Island Arc最新文献

The Qushandao Granite, mainly composed of alkali-feldspar granite, is situated in the eastern Zhejiang province of coastal southeast China. In this paper, we present whole-rock geochemistry, zircon U–Pb geochronology, and Hf isotopes to constrain the age, magma sources, and geodynamic setting of the Qushandao Granite. LA-ICP-MS zircon U–Pb dating results revealed that the Qushandao Granite was emplaced in the Late Cretaceous (101–98 Ma). Geochemically, the Qushandao Granite exhibits relatively high silica and alkali contents, metaluminous to weakly peraluminous (A/CNK = 0.98–1.02), and low abundances of phosphorus, titanium, magnesium, and calcium. It is also characterized by enrichment in Rb, K, Th, and depletion in Nb, Ta, P, Ti, and Sr with moderately to weakly negative europium anomalies (Eu/Eu* = 0.71–0.87). Furthermore, the Qushandao Granite displays lower FeO^T/MgO, 10⁴ × Ga/Al, and Zr + Nb + Ce + Y values relative to typical A-type granites. Therefore, we classify the Qushandao Granite as calc-alkaline I-type granite based on a synthesis of geological and geochemical characteristics. The Qushandao Granite shows variable zircon Hf isotopic compositions (ε_Hf(t) = −7.6 to +2.3) and T_DM2 model ages of 1.40–0.83 Ga with a mean value of 1.17 Ga. We argue that the Qushandao Granite was most likely generated by mixing of mantle-derived mafic magma and crust-derived felsic magma in the lower crust, and that it was formed during post-collisional extension in the Late Cretaceous, related to the gradually increasing subduction angle of the Paleo-Pacific plate.

Stalagmite oxygen isotopes (δ¹⁸O) have been used to reconstruct terrestrial paleoclimates during the late Pleistocene and Holocene. However, the interpretation of the δ¹⁸O is not straightforward when determining the factor controlling δ¹⁸O; temperature or water δ¹⁸O. In addition, the water δ¹⁸O changes with rainfall intensity (amount effect), rainfall seasonality, and some other factors. Here, we first review the hydrochemical processes and behaviors of the oxygen isotopes and the other proxies in a cave system, which are fundamental for interpretating the paleoclimatic signals. We then introduce the oxygen isotope records of Japanese caves. Some of the Japanese stalagmites demonstrated a δ¹⁸O profile that represented a similar pattern to the Chinese stalagmite records, but had relatively small δ¹⁸O amplitudes, which can be explained mainly by temperature changes rather than the amount effect. This demands a reversal of the relationship between climate and rainwater δ¹⁸O across the Japanese Islands. Using δ¹⁸O data for rainwater samples from four sites in Japan (in Niigata, Fukuoka, Gifu and Mie Prefectures), we presents the results of model calculations to verify how the rainfall intensity and the seasonality relate with the δ¹⁸O of rainwater. A significant correlation coefficient was observed in Niigata, where the rainfall δ¹⁸O decreases with an increase in the annual amount of rainfall, and with a decrease in the winter rainfall. Similar trends were observed in Fukuoka, whereas while the results of Gifu and Mie exhibited no significant trends. Temperature change was would be the main factor controlling the stalagmite δ¹⁸O at the latter two sites. For a better understanding of the stalagmite δ¹⁸O records, the measurement of fluid inclusions and carbonate clumped isotopes can be used to evaluate the effect of temperature on the stalagmite δ¹⁸O, as well as to reconstruct the water δ¹⁸O. We predict that the ¹⁷O excess in stalagmites reconstructs the seasonal shift in the vapor sources.

The characteristics and pre-Devonian tectonic evolution between the North and South Wuyi terranes are important for understanding the tectonic framework and aggregation of the micro-terrenes in the Cathaysia Block, as well as the evolution of the South China Block. This paper provides systematic studies on the petrologic features, zircon U–Pb geochronology and geochemical characteristics of the low-grade meta-volcanic and metasedimentary rocks between the North and South Wuyi terranes, and proposes an alternative model for their tectonic evolution. Geochronological data show that the metasedimentary rocks have detrital zircon U–Pb ages ranging from 583 to 3284 Ma with a record of the Nanhua period (764–722 Ma). The meta-volcanic rocks have centralized ages ranging from 720 to 760 Ma and yield weighted average ages of 746–733 Ma. Geochemical studies of the meta-volcanic rocks show similar characteristics to island arc magmatic rocks, with relatively high SiO₂, K₂O and Al₂O₃, low TFeO and MgO contents, and enrichment of Rb, Ba, Th, Pb and Hf elements, and depletion in Ti, Nb, Sr and Ta. These features indicate that the North and South Wuyi terranes may have collided at about 746–733 Ma and eventually pieced together at 583 Ma, with the protoliths of the meta-volcanic rocks and the metasedimentary rocks formed in the same tectonic setting of an island arc during Nanhua island arc magmatism related to the collision. The occurrence of Nanhua island arc activity between the North and South Wuyi terranes suggests that multi-micro-terranes and multi-stage collisions may be major characteristics of the tectonic evolution process in the Cathaysia Block.

Lakes of molten sulfur are features sometimes found in seafloor hydrothermal vent systems. Daikoku of the northern Mariana Arc is notable for being home to one of such features inside its summit caldera, the “Sulfur Cauldron” discovered in 2006. A number of oceanographic research cruises since then have revealed significant volcanic activities on Daikoku Seamount, including an eruption event in 2014 leading to the formation of a new basin-like crater. How this event impacted the sulfur lake on Daikoku Seamount remained unclear. Here, we revisited Daikoku Seamount with a remotely operated vehicle to show that the new crater is currently home to a much larger molten sulfur lake than the Sulfur Cauldron, which we name the “Rengoku” sulfur lake. Our samples provided new insights on the structure of submarine sulfur lakes, and contribute to the time-series observation of volcanic and hydrothermal activities on Daikoku Seamount.

The Shuangqishan Au deposit (>15 t) is located in the Dehua-Youxi-Yongtai goldfield of the southeastern Cathaysia Block, South China. There are three stages of granites in the Shuangqishan Au deposit, the pre-mineralization granite porphyry formed in the Middle Jurassic (~166.0 ± 0.8 Ma), the metallogenic granite porphyry and monzonite porphyry formed in the late stage of the Late Jurassic (153–151 Ma), the post-mineralization granites (granite porphyry, the masanophyre, the dioritic porphyrite) formed in the Early Cretaceous (129.0–120.0 Ma). All rocks are characterized by high-K calc-alkaline and shoshonities series, the per-mineralization granites are enriched in Rb, Th, K, Zr, and Hf, depleted in Ba, P, and Ti, significant negative Eu anomaly and distinctly tetrad effect with highly fractionated I-type granitoids characteristics. The metallogenic granites and the post-mineralization granite porphyry are enriched in K, Rb, Th, Ce, Zr, Hf, and Y, enriched in LREE, strongly depleted in Sr, Ba Ta, P, Ti, have moderate negative Eu anomalies with I-type granites characteristics. However, the post-mineralization dioritic porphyrite displays strong depletions Sr, Ta, P, and Ti whereas Rb, Th, Ce, and Sm are enriched, enriched in LREE with weakly negative Eu anomalies. The metallogenic granite porphyry is significantly enriched in Au, As, Sb, W, Mo, and Bi, especially enriched in Au. The Lu-Hf isotopic data indicate that the magma source comes from the partial melting of the Paleoproterozoic crust, the minor part from the mantle in the Early Cretaceous. Therefore, we propose that the granitoids formed in the subducted plate environment from the Middle Jurassic to the Early Cretaceous, the Late Jurassic I-type granites (~153–146 Ma), The Shuangqishan Au deposit was formed in the stage from compression to extension, the Early Cretaceous granites (~129–114 Ma) mainly formed in the extensional environment and destroyed gold ore bodies.

The Nemegt Formation in the Gobi Desert, Mongolia, hosts abundant dinosaur and other vertebrate remains, the ages of which are uncertain due to a lack of radioactive dating. In order to more strictly constrain the depositional age of Nemegt Formation, we conducted U–Pb isotope, trace elements, and Y-screening analyses using laser-ablation inductively-coupled plasma-mass spectrometry (LA-ICP-MS) on apatites of five Tarbosaurus bataar teeth collected from the middle Nemegt Formation. As a result, the age of 66.7 ± 2.5 Ma was obtained from one of the samples which is suggested to be least affected by secondary alteration of the U distribution, and this can be interpreted as a lower limit for fossilization of the tooth samples. Combined with the previously suggested relative age of the Nemegt Formation based on faunal occurrences, our data supports the deposition of the middle–upper Nemegt Formation during the Maastrichtian stage. Given that the depositional age of the Nemegt Formation has not been examined by isotope dating directly using fossils and minerals from the formation, this study is the first to report that the depositional ages of the vertebrate-bearing strata in the Gobi can be discussed by fossil apatite U–Pb dating in combination with the Y-screening method.

The tectonic properties of the Nanping-Ninghua tectonic belt (NNTB) play a crucial role in understanding the tectonic evolution of the Wuyi orogenic belt, which is related to the Neoproterozoic-Paleozoic tectonic evolution of the Cathaysian Block. In this study, We present comprehensive petrological, geochronological, and geochemical data on the metavolcanic rocks from the Louqian Formation in Changting, Fujian Province. Our results show that volcanism in Changting occurred between 740.4 and 723.5 Ma, based on zircon U–Pb data for the metavolcanic rocks. The metavolcanic rocks have SiO₂ contents ranging from 68.22 to 71.56 wt %, high Al₂O₃ and Na₂O + K₂O contents (13.30–15.19 wt % and 7.56–8.25 wt %, respectively), and low CaO, MgO, and FeO_T contents (0.09–1.25 wt %, 0.72–0.96 wt %, and 1.57–3.16 wt %, respectively). These samples are enriched in large-ion lithophile elements (LILEs) such as Ba and Rb, and depleted in high-field-strength elements (HFSEs) including Nb, Ta, and Ti, with significant negative Eu anomalies, indicating an affinity for arc igneous rocks. Based on previous geological studies, we suggest that the NNTB formed in an active continental margin environment during the Nanhua Period. The subduction of oceanic crust led to the formation of the volcanic belt along the Nanping-Ninghua continental margin, which was active until 696 Ma. During the Neoproterozoic, the North and South Wuyi terranes did not merge to form a single Wuyi terrane, and the Cathaysia Block was composed of several terranes.

The end-Permian mass extinction is thought to have greatly altered biogeochemical cycles. The absence of chert and dominance of claystone in low-latitude pelagic deep-sea sedimentary sequences of Early Triassic Panthalassa (the deep-sea chert gap) has been believed to record radiolarian die-off and consequent decline in biogenic silica production. However, recent studies showed that the upper portion of the deep-sea chert gap has sedimentation rates higher than bedded chert, meaning that increased clastic inputs, rather than decreased biogenic silica inputs, resulted in the anomalous lithology. In this study, we focus on the Akkamori-2 section, which preserves a rare sedimentary sequence spanning a large part of the lower portion of the claystone of the deep-sea chert gap. We obtained conodont fossils that allow correlation with sections in South China that have numerous dated tuffs. By projecting the dates of the tuffs to our measured sections, we show that sedimentation rates of the lower portion of the deep-sea chert gap is also higher than bedded chert. Hence, most of the deep-sea chert gap was formed under increased clastic inputs, which likely records disturbance in the terrestrial landscape, probably aridification and/or increased seasonality in arid areas, that lead to elevated dust flux to the pelagic ocean. On the other hand, the idea that the deep-sea chert gap records lingering effects of the mass extinction event on radiolarians cannot explain the high sedimentation rates of the deep-sea chert gap. This previously favored scenario needs to be reconsidered, taking into account the burial efficiency of biogenic silica in the Early Triassic ocean, and also effects of increased clay deposition on preservation of radiolarians.

Granite is fractured according to the stress state during the cooling stage, providing predispositions for later topographic evolution. This study clarified that triangular mesh-like joints can be made during granite cooling and that they can become the structural causes for the formation of rock towers and corestones on the ground. Tengu rock, which consists of rock towers and granite corestones in Hiroshima, was investigated using an unmanned air vehicle. The rock towers were shaped by high-angle mesh-like joints, which were likely made during the cooling of the granite and are dominated by three joint sets. All the joint sets have sharp planar surfaces, which suggests that they are brittle fractures. One joint set is cut by the other two joint sets, frequently accompanies aplite and quartz veins and is developed in the whole exposed granite; this set likely formed first during cooling and then was penetrated by aplite from depth. The other two joint sets are high-angle conjugate joint sets, are limited to the shallower portion of the granite pluton and do not extend deeper, which strongly suggests that they formed in a rapidly cooled shallower portion of the pluton, probably near its roof. These three joint sets form rock columns with parallelogram cross-sections, in which incipient corestones were made. Subsurface weathering along the joints and subsequent exhumation of the weathering products formed the present rock towers and corestones only in the shallower portion of the granite.

The Piranshahr ophiolite is located in northwest Iran. The Mawat ophiolite in northeast Iraq is the western continuation of the Piranshahr ophiolite. The whole rock geochemistry of mafic rocks from the Piranshahr ophiolite (in the Gerdikavalan regions) is studied and compared with mafic rocks from the Mawat ophiolite in northeast Iraq (in the Hasanbag, Walash and Naopurdan regions). Two groups can be identified among the ophiolitic basalts in northwest Iran and northeast Iraq. The first one is represented by Late Cretaceous calc-alkaline basalts, displaying typical active continental margin signatures and thought to have been derived from the melting of enriched mantle sources modified by continental crust and subduction fluids. The second group is characterized by Eocene–Oligocene tholeiitic basalts, displaying signatures intermediate between mid-ocean ridge basalt (MORB) and oceanic island basalt (OIB) and back-arc basin (BAB) like magmas and suggested to have been derived from the melting of mixed asthenospheric-lithospheric mantle sources in a slab window. We propose that the subduction signature is acquired by interaction of the mantle advected through the slab window in the upper part of the subducting plate.