The Karagwe-Ankole belt in Central Africa hosts numerous rare-metal pegmatites and Sn-W mineralised quartz veins, which are related to the granite generation that formed at 986 +/- 10 Ma, i.e. the G4-granites in Rwanda. This early Neoproterozoic granite generation features in the Gatumba area (western Rwanda) a linkage with a zoned cluster of barren and Nb-Ta-Sn mineralised pegmatites. The most distal pegmatite bodies in this area suffered from intense alkali metasomatism, i.e. widespread growth of albite and white mica. A petrographic and geochemical study has been carried out on the pegmatite bodies in order to determine the petrogenetic evolution of the pegmatite zonation. The compositional variation of schorl and elbaite, sampled along the regional zonation sequence, implies that the zonation can be expressed by 4 zones: a biotite, a two-mica, a muscovite and a mineralised pegmatite zone. Alkali element variations and enrichments in muscovite and K-feldspar along the zonation sequence indicate that the origin and the formation of the different pegmatite zones can be explained by a single path of fractional crystallisation. This substantial chemical differentiation accompanies in the Gatumba pegmatite field the evolution from the G4-granite generation to common pegmatites and eventually rare-element pegmatites.
Abstract. This study presents results from apatite fission track (AFT) thermochronology to investigate the thermal history and exhumation dynamics of the Rio Negro–Juruena basement, situated within the western Guiana Shield of the Amazonian Craton. AFT dating and associated thermal history modeling in South America has largely been restricted to the plate's margins (e.g., Andean active margin, Brazilian passive margin, and others). Our paper reports on low-temperature thermochronological data from the internal part of the western Guiana Shield for the first time. This area is part of a vast cratonic lithosphere that is generally thought to be stable and little influenced by Mesozoic and Cenozoic tectonics. Our data, however, show AFT central ages ranging from 79.1 ± 3.2 to 177.1 ± 14.8 Ma, with mean confined track lengths of ca. 12 µm. Contrary to what might be expected of stable cratonic shields, inverse thermal history modeling indicates a rapid basement cooling event in the early Cretaceous. This cooling is interpreted as a significant exhumation event of the basement that was likely driven by the coeval extensional tectonics associated with back-arc rifts in the Llanos and Putumayo–Oriente–Maranon basins. The extensional tectonics facilitated both basement uplift and subsidence of the adjoining basins, increasing erosional dynamics and consequent exhumation of the basement rocks. The tectonic setting shifted in the late Cretaceous from extensional to contractional, resulting in reduced subsidence of the basins and consequential diminishing cooling rates of the Guiana Shield basement. Throughout the Cenozoic, only gradual, slow subsidence occurred in the study area due to regional flexure linked to the Andean orogeny. Comparative analysis with low-temperature thermochronology data from other west Gondwana cratonic segments highlights that exhumation episodes are highly controlled by tectonic inheritance, lithospheric strength, and proximity to rift zones. This study underscores the complex interplay between tectonic events and the response of cratonic lithosphere over geological timescales and highlights extensional settings as an important geological context for craton exhumation.
The Mesoproterozoic Kibara orogen in Central Africa hosts different granite-related rare element deposits that contain cassiterite, columbite-tantalite ( coltan ), wolframite, beryl, spodumene, etc. as typical minerals. The primary deposits of these minerals are formed by pegmatites and quartz veins that have historically been related to the youngest, most evolved G4-granite generation in the northern part of the Kibara orogen. This study focuses on quartz vein-type cassiterite mineralisation in the Rutongo area in Rwanda. The Rutongo area consists of a large anticline that is characterised by the presence of cassiterite-mineralised quartz vein sets that dominantly occur in quartzites. The emplacement of the quartz veins has been related to a later phase in the deformation history of the Kibara orogeny. The mineralised quartz veins are associated with intense alteration, comprising silicification, tourmalinisation, sericitisation and muscovitisation. Cassiterite itself is associated with muscovite in fractures in and along the margins of the quartz veins. Cassiterite crystallisation is followed by the precipitation of different sulphides, such as arsenopyrite, pyrite, chalcopyrite and galena. Cassiterite mineralisation resulted from the circulation of high-temperature and moderate-salinity fluids with a H2O-CO2-(CH4-N2)-NaCl composition. The stable isotopic composition of the cassiterite mineralising fluids indicates precipitation during metamorphic hydrothermal conditions, during which the metamorphic fluids where in isotopic equilibrium with granitic rocks. The circulation of these fluids probably resulted in the remobilisation of the Sn from these magmatic rocks, as indicated by the relative low Sn concentration of the specialised G4-granites. 40Ar-39Ar age dating of muscovite associated with the mineralisation gives an integrated age of 869 ± 7 Ma, which is clearly younger than the age of the G4-granites (~986 Ma) and the pegmatites with associated columbite-tantalite mineralisation (~965 Ma) in the area. Based on this large time gap, the 40Ar-39Ar age is interpreted to reflect a hydrothermal event post-dating the emplacement of the Kigali granite, only indicating a possible minimum age for the formation of the cassiterite mineralisation. Based on the structural setting, petrographical observations, the geochemistry of the G4-granites, stable isotope geochemistry, we therefore propose a model in which Sn was mobilised from primary magmatic rocks by a metamorphic hydrothermal fluid system that was generated after crystallisation of the granites and pegmatites. Cassiterite was precipitated in structurally controlled locations, together with the alteration of the host-rocks.
(2002). 40Ar/39Ar dating of mesothermal, orogenic mineralization in a low-angle reverse shear zone in the Lower Palaeozoic of the Anglo-Brabant fold belt, Belgium. Applied Earth Science: Vol. 111, No. 3, pp. 215-220.
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