Masatsugu Ogasawara

Geochemistry of the Early Proterozoic granitoids in the Halls Creek Orogenic Subprovince, northern Australia

Abstract The Halls Creek Orogenic Subprovince contains various types of Early Proterozoic granitoids. Three suites of tonalite have been recognized in the central part of the Halls Creek Orogenic Domain which is an eastern wing of the subprovince. The oldest tonalite suite characteristically contains orthopyroxene, and includes trondhjemite. The youngest tonalite among the three is the Sally Downs Tonalite, and is a part of the Mabel Downs Granitoid. The tonalites and trondhjemite have low K2O and Rb contents, fractionated REE patterns with HREE depletion. These geochemical characteristics suggest each of the tonalite melts is likely to have been derived from the partial melting of mafic rocks leaving a residue consisting of garnet or amphibole. It is inferred that subduction of an oceanic plate which had been present between the Kimberley and North Australian blocks was responsible for the generation of the tonalitic magma, melting either subducted oceanic crust or lower mafic crust of an island arc. The Bow River Granitoid Suite occurs in the western part of the Halls Creek Orogenic Domain, forming a large batholith. Granitoids of the suite are typically peraluminous and enriched in K2O and Rb. The Sophie Downs Granitoid is an alkali-feldspar granite, and has A-type characteristics. The subduction and subsequent collision of the two blocks are considered to be responsible for the generation of those granitoids.

Tectonic influence on chemical composition of ocean island basalts in the West and South Pacific: Implication for a deep mantle origin

Pb, Nd, and Sr isotopic compositions and the 40Ar-39Ar ages of lavas from seamounts in the Joban seamount chain, which is the northernmost Cretaceous seamount chain in the northwest Pacific, suggest that these seamounts were produced by HIMU-type (high-μ or high-238U/204Pb) magmatism around 120 Ma. In addition, seamount chains in the western Pacific, which are coeval with the Joban seamount chain, also exhibit a HIMU isotopic signature. Since the seamount chains in the western Pacific are considered to have formed in the present-day South Pacific region, these isotopic features suggest that HIMU-type magmatism must have been active in the South Pacific since the Cretaceous. A notable geochemical characteristic of this magmatism is a correlation between the elemental ratios (La/Yb, Sm/Yb, Sr/Y, Nb/Zr, Ta/Zr, and Th/Ta) of the seamounts/islands and a “relative age” that is an index of the thickness of the lithosphere beneath hot spots. These correlations suggest the importance of the tectonic environment in determining the chemical composition of the magmas. They also suggest a genetic relationship between oceanic plateaus and island/seamount chains because the elemental ratios of oceanic plateaus follow the same trends. Another important geochemical feature is that the elemental ratios of mid-ocean ridge basalts (MORBs) from the East Pacific Rise (EPR) do not follow these trends. Since the source material for EPR MORBs is probably representative of the composition of the upper mantle beneath the South Pacific, this difference could imply an upward flow of material from the deep mantle.

Geochemical characteristics of Miocene Fe-Cu–Pb–Zn granitoids associated mineralization in the Chichibu skarn deposit (central Japan): evidence for magmatic fluids generation coexisting with granitic melt

Abstract In this work we study mechanisms and timing of magmatic fluid generation during magma emplacement. Our focus is the Miocene calc-alkaline granitic rocks from the Chichibu mining area, in Japan. The granitoids consist of northern and southern Bodies and of the Daikoku Altered stocks. Cathodoluminescence observation of quartz phenocrysts from the northern body point towards magmatic resorption, which is thought to be caused by mixing between a more differentiated and a more primitive magma. The coexistence of vapour-rich two-phase and halite-bearing polyphase fluid inclusions in a single quartz crystal from the northern Body supports the possibility of pressure decrease during magma emplacement. The magmatic fluids that originated the Chichibu deposit are thought to have been generated by pressure release, related to magmatic differentiation when the SiO2-content reaches about 65 wt%. As a result, heavy metals, such as copper, gold and arsenic, coexisting with the silicate melt, were transported into the sedimentary strata through degassing of magmatic fluids. A later major fault system caused the intercalation between heavy-metal-free limestone and orebodies, as a secondary skarn-building process took place in the dominant limestone area.

西南日本，岡山市の採石場に見られる還元型／酸化型花崗岩類

Two different granites in their oxygen fugacity during the formation, are seen in the drill cores from the northwestern part of Okayama City. One is coarse-grained granite having no or little rock-forming magnetite, while the other is fine-grained aplitic granite occurring in sheet-like form and containing coarse-grained magnetite in miarolitic aggregates of K-feldspar and quartz. The major and trace elements chemistry indicates that the aplitic granite is crystallized from fractionated melts of the coarse-grained granitic magma. It is suggested that the fractionated magma became rich in H2O which dissociated into O2 and H2 and the hydrogen diffused out to the roof rocks; then the oxygen fugacity was increased to form magnetite. Löllingite was discovered along cracks of the coarse-grained granite at one quarry, reflecting a low sulfur fugacity of the post-magmatic hydrothermal activity of these granites.

Petrography and geochemistry of Permian basalts of the Cam Thuy formation and their relation to Song Da and Emeishan magmatic rocks

Cam Thuy Permian basalts consisting of thick lava flows and pyroclastic layers appear along both sides of the Song Ma fault zone in Thanh Hoa and in Son La and Ninh Binh provinces, NW Vietnam. The magmatism has been thought to have genetic relationship with Permian volcanism in the Song Da rift zone, which is believed to be part of the Emeishan large igneous province, having been extruded along the Red River shear zone following Paleogene India-Eurasian collision. A set of Cam Thuy volcanic samples including olivine and alkaline basalts was collected in the Lam Son area (Tho Xuan, Thanh Hoa province) to analyze for geochemical major, trace element and Sr-Nd-Pb isotopic composition. The Cam Thuy basalts are high-TiO2, CaO, FeO*, moderate MgO and SiO2 that plot between the Song Da and Emeishan highand low-Ti basalt distribution fields and closely overlap that of Song Da’s high-Ti field. The primitive mantle and chondrite normalized trace element patterns of Cam Thuy basalts are essentially enriched oceanic island basalt (OIB)-like; this feature, together with crustal contamination-free, chondritic Sr, Nd and Pb initial (255Ma) isotopic composition are certainly of asthenospheric origin. These geochemical and isotopic characteristics are closely analogous to those features observed for the Song Da high-Ti basalts, suggesting similarity in their source of origin. Nevertheless, while the Song Da (and Emeishan) magmatism is signified by the presence of both highand low-Ti basalts, with the latter being derived from heterogeneous and partially crustal-material contaminated sources in the lithospheric mantle, this low-Ti volcanic rock type has yet to be discovered in the Cam Thuy formation.

Geochemistry of Neogene Basalts in the Nghia Dan district, western Nghe An

Nghia Dan Neogene basalts occur as monogenetic volcanoes and thin lava layers (up to tens of meters thick). They are alkaline basalts and basanites, some containing mantle xenoliths such as spinel lherzolite. Compared with Tay Nguyen (Western Highlands) Cenozoic basalts (for example, Pleiku and DacNong) the Nghia Dan basalts show much lower SiO 2 (45-48.5wt.%) and higher FeO*(up to 9-11 wt.%), TiO 2 (2.5-3 wt.%) and CaO (9-10 wt.%); they are very high in trace element contents especially Ba, Th, Nb (up to 130 ppm), Sr (up to 2000 ppm) and Eu (up to 4 ppm). Their rare earth concentrations are high, much higher as compared to those of Tay Nguyen. Melting parameter modeling shows the Nghia Dan melts generated from about 3 - 4% partial melting of a combined garnet- spinel- lherzolite source between a pressure range of 20 to 25 Kb (about 75 km deep). The parameters are consistent with the low SiO 2 and high trace element, including the rare earth, contents in the Nghia Dan basalts. High FeO*, TiO 2 , CaO and Sr may also be a result of interaction with mafic components in the lithospheric mantle by the mantle-derived melts on the way to the surface.

Major products of the international collaborative project on mineral resources, metallogenesis, and tectonics of northeast Asia

A major international collaborative project on the mineral resources, metallogenesis, and tectonics of Northeast Asia is being concluded by the Russian Academy of Sciences, the Mongolian Academy of Sciences, Jilin University (China), the Korean Institute of Geoscience and Mineral Resources, the Geological Survey of Japan, and the U.S. Geological Survey. The project area includes eastern and southern Siberia, the Russian Far East, Mongolia, northern China, South Korea, and Japan. The project is a multi-national study at 5.0 m scale with over 50 international collaborators from six countries. The Northeast Asia project extends and builds on data and interpretations resulting from a similar previous project on the mineral resources, metallogenesis, and tectonics of the circum-north Pacific (Russian Far East, Alaska, and the Canadian cordillera). The Northeast Asia project is associated with a new project of the U.S. Geological Survey entitled a Global Mineral Resource Assessment.