Timing and duration of ultra-high temperature metamorphism in sapphirine-bearing metapelite granulite from Kodaikanal, Madurai block, South India: constraints from mineral chemistry and U-Th - total Pb EPMA age of monazite

The southern part of Peninsular India, popularly known as the Southern Granulite Terrane (SGT) witnessed a pervasive granulite faces metamorphism, ductile shearing and widespread migmatization during Pan-African orogeny, which marks the final assembly of the Gondwanaland during Cambrian (ca 500 Mu). The tectonic evolution of SGT, irrespective of its Gondwana connection, is far-reaching due to occurrences of ultra-high temperature (UHT) granulites in different parts of the SGT. In the present study, dynamics of melting and temporal evolution of sapphirine-bearing metapelite granulite, hosted within the Kodaikanal charnockite massif, during syn - to post - UHT metamorphic conditions are examined. The onset of UHT metamorphism in the rock is marked by the growth of Al-rich orthopyroxene (Al₂O₃ ∼8 wt%) porphyroblast, sapphirine-cordierite symplectite via biotite dehydration melting. Embayment of orthopyroxene porphyroblast and accumulation of melt, now preserved as mesoperthites, in the pressure shadow region around the porphyroblast attest to syn-tectonic melting and crystallization. Monazites of varying sizes (40 to 100 mm) occur pervasively, either as inclusion in peritectic phases or in the biotite-mesoperthite rich matrix. Diagnostic chemical variation diagram, such as 4 (REE + Y + P) versus 4 (Th + U + Si), (Si + Y + REE) versus (Ca + P) suggests that the core and rims domains are linked by dominantly huttonitic (Th⁴⁺ - Si⁴⁺ = REE³⁺ P⁵⁺) and cheralite (Th⁴⁺ + Ca²⁺ = 2 REE⁴⁺) substitutions. An increase in partitioning of Th towards the rim is also accompanied with a decrease in La/Sm ratio towards the rim. This together with core to rim chemical variation in monazite attests to monazite growth (core) and overgrowth (rim) during peak UHT metamorphism and subsequent cooling. U - Th - total Pb monazite ages constrain the timing of dehydration melting as well as peak UHT metamorphism at - 560 Ma and post-peak cooling at ∼510 Ma. This establishes a time span of about 50 Ma for UHT metamorphism in Madurai Block. (author)