Abstract The Deccan Large Igneous Province of India is the product of fissure eruptions, and contains vast lava fields and dyke–sill networks. It is associated with several rift zones in peninsular India, which reflect pre-existing major weaknesses in the Indian lithosphere. In rift-zone eruptions, magma is normally transported to the surface through dykes. However, some injected dykes do not reach the surface but are arrested at depth in the rift zone due to mechanical heterogeneity and anisotropy, or insufficient magma pressure. In the present work, the effects of mechanical layering and regional tension on dyke emplacement in the Narmada rift zone are studied. The results indicate that the distribution of maximum principal tensile stresses was altered by mechanical layering and/or regional tension, which has led to variations in potential dyke propagation pathways. Studies on dyke evolution and emplacement in the Narmada rift zone indicate four evolutionary stages: stage I – arrival of a mantle plume and pre-volcanic extension; stage II – formation of shallow magma chambers; stage III – vertical dyke injection and fissure eruption; and stage IV – the ‘blanket effect’ and lateral dyke propagation.

Abstract: We report here on the occurrence of an interesting mantle-derived ultramafic xenolith entrained in an Eocene ( c. 55 Ma) lamprophyre dyke from the Dongargaon area of the Chhotaudepur alkaline subprovince located within the Narmada Rift Zone, NW India. The mineralogy of the xenolith comprises olivine, clinopyroxene and mica (phlogopite), with the latter occurring essentially as rims around the clinopyroxene. Inclusions of apatite, interstitial sulphide (pyrite) and micron-scale exsolved spinel are widespread. Olivine is forsteritic (Fo 85.34 ), displays little compositional variation and overlaps with that reported from worldwide mantle peridotite xenoliths. Clinopyroxene is a diopside with a compositional range of Wo 48.36 En 43.83 , Fs 6.53 and Ac 1.27 , and is conspicuous by its high CaO (up to 24.4 wt%) and TiO 2 (up to 1.6 wt%) content. Clinopyroxene is also compositionally similar to that reported from ‘enriched’ (metasomatized) peridotite xenoliths rather than those that occur in the ‘normal’ (depleted) peridotitic xenoliths. Phlogopites have a high concentration of fluorine (up to 1 wt%), whereas the apatites show an anomalous enrichment of F (up to 5 wt%), as well as enrichment in Sr (SrO up to 1.9 wt%). Our study provides the first direct petrographical evidence for the modal metasomatism in the post-Deccan subcontinental lithospheric mantle (SCLM) from this domain. From the textural and mineralogical assemblage of the xenolith, we infer that a possible olivine + garnet + orthopyroxene assemblage, in the presence of a metasomatic fluid, has given rise to clinopyroxene + phlogopite + spinel. The paragenesis of apatite essentially as inclusions suggests that it to be the earliest crystallized phase during the metasomatic event. Geothermobarometry of the clinopyroxene in the xenolith reveal temperatures of approximately 1200°C and pressures of approximately 12 kb, which are comparable with such data reported from other Deccan-related xenoliths. Preservation of phlogopite and apatite in the ultramafic xenolith imply that some of the readily fusible metasomatized portions in this domain escaped wholesale melting during the eruption of the Deccan Traps, possibly due to the variable thickness of the underlying SCLM. Supplementary material: A table detailing natural and synthetic standards used for calibration is available at https://doi.org/10.6084/m9.figshare.c.3815296