Objective The Xishadegai molybdenum deposit, located on the northern margin of the North China Craton, is a medium−sized porphyry−type Mo deposit. Its formation exhibits close temporal, spatial, and genetic links with syenogranite intrusions. The rock geochemical characteristics of this syenogranite and its constraints on mineralization warrant further investigation.

Methods This study conducted zircon U–Pb dating, whole−rock major and trace element analysis, as well as strontium (Sr), neodymium (Nd), and hafnium (Hf) isotopic analyses on the syenogranites.

Results LA−MC−ICP−MS zircon U−Pb dating yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 220.2 ± 1.8 Ma, indicating emplacement of the intrusion during the Late Triassic. Geochemically, the rocks exhibit high silica (SiO₂ = 75.09%~77.06%), alkalinity (K₂O + Na₂O = 7.76%~8.34%), and peraluminous character (A/CNK>1), coupled with low Ti, Fe, Mn, Mg, Ca, and P concentrations. They are enriched in Rb, Th, and U, and depleted in Ba, Sr, and Eu. The syenogranites possess low initial ⁸⁷Sr/⁸⁶Sr ratios ((⁸⁷Sr/⁸⁶Sr)_i = 0.703954~0.705263), strongly negative ε_Nd(t) values (−14.91~−13.40) and zircon ε_Hf(t) values (−16.42~−13.30), and ancient two−stage Nd and Hf model ages (2207~2087 Ma, 2292~2103 Ma).

Conclusions The syenogranites are identified as highly fractionated, high−K calc−alkaline I−type granites. They were derived primarily from ancient lower crustal sources. The emplacement occurred under a post−collisional extensional tectonic setting following the collision between the North China Craton and the Siberian Plate. Significant fractional crystallization and moderate degrees of crustal assimilation occurred during magma ascending. Integrated geochemical and zircon−based oxygen fugacity analyses suggest that the highly evolved nature and elevated oxygen fugacity of the magmatic system were key factors controlling the formation of the Xishadegai Mo deposit.