Petrology of Ocellar Lamprophyres in South Delhi Fold Belt, Danva, Sirohi District, Rajasthan, India

Ocellar lamprophyres are reported from Danva, in Sirohi district, Rajasthan which lies in the Pindwara-Watera sector of the South Delhi Fold Belt (SDFB). They intrude mafic metavolcanics and garnetiferous biotite schist of the Ajabgarh Group of the Delhi Supergroup. These lamprophyres are unaltered, show porphyritic and panidiomorphic textures with olivine, clinopyroxene, amphibole, biotite and spinels constituting the phenocryst phase in a ground mass of clinopyroxene, amphibole, mica and analcime. Petrochemically, these lamprophyres can be classified as analcime monchiquite (alkaline lamprophyre), though they have some affinity towards ultramafic lamprophyres and lamproites. Chemistry of the minerals suggests that the clinopyroxenes have a compositional range of diopside and salite, amphiboles are typically kaersutites, biotites are titanium rich and spinels are Al-rich ulvospinels. These minerals plot in discriminant fields of minerals from alkaline and ultramafic lamprophyres. nva lamprophyres are characterized by a variety of ocellar features that include (i) porphyritic ocellus with microphenocrysts of kaersutites and biotite in ground mass of analcime, (ii) zoned ocellus with concentric zones of carbonates and analcime and (iii) composite ocellus in which type (i) ocellus enclose the type (ii) ocellus. These ocellar features are interpreted to represent late stage magmatic segregation and magmatic crystallization involving two immiscible magmatic liquids. nva lamprophyres are in strike continuity of the Pipela lamprophyres and therefore confirm alkaline magmatism for over 20 kms in SDFB during end-Neoproterozoic tectono-thermal event. The close spatial association of the lamprophyres with Cu-Zn-Au deposits of Danva and Pipela area and the first report of monchiquite with affinity towards ultramafic lamprophyres and lamproites may be significant in gold (and possibly diamond) exploration in the SDFB.