TECTONOTHERMAL EVOLUTION OF PELITIC GNEISSES IN THE SOUTH-CENTRAL SANDMATA COMPLEX AND ITS IMPLICATIONS ON THE PROTEROZOIC TECTONICS OF ARAVALLI CRATON

The Sandmata Complex lies in the northern parts of the Archean Mewar gneisses and consists of high-grade granulites and pelitic gneisses, which are important for understanding the Proterozoic tectonic evolution of the northern Aravalli Craton. Detailed structural study, mineral paragenesis, metamorphic history, bulk geochemistry and U-Th-total Pb monazite dating were conducted on the pelitic gneisses from the south-central Sandmata Complex. Synthesis of mesoscale structures reveal that the gneisses have undergone three stages of deformation, D₁, D₂ and D₃. The earliest tectonic foliation in gneisses is indicated by isoclinal folds on S₁ foliation, interfolial to gently to moderately dipping (20°–50°) S₂ foliation. The moderately NNE-plunging asymmetric folds on S₂ foliation develop steeply-dipping (>70°) S₃ axial planar foliation. Microtextural observations suggest that the gneisses underwent three metamorphic stages: 1. Prograde stage (M₁) characterized by inclusion minerals in garnet cores (Bt_i + Ilm_i + Pl_i + Qz), 2. Peak stage (M₂) indicated by growth of garnet mantle and matrix minerals (Grt_mantle + Bt_m + Pl_m + Ilm_m + Qz ± Sill), 3. Post-peak stage (M₃) is represented by compositional adjustments in garnet rim and associated matrix minerals. The combination of conventional thermobarometry calculations and pseudosection modelling decipher a clockwise P–T path, involving the metamorphic conditions from 4.1–4.7 kbar/567–605 °C (M₁), through 5.6–6.5 kbar/605–647 °C (M₂) to 4.7–5.8 kbar/536–598 °C (M₃). Whole rock geochemistry suggests that the rocks are peraluminous, calcic to calc-alkalic and show positive Eu anomaly, indicating crystallization from a plagioclase-rich source. The texturally-constrained monazite grains from gneisses yield three age populations at 1.78–1.73 Ga, 1.73–1.62 Ga and 0.89–0.76 Ga. The correlation of yttrium zoning in monazite and textural setting of garnet suggests that 1.78–1.73 Ga age represents the timing of peak metamorphism, whereas 1.73–1.62 Ga age population indicate the timing of post-peak metamorphism. The 0.89–0.76 Ga age is interpreted to record the recrystallization of monazite during the South Delhi orogeny. Based on the results obtained, the Sandmata Complex records prolonged amalgamation history by preserving Late Paleoproterozoic (Aravalli) and Mid-Neoproterozic (South Delhi) orogenic episodes.