Petrofacies and Paleotectonic Evolution of Gondwanan and Post-Gondwanan Sequences of Nepal

Abstract

The Gondwanan sequences are widely distributed in several intracratonic basins of peninsular India and also as discontinuous patches along the Himalayan orogenic belt. The Tansen Group (2400 m) in western Nepal is thicker and more complete than the Katari (380 m) and the Barahachettra (350 m) groups located in eastern Nepal. In the Tansen Group, modal compositions of sandstone change from arkosic (Permo-Carboniferous unit) to quartz arenitic and then to litharenitic (Eocene-Early Miocene). Similar modal compositions are also present only in alleged Permo-Carboniferous units Katari and Barahachettra groups of eastern Nepal. The quartzofeldspathic sandstones of the Permo-Carboniferous Sisne (western Nepal) and Saptakoshi (eastern Nepal) formations show high Eu values. The Quartzite unit of the Saptakoshi Formation seems similar to the Amile Formation of western Nepal as both contain higher percentages of mature quartz and stable heavy minerals, and show similar REE patterns. High contents of lithic fragments with relatively rare feldspars indicate that the Tamrang sandstones were deposited in the juvenile foreland basin between India and Tibet like the Eocene Bhainskati Formation of western Nepal. Garnet chemistry suggests sediment supply from variously graded metamorphic rocks. Tourmaline chemistry indicates sediments derivation from both lithium-high and lithium-low igneous rocks. Chrome-spinels with low TiO2 contents in the Taltung Formation show evidence of provenance from alkaline basalts. Cooling ages of detrital muscovites from the Permo-Carboniferous units of all three study sections show some older ages (300 Ma to 1700 Ma) and few younger (17 Ma to 25 Ma) ages. The older ages could be related to the magmatism in the Indian craton (e.g., Malani Rhyolite, Erinpura Granite, Chotanagpur Gneissic Terrain). Ages ~ 40 Ma muscovites from the Bhainskati Formation suggest a Himalayan orogenic source. Relatively younger ages, however, could indicate about the possible argon loss from the muscovite or misinterpretation of the stratigraphy. This study suggests that the stratigraphy of the Gondwana sequences in eastern Nepal needs to be re-evaluated. The existing quartzite unit of supposedly Permo-Carboniferous Saptakoshi Formation (eastern Nepal) should belong to Late Cretaceous, equivalent to the Amile Formation of western Nepal. The Tamrang Formation (eastern Nepal), which is currently considered as Permo-Carboniferous to Cretaceous, could be an equivalent unit of the Eocene Bhainskati Formation of western Nepal.