -54 to +10 degrees C with a very prominent histogram peak at -16 to -32 degrees C. Isochores corresponding to th is main histogram peak agree with P-T estimates for granulite-facies r ecrystallization during the Badcallian (750-800 degrees C, 7-8 kbar) a s well as with Inverian P-T conditions (550-600 degrees C, 5 kbar). Th e maximum densities encountered could correspond to fluids trapped dur ing an early, higher P-T phase of the Badcallian metamorphism (900-100 0 degrees C, 11-12 kbar). Homogenization temperatures substantially hi gher than the main histogram peak may represent Laxfordian reworking ( less than or equal to 500 degrees C, <4 kbar). In the pristine granuli tes, aqueous fluid inclusions are of very subordinate importance and o ccur only along late secondary healed fractures. In rocks which have b een retrograded to amphibolite facies from Inverian and/or Laxfordian shear zones, CO2 inclusions are conspicuously absent; only secondary a queous inclusions are present, presumably related to post-granulite hy dration processes. These data illustrate the importance of CO2-rich fl uids for the petrogenesis of Late Archaean granulites, and demonstrate that early fluid inclusions may survive subsequent metamorphic proces ses as long as no new fluid is introduced into the system. (C) 1997 El sevier Science B.V.