Sapphirine bearing Mg – Al granulites from the northwestern Eastern Ghats Belt: Implications of the metamorphic evolution of craton – mobile belt boundary

Naik, Bibhutibhusan (2021) Sapphirine bearing Mg – Al granulites from the northwestern Eastern Ghats Belt: Implications of the metamorphic evolution of craton – mobile belt boundary. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS Dissertation of Bibhutibhusan Naik (16MS049)) 16MS049_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (3MB)

Abstract

The northwestern terrane boundary between Bastar craton and Eastern Ghats Belt (EGB), in the Deobhog region of Chhattisgarh, is marked by a crustal scale shear zone that contains ensemble of rocks from both craton and the mobile belt. This zone forms a key area to decipher the dynamics of juxtaposition of the mobile belt with the craton during Precambrian. The dominant rock types include, quartzo-feldspathic gneisses, charnockites and rare high-grade Mg-Al granulites of the Eastern Ghats belts, which have been thrusted westwardly over the amphibolite facies biotite-hornblende granites of the Bastar Craton. The sapphirine-bearing Mg-Al granulite, occurring as pods in the quartzo-feldspathic gneisses, are critical in understanding deep crustal process owing to their capability to sustain ultra-high temperatures and slow reaction kinetics. These Mg-Al granulites are of two types, (I) Garnet-bearing, having garnet + orthopyroxene + sapphirine + plagioclase + sillimanite + biotite mineral assemblage, and (II) Garnet-absent having orthopyroxene + sillimanite + sapphirine + cordierite + biotite mineral assemblage. The reaction textures and compositional data of the minerals from garnet-bearing assemblage suggest garnet breakdown reaction, garnet → (orthopyroxene + sapphirine)symplectite + plagioclase / sillimanite In garnet-absent assemblage the orthopyroxene and sillimanite breakdown to sapphirine and cordierite symplectite by the reaction as follows, orthopyroxene + sillimanite → (sapphirine + cordierite)symplectite The metamorphic conditions for symplectite formation calculated to be ~890 ± 50 °C and ~6.9 ± 0.7 kbar from garnet-bearing assemblage using various conventional thermobarometry. The pseudosection analysis and isopleth thermobarometry of garnet-absent assemblage constrain the minimum temperature ~960 °C and a maximum pressure of ~7.4 kbar as peak metamorphic conditions. The textural interpretation, thermobarometry, pseudosection analysis and isopleth thermobarometry indicate a decompression event. However, the age of this decompression event can not be ascertained due to the unavailability of geochronologic data of this Mg-Al granulites.

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