Garnet pyroxenite cumulates from Cretaceous alkaline intraplate magmas underplate the Zealandia mantle lithosphere

Abstract

Abstract The elemental and isotopic properties of garnet pyroxenites can yield information on lithospheric mantle composition, thermal state, and evolution. The 34 Ma Kakanui Mineral Breccia in New Zealand contains spectacular but little-studied mantle peridotite and pyroxenite xenoliths that yield new insights into the evolution of a portion of the underlying mantle lithosphere. The moderately depleted and metasomatised spinel peridotites, as judged from spinel and olivine compositions and bulk rock major and platinum group element abundances, give mineral equilibration temperatures < 1020oC and are derived from the middle to shallow (~35 to 50 km) lithospheric mantle when projected on to a 70 mW m-2 geotherm. These residues have low Re/Os and Re-depletion 187Os/188Os model ages that range from Eocene (0.05 Ga) to Paleoproterozoic (1.9 Ga), consistent with extraction from a lithospheric mantle comprising fragments with complex depletion histories. Although the peridotites have restricted δ18O (olivine +5.2 to 6.2), evidence for an isotopically heterogeneous mantle column in addition to the 187Os/188Os is seen in clinopyroxene 87Sr/86Sr (0.70244 to 0.70292), εNd (+4.1 to 18.8), 206Pb/204Pb (17.8 to 20.3) and εHf (+10 to +101). Higher metamorphic equilibrium temperatures of the garnet pyroxenites (Fe-Mg exchange of >1150oC) compared to the peridotites indicate their Eocene extraction was from towards the base of this isotopically heterogeneous mantle lithosphere. Pyroxenite bulk compositions point to cumulate origins, and the mineral isotope ratios of 87Sr/86 Sr (0.70282 to 0.70294), εNd (+5.5 to 8.0) and 206Pb/204Pb (18.1 to 19.3) match many of metasomatized mantle peridotite xenoliths as well as the Zealandia primitive intraplate basalts although not the host magmas. In contrast to many global pyroxenite studies, the garnet pyroxenite 87Sr/86Sr and δ18O (+5.2 to 5.8) data provide no evidence for subducted crustal material in the primary magma source region and Sm-Nd and Lu-Hf isotope data yield mid-Cenozoic ages that are probably related to isotope closure during eruption. An exception is one sample that yields a Lu-Hf isochron age of 111.9 + 9.1 Ma, which corresponds to the convergence of the Lu-Hf isotope evolution curves of three other samples. Liquids calculated to have been in equilibrium with these cumulates have trace element compositions comparable to primitive alkaline intraplate basalts like those found at the surface of Zealandia. The new data therefore reveal that a pulse of intraplate magmatism occurred during or directly after the cessation of long-lived subduction on the former Zealandia Early Cretaceous forearc Gondwana margin, despite any surface exposure having been long eroded away. The lower lithospheric mantle emplacement of the garnet pyroxenites suggests that the source of the alkaline parent magmas was probably the convecting mantle, which supports conclusions that intraplate magmas in Zealandia have asthenospheric and lithospheric mantle sources.