Volume 38: January - December 1997

Issue 5: May 1997

Abstract

A view into the subsurface of Mauna Kea volcano, Hawaii: crystallization processes interpreted through the petrology and petrography of gabbroic and ultramafic xenoliths

RV. Fodor and P. Galar Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695, USA
ABSTRACT
Xenoliths from the southern flank of Mauna Kea volcano form two broad categories. (1) Ultramafic: porphyroclastic dunite, wehrlite, and olivine clinopyroxenite (Fo89.4-83.6, clinopyroxene mg-number 90.3-86.3, spinel mg-number 57-42, spinel cr-number 7-52, no palgioclase); and granular wehrlite and olivine clinopyroxenite (Fo83-76) with plagioclase (An84-69) +- orthopyroxene, and Cr-magnetite. (2) Gabbroic: granular gabbro, gabbronorite, and troctolite composed of olivine + clinopyroxene frameworks (Fo82-74, mg-number 85-79) enclosing plagioclase (An79-69) +-orthopyroxene, and Fe-Ti oxides; and plagioclase (<An77) forming frameworks for, and fine-grained mosaics with, evolved olivine (Fo75-61), clinopyroxene+-orthopyroxene, and Fe-Ti oxides. Most xenoliths are petrographically uniform, but some manifest modal, phase, cryptic, or grain-size layering, and some are composites of two rock types. Whole-xenolith incompatible elements are 'depleted', and there are positive Eu anomalies; ⁸⁷Sr/⁸⁶Sr is 0.70360, and mineral [delta]¹⁸O is 4.05-5.62. Porphyroclastic ultramafic xenoliths are gravity-settled and in situ cumulates from reservoir bottoms. Plagioclase-bearing xenoliths represent modal, phase, and cryptic layering (e.g. wehrlite to gabbronorite) in reservoir-margin solidification zones superimposed with small-scale (centimeter) modal, cryptic, phase, and grain-size layering. Mineral compositions point to tholeiitic parentage for most xenoliths, but alkalic for some (e.g. clinopyroxene (Al2O3 >4 wt %). These Mauna Kea xenoliths are plutonic complements to postshield lavas (Hamakua volcanics), and they identify that stage of volcano development with 15-5 wt % MgO magmas that underwent processes intrinsic to mafic-layered intrusions; e.g. in situ and gravity-settled crystallization, extensive differentiation, varieties of layering, mobilizations of late-stage, evolved liquids, compaction and connective disturbances in reservoirs.

Keywords: Gabbroic xenoliths; ultramafic xenoliths; cumulate; Hawaii petrology; in situ crystallization; gravity settling; Mauna Kea volcano

Pages: 581 - 624

Part of the OUP Journal of Petrology WWW service

General Information

Click here to register with OUP.

This page is maintained by OUP admin

Last updated 13 May 97

Part of the OUP Journals World Wide Web service.