Geochemical diversity in oceanic komatiites and basalts from the late Archean Wawa greenstone belts, Superior Province, Canada: trace element and Nd isotope evidence for a heterogeneous mantle

Abstract

Diverse types of komatiites and basalts are present in the 2.7 Ga Schreiber–Hemlo and White River–Dayohessarah greenstone belts of the Wawa subprovince. Al-undepleted and Al-depleted komatiites coexist. The least altered Al-undepleted komatiites are characterized by variably depleted LREE and flat to slightly fractionated HREE patterns (La/Sm n=0.5–1.2, Gd/Yb n=0.90–1.3). They have small positive Zr (Hf) anomalies relative to MREE. Al-depleted komatiites and komatiitic basalts have fractionated REE (La/Sm n=2.2–2.4), and variable Nb and Zr (Hf) anomalies (Nb/La pm=0.8–1.4; Zr/Zr*=0.8–1.0). They feature pronounced Fe-enrichment in comparison with Al-undepleted komatiites (Fe 2O 3=15–22 vs. 11–16 wt%). Tholeiitic basalts are the prevalent volcanic rock type. Mg# varies from 65 to 33 over a relatively narrow range of SiO 2 (47–53 wt%). Chromium and Ni decrease erratically with Mg#, whereas Fe, Ti, Zr and all REE increase. Compositionally, these volcanic rocks are similar to Phanerozoic ocean plateau basalts such as at Ontong Java. In suites of basalts from each of four areas, Nb and Th anomalies relative to La vary continuously from negative to positive (Nb/La pm=0.56–1.10, Th/La pm=0.32–1.23). Mantle melting processes, crustal assimilation, alteration and metamorphism, or fractional crystallization can all be ruled out as the cause of anomalies. Rather, the anomalies are interpreted to be recycling of lithosphere into the mantle: recycled ocean crust processed though a subduction zone having positive Nb anomalies, and complementary recycled subarc mantle or continental crust having negative anomalies. Alkali basalts show strong REE fractionation (La/Yb n=14–16), and large positive Nb anomalies (Nb/La pm=1.32–1.42) like Phanerozoic HIMU ocean island basalts, whereas transitional counterparts have less fractionated REE (La/Yb n=2.6–5.3), and higher Al 2O 3 contents with convex-up patterns through La–Nb–Th. Both variants share the negative Zr and Hf anomalies with Al-depleted komatiites and komatiitic basalts, and generally have high Zr/Hf ratios. Al-depleted komatiites and associated alkali basalts tend to have more positive ε Nd (+3.3 to +3.5) than Al-undepleted komatiites and associated tholeiites (+0.7 to +0.5). The former association has generally flat trends of Nb/La pm and Th/Nb pm with ε Nd consistent with mixing of more and less depleted mantle components. The latter association shows negative trends of Th/La pm and Th/Nb pm with ε Nd, consistent with mixing between a depleted mantle component and a less depleted or enriched component of recycled crust. The komatiites and spatially associated tholeiitic to alkaline basalts are interpreted to have been an ocean plateau derived from a heterogeneous multi-component plume. Negative Zr (Hf) anomalies are consistent with melt segregation with residual majorite garnet at a depth of >400 km, whereas the tholeiitic basalts melted at shallower depths either in the plume or in entrained upper mantle. The plateau was fragmented in a subduction–accretion complex and tectonically mixed with primitive arc tholeiites.