Magnesian staurolite and kyanite in garnet inclusions: Further evidence of a high-pressure event in the Swedish Eastern Segment.

The Boras Mafic Intrusion is a large zoned magmatic body which melted an envelope of its country rocks to form dacitic magma during intrusion at 1674 Ma. It was affected by Sveconorwegian metamorphism around 970 Ma, dated on zircon by ion probe and on garnet by Sm-Nd and Lu-Hf. Samples of garnet amphibolite with a basaltic composition are found to contain peraluminous assemblages such as Mg-rich staurolite (mg# 31-33), kyanite and sometimes clinozoizite or white mica, included in garnet. These minerals have been analysed by X-ray microanalysis using our scanning electron microscope and confirmed by electron backscatter diffraction. The thermodynamic packages TWEEQU, THERIAK and DOMINO were used to investigate the conditions under which these parageneses formed, and the bulk compositions from which they could form. We conclude that they formed in rocks which experienced plagioclase-out reactions producing kyanite and clinozoizite, due to high-pressure, low-temperature conditions analogous to the blueschist facies. Furthermore the analysed bulk rock composition does not have a stability field for staurolite-bearing parageneses according to our calculations, although some experiments have produced staurolite in mafic rocks at >24kb. This suggests a two-stage process in which an aluminous and hydrous assemblage such as chloritoid, kyanite and clinozoizite first became isolated from the whole rock by inclusion in garnet, followed by the development of the parageneses we observed by reaction of chloritoid with kyanite and garnet to produce staurolite. THERIAK and DOMINO petrogenetic grids show that possible precursur parageneses are stable in the bulk rock at 480 to 580°C and 17 to 24 kilobars, corresponding to depths of 50-70 km. This work has implications for the tectonic evolution of the Eastern Segment. A model has been suggested in which slices of eclogite were tectonically emplaced into the Eastern Segment from depth, implying that the Eastern Segment as a whole did not experience a high-pressure event. This cannot explain our findings in the Boras Intrusion, which is clearly an integral part of the Eastern Segment and shows no evidence of such a tectonic process.