Volcanogenic massive sulphide (VMS) deposits are common in submarine back-arc environments, yet mineralization is unevenly distributed even among volcanic successions formed within the same tectonomagmatic system. Here we investigate felsic volcanic sequences associated with the Murgul VMS district and stratigraphically equivalent barren successions from the Hatila Valley in the Eastern Sakarya Zone, northeastern Türkiye, to evaluate how magma evolution and crustal magma transfer varied between fertile and barren domains within a single Late Cretaceous back-arc basin. Mafic rocks from both domains display comparable juvenile Nd-Hf isotope compositions, with εNd(t) = +1.3 to +3.7 and εHf(t) = +6.8 to +9.8 for Hatila rocks, and εNd(t) = +3.1 to +4.0 and εHf(t) = +8.6 to +10.0 for the Murgul VMS district, together with subduction-modified trace-element signatures, including elevated Th/Yb and Th/Nb ratios. These data indicate broadly similar mantle sources and slab contributions. In contrast, felsic rocks associated with VMS mineralization show restricted whole-rock εNd(t)–εHf(t) variability, with εNd(t) = +3.0 to +3.5 and εHf(t) = +8.3 to +9.2, limited intra-sample zircon εHf(t) dispersion, and coherent zircon trace-element arrays involving U, Th/U, and Nb/Ta versus Hf. Barren felsic suites show broader isotopic and geochemical dispersion, including whole-rock εNd(t) = −9.4 to +0.4, εHf(t) = −11.1 to +5.9, and zircon εHf(t) = −9.2 to +10.2. Zircon oxybarometry indicates broadly overlapping low- to moderately oxidized conditions in barren and fertile suites, with mean ΔFMQ values of +0.79 ± 0.87 and + 0.31 ± 0.34, respectively, suggesting that oxygen fugacity alone did not distinguish fertile from barren systems. Apatite compositions and Nd isotopes broadly parallel the zircon record, with Murgul apatite yielding εNd(t) = +1.6 to +3.6 compared with −1.0 to +2.2 in Hatila apatite. We interpret the fertile domain to reflect coherent juvenile felsic magma evolution, limited open-system crustal modification, and vertically connected magma transfer within a back-arc basin supplied by compositionally similar mafic magmas. These results highlight isotopic focusing, defined here as restricted isotopic dispersion expressed by limited whole-rock Nd-Hf variability and limited intra-sample zircon εHf(t) variation, together with accessory-mineral-scale geochemical coherence, as indicators of magmatic conditions favorable for VMS development in submarine felsic volcanic successions.