A new paper led by Will Hutchison has just been published in the journal Earth & Planetary Science Letters. The paper uses modelling of sulfur isotopes across the entire Gardar Province to understand the sources of rare element enrichment. It presents a model in which Ketilidian subduction enriched the Sub-Continental Lithospheric Mantle, and we propose that it was melting of this mantle that gave rise to the enriched magmas of the Gardar Province.
New S isotope analyses and a compilation of major and trace element data shed light on the mantle origin and magmatic evolution of alkaline magmas in Greenland’s Gardar Province and, other major REE and HFSE-rich provinces around the world. We show that in most cases the primary δ34S values of Gardar melts have not been significantly modified dring ascent and the current value represents that of source. Analysis of various Gardar magmatic suites reveals an overall 34S-enriched but heterogeneous mantle source and commonality between trace element and S isotope systematics. In particular, the Late Gardar dykes of the Tuttutooq-Ilimmaasaq-Narsarsuaq zone, alkaline complexes, and clusters of silica-undersaturated dykes spatially associated with the alkaline complexes show marked enrichment in δ34S, LILE, LREE and HFSE. We propose that subduction metasomatism associated with the Ketilidian orogeny (∼500 Ma before rifting) created heterogeneous enrichment of the mantle lithosphere. This process created enriched mantle domains and a general northward decrease in the degree of metasomatism (consistent with the direction of subduction, Fig.1b). Gardar alkaline complexes are spatially associated with mantle domains enriched in δ34S, REE, HFSE and volatiles. We suggest it is no coincidence and a lithospheric mantle source enriched in incompatible metals, alkalis and volatiles, which promote extreme differentiation of alkaline melts and fluids, is critical to the formation of alkaline ore deposits.
Hutchison W, Finch AA, Borst AM, Marks MAW, Upton BGJ, Zerkle AJ, Stüeken EE & Boyce AJ (2021) Mantle sources and magma evolution in Europe’s largest rare earth element belt (Gardar Province, SW Greenland): New insights from sulfur isotopes. Earth & Planetary Science Letters, 568, 117034.