We obtained the radiometric ages of detrital zircons from two samples of paragneiss from Oki-Dogo Island, Japan, from the 238U/206Pb ratio and isotopic composition of Pb determined using a Sensitive High-Resolution Ion MicroProbe (SHRIMP II). The zircons show two main age clusters around 2200 Ma and 1800 Ma, with some discordant ages. One zircon grain has a rim with a concordant age of 236 ± 3 Ma, which is consistent with the age of peak metamorphism in the Hida Metamorphic Belt. The modal proportions of zircon ages from Oki-Dogo Island are clearly different from those for paragneiss of the Hida Terrane. This indicates that the Oki Gneiss is derived from an Early Proterozoic rock and is different from the Hida Gneiss in terms of provenance and/or the depositional age of the parent sediment.
In Lake Suigetsu, central Japan, greenish/light‐brown granules identified as cytoplasmic masses had been preserved in siliceous cell walls of freshwater diatoms in annual layers of lacustrine muds since the early Holocene. The lacustrine muds consisted of alternating dark‐colored (rich in diatom valves, clay, and organic matter) and light‐colored (mainly diatom valves) laminae. The greenish/light‐brown granules were predominately preserved in frustules of the genus Aulacoseira preserved in the dark‐colored laminae. The dark‐colored laminae were inferred to have formed annually under stratified water caused by surface water warming in summer that caused the formation of an organic‐rich anoxic layer on the lake bottom that favored granule preservation. The good preservation of cytoplasmic masses in dark‐colored laminae suggested a cause for diatom assemblage periodicity, a phenomenon that was commonly noted in temperate lakes: the cells containing these masses could be potential seed stocks for subsequent spring blooms. Frustules of the most abundant granule‐containing species, Aulacoseira nipponica (Skvortzow) Tuji, in the dark‐colored laminae of the Early Holocene muds were abundant in the overlying light‐colored laminae, suggesting that these species reproduced abundantly in springtime yielding a massive diatom bloom.
Munakataite, Pb2Cu2(Se4+O3)(SO4)(OH)4, occurs as a thin coating on a fracture in a quartz vein containing Cu-Zn-Pb-Ag-Au ore minerals in the Kato mine, Munakata City, Fukuoka Prefecture, Japan. It is monoclinic with the space group P21/m and lattice parameters a = 9.766(8), b = 5.666(5), c = 9.291(10) Å, β = 102.40(8)°, V = 502.1(8) Å3, and Z = 2. An electron microprobe analysis gave the empirical formula as Pb2.03(Cu1.94Ca0.01)Σ1.95(Se4+O3)1.00(SO4)1.02(OH)3.92 on the basis of Pb + Cu + Ca + Se + S = 6 apfu and the calculated (OH) with a charge balance. Munakataite is a member of the linarite-chenite group, and it is the first selenite mineral in Japan, corresponding to a (SO4)-dominant analogue of schmiederite. The mineral occurs as light-bluish aggregates composed of minute fibrous crystals up to 30 μm long. The calculated density is 5.526 g/cm3, and Mohs hardness is less than 2.
Abstract Conglomerates of the Kuma Group, central Shikoku, southwest Japan contain Sanbagawa schist clasts with a variety of metamorphic grades and lithologies. K–Ar and 40 Ar/ 39 Ar dating of phengite show all the pelitic schist clasts from low‐ to high‐grade zones have similar phengite ages (82–84 Ma) that are significantly older than those from the in situ Sanbagawa sequence of central Shikoku. This is because the Kuma–Sanbagawa sequence was exhumed earlier than the in situ Asemi sequence with an exhumation process intermediate between those for the Kanto Mountains and the in situ Asemi sequences. 40 A/ 39 Ar plateau ages (103 and 117 Ma) of phengite in amphibolites indicate the timing of the early stage of the exhumation of the metamorphic pile, probably close to the peak metamorphic age.
Time bracketing of thousands-of-meters-thick, unmetamorphosed, fossil-free sedimentary successions is a long-standing challenge in Indian Proterozoic stratigraphy. Limited geochronologic ages from either the basal or the upper part of the successions prevent workers from constraining the precise age interval for individual basin fill, leaving aside the issue of correlatability between different basin sequences. This article deals with silicic tuff units from the basal part of two areally separated Proterozoic sedimentary successions of central India, the Singhora Group of the Chattisgarh Supergroup and the Khariar Group. The Singhora tuff is enriched in large ion lithophile (Rb, Ba, Th, etc.) elements and light rare earth elements and in a discrimination diagram falls within the volcanic arc granite field. U-Th-Pb electron probe microanalyzer geochronology of monazite and zircon grains from the Singhora tuff revealed several age data clusters, namely, ∼2500, ∼2100, ∼1800, ∼1500, and ∼1000 Ma. In contrast, the age data from the Khariar tuff fall within a narrow range of ∼1455 Ma. To isolate the depositional/crystallization age of the Singhora tuff from inherited provenance ages, we also dated monazite and zircon grains from stratigraphic units overlying and underlying the Singhora tuff. The ∼1500-Ma age is proposed as the depositional/crystallization age for the Singhora tuff, considering its most frequent and exclusive presence within the tuffaceous unit. Comparable ages from the tuffaceous units at the basal part of areally separated basins, namely, Singhora (∼1500 Ma) and Khariar (1455 ± 47 Ma), suggest nearly synchronous initiation of the two basins. Considering the recently reported U-Pb SHRIMP zircon age of 1631 ± 5 Ma from the basal part of the Vindhyan Supergroup, the largest Proterozoic sedimentary basin of peninsular India, we presume a crustal-scale event in early Mesoproterozoic time (1600–1500 Ma) on the Indian continent.
The GSJ B326 borehole was drilled in the Joban coastal region, east of the Abukuma Plateau, reached a depth of ∼ 1005 m. Although most of the sequence is composed of Tertiary sediments, plutonic rocks occur from 815.1 m to 1005 m in depth. The lithology of the plutonic rocks is tonalite and subordinate aplite veins. Radiometric ages of zircons in three granitoids were obtained from the 238U/206Pb ratio and isotopic compositions of Pb using a Sensitive High Resolution Ion MicroProbe (SHRIMP II). The weighted mean of the zircon ages of a sample from the shallower part of the borehole is 293 ± 1.8 Ma, whereas that for two samples from the deeper part of the borehole is 300.3 ± 1.5 Ma and 304.3 ± 1.7 Ma. The previously reported U-Th-Pb chemical age of uraninite from the deeper part of the borehole is 285 ± 4 Ma. These data indicate that a melt intruded into the ∼ 300 Ma rock body at ∼ 290 Ma. Granitoid with 300 Ma are scarce in the Japanese Islands, and this is the first report of such granitoids in northeast Japan.
