A petrologic and laser Raman spectroscopic study of sapphirine–spinel–quartz–Mg-staurolite inclusions in garnet from Kumiloothu, southern India: Implications for extreme metamorphism in a collisional orogen

Abstract

We report the occurrence of sapphirine, spinel, quartz and Mg-staurolite as inclusions in garnet from quartzo-feldspathic gneiss and garnet–biotite gneisses in the northernmost sector of the Madurai Granulite Block along the Palghat–Cauvery Shear Zone System in southern India, suggesting metamorphism under ultrahigh-temperature conditions. Poikiloblastic garnet grains contain numerous inclusions of quartz, rutile, apatite, biotite and zircon with rare associations of sapphirine + spinel and sapphirine + staurolite. We characterize the inclusion population based on detailed petrologic, mineral chemical and laser Raman spectroscopic studies. A direct contact of sapphirine and/or spinel with the quartz inclusions within garnet is not seen, which we attribute to the low modal abundances of sapphirine and spinel, and/or the progress of reactions during retrograde metamorphism. Staurolite occurs as inclusions in garnet and is partially mantled by sapphirine, suggesting the progress of the reaction: St → Grt + Spr + Sil or Ky + H 2 O. Such relict occurrences of staurolite with Mg/(Fe + Mg) of 0.42–0.44 are regarded as evidence of prograde high-pressure metamorphism in the northern Madurai Block. Lack of sillimanite or kyanite within garnet implies that all aluminosilicates have been completely consumed by reactions: Grt + Sil → Spr + Qtz and/or Grt + Sil → Spl + Qtz to form possible sapphirine + quartz and/or spinel + quartz assemblages. Our results confirm that the northern Madurai Block along the Palghat–Cauvery Shear Zone System underwent high-pressure and ultrahigh-temperature metamorphism along a decompressional clockwise P – T – time path associated with the collisional amalgamation of southern India with the Gondwana supercontinent assembly.