Orogenic Peridotites in the UHP Western Gneiss Region, Norwegian Caledonides: some new developments

Two belts of garnetiferous, mantle-derived peridotites outcrop in the Western Gneiss Region (WGR), each with distinctive characteristics (Brueckner et al., in review). One occupies the central-west area (CW belt) with the well-known bodies at Tafjord and Almklovdalen marking the eastern and western ends respectively. The other lies in the archipelago in the northwest of the WGR (NW belt) and includes the massifs on the islands of Otrøy, Fjørtoft and Flemsøy. This contribution describes two new findings in these peridotites, one from each belt.
The CW peridotites are generally highly depleted dunites and harzburgites with thin, tabular or lensoid masses of garnet pyroxenite and associated haloes of garnet lherzolite, thought to have been generated by refertilisation of the original dunites and harzburgites by melts (Beyer et al., 2006). CW garnets are generally mm-cm size and evidence for an original majoritic composition has not been previously described, but majorite-stable conditions have been determined from exsolved pyroxenes in the Sandvik peridotite (Lapen et al., 2009), which appears to occupy a position intermediate between the CW and NW belts. A cluster of ~0.5 km dunite masses at Raubergvik, near Stranda encloses small masses of garnet peridotite and pyroxenite (Osland, 1997) with 1-2cm ovoid garnets that define a weak shape-preferred orientation. These garnets have been found to contain patches of small, rationally oriented pyroxene needles associated with rutile needles, giving clear evidence for an original majoritic composition. The Raubergvik mass is part of an extensive swarm of peridotite bodies that include the well-known Tafjord garnet peridotite about 20km to the east (Carswell, 1968), but no similar exsolution features have been described from the other bodies in the swarm. By analogy with other CW belt peridotites, the Raubergvik garnets may belong to the M2 generation (Scambelluri et al., 2008) formed at about 1.4Ga by melt-fed refertilisation, so the exsolution postdates that event. It has been proposed that the CW peridotites are subcontinental lithospheric mantle fragments derived from shallower depths than those in the NW belt (Brueckner et al., in review). The majoritic character of the Raubergvik body suggests that the CW belt either contains mantle fragments derived from a wider range of depths than previously appreciated, or all bodies in this belt had an early majoritic history, the evidence for which has since been substantially erased.
The NW peridotites are more pervasively garnetiferous and less depleted than the CW bodies, and have often evidently undergone intense ductile deformation at HP/UHP conditions associated with cataclastic or plastic deformation of garnet. In low-strain zones garnets are often megacrystic with sizes in the cm-dm size range, and show abundant and complex pyroxene exsolution indicating three stages of majorite formation, the last of which was associated with (Scandian) microdiamond formation (Scambelluri et al., 2008). A small body of very fine-grained, mylonitic, layered garnet lherzolite outcrops at Nytun on the island of Fjojrtoft. It lies a few km east of the majoritic and microdiamond-bearing garnet ultramafite at Bardane (Van Roermund et al., 2002) and is spatially associated with diamond-bearing pelitic gneisses and coesite eclogites. An unusual, folded, Mg-Al spinel-rich layer has been found to exhibit brown prisms of högbomite intergrown and in textural equilibrium with spinel, garnet and olivine. Högbomite-group minerals are modular-structured, mixed-layer Al-Ti-Al oxides found in aluminous bulk compositions, often associated with spinel, corundum, sillimanite and sapphirine. It is known from upper amphibolite and granulite facies rocks (Petersen et al., 1989) and in retrograde assemblages in mafic and ultramafic eclogite facies rocks where it is a product of the breakdown of spinel (Konzett et al., 2005, Liati & Seidel, 1994). The Nytun occurrence appears to be the first where stability of högbomite at eclogite facies is inferred, and thus extends the stability of this mineral group to high pressures. The five-phase ol+opx+cpx+grt+Cr-spinel assemblage in this body yields 719^oC and 2.2 GPa (Cuthbert et al., 2000) and a Sm-Nd mineral age of 511±18Ma (Jamtveit et al., 1991) indicating deformation and metamorphism associated with Caledonide (M3?) orogenesis. Högbomite is thought to be hydroxyl-bearing, and its presence in this rock implies that it is a potential (though rare) carrier of water to at least shallow mantle depths.

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