Mantle plumes and Antarctica-New Zealand rifting: evidence from mid-Cretaceous mafic dykes

Ocean floor magnetic anomalies show that New Zealand was the last continent al fragment to separate from Antarctica during Gondwana break-up, drifting from Marie Byrd Land, West Antarctica, about 84 Ma ago. Prior to continenta l drift, a voluminous suite of mafic dykes (dated by Ar-Ar laser stepped he ating at 107 +/- 5 Ma) and anorogenic silicic rocks, including syenites and peralkaline granitoids (95-102 Ma), were emplaced in Marie Byrd Land durin g a rifting event. The mafic dyke suite includes both high- and low-Ti basa lts. Trace element and Sr and Nd isotope compositions of the mafic dykes ma y be modelled by mixing between tholeiitic OIB (asthenosphere-derived) and alkaline high- to low-Ti alkaline magmas (lithospheric mantle derived). Pb isotopes indicate that the OIB component had a HIMU composition. We suggest that the rift-related magmatism was generated in the vicinity of a mantle plume. The plume helped to control the position of continental se paration within the very wide region of continental extension that develope d when the Pacific-Phoenix spreading ridge approached the subduction zone. Separation of New Zealand from Antarctica occurred when the Pacific-Phoenix spreading centre propagated into the Antarctic continent. Sea floor spread ing in the region of the mantle plume may have caused an outburst of volcan ism along the spreading ridge generating an oceanic plateau, now represente d by the 10-15 km thick Hikurangi Plateau situated alongside the Chatham Ri se, New Zealand. The plateau consists of tholeiitic OIB-MORB basalt, regard ed as Cretaceous in age, and similar in composition to the putative tholeii tic end-member in the Marie Byrd Land dykes. The mantle plume is proposed t o now underlie the western Ross Sea, centred beneath Mount Erebus, where it was largely responsible for the very voluminous, intraplate, alkaline McMu rdo Volcanic Group. A second mantle plume beneath Marie Byrd Land formed th e Cenozoic alkaline volcanic province.