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Summary
Widespread Late Mesozoic granites in South China are known to be younger southeastward, which is important for understanding the tectonic-magmatic evolution in South China. However, previous studies assumed Jurassic or Late Cretaceous ages for some granites in the Nan'ao Island, contradicting with the established trend. To address such controversy, we have carried out geochronological and geochemical studies of several granite plutons in this island. New zircon U-Pb age data show that they were emplaced in the Early Cretaceous from similar to 141 Ma in the Dajianshan area to 123-121 Ma in the Forest Park and Hulu-Guolaoshan areas, confirming that the southeastward younger trend of granitic magmatism had reached Nan'ao Island. These granites are characterized by high SiO2 (74.2-77.8 wt%) and low Al2O3 (12.0-13.1 wt%) contents, pronounced negative Ba-Eu-Sr-Ti anomalies, and moderately high temperatures of magmas (T-Zr = 655-771 degrees C; T-LREE = 675-819 degrees C). The Dajianshan granites have lower epsilon(Nd)(t) (-3.9 to -3.6), Nb/Ta (6.8-12), and Zr/Hf (16-22) than the Forest Park and Hulu-Guolaoshan granites (-1.5 to + 1.1, 9.4-16, and 20-30). The new data support the interpretation that the parental magmas for the Dajianshan granites were derived from a mixture of juvenile crustal rocks and Proterozoic metamorphic rocks whereas the parental magmas for the Forest Park and Hulu-Guolaoshan granites were sourced from juvenile crustal rocks. We envision that partial melting in the crust was induced by under -plating of mantle-derived mafic magma, which in turn was produced by decompression melting of ascending asthenospheric mantle in response to rollback of the subducting Paleo-Pacific plate.
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Institution桂林理工大学; 河海大学; 中国科学院
