Niels Noack

The Palghat-Cauvery Shear Zone (PCSZ) marks the southern margin of the Archean Dharwar Craton in southern India. As the shear zone has been inferred to represent the ancient suture of the Proterozoic Mozambique Ocean, the age of the shear zone is crucial for understanding the paleogeographic assemblage of Precambrian crustal blocks in India. Here we present new constraints on the timing of tectonic activity along the Dharwar Craton margin from Lu–Hf garnet geochronology on garnetiferous mafic gran-ulites from the Kanjamalai mafic complex (KMC), located within the Salem Block of the northern PCSZ system. These mafic granulites are intercalated with BIF metasediments. They reveal horizontal, MORB-like REE patterns with a slight depletion of the LREE (La/Yb cn 0.35–0.68) and an absence of Nb depletion, suggesting a protolith of oceanic crust affinity (MORB). An emplacement age of 2536 ± 300 Ma can be inferred from whole rock Lu–Hf geochronology for the mafic suite. Mafic rocks of the KMC suite display positive ε Hf (2536) values that range between +8.4 and +9.7, indicating a significant mantle source depletion in Neoarchean times. This conclusion is insignificantly affected by the propagated uncertainty of the emplacement age. Lu–Hf dating of ductile deformed high grade garnets forming stretching lineations to regional scale folds yielded a minimum age of 2434 ± 17 Ma for the initial regional deformation. In combination with literature data, a P–T–t path can be compiled for the KMC with peak conditions of 14–16 kbar and 820–860 • C at 2.48 Ga (Anderson et al., 2012) and a retrograde equilibration at 6–7 kbar and 700 • C. Our results indicate that some structural patterns within the PCSZS may represent crustal reworking at a later stage, because the Paleoproterozoic structures are only reworked locally when truncated by regional high strain zones of the northern PCSZ system. Altogether, we propose that subduction–accretion processes in an oceanic setting operated along the southern DC margin at the Neoarchean–Paleoproterozoic boundary. Our results are clearly in contrast to models entirely explaining the PCSZ as a Neoproterozoic–Cambrian suture (500–600 Ma) (Sajeev et al., 2009; Santosh et al., 2009; Yellappa et al., 2012). However, a recently proposed model arguing for a complex, multistage evolution of the PCSZS region (Santosh et al., 2012) is supported by our study