The Lilloise intrusion, east Greenland: Hydrogen isotope evidence for the efflux of magmatic water into the contact metamorphic aureole

Abstract

The epizonal, Tertiary Lilloise layered intrusion (5–9 km diameter) has a well developed amphibolitised basalt aureole, several hundred metres wide. The sequence of main cumulate minerals is olivine, augite, plagioclase, amphibole (Ti-hornblende to hastingsite), with amphibole first appearing as an intercumulus mineral about 2000 m below the present top of the intrusion. The range of δD and δ¹⁸O values (SMOW) of amphiboles and biotites from early intercumulus, and cumulus minerals and late pegmatites is very narrow, -70 to -88‰ and +4.3 to 5.5‰ respectively. Amphiboles or whole rocks from the basaltic country rocks change from δD= -85‰ near the contact to -116 in the outer part of the aureole, and to -118 in the non-metamorphosed basalts up to 9km away. All basalts are depleted in ¹⁸O relative to “normal” values with the largest depletion (up to 2–3‰) in the outer part of the aureole. Meteoric water did not interact with either the magma or the hot plutonic rocks. This is in contrast to the results from most other plutonic complexes in the North Atlantic Tertiary igneous province (Skaergaard, Kangerdlugssuaq, Skye, etc.) many of which were similarly emplaced into basaltic country rocks. A meteoric-hydrothermal convective system was established in the basalts including the inner contact zone prior to the complete crystallisation of the Lilloise magma. The inner part of the aureole was finally modified by magmatic-hydrothermal fluids which evolved non explosively from the Lilloise magma. The meteoric-hydrothermal system in the outer part of the aureole, with integrated water/rock ratios of about 0.2 (atom % oxygen), did not collapse in on the hot pluton or inner contact metabasalts. Several factors may have reduced the permeability of the basalt country rocks. The emplacement of dykes prior to the Lilloise intrusion and possible associated weak hydrothermal activity could have restricted circulation of meteoric water; major fracture permeability was not generated during the quiet non-explosive crystallisation history of the Lilloise single magma pulse; metasomatism and alteration accompanying the magmatic-hydrothermal fluids may have reduced permeabilities in the inner aureole. Meteoric waters, however, did enter locally, at a very late stage, during low temperature serpentinisation of periodotite.