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

• Main Author: HOARE, BRENDAN COLIN
• 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 (?37Cl) halogen data for UML from the North Atlantic Craton(NAC), South Greenland. The UML display positively fractionated ?37Cl 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 sampled by UML at Nigerdlikasik. This could reflect a flux of incompatible trace elements released at greater depth in subduction in addition to fluids/melts released during antigorite dehydration. The similarities between UML from Nigerdlikasik and lamproite from Aillik Bay could demonstrate an as yet unrecognized link between the lamproite source region at Aillik Bay and subduction metasomatism. In either case the ?37Cl composition of MARID type phlogopite from the C-SCLM underlying Nigerdlikasik clearly demonstrates the potential of the formation, or retention, of subducted volatiles in the MARID suite. The constancy of Br/Cl and ?37Cl compositions between UML at Nigerdlikasik and Pyramidefjeld, yet strong variation in incompatible trace element compositions suggest halogens released during antigorite dehydration can be effectively decoupled from traditional indicators used in the discrimination of subduction metasomatism. This work demonstrates that metasomatised C-SCLM domains have a capacity to record and preserve halogen signatures related to subduction metasomatism on billion-year timescales, allowing the discrimination of agents involved in metasomatism of ancient C-SCLM regions.