Neoproterozoic Mafic Magmatism in Nagercoil Block, Southern India and Its Implications on the Gondwana Collisional Orogeny

Abstract

The Nagercoil block situated at the southernmost tip of India occupies a key position in the East Gondwana collisional tectonic models. The Nagercoil block is dominated by Orosirian I-type charnockite massifs that host numerous gabbroic emplacements. Our present understanding about the crustal architecture of the Nagercoil block is derived mostly from these charnockites, while detailed studies on gabbros are lacking. We present new petrology, geochemistry, and zircon U-Pb/Hf isotopic data of gabbros from the Nagercoil block to understand their petrogenesis and tectonic significance. The results reveal that these are formed by the partial melting of a subduction-modified lithospheric mantle source in a continental arc setting. Zircon U-Pb geochronology results reveal that the gabbros were emplaced between 561 and 531 Ma. Hafnium isotopic studies on zircons argue for a mid-Mesoproterozoic melting source with near-juvenile magmatic signatures. The Hf-TDM ages together with the available data from the terrane point to the involvement of the adjacent Achankovil unit as a possible melting source contributor. The genetic link between the Achankovil unit and Sri Lanka together with the remarkable similarity in ages and isotopic characteristics of mafic rocks from both these terranes point to their coeval formation during the East African Orogeny associated with the final stages of the Gondwana supercontinent assembly.

Age and petrogenesis of mafic granulites from central Madurai block, south India: implications on regional tectonics

“…Hafnium isotopic signatures suggest parental magma generation from both Archean and Proterozoic melting sources with different degrees of reworking. The calculated T DM model ages and ϵ Hf (t) values for these rocks are in good agreement with the reported values of associated rocks from the study area (Kumar et al 2017). The remarkable agreement of T DM model ages with the whole-rock Sm-Nd model ages of rocks from this domain (Plavsa et al 2012;Tomson et al 2013) indicates the involvement of Archean and Proterozoic melting sources.…”

“…The remarkable agreement of T DM model ages with the whole-rock Sm–Nd model ages of rocks from this domain (Plavsa et al 2012; Tomson et al 2013) indicates the involvement of Archean and Proterozoic melting sources. Similar meta-gabbros/mafic granulites have been reported from other Gondwanan counterparts such as Trivandrum block (Yang et al 2019), Nagercoil block (Sajna et al 2022; Kroner et al 2015), Sri Lanka (Santosh et al 2014; Takamura et al 2015) and Antarctica (Takamura et al 2020), with similar emplacement/metamorphic ages suggesting that this Neoproterozoic mafic magmatism is a pervasive igneous event that can be traced across different East Gondwana fragments. Together with previously published results, these inferences suggest a collision setting between western Madurai block and eastern Madurai block along the Kambam UHT belt and Suruli shear zone.…”

“…These cores produce older (>750 Ma) strongly discordant ages with variable Th/U ratios. Zircons with similar U-Pb ages and chemical characteristics are reported from basement charnockites in the study area by Kumar et al (2017). These basement charnockites are identified as the prominent contributor to the chemical contamination of these rocks, as revealed from AFC modelling (Fig.…”

“…Southern Granulite Terrane (SGT) is a composite of polymetamorphic terrains with prolonged crustal evolution history ranging from early Archaean to late Neoproterozoic, preserving the evidence for polyphase deformation, multiple tectonothermal and multiphase exhumation and cooling histories (Ghosh et al 2004;Plavsa et al 2012;Tomson et al 2013;Brandt et al 2014;Collins et al 2014;Kumar et al 2017;Dev et al, 2021;Dev et al, 2022b). SGT is bounded to the north by the Archaean granite-greenstone terrain of the Dharwar craton (Chadwick et al 2000;Jayananda et al 2000).…”