Petrogenesis of amphibole-rich ultramafic rocks in the Hida metamorphic complex, Japan: Its role in arc crust differentiation

Amphibole controls major and trace element chemistry of arc magma via fractionation, therefore its formation process is the key to understand arc crustal evolution. Amphibole-rich cumulates, occasionally found associated with felsic plutons or metamorphic rocks, provide complementary insights into the arc magma differentiation. In this study, we conducted a petrological examination of amphibole-rich ultramafic rocks from the Hida metamorphic complex in Japan to better understand its role in arc crust evolution. The studied samples are characterized by large poikilitic amphibole and orthopyroxene enclosing olivine grains with corroded margins. Amphibole showed high Cr2O3 and TiO2 contents (up to 0.9 and 1.2 wt%) and light rare earth element enrichment relative to heavy rare earth elements. Microscopic and geochemical features of amphibole and olivine suggest that the amphibole formed at the expense of early crystalized olivine (± clinopyroxene) from hydrous juvenile magma at a deep crustal level, which is consistent with the estimated crystallization pressure of 0.7–1.0 GPa based on an empirical Al-in-hornblende geobarometer. Considering the arc affinity of parental magma and the amphibole-fractionation trend in bulk chemistry of the Mesozoic plutonic rocks from the Hida Belt, it is implied the formation of amphibole-rich plutonic rocks contributed to arc crustal evolution as amphibole-rich residue complementary to differentiated magma.