Open Geosciences (Nov 2024)
Origin of Late Cretaceous A-type granitoids in South China: Response to the rollback and retreat of the Paleo-Pacific plate
Abstract
The Late Cretaceous granitic rocks are widely distributed in South China; however, the precise geodynamic mechanism remains controversial. To solve this question, we conducted whole-rock geochemistry, mineral chemistry, zircon U–Pb ages, and Lu–Hf isotopic data analyses of the Maqigang beschtauite, which is exposed in southeastern Guangxi. Laser ablation-inductively coupled plasma-mass spectrometry zircon U–Pb dating revealed the beschtauite emplacement at 83–85 Ma, representing Late Cretaceous magmatic activity. The rocks belong to the high-K calc-alkaline to shoshonite series and displayed metaluminous to weakly peraluminous affinity, with an A/CNK ratio ranging from 0.89 to 1.02. Whole-rock geochemical analyses showed that all rocks were enriched in Rb, Th, U, and K but depleted in Nb, Ta, Ti, Ba, and Sr. They were also rich in light rare earth elements with weakly negative Eu anomalies (Eu/Eu* = 0.61–0.77). The beschtauite showed typical A-type granitoid affinities, with rich silica (mean: 64.95 wt%), alkali (Na2O + K2O, mean: 7.55 wt%), and high field strength elements (Zr + Nb + Ce + Y = 427.40–599.60 ppm) contents, high FeOT/MgO (mean: 3.27) and 104Ga/Al (mean: 2.75) ratios, and low Ba, Sr, Ti, and Eu contents. Mineral chemical analysis demonstrated that phenocrytic plagioclase was mainly andesite with an oscillating zone. Zircon Hf isotopic results showed zircon ε Hf(t) values ranging from −10.8 to −0.9 and TDM2 model age range from 1.2 to 1.8 Ga. These results indicate that the beschtauite was generated by the partial melting of ancient crustal materials via the underplating of mantle-derived magma. Based on these regional geological data, the Maqigang pluton was formed in an extensional back-arc setting associated with the continued rollback and retreat of the Paleo-Pacific plate subduction zone.
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