Variable sources for Cretaceous to recent HIMU and HIMU-like intraplate magmatism in New Zealand

Abstract

Continental intraplate magmas with isotopic affinities similar to HIMU are identified worldwide. Involvement of an asthenospheric HIMU or HIMU-like source is contested because the characteristic radiogenic Pb compositions coupled with unradiogenic Sr and intermediate Nd and Hf compositions can also result from in-situ ingrowth in metasomatised lithospheric mantle. Sr–Nd–Pb–Hf isotopic compositions of late Cretaceous lamprophyre dikes from Westland, New Zealand, provide new insights into the formation of a HIMU-like alkaline intraplate magmatic province under the Zealandia continent. The oldest (102–100 Ma) calc-alkaline lamprophyres are compositionally similar to the preceding arc-magmatism (206Pb/204Pb(i) = 18.6, 207Pb/204Pb(i) = 15.62, 208Pb/204Pb(i) = 38.6, 87Sr/86Sr(i) = 0.7063–0.7074, εNd(i) = −2.1 − +0.1 and εHf(i) = −0.2 − +2.3) and are interpreted as melts originating from subduction-modified lithosphere. Alkaline dikes erupted on the inboard Gondwana margin shortly after cessation of subduction (92–84 Ma) have heterogeneous isotopic properties: 206Pb/204Pb(i) = 18.7 to 19.4, 207Pb/204Pb(i) = 15.60 to 15.65, 208Pb/204Pb(i) = 38.6 to 39.4, 87Sr/86Sr(i) = 0.7031 to 0.7068, εNd(i) = +4.5 to +8.0 and εHf(i) = +5.1 to +8.0. Melt compositions point to an amphibole-bearing spinel facies lithospheric mantle source enriched by metasomatism that introduced, amongst many elements, U + Th which lead to rapid ingrowth to HIMU-like compositions. Importantly, this HIMU-like source enrichment appears to have completely originated from the complex local subduction history. A coeval episode of alkaline magmatism (mainly 98–82 Ma) occurred outboard of Gondwana's former active margin and on the Hikurangi oceanic plateau (accreted to Zealandia in the Early Cretaceous) with compositions closer to true HIMU (206Pb/204Pb(i) ≈ 20.5, 207Pb/204Pb(i) ≈ 15.7, 208Pb/204Pb(i) ≈ 40.0, εNd(i) ≈ 4.5 and εHf(i) ≈ 4.0). In contrast to the inboard HIMU-like magmas, the radiogenic 207Pb/204Pb and relatively unradiogenic Nd and Hf require an ancient enriched source component. This magmatism is interpreted to represent melting of a fossilised HIMU source that resided under the Hikurangi Plateau. These genetically distinct but isotopically similar intraplate reservoirs were separated by the down-going slab under Gondwana's former active margin. Ancient HIMU magmatism was locally replaced by the young HIMU-like type which became dominant across Zealandia during the Late Cretaceous. Our research suggests that the sources for alkaline intraplate magmas with compositions similar to ocean island basalts can be formed either with or without the involvement of a plume-derived component.