159 Ma Kjakebeinet lamproites (Dronning Maud Land, Antarctica) and their implications for Gondwana breakup processes

Abstract

Lamproite dykes that cross-cut Middle Jurassic Karoo-related flood basalts in Vestfjella, Dronning Maud Land, Antarctica, have been dated at 158.7±1.6 Ma by ⁴⁰Ar/³⁹ Ar incremental heating of phlogopite. Compared to most lamproites, these dykes are unusually low in SiO₂ (40.09–40.18 wt%) and high in CaO (12.35–16.24 wt%) and P₂O₅ (3.03–3.81 wt%). They have broad affinities to spatially and temporally related group II kimberlites of southern Africa, but the geochemically closest correlatives are CaO- and P₂O₅-rich diopside madupitic lamproites from Leucite Hills, USA. The Kjakebeinet lamproites are notably high in incompatible elements, show negative initial ε_Nd values (−6.0, −6.7) and positive initial ε_Sr values (+14, +17) with T_DM model ages of c. 1.1 Ga, and probably record melting of metasomatized subcontinental lithospheric mantle beneath western Dronning Maud Land. Their emplacement records the youngest recognized magmatic event in the region, coincides with the proposed second and final stage of Gondwana breakup between Africa and Antarctica, and may be associated with deep-seated coast-parallel strike-slip faulting in Dronning Maud Land. Survival of old, easily fusible lithospheric mantle material 20 Ma after the eruption of voluminous flood basalt magmas suggests that the latter were emplaced within narrow zones of lithospheric thinning and that the source of lamproites remained largely intact between these zones.