Petrogenesis of amphibole megacrysts in lamprophyric intraplate magmatism in southern New Zealand

Sr-Nd-Hf-Pb isotopes and major/trace element data are presented for amphibole megacrysts from two separate instances of lamprophyric intraplate magmatism in Zealandia (Cretaceous Westland Dike Swarm and Cenozoic Alpine Dike Swarm). Megacrysts from both regions have comparable geochemical and isotopic signatures (⁸⁷Sr/⁸⁶Sr_(i) = 0.702754–0.703371, εNd_(i) = +3.9 to +5.5, εHf_(i) = +5.0 to +8.2, ²⁰⁶Pb/²⁰⁴Pb_(i) = 18.909–20.526, ²⁰⁷Pb/²⁰⁴Pb_(i) = 15.642–15.678, ²⁰⁸Pb/²⁰⁴Pb_(i) = 38.824–40.271), regardless of them having traversed contrasting crustal terranes at different times and tectonic settings. Several megacryst cores may represent early-crystallised phenocrysts, whereas rims crystallised in equilibrium with amphibole microphenocrysts during final emplacement of host melts. Strontium isotopic offsets between megacrysts and host rocks indicate secondary hydrothermal alteration in some lamprophyres. Correlations between host rock–megacryst geochemistry as well as evidence for amphibole fractionation in Zealandia lamprophyres indicate that megacrysts likely crystallised from melts similar to their hosts. Isotopic (Pb, Nd) and Mg# offsets suggest a non-cognate relationship and reflect interaction between isotopically heterogeneous lamprophyric melts and/or cumulate amphibole in the lower crust. Modelling suggests that most alkaline intraplate magmatism throughout Zealandia may be derived from moderately heterogeneous SCLM sources that experienced localised varying degrees of subduction-related carbonatitic metasomatism between the Jurassic to Middle Cretaceous during Gondwana break-up.