On the origin of mafic magmatic enclaves (MMEs) in syn-collisional granitoids: evidence from the Baojishan pluton in the North Qilian Orogen, China

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• 作者：Shuo Chen ; Yaoling Niu ; Wenli Sun ; Yu Zhang ; Jiyong Li…
• 刊名：Mineralogy and Petrology
• 出版年：2015
• 出版时间：October 2015
• 年：2015
• 卷：109
• 期：5
• 页码：577-596
• 全文大小：2,474 KB
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• 作者单位：Shuo Chen (1) (2)
  Yaoling Niu (1) (3)
  Wenli Sun (4)
  Yu Zhang (4)
  Jiyong Li (1) (2)
  Pengyuan Guo (4)
  Pu Sun (1)
  
  1. Institute of Oceanology, Chinese Academy of Sciences, Qingdao, 266071, China
  2. University of Chinese Academy of Sciences, Beijing, 100049, China
  3. Department of Earth Sciences, Durham University, Durham, DH1 3LE, UK
  4. School of Earth Sciences, Lanzhou University, Lanzhou, 730000, China
  
• 刊物类别：Earth and Environmental Science
• 刊物主题：Earth sciences
  Mineralogy
  Geochemistry
  
• 出版者：Springer Wien
• ISSN：1438-1168

文摘

Mafic magmatic enclaves (MMEs) are abundant in Baojishan syn-collisional granitoids located in the eastern section of the North Qilian Orogen. Zircon U-Pb ages of the host granodiorite (433.7?±-.4 Ma) and their MMEs (431.6?±-.8 Ma) are the same as the time of the Qilian ocean closing and continental collision at ?40-20 Ma, indicating that the granitoids represent a magmatic response to the collision between the Qilian-Qaidam block and the Alashan block. The MMEs have the same mineralogy as the host granodiorite except that they are more abundant in mafic phases (e.g., amphibole and biotite) and thus have higher heavy rare earth element (HREE) abundances. Both the host granodiorite and the MMEs have light REE-enriched patterns and relatively flat HREE patterns (i.e., [Dy/Yb]_N---.1). They are enriched in large ion lithophile elements (LILEs; e.g., Rb, K, Pb) and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, Ti) and show a varying Sr anomaly (i.e., Sr/Sr*--.9-.2) for the host and a negative Sr anomaly (i.e., Sr/Sr*--.4-.6) for the MMEs. Both the host granodiorite and the MMEs have overlapping and indistinguishable Sr-Nd-Hf isotopic compositions (87Sr/86Sr_(i)--.7067-.7082, ε_Nd(t)-??3.9–?3.2, ε_Hf(t)--.0-4.7). The extremely high ε_Hf(t)--4.7 of sample BJS12-06MME likely results from the calculation due to nugget effect of zircons because of the unexpectedly high Hf (3.53 ppm) and too high Zr (128 ppm). All these characteristics are fully consistent with the MMEs being of cumulate origin formed at earlier stages of the same magmatic systems rather than representing mantle melt required by the popular and alleged magma mixing model. The radiogenic Sr and unradiogenic Nd (ε_Nd(t) <0) indicate the contribution of mature continental crust, while variably radiogenic Hf (ε_Hf(t)-gt;-) for both the MMEs and their host granodiorite manifest the significant mantle input. The apparent decoupling between Nd and Hf isotopes are likely caused by partial melting of recycled terrigenous sediments and the remaining part of the North Qilian ocean crust under the amphibolite facies conditions during the collision. Editorial handling: Y. Xu