Permian high-temperature metamorphism in the Western Alps (NW Italy)

During the late Palaeozoic, lithospheric thinning in part of the Alpine realm caused high-temperature low-to-medium pressure metamorphism and partial melting in the lower crust. Permian metamorphism and magmatism has extensively been recorded and dated in the Central, Eastern, and Southern Alps. However, Permian metamorphic ages in the Western Alps so far are constrained by very few and sparsely distributed data. The present study fills this gap. We present U/Pb ages of metamorphic zircon from several Adria-derived continental units now situated in the Western Alps, defining a range between 286 and 266 Ma. Trace element thermometry yields temperatures of 580-890°C from Ti-in-zircon and 630-850°C from Zr-in-rutile for Permian metamorphic rims. These temperature estimates, together with preserved mineral assemblages (garnet-prismatic sillimanite-biotite-plagioclase-quartz-K-feldspar-rutile), define pervasive upper-amphibolite to granulite facies conditions for Permian metamorphism. U/Pb ages from this study are similar to Permian ages reported for the Ivrea Zone in the Southern Alps and Austroalpine units in the Central and Eastern Alps. Regional comparison across the former Adriatic and European margin reveals a complex pattern of ages reported from late Palaeozoic magmatic and metamorphic rocks (and relics thereof): two late Variscan age groups (~330 and ~300 Ma) are followed seamlessly by a broad range of Permian ages (300-250 Ma). The former are associated with late-orogenic collapse; in samples from this study these are weakly represented. Clearly, dominant is the Permian group, which is related to crustal thinning, hinting to a possible initiation of continental rifting along a passive margin

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

International audienceThe 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

Partial melting and P-T evolution of eclogite-facies metapelitic migmatites from the Egere terrane (Central Hoggar, South Algeria)

The Egéré terrane (Central Hoggar, South Algeria) includes mafic eclogite lenses boudinaged in metapelitic rocks with high-pressure relicts. These metapelites show textural records of partial melting, mainly primary melt inclusions enclosed in garnet crystals and later crystallized as "nanogranitoids."Garnet porphyroblasts also contain inclusions of quartz, kyanite, phengite, biotite, staurolite, and rutile and show a smoothed prograde zoning with a Mn bell-shaped profile. The peak high-pressure metamorphic assemblage consists of garnet, kyanite, phengite (Si up to 6.36), quartz, rutile, ±ilmenite, ±feldspars, and melt. Phengite has partially transformed into fine-grained aggregates of biotite, plagioclase, and K-feldspar, a microstructure interpreted as resulting from a dehydration melting during exhumation. Phengite breakdown, along with other retrograde reactions, produced a late paragenesis with biotite, plagioclase, K-feldspar, quartz, almandine-rich garnet, ±sillimanite, ±staurolite, ±muscovite, and ilmenite. The thermodynamic modeling of P-T pseudosections allows us to constrain various steps of the metamorphic history: beginning of the garnet growth at 4.0 kbar and ~600 °C during prograde metamorphism; pressure peak at 14-20 kbar; temperature peak at 800-820 °C; formation of the last assemblage at 6.0-5.5 kbar and 725-685 °C. Partial melting likely started during the prograde path when crossing the H2O-saturated solidus, at T ≥ 650-670 °C and P ≥ 10 kbar, continued upon heating, up to the peak conditions, as well as during decompression. This evolution is interpreted in terms of subduction of the continental crust to mantle depths, followed by an exhumation through a clockwise P-T path during the Pan-African orogeny. The Egéré metapelites are relatively well-preserved eclogite-facies rocks, contain inclusions of "nanogranitoids"hitherto very little known in eclogite-facies metamorphic rocks, and represent an unusual trace of subduction within a Neoproterozoic orogen.This work is a contribution to the project PHC TASSILLI 15MDU943 and the FP7-IRSES-612572 project “MEDYNA” funded under REA Grant Agreement PIRSES-GA-2013-612572. Grateful thanks are due to the Direction Générale de la Recherche Scientifique et du Développement Technologique of Algeria for funding this work that forms part of the PRFU (E04N01UN160420180001) and PNR ATRST research projects in Algeria. We are extremely grateful to Office du Parc National de l’Ahaggar (Tamanrasset, Algeria) for logistic support during fieldwork, to Chunjing Wei and Abderrahmane Bendaoud for constructive reviews and to Fang-Zhen Teng for the editorial work