Determination of Early Paleozoic TTG intrusive rocks at the southeast edge of Dayao Mountain, Guangxi

This paper presents the zircon LA-ICP-MS U-Pb age, petrology, geochemistry and TTG intrusive rock combination of the Early Paleozoic granite from southeast edge of Dayao Mountain on the southwestern margin of Qinzhou-Hangzhou collision belt. The series of intrusive rock occurs as stocks or dikes with different sizes, which aligns in NE direction as an arc-like belt. According to the distribution of time and space, the intrusive rocks can be divided into two tectonic-magmatic belts: i.e., Gulong- Xiacheng Middle-late Ordovician gabbro diorite + TTG intrusive rock combination (445-475 Ma) and Luoping-Gupao Early Silurian granodiorite-granite rock combination (432-436 Ma). The intrusion with wide range of SiO₂ (51.56%-73.12%) is mainly characterized by low K₂O, high CaO and relative enrichment of Na (Na₂O>K₂O) and is dominated by meta-aluminum and weak peraluminum (A/CNK value ≈ 1), which exhibits evolution trends form low-K, cale-alkaline to high-K calc-alkaline series. Furthermore, the rocks have low total REE (ΣREE=67.82×10^-6-214.81×10^-6), showing chondrite-normalized REE patterns with weak-middle negative Eu anomalies and enrichment of light REE. In addition, the MORB normalized multi-element variation diagram clearly displays intensive enrichment of large ion lithophile elements such as Rb, Th, U and LREE and obvious depletion of Ta, Nb. These petro-geochemical characteristics indicate that the intrusive rocks belong to active continental margin granite rock combination. Combined with regional geological characteristics, the authors hold that the granite series, dominated by I-type granite with crust-mantel mixing source, is the product of typical continental margin, which recorded Early Paleozoic ocean-continent subduction-collsion event at the southeast edge of Dayao Mountain area. The polarity of the TTG intrusive rock combination and granodiorite-granite combination shows that, together with the Cathaysia Yunkai continental margin (subducting from northwest to southeast) adjacent to the east of Dayao Mountain, the oceanic subducation basalt slab, subducting from southeast to northwest, constitutes a bi-directional subduction-collision geodynamic mechanism.