Abstract
The Precambrian granulites (T~850°C and p<1.0 GPa) of the Lindås nappe, Bergen Arcs are transformed to Caledonian eclogites (T~670°C and p~1.8 GPa) in shear zones and along fluid pathways. This transition has been studied in numerous papers, but two important minerals have escaped attention. The minerals scapolite and spinel, with 10 and 5 modal% respectively, occur in anorthositic granulites of the Lindås Nappe in the Bergen Arcs. Field relationships, textural analyses, mineral chemistry and reactions are studied. The main objective was to trace scapolite and spinel across the granulite-eclogite facies transition as it occurs in the Lindås Nappe, focusing on Holsnøy Island. The examined scapolite contains high concentrations of the volatile species CO32- and SO42-. Sulphur content in scapolite of preserved granulites in this study, records an average of 6 wt\% SO3.This makes scapolite an important constituent of the carbon- and sulphur-reservoirs in the earth's lower crust. Breakdown of scapolite during eclogitisation leads to decreasing sulphur concentrations and formation of the eclogite phases omphacite, kyanite and clinozoisite. Reaction products formed after scapolite contain micron-sized grains of sulphides like pyrite, pyrrhotite and chalcopyrite or sulphates, e.g. barite and celestine, which delineate the original grain boundary and incorporate the sulphur from scapolite. Eclogite-facies shear zones and pseudotachylytes (frictional melts) are ubiquitous in these rocks and have an important effect on spinel. In general, with increasing pressure spinel transforms to corundum and by this process releasing Fe and Mg. The breakdown commonly occurs when spinel is surrounded by feldspar, resulting in corona textures, and is prevented when spinel is present as inclusions in garnet. Pseudotachylytes affect the growth of new minerals inside the corona textures. Cr-rich spinel and other high-chromium eclogite phases, like Cr-kyanite and ruby, are preserved within an eclogite-facies garnet grain. Garnet and ruby are replaced by Cr-kyanite near Cr-rich spinel. The Cr-kyanite forms pseudomorphs after garnet. Fibrous Cr-kyanite replaces ruby in the vicinity of fractures inside the garnet grain. Fractures that connect the former silica-deficient inner Cr-nodule with the quartz-bearing eclogite facies rock probably facilitated the transport of silica into the nodule and by this caused the transformation of ruby to Cr-kyanite. The formation of this rare mineral assemblage possibly results from an ultramafic xenolith that was present in the former anorthosite, prior to eclogitisation.