Melt-producing and melt-consuming reactions in the Achankovil cordierite gneisses, South India

• Published in: Journal of Metamorphic Geology
• Main Authors: Cenki, B., Kriegsman, L., Braun, I.
• Other Authors: Géosciences Montpellier, Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université des Antilles (UA)-Université de Montpellier (UM), National Museum of Natural History - Leiden, Universität Bonn = University of Bonn
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2002
• Subjects: anatectic migmatites cordierite gneisses dehydration melting Gondwana KFMASH petrogenetic grid South India; anatectic migmatites; cordierite gneisses; dehydration melting; Gondwana; KFMASH petrogenetic grid; South India; [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; Antarc*; Antarctica
• Online Access: https://hal.science/hal-04404304; https://doi.org/10.1046/J.1525-1314.2002.00388.X

International audience Migmatitic cordierite gneisses within the Achankovil Zone (AZ) of southern Pan-African India record melt-producing and subsequent melt-consuming mineral reactions. Early mineral assemblages Bt-Sil-Qtz and Bt-Sil-Spl, deduced from inclusion textures in garnet prophyroblasts, break down via successive dehydration melting reactions to high-T phase assemblages (e.g. Grt-Crd-Liq, Opx-Liq, Spl-Crd-Liq). Later back reactions between the restite and the in situ crystallizing melt resulted in thin cordierite coronas separating garnet from the leucosome, and partial resorption of garnet to Opx-Crd or Crd-Bt-Qtz symplectites. Leucosomes generally display a moderate (low-strain gneisses) to strong (highstrain gneisses) depletion of alkali feldspar attributed to mineral-melt back reactions partly controlled by the degree of melt segregation. Using a KFMASH partial petrogenetic grid that includes a melt phase, and qualitative pseudosections for microdomains of high and low Al ⁄ Si ratios, the successive phase assemblages and reaction textures are interpreted in terms of a clockwise P-T path culminating at about 6-7 kbar and 900-950 °C. This P-T path is consistent with, but more detailed than published results, which suggests that taking a melt phase into account is not only a valid, but also a useful approach. Comparing P-T data and lithological and isotopic data for the AZ with adjacent East Gondwana fragments, suggests the presence of a coherent metasedimentary unit exposed from southern Madagascar via South India (AZ) and Sri Lanka (Wanni Complex) to the Lu¨tzow-Holm Bay in Eastern Antarctica.