An alternative model for ultra-high pressure in the Svartberget Fe-Ti garnet-peridotite, Western Gneiss Region

Abstract: 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 C coexists with Caledonian ultra-high pressure (4.0 GPa at 800 C) grt-cpx-opx-ol 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 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 87Sr/86Sr 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 theWGR 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 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 more) 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 th