市山 祐司

To constrain the incipient Pacific-type orogeny and tectonic processes in the Early Paleozoic proto-East Asian continental margin, the Motai–Matsugadaira–Yamagami (MMY) metamorphic rocks in the South Kitakami belt, northeast Japan were investigated. They are divided into two different types: amphibolite-facies rocks associated with serpentinite and blueschist-facies rocks associated with pelitic and psammitic schists. Three geochemical groups are identified from the MMY metamorphic rocks. Groups 1 and 2 resemble geochemical characteristics of mid-ocean ridge basalt and continental arc rocks, respectively. Group 3 exhibits considerable depletion of highly incompatible elements, which is caused by the high degree of partial melting of a hot mantle plume. The zircon U–Pb ages of Group 1 indicate that the protoliths experienced amphibolite-facies metamorphism soon after their formation in the Early Ordovician. Group 2 exhibits a coeval zircon U–Pb age with Group 1. The age distribution of detrital zircons in the MMY psammitic schists shows a peak of 500–400 Ma, the presence of Archean to Neoproterozoic zircons, and the youngest Late Devonian zircon. The following model is proposed for the tectonic evolution of the proto-East Asian continental margin: (1) the formation of an arc in the eastern margin of the South China craton in the Cambrian to Ordovician; (2) the subduction of a spreading axis and an oceanic plateau at the same time as the continental arc formation; (3) the consumption and subduction of arc materials by tectonic erosion; and (4) the formation of the Carboniferous accretionary complex and high-pressure metamorphic rocks under steady oceanic plate subduction. The proposed tectonic evolution model may also be applicable to equivalent Early Paleozoic rocks in southwest Japan.

The whole–rock chemistry, mineral chemistry, and remote sensing data of the Atud–Um Khasila Neoproterozoic mafic intrusions in the Eastern Desert of Egypt show two different mafic plutons: (1) the metagabbro–diorite complex; and (2) the G. Atud gabbros. Both of these contain two types of Cu–Ni–Fe–sulfide mineralizations. Multispectral remotely sensed images of Landsat 8 OLI/TIRS, Sentinel 2–B, and ASTER1T were used to give an overview of hydrothermal alteration signatures and distinguish different lithological units. The G. Atud gabbros are intruded into the metagabbro–diorite complex and consist mainly of olivine gabbros, while the metagabbro–diorite complex comprises metagabbros, diorites, and quartz diorites. They were formed under high fO2 (ΔFMQ= +1.43 to + 0.33) with a higher crystallization temperature (∼ 900–1100 °C) and pressure (∼ 6.0 kbar on average) at 18 km depth relative to associated metagabbros. Like magmatic sulfides in mafic intrusions, the G. Atud gabbros contain disseminated grains of pyrrhotite, pentlandite, chalcopyrite, and pyrite, up to 5 vol%. On the other hand, sulfide deposits (up to 30 vol%) such as pyrite, As–bearing pyrite, arsenopyrite, and gold with minor sphalerite and galena at the Atud gold mine, are related to the metagabbro–diorite intrusion. They are found as disseminations, patches, microveinlets, and bands. The sulfide deposits and economic gold are spatially concentrated in smoky quartz veins (up to 25 g/t) and metasomatic alteration zones, i.e., silicification and hematization of metagabbros (0.32 g/t), phyllic, argillic, and propylitic alteration, and carbonate–silicified zones, along gabbroic intrusive contacts, which all follow the Najd NW–SE shear zone. They are possibly of hydrothermal origin (epigenetic). They are also precipitated by mineralized fluids (rich in Si, K, Fe, Pb, Ag, Au, As, S, Ni, Zn, Cu, CO2, and H2O) that have been derived from a mixed magmatic–metamorphic source. The high Au contents with As–bearing pyrite and arsenopyrite in both Fe–rich and smoky quartz veins are related to the interaction between Fe from metagabbro–diorites and the Au(HS)-2 compound as well as the crystallization of pyrite, which reduced the sulfur contents in the mineralized fluids and hence led to gold precipitation. The late intrusion of G. Atud gabbros into metagabbro–diorite rocks enhanced the circulation of sulfide-gold-bearing hydrothermal fluids towards the contacts of the latter ones. These fluids along the shear zones cause metasomatic alteration in addition to leaching and the collection of sulfides and gold in the metagabbros. The protoliths of metagabbro–diorite rocks have a calc–alkaline nature and were formed in a volcanic arc setting, while the G. Atud gabbros were crystallized from Mg-rich tholeiitic melts in the extensional rift (e.g., rifted arc) setting as a result of asthenospheric upwelling due to the slab detachment and/or lithospheric delamination. The Atud gold–sulfide mineralization was related to the thermal uplifting–extensional tectonism. The enrichment of the G. Atud gabbros in LILE, alongside LREE over HFSE, reflects their derivation from a metasomatized mantle source during a rifted arc following the subduction processes.

The magmatic and tectonic evolution of the Oman ophiolite was investigated using the core samples obtained in the Oman Drilling Project. The core samples were recovered from three drilling sites (BA1B, BA3A, and BA4A) in the mantle section composed mainly of harzburgite and discordant dunite and wehrlite. A large number of mafic veins crosscut the structures of both mantle lithologies, indicating that they intruded after the formation of discordant dunites. The wehrlites include Fe3+-rich and TiO2-poor chromian spinel, suggesting that the discordant dunites and wehrlites were involved in hydrous, Ti-poor arc magmatism. The composition of plagioclase and clinopyroxene and rare-earth element (REE) patterns of clinopyroxene for the mafic veins suggest that the mafic veins were produced from mid-ocean ridge basalt (MORB)-like parental melts. On the other hand, early crystallization of clinopyroxene and Fe3+-rich and TiO2-poor chromian spinel in the mafic veins imply that the parental melts were also hydrous. Zircon U[sbnd]Pb dating of the mafic veins yields the igneous age of about 91 Ma, which is younger than the V1 and V2 volcanic sequences. Depending on the distance from the mafic veins, the various REE patterns of clinopyroxene and Ca amphibole in the Oman harzburgites indicate local peridotite metasomatism during intrusion and percolation of late-stage melts that formed the mafic veins. Ca amphibole in the harzburgites and mafic veins lack significant enrichment in fluid-mobile elements, indicating that the hydrous phases were formed by fluids associated with hydrothermal activities in a spreading environment instead of slab-derived fluids. The mafic veins were likely produced in a spreading environment on a supra-subduction zone. As their formation took place after the arc magmatism that formed the Oman V2 lava sequence and discordant dunites, as a result of slab roll-back. This seafloor re-spreading on a supra-subduction zone after the V2 magmatism recorded in the mantle section might have been involved in the formation of the plume-related V3 lava sequence overlying the V2 lavas.

The Wadi Ranga sulfidic jasperoids in the Southern Eastern Desert (SED) of Egypt are hosted within the Neoproterozoic Shadli metavolcanics as an important juvenile crustal section of the Arabian Nubian Shield (ANS). This study deals with remote sensing and geochemical data to understand the mechanism and source of pyritization, silicification, and hematization in the host metavolcanics and to shed light on the genesis of their jasperoids. The host rocks are mainly dacitic to rhyolitic metatuffs, which are proximal to volcanic vents. They show peraluminous calc-alkaline affinity. These felsic metatuffs also exhibit a nearly flat REE pattern with slight LREE enrichment (La/Yb = 1.19–1.25) that has a nearly negative Eu anomaly (Eu/Eu* = 0.708–0.776), while their spider patterns display enrichment in Ba, K, and Pb and depletion in Nb, Ta, P, and Ti, reflecting the role of slab-derived fluid metasomatism during their formation in the island arc setting. The ratios of La/Yb (1.19–1.25) and La/Gd (1.0–1.17) of the studied felsic metatuffs are similar to those of the primitive mantle, suggesting their generation from fractionated melts that were derived from a depleted mantle source. Their Nb and Ti negative anomalies, along with the positive anomalies at Pb, K, Rb, and Ba, are attributed to the influence of fluids/melt derived from the subducted slab. The Wadi Ranga jasperoids are mainly composed of SiO2 (89.73–90.35 wt.%) and show wide ranges of Fe2O3t (2.73–6.63 wt.%) attributed to the significant amount of pyrite (up to 10 vol.%), hematite, goethite, and magnetite. They are also rich in some base metals (Cu + Pb + Zn = 58.32–240.68 ppm), leading to sulfidic jasperoids. Pyrite crystals with a minor concentration of Ag (up to 0.32 wt.%) are partially to completely converted to secondary hematite and goethite, giving the red ochre and forming hematization. Euhedral cubic pyrite is of magmatic origin and was formed in the early stages and accumulated in jasperoid by epigenetic Si-rich magmatic-derived hydrothermal fluids; pyritization is considered a magmatic–hydrothermal stage, followed by silicification and then hematization as post-magmatic stages. The euhedral apatite crystals in jasperoid are used to estimate the saturation temperature of their crystallization from the melt at about 850 °C. The chondrite (C1)-normalized REE pattern of the jasperoids shows slightly U–shaped patterns with a slightly negative Eu anomaly (Eu/Eu* = 0.43–0.98) due to slab-derived fluid metasomatism during their origin; these jasperoids are also rich in LILEs (e.g., K, Pb, and Sr) and depleted in HFSEs (e.g., Nb and Ta), reflecting their hydrothermal origin in the island arc tectonic setting. The source of silica in the studied jasperoids is likely derived from the felsic dyke and a nearby volcanic vent, where the resultant Si-rich fluids may circulate along the NW–SE, NE–SW, and E–W major faults and shear zones in the surrounding metavolcanics to leach Fe, S, and Si to form hydrothermal jasperoid lenses and veins.

The Neoproterozoic pyroxene gabbros and gabbronorites in the El-Baroud mafic intrusion in the Northern Eastern Desert (NED) of Egypt host Fe-Ti oxide ore deposits. This study discusses the major and trace elements of both titaniferous iron ores and their host rocks, along with the mineral chemistry (major and in situ trace elements) of interstitial clinopyroxene (Cpx), to gain a deeper understanding of the Fe-Ti oxide genesis. These ores occur as disseminated (55–60 vol.% of Fe-Ti oxides) and massive types (85–95 vol.%) in the form of the dyke, layer, and lens. They are composed of titanomagnetite (80–87 vol.%) with subordinate ilmenite (10–15 vol.%) and magnetite (3–5 vol.%), in accordance with their high Fe2O3 (75.66 wt.% on average) and TiO2 contents (16.30–17.60 wt.%). The Cpx in the investigated ores is diopside composition (Mg#; 0.72–0.83) and exhibits a nearly convex upward REE pattern, similar to Cpxs in the ferropicrite that originated from the primitive mantle. Melts in equilibrium with this Cpx resemble Greenstone ferropicrite melts; the parent melt of El-Baroud gabbros is possibly a ferropicritic melt that was derived from the lithospheric mantle during plume interaction. The El-Baroud gabbroic rocks were generated during the arc rifting and crystallized under a high oxygen fugacity at a temperature of 800–1000 °C and a pressure of 3 kbar with a depth of 12 km. The Fe-Ti oxide ores have been formed from ferropicritic parent melts by two processes, including in situ crystallization that leads to the formation of disseminated Fe-Ti oxides in the iron-rich gabbros at the bottom and liquid immiscibility that is responsible for the formation of thick Fe-Ti ore lenses and layers at the top of the gabbroic intrusion. Initially, titanomagnetite crystallized from the primary Ti-rich oxide melt. As cooling progressed, some of the excess titanium in this melt was exsolved in the form of the exsolution ilmenite lamellae within the titanomagnetite. The Fe-Ti oxide layers in the NED follow the trend of NW-SE (Najd trend), where their distribution is possibly controlled by the composition of parent melts (rich in Ti and Fe), high oxygen fugacity, and the structure related to the Najd fault system. The distribution of Fe-Ti oxide ores increases from the NED to the Southern Eastern Desert (SED), suggesting the dominant mantle plumes and/or shear zones in the SED relative to the NED.