Mineralogy of peralkaline lamproites from the Raniganj Coalfield, India

Abstract

AbstractTwo mineralogically distinct lamproites occurring as dykes in the Raniganj coalfield of eastern India are described in terms of a mineralogical-genetic classification as: (1) peralkaline olivine-ilmenitephlogopite- K-feldspar lamproite (var. Damodar); and (2) peralkaline pseudoleucite-phlogopite-amphibole- K-feldspar lamproite (var. Damodar). Compositional and paragenetic data are provided for major, accessory and trace minerals. Minerals common to both rocks include: chlorite-pseudomorphed phenocrystal olivine, phenocrystal Ti-rich Al-poor phlogopite and tetraferriphlogopite, groundmass potassic amphiboles, Sr-rich apatite and monazite-(Ce), late stage Na-poor K-feldspar and quartz. The rocks differ in terms of the character of the amphiboles (Ti-potassian arfvedsonite vs. K-richterite– K-magnesioarfvedsonite–K-arfvedsonite solid solution), spinel compositions (qandilite–chromite– magnetite vs. chromite–ulvöspinel–magnetite), the presence or absence of: pseudoleucite, microphenocrystal magnesian ilmenite, diopside, titanian aegirine, lorenzenite, an unamed Ti-silicate, an unnamed Mg-Zr silicate, bazirite, rutile, dolomite and norsethite. The rocks are considered to be members of a spectrum of modally-diverse peralkaline rocks, formed from a common parental magma produced by the partial melting of the ancient metasomatized lithospheric mantle of the northern Singhbhum craton. None of the rocks can be considered as aillikites, minettes, orangeites or kimberlites.