The carbonatites of Amba Dongar, Gujarat, India

Abstract

The geology, petrography and major and trace element geochemistry of Amba Dongar carbonatites are described. The ring structure of Amba Dongar carbonatite-alkalic complex has a central core of basalt collared by carbonatite breccia which in turn is surrounded by a sovite rim. Ankeritic carbonatite occurs as disjointed plugs intruding sovite and also as small dikes in surrounding sovite, fenites and basalts. Sideritic carbonatite forms thin veins in ankeritic carbonatite. Plugs of nephelinite and phonolitic nephelinite form the outermost ring of the complex. The complex is intrusive into Bagh sandstone-Deccan Traps sequence (Cretaceous-Eocene) and is situated in the rift zone of Narmada valley. The trace element contents reveal certain characteristic features such as pronounced enrichment in Ce + La ( > 3.0 %) in ankeritic carbonatite and high values of Th, Ba and P. Fenitization of surrounding Bagh sandstone shows two distinct trends: 1. earlier soda fenitization (orthoclase-albiteaegirine rock) and 2. potash fenitization leading to the development of pure feldspar rocks. The paper describes briefly the fenitization of nephelinites for the first time.