Early Palaeoproterozoic structural reconstitution of a suite of rocks from the Mahadevi Layered Complex, Tamil Nadu, India

Abstract

The Palghat‐Cauvery Shear Zone (PCSZ) of the Granulite Terrane of South India is perceived to be a late Neoproterozoic transcontinental shear zone that develops extensive mylonite and sheath folds. The Mahadevi Layered Complex (MLC), the area of present study, is situated in the eastern part of the PCSZ. The studied area exposes vast expanse of felsic orthogneisses with large enclaves of mafic granulite interlayered with meta‐BIF. Our study records two distinct deformational events (D1–2) that produced E‐W closing regional folds with gently curved fold axis. The curved fold axes results in the regional dome‐and‐basin structure. Kinematic markers suggest that a weak shear couple was accompanied with the D2 deformation. Preponderance of the granoblastic fabric in the rocks, restriction of mylonites only in local conjugate shear zones, and the structures produced during D1–2 deformations are at variance with the view that (a) existence of a sheath fold as previously reported from the area and (b) the PCSZ marks the zone of intense shear deformation. Preliminary petrological information from the studied area shows that D1–2 deformational events were accompanied by high‐pressure granulite‐facies metamorphism that was followed by a steeply decompressive retrograde P‐T path. The U–Pb zircon ages from the felsic orthogneiss fixes the date of emplacement of its magmatic protolith and the regional high‐pressure metamorphism at ≥2,530 and ca. 2,479 Ma, respectively. Integrating all the information it is suggested that the evolution of the MLC occurred in a continent‐continent/arc collisional setting at the dawn of the Proterozoic Eon. This study does not support the existence of a “mega sheath fold” in the MLC as has been published in the recent literature.