Pb Isotope Data Research Articles

Widespread Cadomian–Pan-African Ediacaran magmatism across the Moroccan Meseta: Implication for the geodynamic evolution of the NW Gondwana margin

The Moroccan Meseta constitutes a key peri-Gondwanan and Variscan terranes linking the branches between the SW European Cadomian and the north West African Craton (WAC), anti-Atlas metacratonic Pan-African terranes. While the Paleozoic sedimentary and tectono-magmatic record of the different blocks of the Moroccan Meseta is well documented, there is still a lack of systematic absolute ages and geochemical isotopic data of putative Precambrian basement rocks from different Meseta blocks. Here we present new U–Pb zircon ages, petrological, and whole-rock major, trace, and Sr–Nd–Pb isotope geochemical data of (meta)igneous rocks from the Bou-Acila, Jbel-Hadid, Goaïda, Midelt and El Jadida, constituting the basement of two main Moroccan Meseta blocks. Basaltic rocks, unconformably overlain by Cambrian platform sediments, in the Western Meseta Bou-Acila basement yield U–Pb zircon age of c. 581 ± 9 Ma that unveils the existence of Ediacaran magmatism in the Moroccan basement of this area. Alleged early Paleozoic mafic sills in the metasedimentary series of the Midelt Sidi-Saïd yield U–Pb zircon ages of c. 554 ± 12 Ma reveals, for the first time, the presence of Ediacaran magmatism in the basement of the Moroccan Eastern Meseta. U–Pb zircon ages in the Jbel-Hadid and El Jadida rhyolitic basement rocks (c. 604–543 Ma and c. 575–559 Ma, respectively), as well as granitic pebbles (c. 573 Ma) in the Goaïda Cambrian limestone series, further attest for coeval Ediacaran magmatism in the basement of Western Meseta. The geochemical affinity of the investigated Moroccan Meseta Ediacaran magmatic rocks is mainly metaluminous, low-K tholeiites for mafic rocks, and peraluminous, high-K calc-alkaline for the felsic rocks, consistent with a back-arc tectonic setting. Geochemical and Sr–Nd–Pb isotopic data indicate that the source of the Moroccan Meseta Ediacaran magmatism involved sources from the depleted mantle and old WAC crustal basement. Altogether, these data show widespread Ediacaran magmatism in the Moroccan Meseta mostly likely formed in a back-arc south of the Cadomian active margin along the NW Gondwana during the waning stages of the Cadomian–Pan-African orogeny, recording a transitional stage between the Ediacaran subduction and early Paleozoic rifting.

High-precision Pb isotopes of drinking water lead pipes: Implications for human exposure to industrial Pb in the United States

Millions of lead (Pb) pipes are still used in the drinking water distribution systems in many regions in the world. Human exposure to Pb from contaminated drinking water continues to be of concern in the United States (U.S.), as illustrated by the widely publicized “Flint Water Crisis” in 2015. The Pb isotopic composition of Pb-pipes potentially can be useful to identify human exposure to Pb from lead service lines (LSLs). In addition, as the LSLs were likely manufactured from similar industrial Pb sources as other Pb objects and materials in the USA, the Pb-pipes isotope data can provide information about the overall isotopic composition of the U.S. industrial Pb.In this work we present high-precision Pb isotope data from Pb-pipes excavated from different U.S. municipalities. The Pb-pipes show an extremely wide range of Pb isotopic compositions, with 206Pb/204Pb ranging from 17.004 to 22.010, 207Pb/204Pb from 15.460 to 15.921, and 208Pb/204Pb from 36.687 to 41.120. The wide isotope range is observed even in a single town, suggesting that no regional Pb isotope patterns can be expected within the continental USA. However, the high-precision MC-ICP-MS Pb data form a clear linear trend that, depending on the context, can be used to identify human Pb exposure. Furthermore, as the linear trend is a result of utilization of Pb ores from different domestic and international sources and secondary recycling of metallic Pb, it is likely representative of the overall isotopic composition of the U.S. industrial Pb pool. Therefore, the identified trend is the most accurate isotope representation of the U.S. anthropogenic Pb at present and can be used as first-order evaluation to determine if a person with elevated blood Pb levels was exposed to U.S. industrial Pb sources.

U–Pb ages of tuff from the Triassic Yanchang Formation in the Ordos Basin: Constraints on palaeoclimate and tectonics

The Chang 7 member of Triassic Yanchang Formation in the huger Ordos Basin in North China is important economically as a source of oil in the Triassic, but dating the Triassic successions has proved problematic, especially important oil-bearing units such as the Yanchang Formation. Here we analyze depositional ages, palaeoclimate conditions and palaeoenvironments of the Chang 6 to Chang 10 members of Yanchang Formation using SHRIMP U–Pb isotope dating, trace element composition and palynological analyses for samples from well F75 in the northwestern Ordos Basin. The geochronological and palynological data synthetically indicate that the Chang 7 member was deposited in the Ladinian (Middle Triassic) and the Chang 8 to Chang 10 members were deposited in the Anisian (Middle Triassic). Based on the trace element analysis, the palaeoenvironment abruptly changed in the early sedimentary stage of the Chang 7 member, representing warm and humid palaeoclimate, anoxic, brackish lake water and stronger chemical weathering. Therefore a humid paleoclimate event was recorded during the Chang 7 sedimentary period of the Middle Triassic, which provides a basis for the comparison with the global Triassic paleoclimatic event. It was inferred that the paleoclimate event was referred to palaeotopographic (the basin was surrounded by mountains on three sides and low terrain in the east) of the Ordos Basin, which was controlled by the Qinling orogeny and low-latitude warm and humid airflow of the paleo-Pacific Ocean associated with monsoons.

Caribbean–South America interactions since the Late Cretaceous: Insights from zircon U–Pb and Lu–Hf isotopic data in sedimentary sequences of the northwestern Andes

The Northern Andes in Colombia developed along an active, convergent margin with a complex subduction-accretion/collision history since Triassic times. Sedimentary basins that developed within, or close to, the evolving margin provide an important record for the tectonic history. Here we present U–Pb and Hf isotopic data of detrital zircons from Late Cretaceous and Miocene sedimentary strata. Two sedimentary sequences deposited in an active margin setting are analyzed in this study along two roughly E–W trending transects. Targeted sedimentary sequences are the Late Cretaceous-Paleocene siliciclastic member of the Penderisco Formation outcropping in the Western Cordillera and the hemipelagic Miocene Beibaviejo Formation overlying the suture zone between the Western Cordillera and the Panama-Chocó Block (PCB). Provenance and Hf isotopic signatures for sediments and volcaniclastic sandstones associated with the Penderisco Formation indicate a maximum depositional age of ca. 57 Ma with input of detrital material from mixed sources from two major domains: an eastern ancient continental domain, and a western Cretaceous oceanic domain that defines the present Western Cordillera. The record from continental sources suggests a para-autochthonous to autochthonous syn-tectonic history. For the Beibaviejo Formation provenance and Hf isotopic data suggest the Penderisco Formation and Eocene rocks with oceanic affinity within the PCB as the main sources and define a middle Miocene maximum depositional age (∼16 Ma). Our results provide a new insight into the evolution of volcaniclastic sequences at the northwestern margin of South America and provide a new constraint on the timing of collision between the PCB and the Andean margin during the middle Miocene.

Spatial‐Temporal Variability and Sources of Lead (Pb) in the Indian Ocean and Asian Marginal Seas

AbstractSince North America and European countries phased out leaded gasoline, Asia has become the major contemporary lead (Pb) source to the marine environment, at first from leaded gasoline, but more recently from coal burning and other high‐temperature industrial activities (Flegal et al., 2013, https://doi.org/10.1080/10643389.2012.671738; Lee et al., 2014, https://doi.org/10.1016/j.epsl.2014.04.030). Pb in the Indian Ocean remains relatively under‐evaluated after ∼2000 and is further complicated by various oceanic processes (e.g., monsoons, boundary exchange with particulates). Here, we present three annually resolved coral skeletal Pb isotope and concentration records from the central and eastern Indian Ocean (Salomon Atoll, 1989–2009; Diego Garcia Atoll, 1999–2009; and Phuket Island, 1945–2010), and synthesize published coral/sedimentary records to reconstruct the spatial‐temporal variability of anthropogenic Pb around the region. Pb isotopes in all corals coherently fall along the mixing line between Asian aerosols and the natural crust. However, higher contributions of natural Pb are found in corals located in the coastal region than those in the open ocean, despite the greater contributions of anthropogenic Pb expected in coastal water near human emission sources. This geographical difference suggests that exchanges between dissolved Pb and natural particles at ocean boundaries significantly contribute to the Pb isotope compositions in regional seawater and are subsequently recorded in corals. The temporal variability of Pb concentrations in corals and sedimentary records signifies decreasing trends in Southeast Asia but increasing trends in South Asia. This study contributes new and timely Pb and Pb isotope data for the Indian Ocean and illustrates the importance of boundary exchange in marine Pb cycling.

New insights into the genesis of the Sipingshan gold deposit in eastern Heilongjiang, north-eastern China: Evidence from geology, U-Pb and Rb-Sr geochronology, geochemistry, and H[sbnd]O[sbnd]S-Pb isotopes

The Sipingshan gold deposit is intermediately sized and located in the eastern Wandashan Terrane. It has attracted much attention for its uncommon genetic type of hot spring. Gold orebodies mainly occur in the upper and lower siliceous rocks, siliceous cemented breccia is found between the upper and lower siliceous rocks, and silicified rhyolite porphyry is observed below the lower siliceous rocks. Two main mineralisation periods (i.e. exhalative sedimentation and hydrothermal superimposition) have been identified at Sipingshan based on mineral paragenesis and crosscutting vein relationships. In this study, analyses of whole-rock geochemical compositions, pyrite Rb–Sr age, zircon U–Pb–Hf contents, and S–PbOH isotopes were conducted to determine the Au mineralisation time and ore genesis of this deposit. The ore-bearing siliceous rock revealed weighted mean 206Pb/238U ages of 121.8 ± 1.1 Ma and 117.6 ± 1.0 Ma, representing the mineralisation age of the exhalative sedimentation period. The Rb–Sr isochron age of pyrites (106 ± 3 Ma) was consistent with the Au–Cu mineralisation age in the Wandashan area and represented magmatic–hydrothermal mineralisation. The siliceous rocks were characterised by high SiO2 (97.92–99.18 wt%), low TiO2 (0.01–0.03 wt%), low Al2O3 (0.22–0.55 wt%), negative δCe (0.78–1.00), positive δEu (1.12–4.04), and low Sb, As, Cs, Cd and Li contents, indicating the genesis of hydrothermal sediments. The OH isotope results suggested that the ore-forming fluids were mainly derived from magmatic water and mixed with meteoric water in the later stage. The δ34S values of pyrites from the siliceous rocks (–25.8 to –8‰) indicated a biogenic sulfur-dominated source, corresponding to the exhalative sedimentation period. The δ34S values of pyrites (9.8 to 29.7 ‰ and –1.9 to –4.7 ‰) formed in the magmatic–hydrothermal period indicated that sulfur was most likely derived from sedimentary rock and magmatic sulfur sources. The Pb isotope data also implied a mixture of upper crust and mantle sources. Therefore, we propose that the Sipingshan gold deposit is a continental exhalative sedimentation-type gold deposit superimposed by later magmatic–hydrothermal fluids, and formed during the subduction of the Paleo-Pacific Plate during the Early Cretaceous.

Archean Pb isotope variability tracks crust-mantle fractionation, granite production, and ore deposit formation

Various geological processes that affect Earth's crust may be encoded into isotopic tracers preserved in rocks and minerals. The enhanced sensitivity of U, Th, and Pb to crustal fractionation processes allows Pb isotopes to complement information from the Nd and Hf isotope systems. However, melt fractionation, crustal contamination and recycling, hydrothermal fluid flow and fluid-rock interaction, and other processes in the continental crust can lead to mixing of Pb isotopic signatures. Here, we report new Pb isotopic data from granite-K-feldspar and integrate these data with published Pb isotope ratios from granite K-feldspar and Pb-rich ores across the Yilgarn Craton in Western Australia. The aim of this study is to explore how the variability of Pb isotope ratios and derivative parameters can be used to gain information on specific geological processes occurring throughout the crustal column. We develop a model that subdivides different sampling media into chemical process groups and links their initial Pb signatures to Pb source regions and fractionation processes at various locations within the crust and upper mantle. Equilibration of Pb signatures with a primary mantle source reservoir (in part represented by volcanic-hosted massive sulfides) is contrasted with granite formation in the mid to lower crust (granite K-feldspar), and mineralization of ore deposits in the mid to upper crust (Pb-rich ores). Spatial trends similar to those in Nd and Hf isotopic data are recorded by Pb isotopic derivative parameters (μ = source 238U/204Pb, ω = 232Th/204Pb, and Δt - the difference between true sample age and Pb model age) calculated for komatiite-hosted Ni sulfide ores, granite K-feldspar, and volcanic-hosted massive sulfide (VHMS) ores. The significance of subtle differences in absolute values of derivative parameters is supported by the diversity of Pb isotope ratios, Pb model ages, and ∆t as tracked by a statistical metric, quantifying the variability of Pb sources involved in the formation of different chemical process groups. Generally greater variety in an older terrane (Youanmi) documents more ancient and recycled continental crust as compared with more homogeneous Pb isotopic signatures in a younger terrane (Eastern Goldfields Superterrane). The Pb signatures are interpreted, in part, to relate to the timing of source fractionation in the upper mantle with a legacy of this source signal preserved through various depths in the lithospheric section. The least radiogenic VHMS ore samples appear to provide a good approximation of mantle Pb signatures, indicated for example by a deficit in 206Pb and 208Pb relative to the other process groups. A significant heterogeneity recorded in Pb isotopic data from Pb-rich gold ores is explained by the interplay of hydrothermal fluids with diverse sources leading to the mineralization of gold deposits (e.g., leaching of Pb from surrounding rocks or fluid mixing). Such gold ore Pb signatures are distinct from other process groups, which together track sources less heterogeneous in age and/or U and Th.

A PCA-AHC Approach to Provenance Studies of Non-Ferrous Metals with Combined Pb Isotope and Chemistry Data

This paper discusses the applicability of the Principal Component Analysis-Agglomerative Hierarchical Clustering (PCA-AHC) approach to provenance studies of non-ferrous metals using combined Pb isotope and chemistry data. Pb isotopic ratios were converted to the natural abundance of individual isotopes and then to weight units. Next, all relevant variables (Pb isotopes and trace elements) were processed with PCA and AHC to examine the relationships between observations. The method is first verified on three literature-based case studies (1, 2, and 3). It is argued that, as is the case in archaeological iron provenance studies, the PCA-AHC method is also viable for non-ferrous metals. This method can greatly facilitate research, compared to conventional biplots with ratios of Pb isotopes and trace elements. Additionally, PCA-AHC can become part of the initial deposit selection process, and it can help clarify less obvious classification cases. The main problem with a practical application of this approach is insufficient deposit datasets with complete Pb isotopic and chemistry data. In such cases, it is possible to use the PCA-AHC method separately on Pb isotopic and chemistry data and then to compare and contrast results. Alternatively, the proposed approach can be used solely with Pb isotopic data. This application is shown in two additional case studies (4 and 5), which demonstrate the method’s application for tracing artefacts to their parent ores using datasets with a few thousand observations.

Oligo-Miocene subduction-related volcanism of the Loyalty and Three Kings ridges, SW Pacific: a precursor to the Tonga-Kermadec arc

The SW Pacific region contains several ridges and basins that are inferred to represent pre-Quaternary volcanic arcs and back-arc basins. The geology of these features is less well characterized than that of the active Tonga-Kermadec and Vanuatu arcs. We report new major and trace element, and Pb, Hf, Sr and Nd isotope data for 27 lavas dredged from the Loyalty and Three Kings ridges during the 2015 VESPA cruise of R/V l'Atalante. Low-K basalts were dredged from the seabed deeper than 3300 m, and high-K to shoshonitic suites from shallower ridge crests at 2000–3300 m. The samples are mainly basalts, with lesser trachybasalts, basaltic andesites, trachyandesites andesites, dacites, and one granite (anhydrous SiO2 and K2O + Na2O range from ∼47 to 64 and 1.5 to 11 wt% respectively). Trace element patterns allow discrimination of three geochemical signatures, identified as i) depleted, ii) transitional and iii) enriched, based on their light to heavy rare earth element (REE) ratios (with La/Sm ranging from 0.4 to 8). Depleted and transitional samples are basalts, featuring REE concentrations similar to MORB, but with high field strength element and large ion lithophile element contents, typical of back-arc basin basalts. The most enriched samples are basaltic andesites, andesites, trachyandesites and trachytes with island arc magma trace element signatures. Pb isotope ranges are limited (208Pb/204Pb ∼38 to 39.8, 207Pb/204Pb ∼15.51 to 15.64 and 206Pb/∼17.9 to 20.1), while Hf isotopes display more diverse compositions (εHf ranging from +7.7 to +14). Both Nd (εNd = 2.8–9.3) and Sr (87Sr/86Sr = 0.7026–0.7048) isotopes are correlated with Hf data. Trace element and isotopic compositions can be explained in terms of mixing between three distinct geochemical endmembers in the mantle resembling DMM, HIMU and EM-2 sources. Our study confirms voluminous subduction-related magmatism on the Loyalty and Three Kings ridges, mostly of Late Oligocene – Early Miocene age. The issue of polarity of subduction to generate these rocks remains open, but the composition-space-time distribution of the igneous rocks can be explained in the context of SW Pacific geodynamics using a west-dipping Pacific slab model.

Geochronology, geochemistry, and genesis of the Shamai tungsten deposit, Inner Mongolia, NE China

AbstractThe Shamai tungsten deposit is located in the western part of the Xing'an Mongolian orogenic belt of northeastern China and contains quartz‐vein hosted tungsten mineralization. In this paper, we present the results of zircon laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U–Pb dating of the fine‐grained biotite monzogranite, medium‐grained biotite monzogranite, and porphyritic biotite monzogranite samples from the deposit as well as in situ zircon Hf isotopic and in situ plagioclase Pb isotopic analyses. Combining new laser Raman spectroscopic data for fluid inclusions with previously HO isotopic data yields insights into the precipitation of ore minerals. The fine‐grained biotite monzogranite, medium‐grained biotite monzogranite, and porphyritic biotite monzogranite were emplaced at 142.5 ± 1.0 Ma, 141.9 ± 1.1 Ma, and 140.2 ± 1.0 Ma, respectively. The fine‐ to medium‐grained biotite monzogranite and porphyritic biotite monzogranite samples contain zircons with εHf(t) values from 3.2 to 7.9 and from 4.2 to 7.6, respectively, yielding tDM2 model ages from 996 to 692 Ma and 923 to 708 Ma, respectively. The geochemical, zircon Hf isotopic, and in situ plagioclase Pb isotopic data indicate that the magmas were generated by the partial melting of juvenile crustal material mixed with mantle‐derived components. The laser Raman and HO isotopic data for fluid inclusions within tungsten‐bearing quartz veins indicate that the tungsten mineralization formed as a result of fluid mixing. Combining the data with the spatial and temporal distribution and tectonic setting of tungsten deposits in NE China suggests that this area records four stages of mineralization at 300–290, 250–230, 200–180, and 150–130 Ma. The Shamai deposit occurred in an intra‐continental extensional regime.

Metal of the "Cimmerian elite" of the North Caucasus ( on the materials of tomb 3 of the Terese burial ground)

The article publishes the results of a study of bronze items from tomb 3 of the Terese burial ground (Central North Caucasus, Karachay-Cherkessia), one of the richest burials of the Koban culture of the Central North Caucasus during the Cimmerian campaigns in Transcaucasia and Western Asia.A typological analysis of the bridle accessories of a number of imported metal items from this complex shows that the burial belongs to a war-horseman of the late 8th centuryBC, who participated in military campaigns in Transcaucasia. ED XRF analysis reveals the main alloy groups used for production of the bridle and harness attachments from the Terese tomb 3 (26 samples) and Fars burial ground (2 samples). Tin bronzes and lead-tin bronzes with low, middle, and high tin concentration and a relatively low lead content clearly dominated the Terese selection. Lost wax casting into investment clay moulds was the main manufacturing technique. The Pb-isotope data of 23 samples obtained by MC-ICP-MS reveals the different sources of copper and lead as well as the recycling of scrap metal. Polymetallic ores of the Sadonsky ore field, located in North Ossetia was considered as one of the possible ore sources. Within the framework of the studied samples, arsenic bronze was recorded only as part of the inlay of two pins, originating from the Central Georgia which were probably made to order by a highly qualified Transcaucasian jeweller.

Genesis of the Qilinchang Pb–Zn deposit, southwestern China: Evidence from mineralogy, trace elements systematics and S–Pb isotopic characteristics of sphalerite

The Qilinchang (Pb–Zn)–Ag-Ge deposit of the Yangtze Platform, southwest China are hosted in the Early Carboniferous carbonate rocks of the Baizuo Formation. The deposit is located within the Sichuan-Yunnan-Guizhou (SYG) metallogenic province, which is western bounded by the Yangtze Platform in the southwestern part of China. Ore minerals are sphalerite, galena, pyrite, chalcopyrite, and marmatite, whereas gangue minerals are dolomite, calcite, and quartz. This study highlights the results of sphalerite chemistry analyzed by electron microprobe analyzer (EPMA), the laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and isotopic analysis. These techniques were used to investigate the major and trace concentrations in sphalerite from the Qilinchang deposit, in an attempt to provide noteworthy discernments into the ore-forming temperature and metal sources of the deposit. The calculated FeS (mole %) results obtained from Fe (wt. %) with the assumption that the sphalerite is stoichiometric in composition range from 0.02 to 13.48 mol % and mode at 3 mol %. This low FeS content falls within the bornite + pyrite + chalcopyrite field on the Fe–Zn–S system suggesting an increase in sulfur activity. The application of Ga/Ge ratios in sphalerite against the formation temperature was used to determine the homogenization temperature of this deposit. This expression is based on the existing Al/Si geothermometers as (Al/Si)total atomic ratio as a function of temperature calculated using available constants of hydrolysis reactions. The log10 (Ga/Ge) ratios calculated for this study range from −2.8 to 0.87, corresponding to formation temperatures of 100–227 °C, with an average of 180.3 °C. Likewise, correlation between PC1* and the homogenization temperature indicated that the calculated formation temperatures for this study ranged from 119.3 to 217.9 °C, which suggests that the deposit is sourced from a low-temperature environment. The δ34S of sphalerite is related to the thermochemical sulfate reduction (TSR) of Carboniferous seawater sulfate and are mainly derived from evaporites in the host strata producing H2S−rich fluid by TSR with restricted inducement from mantle-derived sulfur. The homogenous composition of Pb isotopic data of sphalerite indicates that the metals were predominantly originated from the basement rocks of the Mesoproterozoic-early Neoproterozoic rocks of the Kunyang Group. From these geochemical signatures, it can be inferred that the Qilinchang deposit was not linked to magmatic activities. It is proposed that the ore-forming fluids responsible for the formation of the deposit were generated from a low-temperature environment. Economic potential reveals the most important attributes of this deposit are high Zn and Pb grade (approximately 17.5% Zn and 6.6% Pb, 3.32 Mt of metal). Overall, the mineralization of the Qilinchang Pb–Zn deposit shows it a high-grade remarkable Pb–Zn deposit transiting between MVT and high-temperature carbonate-replacement Pb–Zn deposits. We suggest that the determination of the genetic type of this exceptional deposit is important to the research and exploration of similar Pb–Zn deposits in other metallogenic provinces of China.

Detrital zircons from Paleoproterozoic (meta-)sedimentary rocks on the northern margin of the North China Craton and tectonic implications

The Paleoproterozoic strata on the northern margin of the North China Craton (NCC) comprise a relatively intact low-grade (meta-)sedimentary sequence, but whether there are breaks in the sequence and the tectonic setting in which the sequence was deposited are uncertain. In this study, we use zircon U-Pb dating and Hf isotopic analysis to constrain the provenance, depositional age, and depositional environment of the Paleoproterozoic strata, and discuss the likely tectonic setting of sedimentation on the northern margin of the NCC during the Paleoproterozoic. The distribution of detrital zircon ages shows that the lower meta-feldspathic sandstone unit of the Paleoproterozoic sequence was deposited after ca. 2.2 Ga, whereas the maximum depositional age of (meta-)quartz siltstone and quartzites were around 1.89 Ga and at from ca. 1.86 to 1.81 Ga, respectively. Integration of our new U–Pb and Hf isotope data with literature data from other regions of the NCC, allow us to demonstrate that the Paleoproterozoic strata experienced a depositional environment between ca. 2.2–1.81 Ga in an extensional basin system. This suggests that the northern margin of the NCC evolved from an extensional tectonic setting, possibly related to slab rollback. Subsequently, the Paleoproterozoic sequence was affected by a ∼1.8 Ga metamorphic event to form these (meta-)sedimentary rocks in the region.

Heavy metal deposition dynamics under improved vegetation in the middle reach of the Yangtze River

Estuarine heavy metal deposition processes in tributaries can reflect the environmental changes in the basin and the contribution of tributaries to the pollution of downstream lakes. The dynamic processes of heavy metal deposition in two major tributaries of the middle reaches of the Yangtze River were explored using sediment cores. The relationships between heavy metals (HMs) and various physicochemical properties were analyzed using Pearson correlation analysis. The sediment chronological sequences were determined using Pb isotope dating, and the sediment fluxes of heavy metals were calculated. The differences in the driving factors of the two watersheds were analyzed using redundancy analysis. The results showed that heavy metal in both sediment cores were significantly higher than the corresponding background values and showed a relatively stable trend from deep to shallow, with Cd being the most exceeded heavy metal in both tributaries. The average sediment deposition rate was 1.31 cm/year. The Pearson correlation analysis (PCA) results between the HMs indicated similar sources, and the correlation analysis between HM and environmental variables showed that the HMs in both cores, especially Cu and Cr, were significantly correlated with phosphorus, suggesting a synergistic loss of HMs and P. Heavy metal deposition and the normalized difference vegetation index (NDVI) in watersheds showed a negative correlation. This negative correlation is more pronounced in watersheds with higher vegetation cover, where heavy metal deposition is more driven by natural factors. The redundancy analysis (RDA) results indicated that the transport of Cd and Cu was influenced by precipitation and runoff. Heavy metal deposition processes in lake estuaries under improved terrestrial vegetation show the historical contribution of tributaries to lakes, which is important for studying pollution and ecological restoration in watersheds.

Age and compositions of garnet in a magnesian skarn Au-Cu deposit, Tibet, implications for ore-fluid evolution

Compared to calcareous skarn Au-Cu deposits, magnesian skarn (Mg-skarn) Au-Cu deposits are scarce; hence the early evolution of the ore-fluids to late mineralization for the kinds of deposits has been inadequately investigated. Located at the southern margin of the Bangong-Nujiang metallogenic belt, the Galale Au-Cu deposit contains 32.59 tonnes of gold (average grade 2.04 g/t) and 105,600 tonnes of copper (average grade 0.66 %). The orebodies are hosted in an Mg-skarn with associated minerals, including olivine, pyroxene, garnet, and serpentine, which developed in the contact zone between the granodiorite and dolomite of the Jiega Formation. However, the genetic relationship between the Mg-skarn, granodioritic magma, and Au-Cu mineralization remains unclear. The contribution of early stage ore-forming fluids to late Au-Cu mineralization is still vague. In this study, we present the major and trace element compositions, and U–Pb isotope data for garnets obtained from the Galale deposit. We used the data to define the timing, nature, and evolution of early ore-forming fluid and described the genesis of the deposit. The color, at macro- and micro-scales, and chemical composition of the garnets can be used to divide them into two categories, namely early Al-rich Grt-I type garnets, and late stage Fe-rich Grt-II type garnets. In practice, Grt-I (And29.5∼56.1Gro41.3∼68.0Pra1.6∼2.8) and Grt-II (And96.7∼99.9Gro0∼2.7Pra0.1∼0.7) are grossular-andradite solid solution system garnets. Based on the chondrite-normalized REE patterns, Grt-I can be further divided into two sub-classes: Grt-I-1 domain in the core, enriched in HREEs and U, and Grt-I-2 material in the rim, relatively depleted in HREEs. Grt-II is enriched in LREEs and depleted in HREEs. Eu anomalies are slightly negative for the Grt-I-1 domain, weakly positive for the Grt-I-2 domain, and strongly negative for the Grt-II type garnets. The variations in texture and composition in garnets indicated that the formation of Grt-I garnets occurred during diffusive metasomatism in a relatively closed system, in a moderately oxidized ore-forming fluid setting. The formation of Grt-II occurred in a strongly oxidized setting during advective metasomatism in an open system. The enrichment of REE and U in Grt-I garnets was mainly controlled by a “menzerite” type substitution mechanism, partially explaining the strong positive correlation between Mg and REE concentrations. In the Grt-II type garnets, enrichment may also have been co-controlled by coupling substitution and adsorption. In situ U–Pb dating of garnet shows that Grt-I-1 and Grt-I-2 material has isotopic ages of 92 ± 2 Ma (2σ, MSWD = 0.85) and 91 ± 2 Ma (2σ, MSWD = 0.96), respectively. The results were consistent with the previous ages obtained from the granodiorite and Molybdenite using Re–Os isotopic dating within error. This indicates that magma, skarn alteration, and mineralization were spatially and temporally related. In addition, it indicates that the Galale deposit is a typical Mg-skarn Au-Cu deposit. In the earliest stages of deposit development, ore-forming fluids with a high oxygen fugacity (fO2) have inherited their characteristics from the magma. The crystallization of many magnetites during the oxide stage has decreased oxygen fugacity in the metallogenetic system. This has resulted in a relatively low fO2 fluid for Au-Cu precipitation. The study demonstrates that the ore-fluid fO2 plays a vital role in metal migration, hence Au-Cu mineralization in Mg-skarn deposits.

Identification of Jurassic pure sediment-derived granites in the Central Lhasa Terrane, Tibetan Plateau: Implications for continental crustal reworking during Mesozoic Tethyan subduction

A bstract Pure sediment-derived granite, a recently recognized crustal geochemical end-member, is relatively rare but provide primary access to probe the process of continental crustal reworking. This paper presents geochemical and geochronological data for granites emplaced in the Asuo area within the central Lhasa Terrane of the Tibetan Plateau at ca. 158 Ma (zircon U − Pb isotopic dating). These granites are two mica-bearing and contain high SiO 2 (75.40–79.11 wt%), high A/CNK (1.06–1.33), heterogeneous ( 87 Sr/ 86 Sr) i (0.705519–0.710891), and relatively uniform and low ε Nd (t) (−13.18 to −14.81) values, which are analogous to typical S-type granites. The low ε Hf (t) (−13.98 to −12.08) and high δ 18 O (8.01‰–8.85‰) values of magmatic zircons, the similar of ε Nd (t) values between the Asuo granites and regional Jurassic slates, and the absence of mafic enclaves, verify that these granites were derived from a pure sedimentary source. The trondhjemitic features, low CaO/Na 2 O (<0.1) and molar CaO/(MgO + FeO T ) (0.06–0.58), relatively constant and low Rb/Sr (0.04–0.32), as well as the heterogeneous ( 87 Sr/ 86 Sr) i ratios, indicate that the Asuo granites formed via fluid-present muscovite-dehydration melting of metapelitic protoliths, with mica as the main residue. Combined with a synthesis of existing tectonomagmatic data on the Lhasa Terrane, we propose that the Jurassic Asuo granites were formed in a continental extensional setting associated with the northward subduction of the Neo-Tethyan lithosphere. The present study highlights that the inboard region of an active subduction zone is probably an important site of crustal reworking, and provides a geochemical end-member of pure sediment-derived melts for the central Lhasa Terrane. • Jurassic two-mica granites are discovered in the Asuo Area of central Lhasa Terrane; • The Asuo granites were derived from a pure sediment protolith of pelite; • The rear-arc region of a subduction zone is an important site for crustal reworking.

Plutonic-subvolcanic connection of the Himalayan leucogranites: Insights from the Eocene Lhunze complex, southern Tibet

The Lhunze region of southeast Tibet uniquely juxtaposes Cenozoic granitic plutons with subvolcanic rocks in the Tethyan Himalayan Sequence. In this study, the potential petrogenetic connection between two-mica granite and subvolcanic rhyolite porphyry from the Lhunze area is investigated using petrology, geochronology, and geochemistry. Monazite Th Pb ages imply the two-mica granite crystallized during the mid-Eocene at c. 43.5–43.1 Ma, consistent with previous zircon U Pb isotope data. Monazite in the porphyry also records Th Pb ages of c. 44.5–42.9 Ma , suggesting contemporaneous emplacement with the two-mica granite. Porphyritic structure, feldspar aggregates and comb alignment demonstrate that the two-mica granite resulted from transient melt flow associated with melt extraction. Some plagioclase phenocrysts from the porphyry have consistent An contents with the two-mica granite, indicating that both the two-mica granite and the porphyry probably originated from a similar melt. However, the porphyry has a higher SiO 2 concentration (72.2–75.9 wt%) than the two-mica granite (69.0–73.0 wt%). In addition, the porphyry is characterized by lower TiO 2 , MgO, CaO, Ba, Sr, and Zr contents as well as Zr/Hf, Sr/Y, Nb/Ta ratios than the two-mica granite. Moreover, strontium and REE contents of monazite in both the two-mica granite and porphyry also imply that crystal fractionation dominated melt evolution. Whole-rock Sr Nd and in situ monazite Nd isotope compositions indicate that the porphyry is slightly more radiogenic than the two-mica granite, implying that the porphyry likely assimilated more ancient country rocks during its evolution. We interpret the two-mica granite and porphyry as comagmatic intrusives linked by fractional crystallization of a cognate magma mush, in which the crystals accumulated to form the two-mica granite, and the complementary high silicic melt segregated to form the porphyry. Plagioclase accumulation is responsible for the generation of the high Sr /Y adakitic geochemical feather within two-mica granite. The scenario of complementary melt extraction and crystal fractionation might also have been an important process in forming the Himalayan leucogranites. Regionally, Neo-Tethyan slab breakoff during the Eocene could have increased mantle heat flow and triggered the generation and extraction of large volumes of silicic melt. • Two-mica granite and subvolcanic rhyolite porphyry have a coeval crystallization age at 44–43 Ma . • Two-mica granite represents cumulate and the porphyry as its complementary highly fractionated melt. • Magma differentiation via melt extraction from crystal mush is an effective mechanism to produce Himalayan leucogranites.

Geochemical Approach to Determine the Possible Precipitation Parameters of the Coniacian–Santonian Mazıdağı Phosphates, Mardin, Turkey

The Tethyan phosphates were formed during the Upper Cretaceous and Eocene interval as a result of the collision of the African–Arabian and Eurasian plates and the closing of the Neo-Tethys Ocean. This study aimed to reveal the possible precipitation parameters of these phosphates by examining the main oxide, trace element, and rare earth element contents of the phosphates in the study region. The mean major oxide concentrations of the phosphates were found to be 51.6 wt.% CaO, 21.2 wt.% P2O5, 8.03 wt.% SiO2, 18.1 wt.% CO2, 0.51 wt.% K2O, 0.12 wt.% Fe2O3, 0.05 wt.% Al2O3, 0.18 wt.% MgO, and 0.02 wt.% MnO. The average trace element concentrations were 79 ppm Ba, 1087 ppm Sr, 0.23 ppm Rb, 14.7 ppm Ni, 108 ppm Cr, 262 ppm Zn, 27 ppm Cd, 21.6 ppm Y, 58 ppm V, 6.43 ppm As, 30.3 ppm Cu, 1.36 ppm Pb, 6.32 ppm Zr, 39 ppm U, 0.21 ppm Th, and 1.33 ppm Co. The average trace element contents were 1742 ppm, with this indicating an enrichment assemblage of Sr, Cd, As, and Zn in comparison to PAAS (The Post-Archean Australian Shale). The total REE concentrations in the Mazıdağı phosphates varied from 3.30 to 43.1 ppm, with a mean of 22.1 ppm recorded. All phosphates showed heavy REE (HREE) enrichments and had similar REE patterns to PAAS (The Post-Archean Australian Shale). All samples had strongly negative Ce and positive Eu, Pr, and Y anomalies. These anomalies indicate the existence of oxic and suboxic marine conditions during the formation of the phosphates. According to the proposed genetic model, the phosphates mostly formed in the oxic and suboxic zones of the Tethys Ocean and were precipitated on slopes that depended on strong upwelling from an organic-rich basin in anoxic/suboxic conditions from deeper seawater. The Pb isotope data obtained also indicate the existence of a deep-sea hydrothermal contribution to this phosphate formation.

The geochronology, geochemistry and petrogenesis of ca. 2.65 Ga tonalites and ca. 2.61 Ga amphibolites in the Jiefangyingzi area: Implications for the northern boundary of the North China craton

The Bayan Obo–Duolun–Chifeng–Kaiyuan (BDCK) Fault is considered the boundary between the North China Craton (NCC) and Central Asian Orogenic Belt (CAOB). However, the site of this boundary from Duolun to Chifeng and Kaiyuan remains highly controversial. Here, we present an integrated study of petrology, whole‐rock geochemistry, and zircon U–Pb and Sr–Nd–Hf isotope compositions for tonalites and amphibolites in the Bainaimiao arc belt (BAB) to better constrain the northern NCC margin. The tonalites are similar to NCC tonalite–trondhjemite–granodiorites (TTGs), characterized by relatively high Na2O/K2O and Sr/Y ratios and fractionated rare earth element (REE) patterns, with high light REE (LREE) but low heavy REE (HREE) concentrations. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U–Pb dating yields formation ages of 2599–2674 Ma, with a weighted average of 2649.6 ± 3.7 Ma. The εHf(t) values (3.5–7.5), two‐stage model ages (2673–2793 Ma), whole‐rock positive εNd(t) values (1.3–1.9), and two‐stage model ages (2841–2880 Ma) suggest that the Neoarchean tonalites likely originated from remelting of thickened Meso‐Neoarchean mafic crust. LA–ICP–MS U–Pb isotope data reveal that the amphibolites formed at ~2610 Ma. They have high‐Fe tholeiitic compositions and are close to normal mid‐ocean ridge basalt (N‐MORB)‐like rocks, indicating that they were generated from partial melting of the depleted mantle. The existence of Neoarchean basement rocks, combined with seismic and isotope data, suggests that the Jiefangyingzi area is part of the Trans‐North China Orogen (TNCO) in the NCC rather than part of the CAOB and that the northern boundary of the NCC in Chifeng is located north to the Jiefangyingzi area.

Miocene adakitic magmatism and conditions of porphyry metallogenesis in the Gangdese magmatic arc, Tibet

Numerous Miocene porphyry deposits related to adakites have been identified in the eastern Gangdese magmatic arc (east of 87°E), but few of these type of deposits have been found in the western part (west of 87°E), even contemporaneous adakitic porphyries were exposed in the western part. A little research has been undertaken on barren adakitic porphyries in the Gangdese magmatic arc, and the key controls on the metallogenic differences remains unclear. This study presents new whole–rock geochemical, Sr–Nd isotopic, zircon U–Pb and Hf isotopic data for barren adakitic rocks of the Yare complex as well as barren porphyritic and adakitic units in the Mayum, Pujue, Nanmuqie, and Qiagong areas. These data are compared with data for fertile adakitic porphyries, providing insights into the origin and evolution of barren and fertile adakitic magmatism as well as the conditions associated with the porphyry mineralization.The barren adakitic porphyries were formed at 18.03–13.29 Ma. These intrusions have higher (87Sr/86Sr)i ratios (0.708752–0.709785) and lower ƐNd(t) values (−8.4 to −6.5) and zircon ƐHf(t) values (−13.81 to 3.23) than the fertile adakitic porphyries, suggesting the former were derived from ancient crustal sources with a little contributions from mantle-derived material. The fertile adakitic porphyries were formed as a result of partial melting of juvenile lower crust (such as Cu–Mo deposits) and with mixing of material from middle-upper crust (such as Mo–(Cu)–Pb–Zn deposits). The fertile adakitic porphyries also contain more water than the barren adakitic porphyries. Other important factors that discriminate between fertile and barren adakitic magmatism include the involvement of parental magma chambers as well as a history of continuous and relatively complex magmatism prior to emplacement.