Garnet peridotites exposed on the islands of Otrøy, Flemsøy and Fjørtoft form an integral part of the northernmost ultra-high pressure terrane of the Western Gneiss Region, SW Norway. Their mantle and crustal evolution is fundamentally different from similar Western Gneiss Region garnet peridotites exposed further south. For this reason the occurrences and interpretation of the northernmost garnet peridotites is reviewed here emphasizing the progress made in recent years.
Re–Os analyses of whole-rock powders of peridotites and garnetites, Sm–Nd analyses of megacrystic garnets and a 5 point Sm–Nd whole-rock errorchron of garnet pyroxenites suggest an Archean origin (M1), involving majoritic (20 %) garnet growth, decompression melting at high temperature, accretion to the Laurentian subcontinental mantle with simultaneous growth of a second generation of majoritic garnet (1 %) followed by cooling towards pressure-temperature conditions indicated by a stable cratonic geotherm. Minimal mid Proterozoic Sm–Nd garnet-pyroxene ages are interpreted to reflect intense dynamic recrystallisation of garnet megacrysts (M2). Subsequent crustal emplacement of the garnet peridotites (from around 120 km depth) took place during the early Scandian (~430 Ma) when the continental crust of Baltica subducted beneath Laurentia. A prograde, Scandian, subduction event (M3) can be recognised in the peridotites as recrystallised M3 orthopyroxene crystals with markedly lower Al2O3 contents than M2 orthopyroxene crystals in garnet websterite protoliths. M3 is contemporaneous with the introduction of a supercritical, COH-rich, dense subduction zone fluid that formed the source of carbon that gave rise to diamond crystallisation during subsequent continued continental subduction. Scandian peak metamorphic conditions are indicated by the growth of a “third” generation of majoritic garnet indicating pressures on the order of 5.5–6.5 GPa. Roughly similar PT conditions can be calculated for opx-grt mineral-chemistries using classical geothermobarometric techniques.