Melt-producing and melt-consuming reactions in the Achankovil cordierite gneisses, South India
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, br...
Published in: | Journal of Metamorphic Geology |
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Main Authors: | , , |
Other Authors: | , , , |
Format: | Article in Journal/Newspaper |
Language: | English |
Published: |
HAL CCSD
2002
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Subjects: | |
Online Access: | https://hal.science/hal-04404304 https://doi.org/10.1046/J.1525-1314.2002.00388.X |
Summary: | 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. |
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