Zircon U-Pb geochronology, biotite mineral chemistry and their petrogenetic implications of the Longwo and Baishigang plutons in Guangdong province

The Longwo and Baishigang plutons, locating in the eastern end of the EW-trending Fogang granite belt, are typical examples of granitic rocks formed by crust-mantle interaction in Nanling region. Zircon LA-ICP-MS U-Pb dating yields an age of (169.1 ± 2.5) Ma for the Longwo pluton and an age of (157.8 ± 2.3) Ma for the Baishigang pluton, indicating that both of them were formed in early Yanshanian. The major and trace element contents of biotites are measured by employing EMPA and LA-ICP-MS techniques, and their petrogenetic significances are discussed. The EMPA analyses of major elements demonstrate that biotites of the Longwo pluton are distinguished from that of the Baishigang pluton by higher abundances of TiO₂, MgO, and lower concentrations of FeO, Al₂O₃ and volatile constituents (e.g., F and Cl), indicating that the Longwo pluton was likely to have formed under a relative high temperature and a more oxidated environment. Trace elements analyses by LA-ICP-MS technique illustrate that biotite is an important carrier of Rb, Ba, Nb, Ta, Sc, V, Cr, Co, Ni in granitic melts, but has less ability in hosting U, Th, Pb, Sr, Zr, Hf, Y, and thus the concentrations of these elements in biotite are much lower than that in the host rocks. Biotite in granitic rocks also displays quite low REE concentrations, suggesting that it is not the main mineral phase affecting the REE features of the host rocks. The mineral chemistry of biotite can be used as a powerful tool in identifying the differentiation degree of the host magmas and in evaluating the rock-forming physical-chemical conditions, and can also be employed in tracing the nature of the magma source. But in the latter aspect, other data (especially isotope compositions) should be coupled with in order to obtain more accurate information.