gems, lapis lazuli, Afghanistan, geology, Sar-e-Sang, Badakhshan, scapolite

Scapolite and other halogen-rich minerals (phlogopite, amphibole, apatite, titanite, and clinohumite) occur in some high-pressure amphibolite facies calc-silicates and orthopyroxene-bearing rocks at Sare Sang (Sar e Sang or Sar-e-Sang), NE Afghanistan. The calc-silicates are subdivided into two groups: garnet-bearing and garnet-free, phlogopite-bearing. Besides garnet and/or phlogopite, the amphibolite facies mineral assemblages in the calc-silicates include clinopyroxene, calcite, quartz, and one or more of the minerals scapolite, plagioclase, K-feldspar, titanite, apatite, and rarely olivine. Orthopyroxene-bearing rocks consist of clinopyroxene, garnet, plagioclase, scapolite, amphibole, quartz, calcite, and accessory dolomite and alumosilicate (kyanite?). Retrograde phases in the rocks are plagioclase, scapolite, calcite, amphibole, sodalite, haüyne, lazurite, biotite, apatite, and dolomite. The clinopyroxene is mostly diopside and rarely also hedenbergite. Aegirine and omphacite with a maximum jadeite content of 29 mol % were also found. Garnet from the calc-silicates is Grs₄₅–₉₅Py₀–₂ and from the orthopyroxene-bearing rocks is Grs₁₀–₁₅Py₃₆–₄₃. Peak P–T metamorphic conditions, calculated using available exchange thermobarometers and the TWQ program, are 750 °C and 1.3–1.4 GPa. Depending on the rock type, the scapolite exhibits a wide range of composition (from EqAn = 0.07, XCl = 0.99 to EqAn = 0.61, XCl = 0.07). Equilibria calculated for scapolite and coexisting phases at peak metamorphic conditions yield XCO₂ = 0.03–0.15. XNaCl (fluid), obtained for scapolite, ranges between 0.04 and 0.99. Partitioning of F and Cl between coexisting phases was calculated for apatite–biotite and amphibole–biotite. Fluorapatite is present in calc-silicates, but orthopyroxene-bearing rocks contain chlorapatite. Cl preferentially partitions into amphibole with respect to biotite. All these rocks have suffered various degrees of retrogression, which resulted in removal of halogens, CO₂, and S. Halogen- and S-bearing minerals formed during retrogression and metasomatism are fluorapatite, sodalite, amphibole, scapolite, clinohumite, haüyne, pyrite, and lazurite, which either form veins or replace earlier formed phases.