Abstract:

This paper presents a study of the 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， sector， banded or oscillatory zoning. They contain inclusions， including the mineral assemblages of clinopyroxene+orthopyroxene+hornblende+quartz and plagioclase+Kfeldspar+biotite+quartz. Fifty SHRIMP analyses were carried out on 34 zircon grains and commonly yielded high Th/U ratios(mostly>05). Most analyses are distributed along a concordia from 254 to 225 Ga， with the youngest age of about 195 Ga. Compositions and ages show large variations even in an individual zircon grain. Combined with early studies， conclusions have been drawn as follows： （1) the diorite underwent two episodes of highgrade metamorphism both at the end of the Neoarchean and the Paleoproterozoic(about 250 Ga and about 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 are chronologically meaningless； （3) high Th/U ratios may be common features of the zircons formed during highgrade metamorphic conditions.

Key words: zircon； UThPb system； lead loss； highgrade metamorphism； SHRIMP dating； , Early Precambrian； , eastern Shandong