GEOCHEMISTRY OF THE HICKS BUTTE COMPLEX, CENTRAL CASCADES, WASHINGTON: REMNANTS OF A LATE JURASSIC VOLCANIC ISLAND ARC WITH INTRUDING EARLY CRETACEOUS ADAKITES
The major element geochemistry of the Late Jurassic phase is primarily: mafic to intermediate; metaluminous; magnesian; calcic; and low-K. Their Mg# ranges from 16 to 46 and their Eu/Eu* ranges from 0.80 to 1.25. They have low Sr/Y (2-37) and Nb/Yb trace element ratios. The major element geochemistry of the Early Cretaceous phase is primarily: felsic; peraluminous; magnesian; calcic; and low-K. Their Mg# range from 18-32 and their Eu/Eu* ranges from 1.25 to 5.34. They have high Sr/Y (89-607), low Cr and Ni, and moderate Nb/Yb trace element ratios.
The major element geochemistry, low Sr/Y, and Nb/Yb ratios of the Late Jurassic phase suggests an island arc tectonic setting with magma derived from a depleted mantle source. The major element geochemistry of the Early Cretaceous phase also suggests an island arc however, their Nb/Yb ratios suggest they were derived from a more enriched source than the Late Jurassic phase. The Early Cretaceous phase has very high Sr/Y ratios suggesting that these magmas are adakites as well as meeting all other geochemical classifications for adakites. Their high Eu/Eu* suggests little plagioclase fractionation. The low Mg#, Cr, and Ni for the adakites suggest they were derived from melting of lower arc crust, however, partial melting of subducting oceanic crust could have also resulted in adakite petrogenises.