Early Cretaceous adakitic rocks in the Anqing region, southeastern China: constraints on petrogenesis and metallogenic significance

ABSTRACT

The Anqing region in Lower Yangtze River metallogenic belt is one of the important Cu polymetal producers in China. The origin of Cu polymetallic deposits in the region is closely related to Early Cretaceous adakitic intrusions. To constrain the petrogenetic and metallogenic significance of the adakitic rocks, a detailed geochronological, geochemical, and Sr–Nd–Pb–Hf isotopic study was performed. The Anqing adakitic rocks (SiO₂ = 57.4–64.2 wt.%) consist mainly of quartz monzodiorite, formed at 138.2 ± 1.7 Ma (Mean Standard Weighted Deviation (MSWD) = 0.61). They have high MgO, Al₂O₃, Sr, and low Rb, Y, Yb contents, together with high Sr/Y (50.5–222) and La/Yb (31.9–46.9) ratios. They also show negative whole-rock ε_Nd(t) (−9.8 to −8.5) and zircon ε_Hf(t) (−10.0 to −5.4), and high oxygen fugacity (mainly −17.0 to −8.01) values and radiogenic Pb isotopic compositions with (²⁰⁶Pb/²⁰⁴Pb)_i = 17.692–17.884, (²⁰⁷Pb/²⁰⁴Pb)_i = 15.413–15.511, and (²⁰⁸Pb/²⁰⁴Pb)_i = 37.611–37.943. Coupled with negative Nb–Ta anomalies, low K₂O/Na₂O ratios (0.39–0.62), and high Mg# values (0.44–0.71), these data suggest the adakitic rocks and associated large-scale Cu–Au mineralization of the Anqing region resulted from partial melting of the high oxidized subducted oceanic crust. Addition of mantle-derived magmas and assimilation of crustal materials during emplacement are also possible.

KEYWORDS:

• Adakite
• mantle-derived magma
• slab-derived (adakitic) magmas
• Cu–Au mineralization
• Anqing region

Acknowledgements

This study was supported by the National Key R&D Program of China (No. 2016YFC0600404) and the National Natural Science Foundation of China (Grant No. 41373045). Prof, T Ireland is sincerely thanked for constructive comments and proof-reading the manuscript. We also thank the Anqing Mine of Tongling Nonferrous Metal Industrial Ltd. and the Yueshan Mine of the Anqing City for their help during the field study. We highly appreciate the valuable comments provided by Editor-in-Chief Robert J. Stern, Guest Editor, Dr X.Y. Yang, and two anonymous reviewers.

Disclosure statement

No potential conflict of interest was reported by the authors.

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Funding

This study was supported by the National Key R&D Program of China (No. 2016YFC0600404) and the National Natural Science Foundation of China (Grant No. 41373045).