Mineralogy of a New Occurrence of Lamprophyre Dyke from the Saurashtra Peninsula of Gujarat, Northwest Deccan Trap, India

A new occurrence of lamprophyre dyke is being reported from Saurashtra peninsula of Gujarat, north-western Deccan trap. The paper presents and discusses the detail mineralogy, field setting and whole rock major and trace element geochemistry. The newly discovered lamprophyre dyke intruded into the porphyritic olivine basalt flow. It is E-W trending dyke paralleling with Narmada rift trend, 200 m long and ∼1 m in width. Petrographic observations suggest the lamprophyre dyke is fine grained, porphyritic and shows typical panidiomoprphic texture. It contains phenocrysts of euhedral to subhedral amphibole (kaersutite), subhedral clinopyroxenes (diposide augite) and olivine pseudomorph enveloped in the groundmass composed of plagioclase, clinopyroxene and amphibole with carbonate, Ti-magnetite, zeolite and apatite occurring as accessory phases. Few clinopyroxenes show reaction and embayed margins thus indicating xenocrystic orgin. The normative chemistry of the lamprophyre dyke suggests alkaline nature with significant amount of olivine and nepheline. Whole rock geochemical data shows comparatively higher SiO 2 than the other lamprophyres of the Deccan and similar to the MgO depleted lamprophyres of the Chhota Udepur alkaline province. The primitive mantle normalized multielements pattern shows absence of any high field strength element (HFSE) anomaly. Studied dyke displays more similarity with the earlier reported pre- Deccan alkaline lamprophyres and more particular with the Chhota Udepur lamprophyre. Occurrence of this new lamprophyre dyke from the Saurashtra region extends and confirms the presence of lamprophyre magmatism in the far north-western part of the Deccan.