PHULAD SHEAR ZONE, RAJASTHAN, NW INDIA: THE PROPOSED MID-NEOPROTEROZOIC SUTURE FOR THE GREATER INDIA LANDMASS

The position and participation of the Greater India landmass within the Rodinia supercontinent has been widely debated. Profound similarities between the Grenvillian age domain in the Eastern Ghats Granulite Belt, India and the Rayner Complex, Antarctica have prompted researchers to suggest that India and Australo-Antarctic Block formed a coherently evolving landmass within Rodinia during the Grenville. Palaeomagnetic studies from Malani Igneous Suite (MIS) in the Marwar Craton of northwestern India, on the other hand, place India far removed from the Rodinia landmass. Further it is suggested that the MIS, the Seychelles and the central-north Madagascar comprise a separate terrane during the mid-Neoproterozoic, ~750 Ma. However, the timing and suturing of the MIS hosting Marwar Craton with the remaining India remains speculative.

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.