Petro-geochemistry, Sr[sbnd]Nd isotopes and 40Ar/39Ar ages of fractionated alkaline lamprophyres from the Mount Girnar igneous complex (NW India): Insights into the timing of magmatism and the lithospheric mantle beneath the Deccan Large Igneous Province

Abstract

Mount Girnar is one of the most conspicuous alkaline complexes (gabbro-diorite-syenite-lamprophyre), which intrude some of the earliest erupted basalts (ca. 69 Myr) of the Deccan Large Igneous Province (Deccan LIP) in the Kathiawar plateau of NW India. Petrography, bulk-rock geochemistry, Sr and Nd isotopes and 40Ar/39Ar mineral ages of fractionated (Mg#: 36.3–43.6) lamprophyre dykes (younger intrusives) are reported from two widely separated domains from Mt. Girnar. Petrography and mineral chemistry reveal that major mineral assemblages (pargasite and kaersutite varieties of amphibole, diopside, biotite and feldspar) in the lamprophyres are pristine and devoid of alteration. The lamprophyres belong to the alkaline variety in general and the camptonite - monchiquite series in particular. The bulk-rock major and trace elements of the Girnar lamprophyres display very good correlation with each other and also with those of associated rocks (syenites, diorites and gabbros) which support their genetic relationship. Trace-element ratios do not evidence crustal contamination and reveal derivation of the lamprophyres from partial melting of a lithospheric mantle source significantly modified by interaction with asthenospheric-derived melts, and resembles other alkaline rocks from the Deccan LIP in this regard. Initial 87Sr/86Sr (0.7052–0.7053) and 143Nd/144Nd (0.5125–0.5127) of the Girnar lamprophyres and associated rocks such as syenite, diorite and gabbro are tightly clustered and further attest to their derivation from a cogenetic parental melt. Their positive εNdi values (+0.8 to +3.4) require a mantle source that has experienced moderate long-term depletion of light rare-earth elements. Neoproterozoic/Early Cambrian depleted-mantle (TDM) Nd model ages of ~414–588 Myr are closer to the timing of break-up of the Rodinia supercontinent as well as coincide with the Ediacaran-Cambrian Malagasy orogeny. 40Ar/39Ar dating of three mineral separates (amphibole and biotite) from the lamprophyres gave precise plateau ages of 65.9 ± 0.3 Myr to 66.1 ± 0.4 Myr demonstrating that the emplacement of the Mt. Girnar igneous complex was close to the Cretaceous-Paleogene boundary. The emplacement of a range (110 Myr to 68.5 Myr) of other spatially related alkaline as well as silicic plutonic complexes, such as the Mundwara, Sarnu-Dandali, and Barda complexes prior to the main flood basalt event at ca. 66.0–65.1 Myr, highlights the role of extensional events in pre-existing rift/fault zones preceding the plume-lithosphere interaction in the Deccan LIP.