Diabasic intrusion and lavas, segregation veins, and magma differentiation at Kahoolawe volcano, Hawaii

摘要

A mafic sill-like intrusion, ～5 × 30 m, exposed along the eastern shoreline of Kahoolawe Island, Hawaii, represents tholeiitic magma emplaced as diabase among caldera-filling lavas. It differentiated from ～7.8 wt.% MgO to yield low-MgO (2.9 wt.%) vesicular segregation veins. We examined the intrusion for whole-rock and mineral compositions for comparison to Kahoolawe caldera-fill lavas (some also diabasic), to the Uwekahuna laccolith (Kilauea), and to gabbros, diabases, and segregations and oozes of other tholeiitic shield volcanoes (e.g., Mauna Loa and Kilauea lava lakes). We also evaluate this extreme differentiation in terms of MELTS modeling, using parameters appropriate for Hawaiian crystallization environments. Kahoolawe intrusion diabase samples have major and trace element abundances and plagioclase, pyroxene, and olivine compositions in agreement with those in gabbros and diabases of other volcanoes. However, the intrusion samples are at the low-MgO end of the large MgO range formed by the collective comparative samples, as many of those have between 8 and 20 wt.% MgO. The intrusion's segregation vein has SiO_2 53.4 wt.%, TiO_2 3.2 wt.%, FeO 13.5 wt.%, Zr 350 ppm, and La 16 ppm. It plots in compositional fields formed by other Hawaiian segregations and oozes that have MgO <5 wt.%-fields that show large variances, such as factor of ~2 differences for incompatible element abundances accompanying SiO_2 from ～49 to 59 wt.%.