First Evidence of Late Paleoproterozoic/Early Mesoproterozoic Sediment Deposition and Magmatism in the Central Aravalli Orogen (NW India)

The nature and stratigraphic position of the Beawar gneiss unit (BGU) within the central Aravalli orogen (NW India) remain unresolved. These rocks are currently assigned to the South Delhi Supergroup, assumed to be deposited at 1200-1000 Ma. This interpretation is challenged by new results of petrography, combined with U-Pb dating, Hf isotope analyses of zircon, and whole-rock geochemistry. These data reveal that the BGU consists of both ortho- and paragneisses and that the protoliths were formed and/or deposited at the boundary between the late Paleoproterozoic and early Mesoproterozoic at ca. 1600 Ma. U-Pb ages of detrital zircon grains from a paragneiss sample indicate a maximum deposition age of 1614 +/- 14 Ma and major age peaks at 2528, 1916, 1840, 1725, and 1617 Ma. These ages, in combination with Hf isotope data, suggest that the source area consisted mainly of felsic magmatic rocks similar to those exposed within the Aravalli orogen and in the Bundelkhand Craton and/or in terranes within the Central Indian Tectonic Zone. A single detrital zircon with an Eoarchean crystallization age of ca. 3630 Ma and Hf model age of 4.05 Ga point to the existence of the Hadean crust. Magmatic zircon in an orthogneiss sample yields an upper-intercept age of 1581 +/- 24 Ma, indicating granite emplacement during the early Mesoproterozoic. Combined geochemical and Hf isotope data suggest that the orthogneiss protolith was a strongly peraluminous S-type granite, formed by melting of surrounding country rocks perhaps within a convergent setting. U-Pb ages (980-960 Ma) of zircon overgrowths and monazite grains suggest that all the rocks were affected by a Grenvillian metamorphic overprint during the assembly of Rodinia.