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

摘要

Comprehensive trace element analyses of mafic and felsic volcanic rocks from the 2.85-2.74 Ga Meen-Dempster greenstone belt reveal a wide compositional diversity. The approx 2.85 Ga Kaminiskag assemblage is dominated by mafic tholeiite characterized by predominantly unfractionated REF (La/Sm_n = 0.8-1.1). Rare intermediate and felsic pyroclastic flows (SiO_2 = 62-74) with moderate to pronounced LREE enrichment are intercalated with the tholeiite. The Kaminiskag assemblage is overlain by the approx 2825 Ma Meen assemblage, comprising dominantly dacitic tuffs and pyroclastic breccia, displaying enriched LREE (La/Sm_n = 3.7-7.2) and moderately fractionated HREF, in conjunction with pronounced negative Nb anomalies. Five distinct suites have been recognized in the approx 2740 Ma Confederation assemblage: (1) tholeiitic basalt with flat to smoothly depleted REF, (2) tholeiite with flat to weakly depleted LREE in conjunction with pronounced negative Nb anomalies, (3) Fe-rich basalt with elevated Ti and P contents, LREE enrichment, and fractionated HREE, (4) LREE enriched basalt and andesite with negative Nb anomalies, and (5) dacite and rhyolite with enriched LREE, moderately fractionated RREE, and variable high field strength element anomalies. The geochemistry and geochronology of the Kaminiskag and Meen assemblages are consistent with the formation of an oceanic back arc (Kaminiskag assemblage), which formed the basement for a subduction-related arc complex (Meen assemblage) after a 15 Ma hiatus. The Confederation assemblage is interpreted to represent an Archean back arc, where the complex interplay of mantle sources allows for the eruption of tholeiite, subduction-modified tholeiite, ocean island basalt-like basalt, and subduction-related arc-type volcanic rocks. The recognition of back-arc basins within the Meen-Dempster greenstone belt emphasizes a continuity of crustal growth processes from the Archean to the present day.