Boulders of shonkinite, syenodiorite, and nepheline monzoni te occur on Kauai, Hawaii. Boulders of shonkinite and syenodiorite outcrop 1 mile north of Kalaheo and nepheline monzonite boulders have been found in Wailooa Canyon along Koaie and Waialae streams. Detailed petrographic and bulk-rock and mineral geochemical study of samples collected from each locality suggest crystallization in supercooled nephelinic magmas. Shonkinite probably formed after a moderate degree of supercooling, syenodiorite after a high degree of supercooling, and nepheline monzonite after minimal supercooling. Shonkinite, syenodiorite, and nepheline monzonite probably crystallized in hypabyssal magma chambers beneath vents for Koloa Volcanic Group lavas.
Shonkinite formed as a result of single stage crystallization of supercooled magma. Initial homogeneous nucleation of phenocrysts was followed by heterogeneous nucleation of groundmass minerals.
Two stage origin of syenodiorite is suggested: (1) initial rapid rise of magma, incorporation of peridotite nodules, and crystallization of the equigranular border zone of the syenodiorite around the nodules, followed by (2) supercooling of volatile rich magma and rapid crystallization of the central porphyritic zone of the syenodiorite. Primary oxidation of mineral components is suggested. Deep seated origin of the peridotite nodules embedded in syenodiorite is suggested by mineral geochemistry, brecciated and recrystallized texture, and similarity to nodules postulated to have upper mantle sources. Reduction in degree of supercooling during crystallization probably resulted in the large zoned mineral grains in the nepheline manzonite. Crystal settling may have resulted in a zoned intrusive body which has not ·yet been located.
Earth and Planetary Sciences
Level of Degree
Department of Earth and Planetary Sciences
First Committee Member (Chair)
Second Committee Member
Albert Masakiyo Kudo
Third Committee Member
Wolfgang Eugene Elston
Brown, Henry Gassaway IV. "Petrology And Geochemistry Of Shonkinite, Syenodiorite, And Nepheline Monzonite Of The Island Of Kauai, Hawaii." (1977). https://digitalrepository.unm.edu/eps_etds/309