PHULAD SHEAR ZONE, RAJASTHAN, NW INDIA: THE PROPOSED MID-NEOPROTEROZOIC SUTURE FOR THE GREATER INDIA LANDMASS
In northwestern India the Phulad Shear Zone (PSZ) demarcates the boundary between Grenvillian-age South Delhi Fold Belt (SDFB) to the east and Neoproterozoic granitoids of the Marwar Craton to the west. The PSZ is characterized by NE-SW trending and steep easterly dipping mylonitic foliation with a strong oblique stretching lineation in calcareous and quartzofeldspathic mylonites in and around Phulad. The shear zone has developed in a ductile transpressive regime with a top-to-the-west reverse sense of movement. The paragenetic relations among metamorphic minerals in the hanging wall mica schists suggests that the reverse motion of SDFB over the Marwar Craton at mid-crustal condition occurred along the retrograde arm of a clockwise P-T path. Electron microprobe dating of monazite grains in the mica schist yield two age populations, at 970±9Ma (MSWD = 0.92) and 809±7Ma (MSWD = 1.6). Pegmatite veins emplaced synchronous with the PSZ show a single population of chemical ages at 810±6 Ma (MSWD = 0.53). Textural relation of monazite grains in mica schists and pegmatite veins indicate that the reverse motion on the PSZ occurred at 810±6 Ma. The pre-shearing Grenvillian ages (~970 Ma) retrieved in the PSZ mica schists are recorded in embayed cores within the younger monazite grains. 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.