Island Arc最新文献

The timing of mylonitization of the Ryoke Plutonic Rocks in the Tsukide area, eastern Kii Peninsula, Southwest Japan, which is part of a Late Cretaceous volcanic arc on the Asian continental margin, was investigated based on geological relationships and zircon U–Pb dating. The mylonite zone is located north of the Median Tectonic Line (MTL) and has accommodated a large displacement between the low-P/T-type Ryoke Metamorphic Complex, including the Ryoke Plutonic Rocks, and the high-P/T-type Sanbagawa Metamorphic Rocks. The mylonitization in this area was previously dated to 74–67 Ma, as determined from the K–Ar cooling ages of hornblende, biotite, and alkali feldspar. However, these ages might represent the most recent cooling ages that reflect thermal resetting by later intrusions. In this study, the timing of mylonitization was constrained by zircon U–Pb ages of a deformed granitoid and undeformed granitoid that intrudes it. Three stages of mylonitization, M1–M3, were recognized in this area. M1 is high-T-type mylonitization that occurred between 87.5 ± 1.7 and 83.2 ± 1.2 Ma. M2 and M3 are medium- to low-T-type mylonitization that occurred at ca. 77.5 Ma and after ca. 70 Ma, respectively. Top-to-the-west sense shearing occurred after M2 along shear zones that converge obliquely with the present-day MTL. The shear zone became narrower than that of M1, due to uplift and cooling, and eventually transitioned into discrete faults. M1 coincided with the change in the subducting plate beneath proto-Japan from the Izanagi Plate to the Kula Plate at 85 Ma, and M2 occurred during subduction of the Kula Plate.

Manselle, P., S. Foley, and N. Daczko. 2025. “Origin of Amphibole Cumulates at the Base of an Exposed Arc Crustal Section: Perspectives From Fiordland, New Zealand.” Island Arc 34, no. 1: e70030. https://doi.org/10.1111/iar.70030.

In Table 3 on page 12, the column header “P(kbar)” is incorrect as it indicates that pressures are given in kbar. This should be corrected to “P(GPa).”

Below is the correct Table 3.

We apologize for this error.

The origin of extremely diverse arc magmas erupted along the 600-km-long Quaternary frontal arc of NE Japan was examined using Sr–Nd–Hf–Pb isotopic and major/trace elemental compositions. The major/trace element chemistry of the magmas varied with the various combinations of low-K to medium-K and calc-alkaline to tholeiitic rock suites, even within a single volcano. Nevertheless, magmas from individual volcanoes showed quasi-linear isotopic variations in the Pb–Pb and Nd–Pb isotope spaces irrespective of their complex rock suites. The isotope data indicate a binary mixing origin of the magmas derived from a mostly homogeneous mantle source and the others from an isotopically diverse crust represented by Cretaceous to Paleogene basement granitoids. Assimilation-fractional-crystallization (AFC) and melting-assimilation-storage-hybridization (MASH) processes formed the tholeiitic and calc-alkaline series, respectively. These would have occurred in the amphibolitic lower crust that constitutes the roots of the basement granitoids. This study demonstrates that the mafic lower crust extensively contributes to the chemical diversity of arc magmas, even for relatively undifferentiated basaltic andesites.

The Cretaceous high-P/T metamorphic complex such as Sanbagawa schists and Mikabu greenstones is located in the outer zone of the Median Tectonic Line in the Shibukawa area, northwestern Shizuoka Prefecture, central Japan. In this area, a 2 × 1 km Shibukawa ultramafic body, a member of the Mikabu greenstones, occurs within the chlorite zone of the low-grade Sanbagawa schists. The upper part of this ultramafic body is covered by pelitic schists (classified as the Upper unit). The peak temperatures estimated from the Raman spectra of carbonaceous material (CM) in the pelitic schists were 277°C–354°C. The temperatures of the Upper unit range from 277°C to 293°C, which are lower than those of the surrounding Sanbagawa schists, ranging from 295°C to 354°C. Considering the elevation and large-scale structures of the body, the Upper unit is inferred to be a shallow, lower-grade Sanbagawa schist unit preserved in the ultramafic body, as it has a geological structure similar to that of the surrounding schists. This suggests that although the occurrence of the Shibukawa ultramafic body has complicated the geological structure of the Sanbagawa schists, the metamorphic temperature increases toward apparently lower structural levels. In addition to constraining metamorphic temperatures, it shows that the Raman CM geothermometer is a powerful tool for revealing structural characteristics based on geological and thermal relationships within low-grade metamorphic rocks in shallow subduction zones.

During the Miocene opening of the Japan Sea, volcanic activity expanded greatly toward the trench due to the injection of the hot asthenosphere into the mantle wedge. The Ishimoriyama and Iritono volcanic rocks, both erupted at around 17.5 Ma in the Iwaki district on the Pacific coast of NE Japan, are products of this event. Ishimoriyama is a small composite volcano comprising calc-alkaline basaltic to andesitic volcaniclastic rocks. Iritono is a monogenetic volcano composed of low-K aphyric pillow basalts with high TiO₂ contents. The Pb isotopic compositions of volcanic rocks from both volcanoes (²⁰⁶Pb/²⁰⁴Pb = 18.39–18.40, ²⁰⁷Pb/²⁰⁴Pb = 15.61–15.62, and ²⁰⁸Pb/²⁰⁴Pb = 38.53–38.57) are more radiogenic than the Indian MORB-like trend defined by other Miocene volcanic rocks in the fore-arc region of NE Japan and overlap those of the most enriched Japan Sea Miocene basalts. The variation among the Japan Sea basalts can be explained by the mixing of depleted Indian MORB-like mantle and enriched preexisting subcontinental lithospheric mantle (SCLM). Therefore, the Ishimoriyama and Iritono volcanic rocks were derived from SCLM, and the reason why magma was generated by partial melting of the SCLM, which does not normally melt spontaneously, was due to the injection of the hot asthenosphere. The trace element abundances in the Ishimoriyama volcanic rocks match those in normal arc-type volcanic rocks, but their compositional variations suggest the fractionation of large amounts of amphibole from basaltic andesite magma. Because this differentiation process requires a high pH₂O in the parental magma, the source could have been hydrous SCLM. In contrast, the Iritono volcanic rocks are depleted in fluid-mobile elements and have convex rare earth element patterns peaking at Sm. Accordingly, this magma formed when a residual SCLM domain was re-melted by the hot asthenospheric injection. These distinct geochemical differences between two nearly contemporaneous volcanic rocks only 20 km apart indicate that the SCLM was heterogeneous, with adjacent hydrous and residual domains.

The formation of amphibole-rich cumulates in arc settings plays a crucial role in influencing the chemistry of ascending arc magmas. However, understanding the processes of formation of these cumulates is challenging due to the subtle nature of their fractionation. This study presents new petrographic and geochemical analyses of amphibole-rich cumulate rocks from outcrops across Fiordland, New Zealand, an exposed section of the lower arc crust. Our findings show two primary styles of formation. Style 1 represents direct fractionation from a melt, and is typified by adcumulate textures, similar rare earth element concentrations, and minor mineral phases. Style 2 amphiboles show greater variations in their rare earth element contents and typically lack adcumulate textures. Our results indicate that direct igneous fractionation from a melt is the primary formation mechanism for amphibole cumulates, with minor contributions from melt–rock interaction, aligning with the amphibole sponge model. This research underscores the significance of amphibole cumulate formation in the evolution of the lower crust in arc systems and illustrates how co-crystallizing minerals and precursor assemblages can influence the chemistry of ascending magmas during metasomatism.

The Ryoke metamorphic belt records the conditions of the middle–upper crust during Cretaceous crustal deformation and magmatism. To understand the development and evolution of the middle–upper crust, it is important to investigate the thermal structure of the Ryoke metamorphic belt. The Ryoke metamorphic rocks crop out widely in the Takato–Komagane area in Nagano Prefecture, central Japan. However, the spatial variations in metamorphic conditions in both areas are still unclear. We therefore investigated the metamorphic zoning and conducted Raman spectroscopy of carbonaceous material (RSCM) thermometry, chemical analyses, and pseudosection modeling of the Ryoke metamorphic rocks. The Bt, Sil–Kfs, and Grt–Crd zones in the Takato area, and the Bt, Kfs–Crd, and Grt–Crd zones in the Komagane area were redefined based on the mineral assemblages in metasedimentary rocks. The metamorphic zoning is different between the Takato and Komagane areas. We revealed that sillimanite occurs throughout both areas, although its occurrence changes gradually from in the matrix to inclusions in cordierite, between the Takato and Komagane areas. Peak P–T conditions of the Sil–Kfs zone are estimated to be 630°C–650°C and 320–380 MPa using RSCM thermometry and pseudosection modeling. Pseudosection modeling, microscopic observations, and thermometry results indicate higher temperatures in the Komagane area than in the Takato area. It is interpreted that part of the regional metamorphic belt underwent thermal overprinting induced by a late-stage pulse of the gneissose Ryoke granitoids, resulting in the formation of a metamorphic zone distinct from that in the Takato area, characterized by the occurrence of sillimanite inclusions.