Leucocratic and gabbroic xenoliths from Hualalai volcano, Hawai'i
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A diverse range of crustal xenoliths is hosted in young alkali basalt lavas and scoria deposits (erupted 3-5 ka) at the summit of Huallai. Leucocratic xenoliths, including monzodiorites, diorites and syenogabbros, are distinctive among Hawaiian plutonic rocks in having alkali feldspar, apatite, zircon and biotite, and evolved mineral compositions (e.g. albitic feldspar, clinopyroxene Mg-number 67-78). Fine-grained diorites and monzodiorites are plutonic equivalents of mugearite lavas, which are unknown at Huallai. These xenoliths appear to represent melt compositions falling along a liquid line of descent leading to trachyte - a magma type which erupted from Huallai as a prodigious lava flow and scoria cone at 114 ka. Inferred fractionating assemblages, MELTS modeling, pyroxene geobarometry and whole-rock norms all point to formation of the parent rocks of the leucocratic xenoliths at 3-7 kbar pressure. This depth constraint on xenolith formation, coupled with a demonstrated affinity to hypersthene-normative basalt and petrologic links between the xenoliths and the trachyte, suggests that the shift from shield to post-shield magmatism at Huallai was accompanied by significant deepening of the active magma reservoir and a gradual transition from tholeiitic to alkalic magmas. Subsequent differentiation of transitional basalts by fractional crystallization was apparently both extreme - culminating in >5.5 km3 of trachyte - and rapid, at 2.75 106 m3 fmagma crystallized/year. 2006 Oxford University Press.
