Search

Three structural profiles across the Coastal Terrane, the Boundary Igneous Complex and the Orogen Core have been studied in the Kaoko Belt of northwestern Namibia. The oldest known Si fabric is inherited from an older tectono-metamorphic event. It occurs in the Coastal Terrane only and the extent of its reworking increases from south to north. The S1 foliation reactivates or folds Si fabric in the Coastal Terrane and appears as an early planar fabric in granitoids of the Boundary Igneous Complex and migmatites of the Orogen Core domain. Superimposed subvertical S2 fabric corresponds to axial plane cleavage of upright close to isoclinal folds and the extent of its development also increases from south to north. Active migration of partial melt during S2 development in the Orogen Core dates the onset of this deformation at ~550 Ma. Distribution of F2 fold axes and L2 stretching lineations suggests pure shear-dominated deformation associated with development of N-S trending S2 cleavage preserved in the central profile, followed by sinistral simple shear-dominated deformation on newly developed NW-SE trending pervasive cleavage in the northern part of the area. Such spatial variation in the deformation record is attributed to the irregular shape of the Congo Craton indenter that is reflected by heterogeneous development of the S2 cleavage front in the Coastal Terrane and the Boundary Igneous Complex. Common orientation of L1 and L2 stretching lineations and solid-state reworking on both S1 and S2 planes suggest single event of sinistral transpression since 550 Ma with strain partitioning into domains of oblique thrusting (reactivated S1) and transcurrent sinistral shearing (S2 and S3). Such succession of deformation structures suggests that major subvertical shear zones in the Kaoko Belt do not correspond to early crustal discontinuities, but rather reflect late strain localization during cooling. [ABSTRACT FROM AUTHOR]

Gallium (Ga) and germanium (Ge) are technologically important critical elements. Lead blast furnace slags from Tsumeb, Namibia, comprise over two million metric tons of material that contains high levels of Ga (135–156 ppm) and Ge (128–441 ppm) in addition to significant Zn concentrations (up to 11 wt.%) and represent a potential resource for these elements. A combination of mineralogical and chemical methods (PXRD, FEG-SEM-EPMA and LA-ICP-MS) indicated different partitioning of Ga and Ge within the individual slag phases. Gallium is predominantly bound in small euhedral crystals of Zn–Fe–Al spinels (<10 μm in size), exhibiting concentrations in the range of 480–1370 ppm (up to 0.004 atoms per formula unit, apfu). Concentrations of Ga in other phases (e.g. melilite) are systematically below 90 ppm. The principal host of Ge is the silicate glass and, to a lesser extent, silicates (melilite and olivine group phases). Concentrations of Ge in glass attained a concentration of 470 ppm (EPMA), but the LA-ICP-MS analysis of glass matrix containing submicrometre spinel crystallites indicated that average Ge levels vary in the range of 113–394 ppm. In the potential extraction of Ga and Ge, the results indicate that ultrafine milling is needed to liberate the Ga- and Ge-hosting phases prior to metallurgical processing of the slag. [ABSTRACT FROM AUTHOR]

Ediacaran syn-tectonic plutonic rocks (amphibole gabbros, quartz diorites/tonalites to biotite- and muscovite-bearing granites) of the Angra Fria Magmatic Complex (Kaoko Belt, north-western Namibia) belong to two compositionally similar, magnesian, transitional tholeiitic–calc-alkaline suites, the Older (∼625–620 Ma) and the Younger (∼585–575 Ma). Both have counterparts in the broadly contemporaneous Florianópolis Batholith (southern Brazil), from which they were separated during the Cretaceous opening of the southern Atlantic. In the Angra Fria Magmatic Complex, the only unequivocal mantle contributions are identified in mingling zones of the Younger Suite and hybrid mafic–intermediate dykes of uncertain age. Previously published Hf-in-zircon isotopic data, together with new whole-rock geochemical and Sr–Nd isotopic signatures, underline an important role of crustal anatexis of a material with late Palaeoproterozoic to early Mesoproterozoic mean crustal residence (1.9–1.5 Ga). This interval resembles some of the published Nd model ages for Tonian 'Adamastor Rift'-related felsic magmatic rocks in the Namibian Coastal and Uruguayan Punta del Este terranes. In detail, the Older Suite probably originated mainly by fluid-present melting of metabasalts and metatonalites, followed by (near) closed-system fractional crystallization (with or without accumulation) of amphibole ± plagioclase. For the Younger Suite, the principal process was the dehydration melting of relatively felsic lower crustal protoliths (metagreywackes or intermediate–acid orthogneisses >> metapelites), leaving garnet in the residue. Based on the geological context, the conspicuous enrichment of hydrous-fluid-mobile large ion lithophile over the conservative high field strength elements is not interpreted through a classic model of oceanic plate subduction, devolatilization, and fluxed-melting of the overriding mantle wedge. Instead, it is thought to reflect high-grade metamorphism of deeply buried continental crust and attendant water-fluxed melting of the overlying crustal lithologies, connected with inversion of the Tonian 'Adamastor Rift'. [Display omitted] • Ediacaran magmatism in Angra Fria Complex is correlated with Florianópolis Batholith. • Mature crustal material (1.9–1.5 Ga model ages) remelted with limited mantle input. • Syn- to post-collisional intracontinental setting, unrelated to oceanic subduction. • Arc-like LILE/HFSE enrichment may reflect H 2 O-fluxed melting in the crust. [ABSTRACT FROM AUTHOR]