Melt inclusions in xenolith granulite minerals from the diatremes of the Eastern Pamir (Tajikistan): evidence of incongruent melting at the crust-mantle boundary

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Abstract

Melt inclusions in migmatites and granulites provide crucial information about the anatexis of the Earth's crust in various geological settings. This study provides the first data on the composition (including trace elements and H2O) and age of inclusions in peritectic garnets, kyanite, quartz and accessory minerals (zircon, monazite, apatite) from xenoliths of various granulites (garnet-kyanite, garnet-clinopyroxene and garnet-orthopyroxene) from the alkali-basalts of the "Eclogite" pipe (Eastern Pamir). The composition of these inclusions corresponds to potassium-rich acid melts from rhyodacites to rhyolites. The concentration of H2O in the melts varies from 1 to 4 wt. %, and the concentration of CO2 is estimated as high as ~ 1 wt. %. The acidic composition and low water content in these inclusions indicate their formation as a result of dehydration (incongruent) melting of mica (muscovite and/or biotite). Garnet and melt are products of these reactions, therefore, low HREE contents in melt inclusions most likely indicate an equilibrium between the melt and garnet. The findings of melt inclusions in zircon and monazite indicate the equilibrium of the melt and accessory minerals. The presence of melt inclusions (with a high CO2 content) and syngenetic CO2 inclusions with a high density indicates that the carbon dioxide fluid played an important role in the melting of crustal material and the petrogenesis of these melts. According to the data obtained by us on microthermometry of melt inclusions, mineralogical thermobarometry and SHRIMP dating of zircon with melt inclusions, the melts of inclusions and host minerals were formed at a temperature of 950–1000°C and a pressure of >1.5 GPa (14.5 million years) shortly before the exhumation of these xenoliths by alkaline basalt melts, whose age is 11 million years.

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About the authors

A. V. Korsakov

V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences

Author for correspondence.
Email: korsakov@igm.nsc.ru
Russian Federation, Novosibirsk

V. P. Chupin

V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences

Email: korsakov@igm.nsc.ru
Russian Federation, Novosibirsk

D. V. Kuzmin

V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences

Email: korsakov@igm.nsc.ru
Russian Federation, Novosibirsk

N. P. Pokhilenko

V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences

Email: korsakov@igm.nsc.ru

Academician of the RAS

Russian Federation, Novosibirsk

References

  1. Stepanov A.S. A review of the geochemical changes occurring during metamorphic devolatilization of metasedimentary rocks // Chemical Geology. 2021. V. 568. P. 120080.
  2. Stepanov A.S. et al. Geochemistry of metasedimentary restitic rocks and implications for melting conditions and metal potential of crustal felsic magmas // Earth-Science Reviews. 2024. V. 254. P. 104799.
  3. Cesare B. et al. What can we learn from melt inclusions in migmatites and granulites? // Lithos. 2015. V. 239. P. 186–216.
  4. Cesare B. et al. “Nanogranite” and glassy inclusions: The anatectic melt in migmatites and granulites // Geology. 2009. V. 37. № 7. P. 627–630.
  5. Chupin V.P., Kuzmin D.V., Touret J.L.R. High-pressure melt and fluid inclusions in minerals of garnet granulites/eclogites (Eastern Pamir). // Memorias. 2001. V. 7. P. 95–98.
  6. Чупин B.П., Кузьмин Д.В., Мадюков И.А. Расплавные включения в минералах скаполитсодержащего гранулита (нижнекоровые ксенолиты из диатрем Памира) // Доклады Академии Наук. 2006. V. 407. № 6. P. 823–827.
  7. Чупин В.П., Томиленко А.А., Чупин С.В. Происхождение гранулитовых комплексов: результаты изучения расплавных и флюидных включений в цирконе и породообразующих минералах // Геология и геофизика. 1993. Т. 34. № 12. С. 116–131.
  8. Мадюков И.А., Чупин В.П., Кузьмин Д.В. Генезис скаполита из гранулитов (нижнекоровые ксенолиты из диатрем Памира): Результаты изучения расплавных включений // Геология и геофизика. 2011. Т. 52. № 11. С. 1677–1694.
  9. Chupin V.P., Tomilenko A.A. Melt and fluid inclusions in high-pressure minerals (kyanite, garnet, quartz): features of study and interpretation. // Bol. Soc. Espan. Miner. 1995. V. 18. № 1. P. 39–40.
  10. Дмитриев Э.А. Кайнозойские калиевые щелочные породы Восточного Памира. Дониш. Душанбе, 1976. 167 p.
  11. Hacker B.R. et al. Near-Ultrahigh Pressure Processing of Continental Crust: Miocene Crustal Xenoliths from the Pamir // Journal of Petrology. 2005. V. 46. № 8. P. 1661–1687.
  12. Лутков В.С. Петрохимическая эволюция и генезис калиевой пироксенит-эклогит-гранулитовой ассоциации: мантийные и коровые ксенолиты в неогеновых фергуситах южного Памира, Таджикистан // Геохимия. 2003. № 3. P. 254–265.
  13. Stepanov A. et al. The key role of mica during igneous concentration of tantalum // Contrib Mineral Petrol. 2014. V. 167. № 6. P. 1009.
  14. Stepanov A.S., Hermann J. Fractionation of Nb and Ta by biotite and phengite: Implications for the “missing Nb paradox” // Geology. 2013. V. 41. № 3. P. 303–306.
  15. Hermann J., Green D.H. Experimental constraints on high pressure melting in subducted crust // Earth and Planetary Science Letters. 2001. V. 188. P. 149–186.
  16. Huang W.L., Wyllie P.J. Melting reactions in the system Na-AlSi₃O₈–KAlSi₃O₈–SiO₂ to 35 kbar, dry and with excess water // Journal of Geology. 1975. V. 83. P. 737–748.
  17. Auzanneau E., Vielzeuf D., Schmidt M.W. Experimental evidence of decompression melting during exhumation of subducted continental crust // Contrib Mineral Petrol. 2006. V. 152. № 2. P. 125–148.
  18. O’Brien P.J., Rötzler J. High-pressure granulites: formation, recovery of peak conditions and implications for tectonics // Journal of Metamorphic Geology. 2003. V. 21. P. 3–20.

Supplementary files

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2. Fig. 1. Molten and fluid inclusions in rock-forming minerals of xenoliths of granulites from the Eclogite tube: Grt – garnet, Ky – kyanite, Qtz – quartz, Cpx – clinopyroxene

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3. 2. Compositions of molten inclusions in garnet crystals from various types of xenolith granulites in the Ab‒Ort‒Qtz diagram: 1, 2 – Grt-Ky granulites: 1 – massive, 2 – gneiss-like; 3 – Grt-Cpx-Pl granulites; 4 – Grt-Cpx-Scap granulites. The position of the eutectic at various pressure values is borrowed from [16]

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4. 3. a is a high–pressure combined inclusion (potassium-rich acidic melt + CO2 bubble + kyanite) in zircon aged 14.4±1.6 million years (SHRIMP data) from massive Grt‒Ky granulite. b is a cathodoluminescent image of the same zircon with SHRIMP analysis ovals

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