The Orange River Group: a Major Proterozoic Calcalkaline Volcanic Belt in the Western Namaqua Province, Southern Africa

Abstract

Summary The Orange River Group (ORG) consists of a thick volcano-sedimentary pile (> 8 km) preserved as a subprovince within the Namaqua Province of southern Africa. Volcanic activity during deposition of the ORG was predominantly subaerial, and involved the eruption of an andesite dominated calcalkaline suite ranging from basalt through to rhyolite. Agreement between Rb-Sr, Th-Pb, Pb-Pb and Sm-Nd isochron ages indicate extrusion of the ORG around 2000 Ma ago. Compared with modern arc suites, the ORG is characterized by high K and Mg, together with related trace elements (Rb, Sr, Ba, U, Th, LREE; Ni, Cr). Basic lavas differ from many modern calcalkaline types in having lower A1 contents reminiscent of tholeiites. When normalized to N-type MORB, the ORG basic lavas show enrichments in K, Rb, Ba, Th, and the LREE, and depletions in Nb, Ta, Zr, Hf, Ti, Y and the HREE). Metamorphic overprinting associated with the ∼ 1000 Ma Namaqua tectonothermal event are evident to varying degrees throughout the subprovince, and precludes precise monitoring of initial isotope ratios at time of eruption. Evidence from the rocks least affected by metamorphic overprinting suggests common initial Sr, Pb and Nd isotopic compositions for the entire basalt to rhyolite suite in this particular area, which would support a fractional crystallization model and rule out major involvement of old Archaean crust to produce the voluminous andesites. Initial isotopic data translate to ɛ sr (T) + 10), ɛNd(T)∼0, and an apparent high μ value, which, when taken together with the geochemical features described previously, indicate that the source region to the mafic parental magmas of the ORG shows patterns similar to those in the mantle wedge overlying subduction zones. The calcalkaline ORG volcanic suite may therefore represent the remnants of a 2000 Ma old Proterozoic arc complex. The high K character, the intimate association of comagmatic granitoids, the development of copper porphyries, and the lack of any direct or indirect evidence for underlying Archaean crust all point to a mature island arc environment. Active continental margins such as the Andes show similar geochemical and isotopic features, but it is argued that the high elevation of such margins will cause them to be preferentially eroded and lack representation in ancient terrains.

Geochemistry and isotopic evolution of the Mesoproterozoic Cape Meredith Complex, West Falkland

“…Fields for the evolution of~1400 Ma and ~2000 Ma crust in southern Africa are provided for comparison. Data defining these fields have been compiled from the literature Barton, 1983;Barton & Burger, 1983;Reid & Barton, 1983;Cornell et al 1986;Clifford et al 1995;Eglington, Harmer & Kerr, 1989;Moyes & Groenewald, 1996;Reid et al 1987;Reid et al 1997;Reid, 1997;Wareham et al 1998 Cornell et al 1986;Clifford et al 1995;Eglington, Harmer & Kerr, 1989;Moyes & Groenewald, 1996;Reid et al 1987;Reid et al 1997;Reid, 1997;Thistlewood et al 1997;Wareham et al 1998 Haag Nunatak block and western Dronning Maud Land sector, but includes a large component of older crust in its genesis to the west in Namaqualand (as an influence of the Palaeoproterozoic Richtersveld Province) and to the east in the Eastern Sverdrupfjella (as an influence of the East Antarctic craton). Furthermore, consideration of Figure 10 suggests that the generation of juvenile crust in the Falkland Islands was actually slightly younger than noted elsewhere in the Namaqua-Natal-Maudheim belt.…”

The Kalahari Craton during the assembly and dispersal of Rodinia

“…Richtersveld Subprovince represents a Palaeoproterozoic arc terrain within the Namaqua Sector that largely escaped Mesoproterozoic reworking (Reid, et al, 1987). As noted before it is not known if this crustal fragment formed in its present position, or evolved elsewhere and was accreted as a micro-continental block during the Mesoproterozoic.…”

Provenance and reconnaissance study of detrital zircons of the Palaeozoic Cape Supergroup in South Africa: revealing the interaction of the Kalahari and Río de la Plata cratons

“…6, 7). However, perhaps more importantly is to note that the Richtersveld Subprovince along the border between South Africa and Namibia represents an ancient *2.0-1.7 Ga volcanic arc complex (Reid 1982;Reid et al 1987;Cornell et al 2006) and could also have contributed zircons of that age to especially the western part of the Cape Basin (Fig. 7).…”