Mineralogy and 40Ar/39Ar Geochronology of Orangeites (Group II kimberlites) from the Damodar Valley, Eastern India

Ray W. Kent, Simon P. Kelley and Malcolm S. Pringle
Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK
Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow, G75 0QF, UK

Abstract: A suite of ultramafic-mafic alkaline igneous rocks in the Damodar Valley, eastern India, contains carbonate, phosphate and titanate minerals that are not characteristic or common in minettes or lamproites, but are typical of orangeites (Group II kimberlite) from southern Africa. Phlogopite grains from the Damodar alkaline rocks yield mean 40Ar/39Ar ages of 116.6±0.8 Ma, 113.5±0.5 Ma and 109.1±0.7 Ma (1σ errors) using laser dating techniques. These ages are similar to the Rb-Sr ages of African orangeites, which lie mostly in the range 121 to 114 Ma. Prior to this study, only one possible occurrence of orangeite (the ∼820 m.y.-old Aries pipe, Western Australia) was known outside the Kaapvaal craton and its environs. If the Damodar alkaline rocks are bona fide orangeites, it is likely that they were generated at depths of >150 km, within the stability field of diamond.

Keywords: orangeite • Group II kimberlite • 40Ar/39Ar • craton • diamond

Mineralogical Magazine; June 1998 v. 62; no. 3; p. 313-323; DOI: 10.1180/002646198547701
© 1998, The Mineralogical Society
Mineralogical Society (www.minersoc.org)