Petrotectonic framework of granitoids and associated granulites at Nagavalli Shear Zone (NSZ), Eastern Ghats Belt: Evidence of a late transpression orogeny

• Published in: Journal of Earth System Science Vol. 124; no. 4; pp. 707 - 727
• Main Authors: Saha, Tamoghna, Karmakar, Subrata
• Format: Journal Article
• Language: English
• Published: New Delhi Springer India 01.06.2015; Springer Nature B.V
• Subjects: Earth and Environmental Science; Earth Sciences; Granular materials; Metamorphic rocks; Metamorphism; Minerals; Orogeny; Rocks; Sedimentary geology; Shear strength; Space Exploration and Astronautics; Space Sciences (including Extraterrestrial Physics; tectonothermal events; transpression; Granitoids; Nagavalli Shear Zone (NSZ); peak metamorphism; sinistral shear
• ISSN: 0253-4126; 0973-774X
• DOI: 10.1007/s12040-015-0579-4

Megacrystic granitoids associated with migmatitic and metasedimentary gneisses occurring around Nagavalli Shear Zone (NSZ) preserve complex metamorphic and deformation history. Thinly laminated discontinuous banding of quartzofeldspathic layer (S 1 ) in the migmatites is the product of first incipient melting during prograde M 1 –D 1 tectonothermal event. Peak M 2 –D 2 event is manifested by the development of S 2 gneissic foliation in all rocks, which is axial planar to rootless folds on S 1 . Porphyroblastic garnet mantled by leucosomal melt fraction in granitoids, suggest that the rock suffered peak granulite facies metamorphism along with host migmatitic gneisses. The subsequent D 3 event deforms differently the massive granitoids and the migmatitic granulite gneisses. The D 4 deformation acted as transpression with broad northwest–southeast compression that develops strong discontinuous regional-scale anastomosing shear zones transecting the earlier gneissosity (S 2 ) in the granitoids with prominent sinistral shear sense. It deforms the axial plane of regional folds in migmatites and develops superposed non-plane non-cylindrical folds in outcrop to regional scale. Thus we infer megacrystic granitoids were possibly emplaced in pre- to syn-peak metamorphic event within the host granulites. Granitoids and associated migmatitic gneisses of Late Meso- to Neoproterozoic age suffered subsequent petrotectonic events followed by a sinistral transpression acted along NSZ.