All subaerial lavas at Mauna Kea Volcano, Hawaii, belong to the postshield stage of volcano construction. This stage formed as the magma supply rate from the mantle decreased. It can be divided into two substages: basaltic (~240--70 ka) and hawaiitic (~66--4 ka). The basaltic substage (Hamakua Volcanics) contains a diverse array of lava types including picrites, ankaramites, alkalic and tholeiitic basalt, and high Fe-Ti basalt. In contrast, the hawaiitic substage (Laupahoehoe Volcanics) contains only evolved alkalic lavas, hawaiite, and mugeraite; basalts are absent. Sr and Nd isotopic ratios for lavas from the two substages are similar, but there is a distinct compositional gap between the substages. Lavas of the hawaiitic substage cannot be related to the older basalts by shallow pressure fractionation, but they may be related to these basalts by fractionation at moderate pressures of a clinopyroxene-dominated assemblage. We conclude that the petrogenetic processes forming the postshield lavas at Mauna Kea and other Hawaiian volcanoes reflect movement of the volcano away from the hotspot.

Specifically, we postulate the following sequence of events for postshield volcanism at Mauna Kea: (1) As the magma supply rate from the mantle decreased, major changes in volcanic plumbing occurred. The shallow magma chamber present during shield construction cooled and crystallized, and the fractures enabling magma ascent to the magma chamber closed. (2) Therefore subsequent basaltic magma ascending from the mantle stagnated within the lower crust, or perhaps at the crust-mantle boundary. Eruptions of basaltic magma ceased. (3) Continued volcanism was inhibited until basaltic magma in the lower crust cooled sufficiently to create relatively low-density, residual hawaiitic melts. Minor assimilation of MORB-related wall rocks, reflected by a trend toward 206Pb/204Pb in evolved postshield lavas, may have occurred at this time. A compositional gap developed because magma ascent was not possible until a low-density hawaiitic melt could escape from a largely crystalline mush. Eruption of this melt created aphyric hawaiite and mugearite lavas which incorporated cumulate gabbro, wherlite, and dunite xenoliths during ascent. ¿ American Geophysical Union 1990