Formation of low-δ18O magmas of the Kangerlussuaq Intrusion by addition of water derived from dehydration of foundered basaltic roof rocks

Abstract

The Kangerlussuaq Intrusion in East Greenland is concentrically zoned from quartz nordmarkite (quartz syenite) at the margin, through pulaskite, to foyaite (nepheline syenite) in the centre, with no apparent intrusive contacts. The δ18O values of coexisting minerals are consistent with oxygen isotope equilibrium at magmatic temperatures. Most of the intrusion formed from low-δ18O magma; magma δ18O values generally increased upwards from about 3.3 ‰ in the quartz nordmarkites to 5.6 ‰ in the foyaites. The lowest magma δ18O value of about −1.0 ‰ is from the upper part of the nordmarkites, where there is a high concentration of foundered basaltic xenoliths (stoped from the roof of the intrusion). The amphiboles in the syenites have δD values that range from those typical of hydrous mantle-derived minerals to much lower values (−86 to −157 ‰), as do whole-rock samples of xenolith and country rock (−125 to −148 ‰). The low magma δ18O and δD values are consistent with continuous incorporation, exchange and upward escape of low-δ18O and δD fluids released from stoped basaltic roof material. Mass balance suggests that the integrated amount of water involved was 7 wt% of the volume of the magma, but locally reached 30 wt% water. The requirement for large amounts of water with low δ18O value is satisfied only if the foundered basalt contained most of its water in cavities as opposed to hydrous minerals. Even with this requirement, the volume of stoped basalt would have been equal to the volume of the magma. Repeated recharge of the residual magma with progressively less contaminated silica undersaturated melt resulted in a gradual shift across the low-pressure thermal divide. Crystallisation was suppressed by the depression of the liquidus due to water saturation of the residual magma (pH2O ~1 kbar).