Non-depleted sub-continental mantle beneath the Superior Province of the Canadian Shield: Nd-Sr isotopic and trace element evidence from Midcontinent Rift basalts

Abstract

Midcontinent Rift flood basalts represent a sample of the relatively shallow, sub-continental upper mantle beneath the Canadian Shield at 1.1 Ga. A thick sequence of olivine tholeiite lavas, including minor intermediate to rhyolitic lavas, from the Portage Lake Volcanics (PLV) in northern Michigan have initial Nd and Sr isotopic compositions which cluster near Bulk Earth values. The effects of assimilation of old LREE-enriched continental crust into mantle-derived fractionating liquids are isotopically discernible in evolved lavas as well as in olivine tholeiites from the lowest portion of the volcanic pile. However, the effects of crustal contamination decrease with stratigraphie height and are absent in more primitive lavas in the upper half of the section. Therefore, the youngest olivine tholeiites preserve the isotopic characteristics of their sub-continental mantle source area which, at 1095 Ma, had ϵ Nd (T) and ϵ Sr (T) values of about +0.8 and +2, respectively. Incompatible trace element compositions from the PLV olivine tholeiite suite support the interpretation of a mantle source slightly enriched in LIL elements relative to chondritic compositions as opposed to a suite of hybrid magmas resulting from mixtures between depleted mantle and enriched crustal end-members. The source for PLV tholeiites is substantially less depleted than previously reported mantle values from the Superior Province. An origin for the PLV source is compatible with either of several mantle evolution models. The PLV source may have been associated with upwelling of a LIL element-enriched, asthenospheric plume which emplaced non-depleted material from deeper sources into the shallow subcontinental mantle beneath the Midcontinent Rift during continental break-up. Alternatively, the PLV source may have originated by enrichment of refractory sub-continental lithospheric mantle which was previously depleted in incompatible trace elements during Archean-aged melt extraction and continental crust formation. Concurrent generation of carbonatite magmas in other areas beneath the Superior Province indicates the widespread presence of sub-continental mantle with substantially higher ϵ Nd (T) and lower ϵ Sr (T) than the PLV source. Combined tholeiite and carbonatite data indicates the presence of large, chemically distinct regions in the upper mantle beneath the Superior Province at 1.1 Ga.