For decades, it has been unclear as to how the world's first cities, in southern Mesopotamia, not only arose in a fluvial environment but also how this environment changed. This paper seeks to ...understand the long-term fluvial history of the region around Uruk, a major early city, in relation to water-human interactions. This paper applies geomorphological, historical and archaeological approaches and reveals that the Uruk region in southern Mesopotamia had been under the influence of freshwater fluvial environment since the early Holocene. It further demonstrates how canals and long-term human activities since the mid Holocene have been superimposed on the natural river channel patterns. Fieldwork has been conducted to ground-truth features identified applying remote sensing techniques. Five sediment cores were analysed to elucidate palaeoenvironmental changes. Radiocarbon ages for organic samples suggest that the oldest sediment layers, at a depth of 12.5 m, are from the Early Holocene, while results from diatom analyses imply that the whole sediment column was deposited in a freshwater environment. Intensive networks of palaeochannels and archaeological sites within the study area have been reconstructed and these networks have been divided into four different time intervals based on changes in channel courses. The first is from the early 4th to the late 1st millennium BCE; the second is from the late 1st millennium BCE to the middle 2nd millennium CE; the third lasted from after the Islamic period until the 1980s; the fourth is from the 1980s until the present. Key results include evidence for freshwater environments and favourable settlement conditions had already formed by the 8th millennium BCE. The favourable settlement environment resulted in stable (long-lived) canals between the 4th millennium BCE and 1st millennium CE. A significant settlement and irrigation expansion occurred in the early 1st millennium CE. Major abandonment ensued in the late 1st millennium CE and lasted until the mid 2nd millennium CE.
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•Zircon U-Pb dating shows that the Ardakan granitic plutons crystallized at ∼550 Ma and a doleritic sill at ∼528 Ma.•Granitewas produced byinteraction of mantle-derived melt with ...continental crust.•Dolerite was generated by partial meltingof asthenospheric mantle.•The Ardakan magmatic rockswere formed in a long-lived continental arc and neighboring back-arc.
Ediacaran-early Cambrian magmatic rocks from the Ardakan region of central Iran include deformed granites from deformed granitic plutons and dolerites from doleritic sills. The granites contain zircon grains with U-Pb ages of 552–550 Ma, and dolerites have a zircon U-Pb age of 528 Ma. These granites are high K calc-alkaline in nature and have variable concentrations of SiO2 (69.1–76.3 wt%), Na2O (2.06–4.82 wt%), K2O (3.08–4.79 wt%) and MgO (0.57–2.02 wt%). They represent I-type granite and are metaluminous to weakly peraluminous, with negative primitive mantle-normalized Nb, Ti and Eu anomalies. 87Sr/86Sr(i) and εNd(t) in the granites vary from 0.7075 to 0.7120 and from −3.0 to +7.3, respectively. Dolerites are alkaline, with low contents of SiO2 (45.7–48.6 wt%), Cr and Ni (13.6–313 and 15.7–146 ppm, respectively) and overall high contents of TiO2 (2.0–4.6 wt%). TiO2 contents define high-Ti and low-Ti dolerite types, which show similar high field strength and rare earth element abundances. The high-Ti dolerites may be evolved equivalents of the low-Ti type. The 87Sr/86Sr(i) and εNd(t) values of both types are highly variable (0.7029 to 0.7077 and −6.1 to +7.7, respectively). The data indicate melting of an asthenospheric mantle source, with additional fractionation, and mixing with crustal melts to produce the evolved dolerites. Geochemical data from granites in NE Ardakan are consistent with late Ediacaran arc magmatism ∼550 Ma. The ∼528 Ma OIB-like dolerites may indicate continental extension at this time.
Studies of oceanic crust, which covers a large proportion of the Earth's surface, have provided significant insight into the dynamics of crustal accretion processes at mid‐ocean ridges. It is now ...recognized that the nature of oceanic crust varies fundamentally as a function of spreading rate. Ocean Drilling Program (ODP) Hole 1256D (eastern Pacific Ocean) was drilled into the crust formed at a superfast spreading rate, and hence represents a crustal end member. Drilling recovered a section through lava and sheeted dykes and into the plutonic sequence, the study of which has yielded abundant insight into magmatic and hydrothermal processes operating at high spreading rates. Here, we present zircon U‐Pb dates for Hole 1256D, which constrain the age of the section, as well as the duration of crustal accretion. We find that the main pulse of zircon crystallization within plutonic rocks occurred at 15.19 Ma, consistent with magnetic anomalies, and lasted tens of thousands of years. During this episode, the main plutonic body intruded, and partial melts of the base of the sheeted dykes crystallized. One sample appears to postdate this episode by up to 0.25 Myr, and may be an off‐axis intrusion. Overall, the duration of crustal accretion was tens to several hundreds of thousands of years, similar to that found at the fast‐spreading East Pacific Rise and the slow‐spreading Mid‐Atlantic Ridge. This indicates that crustal accretion along slow‐ to superfast‐spreading ridges occurs over similar time scales, with substantially longer periods of accretion occurring at ultraslow‐spreading ridges characterized by thick lithosphere.
Plain Language Summary
The oceanic crust paves approximately 2/3 of the Earth's surface. It is formed at mid‐ocean ridges, where tectonic plates separate and new crust is formed by the solidification of magma. This magma is formed by partial melting of the upper mantle beneath the ridge axis. Plates spread at different rates at different mid‐ocean ridges, and the fastest‐known spreading occurred some 11–18 million years ago in the eastern Equatorial Pacific. A section of the crust formed during this episode of superfast‐spreading was recovered by scientific drilling in the framework of the Integrated Ocean Drilling Program (IODP). This study presents age data that determine when this section of superfast‐spreading crust formed, and how long it took to build the crust. We find that the age of the section is 15.19 Ma, and that crustal formation lasted between tens and several hundreds of thousands of years. This duration is similar to that found at mid‐ocean ridges with slow‐ to fast‐spreading rates, such as the Mid‐Atlantic Ridge and East Pacific Rise. However, it is much shorter than the formation of crust at ultraslow‐spreading ridges, where the cool and thick nature of the lithosphere leads to prolonged episodes of crustal formation.
Key Points
The main pulse of crystallization of superfast‐spreading crust at Hole 1256D occurred at 15.19 Ma
Crustal accretion lasted between tens and several hundreds of thousands of years
Crustal accretion along slow‐ to superfast‐spreading ridges occurs over similar time scales
Abstract
The Chile Triple Junction, where the hot active spreading centre of the Chile Rise system subducts beneath the South American plate, offers a unique opportunity to understand the influence ...of the anomalous thermal regime on an otherwise cold continental margin. Integrated analysis of various geophysical and geological datasets, such as bathymetry, heat flow measured directly by thermal probes and calculated from gas hydrate distribution limits, thermal conductivities, and piston cores, have improved the knowledge about the hydrogeological system. In addition, rock dredging has evidenced the volcanism associated with ridge subduction. Here, we argue that the localized high heat flow over the toe of the accretionary prism results from fluid advection promoted by pressure-driven discharge (i.e., dewatering/discharge caused by horizontal compression of accreted sediments) as reported previously. However, by computing the new heat flow values with legacy data in the study area, we raise the assumption that these anomalous heat flow values are also promoted by the eastern flank of the currently subducting Chile Rise. Part of the rift axis is located just below the toe of the wedge, where active deformation and vigorous fluid advection are most intense, enhanced by the proximity of the young volcanic chain. Our results provide valuable information to current and future studies related to hydrothermal circulation, seismicity, volcanism, gas hydrate stability, and fluid venting in this natural laboratory.
Slab-melting is considered to have played an important role in the formation of continental crust. The combination of oxygen (O) and hafnium (Hf) isotope signatures can provide key information ...relating to the melting components during slab-melting. To reveal the melting components, we used SIMS and LA-ICP-MS to determine O isotope ratios at 220 spots and Hf isotope ratios at 61 spots in zircons from five plutons in the Taitao Peninsula, where slab-melting occurred at ca. 4–5 Ma. In addition, we measured whole-rock HfO isotope ratios in 11 rocks that were intruded by the granitoids. The zircon δ18O values of the Seno Hoppner pluton (5.37 ± 0.44‰) are identical to those of mantle-equilibrated zircons, whereas those of the other four plutons are relatively high (6.09–6.53‰). The ɛHf(t) values of zircons in the granitoids are negatively correlated with δ18O, and they fell along the mixing line between a juvenile component and sedimentary rocks. Therefore, the HfO profiles can be attributed to mixing of juvenile granitoid magma and the sedimentary rocks, the amount of which was likely minimal in the Seno Hoppner pluton. As such, the δ18O values of the Seno Hoppner pluton could best reflect the juvenile component. In view of the δ18O variation in whole-rock samples of the Taitao ophiolite, the melting of hydrothermally altered basalt and dolerite is considered responsible for the juvenile magma generation. This further implies that juvenile granitoid magmas can be generated by the melting of the upper half of subducted oceanic crusts.
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•Taitao granitoids are useful to constrain magmatic composition formed by slab-melting•We obtained new zircon oxygen and hafnium isotopic data from Taitao granitoids•Melting of hydrothermally altered dolerite is required to account for O isotopic data•The HfO profiles can be explained by mixing of juvenile granitoid magma and sediment.•Partial melting of subducted oceanic slab reached the upper half during the Archean
Late Neoproterozoic basement of the Soursat complex to the southeast of Shahin-Dej, NW Iran, spans an area wider than 100 km2 and is cut by Paleogene monzonite and granodiorite bodies. Zircon U-Pb ...dating yields monzonite and granodiorite crystallization ages of 54.7 ± 3.8 Ma and 57.7 ± 2.9 Ma, respectively. The monzonite group has relatively high MgO (2.40–7.01 wt%), Cr (37.5–233 ppm) and Ni (30.6–177 ppm), and high Sr/Y (103−132) and La/Yb (68.0–102) similar to low silica-adakites (LSA). The granodiorite group has low MgO (<1 wt%), Ni (<10 ppm) and Cr (<33 ppm), and high Sr/Y (192–361) and La/Yb (37.3–156) ratios, and are consistent as high silica-adakites (HSA). The monzonite and granodiorite have 87Sr/86Sr(i) values of 0.7045 to 0.7047 and 0.7053 to 0.7058, and εNd(t) values of +1.1 to +1.8 and − 1.9 to −0.8, respectively. The differing chemical compositions and Sr-Nd isotope ratios of these two groups of coeval adakites suggest different sources. Upon consideration of the geodynamic evolution of NW-Iran during the Cenozoic, we infer a two-stage melting process. LSA melts were first produced due to partial melting of delaminated lithosphere and then modified by fractional crystallization in the lower crust. Subsequently, HSA melts were generated during partial melting of lower crustal mafic segments in the hot root zone.
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•The Soursat complex comprises coeval low (LSA) and high silica-adakite (HSA).•Zircon U-Pb dating shows that the LSA and HSA crystallized at 57–54 Ma.•LSA was produced by partial melting of delaminated and metasomatized lithosphere.•HSA melts were generated by partial melting of lower mafic continental crust.
•Zircon U-Pb dating shows that the Zarin granite crystallized 531–530 Ma ago.•The Zarin granite has a shoshonitic to high-K calc-alkaline chemistry with highly fractionated characteristics.•Sr-Nd ...isotope ratios suggest a magma was mainly derived by melting of continental crust.•Results are consistent with emplacement of the Zarin granite during the extension of a Cadomian continental arc.
The Zarin granitic body is located in the eastern part of the Yazd Block in the Central Iran microcontinent. The granite has variable granular to mylonitic textures. New zircon U-Pb dating shows that the granitic rocks crystallized at 535–530 Ma. The pluton has high SiO2 abundances (72.1–77.5 wt%), K2O (4.4–6.3 wt%) and low MgO (0.2–0.7 wt%) contents, as well as high K2O/Na2O (1.10–1.97) and A/CNK (molar ratio of Al2O3/(CaO + Na2O + K2O)) 0.9–1.1 ratios, and is dominantly peraluminous. The granite is enriched in light rare earth elements (LREEs) relative to heavy REEs (HREEs), has negative Eu anomalies, and is extremely depleted in Nb, Ba, P, Ta and Ti. It is also characterized by variable but commonly high 87Sr/86Sr(i) (0.7018–0.7093) and negative εNd(i) values of −1.1 to −2.6. The geochemical features indicate that the rocks are highly fractionated I-type granite and have undergone extensive fractional crystallization of hornblende, biotite and feldspar. Results are consistent with melting of an evolved lower crust source during the subduction of the Proto-Tethys Ocean. The emplacement of the Zarin granite is contemporaneous with the extension of a Cadomian continental arc on the north side of Gondwana, that rifted away to create the Rheic Ocean to its south. Cadomian basement rocks in Iran are interpreted as an important link between western and eastern sectors of the Late Proterozoic-Early Paleozoic orogenic belts along the northern side of Gondwana, and the histories of the Rheic, Iapetus and Proto-Tethyan oceans
Intermediate to felsic volcanic and granitic rocks with high Na2O concentrations (5.2–9.1wt.%) are widely distributed in the Harsin area along the Zagros thrust zone in western Iran. Most of these ...rocks are classified as low-potassium tholeiite, display affinity with oceanic plagiogranite and contain somewhat high Na content and low concentrations of K2O, Al2O3, Rb, Sr and Ca. Thus, we prefer to apply the term Na-rich felsic rocks to this complex. U–Pb dating yielded ages of 94.6±2.7Ma (2σ) from baddeleyite and 95.0±2.4Ma (2σ) from zircon grains, indicating that the complex crystallized in the Late Cretaceous. Based on the mineral compositions, the crystallization occurred at low pressures (mostly <2kbar) and low temperatures (<750°C). High initial ratios of 143Nd/144Nd (0.51288–0.51304) and positive values of ɛNd(t) (+7.0 to +11.5) are consistent with those of mid-oceanic ridge basalt (MORB). During collision of the Arabian plate and Biston-Avoraman block in the Late Cretaceous, an increasing geothermal gradient was responsible for partial melting of altered mafic rocks and for producing the Na-rich felsic rocks in the Harsin area. The presence of these types of rocks along the main Zagros fault indicates local collisions. These collisions were caused by southwestward subduction under the Arabian plate in the southern branch of the Neo-Tethys. This event was the first stage of the Zagros collision, which was followed by collision of the Arabian and Iranian plates during the Eocene through Neogene.
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•The acidic rocks with high concentration of Na2O crystallized 95million years ago•Oceanic basalts and altered rocks were the main source for these rocks.•Partial melting occurred during the Arabian plate and Biston block collision.•Our founding infers the SW subduction of Neo-Tethys during the Jurassic.•Multistage collisions occurred in Zagros orogeny during Mesozoic and younger times.
To understand the processes of melt eduction in a ridge subduction zone, we performed U-Pb dating on zircons separated from igneous and sedimentary rocks that were newly dredged from the Chile Triple ...Junction area and from volcanic rocks collected from the Taitao peninsula, southern Chile. The youngest fraction of the U-Pb age population was used to estimate the age of magmatism or sedimentation. Our new results indicate that the fore-arc region became volcanically active over a period of ~0.4 m.y., after obduction of the Taitao ophiolite (~5.7 to 5.2 Ma) from the west and after granite intrusions related to ridge subduction at ~6 Ma. Fore-arc volcanism produced ejecta of basaltic to dacitic compositions and migrated from offshore (~5.3 Ma) to inland (~4.6 Ma) along the Chile Margin Unit that trends northeast-southwest. The volcanism further extended east to produce the dacitic volcanic plug of Pan de Azucar (~4.3 Ma) and lavas in Fjord San Pedro (~2.9 Ma). The migration took place at a rate of ~2.3 cm/y to ~5.3 cm/y. Another intrusion of a granite pluton, widely distributed offshore of the Taitao ophiolite, took place at ~4.0 Ma. Distributions of old detrital zircon and zircon xenocryst ages were used to evaluate, respectively, the influence of subducted sediments and igneous crustal material. Our results indicate that crustal material influenced only Pan de Azucar and Fjord San Pedro dacites; other acidic magmatism shows moderate evidence for incorporation of subducted sediments. Therefore, melts were formed at shallow depths near the triple junction, due mainly to partial melting of the subducted slab and sediments, and then ejected instantaneously. Depleted oceanic materials became anhydrous, and a volcanic gap was formed along the Andean arc.
In order to better understand the encroachment of seawater into the groundwater system a total 115 groundwater samples were collected in an area of ∼7 km2 in Cox's Bazar paleobeach and its vicinity, ...from different types of tubewells, which included hand pump fitted bore wells (<50 m depth), moderately deep energized bore wells (50 m–100 m depth) and deep bore wells (>100 m depth) during June 2013. A combined hydro-geochemical indicators, selected trace elements (Sr and B) and in addition to δ18O and δ2H were used in this study. Results show that the high values of EC (202 μS/cm to 6730 μS/cm) and TDS (135 mg/L to 4509 mg/L) are found in groundwater of wells closer to the coast and few wells from the northern and central part of the study area. The cross plot of HCO3/Cl and TDS shows that groundwater along the paleobeach and some isolated areas are saline with TDS > 1000 mg/L, associated with high ratios of Cl/TDS (0.065). The groundwater facies generally evolves from freshwater (Ca-Mg-HCO3) to saline water (Na- Cl) type with an intermediate Ca-Mg-Cl type indicating that the aquifer system interacts with seawater and undergoes cation exchange. Results also reveal that the area affected by marine water intrusion has ionic ratios of Br/Cl (0.0006–0.0021) similar to seawater (0.0015) with some higher values. Groundwaters along the paleobeach and some isolated areas, low ionic ratios of SO4/Cl (0.01–6.53) and Na/Cl (0.20–152.09), relative to marine ratios (0.05 and 0.86 respectively), are also observed. Groundwaters with Seawater Mixing Index >1.0 and TDS >1000 mg/L constitute about 20% of the studied groundwaters and have relatively high δ18O (>- 4.0‰) values and the linear relationships between TDS and most of the ions, including B and Sr, and the chemical signature of the saline plumes (e.g., marine SO4/Cl, Na/Cl and Br/Cl ratios) suggest that mixing processes control the chemical composition of the seawater within the aquifers. However, these geochemical variations also reveal that the quality of potable water has deteriorated to a large extent due to seawater intrusion along the paleobeach, migrating inland toward the heavily groundwater exploited areas from the coast line and may entail various future health hazards.
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•Cox's Bazar paleobeach aquifers are prone to contamination from seawater intrusion.•Hydrogeochemical and isotopic tracers were used to delineate the seawater intrusion in the study area.•Shallow wells (<50 m depth) are contaminated and deep wells (>100 m depth) are safe for drinking water supplies.•Groundwater quality has deteriorated to a large extent along the paleobeach, migrating towards inland.
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