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 co nformity 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-ion -formation or carbonate, (5) conglomerate-wacke and (6) conglomerate-a renite. Examples of the associations are described from the Kaapvaal, Superior and Zimbabwe Provinces and the Pilbara Block. Each associatio n differs only slightly between the four regions, thereby emphasizing a common tectonostratigraphic theme. Sedimentary rocks are a minor com ponent of the mafic-ultramafic volcanic association. They were deposit ed in two types of volcanic-basin environments: one similar to barred lagoons and bays around oceanic volcanic islands, and the other simila r to sediment-starved platforms adjacent to coalesced volcanics in int er-arc, intra-arc and back-are basins. In contrast, sedimentary rocks in the calc-alkaline volcanic association comprise thick wedges of epi clastic and volcaniclastic detritus deposited in elongate sedimentary basins, and thick wedges of pyroclastic and volcaniclastic detritus de posited adjacent to volcanic centers. Sedimentary facies were identica l 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 s electively preserved in some terrains. Sedimentary rocks of the bimoda l volcanic association are thick wedges of siliciclastic and volcanicl astic deposits interbedded with subaerial to submarine erupted flows o f komatiite, basalt, and rhyolite. Depositional environments ranged fr om braid-plain and braid-delta to coastal marine and submarine fan. St arved-basin deposits include banded iron-formation and sulfidic black shale. The association is identical to that of cratonic extensional ba sins in arc-continent and intracontinental rifts. Sedimentary rocks of the quartz arenite-iron-formation or carbonate association onlap weat hered basement or bimodal-volcanic association rocks. Depositional env ironments were either transitional from fluvial to siliciclastic and c arbonate marginal-marine to offshore pelagic-hemipelagic, or from insh ore intrabasinal-clastic and volcaniclastic to offshore pelagic-hemipe lagic. Siliciclastic deposits are supermature. Two tectonic sites are represented: those with siliciclastic facies represent continent-adjac ent syn- to post-rift stable shelves and those dominated by banded iro n-formation, with volcaniclastic deposits, represent arc-adjacent post -rift stable shevles. Sedimentary rocks of the conglomerate--wacke ass ociation are represented by thick wedges of siliciclastic or volcanicl astic sandstone, with minor volcanic components. These were deposited in braid-plain, braid-delta and submarine-fan environments within elon gate sedimentary basins. Coastal-plain and pelagic basin facies are al so present. Siliciclastic detritus was derived from syndepositional, m agmatic and metasedimentary thrust-belts. Two stratigraphic styles are preserved: one that records a flysch-like stage of deep- to marginal- marine environments, and the other that records a molasse-like stage o f marginal-marine to fluvial environments. The association is identica l to that of compressional-foreland basins of arc-continent collisiona l and compressional-arc tectonic affinities. The conglomerate-arenite association is preserved in elongate structural basins that transgress crustal components and all other supracrustal units. It is dominated by thick wedges of siliciclastic sandstone, although bimodal volcanic and volcaniclastic rocks can also be present. Depositional environment s ranged from talus and alluvial fan, and braid-plain to either lacust rine or coastal marine. Local basin margins often show lateral offsets from source terrains and vertically skewed facies patterns. Longitudi nal facies geometries are typical. The association is similar to that of strike-slip collisional graben in hinterland tectonic-escape and te rrane-accretion orogens. The examples show that, despite all the ramif ications of secular geophysical, geothermal and geochemical global cha nge, the stratigraphic style of sedimentary rocks in Archean greenston e belts can be matched with modern tectonic analogues, thereby emphasi zing the conformity of stratigraphic style through time. The inseparab le link between stratigraphic and tectonic styles implies that the tec tonic style of greenstone belts was not temporally unique. Major diffe rences exist with regards to the chemical composition of certain magma tic and sedimentary products, but these are compensated for by their c onstant stratigraphic function in terms of depositional processes, dep ositional environments and tectonic sites.