Micro-Meso and Macro Scale Structures in syn-tectonic granite emplaced in a ductile transpression shear zone: A Case Study from the western margin of the South Delhi Fold Belt, Rajasthan, India

In the northwestern Indian shield, the northeast-southwest trending South Delhi Fold Belt (SDFB) is a multiply folded and poly-metamorphosed rock of the Proterozoic age. Phulad Shear Zone (PSZ) is described as a terrane boundary shear zone that separates the SDFB to the east and Marwar craton to the west. This shear zone is defined by steep easterly dipping mylonitic foliation and strong downdip stretching lineation. The PSZ has developed a ductile transpressive regime with a top-to-the-north-north-west reverse sense of movement during 810Ma. The PSZ shows regional NE-SW trends with small bends of N-S orientation. The present study deals with a variably deformed porphyritic granite named Phulad granite that occurs about 200 by 6 km along and across the PSZ.

 

The Phulad granite is characterized by a bi-modal grain size population with prominent euhedral grains of feldspar clasts (2-6 cm long) in a fine-grained (< 3 mm) mosaic of recrystallized feldspar and quartz aggregates. It consists of phenocrysts of k-feldspar that show characteristic features of magmatic origin. Microstructural study reveals a series of magmatic, sub-magmatic, high-temperature and solid-state deformation structures in this granite. Mesoscopic field relations show evidence of magmatic fabric in the studied granite. The granite also preserves tectonic foliation parallel to this magmatic fabric. Strong foliation developments with mean attitude 24˚/85˚E and prominent stretching lineation have been developed in the granitic rock. A detailed study of structural elements of Phulad granite and PSZ demonstrates a similarity in geometry and style, signifying that the deformation in both units is synchronous, and this granite is emplaced during the regional deformation prior to its complete crystallization. The N-S orientation of the PSZ acted as releasing bends and provided the space required for the emplacement of the granite in a transpressional ductile regime. Monazite chemical age data and conventional zircon age data suggest a magmatic age of 819.1 ± 4 and 818 ± 18 Ma, respectively. Integrating micro-meso and macro scale structures along with geochronology of Phulad granite we suggest that the Phulad granite acted as a stitching pluton at the time of suturing around 810-820Ma.