鹿児島県奄美深成岩類の岩石学

Based on Sr isotopic data for Tertiary and Quaternary basaltic rocks from the NE Japan arc, relationships are discussed between the temporal variation of magma source characteristics and the opening of the Japan Sea. The basaltic rocks from the trench side and from the transitional zone show initial87Sr/86Sr ratios (Sri ratios) in the range of 0.70411–0.70546 but no temporal variation in Sri ratios. The back-arc side basaltic rocks with ages of 29.8 to ≈ 15 Ma have Sri ratios similar to those of the trench side and the transitional zone, and these values also show no temporal change. In contrast, the basaltic rocks from the back-arc side, with ages younger than ≈ 15 Ma, show significantly lower Sri ratios (0.70396 to 0.70290), which are slightly higher than those of N-type MORB. These Sr isotopic features may imply that at least before ≈ 15 Ma the magma source regions (the sub-continental mantle) beneath the NE Japan arc had an enriched chemical character and that after ≈ 15 Ma, the magma sources for volcanic rocks from the back-arc side show a drastic change in Sr isotopic character, from an enriched nature to a depleted one. The depleted magmas may have been formed as a result of injection of depleted asthenosphere (or of a depleted mantle diapir) into the subcontinental mantle under the back-arc side of the NE Japan arc, during the spreading of the Japan Sea back-arc basin. The middle Miocene basaltic rocks from the back-arc side are characterized by lower contents of LIL elements such as K2O and Rb compared with those from the trench side, suggesting that during the middle Miocene (syn-opening stage of the Japan Sea) the degree of partial melting may have been higher in the back-arc side mantle than in the trench side mantle. High degree of partial melting in the back-arc side mantle can be attributed to an increasing geothermal gradient in the mantle due to the injection of hot asthenosphere. This injection might also have caused the melting of the lower crust from which the voluminous middle Miocene acidic volcanics in the back-arc side and transitional zone may have been produced.

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.

Radiometric ages of granitic rocks in Outer Zone of Southwest Japan and its extension; 1968 summary and north-shift hypothesis of igneous activity

This article presents geochemical characteristics of trace and rare earth elements in the late Cretaceous granitic plutons, southern part of the Kyongsang Basin, and discusses related magmatic processes and possible source materials. The granitic rocks in the study area were formed in volcanic-arc environments, and can be divided into four plutons (Masan, Jinhae, Kimhae, and Busan plutons). According to LILE/HFSE and initial Sr ratios, the plutons are grouped into the western, HFSE-depleted (Masan/Jinhae) and the eastern, HFSE-enriched (Kimhae/Busan) suites. In each suite, the younger plutons (Jinhae and Busan plutons) are chemically more felsic, and occur as smaller bodies. Plagioclase, apatite, and Fe-Ti oxides have been fractionated in all plutons. Zircon fractionation is recognized in the Masan pluton, whereas small amounts of allanite fractionations are detected in the Jinhae, Kimhae, and Busan plutons. The primary compositions of the plutons were probably LREE-enriched with almost unfractionated HREE and negative Euanomalies. The plutons were, therefore, likely to be generated from abundant-plagioclase source materials. Taking into account the critical melt fraction which controls magma segregation, the intermediate Masan and Kimhae plutons formed earlier in each suite seem to have been derived directly from the gabbroic lower crust. The later, felsic Jinhae and Busan plutons appear to have been formed by remelting of modified protoliths which were chemically very similar to the Masan and Kimhae plutons, respectively. This reworking process may have occurred during comparatively short period of time (<30 m.y.).

Middle–late Permian mantle plume/hotspot–ridge interaction in the Sumdo Paleo-Tethys Ocean region, Tibet: Evidence from mafic rocks

Evolution of the crust in the northeast Honshu arc is discussed on the basis of the melting experiments of a peridotite under hydrous conditions and the crustal compositions estimated from deep crustal xenoliths and seismic velocity data. The chemical compositions of melts formed by partial melting of a hydrous peridotite within the pressure‐temperature conditions for shallow parts of the mantle wedge (1.2–2.0 GPa) have been experimentally determined with the “sandwich experiment” technique. The melts range from olivine tholeiitic to magnesian andesitic compositions (SiO2 48–56 wt %; MgO 9–14 wt %) beneath the volcanic front in the northeast Honshu arc and are slightly less silicic beneath the coast of the Sea of Japan. The crustal compositions of the volcanic zone in the northeast Honshu arc estimated from deep‐seated crustal inclusions in volcanic rocks and the velocity structures of the crust are andesitic in the central zone and high‐alumina basaltic near the coast of the Sea of Japan. The compositions of possible primary melts formed in the mantle wedge are significantly more mafic than these crustal compositions. Consequently, if the major part of the crustal materials in the volcanic zone was formed by magmas generated in the mantle wedge, thick (up to 30 km) ultramafic cumulates including olivine pyroxenite must have been formed beneath Moho during fractional crystallization and would constitute the upper part of the mantle wedge. The volume of the mantle wedge consisting entirely of even fertile peridotite was not sufficient to produce crustal materials and cumulates in the volcanic zone by such processes in the northeast Honshu arc. A static mantle wedge model is, therefore, not satisfactory for the growth of the crust in northeast Honshu arc. A dynamic model is suggested in which the mantle materials have been supplied into the mantle wedge from the back arc side by a convective flow.

Intrusion-hosted gold deposits of the southeastern East Sayan (northern Central Asian Orogenic Belt, Russia)

Volcanic–plutonic connections and metal fertility of highly evolved magma systems: A case study from the Herberton Sn–W–Mo Mineral Field, Queensland, Australia

Geology and petrology of the Hormuz dolomite, Infra-Cambrian: Implications for the formation of the salt-cored Halul and Shraouh islands, Offshore, State of Qatar

Comparison of petrological and geochemical characteristics of three different types of Eocene copper-gold mineralization in eastern Iran

About 150 samples of the plutonic rocks of the Raymond quadrangle were collected during geologic mapping. Of these, 137 were analyzed modally, 3 were analyzed chemically, and 4 were dated isotopically, 2 by the U-Pb method and 2 by the K-Ar method. In general, the different plutonic rocks show little internal compositional variation, but the hornblende-poor facies of the tonalite of Blue Canyon is zoned from more mafic in the margins to less mafic in the interior. The presence of subhedral to euhedral hornblende prisms and biotite books in the hornblende-rich facies of the tonalite of Blue Canyon is interpreted to indicate abundant H20 in the melt phase of the magma during crystallization, perhaps about 3 percent. However, a paucity of magnetite indicates low oxygen fugacity (FQ).

The Bangong-Nujiang suture zone lies in the central part of the Tibetan Plateau. It is not only an important plate suture zone, which has direct controlling effect on magmatic activities and tectonic evolution of South Qiangtang and North Gangdese, but it is also an important metallogenic belt. Its tectonic evolution is closely related to the evolution history of the whole Tibetan plateau. In this paper, zircon U-Pb dating of the acid intrusive rocks in Lhaguo Tso has been conducted. Chronologically, the rock types are medium-fine-grained granodiorite (γδaK1) → medium-grained granodiorite (γδbK1) → fine-grained biotite adamellite (ηγβaK1) → medium-fine-grained biotite adamellite (ηγβbK1). The formation time ranges from 113.7 to 100.3 Ma, and the magmatic activity is concentrated in Lower Cretaceous, coincided with the Lower Cretaceous magmatism of the Northern Gangdese terrane. The intrusive rocks in the research area are characterized by high silica (SiO2 = 59.96~78.03%), potassium-rich (K2O = 2.04~4.71%), alkali-rich (K2O + Na2O = 6.08~8.5%), peralkaline to peraluminous (A/CNK = 0.89~1.25), and high calc-alkaline to shoshonitic. The intrusive rocks in each stage are obviously enriched with LREE and high field elements (such as Th, Ta, Hf), and relatively depleted large ion lithophile elements (such as Ba, K, Sr). Total REE ranges from 82.53 × 10−6 to 203.91 × 10−6, with LREE/HREE of 82.53 × 10−6~203.91 × 10−6, (La/Sm)N of 1.68~14.77, (Gd/Yb)N of 0.92~1.84, and middle negative Eu anomalies (δEu = 0.07~0.94). Based on the above discussions, we propose that the geochemical characteristics of these intrusive rocks are consistent with those of arc-type magmas worldwide. Comprehensive research shows that Lhaguo Tso intrusive rocks in the Bangong-Nujiang Tethys ocean crust of southward subduction background, formed in a typical island arc tectonic background, suggesting that the Bangong-Nujiang ocean in the Lower Cretaceous (113~100 Ma) is still under the southward subduction of the Northern Gangdese terrane, may not have been closed.

Pegmatite-hosted garnets from four localities in the Boroujerd region, Lorestan (Western Iran), have been analysed for major and selected trace element compositions. The mineral assemblage of the granitic pegmatites is primarily quartz, plagioclase (albite), and alkali feldspar (orthoclase-microcline), as well as garnet, muscovite, fluorapatite, tourmaline (schorl-foitite), andalusite and zircon. The mineralogical and geochemical characteristics of the pegmatites indicate that they are peraluminous to slightly metaluminous I-type granites. Based on mineral assemblages and whole-rock geochemistry, the pegmatites are classified as muscovite-type pegmatites. Electron-probe micro-analysis reveals that garnets have concentric compositional zoning and are almandine-spessartine solid solutions with lesser pyrope, grossular and andradite components. Concentric zoning of major elements in the garnet is attributed to magmatic growth from a melt. On a MnO + CaO versus FeO + MgO (wt%) plot, the composition of garnet is consistent with crystallisation from weakly to moderately evolved melts. The garnets from the Boroujerd pegmatites are characterised by decreasing Y, HREE, Ti, Zr, Nb, Ta, Hf, and U abundances from core to rim. The garnets also have high chondrite normalized HREE abundances with nearly flat patterns (YbN/SmN = 0–508), lower LREE contents, and negative Eu anomalies (Eu/Eu* < 0.3). Variation in these elements from core to rim is attributed to increasing magma fractionation. The composition and major and trace element zoning patterns in the garnet of the Boroujerd pegmatites are compatible with a magmatic origin and crystallisation from variably fractionated I-type magmas demonstrating that garnet crystal-chemistry is an important tool for deciphering the origins of pegmatite magmas.

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.

Detail mineralogical study and geochronological framework of Bayan Obo (China) Nb mineralization recorded by in situ U-Pb dating of columbite