Abstract
The In Ouzzal Archaean craton represents a succession of intrusive and metamorphic events since Eburnean, and an important marker of geodynamic processes through geological time. The Ihouhaouene area located in the N-W of In Ouzzal terrane in Algeria is unique by the presence of Proterozoic carbonatite intrusions associated with silica-saturated alkaline rocks. These intracontinental carbonatites are among the oldest and exceptional because of their diversity and the presence of unusual rare earth minerals. Carbonatites are pegmatitic or brecciated with fragments of syenite. They are calciocarbonatites with calcite (> 50 vol.%), apatite, clinopyroxene and wollastonite and are associated with red or white syenites in massive outcrops. Syenites are composed of alternating light levels of red alkaline feldspar or wollastonite associated with white feldspar and dark levels of apatite and clinopyroxene. Carbonatites and syenites form a cogenetic suite characterized by an increase in silica and decrease in calcium and CO2 content. The carbonatites have silica content ranging from 5 to 35 wt.%, 28 to 53 wt.% CaO, and 11 to 36 wt.% CO2. Syenites have high K2O (12 wt.%) and low Na2O content (1 wt.%). Carbonatites and syenites have high incompatible element concentrations with high REE content (7000*chondrites and 1000*chondrites, respectively) and high U, Pb, Sr and Th content. Trace elements (eg. Rare Earths, Nb-Ta, Zr-Hf) in magmatic minerals (apatite-pyroxene) of carbonatites and syenites reveal complex magmatic processes at the origin of these rocks involving several stages of fractional crystallization and immiscibility from a CO2-rich melilititic magma. Silica-rich carbonatites and white syenites are characterized by high Nb/Ta, Y/Zr and Rb/Sr ratios, typical of carbonate-rich magmas by immiscibility. The red syenites have characteristics of immiscible differentiated silicate melt. Silica-poor carbonatite minerals have variable subchondritic Nb/Ta (<10) indicating crystallization from highly evolved liquids and the presence of late carbonatitic magmas. Apatites, in particular, record various magmatic and supergene processes. They present, in some rocks, redistribution and enrichment in rare earth elements, which are characterized by exsolutions of britholite in silica-rich carbonatites and monazite-quartz-calcite inclusions in silica-poor carbonatites. These minerals reflect local sub-solidus re-equilibration with late-magmatic fluids rich in Cl-Th-REE for the exsolution of britholite and S-Ca-P-CO2 for monazite inclusions. The apatite and zircon present in these alkaline and carbonatite rocks, allow determination of the syn-metamorphic crystallization age of the Ihouahouene magmatic complex at 2100 Ma and confirm the pan-African age of its exhumation. The petrological, geochemical and geochronological study of Ihouhaouene carbonatites and syenites highlights the magmatic origin of these rocks and constrains the fluid-rock interactions at sub-solidus conditions leading to REE-enrichment. The carbonatites and syenites result from a low partial melting rate of a CO2-rich Precambrian mantle. Several fractional crystallization and immiscibility stages allowed the genesis of these hybrid magmas, trapped along large shear-zones during the Archean/Eburnean transition period in the In Ouzzal terrane, characterized by extensive deformation in ultra-high-temperature granulitic environment.