Significance and dynamics of the Neoproterozoic (810 Ma) Phulad Shear Zone, Rajasthan, NW India

Abstract

AbstractThe Phulad Shear Zone (PSZ) in northwestern India demarcates the boundary between Grenvillian age para‐metamorphics of the South Delhi Fold Belt (SDFB) to the east and variably deformed Neoproterozoic granitoids of the Marwar Craton to the west. In calcareous and quartzofeldspathic mylonites in and around Phulad, the shear zone is characterized by steep SE dipping mylonitic fabric and a steep oblique stretching lineation. The shear zone is developed in a ductile regime with top‐to‐the‐west reverse sense of movement coupled with pronounced flattening across the PSZ. MnNCKFMASH pressure‐temperature pseudosection analysis of closely spaced specimens of garnet‐bearing mica schists reveals that the stabilization of syn/post‐garnet porphyroblasts in the hanging wall experienced near‐isothermal decompression followed by cooling in the retrograde arm of a clockwise P‐T path. The retrieved P‐T paths are consistent with reverse motion of SDFB mica schist over the cratonic foreland. U‐Th‐Pb (total) monazite age determinations in hanging wall mica schists indicate that reverse motion on the PSZ occurred at 810 ± 6 Ma. The pre‐shearing Grenvillian age (~970 ± 9 Ma) metamorphic monazites of the SDFB occur as embayed cores, in the interiors of younger monazite grains. Intense high‐temperature deformation and the emplacement of mica‐bearing pegmatites during reverse shearing caused recrystallization of monazites by dissolution‐precipitation, mantling older monazites in the hanging wall SDFB schists. It is suggested that the PSZ may represent a terrane boundary shear zone along which the Greater India landmass accreted with the Marwar Craton at ~810 Ma.