Trace element geochemistry of the Meen-Dempster greenstone belt, Uchi subprovince, Superior Province, Canada: back-arc development on the margins of an Archean protocontinent

Comprehensive trace element analyses of mafic and felsic volcanic rocks fro m the 2.85-2.74 Ga Meen-Dempster greenstone belt reveal a wide compositiona l diversity. The similar to 2.85 Ga Kaminiskag assemblage is dominated by m afic tholeiite characterized by predominantly unfractionated REE (La/Sm-n = 0.8-1.1). Rare intermediate and felsic pyroclastic flows (SiO2 = 62-74) wi th moderate to pronounced LREE enrichment are intercalated with the tholeii te. The Kaminiskag assemblage is overlain by the similar to 2825 Ma Meen as semblage, comprising dominantly dacitic tuffs and pyroclastic breccia, disp laying enriched LREE (La/Sm-n = 3.7-7.2) and moderately fractionated HREE, in conjunction with pronounced negative Nb anomalies. Five distinct suites have been recognized in the similar to 2740 Ma Confederation assemblage: (1 ) tholeiitic basalt with flat to smoothly depleted REE, (2) tholeiite with flat to weakly depleted LREE in conjunction with pronounced negative Nb ano malies, (3) Fe-rich basalt with elevated Ti and P contents, LREE enrichment , and fractionated HREE, (4) LREE enriched basalt and andesite with negativ e Nb anomalies, and (5) dacite and rhyolite with enriched LREE, moderately fractionated HREE, and variable high field strength element anomalies. The geochemistry and geochronology of the Kaminiskag and Meen assemblages are c onsistent with the formation of an oceanic back arc (Kaminiskag assemblage) , which formed the basement for a subduction-related arc complex (Meen asse mblage) after a 15 Ma hiatus. The Confederation assemblage is interpreted t o represent an Archean back arc, where the complex interplay of mantle sour ces allows for the eruption of tholeiite, subduction-modified tholeiite, oc ean island basalt-like basalt, and subduction-related arc-type volcanic roc ks. The recognition of back-arc basins within the Meen-Dempster greenstone belt emphasizes a continuity of crustal growth processes from the Archean t o the present day.