Ba-rich phlogopite, dissakisite-(Ce), and fluorine-rich clinohumite in granulite-facies marbles from the In Ouzzal terrane, Western Hoggar, Algeria

Abstract

Granulite-facies marbles are part of the Archaean crust of the In Ouzzal terrane (Western Hoggar). Besides calcite and dolomite, these marbles are characterized by forsterite, spinel, phlogopite, diopside, amphibole, and magnetite. High-Mn ilmenite (MnO: 19.43 wt%), pyrophanite (MnO: 25.83 wt%), dissakisite-(Ce) (containing 12.2–14.38 Ce₂O₃ wt% and 8.3–12.3 wt% La₂O₃), F-rich clinohumite and Ba-rich-phlogopite occur locally. Ilmenite-pyrophanite and Ba-rich phlogopite (BaO: 11.68 wt%) are interpreted to be resulted of the hydrothermal processes during carbonate deposition; whereas, dissakisite-(Ce) is inferred to have resulted from light rare earth element (LREE) enrichment of these marbles by fluids released from the syenite intrusions. Clinohumite with X_Mg [= Mg/(Mg + Fe)] of 0.89–0.91, generated by hydration of forsterite-bearing dolomitic marbles, has the highest fluorine contents ever reported in the literature (3.25–4.62 wt%), resulting in a F/(F + OH) ratio as high as 0.8. Clinohumite is the only phase incorporating F where phlogopite, amphibole and apatite are absent. Thus, all the F present in the rock would be concentrated in clinohumite, resulting in extreme fluorine enrichment in some grains.