Lithology and ages of Cretaceous dikes intruding into the Paleozoic-Mesozoic accretionary complexes in the mid-western Kitakami Massif, Iwate Prefecture, and extension tectonics from stress analysis

Abstract

The Nedamo and North Kitakami belts are composed of Paleozoic-Mesozoic accretionary complexes, forming part of the Kitakami Massif in Sotoyama district, east of Morioka City, Iwate Prefecture. These accretionary complexes are intruded by abundant dikes classified into nine rock-types. Mela-quartz diorite, porphyritic fine mela-quartz diorite, and hornblende andesite account for half of the dikes, with the remainder composed of quartz diorite, porphyritic fine quartz diorite, dacite, porphyritic fine tonalite, rhyolite, and mega-porphyritic rhyolite. The dikes are typically several meters thick, mostly high-angle, and trend roughly NNE-SSW. Hornblende andesite and fine quartz diorite dikes yield hornblende with K-Ar ages of 131±3 Ma and 122±6 Ma, respectively, and a rhyolite dike yields zircon with a U-Pb age of 120±1 Ma. Therefore, the dikes in the Sotoyama district are approximately 130-120 Ma (Barremian-early Aptian), and would have been intruded just prior to intrusion of the Early Cretaceous Kitakami Granitoids. Paleostress analysis of dike attitudes indicates that they formed in a WNW-ESE trending extensional stress field, marking a temporary change in the generally E-W compressional stress field within the Kitakami Massif during the Early Cretaceous.