Geochronological evolution of the Moyar, Bhavani and Palghat shear zones of southern India: implications for east Gondwana correlations

Abstract

The amphibolite facies Moyar, Bhavani and Palghat shear zones separate the South Indian Dharwar Craton (DC) from the Nilgiri and Madurai high-grade crustal provinces. We present Sm–Nd and Rb–Sr isotopic whole rock, mineral and small slab data for remnant granulites, mylonitic gneisses and granitoids that reveal a Neoproterozoic tectonothermal imprint in these shear zones. Sm–Nd garnet crystallisation ages and Rb–Sr mica cooling ages from reworked rocks suggest that Pan-African structurally-controlled retrogression and successive cooling in the Moyar shear zone (MSZ) (garnet 624–591 Ma; muscovite 594 Ma; biotite 604–540 Ma) predates tectonometamorphism in the Bhavani shear zone (BSZ) (garnet 552 Ma; biotite 521–491 Ma) and Palghat shear zone (PSZ) (garnet 521 Ma; biotite 488–485 Ma). A Rb–Sr small slab isochron age of ∼624 Ma indicates that Pan-African retrogression in the MSZ also led to isotopic reequilibration of the Rb–Sr system on a cm-scale. A Sm–Nd garnet crystallisation age of 513±5 Ma reflects emplacement of a postdeformative pegmatite in the BSZ during the late Cambrian. A Sm–Nd garnet age of 2355±18 Ma obtained from a granulite remnant in the MSZ records early Palaeoproterozoic high-grade metamorphism, which may be related to the amalgamation of the Nilgiri Block and DC along the palaeo-Moyar Zone. A relict charnockite from the southern BSZ yielded a reproducible Sm–Nd garnet age of 1705±11 Ma, presumably recording late Palaeoproterozoic metamorphism. Neoarchaean and Palaeoproterozoic Nd model ages (3.0–2.2 Ga) suggest that the shear zone rocks are mainly reworked material from the adjacent Nilgiri and Madurai crustal domains. Nd model ages of granites and tonalites, which intruded the MSZ syntectonically, provide evidence for ≤1.7 Ga crust generation. Unsheared metasediments, exposed between the Bhavani and Palghat high-strain zones, were subjected to Pan-African high-grade metamorphism, as constrained by Sm–Nd age data of garnets grown during coaxial deformation at ∼610–560 Ma. Sm–Nd whole rock data from these supracrustal rocks indicate average crustal residence ages of ∼1.9 Ga, which implies contributions from an unknown Proterozoic crustal source and disproves their previously suggested correlation to the Archaean Sargur Group. These model ages provide an upper limit for the juxtaposition of DC and Madurai Block along the BSZ and PSZ.