A lower crustal mafic source for the ca. 2550 Ma Qôrqut Granite Complex in southern West Greenland

Abstract

The late Neoarchaean Qôrqut Granite Complex is the youngest large igneous intrusion in the Nuuk region in southern West Greenland, where basement is primarily of Eoarchaean and Mesoarchaean age with a tonalite–trondhjemite–granodiorite (TTG) composition. The Qôrqut granite is generally undeformed and it intruded during a prolonged period, starting at ca. 2730Ma, characterised by crustal reworking, possibly related to syn- or post accretion tectonics or continental collision. We present major and trace element whole rock chemistry and combined U/Pb, Hf and O isotope data from zircon. We obtained a mean zircon U/Pb age of 2547±4Ma (MSWD=0.63). Initial εHf values range from −12 to −18 requiring a long residence time and a rather homogeneous source. Sample averaged zircon δ18O values range from 6.1±0.2‰ to 6.5±0.3/0.7‰ best interpreted with a source region of mainly unweathered mantle derived igneous rocks. Compared to the regional TTG basement, the QGC is characterised by low CaO and Na2O and high K2O, LREE and Rb contents, and a stronger fractionated REE pattern with a negative Eu anomaly. We show that the homogeneous Hf isotope signature of the granite together with its low epsilon value and its pristine oxygen isotope composition are best explained with an Eoarchaean mafic source with a 176Lu/176Hf around 0.015–0.019. Trace element modelling confirms that a mafic source in REE and with an eclogitic residue and with plagioclase as a fractionating phase would generate appropriate melt compositions. Modelling requires residual rutile in the source which constrain the pressures to >ca. 13–18kbar. Zirconium saturation temperatures suggest magma temperatures in the range 750–850°C. The obtained P–T conditions suggest a lower crustal source region in a thickened crustal unit consistent with a post or late continental collisional setting.