A Sr, Nd, and Pb isotope study of alkaline lamprophyres and related rocks from Westland and Otago, South Island, New Zealand

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Abstract
The Westland dike swarm of South Island, New Zealand, consists of a variety of ultrabasic, alkaline rock types including camptonite lamprophyres, ouachitite peridotites, and carbonatites. At least part of the swarm is as young as Oligocene-Miocene age. The majority of Nd and Sr isotope analyses for 16 dikes εNd = +3.5 to +5.2, 87Sr/86Sri = 0.7028 to 0.7035) fall to the left of the mantle array in Nd-Sr isotope correlation diagram. The data are similar to the ocean islands St. Helena and Tubuai and to some continental volcanics and mantle nodules (Menzies and Wass, 1983). The dikes contain radiogenic Pb (206Pb/204Pb = 19.19 to 20.59, 207Pb/204Pb = 15.64 to 15.71, 208Pb/204Pb = 39.0 to 40.2), which also is similar to the most radiogenic Pb in ocean islands. The trace element and isotope data are consistent with an origin involving a Paleozoic enrichment event, which added CO2, Sr, light rare-earth elements, and U to a depleted mantle. We propose that this first event was the manifestation of a deep-seated intrusion of carbonated hyperalkaline magma. A second event triggered the melting of this newly metasomatized mantle and the emplacement of the Westland dikes.

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