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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 are
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 + Fifty SHRIMP analyzes were performed on 34 zircon grains, which commonly yielded high Th / U ratios (mostly> 0.5). Host 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 the same zircon grain. Cined with early studies, conclusions can be drawn as follows: 1) the diorite underwent two episodes of high-grade metamorphism . 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 are