Petrographic Constraints on the Metamorphic History of Garnet Amphibolites in the Heia Nappe, Norwegian Caledonides

Sammanfattning

This project focuses on the Heia Nappe, a part of the Nordmannvik Nappe in the Caledonides of Norway.The unit is composed predominantly of metasedimentary rocks and mafic intrusions. The primaryobjective of this study was to identify evidence of high-grade metamorphic conditions in lithologiesassociated with the ultra-high-pressure (UHP) gneisses of the unit and to constrain the pressure–temperature (P–T) conditions during the metamorphic evolution of the rocks.Field investigations aimed at identifying high-pressure (HP) lithologies, such as eclogites,instead revealed only garnet amphibolites. While these do not represent classic HP assemblages,their mineral assemblage, coupled with mineral chemistry, provides important constraints onmetamorphic evolution during orogenesis. The methods employed included optical microscopy,electron microprobe analysis, and geothermometry. The investigated amphibolites are composedof garnet, amphibole, pyroxene, plagioclase, quartz, +/-biotite, +/-zoisite, +/-carbonates, and +/-graphite. Geothermometric analysis of garnet-amphibole yielded a temperature of approximately600 °C. Preserved zoisite–quartz symplectites, in association with amphibole and clinopyroxene,likely reflect the breakdown of high- pressure phases and thus point to former HP metamorphicconditions.