Structure and monazite geochronology along the Bavali Shear Zone, the western extension of the Moyar Shear Zone, southern India

Abstract

Bavali Shear Zone (BSZ), the western extension of the Moyar Shear Zone (MSZ), located between the Coorg Block in the north and Nilgiri Block in the south is a 100 km long, steep-dipping and WNW-striking dextral shear zone, which is a less studied part of the MSZ. The majority of the rocks in the BSZ are schistose, including hornblende-biotite±epidote schist, garnet-biotite-sillimanite±muscovite±chlorite schist and talc-tremolite-actinolite-chlorite schist. The gneissic varieties comprise of amphibolite gneiss, granulite gneiss, quartz-feldspar gneiss, hornblende-biotite gneiss, and garnet-biotite gneiss. Other rock types include high-grade metamorphic rocks such as pyroxene granulite and charnockite, banded magnetite quartzite, micaceous quartzite with/without sillimanite, metapyroxenite, amphibolite and mylonite. Several felsic intrusive like granite, diorite, syenite, quartz-feldspar leucosomes and mafic/ultramafic intrusive such as gabbro and anorthosite are found in some places.The dominant structural trend of the BSZ is WNW with strikes varying between 110⁰N and 130⁰N. The steep-dipping and variably oriented pre-shear zone fabrics are preserved in low-strain domains. The BSZ is steep dipping and characterized by steeply-plunging stretching lineation with a persistent dextral sense of shear. The shear zone shows N-down kinematics in vertical sections perpendicular to the shear zone fabrics. The β-axis of poles to the shear zone fabric and the orientations of the hinges of the folds related to shearing share low-angle obliquities with the stretching lineations. It indicates that the shear-related folds have a reclined to steeply-inclined geometry, and the fold hinges are broadly collinear with the stretching direction. The last deformation in the BSZ with down-dip stretching lineation clearly shows the features of the transpressional shear zone with a dextral sense of movement and top-to-the-north kinematics.U-Th-(total) Pb monazite chemical dating was performed on structurally constrained monazites from the BSZ. Monazites from one garnet-biotite-sillimanite-chlorite schist, one garnet-biotite gneiss and one mylonite close to the BSZ were dated. The monazite hosted in the garnet-biotite-sillimanite-chlorite schists provide prominent ages of 751±40 Ma, the garnet-biotite gneiss yielded a peak at 742±15 Ma and the single mylonite sample collected close to the BSZ yield a distinct age peak at 745±67 Ma. All the metamorphic monazites from the BSZ show a prominent mid-Neoproterozoic age, lacking in the adjoining Coorg Block, Nilgiri Block and Western Dharwar Craton. We, therefore, assign this mid-Neoproterozoic metamorphic chemical ages retrieved from monazites to the ductile deformation in the BSZ that reoriented all the pre-shearing fabrics and speculate that the BSZ collision orogeny preceded the eventual integration of the Greater India landmass with the Gondwanaland during the early-Palaeozoic.