• Sadhana M Chatterjee

Articles written in Journal of Earth System Science

• Deformation of footwall rock of Phulad Shear Zone, Rajasthan: Evidence of transpressional shear zone

Phulad Shear Zone (PSZ) of Delhi Fold Belt in Rajasthan is a northeasterly striking ductile shear zonewith a well developed mylonitic foliation (035/70E) and a downdip stretching lineation. The deformationin the PSZ has developed in a transpressional regime with thrusting sense of movement. The northeasternunit, i.e., the hanging wall contains a variety of rocks namely calc-silicates, pelites and amphibolites andthe southwestern unit, i.e., the footwall unit contains only granitic rocks. Systematic investigation ofthe granites of the southwestern unit indicate a gradual change in the intensity of deformation from adistance of about 1 km west of the shear zone to the shear zone proper. The granite changes from weaklydeformed granite to a mylonite/ultramylonite as we proceed towards the PSZ. The weakly deformedgranite shows a crude foliation with the same attitude of mylonitic foliation of the PSZ. Microscopicstudy reveals the incipient development of C and S fabric with angle between C and S varying from15◦ to 24◦. The small angle between the C and S fabric in the least deformed granite variety indicatesthat 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 astrong downdip lineation and the rock becomes a true mylonite. In mesoscopic scale, the granite showsstretched porphyroclasts in both XZ and YZ sections indicating a flattening type of deformation. Theangle 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 (Wn) fromthe protomylonitic and mylonite/ultramylonite granites varies from 0.3 ± 0.03 to 0.55 ± 0.04 indicatinga strong component of pure shear. The similarity of the geometry of structures in the PSZ and thegranites demonstrates that the deformation of the two units is broadly synchronous and the deformationin both the units is transpressional.

• Reaction enhanced channelised fluid-flux along midcrustal shear zone: An example from Mesoproterozoic Phulad Shear Zone, Rajasthan, India

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 tothe formation of amphibole-rich bands. The computed balanced reaction is 70.74 Clinopyroxene + 27.23 Plagioclase + 22.018 H$_2$O+5.51K$^+$+1.00Mg2$^+$+ 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.

• # Journal of Earth System Science

Volume 131, 2022
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