1988 年 42 巻 4 号 p. 220-236
The Early Cretaceous igneous activity in the Kitakami Mountains is one of the most important events during the geologic development of Northeast Japan. This activity has been recently discussed as a sequence of the Mesozoic subduction system in the circum-Pacific region. In the present paper, characteristics of the igneous activity of the Kitakami Mountains are reviewed from the viewpoint of petrography and geochemistry. The Early Cretaceous volcanism preceded slightly the plutonism in the district, are closely associated with plutonism and include those of various kinds of rock series, namely calc-alkaline, tholeiitic high-alumina basalt and alkaline, but the relationship among them and their succession is still uncertain. Granitic rocks of the district were divided into six zones, i. e., Zones I, II, III, IV, V and VI; the last one is divided into subzones of VIa and VIb, from their distribution, mode of occurrences and petrography. These zonal arrangements coincide with the geochemical nature of the rocks showing systematic increase of incompatible elements from rocks of Zone I on the Pacific Coast side to those of Zone IV of the western side of North Kitakami Mountains, but the boundary between Zones V, Via and VIb in ths South Kitakami Mountains is complex. The relation of zonal arrangements between the North and South Kitakami is considered to be as follows: Zones I, II and III of the North Kitakami correspond with Zones Via, V and VIb of the South Kitakami respectively. Some granite bodies such as the Tanohata, Miyako, Tono and Orikabe bodies are typical zoned pluton having felsic central facies and ferromagnesian marginal one. Many granitic rocks are closely associated with gabbroic rocks. Gabbroic rocks of Zones I, II and V are small in amount and are usually composed of hornblende gabbro, and sometimes contain olivine which is relatively rich in Fe and coexists with anorthite. On the other hand, gabbroic rocks of Zones III, IV and VIb occupy a larger part of body, and some of them contain alkali-feldspar and biotite coexisting with olivine which is also rich in Fe and coexists with An poor plagioclase. From these characters, it is suggested that the former gabbroic rocks have been crystallized from more hydrous magma than the latter. The rocks of Zone IV are similar to those of shoshonite association. Field evidence, major and trace element geochemistry of granitic and gabbroic rocks of Zones III, IV and Vlb show continuous relationship in different from that of Zones I, II and V. However, the latter is thought to have possibility of cogenetic in origin at deeper part. N-MORB normalized trace element patterns indicate that all the granitic rocks are rich in LILE and depleted in HFSE and show Nb anomaly similar to those of the rocks of island arcs and active continental margins, but patterns of gabbroic rocks are irregular due to difference in modal proportion of minerals.