Post-paleozoic magmatism in Angola e Namibia: a review.

Post-Paleozoic magmatism in Angola and Namibia (SW Africa) is widespread along the continental margin (fl ood tholeiites of the Paran\ue1-Etendeka system), and along transverse lineaments (alkaline and alkaline-carbonatitic complexes; sodic and potassic suites). These different magmatic suites are strictly associated in space and/ or time. Variable melting degrees of a veined lithospheric mantle are proposed for the most \u201cprimitive\u201d magmas from geochemical modeling and Sr-Nd isotope systematics. A complex evolution emerges for some ultramafi c rocks (cumulus processes) and for differentiated rock compositions (assimilation and fractional crystallization, AFC, magma mixing), which may also involve anatexis of the crystalline basement and emplacement of S-type granites and rhyolites. Melting of a lithospheric mantle, without an appreciable contribution of the asthenosphere (thermal input excepted), is consistent with regional thermal anomalies in the deep mantle, mapped by gravity of the geoid, seismic tomography, and paleomagnetic analysis. The Walvis Ridge and Rio Grande \u201chotspot tracks\u201d are interpreted as stress response in the lithosphere during rifting. A plume-related heat source is not favored by our results

Petrology of the Proterozoic mafic dyke swarms of Uruguay and Constraints on their mantle source composition

Three dyke swarms in Uruguay range in age from Palaeoproterozoic (1.86 Ga, Florida region) to Neoproterozoic (similar to 0.7 Ga, Nice Perez and Treinta y Tres regions). The Florida and Nice Perez swarms are basalts, basaltic andesites and andesites with tholeiitic affinity, characterized by LILE and LREE enrichment with respect to HFSE, Nb depletion with respect to K and La, K/Rb 13, (La/Yb)(n)> 4.5. Both swarms have positive epsilon(Sr) and negative epsilon Nd, but the Florida isotopic array is dominated by epsilon(Nd) variations (EM1 type) and that of Nice Perez by epsilon(Sr) variation (EM2 type). The Treinta y Tres swarm consists of slightly ne-normative transitional or alkaline basalts with geochemical characteristics resembling those of OIB (e.g., Gough Island),but suggestive of a less enriched OIB source, and have slightly positive epsilon(Nd) and epsilon(Sr). Crustal contamination does not appear to be important in the petrogenesis of the swarms. It is proposed that the geochemical characteristics of the Florida swarm derive from melting of lithospheric mantle infiltrated by hydrous fluids released from the thermal breakdown of hydrous phases, either contained in underplated oceanic crust or formed by interaction of asthenospheric fluids with the lithospheric mantle. The EM1-type isotopic features are considered as unrelated with possible fluid addition, but as a pre-existing feature. The geochemical and isotope characteristics of the Nice Perez swarm are attributed to time-integrated enrichment processes, which affected the Nice Perez lithospheric mantle during the Palaeoproterozoic Florida episode, The geochemistry of the Treinta y Tres swarm may be explained by partial melting of a lithospheric mantle which did not suffer Palaeoproterozoic LILE and LREE enrichment but which was isotopically reset in Palaeoproterozoic times. Alternatively, a residual mantle related to the Florida event is required. Mantle melting and dyke intrusion probably occurred in an ensialic environment. The geochemical and isotopic characteristics of the Uruguay dyke swarms are similar to those of the Mesozoic basalts related to the Gondwana break-up, supporting the possibility that the latter derived from a heterogeneous lithospheric mantle source which recorded Proterozoic enrichment processes

Iron-rich formations at the Cerro Manom\uf3 region, Southeastern Bolivia: remnant of a BIF?

In Southeastern Bolivia, the field relations, geochemical and isotopic data collected in the Manom\uf3 area, along with the occurrence of apatite-rich lenses, carbonate blocks and the extensive enrichment in U, Th and REE probably point to a complex history: 1. An old BIF-like formation with E-W orientation, formed and metamorphosed during Mesoproterozoic times, cuts an older compressive structure at the border of the Amazonic craton; 2. Probable intrusion and fenitization by carbonatitic magma in (Early, Late?) Cretaceous times; 3. Lateritization processes and formation of a thick duricrust in Tertiary-Quaternary times. A more detailed field sampling, as well as additional systematic and extensive geological, mineralogical, petrographic and geochemical studies are required to obtain a more accurate regional picture