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 Sbandong. The studied sample has a mineral assemblage of plagioclase ＋ K-feldspar ＋ clinopyroxene ＋ biotite ＋ quartz and its pro- tolith 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 chronologically meaningless; and 3） high Th/U ratios may be common features of zircons formed during high-grade metamorphic conditions.
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 Sbandong. The studied sample has a mineral assemblage of plagioclase ＋ K-feldspar ＋ clinopyroxene ＋ biotite ＋ quartz and its pro- tolith 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 chronologically meaningless; and 3） high Th/U ratios may be common features of zircons formed during high-grade metamorphic conditions.
基金
supported by the Key Program of the Ministry of Land and Resources of China(Grant Nos.1212010811033, 1212010711815)
