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

摘要

Strain 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, G(0-3). The granulites were deformed by three phases of folding, F1-3, during South Delhi orogeny and marked by a subhorizontal pervasive fabric, S-1, axial planar to isoclinal-recumbent F-1 folds that developed during granulite facies metamorphism. S-1 is overprinted by discrete sets of subvertical shear zones associated with a mylonitic fabric, S-2, that were developed axial planar to NE-SW striking upright F-2 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 F-3 folds also affected the granulite facies rocks resulting in interference patterns and variations in regional structural trends. Brittle strike slip, and normal fault (S-f fabric) that developed post F-3, led the final exhumation of the granulite facies rocks to the surface. The S-1 monazites are Y-depleted and recrystallized through dislocation creep, and the S-2-S-f 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 S-1, S-2, and S-f 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.