Search

• Microstructural observations and EBSD measurements were applied to nine mylonites from the 4°50′ shear zone. • Zircon U-Pb ages provide Pan-African ages and are related to dextral deformation accommodated with fluids. • Partial melting of lower crust/upper mantle triggered granitic intrusions from 661 ± 2.8 to 639 ± 2 Ma. • Successive magma batches during migmatization from 623 ± 4 Ma to 609 ± 2 Ma. • Local sinistral reactivation of the 4°50′ shear zone occurred posteriorly to 609 ± 2 Ma. • The 4°50′ shear zone remained active under high-temperature over ≥ 20 My. The 4°50′Shear zone (SZ), one of the major structures of western Gondwana, formed in Hoggar during the Pan-African orogeny is still poorly studied; especially ages and deformation characteristics are lacking. We performed microstructural analysis and EBSD measurements on nine selected mylonites from the 4°50′SZ, and zircon U-Pb dating on two granitoid samples. The protolith of these mylonites are granitoids. Most samples display segregation of dominant feldspar and minor quartz in separate layers that underline the N-S subvertical mylonitic foliation. Feldspar grains display evidence of intracrystalline deformation, supporting dextral shear-sense. Plagioclase and K-feldspar are frequently corroded with embayments infilled by secondary K-feldspar and plagioclase respectively. Interstitial quartz grains disseminated in feldspar aggregates may infill fractures in feldspar grains. Quartz ribbons, usually parallel to the foliation, locally crosscut feldspar layers. Quartz is partially recrystallized and elongate crystals form a secondary foliation at ∼ 30° to the main foliation, supporting a later sinistral shearing. In some samples, amphibole is associated with quartz and oriented parallel to the foliation. In all samples, quartz CPO suggests a dominant prism- slip-system, activated at medium/high temperature (∼500-700°c), but stress-induced oriented crystallization may have contributed to this CPO. K-feldspar CPO supports the activation of the [1 0 0](0 1 0) slip system while the plagioclase CPO points to activation of [1 0 0](0 0 1) system. These CPO support dislocation creep under amphibolite facies conditions. These new data, in addition to U-Pb dating, suggest the following evolution: 1) successive intrusions of granitoids in the SZ (∼661-∼639 Ma) due to partial-melting of the lower-crust and, possibly also of the lithospheric-mantle, 2) the 4°5 0′SZ was rooted in the lower-crust or the upper-mantle, and 3) dextral shearing lasted over ≥ 20My. This was followed by successive magma batches that intruded these rocks during a late migmatitic episode (∼623-∼609 Ma). After ∼ 609 Ma, a moderate sinistral reactivation of the 4°50′SZ occurred and was accommodated through dislocation creep mainly localized in quartz ribbons. During accretion of the Western Gondwana, the 4°50′SZ was active during subduction. During ≥ 20My, it was successively intruded by granitoids that cooled down slowly, and have been deformed in the submagmatic state then in the solid-state. [ABSTRACT FROM AUTHOR]

The Egéré domain belongs to the Egéré-Aleksod terrane (LATEA metacraton), which has the peculiarity in the Tuareg Shield of being practically devoid of Pan-African batholiths allowing the preservation of pre-Neoproterozoic and early Pan-African events. It is composed of amphibolitic to eclogitic facies metamorphic rocks with the mostly metamagmatic Arechchoum Group, the mostly metasedimentary Egéré Supergroup and the here defined metasedimentary Oneral Group. Petrological P-T-t study, U-Pb on zircon dating, geochemical composition and Sr-Nd isotopic compositions decipher largely unsuspected Paleoproterozoic, Mesoproterozoic and Neoproterozoic evolution for the Tuareg Shield in addition to an Archean cratonic nucleus. We show here that (1) the Arechchoum Group, now composed mainly of garnet orthogneiss, is an alkaline silicic large igneous province (SLIP) of early Rhyacian age (c. 2.28 Ga) underlain by the Archean Amadror craton, defined here. Additional alkaline pulses occurred during the Siderian (c. 2.37 Ga), the late Orosirian (c. 1.89 Ga, gabbro) and the Statherian (c. 1.74 Ga, rhyolitic dykes). (2) The Lower Egéré Group is made of marbles and quartzites interlayered with a large volume of bimodal magmatic rocks during the late Rhyacian (c. 2.1Ga) and the Orosirian (c. 2.0 Ga) reminiscent of the Karoo LIP and emplaced during an extensional continental setting. (3) The Upper Egéré Group consists of paragneiss, quartzite and marble deposited at the end of the Orosirian (c. 1.8 Ga) with a majority of Archean zircon grains (Amadror craton) and a complete record from Arechchoum and Egéré groups. It houses also a Calymmian high-Fe intraplate tholeiitic basaltic event (c. 1.55 Ga) marking a rifting event. (4) The newly defined Oneral Group deposited at c. 0.7 Ga comprising sediments whose source was located to the west of LATEA named Tin Hinan terrane, a continental arc made of a Stenian basement and Tonian subduction-related rocks. At c. 0.68 Ga, all these groups were subducted (continental subduction) generating an eclogitic facies (c. 650 °C/20 kbar) in the Egéré Supergroup and the Oneral Group. The exhumation, triggered by slab breakoff (c. 800 °C/14 kbar) at c. 0.65 Ga, put these groups back on the Arechchoum Group at c. 0.63 Ga, the whole package recording c. 690 °C and 8 kbar. During the long post-collisional transpressive period (630–580 Ma), corresponding to the general northward Tuareg tectonic escape, they were slowly exhumed up to c. 610°/6 kbar, during which time the rigid Amadror craton generated the spectacular NW-verging thrust structure of the Egéré region while the Tuareg Shield is dominated by NE verging structures. [ABSTRACT FROM AUTHOR]

Abstract: Enormous masses of highly mature quartz sands were deposited in Western Gondwana during the Cambrian–Ordovician time, and provide a wide range of information concerning magmatic events through time, provenance, paleoclimate, and basin history. We present a provenance study based on 630 U–Pb (LA-ICP-MS) ages of detrital zircon from the latest Cambrian to Ordovician siliciclastic rocks of the Tassili Ouan Ahaggar basin situated in the Algerian Sahara. Most authors suggest local sources only for the sandstones. Instead, we demonstrate that the detritus is derived from different cratons and terranes which contributed to the deposition of a Cambrian–Ordovician overstep sequence covering western and northern Africa. Most zircon ages (61.0%) fall in the range of ~540 to 740Ma and are interpreted to have been derived from Pan-African orogenic belts such as the Trans-Saharan Belt of NW Africa and previously from the Brazila belt of South America. Other potential sources for this zircon population are terranes of Cadomian affinity situated marginal to West Africa. The second-largest zircon population (20.2%) is 2.0 to 2.2Ga, and is attributed to sources in the West African craton, such as the Birimian basement and the Eburnean orogenic belt, with possible partial input from the Amazonian craton. A zircon population of 7.1% yields Mesoproterozoic and early Paleoproterozoic ages in the range of ~1.3 to ~1.8Ga and was probably derived from source rocks outside of the West African basement, the Tuareg shield and other adjoining areas. The Amazonian craton is a potential source region. A population of 6.7% of all zircon ages scatter from ~750Ma to ~980Ma and may reflect input from latest stages of the formation of Rodinia and its subsequent dispersal. A smaller population (3.2%) of zircon ages lie between ~2.3 and 2.65Ga, and may be derived from late Paleoproterozoic to early Archaean rocks from the West African craton and possibly from Amazonia. Less than 1% of all zircons are Meso- to Paleoarchaean ones and provide evidence for the input of very old cratonic basement, most likely from cratonic inliers of the West African craton (Leonian, Liberian). Because of the potential input of detrital zircon from the Amazonian craton, which is reflected in the Mesoproterozoic and late Paleoproterozoic grains, we speculate that some of the Paleoproterozoic to Neoarchean (2.0Ga to 2.6Ga) zircons were also derived from Amazonia. Due to the total lack of 1.0–1.2Ga old zircon, our data set excludes all crustal domains situated in the Arabian-Nubian shield and the East African belt, as well as the Sunsás belt of Amazonia (“Sunsás-Grenvillian”) as potential sediment sources. Sedimentation in the Tassili Ouan Ahaggar basin started in uppermost Cambrian to Ordovician time due to the opening of the Rheic Ocean. This event led to subsidence related to the rift and drift of Avalonia and related terranes from the northwestern Gondwanan margin. The basal Early Tassili quartzite has detrital zircon populations that suggest a local provenance either from West African or from a related terrane in the Tuareg shield. A dramatic change occurs in the deltaic to shallow marine strata of the Lower Ordovician Ajjers Formation and in the overlying marine sandstones of the Middle Ordovician d''In Azaoua Formation. Our data for both formations indicate the Pan-African orogen, and very likely Cadomian terranes as the main source for the detritus. During this time, the region was affected by rift tectonics due to the opening of the Rheic Ocean and therefore amenable to erosion at rift shoulders and escarpments. Our data also indicate that glacial erosion in Upper Ordovician (Hirnantian) time must have affected larger areas of old cratonic surfaces as the populations of Paleoproterozoic to Archaean zircons are significantly higher than in other age clusters. Large parts of highly mature sands of the Cambro-Ordovician section in the Tassili Ouan Ahaggar basin were derived from a peneplain in the interior of Gondwana, that formed during Cambrian times. This peneplain was formed under warm-humid climate on a vegetation-free land surface and in an extreme corrosive environment that was influenced by high atmospheric pCO2 caused by Pan-African and Avalonian–Cadomian volcanism, volcanic activity related to the opening of the Iapetus, and Late Cambrian–Early Ordovician rift volcanism as well. [Copyright &y& Elsevier]

Abstract: The Eastern Hoggar is by far the least well known part of the Tuareg Shield. It is composed, from west to east, of the Aouzegueur, Edembo and Djanet Terranes. The Djanet Terrane is the easternmost Hoggar terrane and comprises a greenschist-facies clastic sedimentary sequence, the Djanet Group, intruded by granitoids. Laser ablation (LA-) ICP-MS U–Pb zircon ages of detrital zircons from these sediments yield a large range of ages similar to ages known in Central and Western Hoggar; the youngest is 590±10Ma (2σ), which is the maximum age of deposition of the Djanet Group. The Djanet Group is intruded by the Djanet Batholith (571±16Ma), by high-level subcircular plutons such as the Tin Bedjane Pluton (568±5Ma) and finally by the felsic Tin Amali Dyke Swarm (558±5Ma), all ages being SHRIMP U–Pb zircon. The deposition and metamorphism of the Djanet Group thus occurred between 590Ma (the age of the youngest detrital zircons analyzed in the Djanet Group) and c. 570Ma (the age of intrusive granitoids). Nd T DM two-stage model ages (1.30–2.03Ga), initial 87Sr/86Sr ratios (0.7035–0.7095) and ɛ Nd (−3 to −11) of the three intrusive suites, all being high-K calc-alkaline in composition, indicate mainly an old, Rb-depleted continental source. Migmatization in the adjacent Edembo Terrane is dated at 568±4Ma (U–Pb zircon SHRIMP age), contemporaneous with the intrusion of the Djanet plutons. Rocks and events in the Edembo and Djanet Terranes are all intracontinental. We propose that the Djanet Terrane lies on the boundary of a craton located to the north-east, that we defined here as the Murzuq craton. The existence of the Murzuq craton is supported by sedimentary and geophysical data. We suggest that the deformation of Eastern Hoggar occurred 575–555Ma, due to the indentation of the Murzuq craton and has no link with the older convergence with the West African craton as was the case for the central and western parts of the Tuareg Shield. This determines a late Ediacaran intracontinental Murzukian event that probably also occurred to the east in the Tibesti area. [Copyright &y& Elsevier]

Two major granulitic units are recognized in the Gour Oumelalen area. One of the units is composed partially of Archean gneisses (Red Gneiss complex) with U–Pb zircon SIMS and TIMS ages of approximately 2.7 Ga. Although they were formed from 3.0- to 3.2-Ga-old precursors, as indicated by Nd model ages, we find no evidence of any older history (≈3.5 Ga) as suggested by previous Pb–Pb ages. The other formation (Gour Oumelalen supergroup) is a metasedimentary sequence at least partly of Paleoproterozoic age, as indicated by zircon dates of a metavolcanic rock at approximately 2.2 Ga. A later magmatic event is recorded at approximately 1.9 Ga in both units and related to coeval granulite-facies metamorphism that affected both units. Nd model ages at approximately 2.0 Ga suggest an accretion of juvenile crust formation at that time. The existence of TDM Nd model ages intermediate between 2.5 and 2.9 Ga could result from the mixing of 3.2 and 2.0-Ga-old material or may reflect separate events. [Copyright &y& Elsevier]

• The Egéré/Aleksod Terrane (Tuareg Shield, Algeria) contains high-pressure rocks. • A zircon megacryst occurs in a high-pressure leucosome within migmatitic metapelites. • Leucosome formed by melting of surrounding metapelites (εHf i = −21; high δ18O). • Melting occurred during decompressional heating (c. 800 °C) at 654 ± 5 Ma. • Fluid-driven recrystallisation at 584 Ma (700 °C) pervasively modified δ18O (12 to 7‰). The Tighsi area of the Egéré/Aleksod Terrane (Tuareg Shield) contains mafic eclogites interlayered within anatectic metapelites corresponding to metabasalts coeval with a shallow water marine sedimentation. In this study we present in-situ geochronological (U-Pb) and geochemical (major and trace elements, Hf and O isotopes) analyses from a 2.6 mm zircon megacryst found in a high-pressure kyanite pegmatite enclosed in the metapelites. Oscillatory zoning, HREE-enriched patterns, positive Ce anomalies and the absence of negative Eu anomalies are consistent with crystallization of the megacryst from the anatectic melt. Ti-in-zircon temperatures indicate crystallization at 811 ± 15 °C in an allanite/monazite buffered anatectic melt as evidenced by the low Th/U ratios. Oxygen isotopes yield large intra-grain variations (7.1–12.3‰) with a gradual lowering towards the edge of the grain in contact with secondary feldspathic veinlets. These results indicate fluid-assisted oxygen isotope disturbances, consistent with the low retentivity of O in zircon under wet conditions. Hf isotopes do not display intra-grain variations (mean εHf i = −20.7 ± 1.0) and support production of the leucosome by melting of crustal material. U-Pb analyses of the center of the crystal provide an age of 654 ± 5 Ma (2σ), attributed to post-peak decompression and heating. HP eclogite facies conditions in the Egéré terrane are thus significantly older than HP metamorphism in the western part of the shield (610–625 Ma) in agreement with multiple subduction events along the margins of the Tuareg Shield. The rim of the megacryst, close to feldspathic veinlets, is characterized by a significant decrease of trace elements (U, Y, HFSE), but preserved identical Hf isotope ratios, which is consistent with recrystallization processes. The rim displays a Ti-in-zircon temperature of 717 ± 28 °C and a U-Pb age of 584 ± 6 Ma (2σ) coeval with the climax of batholith intrusion in Central Hoggar. Reheating and softening of the lower/middle crust at that time may have assisted and favored upward viscous flow of basement domes and escape tectonics along lithospheric shear zones. We propose that the final push of the Saharan metacraton in the east was responsible for the observed architecture of the Egéré terrane, where anatectic elongated domes of basement gneisses alternate with HP metasedimentary synforms. [ABSTRACT FROM AUTHOR]