The 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.

In northwestern India, Phulad Shear Zone (PSZ,~810 Ma) demarcates the boundary between South Delhi Fold Belt to the east and Marwar craton to the west. This shear zone has regional NE-SW trend with small bends of N-S orientations. PSZ is characterized by steep southeasterly dipping mylonitic foliation with steep oblique stretching lineation. The PSZ has developed in a ductile transpression with top-to-the-NNW reverse sense of movement associated with a component of sinistral-slip movement on horizontal section. The 200 by 6 km porphyritic Phulad granite occurs along and across the PSZ, and it is variably deformed. Phulad granite shows evidence of magmatic foliation with preservation of parallel alignment of euhedral feldspars phenocrysts and microgranitoid enclaves. The feldspar phenocrysts show simple twin interfaces parallel to the direction of elongation. This granite also shows development of solid-state foliation parallel to this magmatic foliation. Detailed study of structural elements suggests that Phulad granite has formed during the regional deformation in the country rock shear zone prior to its complete crystallization. Our data indicate that the releasing bends of N-S orientation within the PSZ have provided the space required for emplacement of Phulad granite in a transpressional regime. EPMA U-Pb-Th monazite and U-Pb LA-ICP-MS zircon ages in Phulad granite indicate a magmatic age of 819.1 ± 4 and 818 ± 18 Ma, respectively. PSZ represents the suture along which Marwar craton accreted with remaining India to form the Greater India landmass. The present study suggests that Phulad granite acted as a stitching pluton during this suturing along PSZ at 810-820 Ma.

Delhi Fold Belt in Rajasthan is a northeasterly striking ductile shear zone with a well developed mylonitic foliation (035/70E) and a downdip stretching lineation. The deformation in the PSZ has developed in a transpressional regime with thrusting sense of movement. The northeastern unit, i.e., the hanging wall contains a variety of rocks namely calc-silicates, pelites and amphibolites and the southwestern unit, i.e., the footwall unit contains only granitic rocks. Systematic investigation of the granites of the southwestern unit indicate a gradual change in the intensity of deformation from a distance of about 1 km west of the shear zone to the shear zone proper. The granite changes from weakly deformed granite to a mylonite/ultramylonite as we proceed towards the PSZ. The weakly deformed granite shows a crude foliation with the same attitude of mylonitic foliation of the PSZ. Microscopic study reveals the incipient development of C and S fabric with angle between C and S varying from 15 • to 24 •. The small angle between the C and S fabric in the least deformed granite variety indicates that the deformation has strong pure shear component. At a distance of about 1 m away from the PSZ, there is abrupt change in the intensity of deformation. The granite becomes intensely foliated with a strong downdip lineation and the rock becomes a true mylonite. In mesoscopic scale, the granite shows stretched porphyroclasts in both XZ and YZ sections indicating a flattening type of deformation. The angle between the C and S fabric is further reduced and finally becomes nearly parallel. In most places, S fabric is gradually replaced by C fabric. Calculation of sectional kinematic vorticity number (W n) from the protomylonitic and mylonite/ultramylonite granites varies from 0.3 ± 0.03 to 0.55 ± 0.04 indicating a strong component of pure shear. The similarity of the geometry of structures in the PSZ and the granites demonstrates that the deformation of the two units is broadly synchronous and the deformation in both the units is transpressional.

Fluid infiltration at great depth during regional metamorphism plays a major role in mass transport and is responsible for significant rheological changes in the rock. Calc-silicate rocks of the Kajalbas area of Delhi Fold Belt, Rajasthan, are characterised by foliation parallel alternate bands of amphibolerich and clinopyroxene-plagioclase feldspar-rich layers of varying thicknesses (mm to decimetre thick). Textural relation suggests that the amphibole grains formed from clinopyroxene and plagioclase in the late phase of regional deformation. Algebraic analysis of the reaction textures and mineral compositions was performed with the computer program C-Space to obtain the balanced chemical reactions that led to the formation of amphibole-rich bands. The computed balanced reaction is 70.74 Clinopyroxene + 27.23 Plagioclase + 22.018 H 2 O + 5.51 K + + 1.00 Mg 2+ + 27.15 Fe 2+ = 22.02 Amphibole + 67.86 SiO 2 aqueous + 36.42 Ca 2+ + 8.98 Na +. The constructed reaction suggests that aqueous fluid permeated the calc-silicate rock along mm to decimetre thick channels, metasomatized the clinopyroxene-plagioclase bearing rocks to form the amphibole-rich layers. The regional deformation presumably created the fluid channels thereby allowing the metasomatic fluid to enter the rock system. The above reaction has large negative volume change for solid phases indicating reaction-induced permeability. Thermodynamic calculations suggest that the fluid-rock interaction occurred at 665±05 • C and 6.6±0.25 kbar (corresponding to ∼20 km depth). Textural modeling integrating the textural features and balanced chemical reaction of the calc-silicate rocks of Mesoproterozoic Phulad Shear Zone thus indicate that extremely channelled fluid flow was reaction enhanced and caused major change in the rock rheology.

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