A VIEW INTO THE SUBSURFACE OF MAUNA-KEA VOLCANO, HAWAII - CRYSTALLIZATION PROCESSES INTERPRETED THROUGH THE PETROLOGY AND PETROGRAPHY OF GABBROIC AND ULTRAMAFIC XENOLITHS

Xenoliths from the southern flank of Mauna Kea volcano form two broad categories. (1) Ultramafic: porphyroclastic dunite, wehrlite, and oliv ine clinopyroxenite (Fo(89.4-83.6), clinopyroxene mg-number 90.3-86.3, spinel mg-number 57-42, spinel cr-number 70-52, no plagioclase); and granular wehrlite and olivine clinopyroxenite (Fo(83-76)) with plagioc lase (An(84-69)) +/- orthopyroxene, and Cr-magnetite. (2) Gabbroic: gr anular gabbro, gabbronorite, and troctolite composed of olivine + clin opyroxene frameworks (Fo(82-74), mg-number 85-79) enclosing plagioclas e (similar to An(79-69)) +/- orthopyroxene, and Fe-Ti oxides; and plag ioclase (<An(77)) forming frameworks for, and fine-grained mosaics wit h, evolved olivine (Fo(75-61)), clinopyroxene +/- orthopyroxene, and F e-Ti oxides. Most xenoliths are petrographically uniform, but some man ifest modal, phase, cryptic, or grain-size layering, and some are comp osites of two rock types. Whole-xenolith incompatible elements are 'de pleted', and there are positive Eu anomalies; Sr-87/Sr-86 is 0.70360, and mineral delta(18)O is 4.05-5.62. Porphyroclastic ultramafic xenoli ths are gravity-settled and in situ cumulates from reservoir bottoms. Plagioclase-bearing xenoliths represent modal, phase and cryptic layer ing (e.g. wehrlite to gabbronorite) in reservoir-margin solidification zones superimposed with small-scale (centimeter) modal, cryptic, phas e and grain-size layering. Mineral compositions point to tholeiitic pa rentage for most xenoliths. but alkalic for some (e.g. clinopyroxene A l2O3 >4 wt %). These Mauna Kea xenoliths are plutonic complements to p ostshield lavas (Hamakua Volcanics), and they identify that stage of v olcano development with 15-5 wt % MgO magmas that underwent processes intrinsic to mafic-layered intrusions; e.g. in situ and gravity-settle d crystallization, extensive differentiation, varieties of layering, m obilizations of late-stage, evolved liquids, compaction and convective disturbances in reservoirs.