Geochronology and geochemistry of Neoproterozoic Mt. Abu granitoids, NW India: Regional correlation and implications for Rodinia paleogeography

Abstract

The ∼125km2 Mt. Abu granitic pluton in Rajasthan, northwestern India, consists of variably deformed, subsolvus, dominantly metaluminous, I-type, porphyritic hornblende-biotite granitoids that are crosscut by both rhyolitic and mafic dykes. Weakly deformed varieties occur mainly in the central regions of the pluton, and gneissic varieties, including distinctive augen gneisses are found on the margins, especially on the northwestern side. Modest diversity in major and trace element chemistry (SiO2=72.8±2.8wt.%; La=100–400× chondrites) is attributed to variable partial melting processes rather than to fractional crystallization. U–Pb zircon analyses (TIMS method) for three samples representing both deformed and undeformed granitoid types yield nearly concordant dates of 765±5Ma, which we interpret as the time of magmatic crystallization. This demonstrates that the Mt. Abu granitoids are coeval and correlative with volcanic and plutonic rocks of the nearby Malani Igneous Suite, rather than with the Erinpura granites, which are demonstrably older by ∼100m.y. We further show that Mt Abu granitoids are geochemically and petrologically distinct from Erinpura granitoids, but match those from the Malani Igneous Suite and Praslin Group granitoids of the Seychelles, which, along with northern Madagascar, formed now-fragmented components of an Andean-type magmatic arc on the margin of the Rodinia supercontinent. 40Ar/39Ar isotopic data on a hornblende separate from a weakly deformed Mt. Abu granite yield an age of 509±2Ma, and a biotite separate from the type locality of Erinpura granitic augen gneiss yields an age of 514±2Ma; we interpret these ages as representing the time of a fabric-forming event that reached amphibolite grade. This late Pan-African age demonstrates that the effects of Gondwana assembly and collapse extend into northwestern India, and must have also affected sedimentary rocks of the Marwar Supergroup, which are likely equivalents of the extensive earliest Paleozoic sandstones of North Africa. Deformation of the Mt. Abu granitoids and Marwar supracrustal rocks may be related to late Pan-African collisional events, and/or to collapse of the mountains formed during the final assembly of the Gondwana supercontinent.