京都府北部舞鶴帯北帯夜久野岩類のＲｂ‐Ｓｒ年代

Abstract

Rb-Sr isotopic age determinations were made on the basement rocks of the Northern subzone of the Maizuru Terrane in the western Maizuru City and Oe Town area. The rocks of the area consist mainly of granitic rocks with minor amounts of quartz-diorite, gabbro, diabase, and pelitic schist and amphibolite (Komori metamorphic rocks), being called the Northern Yakuno rocks. They are distinguished from the Southern Yakuno rocks which consist of ophiolitic complex (Yakuno ophiolite) lying in the Southern subzone of the terrane by their lithofacies and whole rock chemistry. The Northern Yakuno rocks are characterized by the abundance of K-feldspar rich granitic rocks, whereas the Southern Yakuno rocks do not contain such rocks. The Northern Yakuno rocks are thrust over the Shimomidani Formation of the Akiyoshi Terrane to the north in the area. The Rb-Sr mineral isochron ages were determined upon six granitic rocks and one quartzdiorite. They can be divided into two age groups, the older (around 300 Ma) and the younger (around 150 Ma). The rocks give whole rock isochron ages of 327 Ma for the older group and 149 Ma for the younger one, whose initial 87Sr/86Sr ratios are 0.70669 and 0.70779 respectively. Whole rock isochron age for the gabbros and diabases is 163 Ma, and initial 87Sr/86Sr ratio is 0.70687. Nearly coincidence between whole rock and mineral isochron ages for each age group explains the existence of two stage igneous activities at those ages in this area, despite their mylonitization and recrystallization. Both whole rock chemistry and the initial 87Sr/86Sr ratio of those rocks indicate that the Northern Yakuno rocks was evolved as an matured crust of island-arc or continent by the late Carboniferous time. In this respect, the evolutional history of the Northern subzone is quite different from that of the Southern subzone in the terrane.