Neogene Yamadagi volcanic rocks consist of basaltic trachyandesite, trachyandesite, andesite, and dacite. The major- and trace-element chemistry indicates that the lavas are dominantly calc-alkaline and mildly alkaline in character, sodic in nature, and intermediate to acidic in composition. REE and trace-element patterns of volcanic rocks are similar to those typical of within plate magmatics. Volcanic rocks have low Sr-87/Sr-86 (0.70389-0.70633) and high Nd-143/Nd-144 ratios (0.51267-0.51276) and mostly plot within the mantle array of the isotope ratio diagram. The linear correlations among Sr-87/Sr-86 - Nd-143/Nd-144, SiO2 - Sr-87/Sr-86 and SiO2 - Nd-143/Nd-144 isotope ratios in the volcanics suggest that fractional crystallization combined with minor assimilation was an important process within the collision zone. (C) 1998 Elsevier Science B.V. All rights reserved.