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摘要
A synthesis of new and existing data reveals that at least seven]major shear zones and an unconformity separate the rocks of the southern Barberton Greenstone Belt into seven units with different geological histories. Current views that these units constitute a continuous stratigraphic sequence and their subdivision into successive formations cannot be maintained. We argue here that the status of the traditional 'formations' of the Onverwacht Group of the Barberton Greenstone Belt be changed to 'complexes', and collectively these complexes be referred to as the Onverwacht Suite. The total age range and tectonostratigraphic thickness of the suite is about 120 million years and 15 km, respectively. The precise age range of the rocks present in each complex is largely unknown. The original spatial relationships between the complexes can in most cases only be inferred. All of the complexes include significant volumes of intrusions that were emplaced into the volcanic and volcaniclastic host rocks. The ages of the intrusions in the differen : complexes vary. In some complexes the intrusions and volcanic rocks have similar geochemistry and are probably co-magmatic. Others provide 'stitching' ages between complexes. Four of the complexes display internal relationships that resemble ophiolites. Basaltic pillowed and massive lavas are the most corn mon rock types. Sheeted dykes are generally absent whilst multiple mafic sills are common and dominate one of the complexes. The volcanology and geochemistry of the complexes suggest that they formed predominantly in oceanic spreading and intra-oceanic arc environments at water depths of 1.7-4 km. The complexes were tectonically stacked during the final stays of their development, and during two subsequent orogenic events. One complex (the Hooggenoeg Complex) was uplifted and deeply eroded between about 3460 and 3470 Ma, with a minimum uplift rate (ca. 0.5 mm/yr) similar to that during emplacement of Phanerozoic ophiolites. Unconformably overlying, sub-aerial, coarse clastic sedimentary rocks (part of the newly distinguished Noisy Complex) overlap in age with felsic-intermediate igneous activity dated between 3445 and 3460 Ma that, in turn, is genetically related to tonalite-trondjemite plutons exposed farther to the south. The emergence of the deep water Hooggenoeg Complex above sea level, the start of its erosion and fluvial sedimentation on top of it at ca. 3458 Ma, represents the onset of its obduction onto older complexes, some of which are now preserved as enclaves in the grantoid terrains to the south. Other oceanic slabs (the Kromberg and the Mendon Complexes) were subsequently emplaced onto the Noisy Complex, during latitudinal displacements (ca. 12 cm/yr) that were not notably faster than today. Exhumation, with up to 18 km of unroofing along the southern margin of the greenstone belt, took place during a second orogenic episode of subduction and accretion between 3140 and 3230 Ma. This episode was associated with folding, thrusting and molasse-type sedimentation within the greenstone belt, coupled to extension and ductile faulting at a lower crustal level. During this period, younger complexes were assembled, and the older complexes re-cycled and eroded, at rates and with geotherms comparable to those in modern orogenic belts. In the Barberton region between 3.2 and 3.5 Ga tectonic processes appear to have operated at rates similar to those within modern evolving plate boundaries.
- 出版日期2011-4
