Post-tholeiite rifting and genesis of ultramafic lamprophyres at ~65 Ma in the Deccan Large Igneous Province, India

Small-volume alkaline magmatism has now garnered due attention in geosciences, wherein these rocks provide essential information about deep mantle processes, geochemical composition, and the prevailing geodynamics. The continental alkaline magmatism of deep-seated ultramafic lamprophyres (UML) is spatially and temporally linked with continental breakup and rifting. Additionally, they have gained significant attention in terms of mineralization.

Deccan hosts one of the most extensive and peculiar suites of lamprophyre complexes in India, and this magmatism holds valuable geological information to unveil the geodynamic processes and provide significant implications on the enrichment/depletion processes of SCLM during Deccan times. The present research delves into the mineral and whole-rock geochemical compositions and paleomag dating of lamprophyres from the West Coast Alkaline Complex (WCAC) of the Deccan Large Igneous Province. The study has implications for the relation of lamprophyre magmatism with the Réunion mantle plume, the associated main phase of Deccan tholeiite magmatism, the initiation of rifting in the western Indian subcontinental margin, the separation of Seychelles from the Indian subcontinent, as well as the role of lithosphere thinning in triggering lamprophyre magmatism. WCAC lamprophyres are principally composed of olivine and phlogopite phenocrysts/ macrocryst embedded in a mesostasis of carbonate, clinopyroxene, olivine, nepheline, spinel, and melilite groundmass. The presence of titanian aluminous phlogopite, clinopyroxene, and Al-enriched spinels with high Fe²⁺/(Fe+Mg) ratios are the characteristic features of WCAC lamprophyres. They are primitive, undersaturated in silica (SiO₂: 38.43 – 38.99), and rich in MgO (up to 10 wt. %), TiO₂ (up to 4.2 wt. %), and light rare earth elements. Mineral genetic classification schemes and geochemical compositions demonstrate a resemblance with ultramafic lamprophyres. 

High LREE/HREE (La/Yb= 75-82) ratios, similar to UMLs reported globally, and low Ba/Rb and Rb/Sr ratios exhibit the predominance of phlogopite and amphibole in the mantle source. Geochemical investigations inarguably trace their genesis back to the enriched garnet lherzolite mantle, metasomatised by silicate and carbonate veins. They show compositional similarity with global damtjernites and derivation from moderate pressure depth (3-4 GPa) corresponding to 90-100 km thick lithospheric mantle. Paleomagnetic studies support the intrusion of lamprophyres, predominantly during the chron C29n, thereby substantiating their radiometric eruption age at ~65 Ma. Seismic tomography models reveal a current lithospheric thickness of approximately 50 km beneath the WCAC, suggesting delamination of the lithosphere after the intrusion of lamprophyres at 65 Ma. UML magmatism in WCAC resulted from the initiation of a rift, which ultimately led to the separation of the Indian subcontinent and the Seychelles. The impetus behind the emplacement of UML dykes is intricately tied to passive rifting and external plate boundary forces, surpassing the influence of the Reunion mantle plume. The lithospheric thinning presumably occurred after the emplacement of lamprophyres and other alkaline rocks and continued with continental rifting in response to greater plate-tectonic stresses in the region of persistent lithospheric weakness.