Petrogenesis of Dashenshan I-type granodiorite: implications for Triassic crust–mantle interaction, South China

Abstract

Triassic granodiorites in South China (SC) provide an opportunity to examine crust–mantle interactions that may have been caused by a mantle plume. Here we present a combined study of chronological, geochemical, and Sr–Nd–Hf isotopic compositions for Dashenshan granodiorites. These are high-K, calc-alkaline, I-type granodiorites that yield a U–Pb zircon age of 211 ± 3 Ma. They are metaluminous to weakly peraluminous (A/CNK < 1.1), with 3.04–3.89 wt.% Na₂O and 3.24–3.86 wt.% K₂O, and Na₂O/K₂O ratio ranging from 0.79 to 1.11. These granodiorites contain 67.7–72.6 wt.% SiO₂ but show moderate Mg^# values (44.2–57.8) and variable contents of Ni (3.6–29.9 ppm) and Cr (7.6–53.5 ppm). They exhibit light rare earth element (REE) enrichment and flat, heavy REE patterns with negative Eu anomalies (Eu/Eu* = 0.52–0.87). They also display strongly negative Ba, Sr, Nb, Ta, P, and Ti anomalies and positive Rb, Th, K, and Pb anomalies. Dashenshan granodiorites have high whole-rock initial ⁸⁷Sr/⁸⁶Sr ratios (0.7121–0.7172), negative ε_Nd (t) values (–8.8 to –6.8), and negative zircon ε_Hf (t) values (–6.6 to –3.3). These results suggest that the Dashenshan granodiorites were generated by a mixing between crustal melt and mantle-derived magma in an extensional setting. We conclude that generation of the Dashenshan pluton may reflect an interaction between a mantle plume and the overlying SC crust.

Keywords:

• Triassic
• I-type granite
• crust–mantle interaction
• South China

Acknowledgements

Constructive comments from Professors Wenliang Xu and Qiang Wang improved the article significantly. We thank Professor Robert J Stern for editorial input; Dr Zhaochu Hu (China University of Geosciences, Wuhan) for his help with zircon U–Pb dating and Hf isotopic analysis; and Drs Zhuyin Chu and Xindi Jin (Institute of Geology and Geophysics, Chinese Academy of Sciences) for their help with whole-rock Sr–Nd isotopic and chemical analysis, respectively.

Funding

This research was supported by the National Science Foundation of China (Nos 41372076, 40603002, and 41072046) and Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (CUG120121).