ARCHEAN TO PALEOZOIC EVOLUTION OF THE SANDVIK GARNET PERIDOTITE, WESTERN GNEISS REGION, NORWAY

Mantle-derived, Mg-Cr ultramafic bodies are a conspicuous feature of the Western Gneiss Region (WGR) of Norway. Such ultramafic rocks contain multistage mineral assemblages that reflect the complex evolution of Baltica mantle during several Precambrian and Paleozoic tectonothermal events. Most of the ultramafic bodies were recrystallized to chlorite peridotite during Caledonian exhumation, but 14 bodies contain relict garnet peridotite, which yield Sm-Nd grt-wr-cpx isochron ages of ~1700 to ~1000 Ma.

A small outcrop of peridotite and associated pyroxenite, at Sandvik on the island Gurskøy, contains four of the numerous mineral assemblages that have been identified in WGR peridotites, including two garnet-bearing stages: 1) ol + grt + opx + cpx: (porphyroclasts) 1000 °C, 50 kbar; 2) ol + grt + spl + am ± opx: (kelyphite around Stage 1 garnet) 700-750 °C, 12-18 kbar; 3) ol + spl + opx + am: (matrix for Stage 1 porphyroclasts) ~700 °C, ~10 kbar; and 4) ol + chl + opx + am: (the predominant, Caledonian assemblage in WGR peridotites) ~600 °C, ~5 kbar.

Two layered samples, one containing grt-pyroxenite and grt-peridotite and the other containing grt-pyroxenite and ol-grt-pyroxenite, yield Lu-Hf grt-wr-cpx ages of 1397 ± 7.3 and 1378 ± 7.0 Ma for Stage 1 porphyroclasts in grt-pyroxenite and 1470 ± 7.3 Ma and 1485 ± 7.4 in peridotite and ol-grt-pyroxenite, respectively. The difference in ages between lithologies may be related to timing of formation, differences in age resetting due to diffusion, or preferential sampling of garnet cores in peridotite and olivine pyroxenite, due to formation of abundant kelyphite at the expense of the Stage 1 garnet used for geochronology. These ages likely represent the minimum time this material has resided in the lithospheric mantle, away from the effects of convecting asthenosphere. The Stage 2 kelyphitic garnet remains undated, but is likely related to Caledonian subduction.

Initial eHf and f Lu values for bulk rock pyroxenite, ol-pyroxenite, and peridotite are depleted relative to CHUR eHf=33-45; f Lu=0.15-1.35), and this material likely experienced a complex history of depletion and refertilization, possibly beginning in Archean time. An additional garnet peridotite sample from the Sandvik outcrop is extremely depleted eHf0=~1800; f Lu=~22) and defines a CHUR model age of 3.2-3.5 Ga.