Pan-African Rifting and Orogenesis in Southern to Equatorial Africa and Eastern Brazil

ArticleinPrecambrian Research 44(2):103-136 · August 1989with 243 Reads
DOI: 10.1016/0301-9268(89)90078-8
Abstract
The Upper Proterozoic Pan-African belts of Africa and Brasiliano belts of South America are assumed to have evolved from an elaborate system of continental rifts which formed on the West Gondwana continent 1100-1000 Ma ago. Reviews of the Damara belt, Kaoko belt, Gariep belt, Saldania belt, West Congolian belt, Lufilian Arc and Zambezi belt of southern and equatorial Africa and the Ribeira-Mantiqueira belt of Brazil show the applicability of the continental rifting model to all these belts.Strict application of the rifting model describing the formation and further evolution of rift structures in the continental crust leads to the conclusion that the West Congolian belt, as traditionally described, is incomplete and has to be supplemented by the Mayumbian belt and a rift structure situated farther to the west. The Lufilian Arc is assumed to have evolved from two rift structures separated from each other by the later ‘Domes region’. The ‘Golfe du Katanga”, which branches off northeastwards from the Lufilian Arc is interpreted as an aulacogen (‘Shaba aulacogen’).The Pan-African-Brasiliano rift system is comparable to the Mesozoic Arctic-North Atlantic rift system with regard to size and distribution of rifts. Ocean floor spreading and opening of a proto-South Atlantic Ocean most probably occurred along a main line of rifting situated between the west coast of Africa and eastern Brazil. The wedge-like ocean may have terminated at the ‘São Francisco-Congo cratonic bridge’ between northeastern Brazil and Gabon-Cameroon. There is no evidence of ocean-floor spreading in the Zambezi belt, the Lufilian Arc and the traditional West Congolian belt, which are arranged along another major line of rifting.Closure of the Pan-African-Brasiliano rift system took place during two successive orogenic episodes. The Zambezi belt, Lufilian Arc and West Congolian belt underwent their main orogenic deformation with prevailing ENE- and WSW-oriented tectonic transportation directions during the Katangan episode, at ∼900−750 Ma. The transcontinental sinistral Mwembeshi Shear Zone adjusted opposite movement directions in the Zambezi belt and Lufilian Arc and, at its western prolongation, concurrently supported opening of the Khomas Trough in the southern Damara belt.The proto-South Atlantic Ocean, still opening during the Katangan episode, was gradually closed during the Damaran episode, at ∼750−500 Ma. Closure proceeded from north to south and was accompanied by northwest-directed subduction underneath the Brazilian plate and southeast-directed tectonic transportation in the Kaoko, Damara and Gariep belts. In the previously formed West Congolian belt, Lufilian Arc and Zambezi belt a second deformation phase characterized by strike-slip faulting and local emplacement of nappes occurred during this episode.

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  • ... No cenário geotectônico do Gondwana Oeste, o orógeno Araçuaí-Ribeira-Oeste Congo ocupa um lugar peculiar, confinado na reentrância cratônica entre os cratons São Francisco e Oeste Congo ( Fig. 3.4). A natureza confinada deste orógeno decorre do fato de que esses cratons permaneceram parcialmente ligados, pelo menos desde a tafrogênese toniana (mas, muito provavelmente desde 2 Ga) até a abertura do Atlântico Sul, no Cretáceo, por meio da ponte continental que unia as regiões da Bahia e do Gabão (Torquato & Cordani, 1981;Porada, 1989;D'Agrella et al., 1990D'Agrella et al., , 2004Ledru et al., 1994;Brito Neves et al., 1999). O craton São Francisco é formado por rochas magmáticas e metamórficas, arqueanas a paleoproterozóicas ( Cordani et al., 1988;Teixeira & Figueiredo, 1991 (Fig. 3.5). ...
    Thesis
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    The Araçuaí belt was formed by the collision between South American and African protocontinents during the Neoproterozoic. Its eastern part consists of an extensive migmatitic area (~300 km long x 50-100 km wide) where crop out anatexites and leucogranites (Carlos Chagas unit), migmatitic kinzigites and granulites that probably are the record of a widespread partial melting of the middle to lower crust. Field observations associated with microstructural evidences indicate that the deformation occurred when the rocks were incompletely solidified. Synkinematic temperature estimates realized using the TitaniQ (titaniun-in-quartz) geotermomether suggest that the minimum temperature for the quartz crystallization is ~750°C. Such temperatures combined with bulk rock composition of leucosome in the anatexites suggest that the viscosity of crustal rocks was dropped to at least 108 Pa s. Low viscosity values associated with field and microstructural evidences are consistent with the generation of at least 30% volume of melt during the orogeny. The presence of large volumes of melt promotes a drastic weakening of the mechanical strength of rocks and suggests that the anatectic crust of the eastern Araçuaí belt represents an analogue of present day hot orogen such the Himalayas. Detailed mineralogy investigation permitted to characterize the paramagnetic behaviour of the anatexites and the ferromagnetic behaviour of the granulites. Crystallographic preferred orientation (CPO) measurements using the EBSD (Electron Backscatter Diffraction) technique reveal that the magnetic foliation results from the preferred orientation of the biotite [001] oriented normal to the flow plane. However, given the feeble linear anisotropy of this mineral, only a subsidiary contribution of its subfabric to the origin of the magnetic lineation (k1) was observed. Correspondence between [001] of feldspars and k1 is due to the CPO of small inclusions of ilmenite that mimic the CPO of their host minerals. Correlation between k1 of the Anisotropy of Anhysteretic Remanent Magnetization (AARM) and k1 of the Anisotropy of Magnetic Susceptibility (AMS) demonstrate that, at the specimen scale, the magnetic lineation has a contribution of the anisotropy of the ferromagnetic minerals. AMS measurements realized to recover the mineral fabric and investigate the migmatitic flow field revealed a complex strain pattern in which, considering the lineation trends, especially, it is possible to characterize three structural sectors. The north region (structural sector 1) with ! V! ! foliations dominantly sub-horizontal and lineation trending NW-SE is interpreted as a region of tectonic escape that may represent a horizontal channel flow. This oblique tectonic escape probably results from gravity forces (gravity-driven flow). The Southern region (structural sectors 2 and 3) with variable trending foliations (NE-SW, E-W and NW-SE) and lineation plunging to North and West, probably reflect a flow regime dominantly influenced by the E-W convergence of the African and South-American continents (collision-driven flow). Altogether, the characteristics of the various domains suggest that the deformation of the partially molten middle crust of the Araçuaí belt was the result of the combination of gravity forces due to the topographic load and tectonic forces due to the convergence between the African and South-American continents.
  • Article
    The Dom Feliciano Belt developed during the Brasiliano–Pan-African orogenic cycle due to the tectonic interaction between the Rio de la Plata, Congo and Kalahari cratons, along with the amalgamation of smaller continental fragments. Together with its prolongations to the south, the Major Gercino Shear Zone constitutes one of the main lineaments of the orogenic system, establishing a more than 1000 km long NE-trending tectonic boundary between a granitic batholith and a metavolcano-sedimentary association. Based on combined field, structural, microstructural and textural data, together with new and published geochronological data, a refined model for the geological evolution of the Major Gercino Shear Zone is presented. Regional NW-verging, low-dipping structures were generated between 650 and 615 Ma, preceding the formation of the shear zone. This was followed by the main deformation phase, corresponding to pure-shear dominated dextral strike-slip, interpreted do be controlled by regional transpression during oblique convergence of the continental terranes. This stage lasted until ca. 585 Ma and was coeval with the continuous emplacement of granitic magmatism along the structure. Strain partitioning and localization led to the development of mylonitic belts along the intrusion borders mostly under greenschist facies metamorphic conditions. Late-stage ductile deformation along the structure was active during the cooling of the intrusions until ca. 550 Ma, while the deformation front of the orogenic system migrated to counterpart orogenic belts in the African continent. After tectonic stabilization, the Major Gercino Shear Zone recorded episodic brittle reactivation, possibly related to the Phanerozoic evolution of the active margins and intracratonic basins in Gondwana.
  • ... The system of Pan-African orogenic belts formed during the collision of cratonic blocks and the amalgamation of the Gondwana supercontinent in the outgoing Proterozoic (e.g. Miller, 1983;Porada, 1989; Oliver, 2001). The belts evolved through successive stages of intracontinental rifting, continental break-up, spreading, reversal of plate motion, subduction and continental collision (Miller et al., 2009). ...
    ... Intracontinental rifting commenced at ca. 750 Ma, evidenced by the emplacement of rift-related syenites (e.g. Jung et al., 2007) and the deposition of the thick succession of the coarse-clastic, rift-type Nosib Group and the accompanying felsic volcanism preserved in the Naauwpoort Formation (Porada, 1989;Hoffman et al., 1996;Miller et al., 2009). Progressive rifting led to the formation of two oceanic basins, namely the N-S trending Adamastor Ocean and so-called inland sea of the Khomas Ocean, the latter marking the future trace of the Damara Belt. ...
    Thesis
    Numerous quartz-vein hosted gold prospects in the Karibib district in central Namibia testify to the presence of a large hydrothermal system during Pan-African times in rocks of the Damara Belt and centred around the Navachab Gold Mine. This study presents the results of the regional and detailed mapping of the Navachab synform, a NE-SW trending, regional-scale structure hosting a number of these gold prospects, locally referred to as the “zoo prospects”, in the direct vicinity of the main Navachab Gold Mine. The zoo prospects are located in the marble-dominated, up to 800 m thick Karibib Formation, which forms the core of the Navachab synform. Regional mapping of the synform identified six main lithological units comprising massive and banded dolomitic and calcareous marbles, intraformational breccias and interlayered calc-silicate felses. Despite characteristic thickness variations, the six units can be correlated throughout the synform, allowing for a lithostratigraphic correlation of units in the otherwise monotonous marble sequence. All of the prospects are spatially closely associated with and adjacent to unit 5, an up to 100 m thick, competent dolomitic marble unit. This suggests a strong lithological control of the mineralisation. The first-order Navachab synform formed during the regional D2 phase of deformation. The strongly non-cylindrical, doubly-plunging fold shows open- to close interlimb angles and a pronounced NW vergence in the SW, but is tight- to isoclinal and upright in the NE. Higher fabric intensities and detachment folding are confined to the subvertical limbs of the synform in the north. Here, structures record a subhorizontal, NW-SE directed co-axial shortening strain interpreted to have developed in response to the geometric hardening and layer-normal shortening following the rotation of the fold limbs to subvertical attitudes during progressive D2 shortening. The mineralisation of the zoo prospects is located where D2 high-strain zones intersect unit 5. This suggests an additional structural control of the mineralisation. The detailed mapping of the zoo prospects identified five distinct quartz-vein sets. The geometry, orientation and progressive deformation of the vein sets indicate that veining occurred during the late stages of the D2 event and during NW-SE directed, subhorizontal shortening. Areas of increased veining and mineralisation occur preferentially in areas of strain incompatibilities, where the combined effects of D2 strains and prominent lithological contacts created zones of localized dilatancy. This includes most prominently dilational jog geometries developed between anastomosing D2 shear zones, where hydrothermal fluid flow has produced pervasive quartz-vein stockworks. This also includes areas of detachment folding, where deformation of the rheological stiffer dolomitic marbles and less competent calcareous marbles has led to detachment surfaces and, locally, dilatancy. Zones of increased permeability are also created where two or more vein sets intersect, which is particularly common within and adjacent to boudin interpartitions of competent dolomite units and along rheologically prominent contacts. The zoo prospects illustrate the interplay of (1) prominent rheological contrasts between adjacent lithologies, (2) the presence of high-strain zones, and (3) the geometry of host structures for the formation of auriferous quartz veins in the Karibib district.
  • ... A unique link formed long before the assembly of West Gondwana was the Bahia-Gabon cratonic bridge ( Fig. 1; Porada, 1989;PedrosaSoares et al., 2008). This long-lasting continental landmass once connected the São Francisco and Congo counterparts between c. 2 Ga and c. 140 Ma ( Ledru et al., 1994;Trompette, 1994;D'AgrellaFilho and Cordani, 2017;Heilbron et al., 2017). ...
    ... A promising option is, indeed, through the E-ROOS segment in the Bahia-Gabon cratonic bridge. Actually, this long-lived continental bridge has provided reliable correlations between the São Francisco and Congo palaeocontinental regions (Fig. 1), since the pioneer studies of the 1960's ( Hurley et al., 1967;Porada 1989;Ledru et al., 1994;Trompette, 1994;Pedrosa-Soares et al., 2008;D'Agrella-Filho and Cordani, 2017). Accordingly, the coastal Eastern Bahia belt has been correlated with the Eburnean belt of the Congo craton at western Gabon (Caen-Vachette et al., 1988;Ledru et al., 1994, Feybesse andMilési, 1998;Barbosa and Sabaté, 2004;Lerouge et al., 2006;Thiéblemond et al., 2009;Barbosa and Barbosa, 2017). ...
    Article
    Full-text available
    Assembled during the Rhyacian-Orosirian boundary, the Congo-São Francisco palaeocontinent remained linked by a continental bridge from ca. 2 Ga to the Atlantic Ocean opening in the Cretaceous. To the south of that bridge, precursor basins and orogenic outcomes encompassed by the Neoproterozoic-Cambrian Araçuaí-Ribeira orogenic system (AROS) and its African counterparts, developed. The AROS discloses reworked basement units, like the Juiz de Fora, Pocrane and Quirino complexes, which are key units to retrieve the Palaeoproterozoic history of the Congo-São Francisco palaeocontinent. This paper presents new U-Pb ages, and the first Lu-Hf in zircon signatures from those complexes. All magmatic zircon grains show Rhyacian and Orosirian ages, ranging from c. 2250 Ma to c. 1850 Ma. The best Concordia ages range from 2184 ± 9 Ma to 2080 ± 19 Ma. Hf in zircon and whole-rock Sm-Nd data disclose mantle-related signatures for both, the Pocrane and Juiz de Fora complexes, suggesting igneous protoliths with no or very few involvement of continental crust, similarly to juvenile magmatic arcs. Conversely, Hf in zircon data for the Quirino complex show an evolved signature compatible with a magmatic arc developed on a continental older crust with Archaean components. Together with a thorough compilation on Rhyacian-Orosirian orogenic systems (ROOS), our data allow us to envisage links within the Congo-São Francisco palaeocontinent, comprising orogens characterised by mostly evolved isotopic signatures and zircon inheritance. They are grouped into the: (i) W-ROOS, including the Mantiqueira complex, the west segment of the Eastern Bahia belt, intrusions in the south Gavião and Porteirinha blocks, and parts of the Mineiro belt; and (ii) E-ROOS, comprising the Quirino complex, basement rocks of the Cabo Frio tectonic domain, the coastal Eastern Bahia belt, and, in Africa, Eburnean and Kimezian complexes, as well as the western Angola shield. Assembled between the W-ROOS and E-ROOS, an orogenic system with prominent juvenile to moderately juvenile isotopic signature (JU-ROOS) includes the Juiz de Fora and Pocrane complexes, and the Buerarema complex at the central segment of the Eastern Bahia belt.
  • ... Koester et al., Philipp et al., 2016). Previous models suggest that this subduction was from E (paleo-Africa) to W (paleo South America) during the closure of the Adamastor paleo-ocean (Porada, 1989;Fragoso-César, 1991;Germs, 1995;Frimmel et al., 1996;Frimmel and Frank, 1998;Gaucher et al., 2009;Chemale et al., 2012;Diener et al., 2017). The Pinheiro Machado arc was developed over a former subduction- related magmatic arc (based on bulk rock and Sr-Nd-Pb isotope geochemistry; see Lenz et al., 2013;Tambara, 2015;Koester et al., 2016;Martil et al., 2017) with Brasiliano I ages (ca. ...
    ... Silva et al., 1999;Loureiro et al., 2015;Philipp et al., 2016). Ramos et al. (2017a) followed the models of Porada (1989), Fragoso-César (1991), Germs (1995), Frimmel et al. (1996), Frimmel and Frank (1998), Gaucher et al. (2009) and Chemale et al. (2012), where the Pelotas-Cuchilla Dionisio magmatism is related to a E to W subduction of the Adamastor oceanic plate underneath Río de la Plata Craton/Nico Perez Terrane. Blueschists found recently in the Gariep Belt by Diener et al. (2017) strengthen the E to W subduction model; (b) Syn-kinematic intrusion of the Três Figueiras granite at ca. 610-585 Ma. ...
    Article
    This paper presents the first Rb-Sr isotopic geochemistry study and SHRIMP zircon U-Pb dating in the Arroio Grande Ophiolite (southeasternmost Dom Feliciano Belt in Brazil), an extension of the Uruguayan Punta del Este Terrane. The metamafic units of the ophiolite show 87Sr/86Sr ratios ranging from 0.7036 to 0.7070 (recalculated at 630 Ma), which suggest a MORB magmatic source for the amphibolite protoliths and a volcanic magmatic arc source (with possible crustal contamination) for the metagabbro protoliths. Together, these features suggest a supra-subduction zone origin for these rocks, corroborating previous studies based on bulk-rock chemistry. In the metaultramafics, the 87Sr/86Sr630 ratio of the chloritite sample (0.7152) is comparable to that of the Três Figueiras granite (0.7139), a syn-kinematic peraluminous granite related to the Ayrosa Galvão-Arroio Grande Shear Zone (which also affected the southern portion of the ophiolite, where the studied samples are located). The emplacement of the latter granite could be responsible for the metasomatism of the former serpentinites, generating talc- and chlorite-rich reaction zones. The SHRIMP U-Pb analysis of twelve chloritite zircons strengthens that assumption, as zircons with ca. 610-585 Ma (the age of the granite) were found in association with metasomatic microtextures. Zircon ages ranging from ca. 2000 to 660 Ma were also found in the chloritite. These latter ages are analogous to those found in the metasiliciclastic rocks that envelop the metaultramafic rocks of the ophiolite, so that the chloritite probably represents a blackwall reaction zone between (i) a former serpentinite, (ii) the Três Figueiras granite, and (iii) metasiliciclastic rocks. Additionally, we perform a SHRIMP U-Pb dating in a quartz-syenite melt, which are related to a tonalitic intrusion within the metasedimentary unit (the Matarazzo Marbles) of the ophiolite. The obtained concordia age of 640 Ma constrains the minimum age of ophiolitic mélange emplacement, as the tonalitic intrusion is related to continental arc magmatism (Pinheiro Machado Intrusive Suite).
  • Article
    The study of the geology of southern Egypt, in-between the Red Sea and the Libyan borders, south of latitude 24°30′N reveals a succession of the Precambrian Arabo–Nubian Shield along the Red Sea coast overlain by Paleozoic–Mesozoic sediments. The Paleozoic section in the study area is well developed in three sub-basins, namely, Uweinat–Gilf, South Nile, and Etbai. Paleozoic sediments are well developed in the three sub-basins mostly sandstones of Cambrian overlain by glaciogene sediment conglomerates at base namely Gabgaba Formation and by the Naqus Sandstone at top. The tectonic events during the Early Paleozoic Caledonian Orogeny are marked by several unconformities and tectonic uplift and down faulting expressed in the many faults in the Uweinat–Gilf and South Nile sub-basins. The Carboniferous is well-developed sandstones in the three studied sub-basins. The glaciation at the Permian is reflected in sea-level fall, hence continental sediments are well developed in many parts of Egypt—a phase of the Hercynian Orogeny. Volcanics are very well common in the study areas ranging in age from 48 to 34 Ma at Uweinat and Gebel El-Asr. Vulcanicity continued during the Paleozoic and Mesozoic at the Triassic of Nasab El-Balgum and in the south Western Desert, the south Eastern Desert, and Etbai area. The highly seismic conditions in southern Egypt continued up to very recent times where tremors were noticed in the 80s and 90s of the last century pointing to very unstable area.
  • Article
    The Kamoa sub‐basin, in the south‐eastern part of the Democratic Republic of Congo, is a rift basin that hosts a world‐class stratiform copper deposit at the base of a very thick (1.8 km) succession of matrix‐supported conglomerates (diamictite) (Grand Conglomérat Formation) that has been interpreted by some as the product of deposition in the aftermath of a planet‐wide glaciation. Newly available subsurface data consisting of more than 300 km of drill core throws new light on the origin of diamictite and associated facies types, and their tectonic, basinal and palaeoclimatic setting. Initiation of rifting is recorded by a lowermost subaqueous succession of fault‐related mass flow conglomerates and breccias (the ‘Poudingue’) with interdigitating coeval and succeeding sandstone turbidites (Mwashya Subgroup). Overlying diamictites of the Grand Conglomérat were deposited as subaqueous debrites produced by mixing and homogenization of antecedent breccias and gravel from the Poudingue and Mwashya sediments with basinal muds. Failure of over‐steepened basin margins and debris flow was likely to be triggered by faulting and seismic activity and was accompanied by syn‐depositional subaqueous basaltic magmatism recorded by peperites and pillow lavas within diamictites. The thickness of diamictites reflects recurring phases of faulting, volcanism and rapid subsidence allowing continued accommodation of rapidly deposited resedimented facies well below wave base. A distal or glacial influence in the form of rare dropstones and striated clasts is evident, but tectonically‐driven mass flow destroyed any primary record of glacial climate originally present in basin margin sediments. Such basin‐margin settings were common during Rodinia rifting and their stratigraphy and facies record a dominant tectonic, rather than climatic, control on sedimentation. Deposition occurred on tectonic time scales inconsistent with a Snowball Earth model for Neoproterozoic diamictites involving a direct glacial contribution to deposition. This article is protected by copyright. All rights reserved.
  • ... F. DELPOMDOR, S. SCHRÖDER, A. PRÉAT, P. LAPOINTE C. BLANPIED AND final amalgamation of India to Australia-East Antarctica (e.g., Meert, 2003;Collins and Pisarevsky, 2005), the Pan African orogeny ended. The polyphase assembly of the Gondwana supercontinent during the East Africa, Brasiliano, Kuungan and Damaran orogenic events thus extended from about 1000 Ma to at least the end of the Cambrian (~540 Ma to 490 Porada, 1989;Stanistreet et al., 1991;Meert, 2003;Begg et al., 2009;Nascimento et al., 2016). ...
    Article
    The Neoproterozoic Marinoan climatic event corresponded to the Snowball Earth-type glaciation, and is commonly marked by the deposition of diamictites and by a negative carbon isotope anomaly. This event was followed by a sudden return to a greenhouse climate and a rapid post-glacial transgression with deposition of cap carbonates. Although the cap carbonates and marine carbonate sediments at the base of the post-glacial period are well known in the literature, few studies focused on the end of the marine transgression, which is a prelude to the Pan African Orogeny in Central Africa. In this paper, we present new descriptions of these carbonate rocks and a sedimentological study from key cores and outcrops in the Otavi Mountainland (Namibia) and West Congo belt (DRC) of the Hüttenberg Formation and the C5 Formation, respectively. Both successions show five main facies: (i) microbial 'mounds' and pinnacles; (ii) ooid-shoal barrier; (iii) evaporitic brackish lagoon; (iv) beach and (v) coastal sabkha. The Hüttenberg Formation consists of an open-marine mid-inner carbonate ramp setting including microbial mounds and pinnacles, and oolitic shoal-barrier Islands. The C5 Formation exhibits a hypersaline inner carbonate ramp including an ooid shoal barrier, an evaporitic brackish lagoon, a beach and a coastal sabkha plain. Sedimentological, chemostratigraphical and biostratigraphical comparisons between the C5 and Hüttenberg formations suggest these are coeval carbonate shelf deposits on the margins of the Congo Craton, with a depositional age between 580 Ma and 540 Ma for both formations.
  • ... In the Southern Zone, metamorphic conditions increased from south to north reaching up to ~8 kbar and ~600 °C close to the OLZ ( Hartmann et al., 1983;Kasch, 1983;Cross et al., 2015; Fig. 1). The Okahandja Lineament Zone (OLZ; Fig. 1), which separates the Central Zone from the Southern Zone, is supposed to represented the suture of a Neoproterozoic intercontinental subduction of the Southern Zone be- neath the Central Zone (Porada, 1989;Prave, 1996). The Southern Zone and the adjacent Southern Margin Zone, representing an accretionary prism, is essentially composed of metasedimentary rocks, mainly greywackes and some pelites that were progressively metamorphosed from biotite-, garnet-, staurolite-to staurolite/kyanite facies during the Pan-African orogeny. ...
    Article
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    The 541 ± 4 Ma-old magnesian, weakly peraluminous, calc-alkalic Donkerhoek Onanis granite is part of the ca. 6000 km² large Donkerhoek batholith in the Southern Zone of the Damara orogen of Namibia. Linear major and trace element variations and decreasing MgO, FeO, Al2O3, CaO, K2O, Na2O, Ba and Sr concentrations with increasing SiO2 indicate that this part of the batholith represent a coherent mass and underwent fractional crystallization processes. The Donkerhoek Onanis granites are isotopically evolved (initial εNd: −4.7 to −12.3, initial ⁸⁷Sr/⁸⁶Sr: 0.7099–0.7157) with moderately radiogenic Pb isotope ratios (²⁰⁶Pb/²⁰⁴Pb: 17.26–18.22; ²⁰⁷Pb/²⁰⁴Pb: 15.59–15.67; ²⁰⁸Pb/²⁰⁴Pb: 37.60–38.06). Beside heterogeneities imparted by the sources, an evaluation of LREE fractionation and Nd isotope data suggests that AFC processes also modified some samples. Based on the chemical and isotope data, the Donkerhoek Onanis granites cannot be derived by partial melting of Al- and Fe-rich metasedimentary rocks of the Kuiseb formation in which they intruded. Instead, melting of meta-igneous crustal sources with Proterozoic crustal residence ages is more likely. Three igneous to meta-igneous rock suites from the area (Matchless amphibolites, Proterozoic mafic to felsic gneisses from the southern Kalahari craton basement, syn-tectonic Salem granodiorites to granites) are potential sources. An evaluation of chemical and isotope data suggests that remelting of early syn-orogenic Salem-type granites is the most likely process which would also explain the existence of ca. 563 ± 4 Ma-old zircon in the Donkerhoek Onanis granites. Comparison of the Donkerhoek Onanis granites with experimentally derived melt compositions from an intermediate igneous parent indicates temperatures between 800 and 850 °C. It is suggested that the Pan-African igneous activity in this part of the Damara Belt was a moderate-temperature intra-crustal event. Although there are some compositional similarities with juvenile granites generated in subduction zones, unradiogenic Pb isotope ratios and moderately radiogenic Sr and unradiogenic Nd isotopes suggest that reprocessed crustal rocks are more likely sources. Previously obtained high δ¹⁸O values of the Donkerhoek Onanis granites ranging from 11.8 to 13.6‰, covering the range of δ¹⁸O values obtained on Salem-type granites from the area (12.5–13.3‰) confirm this view. In contrast to igneous processes along active continental margins that produce juvenile batholiths with calc-alkaline affinities, this igneous event was not a major crust-forming episode and the Donkerhoek Onanis granites represent reprocessed crustal material.
  • ... Tai Tao (sul do Chile), representa a subducção de uma dorsal meso-oceânica e sua interação com a trincheira; (c) Ilha de Macquarie (Austrália), fragmento de dorsal meso-oceânica exposta acima do nível do mar por empurrões tectônicos (ainda não está alojada no continente); (d) Ofiolitos ligurianos -crosta oceânica formada durante e após o rifteamento entre Ádria/África e Europa (abertura do paleo-oceano Lígure- Piemontês), posteriormente imbricada em depósitos de trincheira e alojada ao longo de falhas de empurrão resultantes de encurtamento regional; (e) Arcos insulares de Magnitogorsk (Montes Urais, Rússia) -consistem em um complexo vulcânico alojado no continente Europeu Oriental como resultado de subducção parcial de rochas de margem continental sob o antearco; (f) Ofiolito de Omã -fragmento de litosfera oceânica tetiana que forma a porção superior de uma nappe, em um pacote imbricado de rochas oceânicas. O antearco da litosfera oceânica tetiana foi desalojado do seu ambiente original e realocado na margem continental Árabe durante a colisão trincheira-continente (modificado de Dilek & Furnes, 2014 Ceará Central da Província Borborema (Fetter, 1999;Fetter et al., 2000;Bizzi et al., 2003 paleorreconstruções e modelos podem ser vistos em, e.g., Porada, 1989;Frimmel et al., e 2013Santos et al., 2017 -Brasiliano III (em torno de 595-560 Ma, com geração de magmatismo granítico pré-a sintectônico, relacionado aos eventos colisionais finais do ciclo orogênico Brasiliano-Panafricano). ...
    Thesis
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    The Arroio Grande Ophiolite, located in the southeastern region of the Dom Feliciano Belt, near the Brazil/Uruguay border, is a metaultramafic-mafic-sedimentary association which represents slices of an ophiolitic mélange, related to the Western Gondwana amalgamation during the late stages of the Brasiliano-Panafrican orogenic cycle. The Arroio Grande Ophiolite rocks are enveloped by metasiliciclastic units of the Arroio Grande Complex and occur as xenolyths within granitoids of the Pinheiro Machado Suite and within the Três Figueiras Granite (units of the Pelotas-Aiguá Batholith). The metaultramafites of the ophiolite comprise serpentinites and Cr-rich magnesian schists. The metamafites comprise amphibolites, metagabbros and metadiorites. The metasedimentary unit comprises calcitic marbles, which are intruded by mafic dykes. The ophiolite is found along the Ayrosa Galvão- Arroio Grande Shear Zone (transcurrent, ductile, high angle), responsible for the mylonitization of this association. The investigations developed in this ophiolite had the objective of identify the magmatic sources of the protoliths and the processes that occurred since their generation within the mantle/oceanic crust until their incorporation into the continental crust, including their absolute and relative ages. The bulk-rock chemistry of the metaultramafites (e.g. Ni >1000 ppm; Cr > 1500 ppm), together with the mineral chemistry of the chromites (e.g. Cr# 0.6-0.8; TiO2 0.01-0.20 wt%; Fe2+/Fe3+ ± 0.9), suggested harzburgitic protoliths, attributed to a depleted mantle source under a back-arc spreading region, which experienced high degrees of partial melting. These harzburgites were serpentinized in an oceanic setting, as suggested by the 87Sr/86Sr630 ratio of a serpentinite (ca. 0.707). The bulk rock chemistry of the metamafites suggested oceanic tholeiitic protoliths, generated in a supra-subduction setting in a back-arc environment (e.g. Cr 260-600 ppm; Nb/Y 0.1-0.5; Ti/Y ± 500; La/Nb 2-5; Th/Yb 0.1-5 and Nb/Yb 1-5; REE patterns; 87Sr/86Sr630 ratios ranging from MORB – 0.703 – to IAT – 0.705-0.707), whose magmatic source was contaminated by crustal material and subduction-related fluids. The minimum age for the obduction and metamorphism of the Arroio Grande Ophiolite rocks was estimated around 640 Ma from the U-Pb age of a quartz-syenite. The latter is the result of melting, related to dioritic-tonalitc intrusions, attributed to the continental magmatism of the Pinheiro Machado Suite. These intrusions affected both the marbles and the amphibolites (fragments of the mafic dykes), in order that, at least around 640 Ma, rocks of the ophiolitic mélange (already metamorphosed) were emplaced on the continent. A late metasomatic event (related to the emplacement of the Três Figueiras Granite, syn-kinematic to the abovementioned shear zone) affected the serpentinites, generating zones of talcification, tremolitization and chloritization, the latter representing a blackwall which also involved metasiliciclastic rocks of the Arroio Grande Complex. The Arroio Grande Ophiolite was inserted in the geotectonic context of the Marmora back-arc basin, whose fragments are found in Namibia (Marmora Terrane) and Uruguay (Paso del Dragón Complex and Rocha Basin – Punta del Este Terrane).