TEMPORAL EVOLUTION OF A LONG-LIVED SYENITIC CENTRE: THE KANGERLUSSUAQ ALKALINE COMPLEX, EAST GREENLAND

Abstract

We present new mineral and whole-rock compositions and Sr-Nd-Hf-Pb-O-H isotope data on samples from the Kangerlussuaq Alkaline Complex (~ 1000 km2) in central East Greenland, part of the North Atlantic Igneous Province. This complex mainly consists of the Kangerlussuaq Intrusion but includes at least 13 separate satellite intrusions emplaced in the uppermost crust close to the unconformity between Archaean gneisses and overlying Palaeogene flood basalts. The complex is divided into (i) older satellite intrusions (~ 55-53 Ma) composed of multiple syenites and granites and minor gabbros and peridotites, (ii) the voluminous Kangerlussuaq Intrusion (~ 50 Ma), which displays a gradual transition from quartz syenites (nordmarkites) at the margin to nepheline syenites (foyaites) in the centre, and (iii) younger satellite intrusions (~ 47-45 Ma) of minor syenites, granites and diorites concentrated southeast of the Kangerlussuaq Intrusion. The complex displays a temporal evolution in which SiO2 decreases (74-56 wt.%) and total alkalis (6-16 wt.%), amphibole Na + K content, 206Pb/204Pbmeas, εNdi and εHfi (+ 3 to + 11) increase from the older intrusions through the nepheline syenites. This is followed by a reversal to higher silica (62-73 wt.%) and lower total alkalis (9-12 wt.%), amphibole Na + K content, 206Pb/204Pbmeas, εNdi and εHfi (- 13 to + 2) in the younger satellite intrusions. Temporal changes in the location of magma plumbing systems and in magma production rates played a profound role in controlling silica content, alkalinity and degree of crustal contamination during development of the complex. Phonolitic magma was only generated after prolonged magmatism had shielded the conduits from interaction with country rock. The parental magmas were probably basanitic to alkali olivine basaltic in composition. The older satellite intrusions and the Kangerlussuaq Intrusion have low δ18O magma values (- 1 to + 6‰) compared to the younger, more crustally contaminated, satellite intrusions (+ 4.5 to + 7‰). It appears that the magmas only had sufficient over-pressure to intrude the basalt cover when larger volumes of less contaminated trachyte magma were produced, resulting in the generation of low-δ18O magmas due to dehydration of hydrothermally altered basalt xenoliths. © 2006 Elsevier B.V. All rights reserved.