ARCHEAN LITHOSPHERE: THE PRODUCT OF INTERACTION BETWEEN PLUME AND CONVERGENT MARGIN MAGMATISM

Volcanic sequences of the ~ 2.7 Ga Superior Province are as diverse as Cenozoic counterparts. Tholeiitic basalts having flat REE patterns, interlayered with komatiites, represent intraoceanic plateaus erupted from mantle plumes. Al-depleted and undepleted-komatiites are present. The former feature negative Zr/Sm anomalies consistent with residual majorite garnet signifying melting at 450-670 km. Komatiites and basalts have Nb/Th consistently > 8 indicative of recycled oceanic crust in the mantle source of plumes. Arc tholeiitic basalts, with negative anomalies at Nb-Ta-P-Ti, plot on the low Ce-Yb trend of intraoceanic low-K tholeiites such as the South Sandwich and Marianas arcs. A 300 km belt of boninites and low-Ti tholeiites in the Abitibi terrane are characterized by high Al2O3/TiO2 but low Gd/Yb ratios. These flows interfinger with plume-related komatiites-basalts in keeping with coeval eruption of convergent margin and plume magmas. Some volcanic sequences include low- to high K-series basalts, shoshonites and leucitites, possibly recording an h-K trend. Given Th/Ce < 0.1, there is no evidence of melting of pelagic sediments. Other complex arcs have an association of tholeiitic basalts-adakites-Mg rich andesites-Nb rich basalts, as in the Cenozoic Phillipine arc. Adaites are slab melts, and the latter two volcanic types hybridization of adakite liquids with wedge peridotite.

Archean cratonic crust is imbricated arc and plateau volcanic sequences, intruded by tonalite series plutons, whereas the deep refractory mantle lithosphere has the composition of the residue of plume melting. These observations are consistent with migrating arcs capturing thick ocean plateaus, where the residue of plume melting couples buoyantly with imbricated crust