Plutonic xenoliths reveal the timing of magma evolution at Hualalai and Mauna Kea, Hawaii

Hawaiian volcanoes evolve through stages that have been delimited by the compositions of their erupted lavas. New in situ [sup.238]U-[sup.230]Th and U-Pb dating of single zircons from leucocratic plutonic xenoliths erupted at Mauna Kea and Hnalalai volcanoes, Hawaii, reveals that important episodes of magmatic evolution are not necessarily reflected in the stratigraphy or compositions of erupted lavas. Zircons from Mauna Kea diorites form a heterogeneous population with apparently bimodal ages of ca. 125 ka and ca. 65 ka, suggesting fractionation, intrusion, and crystal recycling about the time of transition of post shield volcanism from basaltic to hawaiitic compositions. Hualalai syenogabbros and diorites record extreme fractionation and generation of alkalic magma at 41 [+ or -] 9 ka and 261 [+ or -] 28 ka, indicating that alkalic magma was generated ~130,000 yr before the shield to postshield transition inferred from lava stratigraphy, and that coeval evolution of chemically distinct magma reservoirs at shallow and deep levels may have characterized the shield stage. These episodes at Hualalai did not cause eruption of evolved lavas, indicating that extreme differentiation in Hawaiian volcanoes is not necessarily followed by eruption of highly evolved magma. Keywords: uranium-series method, magma chambers, magmatic differentiation, cumulates, volcanism.