Geochemical evidences of subduction along the Achankovil terrane boundary shear zone, Southern Granulite Terrane, South India

Madurai Block (MB) and Trivandrum Block (TB), composed of similar high-grade gneisses separated by the Achankovil shear zone (AKSZ), represent two different crustal domains with distinct crustal evolutionary history. Geochemistry of garnet-biotite gneisses and charnockites from both the blocks shows that the studied rocks are magnesian and calcic to calc-alkaline in nature, indicating magmatic-arc settings. Both the litholunits from the TB and MB displayed an enrichment of light rare earth elements, depletion of high field strength elements and flat heavy rare earth elements pattern. In addition to these, the garnet-biotite gneisses of the TB shows a strong negative Eu anomaly, suggesting the crustal contamination of the source and post-Archean granitoid origin. In the Nb vs. Y diagram, most of the charnockites and garnet-biotite gneisses from the MB and TB fall within the syn-collisional granite and volcanic-arc field. Rb vs. (Y+Nb) and Ta vs. Yb diagrams show a volcanic-arc granite trend for all the lithounits. In the modified La/Yb vs. Th/Yb plot, most of the MB rocks fall within the island-arc field and the TB rocks fall within the continental margin-arc field. Such chemical signatures, characteristic of arc magmatism in active continental margins, suggest subduction-related geodynamic settings along the AKSZ.