New ICP-MS U-Pb Zircon Ages from Khatu Rhyolites in the Jodhpur Group, India: Constraints on the Lower Age Limit for the Marwar Supergroup

Abstract:

The sedimentary sequences of the Marwar Supergroup (NW India) are divided into lowermost Jodhpur Group, middle Bilara Group and uppermost Nagaur Group. Age constraints (based on fossils, detrital zircon and stable isotopic studies) indicate an early Cambrian age for the uppermost Nagaur Group. Although not well-constrained, the Edicaran-Cambrian boundary is placed within the Nagaur Group. The basal Marwar sedimentary units (the Sonia and Girbakhar sandstones) rest unconformably over the ~750 Ma Malani rhyolites and contain enigmatic discoidal fossils. The age for the onset of sedimentation within the lowermost Jodhpur Group remains elusive. In this study we reported new LA-ICP-MS ages from a felsic igneous rock layer within the Jodhpur Group (Sonia sandstone) at Khatu. Cathodoluminescence images showed the zircons from the felsic rock are rounded and fragmentary consistent with an inherited or xenocrystic origin. The age spectra from the zircons are dominated by peaks of around 850 Ma, 1200~1400 Ma, 1500~2000 Ma, 2500 Ma consistent with previous studies. A youngest concordant zircon yielded an age 702 Ma ± 17.2. As the ages in our study are from recycled/inherited ancient grains, the exact crystallization age remains elusive. Nevertheless, the eruption of the felsic rocks must be no older than ~700 Ma. Additional studies are underway to constrain the age of the lowermost Marwar Supergroup and to place the basinal development in a tectonic context.