PETROGENESIS OF THE 1998 MA MINTO LARGE IGNEOUS PROVINCE: EVIDENCE FOR TWO DISTINCT MAGMA SOURCES AND PLUMBING SYSTEM PATHWAYS

The 1998 Ma Minto Large Igneous Province (LIP) of the Superior craton comprises the Watts Group ophiolite and the Povungnituk Group flood basalts of the Cape Smith belt to the north, and the Minto dykes and the Eskimo Formation volcanics of the Belcher islands to the south. We demonstrate that the northern and southern parts have distinct and unrelated compositions, and discuss the petrogenetic significance.

The Povungituk and Watts Group magmatism to the north display compositions ranging between MORB- and OIB-like endmembers. Initial ɛNd isotope values range from +2.6 to +4.3, indicating an absence of crustal contamination; consistent with the lack of a negative Nb-Ta anomaly.

The Minto dykes and the paleomagnetically-linked Eskimo volcanics to the south display a distinct geochemical signature in comparison with the units in the northern part and have ɛNd isotopic compositions between -3.7 and -7.9. Although contamination has been involved in the formation of the Eskimo volcanics, the Minto dykes are nearly uncontaminated, showing that contamination alone could not have produced the different geochemical signature, which is different from the northern units.

Two different processes can explain the subdivision into distinct northern and southern parts, different lithospheric effects or two plume sources:

1) The WNW-trending Minto dykes do not indicate melt generation at great depths beneath the central Superior craton (e.g., high Gd/Yb ratios) and lateral magma emplacement from a plume centre located on the northwestern side of the craton is more compelling. In this scenario, no interaction with lithospheric mantle occurred in the source area for the northern part of the LIP whereas melt generation in, or interaction with, the lithospheric mantle created different geochemical signatures for the southern part.

2) Different signatures are related to the mantle plume source. In this scenario, two different compositions could be sourced on different sides in the deep mantle of the starting mantle plume, and then remain on opposite sides of the plume during ascent to the base of the lithosphere. These two differently sourced portions of the LIP would then spread out in opposite directions from the plume centre, comparable to recent models for parallel chains of Hawaiian volcanoes and other double track hotspot chains.