Petrogenesis of lamprophyres from Chhota Udepur area, Narmada rift zone, and its relation to Deccan magmatism

Abstract

Geological setting, petrology and geochemistry of a new find of lamprophyre dykes, intruding the Precambrian basement gneisses from the Chhota Udepur area, Narmada rift zone, Western India, are presented. Of the three lamprophyre dykes, two of them display E–W trend paralleling that of the Narmada rift, extend up to ∼2.5 km in length and constitute the longest yet recorded lamprophyre dykes from the Indian shield. The Chhota Udepur lamprophyres (CUL) show several overlapping mineralogical and geochemical characteristic features displayed by alkaline- and calc-alkaline lamprophyres and hence it is not straight forward to pigeon-hole them in existing lamprophyre classification schemes. They are porphyritic–panidiomorphic with euhedral to sub-hedral phenocrysts and microphenocrysts of olivine (Fo_{78.33–78.42} Fa_{21.24–21.37}), diopsidic augite to titaniferous augite (Wo_{54.83–59.03} En_{36.75–41.36} Fs_0.1–5.66), biotite and amphibole (kaersutite). Feldspar (orthoclase perthite and plagioclase) is essentially confined to the groundmass along with accessory phases such as apatite and spinel (ti-magnetite). Rare carbonate ocelli are also present. In terms of major element geochemistry the CUL are nepeheline-normative and predominantly sodic (Na₂O > K₂O); yet their K₂O and SiO₂ contents are sufficiently high to signal traits of calc-alkaline lamprophyres. On the other hand, the CUL exclude negative spikes at Nb–Ta which are considered to be a characteristic of subduction-related (calc-alkaline) magmas, are highly evolved (Mg#: 27.82–55.45), enriched in incompatible trace elements (high La/Nb, Zr/Nb) and have fractionated REE distribution patterns (La/YbN ratios: 35–44). Post-emplacement alteration and crustal contamination have had little influence on their geochemistry. Th/Yb, Ta/Yb, Nb/U and Ce/Pb ratios of the CUL imply an OIB type-enriched mantle source, similar to that displayed by other Deccan-related lamprophyres. A metasomatised (enriched) garnet lherzolite mantle source with very low degrees of partial melting that has subsequently undergone a large degrees of fractional crystallisation is capable of accounting observed REE geochemistry of the CUL. We infer the lamprophyres of Chhota Udepur area to be a part of the alkaline rock spectrum of the Deccan Large Igneous Province and their emplacement, together with that of other alkaline complexes in the Chhota Udepur area, is envisaged to be strongly controlled by pre-existing crustal weakness accentuated by the mantle plume–lithosphere interaction during the end-Cretaceous in the Narmada rift zone.