Discovery of large‐scale deep‐seated thrusts in Central Hoggar, with a plurifacial evolution ranging from lower amphibolite facies to upper greenschist facies conditions and linked to a regional refoliation, has led us to reconsider the Pan‐African tectonic and metamorphic history in that region. Two areas are described, and a review of other thrusts leads to an interpretative cross section of a large portion of reactivated continental crust. The age and kinematics of this structural reworking have been approached using U/Pb zircon dating in the Tamanrasset region. Despite the difficulty of estimating the age of the initiation of the assumed intracontinental A‐type subduction, the results provide a time span of 30–40 m.y. between the climax of granitoid emplacement and a late retrogressive offset along the thrust planes. Some key ages were determined: (1) 2075 ± 30 Ma is the age given by granulite facies remnants which escaped from the refoliation, the corresponding lower intercept at 530 ± 70 Ma confirms the Pan‐African imprint; (2) 615 ± 5 Ma reflecting the age of syntectonic to late‐tectonic granitoids emplaced in reworked gneisses and in preserved granulites; (3) 580 Ma, the concordant age of sphenes and monazites from the same granitoids, which is interpreted as corresponding to the end of medium‐grade conditions. No evidence has been found of a ∼1000 Ma age: a Kibaran event does not appear to exist in Central Hoggar. The age similarity between the observed deep intracontinental evolution of Central Hoggar and the collision‐related tectonics of Western Hoggar and Iforas suggests a common origin for both phenomena.
ZusammenfassungEine kristallmorphologische Bearbeitung yon Zirkonen aus Glimmerschiefern des R6niers-Tals (Wesdich yon Poli, Nord-Kamerun) wurde mit Durchlicht-und mit RasterElektronen-Mikroskopie (Rfickstrahl Elektronen) ausgeffihrt, Verbunden mit einer Bearbeitung des strukturgeoIogischen und metamorphen Rahmens ffihrt sie zu der Vorstellung einer massiven Auskristallisation der Zirkone anlSfglich eines tektono-metamorphen Ereignisses mittlerer St/irke in der Granat-Disthen Fazies. Dieses EreighiSS ist yon einer Deformationsphase (D1) und der Platznahme eines basischen bis intermedi~iren Gesteinsverbandes (BIP) begleitet women. Eine U/Pb Datierung dieser Zirkone ergibt praktisch konkordante pan-afrikanische Alter (630 M.J.) Diese Resultate kliiren eine Ungewissheit in Bezug auf das Alter des st/irksten metamorphen Schieferung, die hier als Resultat des Ereignisses D1 gedeutet wird. Je nach der geographischen Stellung innerhalb der zentral-afrikanischen mobilen Zone wurde bis jetzt das Alter dieser Schieferung als pan-afrikanisch oder als unter-bis mittelproterozoisch gegeben. Ein chronologischer Ablauf kann somit ffir die Entwic!dung des nordkamerunischen Gebietes vorgeschlagen werden.
AbstractMorphoscopic studies of zircons, including optical microscopic and SEM (backscattered electron mode), from the R6niers valley (West-Poll, Northern Cameroon) indicate an important crystallization of zircon during a medium-grade garnet-kyanite metamorphic event. This was associated with a deformation (D1) and with the emplacement of a basic and intermediate plutonic suite (BIP). U-Pb dating of these zircon gave almost concordant Pan-African ages (630 Ma) which contribute to a solution of a major regional uncertainty about the age of the regional D1 event, which, in the mobile belt of
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