Formation of wehrlites through dehydration of metabasalt xenoliths in layered gabbros of the Noe-Nygaard Intrusion, Southeast Greenland

Abstract

The Noe-Nygaard Intrusion is a 4 × 2.5 km stock composed of layered gabbros and wehrlites within the Precambrian basement of the coastal mountains west of the Kialineq Plutonic Complex. Transgressive relationships to Tertiary mafic dykes and the occurrence of abundant metabasaltic xenoliths signify a Tertiary age for the intrusion. The intrusion is characterized by alternating zones of gabbro and wehrlite; gabbro is both intruded and replaced by wehrlite, and the wehrlite zones are characterized by abundant metabasaltic xenoliths. Based on 87Sr/86Sr ratios, mica, olivine and oxide gabbros are all cumulates, crystallized at different differentiation stages from a common parental magma. Field relations, together with similarities in strontium isotope ratios, and in the major and rare earth element (REE) mineral chemistry between gabbros and wehrlites, indicate that the wehrlite bodies were formed by the dissolution of plagioclase from a gabbro cumulate mush by H2O derived from dehydration and the partial assimilation of metabasaltic xenoliths. In terms of their REE characteristics, melts from which the Noe-Nygaard Intrusion crystallized are within the compositional range of melts for other early Tertiary mafic/ultramafic complexes of East Greenland. However, they were generated at a greater mean melting pressure, and have less radiogenic strontium isotope ratios than the nearby Imilik mafic/ultramafic complex, supporting existing models for mantle heterogeneity at the time of continental break-up. The abundance of metabasaltic xenoliths in the Noe-Nygaard Intrusion provides further evidence for the lateral extent of the North Atlantic flood basalt province, which onshore has been mostly removed by glacial erosion south of 68° N in Greenland.