Metamorphic Conditions and Fluid Compositions of Scapolite-Bearing Rocks from the Lapis Lazuli Deposit at Sare Sang, Afghanistan

Abstract

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.