| Popis: |
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. Two tectonic sites are represented: those with siliciclastic facies represent continent-adjacent syn- to post-rift stable shelves and those dominated by banded iron-formation, with volcaniclastic deposits, represent arc-adjacent post-rift stable shelves. Sedimentary rocks of the conglomerate-wacke association are represented by thick wedges of siliciclastic or volcaniclastic sandstone, with minor volcanic components. These were deposited in braid-plain, braid-delta and submarine-fan environments within elongate sedimentary basins. Coastal-plain and pelagic basin facies are also present. Siliciclastic detritus was derived from syndepositional, magmatic 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 of marginal-marine to fluvial environments. The association is identical to that of compressional-foreland basins of arc-continent collisional 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 environments ranged from talus and alluvial fan, and braid-plain to either lacustrine or coastal marine. Local basin margins often show lateral offsets from source terrains and vertically skewed facies patterns. Longitudinal facies geometries are typical. The association is similar to that of strike-slip collisional graben in hinterland tectonic-escape and terrane-accretion orogens. The examples show that, despite all the ramifications of secular geophysical, geothermal and geochemical global change, the stratigraphic style of sedimentary rocks in Archean greenstone belts can be matched with modern tectonic analogues, thereby emphasizing the conformity of stratigraphic style through time. The inseparable link between stratigraphic and tectonic styles implies that the tectonic style of greenstone belts was not temporally unique. Major differences exist with regards to the chemical composition of certain magmatic and sedimentary products, but these are compensated for by their constant stratigraphic function in terms of depositional processes, depositional environments and tectonic sites. |
