Insight into the origins of metasomatism along the southern margin of the North Atlantic Craton, South Greenland

• Main Author: HOARE, BRENDAN COLIN
• Other Authors: Tomlinson, Emma, Science Foundation Ireland (SFI)
• Format: Thesis
• Language: unknown
• Published: Trinity College Dublin. School of Natural Sciences. Discipline of Geology 2018
• Subjects: North Atlantic Craton; Subduction; Metasomatism; Halogens; Cratonic sub-continental lithospheric mantle; Greenland; North Atlantic
• Online Access: http://hdl.handle.net/2262/82229; http://people.tcd.ie/hoareb

APPROVED The cratonic sub-continental lithospheric mantle (C-SCLM) has been isolated from mantle convection since it stabilised more than 2.5 billion years ago. Volatiles introduced into the C-SCLM drive metasomatism, ultramafic magmatism and diamond formation, yet the source of volatiles, their abundance and residence in the C-SCLM is poorly unconstrained. The halogen composition of ultramafic lamprophyre (UML) provide an excellent opportunity to investigate the provenance of metasomatic agents in the C-SCLM. This thesis provides novel, combined elemental (F, Cl, and Br) and isotopic (δ37Cl) halogen data for UML from the North Atlantic Craton(NAC), South Greenland. The UML display positively fractionated δ37Cl compositions (+1.0 ‰ to +1.2 ‰), consistent with recycled halogens within the UML source region. The Br/Cl composition of the UML ranges from 0.0047 to 0.0049, precluding derivation from an asthenospheric source, instead suggesting addition of a high Br/Cl metasomatic component from the C-SCLM of the NAC. The data are consistent with a scenario in which halogen-enriched fluids were liberated from a subducting slab beneath the NAC at ~1.8 Ga and provide clear evidence of subduction metasomatism in the NAC. The favored scenario for the halogen enrichment of the NAC follows that Br and Cl were delivered into the overlying C-SCLM of the NAC from fluids/melts released from serpentinized oceanic peridotite during antigorite dehydration. The high F/Cl of both UML comparative to fluids released during antigorite dehydration but low F/Cl comparative to kimberlite likely results from the mixing of a depleted upper mantle melt in terms of F/Cl and a low F/Cl and enriched Br and Cl component relative to depleted upper mantle in the C-SCLM. Variation in whole-rock mantle incompatible trace element compositions of both UML suggest the sampling of a spatially heterogeneous C-SCLM. The spatial position of UML at Nigerdlikasik relative to those from Pyramidefjeld could suggest the trace element enriched MARID source ...