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Isotopic decoupling of K from Sr and Nd in the Saima alkaline complex, NE China: interactions of cratonic roots and asthenosphere

Published online by Cambridge University Press:  05 April 2023

Nan Gao
Affiliation:
Centre for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China University of Chinese Academy of Sciences, Beijing 100049, China
Yingkui Xu*
Affiliation:
Centre for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China CAS Centre for Excellence in Comparative Planetology, Hefei 230026, China
Dan Zhu
Affiliation:
CAS Centre for Excellence in Comparative Planetology, Hefei 230026, China State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
Yang Li
Affiliation:
Centre for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China CAS Centre for Excellence in Comparative Planetology, Hefei 230026, China
Xiongyao Li
Affiliation:
Centre for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China CAS Centre for Excellence in Comparative Planetology, Hefei 230026, China
Jianzhong Liu
Affiliation:
Centre for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China CAS Centre for Excellence in Comparative Planetology, Hefei 230026, China
Jin-Cheng Luo*
Affiliation:
State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
*
Authors for correspondence: Yingkui Xu, Email: xuyingkui@vip.gyig.ac.cn; Jin-Cheng Luo, Email: luojincheng@mail.gyig.ac.cn
Authors for correspondence: Yingkui Xu, Email: xuyingkui@vip.gyig.ac.cn; Jin-Cheng Luo, Email: luojincheng@mail.gyig.ac.cn

Abstract

We report high-precision K isotopes, apatite U–Pb ages, whole-rock elements and Sr–Nd isotopes for the Saima nephelite syenite in the North China Craton. Trace-element and Sr–Nd–Hf–O isotope data indicate the presence of subducting sediments in the source region, while K isotopic compositions show a narrow range between –0.54 ‰ and –0.28 ‰, with an average of –0.41 ± 0.06 ‰, identical to the value of the asthenosphere. The nearly identical K isotopic compositions are low probability events compared with the K isotopic compositions of island arc lavas reported previously (–1.55 ‰ to +0.2 ‰). Although crustal contamination is consistent with the Sr–Nd–K isotopic data, alternatively we propose that the isotopic data also reconcile with the interaction between cratonic roots and the underlying convective asthenosphere, if this interaction is over prolonged periods of time. Numerical simulations successfully reproduced the observed data, if the metasomatism of the lithospheric mantle root, the source of the Saima alkaline rocks, occurred 500 Ma ago. Our study reveals that the isotopic compositions of fast-diffusion components in a lithospheric mantle metasomatized by ancient subducting melts can be effectively homogenized by convective asthenosphere through diffusion over a long time interval.

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Type
Original Article
Copyright
© The Author(s), 2023. Published by Cambridge University Press

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