Assimilation of lower-crustal dunite xenoliths into adakite-related felsic magma: New insights into the production of bajaitic high-Mg andesites

The close spatial association and compositional similarity between bajaites and adakites suggest a possible ge-netic relationship between these rock types. To constrain the petrogenesis of bajaite, this study examines a microgabbro boulder derived from a bajaitic dike in the Kitakami Massif, northern Japan, a well-studied area with respect to adakite-bajaite magmatism. The microgabbro contains numerous dunite xenoliths and displays very fine-grained texture in some parts, representing solidified melt. Equilibrium temperatures (<= 1131 degrees C) derived from the pyroxene thermometer and pressure (similar to 0.95 GPa) calculated using the clinopyroxene barometer indicate a crustal cumulate origin of the dunite xenoliths. The overall whole-rock major-element chemistry of the microgabbro is very similar to the composition of olivine, and trace elements do not show patterns that suggest specific mineral accumulations but rather indicate a melt composition. Equilibrium melt compositions estimated from the trace-element geochemistry of clinopyroxene in the microgabbro are in good agreement with the compositions of bajaite. The major-and trace-element compositions of bajaite are reproduced by incorporation of the dunite xenolith component into a non-adakitic felsic melt prior to typical adakitic magmatism with the weight percent fraction of microgabbro (F) = 0.2. These investigations based on obtained samples robustly constrain the petrogenesis of at least some bajaites and suggest that the geochemical characteristics of bajaites are explained by the assimilation of a significant amount of an olivine component into adakite-related felsic melts within crustal levels.