Pierre Barbey

Origin and evolution of the late precambrian high-grade Yaounde Gneisses (Cameroon)

Abstract The Yaounde series is composed of low to high-grade garnet-bearing schists and gneisses belonging to the Pan-African North-equatorial fold belt. The high-grade rocks consist of kyanite—garnet gneisses and garnet—plagioclase gneisses containing layers of calcsilicate rocks, marbles, quartzites and magnetite-rich orthopyroxenites. The chemical patterns of these rocks are those of a sedimentary sequence of shales and greywackes and minor dolomite-rocks, dolomitic marls, evaporitic beds, quartzites and iron-rich sediments. Some volcanogenic greywackes may have been of alkaline affinity. This sequence was probably deposited in shallow-water, near-shore, semi-barred conditions which can be related either to an intracontinental distensive environment or to a passive margin at the northern edge of the Congo craton. The Yaounde gneisses were deformed during two main tectonic phases: the first phase (D1) is marked by a layering (S0-S1) resulting from tectonic transposition; the second one (D2) corresponds to tangential tectonics with isoclinal folds and flat-lying shear zones associated with a S2 schistosity and a L2 lineation; a third phase (late-D2) is marked by the development of low temperature mylonitic shears. Scarce wrench faults indicate later E—W compression. The D1-D2 transition is characterized by the emplacement of mafic and ultramafic rocks and by the development of high-pressure granulite facies conditions (T=750–800°C, P=10–12 kbar). These peak metamorphic conditions triggered a widespread migmatization dated at 565±22 Ma and coeval with the D2 event. The gradual decrease of the metamorphic conditions toward the south and the existence of a syn-D2, retrograde, inverse metamorphic gradient, suggest that the overall structure of the Yaounde series is a large tectonic nappe thrust onto the Congo craton. A geotectonic scheme for the North-equatorial fold belt is discussed.

Source, genesis, and timing of giant ignimbrite deposits associated with Ethiopian continental flood basalts

Abstract The Ethiopian continental flood basalt (CFB) province (∼30 Ma, > 3 × 105 km3) was formed as the result of the impingement of the Afar mantle plume beneath the Ethiopian lithosphere. This province includes major sequences of rhyolitic ignimbrites generally found on top of the flood basalt sequence. Their volume is estimated to be at least 6 × 104km3, which represents 20% of that of the trap basalts. Their phenocryst assemblage (alkali feldspar, quartz, aegyrine-augite, ilmenite ± Ti-magnetite, richterite, and eckermanite) suggests temperatures in the range of 740 to 900°C. Four units were recognized in the field (Wegel Tena, Jima, Lima Limo, and Debre Birhan areas), each with its own geochemical specificity. Zr/Nb ratios remain constant between basalt and rhyolite in each area, and rhyolites associated with high-Ti or low-Ti basalts are, respectively, enriched or depleted in titanium. Their trace element and isotope (Sr, Nd, O) signatures (high 143Nd/144Nd and low 87Sr/86Sr ratios, compared to those of rhyolites from other CFB provinces) are clearly different from those of typical crustal melts and indicate that the Ethiopian rhyolites are among the most isotopically primitive rhyolites. Their major and trace element patterns suggest that they are likely to be derived from fractional crystallization of basaltic magmas similar in composition to the exposed flood basalts with only limited crustal contribution. Since Ethiopian high-Ti basalts have been shown to form from melting of a mantle plume, it is likely that Ethiopian ignimbrites, at least those that are Ti-rich, also incorporated material from the deep mantle. Rb-Sr isochrons on whole rocks and mineral separates (30.1 ± 0.4 Ma for Wegel Tena and 30.5 ± 0.4 Ma for Jima ignimbrites) show that most of the silicic volcanism occurred within 1.4 × 1015 mol) and Cl (6.4 × 1015 mol) into the atmosphere over a short time span, with the global cooling event at 30.3 Ma suggests that this volcanism might have accelerated the climate change that was already underway.

Petrology and U/Pb geochronology of the Telohat migmatites, Aleksod, Central Hoggar, Algeria

The Aleksod region is composed of metasedimentary rocks and large areas of biotite and hornblende-bearing migmatites. Anatexis associated with the main deformation stages, occurred under high pressure and temperature conditions estimated at 13±2 Kbar and 750±50° C. The bulk mineralogical composition of the Telohat migmatites shows that their protolith was granodioritic. Internal structures of zircons and U-Pb data suggest a polyphased evolution, with a 2131±12 Ma age for the protolith and a 609±17 Ma age for the Pan-African tectono-metamorphic evolution, thus precluding any Kibaran event in the Aleksod area. Leucosomes are richer in Sr and display lower Rb, Zr, Nb, Y, Th, U and REE contents than melanosomes wherein accessory phases are stored. Eu contents are also lower in the leucosomes but in lesser proportion than the other rare earths, leading to a significant positive anomaly. Petrogenetic modelling accounting for accessory mineral phases clearly shows that the trace element contents of leucosomes and melanosomes follow a distribution law consistent neither with equilibrium nor fractional melting. Their trace element patterns are best explained by the model of disequilibrium melting, with mixing of a few residual phases. The present results and previous Sr isotopic data as well raise the question of disequilibrium melting in anatexis of crustal material

2.8–3.0 Ga plutonism and deformation in the SE Amazonian craton: the Archaean granitoids of Marajoara (Carajás Mineral Province, Brazil)

Abstract The granitoids of Marajoara in the Rio Maria terrain (Carajas Mineral Province, Brazil) consist of: (i) a broad unit of 2.96 Ga syntectonic tonalites (Arco Verde Tonalites) displaying a trondhjemitic differentiation trend; (ii) 2.93 Ga syntectonic monzogranites (Guaranta); and (iii) 2.87 Ga post-tectonic monzogranites (Mata Surrao) and granodiorites (Rio Maria), displaying a calc-alkaline differentiation trend. Deformation of the Arco Verde tonalites is heterogeneous with low strain domains (well preserved magmatic banding and textures) and orthogneissic domains displaying an E–W trending mainly subvertical foliation, associated with horizontal lineations, upright folds and subvertical shear zones. Microstructures and phase assemblages suggest that deformation occurred within a large temperature range (i.e. during magma emplacement and cooling), from high-T conditions (synmagmatic shear zones and subsolidus ductile deformation with intense quartz and feldspar recrystallization; Pl+Qtz+Hbl+Bt assemblages) to medium- and low-T conditions (ductile to brittle deformation with weakly recrystallized quartz and undulose extinction in feldspars; Qtz+Pl+Bt+Mu or Chl+Ep+Ab+Qtz assemblages). These data, finite strain analysis and structures reported from the surrounding greenstone belts suggest that deformation did not result from a post-emplacement prograde tectono-metamorphic event as considered previously, but that the Marajoara granitoids are synkinernatic intrusions which were deformed together with the supracrustal rocks during a regional NS horizontal shortening. Although the Rio Maria terrain presents similarities with Archaean domains controlled by diapiric processes (lithologies dominated by thick greenstone sequences and TTG plutons, and forming a dome-and-keel structure), its structural evolution is controlled dominantly by a transpressional event which shaped the granite–greenstone terrains. The Rio Maria area, probably as many Archaean ‘grey gneisses’ domains, represents an intermediate case between terrains controlled by Raleigh–Taylor instabilities in a thermally softened crust with insignificant external forces related to plate convergence (e.g. east Pilbara craton) and those controlled by thrust tectonics related to convergence of rigid plates (e.g. Superior Province). The closest analog to the Rio Maria terrain seems to be the Chilimanzi area in the Zimbabwe craton.

Rare-earth patterns in zircons from the Manaslu granite and Tibetan Slab migmatites (Himalaya) : insights in the origin and evolution of a crustally-derived granite magma

The Tibetan Slab gneisses are currently considered as the source for the High-Himalayan leucogranites. The rare-earth element distributions in zircons from the Tibetan Slab migmatites and the Manaslu leucogranite (Nepal Himalaya), were investigated by in situ ion probe analysis. These data combined with textural information have been used to elucidate the zircon growth conditions and, indirectly, the processes involved in incipient anatexis and evolution of granitic magmas. In the migmatites, the zircons from gneisses and melanosomes have rounded shapes and variable REE patterns with low Yb contents (145–700 ppm) and chondrite-normalized (Yb/Sm)N ratios (≤81). Zircons from low-Zr tonalitic leucosomes are morphologically and chemically indistinguishable from those of the gneisses and melanosomes. Zircons from the high-Zr tonalitic leucosomes and granitic leucosomes are euhedral and show higher Yb contents (409–2820 prim) and (Yb/SM)N ratios (≥145) than those of the gneisses and melanosomes. The euhedral shapes and distinctive REE patterns of zircons from the high-Zr leucosomes and granitic leucosomes are consistent with crystallization from a melt, whereas the morphological and chemical similarities of zircons from the low-Zr leucosomes with those from the gneisses and melanosomes suggest inheritance without significant chemical change. In the Manaslu granite, zircons have rounded cores with REE patterns distinct from those of the rims (e.g., 250 ppm ≤Yb≤710 ppm in the core, 965 ppm ≤Yb≤ 2775 ppm in the rim) but comparable to those from the Tibetan Slab gneisses suggesting inheritance. The rim compositions, however, are distinct from those of either zircon types of the Tibetan Slab leucosomes, indicating that the leucosomes cannot be the unsegregated equivalent to the Manaslu granite parental magma. Comparison of the rim compositions with fractional crystallization models suggests that the range in zircon Sm and Yb contents are consistent with zircon crystallization from a monazite-saturated, xenotime-undersaturated melt. The Yb contents in the different zircons studied, and their variation within a single zircon, further suggest boundary-layer effects and magma compositional heterogeneity, in agreement with previous models which considered that the Manaslu granite resulted from the aggregation of magma batches.

Granite-migmatite genetic link: the example of the Manaslu granite and Tibetan Slab migmatites in central Nepal

In central Nepal, the Tibetan Slab is made up of biotite-gneisses (metapelites and metagreywackes), orthogneisses (metagranites) and migmatites. Melanosomes are generally biotite-(± muscovite)-bearing, but locally they may be tourmaline-rich when associated with boron-rich granitic material. Leucosomes occur as lenses conformable with the foliation, veins, patches, or as fillings in shear zones and extensional structures. Field relationships, and mineralogical and chemical data show that three processes may have contributed to the formation of the Tibetan Slab leucosomes: metamorphic differentiation or disequilibrium partial melting (low-Zr tonalitic leucosomes), in-situ equilibrium partial melting (high-Zr leucosomes and some granitic leucosomes) and injection of externally-derived melts (most granitic and some tonalitic leucosomes). The Manaslu pluton belongs to the High Himalayan leucogranite belt and was emplaced at the top of the Tibetan Slab. It corresponds to a muscovite-biotite leucogranite that has been assumed to derive from melting of the Tibetan Slab gneisses (Formation I). Phase relationships, a more magnesian chemistry of the ferromagnesian minerals from the Tibetan-Slab migmatites as compared to the Manaslu leucogranite, the microtextures of accessory phases, and trace-element compositions (lower U, Li, F and higher Sr, Eu, Y, Yb contents in the migmatite leucosomes) show that the in situ Himalayan migmatites, at the crustal level presently exposed, have not been produced under the same P-T-XH2O conditions as the Manaslu leucogranite magma. While the Formation I was the probable source for the Manaslu granite, migmatites within the formation are not the remanants of a melting process from which the Manaslu granite was derived. Both the Tibetan Slab migmatites and the Manaslu leucogranite may be considered as evidence of dehydration and melting at deeper crustal levels, and of percolation of melts and hydrothermal fluids through the crust.

Role of magma pressure, tectonic stress and crystallization progress in the emplacement of syntectonic granites. The A-type Estrela Granite Complex (Carajas Mineral Province, Brazil)

The Archaean, syntectonic, A-type Estrela Granite Complex (Carajas Mineral Province, Brazil) consists of three plutons emplaced in a greenstone sequence under low-pressure conditions (180<P<310 MPa). It is composed mainly of annite-, ferropargasite (±hedenbergite)- and ilmenite-bearing monzogranites. The contact aureole is affected by a subvertical penetrative schistosity conformable with the limits of the plutons. Meso- to microstructures and mineral reactions in the granites indicate that deformation occurred in a continuum from above-solidus to low-T subsolidus conditions. Two distinct planar structures are observed: (i) a concentrical primary foliation (S0) corresponding to rhythmic, isomodal, phase layering associated with a faint grain shape fabric; it is horizontal in the centre and vertical towards the edges of the plutons; and (ii) a steep to subvertical foliation (S1) associated with the deformation of S0 and accompanied with emplacement of synplutonic dykes and veins of leucocratic granites and pegmatites. Emplacement, differentiation and consolidation of the Estrela Granite Complex are considered to result from a continuous evolution under decreasing temperatures in a single-stage strained crust (transpression), with two main periods. (1) The first period is controlled by body forces, and it corresponds to inflation with magma ponding. As long as the rheology is melt dominated, magma pressure is the critical parameter and almost no strain is recorded. With decreasing T, magmas crystallize and differentiate leading to a concentrical magmatic phase layering. The growing magmatic bodies are mechanically decoupled from the country rocks and their evolution depends on internal magma chamber processes. (2) For higher amount of crystallization (residual melt fraction F<0.5), the role of magma pressure becomes insignificant. Establishment of a continuous crystal framework leads to the coupling of plutons with their surroundings, and deformation in response to tectonic stress. Most of the strain is recorded during this period which starts from the rigid percolation threshold, and extends to subsolidus low-grade conditions. This leads to deformation of the partially crystallized volume and redistribution of fluid-enriched differentiated melts. The amount of crystallization through the rheological thresholds appears as the critical parameter determining the transition from magma-controlled processes (inflation and differentiation of the magma chamber, with development of a phase layering) to tectonic-controlled processes (deformation of the phase layering and redistribution of residual melts). This accounts for the fact that syntectonic plutons commonly display intermingled, boudinaged layers with distinct modal compositions and in some cases well-preserved rhythmic layering.

The Ntui-Betamba high-grade gneisses: a northward extension of the Pan-African Yaoundé gneisses in Cameroon

Abstract The Ntui-Betamba area (southern Cameroon) is composed of high-grade migmatitic gneisses in which two lithological units are distinguished: (i) a metasedimentary unit (kyanite-biotite-garnet gneisses, biotite-muscovite-garnet gneisses, calc-silicate rocks and quartzites) interpreted as a continental margin sedimentary series; and (ii) meta-igneous rocks comprising alkaline ultramafic to mafic pyroxene gneisses and amphibolites and amphibole-bearing alkaline orthogneisses. These units recrystallised under HP-HT conditions ( T =750–800 °C, P ≥0.9–1.3 GPa) and were deformed in relation to a major tangential tectonic event with a north-northeast-south-southwest kinematic direction. This lithological association and its tectono-metamorphic evolution show striking similarities with the Yaounde gneisses, suggesting that the extensional depositional environment envisaged for this formation can be extended farther north, towards the Adamawa Shear Zone (Lorn series). The contrasted metamorphic evolution between areas located to the south of the Adamawa (high pressure: Yaounde, Ntui-Betamba), and those located to the north (low pressure: Banyo), along with widespread remains of a Palaeoproterozoic crust, suggest important crustal thickening during tangential tectonics in southern Cameroon. As a consequence, the Adamawa Shear Zone is not simply a late Pan-African transcurrent or transpressive shear zone but appears to have been formerly a major (possibly intracontinental) thrust zone.

Timing of granite emplacement, crustal flow and gneiss dome formation in the Variscan segment of the Pyrenees

Abstract The Variscan segment of the Pyrenees is well suited to study the timing of crustal-scale deformations as crustal flow and gneiss dome formation. This has been constrained from a synthesis of available structural and geochronological data of intrusive rocks, as well as new zircon U–Pb age determinations via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). After a stage of moderate thickening by fold–thrust belt development in the upper crust between 323 and 308 Ma, the Variscan segment of the Pyrenees experienced crustal flow at c. 306 Ma and then gneiss dome formation at c. 304 Ma. Localization of the deformation along reverse-dextral shear zones occurred at c. 300 Ma. The Variscan segment of the Pyrenees recorded a high-temperature regime, which allowed crustal flow of the middle crust, but with limited amounts of heat which induced rapid cooling. The development of this enigmatic orogenic segment of the Variscan belt is closely contemporaneous with the formation of the Cantabrian Orocline and could correspond to a lithospheric-scale shear zone that accommodated buckling of the orocline. Late Variscan lithospheric delamination and asthenospheric upwelling associated with buckling in the core of the Cantabrian Orocline could explain the short-period high-temperature regime in the Variscan segment of the Pyrenees. Supplementary material: Review of published U–Th–Pb, Ar–Ar and K–Ar ages of granites from the Axial Zone of the Pyrenees; LA-ICP-MS U–Pb dating methodology (Clermont-Ferrand, France); and zircon LA-ICP-MS U–Pb data for Ax-les-Thermes, Carançà, Mont-Louis and Cauterets granites are available at http://www.geolsoc.org.uk/SUP18729.

On the non-existence of a Cadomian basement in southern France (Pyrenees, Montagne Noire): implications for the significance of the pre-Variscan (pre-Upper Ordovician) series

The deepest Hercynian metamorphic terrains in the Pyrenees and in the nearby Montagne Noire are made up of medium-grade orthogneisses and micaschists, and of high-grade, often granulitic, paragneisses. The existence of a granitic-metamorphic Cadomian basement and of its sedimentary Lower Paleozoic cover was advocated from the following main arguments: (i) a supposed unconformity of the Lower Cambrian Canaveilles Group (the lower part of the Paleozoic series) upon both granitic and metamorphic complexes; (ii) a ca . 580 Ma U-Pb age for the metagranitic Canigou gneisses. A SE to NW transgression of the Cambrian cover and huge Variscan recumbent (“penninic”) folds completed this classical model. However, recent U-Pb dating provided a ca . 474 Ma, early Ordovician (Arenigian) age for the me-tagranites, whereas the Vendian age (581 ± 10 Ma) of the base of the Canaveilles Group was confirmed [Cocherie et al. , 2005]. In fact, these granites are laccoliths intruded at different levels of the Vendian-Lower Cambrian series. So the Cadomian granitic basement model must be discarded. In a new model, developed in the Pyrenees and which applies to the Montagne Noire where the orthogneisses appear to be Lower Ordovician intrusives too, there are neither transgression of the Paleozoic nor very large Hercynian recumbent folds. The pre-Variscan (pre-Upper Ordovician) series must be divided in two groups: (i) at the top, the Jujols Group, mainly early to late Cambrian, that belongs to a Cambrian-Ordovician sedimentary and magmatic cycle ; the early Ordovician granites pertain to this cycle; (ii) at the base, the Canaveilles Group of the Pyrenees and the la Salvetat-St-Pons series of the Montagne Noire, Vendian (to earliest Cambrian?), are similar to the Upper Alcudian series of Central Iberia. The Canaveilles Group is a shale-greywacke series with rhyodacitic volcanics, thick carbonates, black shales, etc. The newly defined olistostromic and carbonated, up to 150 m thick Tregura Formation forms the base of the Jujols Group, which rests more or less conformably on the Canaveilles Group. The high-grade paragneisses which in some massifs underlie the Canaveilles and Jujols low- to medium grade metasediments are now considered to be an equivalent of the Canaveilles Group with a higher Variscan metamorphic grade; they are not derived from metamorphic Precambrian rocks. So, there is no visible Cadomian metamorphic (or even sedimentary) basement in the Pyrenees. However, because of its age, the Canaveilles Group belongs to the end of the Cadomian cycle and was deposited in a subsident basin, probably a back-arc basin which developed in the Cadomian, active-transform N-Gondwanian margin of this time. The presence of Cadomian-Panafrican ( ca. 600 Ma) zircon cores in early Ordovician granites and Vendian volcanics implies the anatexis of a thick (> 15 km?) syn-Cadomian series, to be compared to the very thick Lower Alcudian series of Central Iberia, which underlies the Upper Alcudian series. Nd isotopic compositions of Neoproterozoic and Cambrian-Ordovician sediments and magmatites, as elsewhere in Europe, yield Paleoproterozoic ( ca. 2 Ga) model-ages. From the very rare occurrences of rocks of this age in W-Europe, it can be envisionned that the thick Pyrenean Cadomian series lies on a Paleoproterozoic metamorphic basement. But, if such a basement does exist, it must be “hidden”, as well as the lower part of the Neoproterozoic series, in the Variscan restitic granulites of the present (Variscan) lower crust. So a large part of the pre-Variscan crust was made of volcano-sedimentary Cadomian series, explaining the “fertile” characteristics of this crust which has been able to produce the voluminous Lower Ordovician and, later, Upper Carboniferous granitoids.