Geochemical aspects of the assimilation of host rocks by basaltic magmas during the formation of Noril’sk Cu-Ni ores

In models for the genesis of the Noril’sk Pt-Cu-Ni ore deposits, much importance is attached to the processes of assimilation of host rocks by basaltic melts. This idea is based on unusual relations between the silicate and sulfide constituents of this type of ore deposits and also on the heavy sulfur isotopic composition of the sulfide ores. The reason for this unusual composition is thought to be the assimilation of anhydrite from the host rocks. However, no other factors able to influence this process have ever been analyzed in the literature. We were the first to thoroughly analyze the inner structure of contact aureoles of the intrusions hosted in various rocks: the Maslovsky intrusion in Early Triassic basalts of the Ivakinsky and Nadezhdinsky formations and the Talnakh intrusion in Devonian anhydrite-bearing carbonate-terrigenous rocks. The distributions of trace elements, the ⁸⁷Sr/⁸⁶Sr isotopic ratio, and Sm and Nd isotopes indicate that host rocks were either not assimilated at all, or their effect is perceptible only within a very narrow (1 m) zone in the eastern apophyse in the southern portion of the Maslovsky intrusion. The Sr, Nd, and particularly, Pb isotopic composition indicate that the anhydrite could not be the source of isotopically heavy sulfur for sulfides at Noril’sk deposits. The ores of the Maslovsky and Talnakh intrusions have similar sulfur isotopic composition of their sulfides (the maximum δ³⁴S values of these sulfides reach +10.8 and +14.2‰, respectively), in spite of the significant differences in the rocks hosting these intrusions. Our newly obtained data indicate that assimilation was insignificant and could not affect the origin of the ores.     

Geochemical characteristics of the high- and low-Ti basaltic rocks from the uplifted shoulder of the Ohře (Eger) Rift,Western Bohemia

High-Ti melanephelinite (3.8–5.9 wt% TiO₂), medium-Ti (phono)tephrite (2.7–3.1 wt% TiO₂), and low-Ti olivine melanephelinite/basanite (1.9–2.3 wt.% TiO₂) are subordinate rock types in the central European Cenozoic Volcanic Province. A contrasting melanephelinite to (phono)tephrite series occurs in the Lou?ná–Oberwiesenthal Volcanic Centre (37–28 Ma) and also as satellite volcanic bodies (26–12 Ma) together with olivine melanephelinite/basanite (17–20 Ma) on the southwestern periphery of the Kru?né hory mountains (Erzgebirge). The volcanic rocks intrude the Variscan basement of the uplifted shoulder of the Oh?e/Eger Rift in the Kru?né hory mountains of the Bohemian Massif. Low Mg# (44–59) and Cr, Ni contents and enrichment of LILE, Zr, Hf, Nb, Ta, U, Th and LREE in the high-Ti melanephelinites contrast with the composition of primitive low-Ti olivine melanephelinites/basanites displaying high Mg# (63–74) and high contents of compatible elements. The high-Ti melanephelinites reveal a wide range in initial ⁸⁷Sr/⁸⁶Sr of ca. 0.7034–0.7038 and ε_Nd of 2.4–4.9. The low-Ti melanephelinites show an overlapping range of initial ⁸⁷Sr/⁸⁶Sr of ca. 0.7035–0.7036 and ε_Nd of 4.3–5.5. The large variation in initial ⁸⁷Sr/⁸⁶Sr ratios at similar ε_Nd values in those rock types is interpreted as evidence for melting of metasomatized lithospheric mantle sources comprising K-bearing phases with radiogenic Sr. Modification of the olivine-free alkali basaltic magmas by differentiation or crustal contamination could give rise to the medium-Ti (phono) tephrites. The initial isotope ratios of all samples are consistent with HIMU-mantle sources and contributions from lithospheric mantle. The olivine-free melanephelinitic rocks often contain alkali pyroxenite–ijolite xenoliths with initial ⁸⁷Sr/⁸⁶Sr ratios of ca. 0.7036 and ε_Nd of 3.0. We interpret these xenoliths as samples of an intra-crustal alkali complex derived from similar mantle sources as those for the basaltic volcanic rocks.     

Chemistry of chromium spinel in high-Mg rocks from the Morungava Intrusion,Cretaceous Paraná Igneous Province,southernmost Brazil

Mineralogy and Petrology - Mafic-ultramafic intrusions within continental flood basalt terrains are frequently associated with Cu-Ni-PGE mineralization. This study aims to constrain the...     

Isotope (δ13C and δ18O) compositions of dolomites from the Permian evaporitic sequences of the Eastern Russian Plate: Evidence from the Syukeevo gypsum deposit

Carbon and oxygen isotope compositions has been studied in dolomites of the Syukeevo gypsum deposit located in the Eastern Russian Plate. Values of δ¹³C in the dolomites vary from 0.3 to 6.6 ‰; δ¹⁸O, from 28.0 to 36.6‰. It is shown that the dolomites were formed in the epicontinental evaporitic basin in different paleoecological settings. This led to the formation of diverse lithological types of dolomites with peculiar isotope-geochemical features.     

Formation of juvenile continental crust in the Arabian–Nubian shield: evidence from granitic rocks of the Nakasib suture, NE Sudan

Major and trace element data, U–Pb zircon ages, and initial isotopic compositions of Sr, Nd, and Pb are reported for ten granitic and one rhyolitic rock sample from the neo-Proterozoic Nakasib suture in NE Sudan. Chemical data indicate that the samples are medium- to high-K, "I-type" granitic rocks that mostly plot as "volcanic arc granites" on discriminant diagrams. Geochronologic data indicate that rifting occurred 790±2?Ma and constrain the time of deformation associated with suturing of the Gebeit and Haya terranes to have ended by approximately 740?Ma. Isotopic data show a limited range, with initial ⁸⁷Sr/⁸⁶Sr=0.7021 to 0.7032 (mean=0.7025), εNd(t) =+5.5 to +7.0 (mean=+6.4), and ²⁰⁶Pb/²⁰⁴Pb = 17.50–17.62. Neodymium model ages (T_DM; 0.69–0.85?Ga; mean = 0.76?Ga) are indistinguishable from crystallization ages (0.79–0.71?Ga; mean=0.76?Ga), and the isotopic data considered together indicate derivation from homogeneously depleted mantle. The geochronologic data indicate that the terrane accretion to form the Arabian–Nubian shield began just prior to 750?Ma. The isotopic data reinforces models for the generation of large volumes of juvenile continental crust during neo-Proterozoic time, probably at intra-oceanic convergent margins. The data also indicate that crust formation was associated with two cycles of incompatible element enrichment in granitic rocks, with an earlier cycle beginning approximately 870?Ma and culminating approximately 740?Ma, and the second cycle beginning after pervasive high-degree melts – possibly hot-spot related – were emplaced approximately 690–720?Ma.     

Isotopic Composition (δ13C, δ18O) of Carbonate Concretions from Terrigenous Deposits in the Northern Caucasus

The isotopic composition of carbon and oxygen was studied in carbonate concentrations with different compositions (calcite, dolomite, and siderite) from Jurassic, Cretaceous, and Tertiary terrigenous deposits of the Northern Caucasus. Wide variations in the isotopic composition (from 41.4 to 18.1 for ¹³C and from 11.7 to 33.5 for ¹⁸O) point to different formation conditions in the early diagenesis zone and the later catagenesis zone.     

Petrology of mafic and ultramafic layered rocks from the Jaboncillo Valley,Sierra de Valle Fértil,Argentina: Implications for the evolution of magmas in the lower crust of the Famatinian arc

This work presents the field setting, petrography, mineralogy and geochemistry of a gabbroic and peridotitic layered body that is lens-shaped and surrounded by gabbronorites, diorites, and metasedimentary migmatites. This body exposed at Jaboncillo Valley is one among several examples of mafic and ultramafic layered sequences in the Sierras Valle Fértil and La Huerta, which formed as part of the lower crust of the Ordovician Famatinian magmatic arc in central-western Argentina. The layered sequence grew at deep crustal levels (20–25 km) within a mafic lower crust. The base of the layered body was detached during the tectonic uplift of the Famatinian lower crust, whereas the roof of the layered body is exposed in the eastern zone. In the inferred roof, olivine-bearing rocks vanish, cumulate textures are less frequent, and the igneous sequence becomes dominated by massive or thinly banded gabbronorites. Mainly based on the petrographic relationships, the inferred order of crystallization in the gabbroic and peridotitic layered sequence is: (1) Cr–Al-spinel + olivine, (2) Cr–Al-spinel + olivine + clinopyroxene + magnetite, (3) Cr–Al-spinel + olivine + plagioclase + magnetite ± orthopyroxene, and (4) Al-spinel + orthopyroxene + amphibole. A strong linear negative correlation between olivine and plagioclase modal proportions combined with field, petrographic and geochemical observations are used to demonstrate that the physical separation of olivine and plagioclase results in rock diversity at scales of a few centimeters to tens of meters. However, the composition of olivine (Fo ～ 0.81) and plagioclase (An > 94%) remains similar throughout the layered sequence. Spinels are restricted to olivine-bearing assemblages, and display chemical trends characteristic of spinels found in arc-related cumulates. Gabbroic and peridotitic layered rocks have trace element concentrations reflecting cumulates of early crystallizing minerals. The trace element patterns still retain the typical features of subduction-related arc magmatism, showing that the process of cumulate formation did not obscure the trace element signature of the parental magma. Using the composition of cumulus minerals and whole-rock chemical trends, we show that the parental magma was mafic (SiO₂ ～ 48 wt.%) with Mg-number around 0.6, and hydrous. The oxygen fugacity (fO₂) of the parental magma estimated between +0.8 and ?0.6 log fO₂ units around the fayalite–magnetite–quartz (FMQ) buffer is also characteristic of primitive hydrous arc magmas. The initially high water content of the parental magma allowed amphibole to crystallize as an interstitial phase all over the crystallization evolution of the layered sequence. Amphibole crystallization in the inter-cumulus assemblage gives rise to the retention of many trace elements which would otherwise be incompatible with the mineral assemblage of mafic–ultramafic cumulates. This study shows that there exist strongly mafic and primitive magmas that are both generated and emplaced within the lower crustal levels of subduction-related magmatic arc. Our findings together with previous studies suggest that the Early Ordovician magmatic paleo-arc from central-northwestern Argentina cannot be regarded as a typical Andean-type tectono-magmatic setting.     

Early Paleozoic tectonic evolution of the Chinese South Tianshan Orogen: constraints from SHRIMP zircon U–Pb geochronology and geochemistry of basaltic and dioritic rocks from Xiate,NW China

Traditionally the Chinese South Tianshan has been regarded as a late Paleozoic orogenic belt. However, little is known about the early Paleozoic tectonic architecture of the region. This paper presents the first evidence of Cambrian–Ordovician MORB-type basalts and adakitic diorites on the southern margin of the Yili plate in China. Basalts from Xiate in southwestern Tianshan show a typical transitional (T-) MORB and ferrobasalt composition, which indicate a formation at a propagating spreading ridge. The basalts give a weighted mean ²⁰⁶Pb/²³⁸U crystallization age of 516.3 ± 7.4 Ma by SHRIMP U–Pb zircon dating and have experienced contact metamorphism due to the intrusion of a dioritic pluton. The dioritic pluton has a weighted mean ²⁰⁶Pb/²³⁸U crystallization age of 470 ± 12 Ma and geochemical characteristics resembling that of adakitic rocks. The pluton is considered to have been formed by partial melting of garnet amphibolites from thickened lower crust in arc or continental collision settings. The basalts and diorites are considered to outline the eastern extension of the early Paleozoic suture zone, the Nikolaev Line, which stretches east–west for hundreds of kilometers between the Northern Tianshan and Central Tianshan terranes of Kyrgyzstan. Our findings substantiate that the Yili and Central Tianshan plates were separated by the early Paleozoic Terskey ocean. The Terskey ocean probably closed during the early stage of the late Ordovician (Lomize et al. in Geotectonics 31(6):463–482, 1997), resulting in the final amalgamation of the Yili and Central Tianshan plates. Consequently, an early Paleozoic suture zone is documented in the Chinese Tianshan region, which is most likely represented by the North Nalati fault.     

Sr–Nd–Hf–Pb isotope geochemistry of basaltic rocks from the Cretaceous Gyeongsang Basin,South Korea: Implications for basin formation

To better understand the formative mechanism of the Cretaceous Gyeongsang Basin in South Korea, we determined the geochemical compositions of Early Cretaceous syntectonic basaltic rocks intercalated with basin sedimentary assemblages. Two distinct compositional groups appeared: tholeiitic to calc-alkaline basalts from the Yeongyang sub-basin and high-K to shoshonitic basaltic trachyandesites from the Jinju and Uiseong sub-basins. All collected samples exhibit patterns of light rare earth element enrichment and chondrite-normalized (La/Yb)_N ratios ranging from 2.4 to 23.6. In a primitive-mantle-normalized spidergram, the samples show distinctive negative anomalies in Nb, Ta, and Ti and a positive anomaly in Pb. The basalts exhibit no or a weak positive U anomaly in a spidergram, but the basaltic trachyandesites show a negative U anomaly. The basalts have highly radiogenic Sr [(⁸⁷Sr/⁸⁶Sr)_i = 0.70722–0.71145], slightly negative ε_Nd, positive ε_Hf [(ε_Nd)_i = −2.7 to 0.0; (ε_Hf)_i = +2.9 to +6.4], and radiogenic Pb isotopic compositions [(²⁰⁶Pb/²⁰⁴Pb)_i = 18.20–19.19; (²⁰⁷Pb/²⁰⁴Pb)_i = 15.60–15.77; (²⁰⁸Pb/²⁰⁴Pb)_i = 38.38–39.11]. The basaltic trachyandesites are characterized by radiogenic Sr [(⁸⁷Sr/⁸⁶Sr)_i = 0.70576–0.71119] and unradiogenic Nd, Hf, and Pb isotopic compositions [(ε_Nd)_i = −14.0 to −1.4; (ε_Hf)_i = −17.9 to +3.7; (²⁰⁶Pb/²⁰⁴Pb)_i = 17.83–18.25; (²⁰⁷Pb/²⁰⁴Pb)_i = 15.57–15.63; (²⁰⁸Pb/²⁰⁴Pb)_i = 38.20–38.70]. The “crust-like” signatures, such as negative Nb–Ta anomalies, elevated Sr isotopic compositions, and negative ε_Nd(t) and ε_Hf(t) values, of the basaltic trachyandesites resemble the geochemistry of Early Cretaceous mafic volcanic rocks from the southern portion of the eastern North China Craton. Considering the lower-crust-like low U/Pb and high Th/U ratios and the unradiogenic Pb isotopic compositions, the basaltic trachyandesites are considered to be derived from lithospheric mantle modified by interaction with melts that originated from foundered eclogite. Basaltic volcanism in the Yeongyang sub-basin is coeval with the basaltic trachyandesite magmatism, but it exhibits an elevated ⁸⁷Sr/⁸⁶Sr ratio at a given ¹⁴³Nd/¹⁴⁴Nd and highly radiogenic Pb isotopic compositions, which imply an origin from an enriched but heterogeneous lithospheric mantle source. Melts from subducted altered oceanic basalt and pelagic sediments are considered to be the most likely source for the metasomatism. An extensional tectonic regime induced by highly oblique subduction of the Izanagi Plate beneath the eastern Asian margin during the Early Cretaceous might have triggered the opening of the Gyeongsang Basin. Lithospheric thinning and the resultant thermal effect of asthenospheric upwelling could have caused melting of the metasomatized lithospheric mantle, producing the Early Cretaceous basaltic volcanism in the Gyeongsang Basin.     

The Origin of Paleokarst in the Huanglong Formation of the Eastern Sichuan Basin: Evidence from δ13C, δ18O and 87Sr/86Sr

Karst rocks from the Huanglong Formation exposed at the margin of the Eastern Sichuan Basin can be divided into four types:slightly corroded, moderately corroded porous, intensely corroded brecciated and intensely corroded and replaced secondary calcic karstic rocks. The carbon, oxygen and strontium isotope compositions of the various karst rocks are analyzed systematically and compared to rocks without karst corrosion. The results indicate that(1) the Huanglong Formation in the eastern Sichuan Basin was a restricted bay supplied and controlled by freshwater in which mudmicrite and mud-dolomicrite exhibit low δ13C and δ18O values and high 87Sr/86 Sr ratios;(2) all types of karstic rocks in the paleokarst reservoirs of the Huanglong Formation in the research area are affected by atmospheric freshwater with the δ13C and δ18O values and 87Sr/86 Sr ratios in the original formation approaching those of atmospheric freshwater, which reflects ancient hydrological conditions, fluid properties, isotopic source and the fractionation effect;(3) the intensely corroded and replaced secondary limestone is affected by a variety of diagenetic fluids, often reflected by δ13C and δ18O values, while the 87Sr/86 Sr ratios exhibit the strong degree of the corrosion;(4) after comparing the 87Sr/86 Sr ratios of each type of karst rock, the diagenetic fluids are determined to be mainly atmospheric freshwater, and depending on the strength of corrosion, and the low 87Sr/86 Sr ratio fluids in the layer will participate in the karst process. The carbon, oxygen, and strontium isotopes of different karstic reservoirs can provide meaningful geochemical information for forecasting and evaluating the development and distribution rules of the Huanglong Formation at the margin of the eastern Sichuan Basin in time and space.     

Formation process of mid-Neoproterozoic mafic rocks from the western Jiangnan Orogen,South China: insights from SHRIMP U–Pb dating and geochemical analysis

The mid-Neoproterozoic tectonic setting of the Jiangnan Orogen (JO) is uncertain due to the ongoing debate regarding the history of interactions between the Yangtze and Cathaysia Blocks. Extensive magmatic rocks with ages >830 Ma are observed in the eastern JO and are reported to indicate their formation conditions; however, such magmatic rocks are rare in the western JO. This paper presents data from samples collected from two ultramafic intrusions in northern Guangxi province that yield SHRIMP U–Pb ages of 848 ± 7 and 836 ± 5 Ma. These two intrusions have similar geochemical compositions; are enriched in LILE (Rb, Ba, Th, and Pb) relative to HFSE (Hf, Zr, Nb, Ta, and Y), reflecting an arc-like signature; and are derived from the same source. The intrusions have positive εNd(t) and εHf(t) values, implying a depleted mantle source. Comparing the Neoproterozoic mafic–ultramafic rocks of the JO, differences exist between the rocks that formed at 860–830, 830–800, and 800–740 Ma in terms of their mantle sources and formation conditions. Considering the geochemical composition and ages of formation of the strata and the deep structure of the western JO, we speculate that the western JO was a back-arc foreland basin, which experienced shallow subduction-related and arc-like magmatism during the period between 860 and 830 million years. In contrast to the western JO, the eastern JO may have been a back-arc basin with oceanic crustal basement during that time. The JO formed between 830 and 800 million years in association with subduction-related collisional magmatism due to the amalgamation of the Yangtze and Cathaysia Blocks. Subsequently, magmatism occurred in the JO that resulted from the post-orogenic extension.     

Petrogenesis of low-δ<Superscript>18</Superscript>O quartz porphyry dykes,Koegel Fontein complex,South Africa

This paper investigates the origin of low-δ¹⁸O quartz porphyry dykes associated with the 144–133 Ma Koegel Fontein Igneous Complex, which was intruded during the initial phase of breakup of Africa and South America. The 25-km diameter Rietpoort Granite is the largest and youngest phase of activity, and is roofed by a 10-km diameter pendant of gneiss. Quartz porphyry (QP) dykes, up to 15 m in width, strike NW–SE across the complex. The QP dykes that intruded outside the granite have similar quartz phenocryst δ¹⁸O values (average 8.0‰, ± 0.7, n?=?33) to the granite (average 8.3?±?1.0, n?=?7). The QP dykes that intruded the roof pendant have quartz phenocrysts with more variable δ¹⁸O values (average 1.6‰, ± 2.1, n?=?55). In some cases quartz phenocrysts have δ¹⁸O values as low as ? 2.5‰. The variation in δ¹⁸O value within the quartz crystal population of individual dykes is small relative to the overall range, and core and rim material from individual quartz phenocrysts in three samples are identical within error. There is no evidence that quartz phenocryst δ¹⁸O values have been affected by fluid–rock interaction. Based on a ?_quartz?magma value of 0.6‰, magma δ¹⁸O values must have been as low as ? 3.1‰. Samples collected along the length of the two main QP dykes that traverse the roof pendant have quartz phenocryst δ¹⁸O values that range from +?1.1 to +?4.6‰, and ? 2.3 to +?5.6‰, respectively. These δ¹⁸O values correlate negatively (r = ? 0.96) with initial ⁸⁷Sr/⁸⁶Sr, which can be explained by the event that lowered δ¹⁸O values of the source being older than the dykes. We suggest that the QP dykes were fed by magma produced by partial melting of gneiss, which had been variably altered at high temperature by ¹⁸O-depleted meteoric water during global glaciation at ~?550 Ma. The early melts had variable δ¹⁸O value but as melt pockets interconnected during melting, the δ¹⁸O values approached that of average gneiss. Variable quartz phenocryst δ¹⁸O values in the same dyke can be explained by vertical emplacement, at variable rates of ascent along the dyke. The lateral variation in quartz, and hence magma δ¹⁸O value at a particular point along a single dyke would depend on the rate of ascent of magma at that point along the dyke, and the ‘age’ of the particular magma batch.     

Lake carbonate-δ18O records from the Yukon Territory,Canada: Little Ice Age moisture variability and patterns

A 1000-yr history of climate change in the central Yukon Territory, Canada, is inferred from sediment composition and isotope geochemistry from small, groundwater fed, Seven Mile Lake. Recent observations of lake-water δ¹⁸O, lake level, river discharge, and climate variations, suggest that changes in regional effective moisture (precipitation minus evaporation) are reflected by the lake’s hydrologic balance. The observations indicate that the lake is currently ¹⁸O-enriched by summer evaporation and that during years of increased precipitation, when groundwater inflow rates to the lake increase, lake-water δ¹⁸O values decrease. Past lake-water δ¹⁸O values are inferred from oxygen isotope ratios of fine-grained sedimentary endogenic carbonate. Variations in carbonate δ¹⁸O, supplemented by those in carbonate and organic δ¹³C, C/N ratios, and organic carbon, carbonate and biogenic silica accumulation rates, document changes in effective moisture at decadal time scales during the early Little Ice Age period to present. Results indicate that between ～AD 1000 and 1600, effective moisture was higher than today. A shift to more arid climate conditions occurred after ～AD 1650. The 19th and 20th centuries have been the driest of the past millennium. Temporal variations correspond with inferred shifts in summer evaporation from Marcella Lake δ¹⁸O, a similarly small, stratified, alkaline lake located ～250 km to the southwest, suggesting that the combined reconstructions accurately document the regional paleoclimate of the east-central interior. Comparison with regional glacial activity suggests differing regional moisture patterns during early and late Little Ice Age advances.     

Isotopic Composition (δ13C, δ18O) and Genesis of Carbonates from Phosphorite Deposits in the Lesser Karatau (Kazakhstan)

Lithology and Mineral Resources - The study of structural features along with carbon and oxygen isotope compositions made it possible to discriminate lithological rocks in deposits of the Karatau...     

Recycling of sediment into the mantle source of K-rich mafic rocks: Sr–Nd–Hf–O isotopic evidence from the Fushui complex in the Qinling orogen

Potassium (K)-rich mafic rocks are viewed as being derived from partial melting of an enriched mantle source, but it is controversial about which processes cause the mantle enrichment. The Fushui intrusive complex is the largest early Paleozoic K-rich intrusive complex in the eastern Qinling orogen. Therefore, detailed studies on the Fushui complex can contribute not only to understanding of the petrogenesis of K-rich mafic rocks, but also to unraveling the Paleozoic evolution of the Qinling orogen. In this study, we provide an integrated investigation of in situ zircon U–Pb dating and Hf–O isotopes, in combination with whole-rock major and trace elements, as well as Sr–Nd–Hf isotopes, for the Fushui intrusive complex. In situ zircon secondary ion mass spectrometry (SIMS) / laser ablation induction coupled plasma mass spectrometry (LA-ICPMS) U–Pb dating reveals that different rock types of the Fushui complex have identical formation ages of 488–484 Ma. The Fushui complex belongs to the shoshonitic series, and is characterized by extreme large ion lithophile element (LILE, e.g., Ba, U, Th and Sr) and Pb enrichment and depletion of high field-strength elements (HFSEs, e.g. Nb, Ta, Zr, Hf, P and Ti). It shows high initial Sr isotopic ratios (0.7100–0.7151), negative whole-rock ε _Nd(t) (?3.97 to ?5.68) and negative to slight positive whole-rock (?2.24 to 2.38) and zircon (?2.85 to 0.34) ε _Hf(t) values, as well as high zircon δ¹⁸O values (6.86 ± 0.13 ‰). The Hf–Nd isotopic systems are decoupled with positive Δε _Hf values (3.85–5.37). These geochemical features indicate that the mantle source has incorporated subducted zircon–barren oceanic sediments. A simple two-end-members mixing model constrains the amount of subducted sediments in the Fushui mantle source to 5–8 %. The Fushui complex originated from 1 to 6 % equilibrium melting of a phlogopite-bearing garnet lherzolite by non-modal melting. As shoshonitic magmas have been discovered in modern nascent arcs, we suggest that the generation of the Fushui complex was induced by the subduction of the Paleotethyan Ocean, when it jumped from the northern to the southern boundary of the North Qinling microcontinent.     

Freshwater Sources in the Bays of Yuzhnyi Island of the Novaya Zemlya Archipelago According to Isotopic (δD, δ18O) Data

Doklady Earth Sciences - An isotope–geochemical study was performed on the waters of Abrosimov and Stepovoi bays along the southeastern coasts of the Novaya Zemlya Archipelago. The waters...     

Paleoproterozoic crustal evolution of the Tarim Craton: Constrained by zircon U–Pb and Hf isotopes of meta-igneous rocks from Korla and Dunhuang

The Tarim Craton is one of three large cratons in China. Presently, there is only scant information concerning its crustal evolutionary history because most of the existing geochronological studies have lacked a combined isotopic analysis, especially an in situ Lu–Hf isotope analysis of zircon. In this study, Precambrian basement rocks from the Kuluketage and Dunhuang Blocks in the northeastern portion of the Tarim Craton have been analyzed for combined in situ laser ablation ICP-(MC)-MS zircon U–Pb and Lu–Hf isotopic analyses, as well as whole rock elements, to constrain their protoliths, forming ages and magma sources. Two magmatic events from the Kuluketage Block at ∼2.4 Ga and ∼1.85 Ga are revealed, and three stages of magmatic events are detected in the Dunhuang Block, i.e., ∼2.0 Ga, ∼1.85 Ga and ∼1.75 Ga. The ∼1.85 Ga magmatic rocks from both areas were derived from an isotopically similar crustal source under the same tectonic settings, suggesting that the Kuluketage and Dunhuang Blocks are part of the uniform Precambrian basement of the Tarim Craton. Zircon Hf model ages of the ∼2.4 Ga magmatism indicate that the crust of the Tarim Craton may have been formed as early as the Paleoarchean period. The ∼2.0 Ga mafic rock from the Dunhuang Block was formed in an active continental margin setting, representing an important crustal growth event of the Tarim Craton in the mid-Paleoproterozoic that coincides with the global episode of crust formation during the assembly of the Columbia supercontinent. The ∼1.85 Ga event in the Kuluketage and Dunhuang Blocks primarily involved the reworking of the old crust and most likely related to the collisional event associated with the assembly of the Columbia supercontinent, while the ∼1.75 Ga magmatism in the Dunhuang Block resulted from a mixture of the reworked Archean crust with juvenile magmas and was most likely related to a post-collisional episode.