Alkaline magmatism in the Deccan Traps, India

 Hetu C. Sheth

Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400076 India

 

The ~65 MY old Deccan Traps of India are a largely tholeiitic continental flood basalt province with a present-day areal extent of ~500,000 km² and an original extent estimated at three times this size. The flood basalts are up to 2 km thick in the Western Ghats escarpment parallel to the western Indian rifted margin. Large tholeiitic dyke swarms also occur. However, alkaline picrites, basalts and more evolved rocks, forming both lava flows and intrusions, are abundant in certain parts of the Deccan Traps. Intrusions in the Deccan Traps dominate the western Indian rifted margin and the northwestern Deccan region. These intrusions are of great petrological interest owing to their great compositional diversity compared to the Deccan lava flows, and even within individual intrusive complexes. These intrusions cover the entire spectrum of igneous rock compositions, including gabbros (Girnar, Chogat-Chamardi, Mundwara), diorite and monzonite (Girnar), alkali gabbros, syenites and nepheline syenites (e.g., Mundwara), basanites and melanephelinites (Kachchh), trachytes (e.g., Mumbai), granophyres (e.g., Barda, Chogat-Chamardi), rhyolite and pitchstone (e.g., Rajpipla, Rajula), lamprophyres (several of the above), as well as carbonatites (e.g., the Amba Dongar diatreme hosting a commercial fluorspar deposit). Basanite-melanephelinite intrusions and a maar-diatreme, in Mesozoic sandstone in the Kachchh region (northwestern Deccan), have been identified as the plumbing system of a monogenetic volcanic field. The largest Deccan intrusion is that forming Mount Girnar in the Saurashtra region of the northwestern Deccan. This is a major gabbroic intrusion, dioritic-monzonitic in its upper parts, and has domed up the overlying lava flows as a laccolith. Many tholeiitic and alkalic (e.g., lamprophyre) dykes occur around and inside it, and a large ring dyke of granophyre and silicic porphyry encircles it. My talk will give an overview of the Deccan flood basalt province and its intrusive substructure including the alkaline rocks.