The white ignimbrite layer on top of volcanic sequences in the Coastal Range is the youngest eruption of the northern Luzon Arc in the Taiwan region. A gabbroic enclave was coexisting with the andesitic ignimbrite in this sequence. This study reports the dating results, textural and petrographic descriptions, and geochemical characteristics of the gabbroic enclave and andesitic breccias in the ignimbrite. Both components erupted approximately at 4 Ma, and show enrichment in large ion lithophile elements and depletion in high field strength elements, which are important island arc characteristics. Major and trace elements indicated that the andesitic magma shows larger fractionation and lower degrees of partial melting than the basaltic magma which formed the gabbroic enclaves. We suggest that three sub-stages of magmatism occur during the youngest eruption of the northern Luzon Arc at ~4 Ma. In the first sub-stage, the basaltic magma generated by higher degrees of partial melting from the source rose to a shallow reservoir and cooled to build up the gabbroic wall rocks. Later, lower degrees of partial melting magma were injected into the shallow reservoir and fractionated to intermediate magma causing the second substage. The third sub-stage was activated by the recharged magma, which triggered the intermediate magma to rise along the earlier pathway and assimilated within the gabbroic wall rock. This magma eventually erupted to form the andesitic ignimbrite with gabbroic enclaves within.