Geochemical Evolution of Intraplate Volcanism at Banks Peninsula, New Zealand: Interaction Between Asthenospheric and Lithospheric Melts

• Published in: Journal of petrology 2009-06, Vol.50 (6), p.989-1023
• Main Authors: Timm, Christian, Hoernle, Kaj, Van Den Bogaard, Paul, Bindeman, Ilya, Weaver, Steve
• Format: Article
• Language: English
• Subjects: 40Ar/39Ar dating; intraplate volcanism; lithospheric detachment/delamination; major and trace element and Sr–Nd–Pb–Hf–O isotope geochemistry; peridotite and pyroxenite melting
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/egp029

Intraplate volcanism was widespread and occurred continuously throughout the Cenozoic on the New Zealand micro-continent, Zealandia, forming two volcanic endmembers: (1) monogenetic volcanic fields; (2) composite shield volcanoes. The most prominent volcanic landforms on the South Island of New Zealand are the two composite shield volcanoes (Lyttelton and Akaroa) forming the Banks Peninsula. We present new 40Ar/39Ar age and geochemical (major and trace element and Sr–Nd–Pb–Hf–O isotope) data for these Miocene endmembers of intraplate volcanism. Although volcanism persisted for ∼7 Myr on Banks Peninsula, both shield volcanoes primarily formed over an ∼1 Myr interval with small volumes of late-stage volcanism continuing for ∼1·5 Myr after formation of the shields. Compared with normal Pacific mid-ocean ridge basalts (P-MORB), the low-silica (picritic to basanitic to alkali basaltic) Akaroa mafic volcanic rocks (9·4–6·8 Ma) have higher incompatible trace element concentrations and Sr and Pb isotope ratios but lower δ18O (4·6–4·9) and Nd and Hf isotope ratios than ocean island basalts (OIB) or high time-integrated U/Pb HIMU-type signatures, consistent with the presence of a hydrothermally altered recycled oceanic crustal component in their source. Elevated CaO, MnO and Cr contents in the HIMU-type low-silica lavas, however, point to a peridotitic rather than a pyroxenitic or eclogitic source. To explain the decoupling between major elements on the one hand and incompatible elements and isotopic compositions on the other, we propose that the upwelling asthenospheric source consists of carbonated eclogite in a peridotite matrix. Melts from carbonated eclogite generated at the base of the melt column metasomatized the surrounding peridotite before it crossed its solidus. Higher in the melt column the metasomatized peridotite melted to form the Akaroa low-silica melts. The older (12·3–10·4 Ma), high-silica (tholeiitic to alkali basaltic) Lyttelton mafic volcanic rocks have low CaO, MnO and Cr abundances suggesting that they were at least partially derived from a source with residual pyroxenite. They also have lower incompatible element abundances, higher fluid-mobile to fluid-immobile trace element ratios, higher δ18O, and more radiogenic Sr but less radiogenic Pb–Nd–Hf isotopic compositions than the Akaroa volcanic rocks and display enriched (EMII-type) trace element and isotopic compositions. Mixing of asthenospheric (Akaroa-type) melts with lithospheric melts f