The previously reported ``Fe-Ti type'' garnet-peridotite is located in
the northern part of the well known ultra-high pressure (UHP) area of
the Western Gneiss Region (WGR) in Norway. Primary spinel stable up to
only 2.0 GPa at 800 degrees C coexists with Caledonian ultra-high
pressure (4.0 GPa at 800 degrees C) grt-cpx-opx-of assemblages in the
Svartberget garnet-peridotite. The body is cut by a conjugate set of
metasomatic fractures filled dominantly with diamond-bearing
garnet-phlogopite-websterite (5.5 GPa at 800 degrees C) and garnetite.
Single zircon U-Pb dating suggests metamorphic growth of zircon in the
garnetite at 397.2 +/- 1.2 Ma, either coinciding or predating an initial
phase of leucosomes formation at 397-391 Ma. Field observations, major
and trace elements, mineral-chemistry, polyphase inclusions including
microdiamond, coupled with (87)Sr/(86)Sr ratios in clinopyroxene and
whole rock ranging from 0.73 to 0.74, suggest that the Svartberget
garnet-peridotite was infiltrated by melts/fluids from the host-rock
gneiss during the Caledonian UHP event. Present observations in the WGR
document a regional metamorphic gradient increasing towards the NW, and
structures in the field can account for the exhumation of the (U) HP
rocks from similar to 2.5 to 3 GPa. Assuming lithostatic pressures the
diamond-bearing Svartberget peridotite body must have come from a burial
depth of more than 150 km. However, there is a lack of observable
structures in the field to explain exhumation from extreme UHP
conditions (5.5 GPa or mere) to normal HP-UHP conditions (2.5-3 GPa),
which are common pressures calculated from eclogites in western parts of
the WGR Because of the regional and mostly coherent metamorphic gradient
across the WGR terrain, it is difficult to account for local extreme
pressure excursions such as documented from within the Svartberget
peridotite. We introduce here a conceptual model to explain the main
features of the Svartberget body. During burial and heating, rocks
surrounding the peridotite start to melt but surrounding non-molten
rocks confine the space and pressure builds up. When pressure is high
enough, conjugate brittle shear fractures develop in the peridotite.
Melt (or supercritical fluid) that has the same pressure (5.5 GPa) as
the surrounding gneiss can flow in as soon as fractures propagate into
the peridotite. This supercritical fluid is now highly reactive and
metasomatism takes place at UHP conditions along the fractures capturing
micro inclusions of diamond while growing. Finally the lithosphere
holding the overpressurised gneiss constrained breaks due to formation
of large-scale fractures in the crust and decompression melting starts.
Modelling using finite-element method (FEM) shows that melting of the
gneiss results in pressure variations when gneiss is ten to hundred
times weaker than surroundings and peridotite enclave. These pressure
variations can be up to several GPa and are qualitatively similar to
observations in the field.