Abukuma Plateau Research Articles

阿武隈高原の火成活動 : 阿武隈高原の地質学的岩石学的研究(その9)

阿武隈高原の火成活動 : 阿武隈高原の地質学的岩石学的研究(その9)

かつて「新期」「古期」と呼ばれていた花崗岩類の貫入年代差 ―東北日本、阿武隈地域―

かつて「新期」「古期」と呼ばれていた花崗岩類の貫入年代差 ―東北日本、阿武隈地域―

阿武隈変成帯中に露出する沈み込み帯域オフィオライト断片の岩石学

The Abukuma plateau, which is located along the Pacific coast of northeastern Japan, is composed mainly of Cretaceous granitic rocks and regional metamorphic rocks. In the central part of the plateau, the Gosaisho metamorphic rock series in the east overthrust onto the Takanuki metamorphic rock series in the west. The Gosaisho series is mostly composed of mafic and siliceous rocks, and the Takanuki series is mainly composed of pelitic-psammitic rocks. In the Gosaisho series, many small ultramafic bodies are present in the areas adjacent to the Takanuki series. These ultramafic rocks are affected by contact metamorphism of the Cretaceous granitic rocks in various degrees, but their protoliths are discerned as mantle peridotites and ultramafic cumulates based on their bulk rock chemistry. The ultramafic cumulates are accompanied by metagabbros. At Mount Otsube, mantle peridotites occupy the foot of the mountain and cumulates comprise its top part. The rock assemblage and geological relationship indicates that the ultramafic bodies in this area are fragments of the lower part of an ophiolite. Bulk rock chemistry of the mantle peridotite as well as its olivine and spinel chemistry suggest that the mantle peridotite is highly depleted and of island arc origin. In view of the spinel chemistry, the ultramafic rocks might be independent of the Gosaisho-Takanuki metamorphic rocks and be fragments of the early Paleozoic Hayachine-Miyamori ophiolite. However, the ultramafic cumulates are much richer in Fe (~ Fo70) than those in other ophiolites in Japan. This dismembered ophiolite, which we call Furudono ophiolite, may be a new type of the Paleozoic ophiolites in Japan.

Spatial U–Pb age distribution of plutonic rocks in the central Abukuma Plateau, northeastern Japan Arc

Spatial U–Pb age distribution of plutonic rocks in the central Abukuma Plateau, northeastern Japan Arc

Petrology and geochemistry of small ultramafic bodies in the Abukuma plateau, northeastern Japan

Petrology and geochemistry of small ultramafic bodies in the Abukuma plateau, northeastern Japan

ジルコンのU-Pb年代を用いた阿武隈花崗岩類の 新期-古期貫入関係の再検

ジルコンのU-Pb年代を用いた阿武隈花崗岩類の 新期-古期貫入関係の再検

U-Pb zircon ages of Abukuma granitic rocks in the western Abukuma plateau, northeastern Japan Arc

U-Pb zircon ages of Abukuma granitic rocks in the western Abukuma plateau, northeastern Japan Arc

Granitoids with 300 Ma in the Joban coastal region, east of Abukuma Plateau, northeast Japan

Granitoids with 300 Ma in the Joban coastal region, east of Abukuma Plateau, northeast Japan

40Ar-39Ar and K-Ar geochronology for plutonic rocks in the central Abukuma Plateau, northeastern Japan

The Abukuma Plateau is one of the major granitic terranes in the Japan Arc. Older granitic rocks (foliated and intermediate), younger granitic rocks (massive and felsic), and minor gabbroic rocks are complexly distributed in this region. In order to examine the cooling histories of the plutons, we have determined four 40Ar-39Ar hornblende ages and five K-Ar biotite ages of the plutonic rocks in the central Abukuma Plateau. The results are as follows: Utsushiga-take gabbroic body: 103.8 ± 0.5 Ma (hbl) Nagaya body (older): 97.3 ± 0.6 Ma (hbl), 93.4 ± 3.1 Ma (bt) Shikayama body (older): 103.0 ± 0.4 Ma (hbl), 86.1 ± 3.8 Ma (bt) Ishimori body (older): 99.0 ± 0.5 Ma (hbl), 94.2 ± 2.6 Ma (bt) Miharu body (younger): 91.3 ± 2.3 Ma (bt) Hatsumori body (younger): 90.5 ± 2.6 Ma (bt) (hbl, 40Ar-39Ar hornblende age; bt, K-Ar biotite age).On the basis of these analytical results and the petrography of the samples, we made the following observations:(1) The cooling rates of the Nagaya and Ishimori bodies (56 and 46 °C/m.y.) are equivalent to the common cooling rates of the Japanese granitic batholiths. The cooling rate of the Shikayama body (13 °C/m.y.) is slow because its volume is large.(2) The excellent 40Ar-39Ar plateau age of and the scarcity of metamorphic minerals in the Utsushiga-take gabbroic body suggest that the hornblende age is not a rejuvenated age but the original cooling age.(3) We were unable to detect any distinct geochronological boundary between the older and younger granitic rocks in the central Abukuma Plateau, in terms of the K-Ar biotite ages.

阿武隈高原南端部に分布する西堂平泥質変成岩中の紅柱石―藍晶石―珪線石集合体の成因と温度―圧力径路

Three polymorphs of Al2SiO5, andalusite, kyanite and sillimanite, occur in contact with each other, forming aggregates in a part of the Nishidohira pelitic metamorphics in the Hitachi area of the southernmost Abukuma Plateau. Textural relationships between the three polymorphs suggest that andalusite formed first probably as chiastolitic porphyroblasts, that kyanite and/or sillimanite grew subsequently at the expense of pre-existing andalusite, and that sillimanite continued to form even in the rock matrix thereafter. The andalusite-forming first stage may have been either a contact metamorphic event by unknown plutons or a regional metamorphic event of low-pressure type. The second kyanite and/or sillimanite-producing stage was induced by high-temperature loading, whereas the third sillimanite-forming stage possibly resulted from the contact metamorphism by the Nishidohira gabbroic intrusion exposed near Hase. Well-preserved growth zoning of garnet containing sillimanite inclusions in the andalusite-kyanite-sillimanite-bearing gneisses also indicates the high-temperature compression. The loading may be attributed to the overthrusting of the nearby Hitachi metamorphics onto the Nishidohira metamorphics. Such a P-T path of the Nishidohira metamorphics is similar to that of the Takanuki metamorphics in the central to southern part of the Abukuma Plateau. The widespread occurrence of texturally sector-zoned garnet in pelitic-psammitic rocks and the inferred short duration of high-temperature conditions are also in common to the Nishidohira metamorphics and the Takanuki metamorphics.

阿武隈変成帯，日立西方地域における十字石の新産地

Staurolite-bearing pelitic rocks have been newly found at two locations in the western Hitachi district, southernmost Abukuma Plateau, as predicted by Uruno based on his river sand survey. One is a sheared pelitic schist occurring next to the Tamadare metamorphics near Zoso. It is composed mainly of sillimanite, cordierite, biotite, muscovite, plagioclase and quartz with metastable andalusite. Staurolite is relic, being entirely enclosed in cordierite. The other is a pelitic xenolith in the Nishidohira cortlandtite mass, consisting mainly of sillimanite, garnet, biotite, plagioclase, and quartz. Staurolite occurs together with sillimanite and spinel as inclusions in idiomorphic, normal-zoned plagioclase. Thus, staurolite in these rocks is a relict enclosed in other minerals, although the matrix mineral assemblages are distinctly different between these two. Staurolite is additionally found in the Nishidohira andalusite-kyanite-sillimanite porphyroblastic pelitic gneisses. In this case, staurolite occurs as inclusions in garnet and plagioclase, being occasionally associated with kyanite, sillimanite and muscovite.

南部阿武隈山地花こう岩体中の断裂系と地下水 （その２）

南部阿武隈山地花こう岩体中の断裂系と地下水 （その２）

Mineralogy of piemontite-bearing schist in the Yamagami metamorphic rocks of northeastern Abukuma Plateau.

Piemontite-bearing schist (Pm-schist) in the Yamagami metamorphic rocks, northeastern Abukuma Plateau, was studied in terms of bulk chemical composition, mineral assemblage and mineral chemistry. The Pm-schist occurs as intercalating member of the albite-porphyroblastic muscovite-quartz schist and epidote-amphibolite. Petrographic features suggest that the primary mineral assemblage is quartz+muscovite+albite+chlorite+piemontite+tourmaline+hematite+rutile. Apatite, epidote and carbonate also occur as minor minerals. Veins formed of euhedral quartz and anhedral K-feldspar intruded into microgranoblastic quartz aggregation. The compositions of piemontite range between Pm11Ps22Cz67 and Pm17Ps18Cz55. Muscovite is phengitic (Si 6.54-6.64 atoms per 22 oxygens) and contains moderate amount of hydromuscovite component. Chlorites are clinochlore with Fe/(Mg+Fe)=0.02 and pycnochlorite with Fe/(Mg+Fe)=0.38. Hematite contains Mn2O3 component. The bulk chemical composition of the Pm-schist is characterized by the high silica content, 82.41 wt.%, and high oxidation ratio, (Fe3+×100)/(Fe3++Fe2+)=93. MnO content is very low at 0.02 wt.%. Appearance of piemontite, Mg-chlorite and hematite containing Mn2O3 component suggests that fO2 condition during the metamorphism was very high, which is consistent with the extremely high oxidation ratio of the bulk composition. The extremely high fO2 condition during metamor-phism may be explained by the dissociation equilibrium of H2O with H2 and O2.

阿武隈山地北端部，丸森地域の花こう岩類のＲｂ‐Ｓｒ全岩年代

Rb-Sr ages were determined on 20 whole-rock samples of granitic rocks from the Marumoridistrict, northernmost part of the Abukuma Plateau. The rocks are divided into two groups onthe basis of Rb-Sr isotopic nature; five samples of granitic rocks near the Marumori Bridge, which is hereby named the Marumoribashi granite, give a Rb-Sr whole-rock isochron age of 111.6±2.4 Ma with an initial 87Sr/86Sr ratio of 0.70645±0.00009, whereas all other samples give an age of 106.4±3.5 Ma with an initial ratio of 0.70553±0.00010. Thus the Marumoribashi granite isslightly but significantly older than and genetically different from other granitic rocks of theMarumori district. The mineral isochrons with biotite, K-feldspar, plagioclase and whole-rock give ages of 98.8±4.5 Ma for a sample of the Marumoribashi granite and 99.3±2.0Ma for that of the other group, both of which are identical to each other and slightly younger than the whole-rock ages.

東北日本島弧‐海こう系の地形と地質

The Japan Trench and the northeast Japan arc form a single arc-trench system. The topographic features and the geological history of this arc-trench system are briefly summarized. Distribution of the morphotectonic elements across the system east to west is as follows: An outer swell on the deep sea floor, a group of horsts and grabens bound by normal faults caused by the bending of the Pacific Plate, a deep, flat trench axis covered by thick sediments of both pelagic and hemipelagic nature, a trench inner slope partly exposing Cretaceous mudstone, a gradually uplifting and ponding bench, deep sea terraces covered by thick terrigenous sediments, a non-volcanic outer arc (Kitakami Massif and Abukuma plateau), a volcanic chain (volcanic front), an intra-arc depression, and a marginal sea (Japan Sea). These elements are distributed parallel along the arc. Geophysical parameters are also traceable along the arc. Every geological and geophysical feature around the Japan Trench and the Tohoku Arc show this polarity from the trench axis westward.In the Paleogene, around DSDP Sites 438 and 439 an ancient landmass is indicated by samples and seismic profilers where short-term calc-alkaline volcanic activity took place, boulders of the volcanic rocks then being transported to the site. The Japan Trench forearc area began to subside during Miocene times and diatomaceous hemipelagic mud and the volcanic ashes accumulated in the forearc basin of the Japan Trench area.Since Pliocene time, the Japan Trench area and the Tohoku Arc have gradually been uplifting together with the intrusion of the Tertiary granites.At about 5 Ma a drastic change of the depositional environment in this region resulted from the change of the mode of subduction of the Pacific Plate into the Japan Trench from Mariana-type since 15 Ma, to Chilean-type subduction from 5 Ma to the present.

阿武くま山地南端部で見い出された，せん孔貝の巣穴をもつれきを含む後期新生代れき層

阿武くま山地南端部で見い出された，せん孔貝の巣穴をもつれきを含む後期新生代れき層

南部阿武くま山地花こう岩類の角せん石Ｋ‐Ａｒ年代

K-Ar age determinations were carried out on 19 homblendes and 4 biotites from 9 granitic masses and two metamorphic rocks in the southern Abukuma Plateau, Northeast Japan. Hornblende ages for granitic rocks range from 96.4 to 119 Ma and are apparently not correlated with area or mass. Biotites show a limited age range of 96.1-97.8 Ma. The variation in hornblende age is attributed to the difference in cooling rate of granitic rocks. Hornblende ages for the Gosaisho and Tamadare metamorphic rocks are 115 and 103 Ma, respectively, and nearly equal to those for the adjacent granites. No old ages were found on hornblendes from the Ishikawa, Miyamoto and Samegawa masses, which constitute the older type granites and gave Rb-Sr whole-rock ages of about 400 Ma. This may impose a severe constraint on the thermal history of the older type granites.

Behaviour of fluorine in granitic rocks from the Abukuma Plateau, the Ryoke Zone, and the San-in Zone, Japan.

Behaviour of fluorine in granitic rocks from the Abukuma Plateau, the Ryoke Zone, and the San-in Zone, Japan.

Discovery of Triassic conodonts from the chert pebbles of the Tertiary conglomerates of Joban coal-field (The conglomerate of the eastern terrain of the Abukuma plateau, part 2)

Discovery of Triassic conodonts from the chert pebbles of the Tertiary conglomerates of Joban coal-field (The conglomerate of the eastern terrain of the Abukuma plateau, part 2)

阿武隈高原・花崗岩及び変成岩のRb-Sr, K-Ar, 法による研究

阿武隈高原・花崗岩及び変成岩のRb-Sr, K-Ar, 法による研究