Scandian ultrahigh-pressure metamorphism of proterozoic basement rocks on Fjortoft and Otroy, Western Gneiss Region, Norway

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 Fjortoft and Otroy 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 Blaho nappe sequence and the underlying Proterozoic Bahica basement gneisses on Fjortoft 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 Ballica basement gneiss sequence exposed on the northern side of Otroy, 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.