Journal of Petrology Advance Access originally published online on May 4, 2006
Journal of Petrology 2006 47(9):1673-1704; doi:10.1093/petrology/egl024
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The Origin of HIMU in the SW Pacific: Evidence from Intraplate Volcanism in Southern New Zealand and Subantarctic Islands
1 DEPARTMENT OF GEOLOGY, BOWLING GREEN STATE UNIVERSITY BOWLING GREEN, OH 43403, USA
2 DEPARTMENT OF GEOLOGY & GEOPHYSICS, WOODS HOLE OCEANOGRAPHIC INSTITUTION WOODS HOLE, MA 02543, USA
3 DEPARTMENT OF EARTH & ENVIRONMENTAL SCIENCE, NEW MEXICO INSTITUTE OF MINING & TECHNOLOGY SOCORRO, NM 87801, USA
RECEIVED JULY 28, 2005; ACCEPTED MARCH 29, 2006
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 of New Zealand as well as portions of West Antarctica and SE Australia. All of these 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.
KEY WORDS: continental alkaline basalts; lithospheric mantle, mantle metasomatism; New Zealand; OIB, HIMU; Sr, Nd and Pb isotopes; West Antarctica
*Corresponding author. Telephone: (1) 419-372-7337. Fax: (1) 419-372-7205. E-mail: kpanter{at}bgsu.edu
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