岡山県井倉地域の変成岩類 : パイルナップと岩相  [in Japanese] Metamorphic rocks in the Ikura area, Okayama Prefecture, Southwest Japan : pile nappes and lithology  [in Japanese]

The Sangun metamorphic rocks and weakly metamorphosed Paleozoic rocks of the Ikura area in Okayama Prefecture, which are of the Late Paleozoic Sangun-Chugoku belt, are divided into five formations on the basis of their lithologies and metamorphic grades. Each formation occurs as a thrust nappe, forming collectively a pile-nappe structure; from below to top, the Iwanaka, Oojaridani, Taniai, Ikura II and Ikura I Formations. The Ikura I Formation consists of schistose and crystalline limestones and underlying greenstones derived from intraplate, alkali basalts. The Ikura I Formation, which occurs as a thin sliver, is composed of alkali amphibole-bearing basic schists. The Taniai Formation is characterized by a sedimentary melange with blocks of various lithologies in a pelitic matrix. The blocks consist largely of limestone, pillow lava with MORB geochemistry, chert, sandstone and mudstone. Other block types include pervasively sheared metagabbroic to metagranitic rocks of the calc-alkalic affinity. The Oo-jaridani Formation is made up mainly of alternating beds of basic and pelitic schists. The Iwanaka Formation shows a lithology of sedimentary melange. The upper three formations corresponding to the north zone of the Chugoku belt seem to have undergone glaucophanitic metamorphism of which grade increases downward from the pumpellyite-chlorite zone (Ikura I F. and upper Taniai F.) through the pumpellyite-actinolite zone (lower Taniai F.) to the epidote-glaucophane zone (Ikura II F.). The Oojaridani Formation which belongs to the Sangun metamorphic rocks proper has also been affected by glaucophanitic metamorphism of the pumpellyite-actinolite to epidote-glaucophane zones. The block association observed in the Taniai melange complex suggests that the complex was formed in a trench setting during Permian time. The calc-alkalic blocks indicate the presence of a nearby mature arc. Probably a part of basement rocks of the arc was exposed at a landward-slope of the trench by some tectonic processes, forminga ridge from which the calc-alkalic blocks were derived. The Ikura I Formationis interpreted as a fragment of an Early Carboniferous seamount capped by limestones, which was acrreted at the front of the arc. The Ikura II Formation may also represent a remnant of the seamount which was emplaced into deeper levels in a subdution zone and was metamorphosed into glaucophane schist facies rocks.