Geology of southern White Island, Antarctica

Adam, Lotte J.

White Island is a glaciated volcanic island in the western Ross Embayment of the Ross Ice Shelf, Antarctica. The interests of this research are the extrusive volcanic rocks, shallow intrusive rocks, deep crusta! xenoliths, and pyroclastic deposits that characterise the southern half of White Island. The flow rocks, dykes, and plugs of southern White Island comprise predominant basanite and minor tephriphonolite. The basanites contain phenocrysts of olivine and titanaugite, and variable abundances of kaersutite, aegirine, magnetite, apatite and plagioclase xenocrysts in a fine grained groundmass. The tephriphonolite comprises phenocrysts and xenocrysts of kaersutite and microphenocrysts of aegirine-augite and plagioclase in a sideromelane groundmass. The rocks of the southern White Island suite are related by fractional crystallisation involving the early removal of olivine and titanaugite and late fractionation of magnetite. Trends are comparable to those from alkali basaltic suites of Antarctica, and to geochemical trends of the Dunedin Volcanic Province, New Zealand. Two basanites are not considered to be related by fractional crystallisation, and originate from different degrees of partial melting of the mantle. Lower crusta! xenoliths are found in the aforementioned basanitic flow rocks, and are defined petrographically into three types: Type One xenoliths, which are unmetamorphosed gabbro; Type Two xenoliths, which are partially recrystallised granulites displaying a mortar texture; and Type Three xenoliths, which are fully recrystallised granulites. Features of their whole rock geochemistry indicate these xenoliths originated as cumulates from an evolving alkaline magma. Crusta! xenoliths do not represent cumulates of their host magmas; however, they may originate from alkaline magmas associated with rift-related volcanism of the McMurdo Volcanic Group. Following igneous crystallisation, these rocks were variably recrystallised under granulite facies metamorphic conditions. Secondary fluid inclusions provide evidence of fluxes of C02- rich fluids both preceding and following recrystallisation. This fluid flux may be related to the crystallisation of metasomatic kaersutite and biotite, which is shown by whole rock geochemistry to occur by the infiltration of a volatile-rich fluid rather than an alkaline melt. Mineral chemistry shows the Type One xenolith analysed does not represent the unmetamorphosed protolith of the Type Two and Three xenoliths analysed; however, this does not necessarily preclude such a relationship between other xenoliths. The Type Two and Three xenoliths analysed are related by different degrees of recrystallisation of the same protolith. Lapilli tuffs rich in basanitic accidental fragments comprise two of the peaks in the field area, and were formed during phreatomagmatic eruption, and subsequently transported and deposited by a series of hydroclastic base surges. Subordinate tuff occurs in one location, and is thought to also have originated during phreatomagmatic eruption. Spatter lapillistone comprising several small outcrops was deposited during Hawaiian or Strombolian eruptions, throughout which quenched juvenile ejecta impacted onto a solid substrate to form welded deposits. Ejecta from the centre of eruption columns retained sufficient heat to coalesce upon landing, forming spatter-fed lava flows. Keywords: White Island, Antarctica, basanite, tephriphonolite, fractional crystallisation, lower crustal xenolith, cumulate, lapilli tuff, spatter, phreatomagmatic, base surge.

156 leaves : col. ill., maps (1 folded) ; 30 cm. + 1 folded map in pocket.

2004Adam

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