GEOCHEMISTRY AND PETROGENESIS OF THE REJUVENATED-STAGE LAVAS, AND STRUCTURE OF THE KOLEKOLE VOLCANIC CENTER, SOUTHWEST RIFT ZONE, EAST MAUI, HAWAII

The Kolekole volcanic center is located in East Maui on the southwest rift, a potentially weak and active extensional zone in the upper crust characterized by parallel fissures with splayed branching covered by lava flows. Kolekole is an asymmetric volcanic cone with steep slopes at the W and NW sides (55-40oW), while the E (25-15oE) and S slopes (35-15oS) are gentler. The N slope is flattened and disturbed. The flanks of Kolekole merge with adjacent cinder cones. The E and S slopes are characterized by composite lava flows agglutinated with spatter, ash and cinder. The central crater is a flattened bowl ponded with ankaramite lava, spatter and pyroclastic ejecta, with a basal zone of sub-columner, phenocryst-rich lava. This zone grades abruptly into a highly vescicular aa-type lava about 1-8 m thick at the northern end, associated with ropy, pahoehoe-type lava as well as ash, scoria, lapilli, and volcanic bombs. The pyroclastic materials often alternate with ankaramite lava, indicating a series of eruptions. The lavas are alkalic, and constitute the post-shield Kula series as well as the initial phase of the rejuvenated Hana series. The Hana lavas are mostly ankaramitic picro-basalts. They contain higher average MgO, and lower average K2O, Zr and Nb than the Hana lavas further southwest along the same rift zone. They are also chemically distinct from the underlying Kula lavas except for Nb/La and La/Sr, which are similar to those of the Kula lavas. The olivine and clinopyroxene phenocrysts are mostly reversely or cyclically zoned, indicating magma mixing during their formation. The phenocrysts are not in chemical equilibrium with the bulk rock, but the cpx rims are in equilibrium with calculated matrix compositions in many samples. Equilibrium pressures calculated from cpx rim-liquid equilibrium, and from equilibrium compositions of multiple phase-saturated liquids, are between 0 and 11.2 kb. In addition, barometry based on cpx chemistry indicates that the cores equilibrated at higher pressures than the rims. Equilibrium pressures of Kula lavas, calculated from previously published cpx rim compositions, are estimated between 0 and 5.6 kb. The rejuvenated stage magma reservoir at Kolekole may thus extend much deeper than the post-shield magma reservoir, which may have extended up to the crust-mantle boundary.