Geological and petrological studies of low-temperature metamorphic rocks in the northern Kitakami Mountains.

北上山北部主要为晚古生代至晚中生代沉积岩，主要受白垩纪花岗岩侵入作用的接触变质作用。低温区域变质岩分布在未受后期接触变质作用影响的个别破碎区。详细研究了“早机器构造带”和北北上构造带中典型的低温变质残余物——久崎、森冈、万足山形和久崎地区。在这些地区，基性岩中多见泵柱石+放光石组合，泥质岩中多见云母石+绿泥石组合。碱角闪洞（不是交叉的，而是菱镁矿）只出现在Kuzumaki地区。基性岩中除绿泥石、绿帘石、钠长石外，变质矿物如前长辉石、泵长辉石、静辉辉石等也广泛存在。这些矿物和矿物组合表明，所有地区均发生过泵柱岩-放线石相变质作用。利用电子探针微量分析仪对上述主要变质矿物进行了分析。另一方面，下白垩统（原山组及其相关组）则以前白云岩+泵压砂岩组合为特征。放线石只存在于白垩纪花岗岩体周围热光环的黑云母带中。因此，它们遭受了前花岗岩-泵状岩相变质作用。

The northern part of the Kitakami Mountains is composed mainly of late Paleozoic to late Mesozoic sedimentary rocks, which were mostly subjected to contact metamorphism by Cretaceous granitic intrusions. However, low-temperature regional metamorphic rocks have been found in separate fragmental areas unaffected by the later contact metamorphism.The typical low-temperature metamorphic remnants, the Kuzakai, Morioka, Manami-yamagata and Kuzumaki areas, in the "Hayachine Tectonic Belt" and North Kitakami Belt were studied in detal. In these areas the pumpellyite + actinolite association is commonly found in mafic rocks and the phengite + chlorite assemblage in pelitic rocks. Alkali-amphibole (not crossite but magnesioriebeckite) occurs only in the Kuzumaki area. In addition to chlorite, epidote and albite, metamorphic minerals such as prehnite, pumpellyite and stilpnomelane are widespread in mafic rocks. These minerals and mineral assemblages suggest that the pumpellyite-actinolite facies metamorphism took place in all the areas. The major metamorphic minerals mentioned above were analysed by means of an electron-probe microanalyser.On the other hand, lower Cretaceous formations (the Harachiyama Formation and its correlatives) are characterized by the prehnite + pumpwllyirw association. Actinolite occurs only in the biotite zone of the thermal aureole around the Cretaceous granitic intrusion. Therefore, they have suffered the prehnite-pumpellyite facies metamorphism.