U–Pb geochronology and REE geochemistry of zircons from Palaeoproterozoic paragneiss clasts in the Mesozoic Kamiaso conglomerate, central Japan: evidence for an Archean provenance

Abstract

Sensitive high resolution ion microprobe (SHRIMP) U–Pb analyses of zircons from a pelitic gneiss clast from the Jurassic Kamiaso conglomerate in central Japan show that most grains are 1850–1950 Ma, but six are >2400 Ma, including a very old one (>3254±14 Ma). The SHRIMP data give chronological evidence for previously unrecognized igneous activity from 1850 to 1950 Ma. The existence of Archean zircons allows us to consider the evolutionary history of the Archean crust in eastern Asia. Chondrite-normalized REE patterns of the individual zircons determined by SHRIMP show that the 2400–3250 Ma zircons have large positive Ce and small negative Eu anomalies, whereas the 1850–1950 Ma zircons have small positive Ce and significantly negative Eu anomalies. These differences in the REE abundances for the 2400–3250 and 1850–1950 Ma zircons may reflect the composition of the melt that produced the zircons, and reveal the possibility of a geological constraint on the origin of Kamiaso zircons.