The earliest Jurassic A-type granite in the Nanling Range of southeastern South China: petrogenesis and geological implications

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posted on 14.11.2016 by Chengshi Gan, Yuejun Wang, Yuzhi Zhang, Jian Zhang

The tectonic transition from the palaeo-Tethyan to palaeo-Pacific dynamic domains in the South China Block (SCB) is still a matter of debate. The A-type granites collected from the southeastern SCB offered an opportunity to illustrate this tectonic transition. This article records a set of petrographic, geochronological, and geochemical data for the Wengong granitic pluton from the eastern Nanling Range. LA-ICP-MS zircon U–Pb dating shows a crystallization age of 196.9 ± 4.4 Ma with εHf(t) values ranging from +2.1 to +7.7. The samples have high SiO2, Zr+Nb+Ce+Y, FeOt/MgO, Ga/Al, and Y/Nb and are depleted in Nb–Ta, Zr–Hf, Ba, Sr, Ti, and Eu, similar to those of the A2-type granite. Their initial 87Sr/86Sr ratios range from 0.70885 to 0.70983 and the εNd(t) values range from −2.9 to −1.1, close to those of the Early Palaeozoic mafic rocks in the southeastern SCB. The Wengong A2-type granite was derived from partial melting of the mafic rocks underplated into the lower crust during the Early Palaeozoic.

The Mesozoic A-type granites in the southeastern SCB can be subdivided into 229–215 Ma (Late Triassic), 197–152 Ma (Jurassic), and 135–92 Ma (Cretaceous). They differ in geochemical and spatial distribution characteristics. The Late Triassic A-type granites were formed in the post-collision extensional setting associated with the palaeo-Tethyan dynamic domain, whereas the Cretaceous A-type granites were under the control of the palaeo-Pacific dynamic domain. The A-type granites were hardly exposed during the Late Triassic–Early Jurassic and Late Jurassic–Early Cretaceous. The Jurassic A-type granites were formed in the intra-plate extensional setting, a response to the tectonic transition from the palaeo-Tethyan to palaeo-Pacific dynamic domains. Thus, the occurrence of the Wengong A2-type granite indicates that this tectonic transition possibly initiated at the earliest Early Jurassic.