Abstract <p>The study of felsic and intermediate volcanites of the Dzhida island-arc system (Southwestern Transbaikalia) has been carried out. The volcanites correspond to low-potassium andesites, dacites, and rhyodacites of the tholeiitic petrochemical series and are comparable to volcanogenic formations predominate within the volcanic fronts of mature island arcs. U–Th–Pb (SIMS) age estimate (527 ± 13 Ma) of crystallization was obtained for dacites for the first time, indicating their formation in the Early Cambrian and, accordingly, the period of transition from the nascent (primitive) arc stage to the mature arc stage. Relatively low isotopic ⁸⁷Sr/⁸⁶Sr (0.7036–0.7038) ratios and positive ɛ_Nd(<i>T</i>) values (4.1 to 6.4) in the intermediate and felsic volcanites are most likely inherited from the juvenile crust, which rocks could have been formed as a result of mantle melt differentiation, i.e. as the island-arc system evolved, additional sources were involved in the magma generation process.</p>