Tholeiitic to calc-alkaline metavolcanic transition in the Archean Nigerlikasik Supracrustal Belt, SW Greenland

• Published in: Precambrian research 2017-11, Vol.302, p.50-73
• Main Authors: Klausen, Martin B., Szilas, Kristoffer, Kokfelt, Thomas F., Keulen, Nynke, Schumacher, John C., Berger, Alfons
• Format: Article
• Language: English
• Subjects: Andesite; Archean; Basalt; Dacite; Island arc; Subduction
• ISSN: 0301-9268; 1872-7433
• DOI: 10.1016/j.precamres.2017.09.014

•Transect through a ∼580m metavolcanic tholeiite to calc-alkaline sequence.•Aplite sheets yield zircon U-Pb minimum age of 2930±4Ma.•Petrogenesis through sub-volcanic processes within an evolving Archean island arc.•Mantle-derived tholeiites evolved through low-pressure fractionation.•Adakitic calc-alkaline suite evolved through high-pressure fractionation±crustal melts. We present bulk-rock geochemical and U-Pb zircon age constraints on a ∼580m thick sequence of Mesoarchean metavolcanic rocks from SW Greenland. The rocks were arguably deformed into a tight synform and metamorphosed under amphibolite facies conditions, where relict volcanic structures testify to their igneous origins. The sequence includes picrites, tholeiitic basalts and calc-alkaline andesitic to dacitic schists, interbedded with syn-volcanic mafic and felsic feeder intrusions. Amongst late aplitic intrusions, four conform to a minimum U-Pb age for the entire succession of 2929±5Ma, which is slightly older than a protolith intrusion age of 2902±4Ma for the regional grey orthogneisses. The metavolcanic section describes an overall inward increase in SiO2 and Al2O3, coupled with a decrease in MgO, total iron (FeOT) and CaO, across a major discontinuity that separates: (1) A tholeiitic suite with high FeOT, low Al2O3/TiO2 coupled with flat REE-patterns, interpreted as mantle-derived signatures; and (2) a more evolved calc-alkaline suite with relatively low FeOT, high Al2O3/TiO2 and steep REE-patterns, yet relatively high Mg, Ni and Cr, which resemble adakites (or high-Mg andesites). A systematic greater negative Nb-anomaly (Nb/Nb∗) and LREE-enrichment (La/Sm)N up through the metavolcanic section is consistent with an increasing subduction zone signature, and thereby a likely arc setting for all rocks. The overall field relationships argue strongly for the deposition of (i) enriched tholeiites (including basal picrites that could be either lavas or cumulates), (ii) more depleted tholeiites that also intruded the enriched tholeiites, before (iii) an abrupt transition into distinctly different calc-alkaline and adakite-like volcanics that evolved from andesites to dacites. Thus, the Nigerlikasik section records tholeiitic magmatism, succeeded by calc-alkaline adakitic magmatism during the evolution of an Archean island arc. In our single arc model, we discuss the possibilities of the early tholeiitic suite being formed through progressively more hydrous melting of a juvenile mantle wedge and s