Monazite Geochronology of Deformational Strain Fabric and its Tectonic Implications of the Lower-Middle Crustal Rocks: A Case Study of Ambaji Granulite, NW India

Monazites crystallize along with silicate minerals, displaying microstructure characteristic of different recrystallization processes. They crystallize as the result of dislocation creep, dissolution creep or dissolution- precipitation creep, depending on pressure-temperature conditions and fluid composition. In dissolution- precipitation creep, newly formed grains replace pre-existing ones and appear as a distinct compositional domain under X-Ray and BSE images. Rock strain-fabric can be correlated with monazite-microstructure and dated using monazite-chemistry. Because it contains a significant amount of Th and U and lacks common Pb and continues to be used for Th-U-total Pb geochronology by chemical analysis in Electron Probe Microanalyser. As the closure temperature of monazite for Th-U-total Pb is ca. 800 °C; each domain retains its age of formation and can be used as geochronometer. Thus, monazite geochronology is used as an effective tool to constrain the timing of fabric development. Fourteen samples of Ambaji granulites of South Delhi Terrane, Aravalli Delhi Mobile Belt, NW India were used for the monazite geochronology and 306 analyses were performed. The microfabric of these samples has been studied to ascertain the category of strain fabric present in them.

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, F_1–3, during South Delhi 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. Brittle strike slip, and normal fault (S_f 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-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 S₁, S₂, 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.