Origin of corundum–tourmaline–phlogopite rocks from Badakhshan, northeastern Afghanistan: a new type of metasomatism associated with sapphire formation

Infiltration of out-of-equilibrium fluids can lead to extensive fluid–rock interactions resulting in unusual and uncommon mineral assemblages that are petrologically and mineralogically important. The corundum (cor)–tourmaline (tur)–phlogopite rocks of the Badakhshan region of Afghanistan provide insights into processes that may result in the formation of blue sapphire. The rocks are mostly comprised of two texturally distinct generations of randomly oriented phlogopitic-eastonitic micas (X_Mg = 0.93–0.96), a coarse generation with variable Al and a finer generation partially replacing corundum and having similar compositions to the earlier generation. Corundum crystals develop in the phlogopitic matrix and exhibit blue and colorless sectors. Large tourmaline porphyroblasts overgrow the matrix micas. Major-element analyses demonstrate that the zoned tourmaline is a Ca-rich dravite to uvite (X_Mg = 0.92–0.97). The oscillatory zoned core region has an average formula of (Na_0.64Ca_0.30□_0.06) (Mg_2.66Fe_0.22Al_0.11) (Al_6.00) (Si_5.97Al_0.03O₁₈) (BO₃)₃ (OH)_3.00 [(OH)_0.61O_0.32F_0.07]. The outer region has an average composition of (Na_0.54Ca_0.39□_0.07) (Mg_2.88Fe_0.11) (Al_5.98Mg_0.02) (Si_5.97Al_0.03O₁₈) (BO₃)₃ (OH)_3.00 [(OH)_0.55O_0.28F_0.17] and is progressively zoned with an increase in Mg and decrease of Fe toward the rim. Compositional variations of tourmaline can be described primarily by the (CaMg)(NaAl)_-1, (Mg(OH))(AlO)_-1 and MgFe_-1 exchange vectors. LA-ICP-MS analyses of trace elements in tourmaline and corundum show unusually low Ti contents (ppmw) of 212–376 (tur) and 2–248 (cor) and undetectable amounts of Cr with <1 in both minerals. Mineral chemistry of tourmaline and sapphire suggests that the rocks likely formed from a metacarbonate protolith subsequently modified by at least two stages of metasomatism. An initial stage of acidic-to-neutral fluids rich in B, Mg, and K infiltrated the original host to produce the phlogopitic mica host, the tourmaline core region, and likely the corundum. Oscillatory chemical zoning in the tourmaline core suggests growth in an environment with dynamically changing boron-bearing fluids. Continued fluxing of the rock with B-, Na-, and Mg-rich fluids resulted in continued tourmaline development that overgrew and included matrix micas. The final stage resulted in finer-grained phlogopites partially pseudomorphing the corundum. Tourmaline compositions and the low amounts of fluid-immobile elements reflect the original protolith. The unusually K-, Mg-, B-, Al-rich and Si-, Ti- and Cr-poor bulk composition is most compatible with a metasomatism of a clay-bearing metacarbonate rock. These K-, Na-, Mg-, B-rich fluids represent a previously unrecognized type of metasomatism associated with the formation of blue sapphire. Such unusual rocks provide new information on fluid compositions and infiltration events occurring in the region and contribute to our understanding of sapphire genesis.