METASOMATIC ORIGIN FOR TOURMALINE-SAPPHIRE-PHLOGOPITE ROCKS OF THE BADAKHSHAN PROVINCE, NE AFGHANISTAN

Tourmaline + corundum-bearing metamorphic rocks from Badakhshan Province, Afghanistan, likely formed from metasomatic alteration. Randomly oriented phlogopites comprise the matrix with porphyroblasts of dark-blue tourmaline and blue-to-colorless corundum with minor spinel, feldspar and apatite. Imaging, electron microprobe (EMP) and laser-ablation inductively-coupled mass-spectroscopy (LA-ICP-MS) analyses characterize the major, minor and trace elements of the minerals.

Tourmalines are zoned: oscillatory in the interior of the grains and patchy on the outer portion reflecting overgrowth of matrix phlogopite. Compositionally tourmalines are Ca-rich dravite (X_Mg = 0.92-0.97); the core zone has an average formula of (Na_0.60 Ca_0.32□_0.07) (Mg_2.70 Fe_0.23Al_0.07) (Al_6.00) (Si_5.93Al_0.07O₁₈) (BO₃)₃ (OH_3.00) (OH_0.63O_0.25F_0.12) whereas the outer zone is (Na_0.64 Ca_0.24□_0.12) (Mg_2.73Al_0.15Fe_0.12) (Al_6.00) (Si_5.97Al_0.03O₁₈) (BO₃)₃ (OH_3.00) (OH_0.69O_0.24F_0.07). Magnesio-foitite occurs at the outer grain margin. Phlogopites are magnesian (X_Mg = 0.94-0.96) with a compositional range (K_1.66-1.80 Na_0.06-0.11 □_0.10-0.27) (Mg_4.95-5.24 Al_0.39-0.73 Fe_0.23-0.37) (Si_5.47-5.91 Al_2.09-2.53 O₂₀) (OH_3.70-3.92 F_0.07-0.27). Minerals have unusually low Ti contents (ppmw); 262 (tur), ~150 (phl), <250 (cor): V contents (ppmw); 27 (tur), 2 – 10 (cor); and Cr contents (ppmw); <150 (tur), <1 (cor). Tourmaline contains Sr (240 ppmw), Ga (21 ppmw), Ce (10 ppmw) and La (5 ppmw). The most abundant trace element in corundum is Fe (400-1300 ppmw) with lesser amounts (ppmw) of Ti (11-250), Mg (10-140) and Ga (44-72).

Compositionally tourmalines fall within the low Ca-ultramafic and metacarbonate fields (Henry & Guidotti 1985 Am Min). However, lack of Cr and V are inconsistent with an ultramafic origin. Textural evidence such as oscillatory zoning in tourmaline suggests the interaction with dynamic boron-bearing fluids during initial tourmaline growth. Continued infiltration of the rock with B-rich fluids resulted in additional tourmaline growth at the expense of matrix micas. The unusually K-, Mg-, Al-rich and Si-, Ti-poor bulk composition is consistent with metasomatism of a clay-bearing metacarbonate rock. A metamorphic overprint of these unusual bulk compositions is commensurate with development of blue sapphire.