Ultramafic nodules from Seiland, northern Norway

Abstract

Peridotitic inclusions were emplaced in synorogenic basic and ultrabasic dykes during the later evolutionary stages of the Seiland petrographic province. Tectonite and cumulate types are recognized. The former show penetrative textural evidence of a series of pre-incorporation mineral assemblages due to partial equilibration of the source along a decreasing P-T gradient. Unmixing of aluminous pyroxenes and reaction with spinel resulted in the appearance of minor plagioclase, which later was replaced by hydrous minerals. The metamorphic fabric eliminates an accumulative relationship with the transporting magma. The rare cumulate-type nodules, however, retain evidence of poikilitic textures and primary plagioclase; orthopyroxene and spinel are due to subsolidus reaction of olivine and plagioclase. A high-temperature alpine peridotite rising diapirically through the upper mantle is postulated to be the parent of the tectonite-type nodules. This intrusion may be the source of the synorogenic magmatism and positive gravity anomaly of the Seiland province.