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Two distinctive differentiation trends, tholeiitic and calc-alkalic, are recognized in Zao volcano, which is located immediately behind the volcanic front of the NE Japan arc. The genetic relationship between these two magma series is critical for a better understanding of andesite genesis, because they often coexist in close spatial and temporal proximity in arc volcanoes. Petrographic features indicative of ‘disequilibrium’, such as reversely zoned pyroxene phenocrysts, the wide and bimodal compositional distribution in Ca/(Ca + Na) of plagioclase phenocrysts, honeycomb textures and dusty zones that these plagioclase phenocrysts often exhibit, and the presence of olivine–pyroxene pairs with different Mg/Fe, are observed exclusively in calc-alkalic rocks. In tholeiitic rocks the Sr isotopic ratios of plagioclase phenocrysts, determined by both micromilling combined with thermal ionization mass spectrometry, and laser-ablation inductively coupled plasma mass spectrometry techniques, are constant at 0·7042–0·7044. On the other hand, those in calc-alkalic rocks (0·7033–0·7042) show more complex characteristics, which can be best understood if at least three end-member components, a calc-alkalic basaltic melt, a tholeiitic basaltic melt and a tholeiitic felsic melt, contribute to the production of mixed calc-alkalic magmas. The 87Sr/86Sr and trace element compositions of the least-differentiated basalt magmas, which are inferred from the composition of the calcic plagioclase [Ca/(Ca + Na) >0·9], suggest that two types of basaltic magma, calc-alkalic and tholeiitic, exist beneath the volcano. The tholeiitic basalt magma has a higher 87Sr/86Sr than the calc-alkalic magma (0·7042 vs 0·7038) and a characteristic trace element signature consistent with the presence of plagioclase and amphibole as melting residues. This suggests that the tholeiitic magmas are produced via anatexis of amphibolitic crust caused by underplating and/or intrusion of mantle-derived calc-alkalic basalt magmas into the sub-Zao crust. The mantle-derived calc-alkalic basalt magma mixes with crust-derived tholeiitic melts to form calc-alkalic andesite magmas. The hypothesis proposed here requires revision (or even abandonment) of the general consensus that calc-alkalic magmas have greater contributions of a crustal component than tholeiitic magmas. [ABSTRACT FROM PUBLISHER]