Analytical study of mineral parageneses of the Abukuma metamorphic rocks

对日本东北部的武间变质岩（高赛世系和高树系）进行了地质岩石学研究，揭示了它们的P（压力）-T（温度）演化。在gosaiisyo系列富铁锰元岩中首次发现了侏罗纪放射虫化石，从而确定了gosaiisyo系列原始岩石的时代为侏罗纪。因此，阿布库马区域变质作用发生在侏罗纪之后，白垩纪侵入杂岩体侵位之前，时间约为120 Ma。戈赛约系列的岩石没有显示出任何高压到中压变质作用的证据。相反，石榴石和斜长石的结构和成分表明其变质作用处于低压环境。高树泥质岩的特征是出露石榴石斑母岩，化学分带复杂，呈扇形结构。基于石榴石、斜长石和黑云母组成的P-T-t路径表明，在区域变质作用的早期阶段，它们在高温下几乎是等温加载的。高温载荷作用还体现在高野村榴辉岩中的硅线石、海绢石以及刚玉内的橄榄石。这可能是由于Gosaisyo系列对Takanuki系列的逆冲作用。

Geological and petrological studies has been carried out on the Abukuma metamorphic rocks (the Gosaisyo series and the Takanuki series), northeastern Japan, to reveal their P(pressure)-T(temperature) evolution.Jurassic radiolarian fossils were found for the first time in a Fe-Mn-rich metachert of the Gosaisyo series and, thus, the age of original rocks of the Gosaisyo series was confirmed to be Jurassic. Consequently, the Abukuma regional metamorphism took place after Jurassic and before the emplacement of the Cretaceous intrusive complexes about 120 Ma ago.Rocks of the Gosaisyo series have not shown any evidence of high- to mediumpressure metamorphism. On the contrary, textures and compositions of garnet and plagioclase suggest low-pressure conditions during metamorphism.The Takanuki pelitic rocks are characterized by the occurrence of garnet porphyroblasts showing complex chemical zoning and a sector structure. Their P-T-t paths based on the compositions of garnet, plagioclase, and biotite suggest nearly isothermal loading at high-temperatures at an early stage of regional metamorphism. The high-temperature loading is also evidenced by the occurrence of sillimanite and hercynite along with staurolite within almandine and corundum in the Takanuki lareritic rocks. It may be due to the overthrusting of the Gosaisyo series on the Takanuki series.