Dynamics, EPMA Th‐U‐Total Pb Monazite Geochronology and Tectonic Implications of Deformational Fabric in the Lower‐Middle Crustal Rocks: A Case Study of Ambaji Granulite, NW India

Abstract

AbstractStrain fabric and monazite microstructure were studied and dated by Electron Probe Microanalyser (EPMA) in situ Th‐U‐total Pb monazite geochronology in the Ambaji granulite, South Delhi terrane, NW India. The Ambaji granulite comprises pelitic, calcareous, and mafic granulites with several phases of granite intrusions, G0‐3. The granulites were deformed by three phases of folding, F1–3, during South De lhi orogeny and marked by a subhorizontal pervasive fabric, S1, axial planar to isoclinal‐recumbent F1 folds that developed during granulite facies metamorphism. S1 is overprinted by discrete sets of subvertical shear zones associated with a mylonitic fabric, S2, that were developed axial planar to NE‐SW striking upright F2 folds and facilitated exhumation of granulite facies rocks to the upper crust. The shear zones show early history of high‐temperature thrust sense shear and late stage low‐temperature sinistral shear. The NW‐SE striking F3 folds also affected the granulite facies rocks resulting in interference patterns and variations in regional structural trends. Brittle strike slip, and normal fault (Sf fabric) that developed post F3, led the final exhumation of the granulite facies rocks to the surface. The S1 monazites are Y‐depleted and recrystallized through dislocation creep, and the S2‐Sf monazites are Y‐enriched and recrystallized through dissolution‐precipitation creep. Different monazite population yielded distinct ages of circa 875‐857, 834‐778, and 764‐650 Ma for S1, S2, and Sf strain, respectively, indicating that the South Delhi orogeny spanned 875‐650 Ma overlapping with the early phase of the Pan‐African orogeny or representing a transition between Grenvillian and Pan‐African orogeny.