K–Ar ages of volcanic rocks from the Himeshima volcanic group, northern Kyushu, Japan

ABSTRACTMixtures of melt and residue in a high-T metamorphic complex have a lower density and viscosity than the surrounding host crust, and the mixtures should ascend due to buoyancy. The mixtures are recognized as migmatites in the high-T metamorphic complex. To confirm ascent of migmatites, we conducted numerical simulations of ascent of a model migmatite (buoyant viscous fluid). The numerical simulations show that the model migmatite could rise to shallow levels of a model crust so long as it is continuously produced at the bottom of the model crust. Otherwise it ceases to rise at depth due to loss of buoyancy by cooling. The numerical simulations also show that the model migmatite experiences vertical thinning during the ascent. The ascent mechanism proposed in this paper requires the continuous production of partially melted rocks at the base of the crust, which is provided by a continuous input of energy into the crust from the mantle. Given that high-T metamorphic complexes are associated with igneous activity beneath a volcanic arc, the igneous activity reflects the energy input into the lower crust from the mantle. A high-grade part (migmatites) of a high-T metamorphic complex in the Omuta district of northern Kyushu, southwest Japan, experienced thinning during ascent. Large amount of igneous rocks, such as plutonic and volcanic rocks, are also distributed in northern Kyushu. Zircon U–Pb ages of igneous rocks from northern Kyushu reveal that igneous activity continued from 115 to 93 Ma, and that peak igneous activity at 110–100 Ma was synchronous with the ascent of migmatites of the high-T metamorphic complex in northern Kyushu. Therefore, the numerical simulations may provide an appropriate model of the ascent of migmatites of the high-T metamorphic complex beneath a volcanic arc, at the eastern margin of Eurasia in the mid-Cretaceous.

We determined the eruption age of basaltic rocks by application of thermoluminescence (TL) method, which is often used for TL dating, to quartz. Mafic magma only rarely includes quartz because of their mutual disequilibration. The basaltic lavas reported herein include quartz as xenocrysts, as corroborated by their rounded or anhedral shape. The basaltic lava used for this study is from the Oninomi monogenetic volcano in northern Kyushu, Japan. The volcano eruption was estimated as occurring 7.3–29 ka because the lava exists between two widespread tephras: Aira-Tanzawa ash (26–29 ka) and Kikai-Akahoya ash (7.3 ka). We succeed-ed in collecting ca. 200 mg of quartz by decomposition of 30 kg of the lava samples. TL measurements for the lava indicate the eruption age as 15.8 ± 2.5 ka, which is fairly consistent with the stratigraphical estimation. Although the TL method has played a considerable part in constraining the timescale of Quaternary events, its application has been limited to silicic samples. The present result demonstrates the availability of quartz for dating even of mafic rock.

K-Ar age determinations have been carried out on 15 hornblendes from the Yakuno Basic Rocks and their affinities in the Inner Zone of Southwest Japan. Gabbroic rocks from the Yakuno Basic Rocks and those from the Sangun Metamorphic Terrane in eastern Chugoku gave ages ranging from 228 to 275 m.y.; more than half of them are concordant with the geologically estimated age of the rocks. Remaining older ages may indicate either the time of plutonism in Early Permian, which might have been contemporaneous and comagmatic with the basic volcanic activity in the same areas, or the existence of inherited argon in hornblendes. Ages of gabbroic rocks from the Sangun Metamorphic Terrane in northern Kyushu are 296-372m.y., all older than those from eastern Chugoku. Hornblendes from gabbroic rocks in the Nagasaki Metamorphic Terrane gave still older ages of 449-472m.y., suggesting that these rocks may be a part of old basement rocks in the Japanese Islands. Graphical presentation of argon and potassium data indicates that most of the hornblende samples contain no significant amount of excess argon.

Abstract-Rb-Sr whole rock isochron ages and K-Ar mineral ages have been determined for late Cretaceous granitic rocks distributed in the southern part of the Kyeongsang basin, Korea. The granitic rocks are grouped into seven in four areas (Masan, Jinhae, Kimhae and Busan) based on their petrological features. In the five plutons, Rb-Sr whole rock isochron ages and isotopic initial ratios were determined as follows: Rb-Sr whole rock age Initial Sr ratioMasan Hbbtgr 100.1 ± 7.1 Ma 0.70489 ± 0.00012Jinhae Btgr 70.5 ± 1.9 Ma 0.70497 ± 0.00006Kimhae Hbbtgr 83.9 ± 9.0 Ma 0.70538 ± 0.00021Busan Btgr (YS) 70.6 ± 4.2 Ma 0.70531 ± 0.00029Busan Btgr (KP) 68.8 ± 3.4 Ma 0.70579 ± 0.00028 Rb-Sr whole rock ages become younger toward more felsic and smaller plutons, which is thought to reflect progressive lowering in melting degree at the source with time. Initial Sr ratios of each pluton progressively increase from the western to eastern areas, regardless of their ages. This means that the granitic magmas have been more evolved toward the eastern area due to much contribution from lower crustal materials. K-Ar ages of biotite are about 3 to 15 m.y. younger than Rb-Sr whole rock isochron ages in each plutonic unit. Average cooling rate of each body is directly proportional to the volume of granitic body. The conduction-loss of heat due to large thermal contrast and the circulating meteoric water between a granitic body and the country rock at shallow levels are thought to have directly affected the average cooling rate of each body. Initial Sr isotopic ratios of the granitic rocks in the study area are well correlated with those of the central to northern Kyushu and the North Zone of Southwest Japan. Rb-Sr isochron ages of the Cretaceous granitic rocks became younger from central Kyushu (120-100 Ma) through northern Kyushu (115-90 Ma) to southern part of the Kyeongsang basin (100-70 Ma). The granitic magmatism having 70 Ma ages on the eastern side of the Yangsan Fault is thought to be better correlated with those of the North Zone of Southwest Japan.

Crustal dynamics for formation of high-temperature metamorphic belt and U-Pb ages of metamorphic and plutonic rocks in northern Kyushu

The Cretaceous granitic rocks, the Kitazaki Tonalite and Shikanoshima Granodiorite, and veneer Oligocene sedimentary rocks, the Tsuyazaki Formation, are distributed in the Watari Peninsula, Fukuoka Prefecture. There are mineral veins composed by calcite and zeolites in those rocks. Rb-Sr, and fission track geochronological analyses were carried out for granitic rocks, in order to determine the cooling process of granitic rocks and timing of hydrothermal activity. Biotite and felsic fraction separated from the Kitazaki Tonalite and Shikanoshima Granodiorite give Rb-Sr isochron age of 108.6 ± 2.5 Ma and 107.0 ± 0.7 Ma, respectively. Fission track ages from the Kitazaki Tonalite are 97 Ma (titanite), 89-88 Ma (zircon), 14 Ma (apatite). Fission track ages from the Shikanoshima Granodiorite are 95 Ma (titanite), 87-84 Ma (zircon), 15 Ma (apatite). Fission track ages of apatite from granitic rocks are younger than the Tsuyazaki Formation, which suggests a thermal event at about 15 Ma to reset the fission track apatite age. The homogenization temperature of fluid inclusions in calcite are 89-111 °C (Kitazaki Tonalite), 95-118 °C (Shikanoshima Granodiorite) and 85-91 °C (Tsuyazaki Formation). Sr isotopic compositions of calcite and zeolites in granitic rocks and Tsuyazaki Formation are overlap with one another. These data suggest that vein minerals in granitic rocks and zeolites and calcite in the IC Member of the Tsuyazaki Formation were formed by a series of hydrothermal activity at about 15 Ma.

Abstract. Determinations of SO3 and Cl contents of igneous accessory apatite were carried out on Late Cenozoic intermediate to silicic intrusive and volcanic rocks in the Japanese island arcs of the western Pacific rim including the southwestern Kuril arc (eastern Hokkaido), Northeast Japan arc (southwestern Hokkaido through northeastern Honshu to central Honshu), Izu‐Bonin arc, Kyushu‐Palau ridge, Southwest Japan arc (northern Kyushu) and northern Ryukyu arc (southern Kyushu). These were compared to those from the Western Luzon arc, Philippines, to better understand the metallogenesis of porphyry Cu deposits in the western Pacific island arcs. In addition, SO3 and Cl contents of accessory apatite in the Cretaceous magnetite‐series granitic rocks in the Kitakami belt (northeastern Honshu) and the Miocene ilmenite‐series granitic rocks in the Outer Zone of Southwest Japan (southern Kyushu) were also examined.Microphenocrystic apatites in shallow intrusions associated with porphyry Cu deposits in the Western Luzon arc contain >0.1 wt% S as SO3. Such high SO3 contents of microphenocrystic apatite are a common characteristic of hydrous mag‐matism in the Western Luzon arc, from 15 Ma old tonalitic plutonic rocks of the Luzon Central Cordillera to present‐day volcanism at Mount Pinatubo. The accessory apatite in intrusive rocks associated with porphyry Cu deposits, especially those at the Santo Tomas II deposit, show significantly high Cl contents (>2 wt%).The SO3 contents of microphenocrystic apatite in most of the hydrous silicic rocks along the volcanic front, in andesites related to native sulfur deposits, and in Miocene and younger shallow granitic intrusions in northeastern Honshu, are generally <0.1 wt%. On the other hand, the SO3 contents of apatite in such rocks from eastern Hokkaido, southwestern Hokkaido, Izu, northern Kyushu and southern Kyushu are similar to those from the Western Luzon arc. The SO3 contents of accessory apatite in the Cretaceous magnetite‐series granitic rocks in the Kitakami belt are variable, whereas those of the Miocene ilmenite‐series granitic rocks in southern Kyushu are extremely low. The Cl contents of accessory apatite in some rocks of the Northeast Japan arc, Izu‐Bonin arc and Southwest Japan arc are significantly high.In terms of the Cl and SO3 contents of microphenocrystic apatite, Cenozoic Japanese arc magmatism show similarities with arc magmatism associated elsewhere with porphyry Cu mineralization, except for the most of northeastern Honshu of the Northeast Japan arc. Apatite commonly occurs as inclusions in other phenocrystic phases. Thus the variation in SO3 contents of apatite is a feature of early stage magmatic differentiation. The SO3 contents of microphenocrystic apatite are considered to reflect the redox state of the magma source region or fluids encountered during magma generation.

Removing a mask of alteration: Geochemistry and age of the Karadag volcanic sequence in SE Crimea

In the volcanic rock fields around Hita City, 19 landslides occurred in the 2017 torrential rains in northern Kyushu, 11 of which triggered debris flows. In order to mitigate debris flows disasters caused by landslides, it is important to assess the points where landslides are most likely to occur. Therefore, topographic profiles of landslide and non-landslide slopes were calculated from digital elevation models obtained before the disaster. The results indicate that there is a topographic criterion for landslide occurrence in the relationship between the topographic position index (TPI) and the tangent Laplacian.

40Ar 39Ar ages of igneous rocks recovered from Daiichi-Kashima and Erimo Seamounts during the Kaiko project

The age and origin of volcanics in the Riphean section of the Siberian craton ( western Baikal area)

Re [sbnd]Os dating of molybdenites from ore deposits in Japan: Implication for the closure temperature of the Re [sbnd]Os system for molybdenite and the cooling history of molybdenum ore deposits

Geochronology of the North Caribou greenstone belt, Superior Province Canada: Implications for tectonic history and gold mineralization at the Musselwhite mine

Committee on science and the arts

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.