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Petrogenesis of Cogenetic Silica-Oversaturated and -Undersaturated Syenites by Periodic Recharge in a Crustally Contaminated Magma Chamber: the Kangerlussuaq Intrusion, East Greenland |
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| Authors: | Riishuus Morten S; Peate David W; Tegner Christian; Wilson J Richard; Brooks C Kent |
| Institution: | 1Department of Earth Sciences, University of Aarhus, 8000 Århus C, Denmark 2Department of Geoscience, University of Iowa, 121 Trowbridge Hall, Iowa City, Ia 52242, USA 3Geological Institute, University of Copenhagen, ØSter Voldgade 10, 1350 Copenhagen K, Denmark |
| Abstract: | The Palaeogene Kangerlussuaq Intrusion ( 50 Ma) of East Greenlanddisplays concentric zonation from quartz-rich nordmarkite (quartzsyenite) at the margin, through pulaskite, to foyaite (nephelinesyenite) in the centre; modal layering and igneous laminationare locally developed but there are no internal intrusive contacts.This is an apparent violation of the phase relations in Petrogeny'sResidua System. We propose that this intrusion is layered, gradingfrom quartz syenite at the bottom to nepheline syenite at thetop. Mineral and whole-rock major and trace element data andSr–Nd–Hf–Pb isotope data are presented thatprovide constraints on the petrogenesis of the intrusion. Radiogenicisotope data indicate a continuously decreasing crustal componentfrom the quartz nordmarkites (87Sr/86Sr = 0·7061;  Ndi= 2·3; Hfi = 5·2; 206Pb/204Pbmeas = 16·98)to the foyaites (87Sr/86Sr = 0·7043–0·7044;Ndi = 3·8–4·9; Hfi = 10·7–11·1;206Pb/204Pbmeas = 17·78–17·88); the foyaitesare dominated by a mantle isotopic signature. The average Mg-numberof amphibole cores becomes increasingly primitive, varying from26·4 in the nordmarkites to 57·4 in the pulaskites.Modal layering, feldspar lamination and the presence of hugebasaltic xenoliths derived from the chamber roof, now restingon the transient chamber floor, demonstrate bottom-upwards crystallization.The intrusion cannot, therefore, have formed in a system closedto magmatic recharge. The lack of gneissic xenoliths in thenordmarkites suggests that most contamination took place deeperin the crust. In the proposed model, the nordmarkitic magmaformed during crustal assimilation in the roof zone of a large,silica-undersaturated alkali basaltic/basanitic, stratifiedmagma chamber, prior to emplacement in the uppermost crust.The more primitive syenites, terminating with foyaite at thetop of the intrusion, formed as a consequence of repeated rechargeof the Kangerlussuaq Intrusion magma chamber by tapping lesscontaminated, more primitive phonolitic melt from deeper partsof the underlying chamber during progressive armouring of theplumbing system. KEY WORDS: Kangerlussuaq; East Greenland; syenite; crustal contamination; magma mixing |
| Keywords: | : Kangerlussuaq East Greenland syenite crustal contamination magma mixing |
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