Abstract: In the Fulin Forest Farm area of northern Daxinganling Mountains, a suite of intermediate volcanic rocks is exposed, primarily composed of trachyandesite and trachyte. The LA-ICP-MS zircon U-Pb dating yields an age of 145.8±1.7 Ma, indicating a Late Jurassic emplacement age. Geochemically, these volcanic rocks belong to a high-K calc-alkaline, metaluminous to weakly peraluminous series, characterized by SiO₂ > 56%, high Al (Al₂O₃ > 15%), low Ca and Mg, high Sr (averaging 676.58×10^－6), and low Yb (averaging 1.42×10^－6) and Y (averaging 15.00 ×10^－6). The rocks show significant fractionation between light and heavy rare earth elements, with (La/Yb)_N ratios of 12.50－26.55, and show weak negative Eu anomalies (δEu＝0.74－1.02). Trace element patterns are marked by enrichment in LILEs (K, Rb, Ba, and Sr) and depletion in HFSEs (Nb, Ta, Ti, and P), consistent with the characteristics of "C-type" high-K calc-alkaline adakitic rocks. Integrated studies suggest that the Late Jurassic adakitic volcanic rocks in the Fulin Forest Farm area were derived from low-degree partial melting of K-rich basaltic metamorphic rocks within thickened lower crust. They formed during the late stage of the closure and collisional orogeny of the Mongol-Okhotsk Ocean, under a transitional tectonic regime shifting from compression to extension. The presence of these rocks indicates that crustal thickening occurred in this region no later than the Late Jurassic, implying that the eastern segment of the Mongol-Okhotsk Ocean had already closed by that time.