関東山地跡倉層の後背地(古領家帯と黒瀬川帯の構成要素と改変過程)  [in Japanese] Provenance of the Atokura Formation, Kanto Mountains, central Japan(Geotectonic evolution of the Paleo-Ryoke and Kurosegawa Terranes)  [in Japanese]

関東山地跡倉層の後背地を, 砂岩モード組成, 重鉱物組成, 砕屑性ザクロ石の化学組成から推定した.跡倉層の砂岩は, Qm-F-Lt図上で主にbasement uplift, 一部はdissected arcおよびtransitional continentalのprovenance typeを示す.岩片は花崗岩類, 酸〜中性火山岩類を主体とする.重鉱物は緑簾石, 黒雲母, 白雲母, チタン石, ザクロ石で特徴づけられる.砕屑性ザクロ石の化学組成から, その源岩として低〜中P/T変成岩および石灰質変成岩が推定される.これらの特徴から, 跡倉層の主な堆積物供給源岩は跡倉層を除く跡倉ナップおよび金勝山ナップ構成要素であったと推察される.また, 酸〜中性火山岩類岩片の供給源岩は, 北上山地の前期白亜紀火山岩類や黒瀬川帯のシルル-デボン系に対比される可能性がある.これらの供給源岩は古領家帯を構成し, それが跡倉層の後背地であったと考えられる.

The Upper Cretaceous Atokura Formation constitutes the Atokura Nappe overlying the Mikabu Unit of the Sambagawa Belt. Framework mode of sandstones, modal composition of heavy minerals, and chemical composition of detrital garnets from the Atokura Formation were investigated. Detritus in the Atokura Formation are characterized by fragments of granitic rocks, hornfels, acidic to intermediate volcanic rocks, while those derived from rocks of accretionary complex such as chert, limestone, and greenstones are rather rare. Sandstone compositions plotted on a Qm-F-Lt diagram show dominant provenance types of the basement uplift, and subsidiary of dissected arc and transitional continental. Composition of heavy minerals is characterized by epidote, biotite, titanite, garnet, and muscovite. The chemistry of detrital garnets implies that low to intermediate P/T metamorphic rocks, plutonic rocks and calcareous metamorphic rocks were present in the provenance, while high P/T metamorphic rocks such as the Sambagawa metamorphic rocks was not exposed. Detrital high-Mg garnets whose source rocks have not been found in present Japanese Island are contained in sandstones, and might have been derived from Precambrian basement of the Asian Continent. Most of detritus in the Atokura Formation probably originated from Permian and mid-Cretaceous plutono-metamorphic complex of the Atokura and Kinshozan Nappes. Correlatives of the Lower Cretaceous in the Kitakami Mountains and the Siluro-Devonian in the Kurosegawa Belt are possible sources of acidic to intermediate volcanic clasts. The Paleo-Ryoke Terrane has been constructed of the source of these detritus.