The Vestfjella Dike Swarm, Western Dronning Maud Land, Antarctica: an Archive of Mantle Heterogeneity in the Karoo LIP

The Vestfjella dike swarm crosscuts Karoo-related flood basalts at the rifted continental margin of western Dronning Maud Land, Antarctica. The dikes range in age from ~180 Ma to ~150 Ma, include rocks that lack geochemical evidence of contamination, and thus provide insights into the evolution of Gondwana breakup magmatism and the mantle sources involved. The Vestfjella dike swarm is mainly composed of subalkaline tholeiitic picrites, basalts, and basaltic andesites (MgO 3-25 wt.%) that show affinities to low-Ti and high-Ti magmas (TiO2 1.2-4.8 wt.%). The initial epsilon Nd values vary from -12 to +8 and Nb/Nb* from 0.3 to 1.2, suggesting involvement of lithospheric and asthenospheric magma sources. Dikes with positive epsilon Nd values, Nb/Nb* >0.8, (Th/Ta)N <0.8 and Nb/Zr <0.06 show geochemical affinities to oceanic island picrites (e.g. Hawaii) and MORB. Based on their mildly enriched or depleted immobile incompatible element (IE) signatures, they were derived from depleted mantle and are designated as D-type dikes. Isotopic and immobile IE ratios of Karoo-related lavas and dikes from Antarctica and southern Africa define two principal trends which extend from the field of D-type dikes towards light REE- enriched, high-Zr and low-Zr end-members, possibly representing lithospheric mantle-derived (lamproite-like) and crust-derived contaminants, respectively. These variations are compatible with a broad 3-component mixing system for the Karoo magmas, with the D-type dikes representing a depleted mantle source component. The majority of D-type dikes exhibit marked positive Ba, K, and Sr anomalies and many show a distinctive positive V anomaly. These anomalies are remarkably similar to those of gabbroic cumulates; it is tempting to interpret this affinity as evidence for recycled oceanic lower crust in the mantle source. On the other hand, similar LILE-enrichments in modern mid-ocean ridge basalts in the southern Atlantic and southwestern Indian ridges have been ascribed to subduction-related enrichment of the upper mantle prior to Gondwana breakup. It is quite possible that both kinds of subduction-related sources have been involved in the petrogenesis of the Vestfjella dike swarm. In fact, the overall geochemical ranges of the D-type dikes can be modeled with binary mixing between depleted mantle plume and LILE-enriched upper mantle. A subset of the Vestfjella dikes is characterized by positive epsilon Nd values, Nb/Nb* >0.8, and (Th/Ta)N <0.7 combined with Nb/Zr >0.1; these E-type dikes show geochemical affinities to ocean island basalts and probably represent a sublithospheric enriched mantle source component. They plot apart from the main field of Karoo magmas in IE diagrams, however, implying that enriched mantle sources may have played a minor role in the Karoo LIP. Overall, geochronological and geochemical data for the Vestfjella dike swarm and the wider Karoo province indicate that the lithospheric overprinting of magmas decreased in the course of time. Depleted mantle sources, possibly influenced by deep and shallow recycling of subducted oceanic crust, were contemporaneously tapped during early (~180 Ma) and late (~165 Ma) stages of Karoo magmatism and may be linked to mantle heterogeneity recorded by modern oceanic lavas associated with Gondwana breakup.