DEVELOPMENT OF PILBARA AND KAAPVAAL GRANITE-GREENSTONE TERRANES IN AN OCEANIC PLATEAU SETTING

Granite-greenstone terranes (GGT) are widely interpreted in terms of modern-day arc-accretion plate-tectonic models. Recent work in the 3.6-3.2 Ga GGTs of the Pilbara (Australia) and Kaapvaal (South Africa) cratons, however, suggest they formed as long-lived oceanic plateaux over continuously upwelling mantle. Key evidence supporting an oceanic plateau origin includes:

• Thick (>10 km), autochthonous, upward-younging stratigraphy;

• An origin of calc-alkaline felsic volcanics through fractionation and contamination of large tholeiitic magma chambers;

• An infracrustal origin for widespread sodic granites (TTG) over a range of high to moderate pressures;

• Contemporaneous development of subcontinental lithospheric mantle (SCLM) from unusually high-degree partial melting events.

Evidence previously used to support arc-accretion development (e.g. thrusts, recumbent isoclinal folds, high-P metamorphic assemblages) are explained as the result of punctuated episodes of partial convective overturn (PCO) of newly erupted, dense greenstones and more buoyant, mid-crustal granites.

Pilbara/Kaapvaal crust formed through three stages. 1) A 35 km thick proto-crust was produced (3.6-3.5 Ga) through voluminous melting of a primitive asthenospheric source, made up dominantly of enriched basalt and with low-Al TTG. 2) Continued melting of upwelling mantle (3.5-3.42 Ga) resulted in a crust ≥50 km thick and extreme melt depletion of the SCLM. Large tholeiitic magma chambers fractionated and were contaminated by older felsic crust (from Stage 1), resulting in calc-alkaline felsic volcanics. Widespread high- and low-pressure melting of Stage 1 enriched basalt and TTG produced large volumes of high- and low-Al TTG. 3) Stage 2-like events were repeated at 3.35-3.29 and 3.27-3.22 Ga, which further thickened and differentiated the crust and SCLM. Stage 3 events were accompanied by PCO, whereby extensive partial melting of granitic mid-crust facilitated sinking of greenstone belts and the rise of granite domes.

This model contrasts with evidence for arc-accretion tectonics in West Greenland, suggesting that early crustal growth followed two end-member processes relating to zones of mantle upwelling (plumes) and zones of crust recycling over primitive (shallow) subduction zones.