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 (XMg = 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 (XMg = 0.92–0.97). The oscillatory zoned core region has an average formula of (Na0.64Ca0.30□0.06) (Mg2.66Fe0.22Al0.11) (Al6.00) (Si5.97Al0.03O18) (BO3)3 (OH)3.00 [(OH)0.61O0.32F0.07]. The outer region has an average composition of (Na0.54Ca0.39□0.07) (Mg2.88Fe0.11) (Al5.98Mg0.02) (Si5.97Al0.03O18) (BO3)3 (OH)3.00 [(OH)0.55O0.28F0.17] 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.
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Research Article|
July 01, 2019
Origin of corundum–tourmaline–phlogopite rocks from Badakhshan, northeastern Afghanistan: a new type of metasomatism associated with sapphire formation
Barbara L. Dutrow;
1
Department of Geology & Geophysics, Louisiana State University
, Baton Rouge, LA70803, USA
Corresponding author, e-mail: dutrow@lsu.edu
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Darrell J. Henry;
Darrell J. Henry
1
Department of Geology & Geophysics, Louisiana State University
, Baton Rouge, LA70803, USA
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Ziyin Sun
Ziyin Sun
2
Gemological Institute of America
, 5345 Armada Dr., Carlsbad, CA92008, USA
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1
Department of Geology & Geophysics, Louisiana State University
, Baton Rouge, LA70803, USA
Darrell J. Henry
1
Department of Geology & Geophysics, Louisiana State University
, Baton Rouge, LA70803, USA
Ziyin Sun
2
Gemological Institute of America
, 5345 Armada Dr., Carlsbad, CA92008, USA
Corresponding author, e-mail: dutrow@lsu.edu
Publisher: Deutsche Mineralogische Gesellschaft, Sociedad Española de Mineralogia, Societá Italiana di Mineralogia e Petrologia, Société Francaise de Minéralogie
Received:
01 Mar 2019
Revision Received:
19 Jun 2019
Accepted:
24 Jun 2019
First Online:
07 Aug 2019
Online ISSN: 1617-4011
Print ISSN: 0935-1221
© 2019 E. Schweizerbart’sche Verlagsbuchhandlung, 70176 Stuttgart, Germany
European Journal of Mineralogy (2019) 31 (4): 739–753.
Article history
Received:
01 Mar 2019
Revision Received:
19 Jun 2019
Accepted:
24 Jun 2019
First Online:
07 Aug 2019
Citation
Barbara L. Dutrow, Darrell J. Henry, Ziyin Sun; Origin of corundum–tourmaline–phlogopite rocks from Badakhshan, northeastern Afghanistan: a new type of metasomatism associated with sapphire formation. European Journal of Mineralogy 2019;; 31 (4): 739–753. doi: https://doi.org/10.1127/ejm/2019/0031-2881
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Index Terms/Descriptors
- Afghanistan
- alkali metals
- alkaline earth metals
- Asia
- Badakhshan Afghanistan
- boron
- chemical composition
- chromium
- corundum
- dravite
- fluid phase
- genesis
- host rocks
- ICP mass spectra
- Indian Peninsula
- infiltration
- magnesium
- mass spectra
- metals
- metamorphism
- metasomatism
- mica group
- mobility
- oxides
- partitioning
- phlogopite
- potassium
- ring silicates
- sapphire
- sheet silicates
- silicates
- sodium
- spectra
- tourmaline group
- trace elements
- uvite
- water-rock interaction
- whole rock
- zoning
- Kokcha Valley
- Sar-e Sang Afghanistan
Latitude & Longitude
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