The newly identified Neoproterozoic Yuanmou high-Nb mafic and adakitic rocks at the western margin of the Yangtze Block in South China are investigated for petrogenesis and geodynamic implications. Zircon U-Pb ages show the adakitic granodiorite host and mafic microgranular enclaves (MMEs) were synchronously emplaced at 811−806 Ma, and were cut by an 802 ± 11.5 Ma mafic dike. The medium-grained granodiorites consist mainly of plagioclase, quartz, K-felspar, biotite, and amphibole. High SiO2 (62.8−67.5 wt%), Al2O3 (13.1−16.8 wt%) contents, high Sr/Y (17.4−49.0), and (La/Yb)N (16.3−52.6) but low Y (6.83−16.1 ppm) and Yb (0.74−1.82 ppm) concentrations of the granodiorites point toward their calc-alkaline and adakitic nature. Further, their high Dy/Yb (1.52−2.31) and Gd/Y (2.35−4.43) and low K2O/Na2O (0.22−0.48) and Th/La (0.14−0.20) ratios, constant bulk rock εNd(t) (−0.5 to −1.5) and zircon εHf(t) (0.0 to +2.3) values, suggest that the granodiorites were derived from partial melting of a mafic lower crustal source. The MMEs in the granodiorites and mafic dike intrusions are composed of amphibole, plagioclase, clinopyroxene, and olivine, and have similar chemical compositions, such as high Nb (15.7−41.9 ppm) and TiO2 (2.13−3.39 wt%) contents and high Nb/U (18.7−30.9) values, akin to alkaline high-Nb basalts (HNB). Their variable Ba/Nb (7.68−79.2), Nb/Zr (0.09−0.16), high Nb/Th (5.47−6.68), and low Th/Zr (0.01−0.03) and La/Nb (1.03−1.37) ratios indicate a mantle source modified by melts derived from subducted oceanic crust and sediments. The Yuanmou HNB with high εNd(t) (+4.8 to +6.9) were probably generated during back-arc extension by decompression melting of a mantle source metasomatized by adakitic slab melts formed during earlier stages of the subduction. This implies that the location of the back-arc was far from the subducted oceanic slab, possibly due to a slab rollback process. In this scenario, HNB intrusions provided the heat for partial melting and adakitic melt formation in the mafic lower crust. The model suggests that the Yuanmou and other similar associations of HNBs and adakitic rocks emplaced along the western margin of the Yangtze Block are products of a widespread Neoproterozoic back-arc extension.
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Research Article|
May 23, 2023
Coupled alkaline high-Nb mafic rocks and adakitic granodiorites: Products of Neoproterozoic back-arc extension at the western margin of the Yangtze Block, South China
Bin Huang;
Bin Huang
1
State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
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Wei Wang;
Wei Wang
1
State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
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Jun-Hong Zhao;
Jun-Hong Zhao
1
State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
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Nimat Ullah Khattak;
Nimat Ullah Khattak
2
National Centre of Excellence in Geology, University of Peshawar, Peshawar 009291, Pakistan
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Si-Fang Huang;
Si-Fang Huang
3
Department of Surveying and Planning, Shangqiu Normal University, Shangqiu 476000, China
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Gui-Mei Lu;
Gui-Mei Lu
1
State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
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Er-Kun Xue;
Er-Kun Xue
1
State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
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Li Sun
Li Sun
1
State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
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Bin Huang
1
State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
Wei Wang
1
State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
Jun-Hong Zhao
1
State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
Nimat Ullah Khattak
2
National Centre of Excellence in Geology, University of Peshawar, Peshawar 009291, Pakistan
Si-Fang Huang
3
Department of Surveying and Planning, Shangqiu Normal University, Shangqiu 476000, China
Gui-Mei Lu
1
State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
Er-Kun Xue
1
State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
Li Sun
1
State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
Publisher: Geological Society of America
Received:
03 Jun 2022
Revision Received:
04 Jan 2023
Accepted:
06 Mar 2023
First Online:
23 May 2023
Online ISSN: 1943-2674
Print ISSN: 0016-7606
© 2023 Geological Society of America
GSA Bulletin (2023)
Article history
Received:
03 Jun 2022
Revision Received:
04 Jan 2023
Accepted:
06 Mar 2023
First Online:
23 May 2023
Citation
Bin Huang, Wei Wang, Jun-Hong Zhao, Nimat Ullah Khattak, Si-Fang Huang, Gui-Mei Lu, Er-Kun Xue, Li Sun; Coupled alkaline high-Nb mafic rocks and adakitic granodiorites: Products of Neoproterozoic back-arc extension at the western margin of the Yangtze Block, South China. GSA Bulletin 2023; doi: https://doi.org/10.1130/B36618.1
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