New Zealand Journal of Geology and Geophysics abstracts
Basement gabbro from the Lord Howe Rise
N. Mortimer
Institute of Geological & Nuclear Sciences
Private Bag 1930
Dunedin, New Zealand
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.
Keywords Lord Howe Rise; gabbro; xenolith; Gondwana;
rifting
G03043; Received 22 April 2003; revised 24 December 2003; Online publication
date 7 September 2004
New Zealand Journal of Geology & Geophysics, 2004, Vol. 47: 501-507
0028-8306/04/4703-0501 © The Royal Society of New Zealand 2004
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