Résumé
The Ihouhaouene alkaline complex (In Ouzzal terrane, Hoggar, Algeria) comprises various calciocarbonatites closely associated with syenites emplaced into the In Ouzzal Archean crust, which later underwent Paleoproterozoic Ultra-High-Temperature (UHT) metamorphism at ∼ 2 Ga. The emplacement ages of carbonatites and syenites and the granulite-facies metamorphism age were determined using the U-Th-Pb LA-ICP-MS system on four geochronometer: zircon, apatite, monazite, and titanite.Zircons from both syenite and carbonatite are magmatic, with high Th/U ratios (mean 0.3) and pronounced HREE-LREE fractionation (YbN/SmN from 3.3 to 87.2). Concordia ages obtained for zircon in syenite and carbonatite, and for monazite in syenite date the emplacement of carbonatite and syenite intrusions between 2061 ± 19 Ma and 2029 ± 7 Ma. This magmatism predates the peak of UHT metamorphism, which is constrained by the upper intercept age of the granulite zircons at 2000 ± 6 Ma. Monazite, apatite, and titanite from carbonatites and syenites yield younger Paleoproterozoic dates (∼1820 to 1680 Ma) and Pan-African dates (∼650 Ma) interpreted as reflecting post-emplacement cooling due to thermal relaxation of the lithosphere, resulting in nascent subsolidus reactions and fluid-rock interactions, probably at high temperatures.The carbonatite-syenite magmatic system, with an estimated emplacement temperature ∼ 716°C (Ti-in-zircon geothermometer), intruded the Ihouhaouene crust 30 to 60 Myr prior to the regional UHT metamorphism (800-1000°C; 2 Ga). The magmatic rocks subsequently recorded subsolidus and fluid re-equilibration during lithospheric and crustal thermal relaxation.Two-D thermal modelling supports a causal link between asthenosphere upwelling (and hence magmatic intrusion) and the regional metamorphism, showing that a lithosphere-asthenosphere boundary at 60 km depth (compared to 120 km for the classical geotherm) could increase Moho temperatures by 430°C within 60 Myr. Comparisons with other Paleoproterozoic carbonatites in northwest Africa and North America underscore the Ihouhaouene complex’s uniqueness, especially with respect to its emplacement in the lower crust and its relationship to UHT metamorphism.