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 epmaage of monazite

The southern part of Peninsular India, popularly known as the Southern Granulite Terrane (SGT) witnessed a pervasive granulite facies metamorphism, ductile shearing and widespread migmatization during Pan-African orogeny, which marks the final assembly of the Gondwanaland during Cambrian (ca 500 Ma). The tectonic evolution of SGT, irrespective of its Gondwana connection, is farreaching 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⁺ = 2REE³⁺) 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.