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

Abstract

Thepreviouslyreported''Fe-Titype''garnet-peridotiteislocatedinthenorthernpartofthewellknownultra-highpressure (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 byaconjugatesetofmetasomaticfracturesfilleddominantlywithdiamond-bearinggarnet-phlogopite-websterite (5.5GPaat800 ! 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 CaledonianUHPevent.PresentobservationsintheWGR documentaregionalmetamorphicgradientincreasingtowardstheNW,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 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 con- strained breaks duetoformationoflarge-scale fracturesinthe crustanddecompression meltingstarts.Modelling usingfinite-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.