Geochemistry in the Uchi subprovince, northern Superior Province : an evaluation of the geodynamic evolution of the northern margin of the Superior Province ocean basin
Hollings, Peter Nigel
Older assemblages of the Uchi subprovince, northern Superior Province, Canada, ranging in age from 2.9 to 3.0 Ga, are dominated by an association of komatiite-tholeiite sequences intercalated with felsic volcanic rocks. Komatiites vary from weakly LREE enriched to strongly depleted. The majority of ultramafic rocks display the conjunction of negative Nb anomalies with trends of increasing SiO2 and La/Smn, indicative of contamination by a felsic component. Many of the komatiites and komatiitic basalts are characterised by high V/Yb and Sc/Yb ratios which are unique amongst Archean komatiites. Tholeiitic flows intercalated with the komatiites typically display flat REE patterns with variable Nb anomalies, consistent with minor degrees of contamination. Two suites of felsic volcanic rocks are intercalated with the komatiite-tholeiite sequences of the older assemblages. Both types display pronounced LREE enrichment and negative Nb and Ti anomalies, but Type 1 has strongly fractionated HREE patterns, whereas Type 2 HREE are generally flat in conjunction with elevated compatible element contents. Collectively, the geochemical, geochronological and stratigraphic evidence from the northern Superior Province greenstone belts provide evidence for both a spatial and temporal association of mantle plumes and subduction zones during generation of the proto-continent nucleus to the Superior Province. The 2.8-2.9 Ga assemblages of the Uchi subprovince comprise mafic tholeiites characterised by predominantly flat REE, along with volumetrically significant units of felsic to intermediate volcanic rocks, that display enriched LREE and moderately fractionated HREE in conjunction with pronounced negative Nb anomalies. The geochemistry and geochronology of these assemblages is consistent with the formation of ocean plateau fragments which were accreted to the Uchi paleo continental margin where they subsequently formed the basement for a subduction related arc complex. Five distinct suites have been recognised in the ~2.7 Ga Confederation assemblage: (1) tholeiitic basalts with flat to smoothly depleted REE; (2) tholeiites with flat to weakly depleted LREE in conjunction with pronounced negative Nb anomalies; (3) Fe-rich basalts with elevated Ti and P contents, enriched LREE and fractionated HREE; (4) LREE enriched basalts and andesites with negative Nb anomalies; and (5) dacites and rhyolites with variably enriched LREE, moderately fractionated HREE and variable HFSE anomalies. The occurrence of this varied range of rocks in a single assemblage is comparable to modern back arc basins.