Two types of scapolite in Evate carbonatite deposit (Mozambique) implications for magmatic versus metamorphic origins

摘要

Two types of scapolite occur in the Evate carbonatite deposit – the largest resource of apatite in south-east Africa. Calc-silicate rocks composed of amphiboles (hastingsite, hornblende, actinolite, and pargasite), diopside, Ba-rich phlogopite, allanite, epidote, apatite, K-feldspar, titanite, and minor calcite, also contain scapolite with 62–74 mol. % of meionite – (Me) end-member, XCl = 0.06–0.17 and the crystallochemical formula corresponding to (Ca2.46–2.99Na1.00–1.47)Σ4.00–4.20Al4.62–5.04Si6.95–7.37O24(Cl0.06–0.17S0.18–0.26C0.64 –0.70)Σ1.00. In contrast, carbonatite contains scapolite ranging from Na meionite to Ca marialite with 46–63 mol. % Me, XCl = 0.19–0.47 and the crystallochemical formula (Na1.43–2.07Ca1.82–2.50)Σ3.97–4.11Al4.29–4.73Si7.27–7.70O24(Cl0.19–0.47S0.00– 0.25C0.45–0.74)Σ1.00. The chemical composition of scapolites mirrors the complex history and multistage evolution of the Evate deposit. The low-Cl, high-S scapolites from calc-silicate rocks are chemically indistinguishable from metamorphic scapolites, whereas the Cl-rich scapolite from carbonatite is interpreted as magmatic in origin. The gradual Cl enrichment reflects an increasing NaCl activity in the scapolite-forming fluid or melt. Both scapolite types are closely associated with amphibole and phlogopite, thus indicating amphibolite-facies conditions during retrograde stages of Late-Proterozoic (Ediacaran) granulite-facies metamorphism and/or Ordovician reactivation.