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| Geochemistry of high-Mg andesites from the early Cretaceous Yixian Formation, western Liaoning: Implications for lower crustal delamination and Sr/Y variations | |
| 基金项目： | 国家自然科学基金;教育部资助项目 |
| 摘 要： | The Yixian Formation at Sihetun in western Liao- ning Province has attracted considerable attention over the last two decades due to discovery of a wide range of well-preserved ‘feathered’ dinosaurs and primitive bird fossils1―4]. This formation is dominated by vol- canic rocks, with fossil-bearing lacustrine sedimentary rocks at the upper part of the section4]. The sedimen- tary rocks contain thin layers of tuff. According to previous studies4], the total thickness of the Yixian Form… |
| 收稿时间： | 25 January 2005 |
| 修稿时间： | 19 May 2005 |
Geochemistry of high-Mg andesites from the early Cretaceous Yixian Formation, western Liaoning: Implications for lower crustal delamination and Sr/Y variations |
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| Authors: | WANG Xiaorui GAO Shan LIU Xiaoming YUAN Honglin HU Zhaochu ZHANG Hong WANG Xuance |
| Institution: | 1. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an 710069, China 2. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an 710069, China;State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China 3. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an 710069, China;Shenyang Institute of Geology and Mineral Resources, Shenyang 110032, China 4. Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China |
| Abstract: | 85 volcanic rocks of the Yixian Formation from the Sihetun type section were collected and analyzed for geochemical and isotopic compositions. Major element compositions indicate that the Sihetun volcanic rocks are high magnesium andesites with some basalts occurring at the bottom of section. The Sihetun high magnesium andesites (SiO2 = 52.82−59.31wt%, Al2O3 = 14.15−16.35wt%) show many characteristics of adakites such as depletion in heavy rare-earth elements (HREE; Yb = 1.03−1.88 μg/g) and Y(12−20 μg/g) and high Sr (620−1323 μg/g) and Sr/Y(32−88), with high LaN/YbN ratio (10–25). They share similar major and trace element characteristics to volcanic rocks from the Xinglonggou Formation except their lower Nd isotope ratios (143Nd/144Nd (130Ma) = 0.5118–0.5119, ɛ Nd (130Ma) = −11.6–−13.8, 87Sr/86Sr (130 Ma) = 0.7058–0.7064. They were interpreted in a way that eclogite that formed at the base of thickened Archean lower crust of the North China craton foundered into the convecting mantle and subsequently melted and interacted with peridotite. However, compared to the Xinglonggou volcanic rocks, the source of the Sihetun magma contained more ancient continental crustal material in order to explain its evolved Nd isotopes. The age of the Sihetun Formation was 120 to 130 Ma, and this indicates that delamination lasted to the early Cretaceous period. The Sr contents and Sr/Y ratios of the Sihetun high-Mg andesites show significant negative correlations with SiO2 for samples with SiO2 > 56%. These suggest that the Sr and Sr/Y values were reduced due to fractional crystallization of plagioclase. Accordingly, the effect of crystallization on volcanic Sr and Sr/Y ratio has to be taken into account. |
| Keywords: | High-Mg andesite Adakite Yixian Formation North China Craton delamination |
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