A melt contamination model is presented to explain the formation of “I-type” granitic rocks, based on studies of migmatitic mafic granulites from the Hidaka Metamorphic Belt (Japan). Analysis of apatite and zircon from leucosomes and mesosomes of the migmatites reveals grain-scale Nd and Hf isotope heterogeneities of >10 epsilon units, inconsistent with closed-system anatexis. We interpret this marked isotopic variability to instead reflect hybridization between metasedimentary-derived partial melts and interlayered mafic granulite horizons during extraction of silicic melt from the lower crust. This open-system melt-rock interaction induces local isotopic modification of the mafic granulites and shifts the Hf isotope signature of the anatectic melt to more radiogenic values, similar to those of hornblende-bearing (I-type) granitic rocks emplaced into the Hidaka sequence at higher crustal levels. This study shows that the generation of broadly I-type granitic magmas does not require extreme temperatures to extensively melt meta-igneous rocks, nor is the direct input of mantle magma essential. Mineral-scale isotopic heterogeneities in such magmas reflect derivation from contrasting crustal sources and the rate and mechanism of magma transfer through the granulitic lower crust.