Leucocratic and Gabbroic Xenoliths from Hualalai Volcano, Hawai'i |
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| Authors: | SHAMBERGER PATRICK J; HAMMER JULIA E |
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| Institution: | DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF HAWAII 1680 EAST–WEST RD, HONOLULU, HI 96822, USA |
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| Abstract: | A diverse range of crustal xenoliths is hosted in young alkalibasalt lavas and scoria deposits (erupted  3–5 ka) at thesummit of Hual lai. Leucocratic xenoliths, including monzodiorites,diorites and syenogabbros, are distinctive among Hawaiian plutonicrocks in having alkali feldspar, apatite, zircon and biotite,and evolved mineral compositions (e.g. albitic feldspar, clinopyroxeneMg-number 67–78). Fine-grained diorites and monzodioritesare plutonic equivalents of mugearite lavas, which are unknownat Huallai. These xenoliths appear to represent melt compositionsfalling along a liquid line of descent leading to trachyte—amagma type which erupted from Huallai as a prodigious lava flowand scoria cone at 114 ka. Inferred fractionating assemblages,MELTS modeling, pyroxene geobarometry and whole-rock norms allpoint to formation of the parent rocks of the leucocratic xenolithsat 3–7 kbar pressure. This depth constraint on xenolithformation, coupled with a demonstrated affinity to hypersthene-normativebasalt and petrologic links between the xenoliths and the trachyte,suggests that the shift from shield to post-shield magmatismat Huallai was accompanied by significant deepening of the activemagma reservoir and a gradual transition from tholeiitic toalkalic magmas. Subsequent differentiation of transitional basaltsby fractional crystallization was apparently both extreme—culminatingin >5·5 km3 of trachyte—and rapid, at  2·75x 106 m3 magma crystallized/year. KEY WORDS: geothermobarometry; magma chamber; xenolith; cumulate; intensive parameters |
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| Keywords: | : geothermobarometry magma chamber xenolith cumulate intensive parameters |
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