The Syenite–Carbonatite Complex of Ihouhaouene (Western Hoggar, Algeria): Interplay Between Alkaline Magma Differentiation and Hybridization of Cumulus Crystal Mushes

The 2 Ga-old Ihouhaouene alkaline complex (Western Hoggar, Algeria) is among the oldest known carbonatite occurrences on Earth. carbonatites are calciocarbonatites hosted by syenites, predominant rock type in complex. Both types characterized medium-grained to pegmatitic textures and contain clinopyroxene, apatite, wollastonite, associated with K-feldspar syenites a groundmass of calcite carbonatites. suite shows continuous range compositions from 57–65 wt.% SiO 0.1–0.4 CO red 52–58 0.1–6.5 white 20–35 11–24 Si-rich (>10% silicate minerals), <20 24–36 Si-poor (<5% minerals). Calculation mineral equilibrium melts reveals that apatite clinopyroxene disequilibrium each other were most likely crystallized different parental magmas before being assembled studied rocks. They subtle whereas bear evidence for highly contrasted compositions. Apatite was equilibrated LREE-enriched (Ce/Lu = 1,690–6,182) carbonate melts, also elevated Nb/Ta ratio (>50), precipitated liquids lower LREE/HREE 49–234) variable ratios (Nb/Ta 2–30). resemble REE contents compared clinopyroxene. However, melt majority subchondritic values (Nb/Ta<10) has chondritic-to-superchondritic 15–50). Although paradoxical at first sight, this Nb-Ta signature may simply reflect segregation evolved extremely low values. Altogether, our results suggest an evolutionary scheme whereby slow cooling silico-carbonated mantle resulted both cumulus minerals immiscible fractions, resulting overall differentiation This process however counterbalanced intermingling partially which formation hybrid cumulates composed phases proportions or K-feldspar.