The stability and origin of sodicgedrite in ultrahigh-temperature Mg-Al granulites: a case study from the Gondwana suture in southern India

  • Tomohito Kanazawa
  • Toshiaki Tsunogae
  • Kei Sato
  • M. Santosh
Original Paper


Mg-Al-rich rocks from the Palghat-Cauvery Shear Zone System (PCSZ) within the Gondwana suture zone in southern India contain sodicgedrite as one of the prograde to peak phases, stable during = 900–990°C ultrahigh-temperature metamorphism. Gedrite in these samples is Mg-rich (Mg/[Fe + Mg] = X Mg = 0.69–0.80) and shows wide variation in Na2O content (1.4–2.3 wt.%, NaA = 0.33–0.61 pfu). Gedrite adjacent to kyanite pseudomorph is in part mantled by garnet and cordierite. The gedrite proximal to garnet shows an increase in NaA and AlIV from the core (NaA = 0.40–0.51 pfu, AlIV = 1.6–1.9 pfu) to the rim (NaA = 0.49–0.61 pfu, AlIV = 2.0–2.2 pfu), suggesting the progress of the following dehydration reaction: Ged + Ky → Na-Ged + Grt + Crd + H2O. This reaction suggests that, as the reactants broke down during the prograde stage, the remaining gedrite became enriched in Na to form sodicgedrite, which is regarded as a unique feature of high-grade rocks with Mg-Al-rich and K–Si-poor bulk chemistry. We carried out high-P-T experimental studies on natural sodicgedrite and the results indicate that gedrite and melt are stable phases at 12 kbar and 1,000°C. However, the product gedrite is Na-poor with only <0.13 wt.% Na2O (NaA = 0.015–0.034 pfu). In contrast, the matrix glass contains up to 8.5 wt.% Na2O, suggesting that, with the progressive melting of the starting material, Na was partitioned into the melt rather than gedrite. The results therefore imply that the occurrence of sodicgedrite in the UHT rocks of the PCSZ is probably due to the low H2O activity during peak P-T conditions that restricted extensive partial melting in these rocks, leaving Na partitioned into the solid phase (gedrite). The occurrence of abundant primary CO2-rich fluid inclusions in this rock, which possibly infiltrated along the collisional suture during the final amalgamation of the Gondwana supercontinent, strengthens the inference of low water activity.


Sodicgedrite Sapphirine Magnesian staurolite High-pressure and ultrahigh-temperature metamorphism Palghat-Cauvery Shear Zone System Southern India Gondwana Supercontinent 



We thank the staff at Gondwana Research Office in Trivandrum and Ms. Preetha Warrier for their helpful support, and Dr. N. Nishida for his assistance on microprobe analyses. Kanazawa thanks his colleagues Messrs. I. Hossain, H. Ohyama, H. Tadokoro, Ms. N. Kondou, and Dr. Y. Miyagi for their helpful suggestions. Prof. Christian Nicollet and an anonymous reviewer provided helpful comments on the earlier version of this manuscript. We thank these referees as well as Prof. J.L.R. Touret for his valuable editorial suggestions. This is a contribution to the Grant-in-Aid from the Japanese Ministry of Education, Sports, Culture, Science and Technology to Tsunogae (no. 17340158, 20340148) and Santosh (no. 17403013) as well as JSPS-INSA Joint Research Program.


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Copyright information

© Springer-Verlag 2008

Authors and Affiliations

  • Tomohito Kanazawa
    • 1
  • Toshiaki Tsunogae
    • 2
    • 3
  • Kei Sato
    • 4
  • M. Santosh
    • 5
  1. 1.College of Natural SciencesUniversity of TsukubaIbarakiJapan
  2. 2.Graduate School of Life and Environmental Sciences (Earth Evolution Sciences)University of TsukubaIbarakiJapan
  3. 3.Department of GeologyUniversity of JohannesburgAuckland ParkSouth Africa
  4. 4.Department of Earth and Planetary Sciences, Graduate School of Science and EngineeringTokyo Institute of TechnologyTokyoJapan
  5. 5.Department of Natural Environmental Science, Faculty of ScienceKochi UniversityKochiJapan

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