Genesis of the Gneissic Biotite Granite in Lanhe, Northern Guangdong: Constraints from Zircon U-Pb Geochronology, Hf Isotopes, and Geochemistry

[Objective] The Lanhe pluton in northern Guangdong is located at the southeastern margin of the Zhuguangshan pluton, primarily composed of gneissic biotite granite, whose petrogenesis has not yet been clearly defined. [Methods] In this study, LA-ICP-MS zircon U-Pb geochronology, rock geochemistry, and zircon Hf isotope analyses were conducted on the Lanhe gneissic biotite granite. [Results] U-Pb dating results indicate that the emplacement age of the Lanhe gneissic biotite granite is 427 ± 2 Ma, representing a product of the Caledonian magmatic activity. The geochemical characteristics show that the granite has SiO₂ contents ranging from 71.53% to 75.41%, high total alkali contents (K₂O + Na₂O = 7.57%–8.23%), and A/CNK values (1.00–1.06). It is enriched in Rb, Th, U, and K, but depleted in Ba, Y, Nb, Ta, Sr, and Yb. The LREE/HREE ratios range from 9.49 to 28.15, with significant Eu negative anomalies (δEu = 0.21–0.76). The zircon ε_Hf(t) values of the samples are all negative (-11.8 to -5.2), with corresponding t_DM2 values of 1806–2129 Ma. [Conclusion]Based on the geochemical and isotopic characteristics, the Lanhe gneissic biotite granite is identified as a highly fractionated I-type granite, primarily formed by partial melting of crustal metasedimentary rocks, including metagraywacke and metapelite. It is likely a product of the multi-stage reworking of the Paleoproterozoic basement during the Neoproterozoic to Early Paleozoic. The comprehensive study suggests that the Lanhe gneissic biotite granite formed in a syn-collisional tectonic setting during the Early Paleozoic in South China. [Significance] Integrating with the Zhuguang magmatic system and regional geological data, the Lanhe pluton likely represents a product of the transition from compressional thickening to post-collisional extension during the Caledonian orogeny in South China. This transition may have been associated with intracontinental tectonic reorganization or external subduction-collision processes.