Dinesh Pandit

Chrome-diopside Megacryst-bearing Lamprophyre from the Late Cretaceous Mundwara Alkaline Complex, NW India: Petrological and Geodynamic Implications

The occurrence of a rare mantle-derived chrome-diopside megacryst (∼8 mm), containing inclusions of olivine, in a lamprophyre dyke from the late Cretaceous polychronous (∼100 – 68 Ma) Mundwara alkaline complex of NW India is reported. The olivine inclusions are forsteritic (Fo: 85.23) in composition, and their NiO (0.09 wt%) and CaO (0.13 wt%) contents imply derivation from a peridotitic mantle source. The composition of the chrome diopside (Cr2O3: 0.93 wt ) (Wo45.27 En48.47 Fs5.07 and Ac1.18) megacryst is comparable to that occurring in the garnet peridotite xenoliths found in diamondiferous kimberlites from Archaean cratons. Single pyroxene thermobarometry revealed that this chrome diopside megacryst was derived from a depth range of ∼100 km, which is relatively much deeper than that of the chrome-diopside megacrysts (∼40–50 km) reported in spinellherzolite xenoliths from the alkali basalts of Deccan age (ca. 66–67 Ma) from the Kutch, NW India. This study highlights that pre- Deccan lithosphere, below the Mundwara alkaline complex, was at least ∼100 km thick and, likely, similar in composition to that of the cratonic lithosphere.

Cobaltoan pyrite in a lamprophyre from the Sidhi Gneissic complex, Mahakoshal belt, Central India

Pyrite containing appreciable content of cobalt (up to 5.61 wt%) and minor amounts of nickel (up to 0.70 wt%) is being reported from electron microprobe studies in a lamprophyre dyke from the Sidhi complex, Mahakoshal belt, Central India. However, cobalt and nickel are conspicuously absent from the associated chalcopyrite reflecting that in a pyrite-chalcopyrite association these elements are preferentially sequestered in pyrite. The microprobe analysis of pyrite grains reveal that their Fe content is variable (40.62 to 46.02 wt%) and reciprocally related to cobalt and nickel concentrations implying the presence of latter as solid solution in the structure of pyrite. The cobalt contents of the pyrite are comparable to those (up to 3.21 wt%) reported from the cobaltbearing pyrites of the Kalyadi copper deposits of Dharwar craton of southern India. Co:Ni (16.05 average) in the studied pyrite appears to be more consistent with a magmatic, rather than a hydrothermal, origin. However, further studies are in progess. This study highlights that besides gold and diamond, lamprophyres are also important carriers of cobalt when available in the magmatic system.