Ca. 2.17 Ga mafic magmatism in the Superior Craton: a case for a rift-related LIP

English Abstract:

Significant volumes of mantle-derived magma, evidenced by extensive mafic dyke swarms and thick gabbroic sills, intruded the Superior Craton during the Paleoproterozoic (ca. 1880-2500 Ma). The Otish Gabbro, Biscotasing, and Payne River suites and the coeval mafic volcanics in the Labrador Trough were emplaced along the southern and eastern present-day margins of the Superior craton at ~2.17 Ga. The ~2.17 Ga magmas were variably enriched in FeO, TiO₂, and incompatible elements relative to modern mid-ocean ridge basalts, suggesting contributions from at least three geochemical reservoirs (continental crust, asthenosphere, and metasomatized lithosphere). The relatively unfractionated Ti/Yb_MORB ratios (0.9-3.0) suggest that the ~2.17 Ga mafic magmas largely equilibrated at depths ≤100 km, near the spinel-garnet transition.

Estimated potential temperatures during the Paleoproterozoic (≥1550ºC) constrain the pressure of melting of ambient mantle to a minimum depth ~50 km under a thin crustal lid. Due to a sharp increase in dP/dT slopes of mineral stability surfaces at higher temperatures, melting in a significantly hotter Proterozoic mantle (ΔT =+100ºC) will cease at significantly greater depths, well within the garnet stability field. Consequently, the relatively shallow depths of equilibration of ~2.17 Ga mafic magmas of the Superior Craton are inconsistent with derivation from anomalously hot mantle plume(s). Instead, the geochemical character of ~2.17 Ga magmas suggests derivation by decompression melting of metasomatized lithosphere and of upwelling asthenospheric mantle during regional lithospheric thinning. The metasomatism of lithosphere is attributed to prior Paleoproterozoic events, possibly involving ca. 2.5 Ga mantle plume activity. The lithospheric thinning along the eastern and southern margins of the Superior Craton resulted in extensive replacement of the depleted Archean lithospheric root by fertile Proterozoic lithospheric mantle to depths of ≤ 100 km.