4:45 PM - 5:00 PM
[SMP30-12] Ultramafic rock in the Hida metamorphic belt: an example of arc plutons
Ultramafic rocks of the Hakusan city (simply call Hakusan Ultramafic rock hereafter) is mainly composed of large poikilitic amphibole and orthopyroxene enclosing olivine grains. These olivine grains contain chromian spinels. Olivine with spinel existed phases before amphibole and orthopyroxene were crystallized. Orthopyroxene are sometimes included in amphibole and amphibole shows euhedral shape against the next orthopyroxene grain indicating amphibole is an early crystallization phase. The Fo content of olivine is around 84 and XMg of amphibole is around 90. High XMg of mafic minerals indicate that the Hakusan Ultramafic rock was formed from a relatively primitive melt. Poikilitic amphibole is pargasitic in composition. Calculated melt compositions from trace element compositions of the poikilitic amphibole are characterized by high in LILEs with negative anomalies of HFSEs, and are similar to arc magmas. We concluded that the Hokuriku Ultramafic rocks are formed from arc-related magma. It is important to study the origin of cortlandite and related amphibole-rich plutons from other metamorphic belts with different ages to understand the differences in mode and conditions of arc formation.