U-Th-Pb behavior of zircons under high-grade metamorphic conditions:A case study of zircon dating of meta-diorite near Qixia,eastern Shandong

摘要

This paper carried out a study on U—Th—Pb behavior of zircons in a'dry'rock system during high-grade metamorphism in the Archean basement of eastern Shandong.The studied sample has a mineral assemblage of plagioclase + K-feldspar + clinopyroxene + biotite + quartz and its protolith is considered to be diorite.The zircons are stubby,equant or irregular in shape and show fir-leaf, sectorial,banded or oscillatory zoning.They contain inclusions,including mineral assemblages of clinopyroxene + orthopyroxene + hornblende + quartz and plagioclase + K-feldspar + biotite + quartz. Fifty SHRIMP analyses were performed on 34 zircon grains,which commonly yielded high Th/U ratios (mostly>0.5).Most analyses are distributed along concordia from 2.54 to 2.25 Ga,with the youngest age being～1.95 Ga.Compositions and ages show large variations even in a same zircon grain.Combined with early studies,conclusions can be drawn as follows:1) the diorite underwent two episodes of high-grade metamorphism.at the end of the Neoarchean and the Paleoproterozoic(～2.50 Ga and～1.95 Ga or slightly later):2) high-grade metamorphism in a'dry'rock system may partially reset the U—Th—Pb system of zircons and.in this case,the ages between the oldest and youngest