Serpentinite enigma of the Rakhabdev lineament in western India: Origin, deformation characterization and tectonic implications

Abstract

Serpentine mineralogy controls fault rheology in the ocean and continental rift settings to subduction settings and hence can be used to discern the paleo deformational conditions. The Rakhabdev lineament from Rajasthan, India, provides a unique opportunity to understand its tectonic evolution inferred from the deformation microstructures. However, the complexity of surrounding calc-silicate rocks had resulted in a long-driven debate on the origins of these serpentinite rocks. The source rocks of the serpentinites also cannot be determined previously due to complete serpentinization and metasomatism rendering complete alteration of the source rocks. In this study, the serpentinite mineral was analyzed using Raman spectroscopy to accurately characterize its molecular structure. The presence of the antigorite-variety of serpentine mineral indicate towards the origin of Rakhabdev serpentinites in the upper mantle condition. The antigorite serpentinite of Rakhabdev is a hydration product of mantle materials showing high Mg# values obtained from EPMA data. The microstructural and EBSD analysis also indicates two stages of deformation, with deformation of antigorite at upper mantle conditions, followed by their shallow crustal carbonate metasomatism and subsequent deformation of the carbonates, with later stage calcite vein intrusion. This resulted in the appearance of antigorite in contact with calcite, dolomite, talc, tremolite, and chlorite. The exhumation of mantle wedge antigorite serpentinite is, therefore, indicating a paleo-subduction zone culminating in a crustal-scale collision boundary expressed as arcuate discontinuous bodies forming the Rakhabdev lineament.