Komatiite-Tholeiite and Boninite Series Volcanic Associations of the Abitibi Greenstone Belt: Plume-Protoarc Interaction, and Crustal Growth

Abstract

Komatiite-tholeiite sequences and boninite series volcanic rocks are spatially associated at several localities along a 300 km strike sector of the late Archaean (2.7 Ga) Abitibi greenstone belt, Canada. Examples include the Kidd volcanic complex (KVC) and volcanic sequences in Tisdale and Whitney Townships near Timmins, Ontario, as well as volcanic sequences near Val d'Or, Quebec. In the field, the different types of flows interdigitate on a scale of 10's to 100's metres. Komatiites include Munro, or Al-undepleted type, and Barberton, or Al-depleted type. Spinifex textured Al-undepleted komatiites at the KVC and Whitney localities, like Munro counterparts, are characterized by near-chondritic A1203/TiO2, Ti/Zr, and Zr/Y ratios. They have (1) near-fiat HREE, (2) variable L R E E depletion (La/Smn 0.4 to 0.8), and (3) small positive Hf anomalies relative to Sm, with Zr/Hf 400 km, whereas Al-undepleted komatiites reflect melt segregation at shallow levels. Mg-, Fe-tholeiites are spatially associated with the komatiites. They have near-flat R E E patterns over a continuous range from 4 to 20x chondritic abundance. Mg#, Cr, and Ni decrease continuously with increasing REE, Th, Nb, Zr, Hf and Y. The most primitive have near-chondritic interelement ratios. This magma series is interpreted to represent dynamic partial melting in the outer envelope of the mantle plume from which the komatiitic liquids were derived. Collectively, the tholeiites have a continuous range from negative to positive anomalies of Nb relative to Th and La, similar to Phanerozoic and Recent ocean plateau basalts and some ocean island basalts (Saunders et al., 1988). Alteration and crustal contamination can be ruled out as the source of the anomalies. The negative anomalies are consistent with recycling of subduction-influenced subarc mantle, or continental crust, into the mantle, and the positive anomalies recycling of ocean crust processed through a subduction zone (Saunders et al., 1988; McDonough, 1991). Consequently, the plume from which the komatiites and tholeiitic basalts were derived was multi-component, as demonstrated from trace element and isotope systematics on Phanerozoic and Recent plume-related basalts (Mahoney et al., 1993). Compositionally, the boninite series flows range from Mg# = 81-45, TiO2 = 0.17-0.41, and Ni = 1020-310 ppm. They are characterized by high A1203 (12-16 wt.%) contents, and A1203/TiO2 (60-84) ratios. In common with Phanerozoic boninites, these flows have negatively fractionated H R E E (Gd/Ybn = 0.4-0.6), Zr/Y (1.4-2.2) less than the primitive mantle value of 2.4, and variably positive Zr and Hf anomalies, where Zr/Hf 53% and Mg# > 60 given by Crawford et al. (1989). Collectively, the boninite series overlap spinifex texatred Al-undepleted komatiites in terms of TiO2 and Zr contents, but are much more aluminous, possess higher A1203/ TiO2 ratios and Sc and Yb contents, but lower Gd/ Ybn and Zr/Y ratios. The boninites are interpreted to have formed in a protoarc from second stage metasomatic enrichment of a mantle source highly