870 Ma age of South Delhi Orogeny: A study on Geochronology of the granites and the metasediments, NW India.

South Delhi orogeny is constrained by correlating the deformational fabric with geochronology of the granites and metasediments around Beawar- Rupnagar-Babra, Rajasthan, NW India. The area consists of metaconglomerate, calcareous schist, mica schist and amphibolite. These were deformed by three stages of deformation(D_1-3) and intruded by four types of granite plutons (G_1-4). The D₁ deformation produced F₁, reclined/recumbent folds with S₁ axial planar fabric in greenschist facies metamorphic condition. The D₂ deformation produced NE-SW trending F₂ folds coaxial with F₁(type 3 interference pattern), crenulations and F₂-axial parallel ductile shear zones.  The D₃ deformation produced NW-SE F₃ folds, which superimposed on F₁ and F₂ to create type 1 and 2 interference pattern. Granites carry pervasive S₁ fabric. In G_1-3 granites, the S₁ is characterized by low temperature deformation fabric marked by bulging recrystallization of quartz. The G₄ granite (namely Sewariya granite) contains magmatic to submagmatic fabric and the S₁ fabric in it is a high temperature deformation fabric and lies parallel to magmatic fabric in the rock. Plagioclase is dynamically recrystallized by subgrain rotation and grain boundary migration and quartz shows chess board twinning. We interpret that the G₄ granite is syntectonic and G_1-3 were pre-tectonic to D₁ deformation.  U-Pb data (SHRIMP method) of G₁, G₂ and G₄ granites yield Concordia age calculated with ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U ratio at ~982 Ma, ~992 Ma and ~878 Ma respectively. Thus the South Delhi orogeny is constrained by the age of G₄ granite at ~ 878 Ma (~ 870 Ma).  The G_1-3 granites are pre- Delhi orogeny and probably constrain the age of rifting of the Delhi basin. EPMA Th-U-total Pb monazite geochronology of the garnet-staurolite-quartz-feldspar-biotite schist from the basal conglomerate zone shows three distinct ages, ca. 1611 Ma, 864 Ma and 718 Ma.  Correlating with granite SHRIMP age, the ~ 864 Ma corresponds to Delhi metamorphism and D₁ deformation (~ 870 Ma). The event ca. 1611 Ga probably belongs to pre-Delhi age, which is observed in nearby pre-Delhi localities like Sandmata terrane.

Keywords: Deformational fabric, geochronology, metaconglomerate, granite and geochronology.