THE STOKKAFJELL TROCTOLITE, SW NORWAY - ITS BEARING ON THE EARLY P-T EVOLUTION OF THE ROGALAND TERRANE

Whereas the P,T conditions of the prominent M2 and M3 metamorphic stag es in the Rogaland terrane, SW Norway are well defined, the nature of earlier metamorphic stages is less well established. The Stokkafjell T roctolite, which is actually a series of boudins ranging from gabbroic and noritic lithologies via troctolitic compositions to ultramafic ro cks, preserved two metamorphic mineral assemblages older than the M2 t wo pyroxene assemblage. The oldest assemblage (M1a) in the Stokkafjell troctolite is composed by pargasite and plagioclase. For this assembl age, P,T conditions are estimated at 640-725 degrees C and 7-10 kb. Su bsequent Mlb metamorphism involved the formation off corona's between olivine and plagioclase. These corona's are composed by an inner shell of orthopyroxene, and an outer shell of symplectitic pargasite and sp inel. For this stage, conditions >700 degrees C and >6kb have been est imated. Orthopyroxene-clinopyroxene geothermometry yields temperatures between 800 and 1000 degrees C for the M2 stage, which is in fair agr eement with earlier studies. Conditions of Mlb agree with estimates fo r M1 as defined by other studies. However, P and T estimates for the M 1a stage do not agree with estimates for M1 metamorphism in Rogaland p rovided by other studies. Therefore, we propose that the M1a stage eit her represents the MO event hitherto only indicated by whole rock geoc hronology, or reflects an earlier stage of the previously defined earl y Sveconorwegian M1 metamorphism.