Metamorphic paragenesis of the Bjørkedalen peridotite, western Norway: implications of amphibole zoning




Pasillas, Nancy

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The Bjørkedalen peridotite in the Western Gneiss Region (WGR) of Norway has a mineral assemblage consisting of abundant hydrous minerals implying significant presence of water during metamorphism. Examining mineral chemistry and textural relationships in peridotite samples leads to inferences about the pressure, temperature and fluid infiltration history of the mantle. This research aims to understand the metamorphic evolution of the hydrous assemblage, the appearance of amphiboles, and the pressure-temperature implications of amphibole zoning. I seek to understand whether this zoning developed on the prograde or retrograde path, and thus whether the peridotite was entrained during subduction or subsequent exhumation. These objectives are accomplished by thermodynamic modeling of whole rock analyses using Perpl_X and comparing these equilibrium models to modal and chemical analyses of chlorite and amphibole obtained by scanning electron microscopy (SEM) and an electron microprobe analyzer (EMPA). Chronology of the WGR is well understood: subduction to peak conditions of 750?C and 3.5 GPa between 425-400 Ma followed by decompression and amphibolite facies metamorphism during near-isothermal exhumation at 750 °C, 1.5-0.5 GPa between 400-380 Ma. Our results indicate that the hydrous mineral development took place on a retrograde path. Retrograde features include chlorite growing at the expense of spinel and anthophyllite overgrowths around tremolite cores. The retrogressive P-T path that produced the mineral assemblage of the Bjørkedalen peridotite is consistent with decompression following ultrahigh pressure metamorphism and the overprint of amphibolite-facies retrogression during exhumation in the WGR.


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Amphibole zoning, Bjørkedalen, Western Gneiss Region, Western Norway, peridotite