Keywords:Shitara igneous complex, Middle Miocene, dike, Shitara volcanic rocks, paleomagnetism
We have mapped volcanic dikes at the western margin of the Miocene Shitara igneous complex in Aichi Prefecture. The dikes consist mainly of pyroxene andesite and dolerite and intrude into pre-Cenozoic metamorphic and plutonic rocks of the Ryoke belt. At least 20 dikes were found in the mapped area. Interestingly, their attitudes (i.e., dips and strikes) differ significantly from each other, although they exist within a north–south-oriented narrow zone as a whole. These observations lead us to interpret that, when the dikes formed, magmas intruded into the host rock through a north–south-oriented planar fracture at greater depth and, at shallower depth, intruded through fractures having extremely different attitudes, possibly due to higher magma pressure than principal stresses. Directions of remanent magnetization determined for 11 dikes are classified into three groups. Site-mean directions tightly or moderately cluster at each group, whereas there are significant differences in direction between groups. This suggests that dikes of each directional group formed within a geologically short period (<102 years) compared to the timescale of geomagnetic secular variation, whereas there was a relatively long interval (>102 years) between groups so that mean directions for groups differed significantly due to the influence of secular variation.