能登半島新第三紀火山岩のＫ‐Ａｒ年代

Nine SHRIMP U/Pb ages on zircon and two Pb/Pb single zircon ages have been determined from Late Paleozoic volcanic rocks from Saxony and northern Bohemia. Samples came from the Teplice-Altenberg Volcanic Complex, the Meissen Volcanic Complex, the Chemnitz Basin, the Dohlen Basin, the Brandov-Olbernhau Basin, and the North Saxon Volcanic Complex. The Teplice-Altenberg Volcanic Complex is subdivided into an early Namurian phase (Mikulov Ignimbrite, 326.8 ± 4.3 Ma), thus older than assumed by previous studies, and a late caldera-forming phase (Teplice Ignimbrite, 308.8 ± 4.9 Ma). The age of the latter, however, is not well constrained due to a large population of inherited zircon and possible hydrothermal overprint. The Leutewitz Ignimbrite, product of an early explosive volcanic episode of the Meissen Volcanic Complex yielded an age of 302.9 ± 2.5 Ma (Stephanian A). Volcanic rocks intercalated in the Brandov-Olbernhau Basin (BOB, 302 ± 2.8 Ma), Chemnitz Basin (CB, 296.6 ± 3.0 Ma), Dohlen Basin (DB, 296 ± 3.0 Ma), and the North Saxon Volcanic Complex (NSVC, c. 300–290 Ma) yielded well-constrained Stephanian to Sakmarian ages. The largest Late Paleozoic ignimbrite-forming eruption in Central Europe, the Rochlitz Ignimbrite, has a well-defined middle Asselian age of 294.4 ± 1.8 Ma. Ages of palingenic zircon revealed that the Namurian-Westphalian magmatism assimilated larger amounts of crystalline basement that formed during previous Paleozoic geodynamic phases. The Precambrian inherited ages support the chronostratigraphic structure assumed for the Saxo-Thuringian Zone of the Variscan Orogen. The present results help to improve the chronostratigraphic allocation of the Late Paleozoic volcanic zones in Central Europe. At the same time, the radiometric ages have implications for the interbasinal correlation and for the geodynamic evolution of the Variscan Orogeny.

Precise in situ zircon U‐Pb dating and Lu–Hf isotopic measurement using an LA‐ICP‐MS system, whole‐rock major and trace element geochemistry and Sr–Nd isotope geochemistry were conducted on the volcanic host rocks of the Tongyu copper deposit on the basis of further understanding of its geological characteristics. Three zircon samples from the volcanic host rocks yielded 206Pb/238U weighted average ages ranging from 436±4 Ma to 440±5 Ma, which are statistically indistinguishable and coeval with the ca. 440 Ma northward subduction event of the Paleo‐Qinling oceanic slab. The volcanic host rocks were products of magmatic differentiation that evolved from basalt to andesite to dacite to rhyolite, forming an integrated tholeiitic island arc volcanic rock suite. The primitive mantle‐normalized trace element patterns for most samples show characteristics of island arc volcanic rocks, such as relative enrichment of LILE (e.g. Th, U, Pb and La) and depletion of HFSE (e.g. Nb, Ta, Ti, Zr and Hi). Discrimination diagrams of Ta/Yb vs Th/Yb, Ta vs Th, Yb vs Th/Ta, Ta/Hf vs Th/Hf, Hf/3 vs Th vs Nb/16, La vs La/Nb and Nb vs Nb/Th all suggest that both the volcanic host rocks from the Tongyu copper deposit and the volcanic rocks from the regional Xieyuguan Group were formed in an island arc environment related to subduction of an oceanic slab. Values of ISr (0.703457 to 0.708218) and ∊Nd(t) (–2 to 5.8) indicate that the source materials of volcanic rocks from the Tongyu copper deposit and the Xieyuguan Group originated from the metasomatised mantle wedge with possible crustal material assimilation. Most of the volcanic rock samples show good agreement with the values of typical island arc volcanic rocks in the ISr–∊Nd(t) diagram. The involvement of crustal‐derived material in the magma of the volcanic rocks from the Tongyu copper deposit was also reflected in the zircon ∊Hf(t) values, which range from –3.08 to 10.7, and the existence of inherited ancient xenocrystic zircon cores (2616±39 Ma and 1297±22 Ma). The mineralization of the Tongyu copper deposit shows syn‐volcanic characteristics such as layered orebodies interbedded with the volcanic rock strata, thus, the zircon U–Pb age of the volcanic host rocks can approximately represent the mineralization age of the Tongyu copper deposit. Both the Meigou pluton and the volcanic host rocks were formed during the ca. 440 Ma northward subduction of the Paleo‐Qinling Ocean when high oxygen fugacity aqueous hydrothermal fluid released by dehydration of the slab and the overlying sediments fluxed into the mantle wedge, triggered partial melting of the mantle wedge, and activated and extracted Cu and other ore‐forming elements. The magma and ore‐bearing fluid upwelled and erupted, and consequently formed the island arc volcanic rock suite and the Tongyu VHMS‐type copper deposit.

At perhaps the coarsest scale of consideration, the rock cycle operates through fluxes associated with tectonic uplift and erosion, which are generally balanced by subsidence/subduction and the burial of mineral materials that leads to volumetric balance among the principal rock reservoirs. At Earth’s surface, net rates of transfer are manifest as the reduction of areas of older rocks during erosional destruction as well as during burial by younger units. Because exposure of continental crust comprises some finite space, decrease of older rock area by erosion and/or burial must be largely balanced by an increase in area of new sedimentary and volcanic successions. Here, we examine relations between the lateral extents of rock units exposed across North America—their lithology, elevation, and the age of formation—to determine rates of geologic “repaving” that are recorded in the age and area of rocks making up the surface of the continent. Moreover, because deposition occurs at lower elevations, one might expect that subsequent episodes of uplift and/or burial would serve to increase the average elevation of currently exposed sedimentary lithosomes. Because plutonic and metamorphic rocks form at depth and are only exposed along orogenic belts and across shields after extended intervals of uplift and/or erosion, one might expect basement rocks to be initially exposed at higher elevations, and that subsequent erosion would decrease elevation with increasing age. Therefore, processes giving rise to associations between outcrop lithology, extent, and elevation may serve as measures of continent-scale rates of rock cycling. We combine data from the 1 arc-minute global relief model with data from the Geological Society of America’s 2005 Geologic Map of North America to assess the actuality and significance of first-order relations among the lithologies, areas, ages, and elevations of rock bodies now exposed across the North American continent and how these may shed light on deep-time rates of rock cycling. Areas of the 23,642 mapped North American lithosomes make up a lognormal frequency distribution (mode = 121 km2). This distribution reflects both natural (lateral extents of individual map units must be limited in space) and anthropogenic (cartographers must render map units within limited range of sizes) causes. Contiguous, lateral associations of the major rock types suggest that the spatial occurrence of volcanic and sedimentary rock bodies is largely independent of the proximity of other rock types, but exposures of plutonic and metamorphic lithosomes are intimately associated in space and thus comprise the “crystalline basement” exposed in cores of younger orogens and across the Canadian Shield. Unlike sedimentary and volcanic rocks that “attain their age” when formed at Earth’s surface, plutonic and metamorphic units are “born” at depth and therefore must have reached some antiquity prior to first exposure. As a result, the ages of volcanic and sedimentary exposures span the full range of ages represented by all lithologies, while those of plutonic and metamorphic suites are more abbreviated and older. Median ages of North American volcanic, sedimentary, plutonic, and metamorphic exposures are 482 Ma, 306 Ma, 1695 Ma, and 2467 Ma, respectively. Although the areal extent of sedimentary rock decreases with increasing age, elevation increases. This change is interpreted as reflecting progressive tectonic uplift from initial accumulation at lower elevations. In contrast, the surfaces of exposed volcanic, plutonic, and metamorphic lithosomes become lower in elevation with increasing age. This change reflects both the erosional lowering of lithosome surfaces as well as the progressive exhumation of larger areas of crystalline basement. Exposed rock area exhibits a power law decrease with increasing age. This change reflects the progressive destruction of older units and their burial by younger units. A simple “repaving” model in which 0.8% area is uplifted and eroded per million years is in good agreement with observed relations of map age versus area, the rate of which is equivalent to the resurfacing of the North American continent every 63 m.y. or ∼56 times over the past 3.5 Ga. The rate of decrease in map area of exposed rocks with increasing age evident in both geologic map data and repaving models is about three times the rate of decrease in total sediment volume as determined from stratigraphic data and repaving models. The decrease in the areas of exposed rock units is primarily the result of burial by younger units rather than erosive destruction.

A large shallow earthquake occurred in the Noto Peninsula on March 25, 2007. The Japan Meteorological Agency (JMA) estimated the hypocenter and origin time to be 136.685E, 37.220N and 09:41:57.9 JST, respectively, and the depth to be 11 km. The magnitude of the main shock is Mj (JMA scale) 6.9 and Mw 6.6, and the focal mechanism of the main shock is a reverse fault with right-lateral components. The strike, dip and rake are 58◦, 66◦ and 132◦, respectively, according to the National Research Institute for Earth Science and Disaster Prevention (NIED). The two largest aftershocks of Mj 5.4 occurred at the northeast and southwest edges of the aftershock area, at 18:11 on May 25 and at 07:16 on May 26, respectively. A maximum seismic intensity, 6 upper on the JMA scale, was observed at several stations situated in the source area. The earthquake caused severe damage on the Noto Peninsula and in the surrounding prefectures of Ishikawa, Toyama, Niigata and Fukui. The final report of the Fire and Disaster Management Agency reported one human fatality, 356 people injured and 29,349 houses completely and partially collapsed. Although several moderate-sized earthquakes had occurred in the northeast and southern parts of Noto Peninsula prior to this event, there is no record of large earthquakes with M > 7 occurring in this area during the last 1500 years. In addition, events of M 6.0 ∼ 6.5 have never occurred in the source region of the March 25, 2007 earthquake, and even the seismic activity of small and microearthquakes has been very low for more than 25 years, based on the high-gain seismic-observations by the Kamitakara Observatory, Kyoto University (Ito et al., 2007). Large active faults have not been identified on land in the source area. Nevertheless, some offshore faults in west off Noto Peninsula have nearly the same strike as that of the aftershock area, which may be related to the March 25, 2007 earthquake. Well-arranged high-density nation-wide networks have been constructed, and these provided abundant data for the sequence of the earthquake. In particular, strong-motion and high-gain earthquake records as well as the GPS data provided fundamental information on the earthquake sequence and related phenomena from immediately after the main shock. In addition, temporary dense seismic stations were deployed for aftershock observations

Potash-rich volcanic rocks and lamprophyres in western Shandong Province: 40Ar-39Ar dating and source tracing

In central-north Kyushu, a large volume of Pliocene-Quaternary volcanic rocks are distributed in and around the east-west elongated graben (40×30 km), which is considered to be a volcanotectonic depression (VTD). The radiometric ages of the volcanic rocks are isochronally zonated in concordant with the outline of the VTD, so that the ages increase from the center to the rim of the VTD. In this study, K-Ar ages were determined upon four volcanic rocks from the northern rim of the VTD. K-Ar ages of the volcanic rocks from the Hikosan area (northwest part of the VTD) were determined as 5.0±0.4, 4.0±0.2 and 4.7±0.3 Ma, which show close agreement with the respective FT dates (Watanabe et al., in press). In the Yamaga area (northeast part of the VTD), the andesite lava from the Yabakei Formation, whose FT age is reported as 12.1 Ma (Hayashi et al., 1983), was dated by K-Ar method as 5.7±0.3 Ma. The K-Ar ages of the Usa Group which is overlain by the Yabakei Formation near the Yamaga area show 4.8±0.4, 5.3±0.3 and 4.9±1.1 Ma (Kamata, 1985b). Accordingly the previous FT age (12.1 Ma) for the Yabakei Formation is apparently too old, which may account for corrections made for thermal alteration of the specimen and/or contamination of accidental particles. On the other hand, the K-Ar age (5.7 Ma) is considered to be close to the eruption age of the Yabakei Formation in this area. These results indicate that the oldest volcanic rocks, which are distributed on the rim of the VTD, were erupted 5-6 Ma ago. Therefore it is suggested that the VTD has been formed since 5-6 Ma ago in centralnorth Kyushu.

40Ar/39Ar ages of volcanic rocks from offshore southern and peninsular California and the Southern California Borderland

Well-crystallized groundmass fractions are commonly used to obtain reliable K-Ar ages for volcanic rocks, as phenocrysts potentially contain excess 40Ar. In general, phenocrysts are removed by magnetic separator, heavy liquid separation, and hand picking. Heavy liquid separation utilizes sodium polytungstate (SPT) solution, which replaced toxic heavy liquids such as bromoform in the 1980s. Recently, a vacuum pump has been used to reduce the filtration time for heavy liquid separation using SPT solution, which has a high viscosity. This note describes the separation procedure for gentle suction filtration using a hand vacuum pump, which can be safely completed in 50-60 minutes per sample. The procedure was tested on andesite and basalt samples from Hakusan, Miyakejima, and Etna Volcanos. Separation of the groundmass fraction from the phenocryst fraction was successful for all andesite samples, whereas separation was incomplete in some of the basalt samples, probably because the gravity of groundmass grains was heavier than the maximum gravity of the SPT solution (3.11 g/cm3).

The K-Ar ages and paleomagnetic stratigraphy of the Plio-Pleistocene volcanic rocks from the Ariake Sea region have been studied to clarify the evolution of volcanism in the western part of Beppu-Shimabara graben, west Kyushu, Japan. This region has seven domains of volcanic rocks. We determined K-Ar ages of ten volcanic rocks from five domains: the Misumi volcanic rocks=4.51 Ma and 3.97 Ma; the Kunisaki andesite=1.45 Ma; the Ibinoodake andesite=3.97 Ma and 4.52 Ma; the Uki volcanic rocks=1.63 Ma; the Yushima basalt=0.82 Ma; the Otake volcanic rocks=1.82, 1.77, and 1.72 Ma. Paleomagnetic measurements were made on volcanic rocks collected from all seven domains. Magnetic property of lavas from these domains can be classified into four magnetic polarity epochs: Brunhes Chron (Ninotake volcano, Takesaki basalt and Yushima basalt), Gauss Chron (Nogamajima volcano), Gilbert Chron (Ibinoodake andesite and Hidake lava dome), and Matsuyama Chron (the other volcano). The age of Ohya tuff, which belongs to the Kuchinotsu Formation, corresponds to Gilsa event. We synthesized our new data together with previously reported ages and stratigraphic data, and examined the volcanic activity in this region in terms of a spatial and temporal distribution. There is no correlation between spatial distribution of the volcanic rocks and eruption ages, but we revealed that the volcanic activity in this region is episodic during the last 5 million years. We can identify the magmatic activities as five episodes: 5-4 Ma, 3.2 Ma, 2.0-1.4 Ma, 1.0 Ma, and <0.9 Ma. The intensity of volcanic activity in the Ariake Sea region increased with time.

Late Cretaceous-early Paleogene magmatism in the Gyeongsang basin, southeast Korea and its implications for middle Paleogene climate change

In the Grandfather Mountain area of northwestern North Carolina, Precambrian schists, gneisses, and amphibolites of the Blue Ridge province have overridden early Precambrian gneiss, and late Precambrian, Cambrian(?) and Lower Cambrian sedimentary and volcanic rocks, which are exposed in the Grandfather Mountain window. Schists, gneisses, and granitic rocks of Precambrian or Paleozoic age, or both of the Inner Piedmont are in fault contact with rocks of the Blue Ridge. The rocks of the area have been deformed 2 or more times, and the different tectonic units have different histories. Older Precambrian rocks both within and above the window were metamorphosed under plutonic conditions 1000 to 1100 million years ago and were metamorphosed retrogressively at least once during the Paleozoic. Younger volcanic and sedimentary rocks were metamorphosed progressively at the same time that the basement rocks in the window were retrograded. Although little is known about the age of the rocks of the inner Piedmont, the are polymetamorphic and probably underwent 2 episodes of metamorphism, one or more in the Paleozoic and possibly one in the Precambrian. Radiometric age determination of minerals have established the 1000 to 1100 million-year age of the plutonic metamorphism and a 350 million-year age for an episode of dynamothermal metamorphism affecting the rocks of the Blue Ridge thrust sheet. More age determinations are needed to clarify completely other details of metamorphic and structural history.

A belt of Palaeoproterozoic, dominantly felsic metavolcanic rocks occurs in south-central Sweden. The volcanic rocks comprise volcaniclastics, lavas and subvolcanic intrusions. The volcanic pile is more than 8 km thick and overlies an unexposed basement. Two new U-Pb zircon age determinations provide precise control on the age of stratigraphically and volcanologically well-constrained volcanic rocks in the belt. One age determination from a pyroclastic flow deposit at Utö in the easternmost part of the volcanic belt yielded an age of 1904±4 Ma. Stratigraphic and depositional facies analyses show that this rock marks the onset of volcanism in the area. The igneous activity was coeval with a change from deep water sedimentation to shallow water and subaerial sedimentation in a broadly continental setting. Another determination of zircons from a similar pyroclastic flow deposit at the lowest known stratigraphic level east of Hällefors in W. Bergslagen in the westernmost part of the volcanic belt gave an age of 1891±4 Ma. This age is consistent with previously published ages for metavolcanic rocks in W. Bergslagen. The age of the onset of volcanism in W. Bergslagen is not possible to determine because the lowermost sections of all known volcanic successions are truncated by intrusive rocks. Consequently, it is unknown whether volcanism also commenced around 1904 Ma in the western part of the region, or if it commenced later, just prior to 1891 Ma. The duration of volcanism also remains to be constrained.

Volcanic rocks such as andesite have popularly been used for aggregates in the Hokuriku District. Therefore, the andesitic aggregates have contributed to a steady occurrence of alkali-silika reaction (ASR) in concrete. Taking into account the effective utilization of local materials, so-called “Chisan-chisho”, both the reduction in the environmental impact and the rationalization of natural resources, positive countermeasures for mitigating ASR by using blended cements with fly ash (FA) or blast-furnace slag (BFS) are most expected. This paper examines the mitigating effects of mineral admixtures on ASR of mortars by the accelerated mortar-bar test (ASTM C 1260). As the result of test, the replacement with FA or BFS at normal percentages recommended by JIS A 5308 decreased significantly the expansion of mortar bars using reactive andesitic aggregates in the Noto peninsula. Especially, a high-quality fine FA produced in the Hokuriku District is hopeful in its practical use of concrete mixtures. Furthermore the ASR mitigating effects by mineral admixtures are assessed with a good performance by the test method “ASTM C 1260”.

U–Pb zircon, titanite, and baddeleyite ages are reported for 20 volcanic and plutonic rock units in the Chibougamau, Matagami, and Joutel areas of the northern Abitibi belt. Pretectonic magmatism occurred in three main pulses. Volcanic rocks of the Obatagamau Formation in the Chibougamau area were erupted at 2759 Ma. Following a possible hiatus in igneous activity, volcanism, accompanied by the emplacement of layered mafic intrusions, occurred in the Chibougamau, Matagami and Joutel areas in the period of 2730–2722 Ma. In the Chibougamau area, a slightly younger period of plutonism between 2720 and 2712 Ma is thought to be comagmatic with felsic volcanic rocks in the uppermost part of the Roy Group. Syntectonic intrusive rocks in the northern Abitibi belt range in age from 2701 to 2693 Ma. These ages are broadly comparable to ages obtained for syntectonic plutons elsewhere in the Abitibi belt, suggesting that tectonism associated with the Kenoran orogeny occurred at approximately the same time throughout the belt.

The volcanic rocks in the Tibetan Plateau have undergone complex evolution, as the collision between the Indian and Eurasian Plates proceeds. The volcanic rock samples collected from a volcanic edifice in the Hoh Xil, Tibetan Plateau, are studied in this paper. The geochemistry and zircon U-Pb chronology of the volcanic rock samples are determined. Our results indicate that the volcanic rocks in this area are mostly alkali-rich intermediate-acidic ones, and consist of quartz trachyandesite, andesite, pyroxene andesite, and andesitic tuff. Geochemically, these rocks feature high light rare earth elements and large-ion lithophile elements concentration, with low Nb, Ta, and Ti concentration, and significant Pb positive anomaly. Based on our analysis, we conclude that the volcanic rocks are adakites. The eruption ages of the volcanic rocks in this area are determined for the first time in the literature: 45.23 ± 0.35 Ma for the hornblende trachyandesite; 41.36 ± 0.45 Ma for the biotite trachyandesite; and 42.97 ± 0.23 Ma for the biotite-quartz trachyandesite. This suggests that the volcanic rocks studied in this paper were formed in the Lutetian (Eocene). It is further concluded that the volcanic rocks originated from a mantle provenance that have assimilated large amount of crustal materials. The formation of the volcanic rocks is attributable to the breakoff of oceanic lithospheric slab.