Transpressional tectonics during the emplacement of Pasupugallu Gabbro Pluton, Western margin of Eastern Ghats Mobile Belt, India: Evidence from AMS fabrics

Abstract

The Pasupugallu Gabbro–Anorthosite Pluton intruded the transition zone between the Mesoproterozoic Eastern Ghats granulites and the Meso- to Paleoproterozoic amphibolites and migmatitic gneisses of East Dharwar Craton, southern India. The pluton is bound by shear zones at its entire eastern, northern and southwestern margins. The objective of this work is to study the internal fabric of the pluton to understand the tectonic setting during its emplacement, essentially by means of Anisotropy of Magnetic Susceptibility (AMS). A total number of 107 samples have been subjected to AMS and IRM studies. The magnetic fabric is principally ferromagnetic in origin as the magnetic susceptibility values are >10−3 SI, which is further supported by IRM studies. The magnetic fabrics correspond well with the field measurements and petrographic studies. They show two lobes in both the northern and southern parts which display concentric inward dipping helicoidal foliation trajectories with interlayered steep and gently dipping domains. The magnetic lineation pattern suggests that the magma might have ascended through the central lobes and extended towards outer margins, particularly to the west. The constrictional strains occur at the eastern margin while flattening strains mark the western margin. AMS studies together with our field structural observations suggest that the magma has been emplaced into a chamber (dilational jog) induced by crustal-scale dextral transpressional tectonic regime, probably during the onset of Grenvilian event.