Structure and petrography of the Bjørkedalen peridotite, western Norway: implications for mantle emplacement during subduction




Vasquez, Colleen

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The Bjørkedalen Peridotite is a 6.5 X 2.5 X 1 km lens lying in an upright, E-W trending, shallowly-plunging synform. It was emplaced into the crust of the Baltica plate during subduction accompanying the Caledonian Orogeny. The interaction between the crust and mantle can be examined by studying the structure and petrography of gneiss surrounding the peridotite body, to understand the relative temperatures, magnitudes of strain, and sequence of events that occurred during the emplacement. These data are used to identify likely intrusion models amongst those proposed by Brueckner (1998). Intrusion most likely occurred in a ductile environment, although brittle emplacement and subsequent subduction allowing high-temperature recrystallization to overprint brittle fabrics cannot be ruled out. The emplacement of hot mantle into crustal rock caused a decreasing gradient in temperature away from the contact; higher temperatures outlasted strain at the contact, while high-strain, fine-grained shear fabrics become dominant beyond 900m from the contact. A different lineation and sense of shear direction on either side of the peridotite body may result from the downward sinking of the intrusion, or may be developed by crustal thinning and bulk coaxial shear during exhumation.


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Earth sceinces, Intrusion, Mantle, Peridotite, Subduction