Zircon U-Pb Dating, Trace Elements Characteristics and Geological Significance of Tonalite in the Huashigou Area, Northwestern Margin of Qaidam Basin

This study focuses on the tonalite in the Huashigou in the western section of the Quanji massif, northwestern margin of the Qaidam basin. Based on detailed field investigations, LA-ICP-MS in-situ isotopic dating and elemental analyses were carried out on the primary zircons to explore the evolutionary history of this area during the important tectonic transition period of the Devonian. The CL images of zircons in Huashigou tonalite show that zircon crystals are mostly columnar or irregularly shaped, euhedral to subhedral grains with clear oscillatory zonations. Zircon U-Pb dating yields a weighted average age of 371.1±3.5 Ma, a concordant age of 370.9±1 Ma, and a Th/U ratio of 0.42～0.83. LA-ICP-MS in-situ elemental analysis shows obvious Ce positive anomalies, strong Eu negative anomalies, and relative enrichment in light rare earth elements (LREE) compared to heavy rare earth elements (HREE). Zircon Ti thermometer calculation reveals zircon crystallization temperatures (t_Zr-Ti) ranging from 628 to 722 ℃, indicating a typical magmatic origin. In the Pb-Th diagram, all zircon spots fall within the I-type granite zircon field, representing that the original magma is crust-mantle mixed. Based on the above information, it is inferred that Huashigou tonalite was formed in an extensional environment after the collision, representing a magmatic event caused by mantle upwelling and crustal reworking.