Extensional tectonics and mafic plutons in the Ketilidian rapakivi granite suite of South Greenland

The early Proterozoic rapakivi suite in the Ketilidian mobile belt of South Greenland is bimodal with an abundance of mafic plutons which mingled and hybridised with the rapakivi granites. Basic rocks lacking signs of hybridisation and with mineralogy and bulk compositions apparently unaffected by proximity to the granites are enriched in large ion lithophile element (LILE) relative to high field strength element (HFSE) and have moderate Nb depletion. These are characteristics common to basic rocks found in back-arc basins. This tectono-magmatic environment supports a model for the late Ketilidian of crustal extension, probably caused by slab roll-back and is in keeping with the low-pressure, high-temperature metamorphic conditions which prevailed when the rapakivi suite was emplaced. Nd and Sr isotopes confirm the juvenile character of the Ketilidian crust and rapakivi granites. Hybrid mafic rocks have low eNd compared to depleted mantle, while apparently unhybridised examples have also undergone some isotopic modification.