THE ORIGIN OF HIMU IN THE SW PACIFIC: EVIDENCE FROM INTRAPLATE VOLCANISM IN SOUTHERN NEW ZEALAND

     

This paper presents field, geochemical and isotopic (Sr, Nd, Pb) results on basalts from the Antipodes, Campbell and Chatham Islands, New Zealand.  New ⁴⁰Ar/³⁹Ar age determinations along with previous K-Ar dates reveal three major episodes of volcanic activity on Chatham Island (85-82, 41-35, ~5 Ma).  Chatham and Antipodes samples comprise basanite, alkali and transitional basalts that have HIMU-like isotopic (²⁰⁶Pb/²⁰⁴Pb >20.3-20.8, ⁸⁷Sr/⁸⁶Sr <0.7033, ¹⁴³Nd/¹⁴⁴Nd >0.5128) and trace element affinities (Ce/Pb 28-36, Nb/U 34-66, Ba/Nb 4-7).  The geochemistry of transitional to Q-normative samples from Campbell Island is explained by interaction with continental crust.  The volcanism is part of a long-lived (~100 Myr), low-volume, diffuse alkaline magmatic province that includes deposits on the North and South Islands as well as portions of West Antarctica and SE Australia.  All of the continental areas were juxtaposed on the eastern margin of Gondwanaland at >83 Ma.  A ubiquitous feature of mafic alkaline rocks from this region is their depletion in K and Pb relative to other highly incompatible elements when normalized to primitive mantle values.  The inversion of trace element data indicates enriched mantle sources that contain variable proportions of hydrous minerals.  We propose that the mantle sources represent continental lithosphere that host amphibole/phlogopite-rich veins formed by plume and/or subduction related metasomatism between 500 and 100 Ma.  The strong HIMU signature (²⁰⁶Pb/²⁰⁴Pb >20.5) is considered to be an in-grown feature generated by partial-dehydration and loss of hydrophile elements (Pb, Rb, K) relative to more magmaphile elements (Th, U, Sr) during short-term storage at the base of the lithosphere.