The Geochemistry and Sm–Nd Isotopic Systematics of Precambrian Mafic Dykes and Sills in the Southern Prince Charles Mountains, East Antarctica

Mafic dykes are a characteristic geological feature of the Ruker Complex in the southern Prince Charles Mountains. We present new geological, geochemical, and Sm–Nd isotopic data for these rocks, which place constraints on their genesis, mantle sources, and relative timing, as well as providing a substantial basis for a comparison between these intrusions and similar rocks from other Archaean terranes of East Antarctica. The oldest of the intrusions form dykes composed of distinctive rocks rich in Mg, Cr, and Ni. These rocks were probably derived via varying degrees of partial melting of a metasomatically enriched and radiogenic mantle source at relatively shallow mantle levels. Younger and volumetrically more significant dykes are composed of subalkaline tholeiite. These tholeiitic dykes strike ENE, NW or NNE and can be divided into two geochemically distinct subgroups: a low-LILE (large ion lithophile element) group (mostly the NNE-trending dykes) and a high-LILE group (NW-trending and partly other directions). The low-LILE group rocks originated from an enriched mid-ocean ridge basalt-like source and are characterized by relatively young Sm–Nd TDM model ages between 1·9 and 2·4 Ma. The high-LILE group rocks have higher LILE/HFSE (high field strength element) ratios, which imply derivation from an enriched subcontinental mantle source. These rocks have generally older TDM model ages between 2·3 and 3·8 Ga and display geochemical similarities to mafic metavolcanic rocks found within the Palaeoproterozoic Ruker Group, a cover sequence to the Archaean Ruker Complex. As was observed by earlier researchers, the subalkaline tholeiitic dykes in the Ruker Complex have many compositional features in common with mafic dykes from the Napier Complex of Enderby Land and from the Vestfold Hills. However, our data do not provide convincing evidence for a direct correlation with these dyke suites. The youngest phase of mafic intrusions comprises distinctive high-Ti–P rocks that occur as sills within the Neoproterozoic Sodruzhestvo Group, another cover sequence to the Ruker Complex. The high-Ti–P rocks do not correlate with any of the dyke suites observed within the Ruker Complex, nor from elsewhere in East Antarctica. They are interpreted to represent manifestations of plume-related magmatism associated with extension, subsidence, and accumulation of the Sodruzhestvo Group.