Search

The Heerenveen batholith exemplifies an aerially extensive group of ∼3·1 Ga felsic potassic batholiths that mark the final magmatic event associated with the cratonization of the central–eastern Kaapvaal Craton, and the transition from the earlier sodic tonalite–trondhjemite–granodiorite (TTG) to the more recent style of monzogranitic and granitic magmatism. As is typical of these bodies, it is geochemically and mineralogically heterogeneous, sheeted in structure, and was assembled incrementally, with eight main units in four assembly stages. Apart from a metaluminous quartz monzonite, all the Heerenveen rocks are peraluminous. There are few identifiable geochemical lineages among the rocks of the batholith. The geochemical heterogeneity is largely due to protolith internal heterogeneity, tapping of multiple sources, pulsed magma extraction related to sequential occurrence of discrete melting reactions, and varying degrees of entrainment of peritectic minerals formed during partial melting of the crustal protoliths, together with varying structural controls on magma extraction and ascent, which allowed distinct magma sources to be tapped, at different periods during batholith assembly. Fractional crystallization, magma mixing and wall-rock assimilation were not significant processes. TTG rocks are not suitable source materials for any of the Heerenveen magmas; nor are highly aluminous metasediments. For magmatic rocks with SiO2 <70 wt %, the likely source rocks were intermediate to mafic igneous rocks, with variable degrees of K-enrichment. For those rocks with SiO2 >72 wt %, the sources were K-enriched intermediate to felsic rocks. It seems likely that the partial melting reactions that produced the Heerenveen magmas occurred at moderate temperatures and that hornblende was present in the residues. This suggests that initial melting may have occurred in the presence of an H2O-rich fluid. There is no evidence that the genesis of the potassic granitic magmas involved either recycling of TTG crust or the presence of aluminous metasediments in the deep crust. The K-enrichment in the protoliths was most probably due to their ultimate derivation from mantle rocks metasomatized by fluids evolved in subduction-like processes. The monzonite in the batholith may have been derived directly by partial melting of this metasomatized mantle. [ABSTRACT FROM PUBLISHER]

A U-Pb zircon LAM-ICP-MS age of 549 ± 11 Ma from the Stinkbank granite in the south Central Zone of the Damara belt represents the first robust age for the crystallization of a member of the regionally widespread suite of syntectonic Salem-type granites. This age confirms previous assertions that suggest that the main phase of crustal shortening related to the collision between the Congo and Kalahari Cratrons occurred between 550 and ∼540 Ma in the south Central Zone, the leading edge of the Congo Craton. Ages from syntectonic granites reported in the literature from the north Central and the Northern Zone record progressively younger ages of ∼530 to ∼515 Ma and late-stage folding in the Northern Zone is suggested to have occurred between ∼510 to ∼500 Ma. The younging of ages that date the main collisional phase towards the north is interpreted to track the northward propagation of the deformation and metamorphic front from the main suture zone to the northern foreland of the Damara belt. Geochronological data are sparse, and the lack of detailed structural data precludes the retrodeformation of the fold-and-thrust belt along this 250 km long section. However, the age brackets suggest a migration of the orogenic front at a rate of at least 5 to 7 mm/a, but more likely in the order of several centimetre per year, if the internal strain is removed. [ABSTRACT FROM AUTHOR]

The Navachab gold mine in central Namibia is situated in amphibolite-facies, marble-dominated metasediments of the Neoproterozoic Damara Sequence in the Pan-African Damara Belt. Gold mineralization is hosted by two main, and at least three minor, sets of auriferous quartz-veins that are developed on the subvertical northwestern limb of the regional-scale, shallow doubly-plunging anticline of the Karibib dome. The orientation, relative timing and progressive deformation of quartz veins indicate that veining occurred during folding and fold amplification of the Karibib dome. This deformation forms part of the main phase of northwest-directed collisional tectonics in the Damara belt at ∼540 to 550 Ma. The two main vein sets include (1) bedding-parallel, shallowly-plunging ore lenses and shoots situated at the base of a prominem marble unit, and (2) a laterally extensive swarm of shallowly-dipping quartz veins that truncate the host rocks at high angles. The bulk of the bedding-parallel lenses represent dilational jogs that opened during flexural flow along bedding-parallel slip planes during the amplification of the Karibib dome. Flexural flow and associated bedding-parallel fluid infiltration were concentrated close to the contact between marbles and the underlying siliciclastic formations. This lithological contact, in particular, represented a pronounced rheological contrast resulting in increased slip rates during fold amplification. The highly discordant sets of shallowly-dipping, sheeted quartz veins were emplaced during the fold lock-up stage of the Karibib dome. Quartz veining occurred when the northwestern limb of the dome was rotated to subvertical attitudes, so that bedding-parallel flexural slip ceased to be active. Extensional fracturing and veining was facilitated by the presence of transiently supralithostatic fluid pressures. The high-amplitude fluid pressure cycling associated with extensional fracturing is likely to have triggered gold precipitation in the shallowly-dipping veins. Although bedding-parallel veins and discordant veins are broadly contemporaneous, related to the regional-scale D2 phase of fold-and-thrust tectonics, cross-cutting relationships between the two main vein systems indicate that the two vein sets have represented separate entities and succeeded each other rather than forming an interconnected fracture mesh. [ABSTRACT FROM AUTHOR]

A new lithostratigraphic subdivision is proposed for the low-grade metamorphic, supracrustal rocks of the Pan-African Saldania Belt of the Western Cape Province South Africa, previously referred to as the Malmesbury Group. Two tectonostratigraphic groups can be distinguished that appear to be separated by an unconformity. The lower of these groups is termed the Swartland Group. There are no absolute age constraints for the Swartland Group, but a penetrative D1 deformation in rocks of the Swartland Group is correlated with the onset of oblique crustal convergence along the Pan-African West Coast Belts at ca. 580 to 570 Ma, which also provides an upper age limit of the Swartland Group. The structural evolution and style of deformation suggest a correlation of the Swartland Group with part of both the Gifberg Group of the Vanrhynsdorp Basin and the Port Nolloth Group of the Gariep Belt to the north of the Saldania Belt. The Swartland Group is unconformably overlain by mainly clastic sediments that lack evidence of the early D1 deformation and that are here referred to as the Malmesbury Group. A lower age bracket of the Malmesbury Group is provided by the presumed age of deformation of the underlying Swartland Group (ca. 575 Ma), while a minimum age of ca. 550 Ma is indicated by the intrusion of the oldest phases of the Cape Granite Suite that cross-cut the lithologies and fabrics of the Malmesbury Group. The lithological evolution and age of the Malmesbury Group suggests a correlation with the late orogenic clastic deposits of the Late-Neoproterozoic to EarlyCambrian Nama Group that formed in the eastern foreland basins of the emerging Pan-African orogenic belts. The new subdivision no longer assumes the presence of three allochthounous terranes in the Saldania Belt and allows for a better correlation of formations with those in Pan-African belts to the north. [ABSTRACT FROM AUTHOR]