Sedimentology of archean greenstone belts: Signatures of tectonic evolution

Stratigraphic styles in Archean greenstone belts are compared to those of modern and Phanerozoic depositional basins in order to test the conformity of tectonic style through time. Six lithological associations in greenstone belts are recognized: (1) mafic-ultramafic volcanic, (2) calc-alkaline volcanic, (3) bimodal volcanic, (4) quartz arenite-iron-formation or carbonate, (5) conglomerate-wacke and (6) conglomerate-arenite. Examples of the associations are described from the Kaapvaal, Superior and Zimbabwe Provinces and the Pilbara Block. Each association differs only slightly between the four regions, thereby emphasizing a common tectonostratigraphic theme. Sedimentary rocks are a minor component of the mafic-ultramafic volcanic association. They were deposited in two types of volcanic-basin environments: one similar to barred lagoons and bays around oceanic volcanic islands, and the other similar to sediment-starved platforms adjacent to coalesced volcanoes in inter-arc, intra-arc and back-are basins. In contrast, sedimentary rocks in the calc-alkaline volcanic association comprise thick wedges of epiclastic and volcaniclastic detritus deposited in elongate sedimentary basins, and thick wedges of pyroclastic and volcaniclastic detritus deposited adjacent to volcanic centers. Sedimentary facies were identical to those of forearc-trench and marine volcanoplutonic-arc settings. Chemogenic lithofacies in both volcanic associations were deposited in shallow to deep-marine waters, although shallow-water profiles were selectively preserved in some terrains. Sedimentary rocks of the bimodal volcanic association are thick wedges of siliciclastic and volcaniclastic deposits interbedded with subaerial to submarine erupted flows of komatiite, basalt, and rhyolite. Depositional environments ranged from braid-plain and braid-delta to coastal marine and submarine fan. Starved-basin deposits include banded iron-formation and sulfidic black shale. The association is identical to that of cratonic extensional basins in arc-continent and intracontinental rifts. Sedimentary rocks of the quartz arenite-iron-formation or carbonate association onlap weathered basement or bimodal-volcanic association rocks. Depositional environments were either transitional from fluvial to siliciclastic and carbonate marginal-marine to offshore pelagic-hemipelagic, or from inshore intrabasinal-clastic and volcaniclastic to offshore pelagic-hemipelagic. Siliciclastic deposits are supermature. Tw