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 (87)Sr/(86)Sr 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.