T282 Gemological Research in the Twenty-First Century—Characterization, Exploration, and Geological Significance of Diamonds and Other Gem Minerals

Wednesday, 7 November 2018
1:30 PM - 5:30 PM
Room 145

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

Paper number: T282-15
Program: Technical Sessions
Time: 4:50 PM – 5:05 PM

Authors

  • Barbara Dutrow Presenting Author
    Louisiana State UnivDept Geology & Geophysics, Baton Rouge, LA
  • Darrell Henry
    Louisiana State University, Baton Rouge, LA
  • Ziyin Sun
    Gemological Institute of America (GIA), Carlsbad, CA

Abstract

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 (XMg = 0.92-0.97); the core zone has an average formula of (Na0.60 Ca0.32□0.07) (Mg2.70 Fe0.23Al0.07) (Al6.00) (Si5.93Al0.07O18) (BO3)3 (OH3.00) (OH0.63O0.25F0.12) whereas the outer zone is (Na0.64 Ca0.24□0.12) (Mg2.73Al0.15Fe0.12) (Al6.00) (Si5.97Al0.03O18) (BO3)3 (OH3.00) (OH0.69O0.24F0.07). Magnesio-foitite occurs at the outer grain margin. Phlogopites are magnesian (XMg = 0.94-0.96) with a compositional range (K1.66-1.80 Na0.06-0.11 □0.10-0.27) (Mg4.95-5.24 Al0.39-0.73 Fe0.23-0.37) (Si5.47-5.91 Al2.09-2.53 O20) (OH3.70-3.92 F0.07-0.27). 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.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-317498

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