Occurrence of Unusual Quartz Xenocryst-Laden Dykes in the Chhota Udaipur Alkaline-Carbonatite Sub-province, Deccan Igneous Province, India

Abstract

Occurrence of unusual basaltic dykes laden with a dense population of quartz xenocrysts within the Chhota Udaipur alkaline-carbonatitic sub-province of the Deccan Igneous Province is reported here. These dykes occur near Rorda (22°07.890 ' N; 74°04.267 ' E) and Samalavat (22°05.586 ' N; 74°07.261 ' E). The xenocrysts vary in size and shape and show effects of corrosion and partial assimilation by the host magma. These dykes have been affected by the carbonatitic fluids which commonly attack quartz xenocrysts, resulting in the formation of a complex growth pattern between quartz and carbonates. Mineralogical study confirms the presence of plagioclase, magnetite, ilmenite, rutile, chlorite, apatite, barytes and hydrated Fe-oxides. Original basaltic texture (sub-ophitic) and mineralogy (plagioclase, magnetite, ilmenite) is preserved, though pyroxenes are converted to chlorite; hematite, ilmenite, calcite and altered glassy material occur profusely in the groundmass. Basaltic dykes of such description have not so far been reported from the Deccan Igneous Province, although there are few references to the occurrence of quartz xenocrysts within basaltic dykes. It appears prima facie that quartzite xenoliths were selectively escalated by the basaltic magma at depth at the time of crustal extension leading to formation of Narmada rift; roughly coinciding with the emplacement of alkaline-carbonatite magma.