Zircon SHRIMP U-Pb age, geochemistry and tectonic setting of Indosinian Wutuan pluton in southwestern Hunan Province

Abstract: The Wutuan granitic pluton in southwestern Hunan is mainly composed of biotite monzogranite and dimicaceous monzogranite. U–Pb zircon dating of a biotite monzogranite sample was carried out, and analyses of 9 zircon grains yielded an average age of (220.5±4.4)Ma, which indicates that the granites were formed in middle Late Triassic. The rocks are silicon-high (SiO2=70.64%~75.32%, 72.33% on average), aluminum-rich (Al2O3=13.24%~16.37%, 14.47% on average), potassium-high (K2O=4.27%~6.66%, 5.10% on average) and alkali-moderate (Na2O+K2O=6.71%~8.46%, 7.65% on average) ones, with K2O/Na2O ratios being 1.54~5.34 (2.23 on average) and ASI value being 1.24 on average, thus belonging to ferric, high-K calc-alkaline series and shoshonite series perluminous granitoids. Ba, Nb, Sr, P and Ti are pronouncedly depleted, while Rb, (Th+U+K), (La+Ce), Nd, (Zr+Hf+Sm) and (Y+Yb+Lu) are comparatively concentrated. The rocks have low ΣREE values of 84.35~222.90μg/g (156.81μg/g on average), δEu values of 0.17~0.52 (0.39 on average), (La/Yb)N values of 1.81~17.12 (10.57 on average), ISr values of 0.71813~0.72110, εSr(t) values of 193.5~235.6, εNd(t) values of -9.78~-9.70 and t2DM ages of 1.78~1.79 Ga. The C/MF-A/MF diagram indicates that the granites were derived from mudstones and clastic rocks. Al2O3/TiO2 ratios of most strongly perluminous granite samples are less than 100. All these data point to S-type granitiods and suggest that the granites might have come mainly from acid rocks of the middle-upper crust. Trace element diagrams for discrimination of the structural environment show that the granites were formed in a post-collisional tectonic setting. According to the petrogenesis, the discrimination of structural environment and regional tectonic evolutional setting, the authors infer that the formation mechanism of the Wutuan pluton should be as follows: the thickening of the crust in Indosinian movement caused the rise of the temperature of the crust, and later the weakening of the stress in the post-collisional tectonic setting caused the melting of the middle-upper crust, with the magma emplaced in a relatively open environment. In addition, upwelling and heat transference of asthenosphere mantle might have played a certain role in the formation of granitic magma.