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

Abstract

ABSTRACT 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 spinel-lherzolite 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.