Rhyolitic Welded Tuff Research Articles

Subaqueous felsic volcanic sequence and its contribution to the ancient alkaline lacustrine deposits in the Mahu Sag, Junggar Basin, NW China

Subaqueous eruptions alter the chemical conditions of the lake by intensifying the interaction between hot volcanic rocks and water, resulting in the exchange of alkalis. The lower part of the Fengcheng Formation in the Mahu Sag, NW China, has abundant rhyolitic welded tuff and related pyroclastic deposits, which were deposited under ancient alkaline lacustrine deposits of the Fengcheng Formation. Despite this, the effect of the eruption process on the formation of the alkali lake remains unclear. To gain a better understanding of the eruptive behaviour and emplacement processes, detailed drilling core descriptions, microscopic observations, and interpretations of textural characteristics of rhyolitic welded tuff units and adjacent volcano‐sedimentary associations have been conducted. The rhyolitic welded tuff can be categorized into seven lithofacies units based on the degree of welding, devitrification characteristics, pore development, and filling characteristics. The age of eruption was determined through zircon U–Pb dating of the rhyolitic welded tuff, revealing it to be approximately 300 Ma, thereby providing constraints on the deposition time of alkaline lacustrine deposits. Mudstones that encase the welded tuff unit and quenched structures such as perlitic fractures, quench cracks of spherulite and lithophysae, and chalcedony spherules indicate that the welded tuff unit was formed through subaqueous eruption. Alkali fractionation (sodium and potassium) occurs in the spherulite and lithophysae zone and at the top condensation boundary, demonstrating that the subaqueous eruption process significantly enhanced devitrification and hydration, causing sodium to escape from the welded tuff and released into the lake water as sodium ions. The rhyolitic welded tuff provides evidence of the evolution of a subaqueous felsic eruption where the water‐rock reaction played a fundamental role in alkalizing the lake by providing the lake with sodium ions in Mahu Sag.

Measuring Gas Transport and Sorption in Large Intact Geologic Specimens via the Piezometric Method

Uptake of noble gases into heterogeneous geologic core samples was measured using a piezometric methodology. In addition to measuring accessible porosity—as with gas pycnometry—by monitoring the rate of pressure decay, this method can also be used to estimate the gas effective diffusivity in the sample. In contrast to previous applications of this method, where milligram quantities of fractured grains are characterized, here approximately kilogram core samples were left intact when tested. In doing so, a more representative sample of the heterogeneous field geology is provided. Additionally, alteration of the pore structure and connectivity during sample preparation is avoided. To scale the piezometric method from milligrams to kilograms, the system was designed to operate at medium vacuum (1 to 100 Pa) to restrict transport in pores less than approximately 60 µm to large Knudsen numbers. To test the system performance, two samples of interest were selected: a rhyolitic welded tuff from Blue Canyon Dome at the Energetic Materials Research and Testing Center and a zeolitized non-welded rhyolitic tuff from the Nevada National Security Site. Three noble gases were utilized in this test series; Argon and xenon as they are of direct interest to nuclear monitoring efforts and helium as it is a weakly adsorbing reference standard. Additionally, mercury intrusion porosimetry measurements were made on subsamples of the core to compare the observed porosity by the two methods and to discuss gas transport rates in the context of the measured pore distribution.

Fluid inclusions, isotopic characteristics and geochronology of the Sinongduo epithermal Ag-Pb-Zn deposit, Tibet, China

The Sinongduo Ag-Pb-Zn deposit is the first documented low-sulfidation epithermal deposit in Tibet and located on the southern side of the Longge’er-Gongbujiangda arc fault belt of the Lhasa terrane. The Ag-rich base metal veins and relatively low-grade breccias and stockworks in the Sinongduo deposit occur in the Paleocene high-potassium calc-alkaline to shoshonitic series volcanic rocks, including rhyolitic welded tuff breccia and volcanic breccia, rhyolitic ignimbrite, rhyolitic crystal fragment tufflava, rhyolite, and rhyolitic tuff. A nearly N-S-trending fault is the most important ore-controlling structure in the deposit. Galena and sphalerite are the dominant ore minerals in quartz-sericite-illite ± adularia veins, with subordinate argentite, pyrargyrite, native silver, chalcopyrite, pyrite and tennantite. The most widespread silver minerals are argentite, pyrargyrite, pearceite and native silver, which occur in fissures in Fe-Mn carbonates, jasper and early sulfides. The mineralization process can be divided into a pre-ore stage, main ore stage, and post-ore stage, and the main metallogenic stage is divided into the galena substage, sphalerite substage and silver minerals substage.Microthermometric analyses of the primary liquid-rich and vapour-rich fluid inclusions (FIs) in the sphalerite (SpII), main ore stage quartz (QzII) and post-ore stage quartz (QzIII) of the Sinongduo deposit indicate that the FIs in SpII, QzII and QzIII are mainly homogenized at temperatures of 197.2–280.8 °C, 185.6–290.4 °C and 148.2–218.7 °C with salinities of 1.40–5.71 wt% NaCl equiv., 1.59–5.71 wt% NaCl equiv. and 2.07–8.94 wt% NaCl equiv., respectively. These data indicate that the deposit was formed in a low-temperature and low-salinity environment, corroborating an epithermal system of mineralization. The O and H isotope compositions of the ore-forming fluids (δ18O = −0.6 to −6.3‰; δD = −177 to −151‰) indicate there was a major contribution from a meteoric water component to metallogenesis. All of the Pb isotope data from the metal sulfides and country rocks in the Sinongduo deposit in this study are similar and imply a close genetic relationship. The δ34S values of galena and sphalerite exhibit narrow distributions, ranging from +4.1‰ to +5.6‰ and from +7.6‰ to +8.4‰ respectively, with average values of +5.0‰ and +8.1‰, respectively, suggesting a uniform source. The Ar-Ar ages determined from sericite in the quartz-sericite-illite-sulfide veins range from 60.9 ± 0.7 to 63.1 ± 0.7 Ma, showing that the timing of epithermal Ag-Pb-Zn mineralization in the study area is consistent with the eruption of the Dianzhong Formation volcanic rocks (DVR) (U-Pb dating, 65–62 Ma), which indicates a genetic relationship. The characteristics of FIs, the H-O-S-Pb isotope data and the mineralization ages suggest that the ore-forming fluids were primarily sourced from circulating meteoric water, while S and Pb were derived from the DVR. Our results, combined with previous data on the regional geology, ore geology, FIs, and tectonic setting, indicate that the Sinongduo Ag-Pb-Zn deposit was produced in an epithermal system that formed on the southern side of the Longge’er-Gongbujiangda fault-block uplift belt, following the northward subduction of the Neo-Tethyan oceanic plate beneath the Asian continent.

Mineralogical characterization and formation process of spherulite in the rhyolitic welded tuff from Aga town, Niigata Prefecture, Japan

Mineralogical characterization and formation process of spherulite in the rhyolitic welded tuff from Aga town, Niigata Prefecture, Japan

Origin of two contrasting latest Permian–Triassic volcanic rock suites in the northern North China Craton: implications for early Mesozoic lithosphere thinning

New geological, geochronological, and geochemical results on volcanic rocks and cobbles from early Mesozoic sedimentary rocks identify two contrasting latest Permian–Triassic volcanic rock suites in the northern North China Craton (NCC). The early rock suite erupted during the latest Permian–Early Triassic at ca. 255–245 Ma and was probably widely distributed in the northern NCC prior to the Early Jurassic. It comprises rhyolitic welded tuff, rhyolite, and tuffaceous sandstone and is characterized by high contents of SiO2 and K2O, moderate initial 87Sr/86Sr, low negative εNd(t) and εHf(t) values, and old Nd-Hf isotopic model ages. It was likely produced by fractional crystallization of lower crustal-derived magmas due to underplating by lithospheric mantle-derived magmas near the crust–mantle boundary in syncollisional to post-collisional/post-orogenic tectonic settings. The late rock suite, erupted during the Middle–Late Triassic at ca. 238–228 Ma, displays adakitic geochemical signatures and consists of intermediate volcanic rocks such as andesite, trachyandesite, and autoclastic trachyandesite breccia, with minor felsic rocks. This suite is characterized by high Al2O3, MgO, Sr, Ba, Cr, V, and Ni concentrations; high Mg# values; low Y and Yb concentrations and high Sr/Y ratios; low initial 87Sr/86Sr; high negative εNd(t) and εHf(t) values; and young Nd-Hf isotopic model ages. The younger suite was generated by mixing of magmas derived from melting of upwelling asthenosphere, with melts of ancient lower crust induced by underplating of basaltic magmas in an intraplate extensional setting. Strong upwelling of asthenospheric mantle and significant involvement of the asthenospheric mantle materials indicate that the lithospheric mantle beneath the northern NCC was partially delaminated during Middle–Late Triassic time, representing the initial destruction and lithospheric thinning of the northern NCC. Lithospheric thinning and delamination are likely the most important reasons for the Triassic tectonic transition and change of magmatism and deformation patterns in the northern NCC.

岐阜県中津川市，小秀山北西斜面に分布する流紋岩質溶岩 (小秀山流紋岩) のK-Ar年代

Mt. Kohide (Nakatsugawa City, Gifu Prefecture) is located ∼ 7 km north of the Atera fault. A partial collapse of the northwestern flank of Mt. Kohide has exposed a 5-m-wide dyke-like conduit ∼ 15-30 m beneath the extrusive rhyolite lava that was fed by it. The rocks of the extrusive lava and the feeder are nearly aphyric (< 0.5 vol% phenocrysts), biotite-rhyolite (∼ 73 wt% whole-rock SiO2). The extrusive lava yields a K-Ar age of 23.01 ± 0.36 Ma. Within the measurement errors, this age coincides with the previously determined K-Ar ages of biotite-rhyolite dykes intruded along the Atera fault (23-22 Ma) and the rhyolitic welded tuff of the Hachiya formation (∼ 40 km southwest of Mt. Kohide; 22 Ma) as well as the K-Ar and fission track ages of the rift-type alkalic moonstone rhyolites from the Hokuriku area (25-22 Ma). This suggests that rhyolitic volcanism occurred contemporaneously in central Japan between the Late Oligocene and Early Miocene.

室生火砕流堆積物の全岩化学組成 その給源への手掛り

The Muro pyroclastic flow deposit (MPFD), distributed in the central part of the Kii peninsula southwest Japan, is composed of rhyolitic welded tuff, representing a single cooling unit. Though the MPFD is a remarkable gigantic igneous unit with the volume of ∼100 km3, its source region is still in dispute. To discuss the problem, bulk rock chemical composition of MPFD were determined by XRF and LA-ICP-MS. The samples used in this study scarcely contain exotic materials. The MPFD welded tuffs indicate rhyolitic compositions with SiO2=74.0-75.5 wt%. They are characterized by LREE enrichments, large Eu negative anomalies (Eu/Eu*=0.30-0.59), and relatively constant chondrite-normalized middle to heavy REE abundances. All the analyzed samples have mole Al2O3/(CaO+Na2O+K2O) ratio higher than 1.1. Garnet is ubiquitously included, and some samples contain cordierite. Some of the rhyolitic rocks in the middle Miocene Setouchi Volcanic Rocks, as found in the lowest horizon of the Nijo Group for example, also have peraluminous bulk rock compositions. However, they are discriminated from MPFD by whole rock chemical compositions, in particular depletion in HREE. Actually, major and trace element compositions of MPFD are very close to those of the middle Miocene S-type granitoids in the Outer Zone of southwest Japan. These facts strongly suggest that MPFD was derived from an S-type felsic igneous body in the Outer Zone, travelling more than 20 km.

石川県に分布する鷲走ヶ岳月長石流紋岩質溶結凝灰岩の 古地磁気とフィッション・トラック年代

石川県に分布する鷲走ヶ岳月長石流紋岩質溶結凝灰岩の 古地磁気とフィッション・トラック年代

新第三系北陸層群の鷲走ケ岳月長石流紋岩質溶結凝灰岩

新第三系北陸層群の鷲走ケ岳月長石流紋岩質溶結凝灰岩

Miocene rhyolitic welded tuff infilling a funnel-shaped eruption conduit Shiotani, southeast of Matsue, SW Japan

At Shiotani, SW Japan, rhyolitic welded tuff forms a steep-sided funnel-shaped body, confined by Paleogene granitic rocks to an elliptical area 1–1.5 km across. The Shiotani welded tuff is pervasively welded and foliated concordantly with the contact that dips inward at angles of 70–90°. In contrast, nearby contemporary volcaniclastic deposits are non-welded and gently inclined. Near the contact with the granite, the tuff is plastically deformed and shows lineations that plunge inward at angles of 40–65°. Lithic and crystal clasts in the rheomorphic outer part are rotated in a plane normal to the foliations and parallel to the lineations indicating downward flow of the welded tuff. The geometry and internal structures suggest that the Shiotani welded tuff was emplaced and welded in a funnel-shaped eruption conduit. Upon collapse of a plinian or phreatoplinian eruption column, the majority of the conduit-filling pyroclasts probably fell back en masse into the conduit. Heat and steam from underlying magma and diffusion of interstitial volatiles into the glass perhaps reduced the viscosity of juvenile pyroclasts and facilitated welding in the conduit, especially at deep levels. The hot welded pyroclasts then flowed down the conduit wall during welding compaction and retreat of the magma. These processes resulted in increased welding toward the contacts and welding foliations concordant with the steep wall. Emplacement of nearby correlative volcaniclastic mass-flow deposits in a shelf to upper bathyal environment suggests a possibility that, when active, the Shiotani conduit was under the sea. Welding compaction would occur even under the sea provided that the steam generated in the upper part of the conduit fill prevented water access.

Glass inclusions in quartz phenocrysts of rhyolites, Mt. Jang, Pusan, Korea

Quartz phenocrysts of rhyolitic welded tuff from Mt. Jang, Pusan, preserve not only clear and homogeneous glass inclusions but also partially devitrified inclusions. The glass inclusion that contains a trapped liquidus phase comprises a vapor bubble, whereas the devitrified inclusions are commonly associated with micro-cracks. These glass inclusions are composed of highly evolved peraluminous potassic rhyolites, and are more differentiated than those of the bulk rocks. Hornblende and plagioclase phenocrysts began to crystallize earlier than potassium feldspar and quartz phenocrysts. Melt temperatures prior to the eruption and quenching were probably 740–765°C, estimated from the homogenization temperature of glass inclusions. Distinctive long prismatic crystal form of glass inclusions suggests that part of the phenocrysts grew in the stability field of α-quartz and thus the tuff unitprobably solidified at least below the β-α inversion temperature. Total volatile contents in melts (obrained by summing H2O, F, and CI) vary from 1.8 to 4.0 wt%. Textural and mineralogical characteristics suggest that magma was water-saturated shortly before or during the eruption. H2O content of the glass (ca. 1.0–3.0 wt%) suggests a water saturation pressure\((P_{H_2 O} )\) of about 200–600 bars. This pressure implies a minimum depth of 0.5–1.6 km for the magma chamber.

亀裂性岩盤の比貯留係数の推定に関する一考察

Specific storage, one of the hydraulic constants, is defined as a rate of volume change caused by the elastic deformation of structural skeleton and pore water in a porous media associating with changing hydraulic pressure. In this report,, the specific storage was theoretically estimated from the combination of compressibility of skeleton of rock mass, compressibility and density of pore water, and porosity.The basement rocks of the studied area, mainly composed of rhyolitic welded tuff, are considerably fractured from the observation of the survey tunnels having a total length of 300 meters and of the vertically drilled four bore-holes to the tunnels. The basement rocks also have high permeability, determined from the Lugeon water tests carried out at about four bore-holes. Hydraulic conductivities and specific storages have precisely been determined by the cross-hole hydraulic pressure tests performed at selected four bore-holes. An apparent principal hydraulic direction can be distinguishable from the differences of hydraulic pressure propagation among horizontal three directions during cross-hole hydraulic pressure tests. The specific storage value is larger in the apparently minimum principal hydraulic direction than that in the maximum principal hydraulic direction.The compressibility of skeleton was determined from the data of elastic wave exploration around survey tunnels and bore-hole horizontal loading test at several bore-holes. The specific storage of a fractured rock mass can be calculated from the combination of the compressibility of skeleton, compressibility and density of pore-water and porosity. Consequently, the calculated specific storages is concordant with that of the apparent minimum principal hydraulic direction. Specific storages of the apparent maximum principal hydraulic direction have smaller of 2 orders and more than those calculated above. The smaller values are explained that volume of the pore in fractures varies with ground water seepage along the apparent maximum principal hydraulic direction without the volume change of the rock skeleton. When the rock skeleton along the apparent maximum principal hydraulic direction has no volume change associating with seepage flow, the specific storage is given by the product of compressibility and density of pore water, porosity and coefficient, α, where α is the ratio of pore volume in the main flow paths, which volume changes during seepage flow, to total pore volume. In the studied rock mass here, α ranges from 0.07 to 1.3. The α is smaller than 1, because ground water flows in a part of fractures which regard as channels, and the storage of the ground water is due to the volume change of a certain part of pore space in the main ground water flow path.

A Rb-Sr geochronological study on the Early Cretaceous Monai volcanic rocks from Okushiri Island, Northeast Japan Sea.

Rb-Sr whole rock isochron age has been determined on the Early Cretaceous Monai volcanic rocks occurring in Okushiri Island, Northeast Japan Sea. Five samples of dolerite, andesite and rhyolitic welded tuff give a well defined whole rock isochron with an age of 101.3±4.8 Ma and an initial 87Sr/86Sr ratio of 0.70532±0.00007 (2σ). These results almost agree with the mineral and whole rock isochron ages and the initial Sr isotopic ratios determined by the previous workers for the pre-Tertiary granitoids occurring in Okushiri Island and the western margin of southwest Hokkaido. This facts indicate the concurrence of the pre-Tertiary volcanism and plutonism in these areas and the similarity in isotopic compositions of the source materials for the Monai volcanic rocks and these granitoids. The initial Sr isotopic ratio of the Monai volcanic rocks are much higher than those of the Early Cretaceous volcanic rocks from the region to the west of Hidaka Mountains and the eastern margin of the Kitakami Mountains. Comparative investigations of initial Sr isotopic ratios reported by the previous and present workers for the Early Cretaceous volcanic rocks and granitoids from the NE Japan arc and Hokkaido indicate that initial Sr isotopic ratios for these igneous rocks increase from the eastern tholeiitic volcanic zone to the western calc-alkaline volcanic-plutonic zone, in contrast to the across-arc variation of the Sr ratios of the Quaternary volcanic rocks from the NE Japan arc.

Thermal annealing characteristics of spontaneous fission tracks in zircon

One-hour isochronal annealing experiments were carried out on zircon crystals KT 06, separated from a rhyolitic welded tuff of the Koto Rhyolites, southwest Japan, in order to study thermal annealing characteristics of spontaneous fission tracks in zircon. Variations of track density, horizontal confined track length and track angular distribution were measured at various temperatures between 400° and 750°C. Track density and track length begin to decrease around 450°C and reach zero around 750°C. Both time and anisotropy of track etching increase drastically for the samples annealed above 500°C because of the loss of accumulated radiation damage derived from α-decay of U and Th. Angular distributions of etched tracks in those samples suggest no significant apparent reduction of etching efficiency due to increased etching anisotropy, so long as adopted etching criteria are strictly maintained. Angular distribution measurements also reveal anisotropic annealing characteristics of tracks, which may produce an additional sampling bias in track length measurement.

Petrology and geochemistry of the Loch Ba ring-dyke, Mull (N.W. Scotland): an example of the extreme differentiation of tholeiitic magmas

The Loch Ba ring-dyke in the Tertiary igneous central complex of Mull, N.W. Scotland is composed predominantly of a banded rhyolitic welded tuff. The rhyolite contains numerous inclusions of dark aphanitic rock. The textural relationships between the different rocks indicate rapid, violent and intimate mixing during emplacement of the dyke. The dark glassy component varies continuously from basaltic andesite to andesite, dacite and rhyolite. These glasses are enriched in FeO and depleted in MgO at a given SiO2 content in comparison to other tholeiitic highly differentiated volcanic rocks. The rhyolite contains an average of 4% phenocrysts and is associated with the mineral assemblage plagioclase (An32 to An21)-sanidine(Or50–60)-hedenbergite-fayalite-magnetite-ilmenite-apatite-zircon. Mineral aggregates involving either plagioclase-hedenbergite-ilmenite or plagioclase-fayalite-magnetite are common, but aggregates containing fayalite and hedenbergite together are scarce. The dark glassy components are either phenocryst free or contain less than 0.2% phenocrysts. The main phenocrysts associated with the dark glasses are plagioclase (An65-An30), high calcium clinopyroxene ranging continuously from augite to pure hedenbergite, pigeonite, magnetite, ilmenite and rare apatite. Zoning in minerals is generally weak or absent. The plagioclase feldspar, high calcium clinopyroxenes and pigeonites have similar compositional ranges to the minerals observed in the Middle and Upper Zones of the Skaergaard Intrusion. The mineral compositions are systematically related to SiO2 content and Mg number of the glasses. The data demonstrate that mineral compositions and assemblages similar to the Skaergaard form from silica-rich andesitic to rhyolitic liquids. The various mafic glasses are interpreted to have been derived from a zoned magma chamber underlying an upper layer of rhyolitic magma. Differentiation is attributed to fractional crystallization of the observed mineral assemblages causing SiO2 enrichment and FeO depletion. However, glasses with less than 57% SiO2 have unusual compositions with very low MgO and P2O5 as well as variable Al2O3 and TiO2. Their peculiarities could be explained by andesitic magmas assimilating cumulate mineral aggregates precipitated from more differentiated dacite and rhyolite magmas. The bulk compositions of these cumulates have high FeO, low SiO2 and negligible MgO and P2O5. It is suggested that the high density of the mineral aggregates containing fayalite-hedenbergite-magnetite and ilmenite caused them to settle through the zoned chamber to be assimilated by high temperature, less differentiated magmas.

Timing and sources of neogene and quaternary volcanism in South-Central Guatemala

Five new and six existing radiometric age dates place constraints on the timing of volcanic episodes in a 1400-km 2 area east of Guatemala City. The source of the voluminous Miocene rhyolitic welded tuffs was the newly discovered Santa Rosa de Lima caldera, in the northern part of the area, not fissure eruptions as was previously believed. Resurgence during the Pliocene included the eruption of more silicic tuffs, followed by post-collapse volcanism around the perimeter. Volcanism in the southern part of the area occurred along the Neogene volcanic front. The sources for these Late Miocene and Pliocene andesitic lavas were not fissure eruptions, as was once believed, but were four large volcanic centers, Cerro Pinula, Ixhuatán, Teanzul, and Cerro La Gabia. The Santa Rosa de Lima caldera structure deflects the Jalpatagua Fault forming tensional fractures along which eruptions in the Quaternary Cuilapa-Barbarena cinder cone field took place. Pleistocene ash flows were erupted from Ixhuatán and Tecuamburro volcanoes in the southern part of the area. Tephras from Ayarza, Amatitlán, and Atitlán blanket the northern and central portions. Present-day activity is restricted to hot springs around the northern and eastern base of Tecuamburro volcano. Based on the work in this area it is proposed that rocks of the Miocene Chalatenango Formation throughout northern Central America were erupted from calderas behind the Neogene volcanic front. Rocks of the Mio-Pliocene Bálsamo Formation in Guatemala and El Salvador were erupted from discrete volcanic centers along the Neogene volcanic front. Pliocene rocks of the Cuscatlán Formation probably represent post-collapse volcanism around earlier caldera structures.

Geology of Iwate volcanic group, northeastern Japan

Iwate volcanic group is situated at the southeastern portion of Hachimantai and surrounding volcanic clusters, northeastern Japan. It consists of several volcanic bodies extending in E-W direction, parallel to the maximum horizontal compressional axis of the tectonic stress in this region. The basement rocks of Iwate volcanic group are composed mainly of the Tertiary rocks and the early Quaternary rhyolitic welded tuff which is a member of the Tamagawa welded tuffs. The activity of Iwate volcanic group is summarized as follows. Matsukawa andesite : this andesite consists of altered lavas and pyroclastics and are exposed at the western portion of the volcanic group. Their K-Ar ages have been reported to be 1.62 and 1.36 Ma. Amihari volcanic group: it is divided into two substages, older and younger, according to preservation of their volcanic primary topography. Older stage: in early to middle Pleistocene, several volcanic bodies, composed of an alternation of lavas and pyroclastics, were built up at the nearly same time. They are arranged in E-W direction 12km in length. Younger stage: after the formation of the volcanic chain, two cinder cones and a lava dome were formed, and lava flows were effused from the western portion of the volcanic chain. It is possible that they were active during the growth of Iwate volcano. Iwate volcano: in middle or late Pleistocene, the activity of Iwate volcano started at the eastern portion of Amihari volcanic group. Its history consists of four substages, and involves collapse of the volcanic body. Older Iwate 1st stage: voluminous pyroclastics and subordinate amounts of lavas formed two stratovolcanoes, Kurokura and Older-Iwate volcanos, with a net height of about 2, 000m. Older Iwate 2nd stage: at about 0.15Ma, the southeastern portion of the Older-Iwate volcano collapsed to form Onimata caldera. After the collapse, lava flows and pyroclastics were erupted from the central and flank vents to fill the Onimata caldera. Older Iwate 3rd stage: west-Iwate caldera was formed at the top of Older-Iwate volcano. From the vent in the caldera, lavas and pumice were effused to built up a composite central cone, Onashiro cone. It seems that these activities started at about 0.05Ma and ceased before 0.03Ma. Younger Iwate stage: after more than 25, 000 years of pause, about 5, 000 years ago, East-Iwate caldera was formed at the eastern portion of Older-Iwate volcano. Pyroclastics and lava flows were eruped from the vents mainly within the caldera, resulting in the formation of the composite Younger-Iwate volcano, Yakushidake volcano (2, 040.5m). In 1719, Yakehashiri lava flow was extruded from the northeastern flank of the volcano. The eruptive rocks are mainly basalt and basaltic andesite and associated with andesite. Two types of mafic phenocryst assemblage in basaltic rocks, clinopyroxene+olivine and orthopyroxene+olivine, are found. Although quartz phenocryst often occur, no hornblende phenocryst is found in the rocks from Iwate volcanic group.

Uranium in Igneous Rocks of Central Davis Mountains, West Texas: ABSTRACT

The central Davis Mountains are remnants of a Tertiary eruptive center composed of layered extrusives and a few shallow intrusions. Rock types range from basalts to trachyandesites, latites, trachytes, syenites, and rhyolites. Delayed-neutron counting was used to determine uranium abundance in 102 rock samples representative of all rock types. Thin sections of 72 samples, 30 with fission-track data, were analyzed for petrography, mineralogy, and uranium distribution and mode of occurrence. Major-element analyses for 23 samples were obtained for comparison with trends in uranium abundance. Uranium abundance increases in general with SiO2 and K2O content, and ranges from a mean of 1 ppm in basalts and andesites to 7 ppm in rhyolites. Uranium is most abundant in welded tuffs, in contrast to lava flows and shallow intrusions. Hydrothermal alteration redistributes uranium; unaltered rocks have significantly more uranium than altered rocks of all types. Glassy rocks contain up to one-third more uranium than their crystalline counterparts. In a vertical section through three rhyolitic-welded tuff units, uranium increases progressively from the oldest unit to the youngest; within each of two of these units, uranium also increases upsection. Further, rhyolitic-welded tuffs from the southwest part of the area contain 50% more uranium than those from the eas ; the eruptive source, however, has not been located. Within a given rock type, uranium occurs preferentially in accessory minerals, in areas surrounding hydrothermally leached zones, and in vein fillings. Coarse-grained rocks have more localized concentrations of uranium than aphanitic or glassy rocks. Ground-water leaching of uranium from igneous rocks of the central Davis Mountains is not considered an effective mechanism for uranium redistribution and enrichment because of the low permeability of the rocks and the nature of occurrence of uranium. Therefore, the probability of occurrence of large secondary uranium deposits in the area is not high. End_of_Article - Last_Page 545------------

節理性岩盤表層部にみられるトップリングの性質とその意義

Toppling phenomena were observed along the cut of forestry road where rhyolitic welded tuff of the Nohi rhyolitic rocks is distributed. The rock mass of the welded tuff is separated into platy segments at the site by joints that strike parallel with the slope surface and dip 80° to 90°. The joints decrease in their number and the rocks near joints change in their texture with increasing vertical distance from the slope surface. These facts show that the joints have developed by weathering action from the slope surface.The toppling phenomena are inferred to have developed through the following process. Firstly the development of joints separated the rock mass into platy segments with narrow width, and decreases the strength of segments against bending and buckling. Secondly segments began to topple when they are buckled mainly by their own weight or bended by the force exerted by adjacent segments.

K-Ar dating on moonstone rhyolite in Toyama Prefecture, central Honshu

K-Ar age determination was carried out on two moonstones from rhyolitic welded tuff which consists of the upper part of the Futomiyama formation. The measured ages gave 24 and 25 m. y. respectively, corresponding to the lower Miocene age. The result agrees well with the stratigraphic succession of the Futomiyama formation (Yamasaki and Miyazima, 1970).