Metamorphosed phosphorites in the Hatrurim Basin, Israel: implications for the Mottled Zone pyro-metamorphic event

Metamorphosed phosphorites are exposed at the base of the Mottled Zone pyro-metamorphic complex in the Hatrurim Basin, Israel. Formed by recrystallization of unusually P-rich protoliths under extraordinary conditions, they provide insight on the effect of metamorphism on globally important natural resource and the behaviour of apatite under extreme high T, low P conditions. By studying the textures and assemblages of meta-phosphorites, measuring the chemical composition of apatite and dating it by U–Pb geochronology, the extent, characteristics and timing of combustion metamorphism around the Dead Sea basin are constrained, and an insight on the trigger of such events is provided. Our field, petrographic and geochemical analysis indicates peak metamorphic temperatures of ≥800°C (locally exceeding 1170°C) in apatite marbles immediately beneath Mottled Zone calc-silicate marbles and ≥525°C in fluorapatite phosphorites at lower stratigraphic levels. Apatite recrystallization at high temperatures involved SiO₂ and CO₂ incorporation into its lattice, and formation of an assemblage dominated by non-silicates. Sulfide formation was constrained by fO₂ and the SO₃ content of the protolith and spurrite formation was controlled by the P₂O₅/SiO₂ ratio. Metamorphism involved partial decarbonation, inhibited by CO₂ incorporation into the apatite lattice. Mobilization of V and U during low T alteration of apatite marble is evident, but P₂O₅ mostly remained immobile. Thermally driven recrystallization of apatite during high T metamorphism resulted in isotopic homogenization of Pb and U–Pb system closure, enabling U–Pb geochronology based on spot analyses of apatite clusters. U–Pb dating of apatite records combustion events in the Hatrurim Basin in the Early Miocene (18 − 14 Ma), Late Miocene (12 − 10 Ma) and Pliocene (4.3 − 3.9 Ma), in accordance with previous age determinations. The first and last events correlate well with major tectonic events along the Dead Sea Transform. Sequential U–Pb ages of apatite in a single outcrop and textural evidence for metamorphic overprint require either partial preservation or replenishment of flammable material between combustion events.