U-Th-Pb behavior of zircons under high-grade metamorphic conditions： A case study of zircon dating of meta-diorite near Qixia, eastern Shandong
U-Th-Pb behavior of zircons under high-grade metamorphic conditions： A case study of zircon dating of meta-diorite near Qixia, eastern Shandong
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DOI：
10.1016/j.gsf.2011.02.004
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发表时间：
2011-04
影响因子：
8.9
通讯作者：
Y. Wan;Dunyi Liu;C. Dong;Shoujie Liu;Shijin Wang;Enxiu Yang
中科院分区：
文献类型：
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作者：
Y. Wan;Dunyi Liu;C. Dong;Shoujie Liu;Shijin Wang;Enxiu Yang
本文研究了胶东太古宙基底高级变质作用过程中“干”岩系中锆石的U-Th-Pb行为。研究样品的矿物组合为斜长石+钾长石+单斜辉石+黑云母+石英，其原岩为闪长岩。锆石呈等长、等长或不规则形状，并呈杉叶状、扇形、条带状或振荡环带。包裹体主要有单斜辉石+斜方辉石+角闪石+石英和斜长石+钾长石+黑云母+石英两种矿物组合。对34个锆石样品进行了50次SHRIMP分析，结果表明，这些样品的Th/U比值普遍较高（大多数>0.5）。大多数分析都沿着协和面分布，年龄为2.54 ~ 2.25 Ga，最年轻的年龄为1.95 Ga。同一颗粒锆石的成分和年龄差异较大。结合前人的研究，可以得出如下结论：1）早白垩世以来，该地区经历了新太古代末和古元古代的两期高级变质作用（约2.50 Ga和约1.95 Ga或稍晚）;（2）“干”岩石系统中的高级变质作用可部分重置锆石的U-Th-Pb系统，在这种情况下，高Th/U比值可能是高级变质条件下形成的锆石的共同特征。
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 chronologically meaningless; and 3) high Th/U ratios may be common features of zircons formed during high-grade metamorphic conditions.