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

Geoscience Frontiers

Yusheng Wan¹  Dunyi Liu¹  Shoujie Liu¹  Chunyan Dong¹  Enxiu Yang²  Shijin Wang² 

[1] Beijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;Shandong Geological Survey, Jinan 250013, China;

关键词: Zircons;    U–Th–Pb system;    Reset;    High-grade metamorphism;    SHRIMP dating;    Early Precambrian;   

DOI  :  10.1016/j.gsf.2011.02.004

来源: DOAJ

【 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 are chronologically meaningless; and 3) high Th/U ratios may be common features of zircons formed during high-grade metamorphic conditions.

【 授权许可】

Unknown