Neoproterozoic magmatism in NW India and its implication for crustal evolution

In northwest India, the South Delhi Fold Belt (SDFB) is a NE-SW trending region of the Neoproterozoic age, consisting of poly-deformed and poly-metamorphosed rocks. To the west lies the Marwar Craton, and the boundary between them is defined by a crustal-scale shear zone, dated to 810 Ma, known as the Phulad Shear Zone (PSZ). The syn-tectonic Phulad Granite, which runs along the PSZ, played a key role in stitching together the Marwar Craton and the SDFB during the 810 Ma tectonic event. Approximately 30 km to the east of the PSZ, a quartz monzonite pluton, emplaced within the calc-silicates of the SDFB, is observed. This study focuses on the meso- and micro-structures, as well as the geochemistry of the quartz monzonite, to better understand its emplacement conditions and the tectonic processes at that time.

In the field, the quartz monzonite exhibits a saccharoidal texture with a crude foliation, defined by the alignment of feldspar grains. The foliation in the monzonite has a mean orientation of 14°/67° E. The quartz monzonite is primarily composed of k-feldspar and plagioclase feldspar, with minor amounts of quartz, amphibole, and titanite. Microstructural analysis reveals features indicative of sub-magmatic, high-temperature deformation, suggesting that the rock underwent solid-state deformation. These microstructural characteristics of the quartz monzonite suggest a syn-magmatic deformation event. The foliation in the monzonite is broadly parallel to the mylonitic foliation in PSZ, further supporting the idea of a syn-tectonic emplacement. The geochemical study of the quartz monzonite displays a syn-collisional granite-type geochemical signature with a distinctly negative REE pattern. The REE pattern features suggest that garnet played a significant role in the petrogenesis. By integrating micro and meso-structural analyses with geochemical data, we infer that the emplacement of the quartz monzonite coincided with the development of the PSZ and the intrusion of the Phulad Granite. Despite the temporal overlap, the quartz monzonite and the Phulad Granite display significant geochemical differences, denoting distinct petrogenetic processes. Based on the integration of all available data, we propose that the quartz monzonite was emplaced during the 810 Ma collisional event, resulting from the partial melting of garnet-bearing mafic crust. While both quartz monzonite and Phulad Granite likely share a common source, the depth of melting was different. The greater depth of melting in the eastern portion suggests an eastward subduction of the Marwar Craton during this tectonic event.