Evidence for diamond-grade ultra-high pressure metamorphism and fluid interaction in the Svartberget Fe-Ti garnet peridotite-websterite body, Western Gneiss Region, Norway

Based on mineral-chemical evidence we propose that the northernmost Scandian ultra-high pressure (UHP) metamorphic domain within the Western Gneiss Region of Norway can be extended 25 km northeastwards. A newly discovered, well preserved, fine-grained, Fe-Ti type garnet peridotite body at Svartberget, located in the Ulla Gneiss of the 'More og Romsdal' area north of Molde, is cut by a network of systematically orientated coarse-grained garnet-websterite and garnetite veins. Standard thermobarometric techniques based on electron microprobe analyses yield pressure (P) and temperature (T) estimates around 3.4GPa, and 800 degrees C for the peridotite body and 5.5 GPa, and 800 degrees C for the websterite veins consistent with UHP conditions. In addition, polyphase solid inclusions, consisting of silicates, carbonates, sulphates and elemental carbon (including microdiamond), are randomly located in garnet and clinopyroxene of the websterite vein assemblage. Garnet-clinopyroxene mineral pairs yield a Sm-Nd cooling age of 393 +/- 3 Ma for the peridotite and 381 6 Ma for the vein assemblage suggesting that the Svartberget body was overprinted during the UHPM of the Scandian Orogeny. The initial ratio of the mineral isochron and Nd model ages suggest a mid-Proterozoic origin for the peridotite body. The polyphase inclusions, coupled with high Sr-87/Sr-86 ratios may indicate that the peridotite body was infiltrated by crustal-derived C-O-H melts/fluids at UHPM conditions to form the websterite veins in the diamond field. We propose that fracturing and vein emplacement were the result of local high fluid pressure during subduction of the Baltic plate.