Basement gabbro from the Lord Howe Rise

Abstract In 1975 a French expedition dredged a seamount on the Lord Howe Rise and recovered basalt with gabbro xenoliths. New analytical work indicates that the basalt is a strongly alkaline lava, similar to Neogene intraplate basalts that are widespread across Zealandia. The gabbro is a cumulate leucogabbro containing augite, plagioclase, titanomagnetite and, in some parts, rare olivine (now entirely pseudomorphed by clay). Whole rock and clinopyroxene element concentrations and ratios indicate that the gabbro is weakly alkaline to transitional in composition and is not petrologically related to the basalt. Attempts to date the gabbro by the Rb‐Sr method have been unsuccessful but clinopyroxene has an initial 87Sr/86Sr of c. 0.7036, that varies little with age. Whole rock and mineral chemistry, and Sr, Nd, and Pb isotopic ratios support an origin for the gabbro as part of an intraplate mafic pluton (of probable Late Cretaceous age), rather than as a continuation of the Devonian‐Cretaceous convergent margin Median‐New England Batholith.

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