Abstract
In order to understand compositional characteristics of zircon crystals with various appearance and to relate them with evolution of S-type, felsic magmatism of the Kumano Acidic Rocks in the Outer Zone of southwest Japan, U-Pb ages and trace element compositions of colorless and reddish zircons were measured using LA-ICPMS technique. Sorted U-Pb ages for these zircons range from 15.5 Ma to 16.7 Ma and ca. 1 m.y. older than fission track ages, implying slightly longer time elapsed between generation and cooling of the felsic melt. The REE patterns for both colorless and reddish zircons have a steep increase in LREE contents from La to Lu with significant positive Ce and negative Eu anomalies and show a common feature with those in granitoids. Scrutiny in the obtained data revealed that the slopes of HREE region of the reddish zircons are gentler than those of colorless zircons. REE composition of coexisting felsic melt estimated from the colorless zircons are relatively similar to the bulk rock composition of the evolved host rocks. Whereas the melt composition estimated from the reddish zircons has an adakitic signature. These suggest existence of roughly two different felsic melts for the origin of the Kumano Acidic Rocks. Large amounts of plagioclase fractionation from the parental magmas are required due to deep Eu troughs in the host melts. The production rate of the adakitic melts might be significantly smaller due to low population of reddish zircon. This compositional variation may reflect variable degree of partial melting of accretionary prism sediments that comprise the basement of the Kumano area. The partial melting of the sediments would have been triggered either by underplating of tholeiitic basalt magma derived from Shikoku basin or felsic magma derived from slab melting of the subducted plate, or melting directly of the subducted sediment.
