Petrogenesis and tectonic setting of the mafic-ultramafic rock association from NW Senja, West Troms Basement Complex

Abstract

The studied mafic-ultramafic rock association occurs within Archean tonalitic gneisses and metasupracrustal suites of the Astridal belt belonging to the West Troms Basement Complex in the northwestern part of the Senja Island. The rock association appears in lens-shaped bodies, up to 200 m long, which are oriented conformal to the major metamorphic fabric of the host rock. The rock association is made up by interlayered units of green spinel-bearing olivine orthopyroxene-hornblendite, hornblende peridotite, olivine hornblendite, hornblendite and amphibolite (metagabbroic rock). Microscope studies and whole rock geochemical data indicate that the ultramafic rocks represent a single magmatic series of mafic and ultramafic olivine-amphibole cumulates, which have been significantly affected by metamorphism. Igneous mineral and whole rock trace element data indicate MORB affinity of the cumulates and suggest their emplacement in oceanic rift or rifted continental margin setting. The studied cumulates may represent a crustal remnant of the Paleoproterozic Svecofennian ocean. The rocks were metamorphosed together with the supracrustal suites of the Astridal belt during the major Svecofennian event about 1.74 Ga ago as indicated by a U-Pb age of metamorphic zircons from the metagabbroic rock belonging to the studied suite.