Alkaline-ultramafic lamprophyre dykes from the Vestfold Hills, Princess Elizabeth Land (East Antarctica): primitive magmas of deep mantle origin

Abstract

Alkaline dykes tentatively dated at ∼1.3 Ga cut the Vestfold Hills in a consistent N–S to N15°E direction. They form a spectrum between more abundant ultramafic lamprophyres (UML) corresponding broadly to H₂O–CO₂-rich nephelinites, and alkaline lamprophyres (AL), representing H₂O–CO₂-rich basanites. Olivine (Fo_46–93, averaging Fo₇₅) is abundant only in the UML, but both types carry primary diopsidic clinopyroxene with complex zoning; amphibole (pargasite, hastingsite, kaersutite with up to 8.6% TiO₂); titanian phlogopite (up to 10% TiO₂); feldspars (orthoclase, anorthoclase, albite and andesine), nepheline (K-poor and Si-rich), ilmenite (up to 1% MgO and MnO), chrome titanomagnetite, and carbonate (magnesian calcite, ferroan dolomite, breunnerite). Lamprophyric peculiarities include the local coexistence of three feldspars, extremely Ti-rich amphiboles and micas, and the presence of globular structures and possibly primary carbonates. Some dykes carry small but abundant lherzolite xenoliths, others carry chromian diopside (1% Cr₂O₃) and En_58–76 orthopyroxene xenocrysts. The dykes represent primitive, mantle-derived magmas which have undergone varying but generally low degrees of polybaric fractionation, together perhaps with mixing of more primitive and fractionated batches, during their ascent through the crust.