Further geochronological and paleomagnetic constraints on Malani (and pre-Malani) magmatism in NW India

Abstract

article i nfo At 750 Ma India was part of a larger fragment of eastern Gondwana blocks that included the Seychelles-Mauritia, Madagascar, Sri Lanka and the Enderby Land-Prydz Bay region of East Antarctica. Sub- duction of the Mozambique Ocean beneath Seychelles-Mauritia, northern Madagascar and northwestern India formed a lengthy continental arc that remained active during the formation of Gondwana. Paleomag- netic data from the Malani rhyolites and associated dykes provide a robust paleomagnetic pole constraining India's position at this time. The rhyolitic and granitic rocks associated with the Malani Igneous Suite (MIS) have robust age constraints; however, the ages of the mafic dykes were inferred solely on the basis of simi- larity in paleomagnetic directions to the rhyolitic units. Here we present new geochronological data from the Malani mafic dykes that yield a minimum age of 704 Ma. The 207 Pb/ 206 Pb ages obtained for the dykes are less-likely to be affected by Pb-loss and yield a more reliable estimate for the age of the mafic dykes of ~750 Ma. We argue that intrusion of these mafic (and minor felsic) dykes represents the final pulse of MIS magmatism. Many of the granitic rocks in the region are reported as 'unclassified' due to limited geochemical data and/or geochronological ages. Some of these 'unclassified' granites are intruded by the mafic dykes sampled in this study near the town of Bilara. The granites yielded zircon core ages of ~1100 Ma with younger rims averaging ~1020 Ma. We argue that this provides further evidence for a significant orogenic event ~1000 Ma that may relate to the collision of the Marwar block with the Banded Gneiss Complex/Bundelkhand craton in north-central India. Other ~1000 Ma orogenesis is also known along the Central Indian Tectonic Zone (CITZ) and the Eastern Ghats Mobile Belt. Globally, this same time interval is thought to represent the amal- gamation of the supercontinent Rodinia and may also have resulted in the closure of the major Purana basins in India.