The Egéré Paleo-Mesoproterozoic rifted passive margin of the LATEA metacraton (Central Hoggar, Tuareg Shield, Algeria) subducted and exhumed during the Pan-African orogeny: U-Pb zircon ages, P-T-t paths, geochemistry and Sr-Nd isotopes

Abstract

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.