Evidence for enrichment of subcontinental lithospheric mantle from Paleoproterozoic intracratonic magmas: Geochemistry and U–Pb geochronology of Martin Group igneous rocks, western Rae Craton, Canada

Abstract

The Paleoproterozoic Martin Group, located in the Rae Craton of northwestern Saskatchewan, is a sequence of intercalated siliciclastic deposits and mafic igneous rocks that were deposited in a fault-controlled intracratonic basin. Based on field relationships, mafic volcanic rocks and sills of the Martin Group have historically been viewed as genetically related to a suite of regionally extensive mafic dikes. In this study, the geochemical characteristics of the three mafic suites (volcanic rocks, sills, and dikes) were used to interpret their respective magmatic histories, to assess the possibility of a genetic relationship between the suites, and to deduce the nature of the magma source. Geochemically, these rocks range from subalkaline (tholeiitic) basalts to alkali basalts or trachyandesites and contain trace element compositions consistent with emplacement in an intracontinental setting. All three suites are highly enriched in light rare earth elements but have anomalously low concentrations of high field strength elements (e.g. Nb, Ta, Hf, Zr) relative to other trace elements. The trace element compositions suggest derivation of all three suites from a common subcontinental lithospheric mantle source that had experienced a complex enrichment history, including modification of depleted mantle by ancient subduction-related components and by small degree asthenospheric melts. A common, enriched source for these magmas is confirmed by their identical initial Nd isotopic compositions ( ɛ Nd at the time of emplacement of −3.7 to −5.3), likely indicative of input of isotopically evolved crust to the source region during the subduction-related enrichment. The overall similarity of chemical compositions for the three suites indicates that regional emplacement of the mafic dikes and (localized?) magmatism within the Martin Group was essentially part of the same mafic magmatic event in the area, though slight intersuite compositional variations might reflect differences in magma evolution histories. A U–Pb baddeleyite date from one dike broadly dates this magmatic event at ca. 1818 Ma, though it must have occurred as a series of magmatic pulses that alternated with periods of sedimentary deposition within the Martin basin. The dike magmas infiltrated pre-existing fractures, whereas the intrabasinal suites might have utilized major faults associated with basin genesis (e.g. the Black Bay fault) as magma conduits.