Scandian Ultrahigh-Pressure Metamorphism of Proterozoic Basement Rocks on Fjørtoft and Otrøy, Western Gneiss Region, Norway

Abstract

Electron microprobe mineral composition data are presented for samples of exsolved and recrystallized garnet websterites within the mantle-derived peridotite bodies and of external orthopyroxene eclogite lenses directly enclosed within the gneisses on the islands of Fjørtoft and Otrøy in the Western Gneiss Region of Norway. These data are utilized to obtain P-T estimates that lie within the coesite stability field, and are mostly also compatible with rare observations of microdiamond formation and preservation for the deformed and re-equilibrated garnet websterite assemblages in the peridotite bodies included within the basement gneiss sequences on both islands. Equivalent P-T estimates for external orthopyroxene eclogite lenses within the basement gneisses on both islands are mostly not well constrained due to fairly extensive modification of orthopyroxene compositions during amphibolite-facies retrogression. Nonetheless, P-T estimates for the least retrogressed samples again indicate the likelihood that these rocks underwent Scandian ultrahigh-pressure metamorphism. We thus conclude that, contrary to a previously published interpretation, both the Caledonian Blåhø nappe sequence and the underlying Proterozoic Baltica basement gneisses on Fjørtoft experienced Scandian ultrahigh-pressure metamorphism, thus denying a previous suggestion that a major, greater than normal continental crust thickness, tectonic break exists between these rock sequences on this island. Similarly, the Baltica basement gneiss sequence exposed on the northern side of Otrøy, with its comparable mantle-derived peridotite bodies and external orthopyroxene eclogite lenses, is also shown to have been subjected to the Scandian ultrahigh-pressure metamorphic event.