Zircon U-Pb SHRIMP dating of the Yematan batholith in Dulan,North Qaidam,NW China
Abstract The Yematan batholith crops out over 120 km2 in the North Qaidam ultrahigh pressure (UHP) metamorphic belt. It consists of granodiorite, monzogranite and biotite granite and forms an irregular intrusion into Neoproterozoic gneiss that has undergone Caledonian UHP metamorphism. Zircons from the Yematan granodiorite yield a SHRIMP U-Pb age of 397±3 Ma. These granitic rocks have geochemical characteristics intermediate between I- and S-type granites，and are post-collisional. We suggest that the Yematan granitic rocks were formed during the last exhumation event of the North Qaidam UHP belt.
SHRIMP U-Pb zircon dating for Qiashikansayi granodiorite,the Northern Altyn Tagh Mountains and its geological implications
Abstract The Qiashikansayi granodiorite is foliated resulting in a granodioritic gneiss. Its geochemical features,such as alumina saturation index(A/CNK) of 0.81—0.99,Na2O/K2O values>1, TiO2 contents <1.0%, LREE enrichment with high fractionation factors, weakly negative or no Eu anomalies, and significant Ba and Ti negative anomalies,suggest that it is similar to a typical island arc pluton. The trace elements of the Qiashikansayi granodiorite are plotted in the island arc field in the tectonic setting discrimination diagrams as well. Cathodoluminescence images demonstrated that the zircons have clear rhythmic crystallized zoning, without any remnant core and new crystallized rim,suggesting the zircons be magmatic ones. Their U and Th contents vary in the range of 574—870 μg/g, and 279—556 μg/g respectively, with the Th/U ratio in the range of 0.52—0.68. SHRIMP zircon U-Pb dating yielded the 481.5±5.3 Ma age for the intrusion of the granodiorite, which is coeval with the island arc volcanic rocks in the northern Qilian Mountain, and confirms that there is an early Paleozoic island arc in the Hongliugou- Lapeiquan area.
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