Geochemical systematics of tholeiites from the 2.86 Ga Pickle Crow Assemblage, northwestern Ontario: arc basalts with positive and negative Nb–Hf anomalies

Abstract

The ∼2.89 Ga Pickle Crow assemblage of the Pickle Lake greenstone belt is located within the central Uchi subprovince of the Superior Province. Tholeiitic basalts at the base of the assemblage have uniform SiO 2 contents and MgO spanning 10.5–3.1 wt.%. They plot with modern intraoceanic arcs in Ce–Yb co-ordinate space. There are positive to negative Nb, P and Hf anomalies, but negative anomalies dominate. Ti anomalies are systematically negative and Nb and Hf anomalies are correlated. These features are consistent with a two stage melting regime in a convergent margin. During first stage melting LREE are more mobile than Nb, P, Hf and Ti, generating basalts with negative anomalies of these elements and a refractory residue with complementary positive anomalies; as a result second stage melts inherit positive anomalies of Nb, P and Hf. The basalts plot on a hyperbolic mixing array from a high Nb (4–8 ppm), high La/Yb n , but low Zr/Nb member to a low Nb (2–4 ppm), low La/Yb n , but high Zr/Nb counterpart, indicative of a heterogeneous subarc mantle source, as depleted to slightly enriched compared to mid ocean ridge basalts, the source of which is prevalent in modern arcs. Collectively, the data can be interpreted as a paired arc–back arc where first stage melting in the back arc (low Zr/Nb), generated higher Nb basalts, and the depleted residue (high Zr/Nb) was drawn by induced convection beneath the arc where second stage melting produced lower Nb basalts. Alternatively, there was a single arc with a heterogeneously depleted subarc mantle wedge.