Abstract

The Abitibi subprovince is widely regarded as a classic example of Neoarchean continental crust formed in an ensimatic setting. However, sialic crust as old as 2.93 Ga occurs in the adjacent Wawa subprovince, and the two subprovinces are thought to have formed a contiguous block since the initiation of Abitibi magmatic activity at 2.75 Ga. A detailed geochemical, U-Pb geochronological, and Nd-Hf isotope study of plutonic and volcanic rocks from the southwestern Abitibi provides evidence for the involvement of older crust during magma genesis near the Abitibi-Wawa boundary (Kapuskasing structural zone). Inherited zircons dated at ca. 2.85 Ga occur both in 2747 Ma tonalite and 2700 Ma granodiorite of the Rice Lake batholith, part of the larger Kenogamissi plutonic complex. Igneous zircons from this batholith display large intrasample variations in 176Hf/177Hf, which we attribute to mixing of magmas derived from mantle ± juvenile crust and older crustal sources. Nd isotope data for felsic volcanic and plutonic rocks in this region indicate a dominantly juvenile source; however, two older felsic volcanic units (2739 and 2729 Ma) show Nd isotope evidence of significant crustal contamination. Post-tectonic plutonic units that were emplaced at 2676 and 2662 Ma also contain inherited zircons, but none are older than 2.76 Ga. In contrast, tonalitic units dated at 2744 and 2715 Ma from the Round Lake plutonic complex, located 150 km east of the Kapuskasing structural zone, lack zircon inheritance and display less intrasample Hf isotope variability in zircons. Our data suggest that the influence of older sialic crust during Abitibi volcanic and plutonic activity is restricted both in space and time. Similar evidence for an older crustal influence occurs elsewhere in the southwestern Abitibi near the Kapuskasing structural zone. The data collectively suggest that an approximately 75-km-wide belt of Abitibi crust adjacent to this structure was underlain by older Wawa crust from 2.75 Ga onward. This crustal geometry is consistent with development of the Abitibi sub-province at the edge of a Wawa protocraton, perhaps as the result of rifting of this older continental block.

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