Trace Element Distribution Patterns Research Articles

Neoproterozoic Ridge Subduction Beneath South China Craton: TTG Suites of Kangding Complex in Western Yangtze Block and its Geochemistry Characteristics

ABSTRACT A multidisciplinary study involving field investigation, petrographic, geochronology, and whole-rock geochemical data is presented for TTG suites in the western Yangtze Block. In this paper, we compiled element geochemistry data that we obtained from our previous works for China Geological Survey Projects and presented the zircon U-Pb age and the whole-rock elemental compositions of the Na-rich TTG rocks to provide constraints on the origin of the Neoproterozoic magmatism in the western Yangtze Block. The studied rocks show a calc-alkaline trend. They are comparable in whole-rock chemistry to Archean TTG suites from Finland, Labrador, India, Zimbabwe, Brazil, and Ontario, as well as the modern calc-alkaline magmas such as volcanic rocks in the Kastamonu area of Northern Turkey and the Cenozoic andesitedacite-rhyolite suites from the North American Cordillera. Zircon U-Pb age shows the crystallization age of the plagioclase gneiss from Kangding Complex is 772.4 ± 6.9 Ma. The granodiorites are more enriched in LREEs but depleted in HREEs, showed strong negative Eu anomalies, and are characterized by fractionated LREE, flat HREE, and depletion of high-field-strength trace elements (HFSE) relative to normal mid-ocean basalts (N-MORB). However, most trondhjemites, all tonalites, and all quartz diorites have no Eu anomalies. Trace element distribution patterns for all samples show typical island arc signatures with relative enrichment in LILE and depletion in HFSE like Nb and Ta. We assume that the TTG suites were derived from melting of metabasalt in a subducted slab heated by a slab window, which provides further support for a Neoproterozoic intra-oceanic ridge subduction along the western margin of Yangtze Block.

Discovery of mafic dikes in northeastern Tibet plateau belonging to the Emeishan large igneous province: Implications for paleocontinental reconstruction and mineral exploration

Due to the lack of paleomagnetic data and paleoclimatic records, the paleocontinental belonging of the Yidun terrane in northeastern Tibet plateau is debated. In this paper, we use the distribution of the Emeishan large igneous province (ELIP) to put the issue to rest. The ELIP is the product of mantle plume activity, which took place at 260 ± 3 Ma in the western margin of the Yangtze craton. The Yangtze craton is the northern part of the South China paleocontinental block. Our new discovery of four mafic dikes in the Yidun terrane with ages and chemistry like the ELIP strongly supports a new interpretation that this terrane was an integral part of the Yangtze craton during the ELIP magmatic event. Baddeleyites from these mafic dikes yield the U–Pb ages of 257.1 ± 4.8, 259.8 ± 8.5, 260.2 ± 4.5 and 261.5 ± 6.8 Ma for each of these dikes, which are similar to the zircon U-Pb ages of the mafic–ultramafic intrusions of the ELIP within the Yangtze craton. The Yidun mafic dikes and some high-Ti basalts of the ELIP within the Yangtze craton have similar Sr-Nd isotope compositions and mantle-normalized incompatible trace element distribution patterns, indicating similar mantle source compositions. Mixing calculation using Sr-Nd isotope compositions shows that the parental magmas for the Yidun mafic dikes experienced only 2–3 % crustal contamination. Partial melting modelling shows that the primary magmas for the Yidun mafic dikes were produced by 5–6 % mantle partial melting at the depths > 85 km below surface, consistent with mantle plume-related magmatism in a continental setting. The Yidun terrane is a new exploration frontier for magmatic Fe-Ti-V oxide deposits that are present elsewhere in the ELIP.

Identification of variable contributions to arc rocks by Li[sbnd]Mo and Sr–Nd–Hf–O isotope compositions: A study of the Mafan mafic rocks in the Hong'an orogen, Central China

The geochemistry of arc rocks provides a window to probe compositional variability in mantle wedge and slab-derived inputs. However, quantitative estimation of their contribution is a great challenge. Here we report an integrated study for determination of variable contributions to early Paleozoic arc plutons (the Mafan diorites and gabbros), from the northern Hong'an orogen, central China. Zircon grains from the diorites yielded crystallization age of 445 ± 3 Ma, whereas the gabbros have zircon UPb age of 432 ± 3 Ma. Although both the two mafic rocks suites exhibit typical arc-like trace element distribution patterns, they show a series of differences in stable Li–Mo–O and radiogenic Sr–Nd–Hf isotope compositions, as well as major and trace element features. Geochemically, the diorites are medium-K calc-alkaline series with high and positive whole-rock εNd(t) (+3.8 to +4.5) and zircon εHf(t) values (+12.4 to +14.3), and normal mantle-like zircon δ18O values (5.1 ± 0.2 ‰). Contrarily, the gabbros are tholeiitic and have relatively low whole-rock εNd(t) (+ 2.3 to +3.1) and zircon εHf(t) (+5.0 to +8.5) values with normal mantle-like zircon δ18O values (5.4 ± 0.3 ‰). The diorites have δ98Mo values of −0.15 to −0.03 ‰, while the gabbros have δ98Mo values of −0.28 to −0.12 ‰. These values are slightly higher than those of the mid-ocean ridge basalts (MORB) (−0.20 ± 0.01 ‰). Whereas, δ7Li values of the studied diorites (2.15–4.49 ‰) and gabbros (2.89–5.33 ‰) are mostly consistent within the range of MORB. In combination with the above results, the primary magma of diorites was inferred to be derived from 1 to 5% partial melting of depleted lithospheric mantle metasomatized by slab-derived fluid with minor sediment-derived melts. While the gabbros were suggested to be generated by 5–10% partial melting of more depleted lithospheric mantle that had undergone perhaps extra 0.3% melt extraction, which had further been metasomatized by more subducted materials. In summary, both the slab-derived components and nature of the mantle wedge sources are quantitatively constrained for the mafic rocks in the Mafan plutons. Our new data suggest that the Mafan plutons exhibited the Mariana arc-affinity and represented products of an intra-oceanic subduction system in the northern Hong'an orogen. Combined with previously published data for the early Paleozoic mafic magmatic rocks along the Qinling-Tongbai-Hong'an orogenic belt, there exists an oceanic arc in the east (Hong'an orogen) and an Andean-type continental arc in the west (Qinling-Tongbai orogen).

Hydrogeochemical characteristics of groundwater contamination in Guwahati city, Assam, India: Tracing the elemental Threads

Groundwater serves as an important resource for drinking and agriculture in many countries, including India. Assessing the quality of groundwater is essential for understanding its chemical characteristics and suitability for consumption. This study aims to explore the factors affecting the hydrogeochemical changes in groundwater within Guwahati City, Assam, India. Groundwater samples were collected and analyzed for major and trace elements, as well as anion concentrations. Concentrations of As, Al, Ba, Cu, F−, Fe, Mn, and Pb exceeded the permissible limits set by both World Health Organization (WHO) and Bureau of Indian Standards (BIS), indicating serious health concerns for the local inhabitants. The distribution pattern of trace elements exceeding the guideline values is intricate, suggesting widespread contamination of groundwater throughout the study area. The Heavy Metal Pollution Index (HPI) and Water Quality Index (WQI) revealed that, except for the central zone, groundwater across the entire study area requires intervention. Piper plot illustrated that the groundwater is predominantly of Ca–HCO3 type, indicating the dominance of alkaline earth and weak acids. Groundwater hydrogeochemistry is mainly controlled by rock-water interaction and evolves through silicate weathering, carbonate weathering, and cation exchange processes. Multivariate statistical analysis identified distinct groups of groundwater based on chemical characteristics, emphasizing the role of both natural processes and anthropogenic activities in influencing groundwater quality. Regular monitoring, management, and intervention of groundwater sources throughout the study area are crucial for long-term use. The findings of this study will assist stakeholders, regulators, and policymakers in formulating strategies for the sustainable use of groundwater.

Recycling of Paleo-Tethyan oceanic crust: Geochemical record from Early–Middle Triassic igneous rocks in the East Kunlun Orogen in western China

Abstract Deciphering the contribution of crustal materials to generation of mafic arc igneous rocks at different subduction stages is of great significance to unravel the fate of the subducted paleo-oceanic crust. Here we present an integrated geochemical study on two types of early Mesozoic mafic arc igneous rocks from the East Kunlun Orogen. Zircon U-Pb isotopic analyses yield ages of 252–248 Ma for lamprophyres and 239–238 Ma for diorite porphyries. All the samples display arc-like trace element distribution patterns, high zircon δ18O values, and variably low zircon εHf(t) values. However, significant geochemical distinctions exist in terms of trace element concentrations, radiogenic isotopes, and other geochemical variations between them. The Early Triassic lamprophyres are characterized by significant enrichment in fluid-mobile trace elements and weakly enriched whole-rock Sr-Nd-Hf isotopes, whereas the Middle Triassic diorite porphyries show high contents of light rare earth elements, Th, Zr, and Hf, and more enriched Sr-Nd-Hf isotopes. Furthermore, the lamprophyres exhibit remarkably higher ratios of Ba/Th, Ba/La, K/La, and Sr/Nd and slightly higher ratios of La/Sm, Th/Yb, and Th/La than mid-oceanic-ridge basalt (MORB), while the diorite porphyries display higher La/Sm, Th/Yb, Th/Nd, and Th/La ratios compared to normal MORB but closer to those of seafloor sediments. Taken together, these differences can be attributed to the incorporation of two distinct slab liquids into their mantle sources, including oceanic slab-derived aqueous solutions and minor sediment-derived hydrous melts for the formation of the lamprophyres, and sediment-derived hydrous melts for the formation of the diorite porphyries. As a result, we suggest the lamprophyres were generated during the Early Triassic subduction of the Paleo-Tethyan oceanic crust, while the diorite porphyries may be generated due to rollback of the subducting oceanic slab in response to the closure of the Paleo-Tethyan Ocean basin. Therefore, the studied Early–Middle Triassic mafic igneous rocks provide important evidence for the recycling of the Paleo-Tethyan oceanic slab at different stages.

Mo-Mg isotopes trace the role of serpentinite in generating arc magmatism

Serpentinite-derived components play an important role in chemical cycling in subduction zones. However, it is difficult to identify recycled serpentinite in subarc mantle due to the overprinting of other slab components, and how serpentinite contributes to arc magmatism is poorly understood. Here we report Mo and Mg isotopic compositions of Early Paleozoic Qushiang arc mafic rocks in the East Kunlun orogen. The Qushiang mafic rocks have variable δ98Mo values of −0.50‰ to 0.78% and δ26Mg values of −0.31‰ to −0.05‰. These features, combining with arc-type trace element distribution patterns, enriched SrNd isotopes, and variable zircon HfO isotopes, suggest the incorporation of different subducting components in their mantle source. Some mafic rocks show high δ98Mo values together with low Mo/Ce ratios and enriched SrNd isotopes, suggesting the contribution of subducted sediment (SSD)-derived melts. Additionally, light Mo isotopes are also identified in mafic rocks, which are closely related to depleted SrNd isotopes. These features are mainly inherited from the subducted dehydrated altered oceanic crust (SAOC)-derived melts. More importantly, the coexisting and significantly high δ26Mg and δ98Mo values of these rocks could not be explained by the SSD or SAOC components, but require serpentinite components in the subarc mantle source. Our study reveals that combined Mo and Mg isotopic studies are potential tools for tracing serpentinite recycling, and provides new insight into the mechanisms by which different subducting components contribute to arc magmatism.

Andesitic arc magmas derived from two contrasting mélange origins: Evidence from central Tibetan dioritic porphyries

Numerous studies have argued that the andesitic model (i.e., the arc directly produces andesitic magmas) rather than the basaltic-input model (i.e., andesitic magmas were derived by fractionation from basaltic magmas) better explains the composition of continental crust. However, it remains controversial as to how andesitic magmas are directly produced in subduction zones, and this weakens the basis of the andesitic model. Recently, the mélange-diapir model has been increasingly considered to be an important mechanism for the direct generation of arc andesites, and would thus support the andesitic model for crustal growth. Moreover, melting experiments on mélange rocks show that varying the composition of mélange rocks (e.g., serpentinite-dominated and sediment-dominated mélange) can produce a range of primary andesitic arc magmas, from tholeiitic, calc-alkaline, to high-K calc-alkaline and shoshonitic magma types. However, it remains challenging to determine the different mélange sources for low-K to high-K arc andesites. Here we verify these experimental results using data from early Cretaceous dioritic porphyries in the southern Qiangtang block of central Tibetan plateau. The generation of these dioritic rocks cannot be linked to any coeval basaltic rocks in this region and so they may represent primary andesitic magmas. These dioritic porphyries are divided into low-K type-1 and high-K type-2. Type-1 has higher δ26Mg values (−0.16 to −0.08‰) than MORBs (mid-ocean ridge basalt, δ26Mg = −0.25 ± 0.06‰), which can be ascribed to the contribution of subducted bulk/un-differentiated serpentinite in their source. Combined with their low K contents and La/Sm ratios, and high NdHf isotope ratios (εNd(t) = 3.36–5.03; εHf(t) = 12.88–13.76), we suggest a serpentine-dominated mélange for their origin. The type-2 and type-1 rocks have similar trace-element distribution patterns, the same ages (∼124 Ma), and are indistinguishable in field outcrops, indicating their common petrogenesis. However, compared to type-1, type-2 has higher K and Th contents, and La/Sm and Th/Nd ratios with lower NdHf isotope values (εNd(t) = −1.61 to +0.30; εHf(t) = 3.78–4.39), and mantle-like δ26Mg values (−0.28 to −0.17‰). This likely indicates derivation from a sediment-dominated mélange for type-2. This study thus verifies the experimental results of melting of different mélange from natural rock record and shows that the mélange model provides an important mechanism for generating variable andesitic arc magmas and so the formation of the andesitic continental crust.

Identification of recycled oceanic crust in the mantle source of sulfide-bearing lower crust of the Gangdese magmatic arc, Southern Tibet: Constraints from Sr–Nd–Li isotopes

Post-collisional porphyry copper deposit (PCD) is a new genetic type of PCDs discovered in the Himalayan–Tibetan orogenic belt. It is believed that the formation of ore-forming magmas results from the re-melting of a sulfide-bearing, thickened, juvenile mafic lower crust in a magmatic arc associated with the oceanic plate subduction. However, the material contribution of subduction processes to the formation of this particular arc magma remains unclear. In this paper, we attempted to present a study that includes petrology, mineral geochemistry, whole rock Sr–Nd–Li isotopes, of Late Cretaceous Lilong ultramafic igneous rocks, south Lhasa sub-terrane, Tibet. The target ultramafic igneous rocks are sulfide-bearing cumulates. They show relative depletion in whole rock Nd isotopic compositions, slightly elevated (87Sr/86Sr)i ratios (0.7043–0.7059), and variable δ7Li values (2.8–9.0‰). Simulation results showed that trace element of melts in equilibrium with cumulus hornblende grains exhibit typical arc-like trace element distribution patterns, indicating that these ultramafic igneous rocks crystallized from arc magma. Additionally, the modeled melts showed enrichment in melt-mobile elements such as Ba, Pb, Sr, Th, and light rare earth elements (LREE), and higher Th/Yb ratios, as compared with normal mid-ocean ridge basalt (MORB). These geochemical properties can be explained by the metasomatism of depleted MORB mantle peridotites by hydrous melts derived from subducted seafloor sediments. Whole rock Sr isotopes show correlations with Ba/Nb and Th/La ratios for the modeled melts, showing that fluids from the basaltic igneous crust, in addition to hydrous melts from the seafloor sediments, contribute to their mantle source. In addition, the heterogeneity of whole rock δ7Li values for ultramafic igneous rocks also indicates that their mantle sources have been metasomatized by terrigenous sediment- and altered oceanic crust-derived fluids. Depleted MORB mantle peridotites react with the mixed fluids at sub-arc depths to produce ultramafic metasomatites, and model calculations for trace element contents and Sr–Nd–Li isotopic compositions further validate that the geochemical compositions of the Lilong ultramafic rocks can be clarified by this process. The results show that the deeply subducted oceanic crust may have played an important role in the formation of sulfide-bearing juvenile mafic lower crust of this magmatic arc.

Magma process of the ore-forming gabbrodiorite porphyry of the Meishan iron deposit, eastern China: Insights from the phenocrysts chemistry

Mesozoic iron deposits in the Ningwu volcanic basin in eastern China are genetically linked to the gabbrodiorite porphyries, which have long been regarded as the subvolcanic intrusive of the andesitic volcanics. Compared with abundant research conducted on the mineralization process, the magmatic process of the ore-forming intrusion has been poorly understood. This study performed petrographic and in situ analyses of representative phenocrysts from the ore-forming gabbrodiorite porphyry of the Meishan iron deposit to gain new insights into the magmatic process. The phenocrysts are composed of clinopyroxene and plagioclase, which commonly exhibit disequilibrium features, such as reverse zoning and sieved textures, suggesting that the gabbrodiorite porphyry evolved in an open-system andesitic-magma reservoir into which mafic magma was repeatedly recharged. The zoned clinopyroxene phenocrysts show consistent trace-element distribution patterns and a narrow range of 87Sr/86Sr of 0.7037–0.7049, which suggest that the recharged mafic magma was the primitive basaltic magma and crystal fractionation dominated the evolution from basaltic magma to andesitic magma. Pressure estimations based on clinopyroxene phenocryst compositions indicate that the magma reservoirs were emplaced at the uppermost mantle. Comparison of clinopyroxene phenocryst compositions between the Meishan gabbrodiorite porphyry and published andesitic volcanics shows distinct rare earth element distribution patterns, which don't support a volcanic–subvolcanic relationship between the gabbrodiorite porphyry and andesitic volcanics. Estimated pressures indicate that the primitive magma of andesitic volcanics evolved in lower crust reservoir, which was much shallower than the reservoirs of the ore-forming gabbrodiorite porphyry. Therefore, the Meishan gabbrodiorite porphyry was not the subvolcanic intrusive of the andesitic volcanics and probably represented an independent magmatic pulse. This understanding suggests that the exploration target of iron deposits in the Ningwu basin is not necessarily limited to the contact zone between gabbrodiorite porphyry and andesitic volcanics.

Trace elements and Li isotopes distinguish between liquid and solid phases contributing to mafic arc magmas

It is generally accepted that metasomatism of the mantle wedge peridotite by subducting slab-derived fluids plays a key role in dictating the composition of mafic arc magmas. However, this fluid metasomatism model has been challenged recently by the mélange diapirism model. In order to distinguish between liquid and solid phases that would exclusively contribute to mafic arc magmas, we have carried out an integrated study of whole-rock and mineral geochemistry for two types of contemporaneous mafic igneous rocks (gabbroic diorites and diorite dikes) from an accretionary orogen. The results indicate primary contributions from liquid phases composed of not only slab-derived aqueous solution but also seafloor sediment-derived hydrous melt. Although these mafic rocks all exhibit typical arc-like trace element distribution patterns, they show a series of differences in stable Li and radiogenic Sr–Nd–Hf isotope compositions as well as other geochemical variables. In general, the gabbroic diorites are characterized by enrichment in fluid-mobile elements (e.g., Rb, Ba, K, Pb, and Sr), high δ7Li and δ18O values, and depletion in Sr–Nd–Hf isotope compositions. By contrast, the diorite dikes exhibit enrichment in melt-mobile elements (e.g., LILE, LREE and Th), relatively low δ7Li values, and slight enrichment in Sr–Nd–Hf isotope compositions. There are also systematic differences and pertinent variations between Li isotope compositions and slab-derived fluid indicators such as whole-rock Rb/Nd, Sr/Nd, Nb/U, Th/La, Th/Nd and Th/Yb ratios for the two types of mafic igneous rocks. These indicate that the mantle sources of these mafic rocks would possess geochemical signatures that were acquired through chemical metasomatism by slab-derived fluids rather than physical mixing with mélange diapirs. Furthermore, the geochemical signatures allow for distinction between the oceanic slab-derived aqueous solution and the seafloor sediment-derived hydrous melt. This qualitative interpretation is quantitatively confirmed by model calculations of trace elements and Sr–Nd–Li isotopes in the two types of mafic igneous rocks. Therefore, the target mafic igneous rocks provide the geochemical evidence for the formation of their mantle sources through chemical metasomatism by incorporating the two types of metasomatic agent into the mantle wedge. This demonstrates a strong preference for the fluid metasomatism model over the mélange diapirism model for the origin of mafic arc magmas.

Recycling of continental crust materials: Geochemical constraints from post-collisional alkaline intrusive rocks in the Dabie orogen

Post-collisional alkaline intrusive rocks from the Dabie orogen were studied for their whole-rock major-trace elements and Sr-Nd-Hf-Pb isotopes, as well as zircon U-Pb ages and Hf-O isotopes. The results provide geochemical constraints on the nature of their mantle sources and thus insight into crust-mantle interaction in the continental collision zone. The alkaline intrusive rocks are composed of syenite and nepheline syenite. Syn-magmatic zircon U-Pb dating by LA-ICP-MS for them yielded Early Cretaceous ages of 131.3 ± 1.4 Ma to 122.6 ± 0.6 Ma, coeval with the post-collisional magmatism in the Dabie orogen. One relict zircon with U-Pb age of 211 Ma is consistent with the timing of metamorphism for the ultrahigh-pressure (UHP) metamorphic rocks in this orogen. They have arc-like trace element distribution patterns, such as enrichment in LILE (large ion lithophile element) and LREE (light rare earth element) but depletion in HFSE (high field strength element), and enriched whole-rock Sr-Nd-Hf isotope compositions with high (87Sr/86Sr)i ratios of 0.7077–0.7131 but negative εNd(t) values of −16.0 to −9.4 and εHf(t) values of −17.5 to −12.7. Their syn-magmatic zircons have three groups of Hf-O isotope compositions comparable to those of UHP metamorphic rocks in Central-South Dabie and North Dabie, which represent the upper and middle continental crust of the subducted South China Block, respectively. In this regard, slab–mantle interaction is evident during the Triassic continental collision. We suggest that the melts derived from the subducted South China Block reacted with the lithospheric mantle wedge of the North China Block, resulting in phlogopite-bearing metasomatites, whose partial melting would generate the post-collisional alkaline intrusive rocks during the Early Cretaceous.

Zircon U–Pb Geochronology, Geochemistry and Geological Significance of the Santaishan–Yingjiang Ultramafic Rocks in Western Yunnan, China

The study of ultramafic rocks in Western Yunnan is of great significance for an understanding of the tectonic evolution of the Neo-Tethys Ocean. The zircon U–Pb data indicated that the Santaishan serpentinized pyroxene peridotite (SSPP) was formed 186–190 Ma, and the Yingjiang hornblende pyroxenite (YHP) was formed 182–183 Ma. The content of MgO in the SSPP is relatively high, but the SiO2, Al2O3, CaO and TiO2 content and ΣREE are low, while the YHP has opposite characteristics. The samples from the SSPP and YHP have similar distribution patterns of trace elements, both being enriched in large ion lithophile elements (LILEs) such as Rb, Ba and Th and depleted in high field strength elements (HFSEs) such as Ti, P and Nb. These characteristics are consistent with the supra-subduction zone (SSZ) type and mid-ocean ridge basalt (MORB) type of ophiolite in the Bangong–Nujiang suture zone. The SSPP rocks have relatively high (87Sr/86Sr)i ratios (0.7091–0.7131) and positive Hf(t) values (11.2–13.8), with εNd(t) values varying from −1.1 to 9.4. The YHP has relatively low εHf(t) values (3.5 to 6.9), with the Nd–Hf isotopic model ages ranging from 610 to 942 Ma. The signatures of Sr–Nd and Lu–Hf isotopes indicate that the SSPP and YHP were derived from the depleted mantle, and the crustal material in the magma source may have originated from the Neoproterozoic Rodinia supercontinent. In the early Middle Jurassic (190 Ma), the Tengchong Block was in the setting of an active continental margin induced by the subduction of the Bangong–Nujiang Ocean, where the SSZ-type SSPP with ophiolite characteristics was formed. With the continuous subduction of the Bangong–Nujiang Ocean, the slab retreated and induced mantle convection, which resulted in the gradual thinning of the continental crust. Meanwhile, the Yingjiang back-arc basin was formed 183 Ma. Under the influence of the upwelling of the asthenosphere and the mixture of crustal materials, the MORB-type YHP was formed.

Neoproterozoic Aksu Diabase Dyke, Chinese South Tianshan: Magma Sources and Implications for Regional Gold Metallogeny

Tianshan is one of the world’s largest gold provinces; however, the relationship between gold mineralization and metasomatized subcontinental lithospheric mantle (SCLM) remains poorly understood. To improve our understanding, we present new bulk-rock geochemistry and platinum group element (PGE) concentrations of the SCLM-sourced Aksu Neoproterozoic diabase dykes in Chinese South Tianshan. These data, combined with in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of hydrothermal pyrite grains in the diabase dykes, are used to discuss the SCLM source characteristics in the region and their potential links to formation of gold deposits. The diabase dykes exhibit high Th/Yb (0.47–0.62) and low Nb/U (13.4–16.3) ratios, indicating that magma evolution involves subduction-related fluid metasomatism and limited contamination of the continental crust. This is consistent with little variation in whole-rock Pd/Zr, Cu/Zr, and Ni/MgO ratios, suggesting that no sulfide segregation was caused by crustal contamination and magma mixing. In addition, the diabase dykes show low PGE and Au contents, with high Cu/Pd (>105) and low Cu/Zr (<0.5) ratios, indicating that magmas were derived from low-degree partial melting of the SCLM under S-saturated conditions. Such source characteristics indicate residual sulfides and chalcophile elements (e.g., PGEs, Au, and Cu) were concentrated at the SCLM reservoir in South Tianshan. Hydrothermal pyrite in the studied dykes has similar Au/Ag ratios and trace element distribution patterns to gold-bearing pyrite of lode gold deposits in Chinese South Tianshan, indicating that metasomatized SCLM may have contributed ore metals during the formation of these gold deposits. Adding to the available data, our study highlights that the SCLM may be a potential metal source reservoir, and it may have contributed to formation of the lode gold deposits in Chinese South Tianshan.

Dolomitisation under an arid climate at low sea-level: a case study of the Lei 4 Member of the Middle Triassic Leikoupo Formation, Western Sichuan Depression, China

The origin of dolostones from the Lei 4 Member (T2l4) of the Middle Triassic Leikoupo Formation in the Western Sichuan Depression is unclear. The occurrence and genetic evolution of dolostones in T2l4 were analysed by polarised thin-sections and cathodoluminescence, major- and trace-element, scanning electron microscope, and carbon, oxygen and strontium isotope analyses. The study results are summarised as follows. (1) The dolostones were mainly precipitated in three stages of <40 °C, 40–60 °C and 60–80 °C, corresponding with three paleodepth ranges of 167–300 m, 433–1000 m and 1067–1433 m. The micritic dolostones and the fabric-retentive dolostones (algal dolostone, granular dolostone) mainly formed in a near-surface–shallow-burial environment, and the crystalline dolostones were mainly formed under intermediate burial conditions. (2) 87Sr/86Sr ratios are equivalent to that of the Middle Triassic seawater, Sr/Ba and V/Ni ratios >1, Na notably higher than that in coexisting limestones of T2l4, and the dolostones always coexist with evaporative minerals such as gypsum, indicating that dolomitisation fluids mainly originated from evaporative concentrated seawater. (3) The dolostones mainly inherit materials from precursor limestones based on trace-element distribution patterns, and carbon and oxygen isotope values that are consistent with coexisting limestones of T2l4. (4) Mediated by micro-organisms during the syngenetic period, micritic dolostones and some algal dolostones were formed by replacing aragonites and calcites. During shallow burial, concentrated seawater rich in Mg2+ from the supratidal–intertidal zone flowed downward owing to gravity along the platform and replaced the underlying carbonate rocks, promoting continuous growth of the early dolomites. In the intermediate burial period, the Mg2+-depleted dolomitisation fluid caused the early micritic and silt-crystalline dolostones to recrystallise into silt- or fine-crystalline dolostones with larger crystals and altered the fabric-retentive dolostones into crystalline dolostones. (5) The evaporative dolostones deposited in the near-surface environment are characterised by maximal enrichment of Fe, Sr and Na, the highest δ18O values, the lowest order degree and the highest Ca/Mg ratios. The reflux dolostones formed in a shallow-burial environment characterised by the lowest Fe, medium δ18O values and the lowest order degree. The burial dolostones that developed in the intermediate burial environment are characterised by relative enrichment of Fe and Mn, minimal Na, the lowest δ18O values, the highest order degree and medium Ca/Mg ratios. KEY POINTS The fabric-retentive dolostones mainly formed in a near-surface–shallow-burial environment, and the crystalline dolostones mainly formed under intermediate burial conditions. Dolomitisation fluids mainly originated from the evaporative concentrated seawater, and the dolostones inherit materials from the precursor limestones. Micritic dolostones were formed by evaporative dolomitisation; algal dolostone and granular dolostone were formed by evaporative dolomitisation and reflux dolomitisation; and silt-crystalline dolostone were formed by reflux dolomitisation and burial dolomitisation.

Origin, transport, and diagenesis of tuffs in organic-rich lacustrine mudstone: An example from the lower part of the Middle–Late Triassic Chang7 Member, Ordos Basin (NW China)

Tuffs, produced by explosive volcanic eruptions, are widely deposited throughout the Precambrian to Cenozoic stratigraphic record. Due to rapid compaction and erosion, preservation and identification of tuffs in distal environments are challenging. However, in deep lacustrine environments, tuffs may be found well-preserved as interlayers in the organic-rich mudstone. This in turn provides valuable insights into the origin, transport, and diagenesis of volcanic material. In this study, we performed an integrated sedimentological, mineralogical, and geochemical analysis of seven tuff samples collected from the lower part of the Chang7 member of the Maquan section to infer on their provenance, transport mechanisms and post-depositional alterations. Immobile element ratios, tectonic discrimination diagrams, trace element distribution patterns, and negative whole-rock εNd(t) reveal an intermediate to felsic continental arc source of the studied tuff. Supported frameworks and ash aggregates in the tuff are in favor of the transport from pyroclastic density currents and ash plumes during multiple volcanic eruptions with various degrees of eruption volume and water content. Tuff diagenesis is characterized by dissolution of plagioclase, illitization of smectite, cementation of carbonate minerals, and presence of buddingtonite. Relatively ordered illite-smectite and low vitrinite reflectance from the adjacent organic-rich mudstone indicate diagenetic evolution of studied tuff have reached mesogenetic stage.

Identifying the clay-size sediment provenance of the radial sand ridges in the southwestern Yellow Sea using geochemical and Sr[sbnd]Nd isotopic tracers

Although the sediment provenance of the Jianggang radial sand ridges (RSRs) in the southwestern Yellow Sea has attracted much attention in recent years, the provenance of the clay-size fraction is rarely known. In this paper, the residual fraction of the clay-size sediments in the Jianggang RSRs was isotopically and geochemically determined to analysis their provenance contributions and transport patterns. The clay minerals of the sediments in the Jianggang RSRs, Yangtze River Mouth, Modern Yellow River Mouth and Old Yellow River Delta were characterized by high illite and low smectite contents, differing from those of the Korean rivers. The isotopic and geochemical data showed that 87Sr/86Sr ratios and εNd(0) values of the clay-size sediments ranged from 0.719 to 0.725 and from −14.5 to −12.2, respectively for the onshore RSRs, and from 0.715 to 0.724 and from −14.6 to −10.9, respectively for the offshore RSRs. The clay-size sediments in the offshore RSRs were represented by high concentrations of Sr, Co, Zr, Nb, Hf, Ta, while in the onshore RSRs were characteristic of high concentrations of other elements (except for Ba). The clay-size sediments showed partially similar Y/Hf and Rb/Sc ratios and upper continental crust (UCC)-normalized distribution patterns of trace elements between the onshore and offshore RSRs. However, there was a clearly SrNd isotopic boundary between the two RSRs, indicating that the offshore clay-size sediments were just partly transported to the onshore area. The UCC normalized patterns of the clay-size sediments in the onshore and offshore RSRs were close to those of the Yangtze River Mouth, Modern Yellow River Mouth and Old Yellow River Delta, but different from those of the Korean rivers and Cheju Island. Based on the analyses of SrNd isotopes and trace elements through MixSIAR model, the clay-size sediments of the onshore RSRs mainly originated from the Yangtze River Mouth, but those of the offshore RSRs were predominantly mixed by the Yangtze River Mouth, the Modern Yellow River Mouth and the Old Yellow River Delta, while the contributions of other sources were tiny. Through the SrNd isotopic comparisons, it was further found that there existed a large-scale transport of the clay-size sediment from the Yangtze River Mouth within the Yellow Sea to reach the Jianggang RSRs finally under the influence of the Changjiang Diluted Freshwater and the Yellow Sea Warm Current. This study not only gives deep insight into the formation and evolution theory of the Jianggang RSRs, but also enriches the sediment migration patterns in the Yellow Sea. Meanwhile, it demonstrates the effectiveness of the SrNd isotopic and geochemical tools for provenance analyses of the clay-size sediments, although 87Sr/86Sr should be used cautiously for provenance apportionment due to its indistinguishability between the Chinese and Korean rivers.

Reconstruction of Paleoenvironment and Paleoclimate of the Neogene Guantao Formation in the Liaodong Sub-Uplift of Bohai Bay Basin in China by Sedimentary Geochemistry Methods

The paleosedimentary environment and paleoclimate of the Neogene Guantao Formation in the Liaodong sub-uplift of Bohai Bay Basin in China, which is an important oil and gas exploration horizon, are not clear owing to the lack of quantitative analysis. The paleosedimentary environment and paleoclimate can be qualitatively or semi-quantitatively reconstructed by sedimentary geochemical indicators sensitive to the environment and climate. Based on the evaluation of whether the elements and isotopes can effectively record paleosedimentary environment and paleoclimate information or not, the paleoclimate (temperature and humidity), paleoenvironment (salinity, water depth, redox conditions), and evolution of the paleoenvironment and paleoclimate are studied by analyzing the trace elements, carbon and oxygen isotopes, strontium isotopes, whole-rock mineral compositions, and clay minerals of mudstones of the Guantao Formation in the Liaodong sub-uplift, Bohai Bay Basin. The study results show that (1) according to the trace element distribution patterns, high concentrations of continental elements (Ti, Zr, Th), clay minerals, and detrital content, the Guantao Formation is a product of proximal deposits, and the provenance mainly originates from the intermediate–acidic magmatic rocks near surrounding uplifts. (2) The paleoclimate during the sedimentation of the Guantao Formation was mainly semi-arid and semi-humid, as revealed by the Sr/Cu ratio. However, the high Rb/Sr and 87Sr/86Sr ratios demonstrate that the paleoclimate is relatively warm and humid. (3) The paleosedimentary temperature calculated by the Sr content and δ18O is roughly 30 °C, indicating that the Guantao Formation was deposited in a subtropical to tropical climate. (4) The Guantao Formation was deposited in continental freshwater according to the Li, Ni, and Sr content, Sr/Ba ratios, 87Sr/86Sr ratios, and salinity index Z. (5) The water depth of the paleoenvironment of the Guantao Formation is mainly in nearshore shallow water, reflected by the Mn/Ti ratio and calculated by the Co content, and the oxidizing condition of the paleoenvironment of the Guantao Formation, reflected by the U/Th, V/Cr, and Ni/Co ratios, is in a good coupling relationship with this environment. (6) As revealed by the variations in the geochemical data in the section, the temperature, humidity, and water depth of the Guantao Formation gradually increased from the early to late period, while the salinity gradually decreased, and the chemical weathering effect gradually increased.

Magma Evolution and Constraints on the Graphite Mineralization Hosted by the Huangyangshan Alkaline Granite Suite in the East Junggar of Xinjiang Province: Evidence from In Situ Analyses of Silicate Minerals

The Huangyangshan super-large graphite deposit, located in the East Junggar area of the Xinjiang Province, is hosted in and has closely temporal, spatial, and genetic relationships with the Huangyangshan alkaline granites. There are such silicate minerals as amphibole, biotite, pyroxene, and plagioclase occurring in the graphite-bearing granites. The integration of the electron microprobe analysis (EMPA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) enabled us to reveal the physicochemical conditions and evolution process, as well as the relationship of alkaline magmatism with graphite mineralization. The results show that the amphiboles generally have low Al and high Ti, K, Si, and Fe contents, as well as similar rare-earth elements (REEs) patterns and trace element distribution patterns to granites with significantly negative Eu anomalies. In the analyzed samples, primary biotite belongs to Fe-biotite and has characteristics of high Si and Fe and low Al and Mg contents. In the graphite orbicules, the pyroxene phenocrysts develop multiple zonal structures and are characterized by high Si and low Ca and Fe contents. The dominant plagioclase phenocrysts in the graphite orbicules are oligoclase and andesine, with normal and occasionally oscillatory zoning. The calculated crystallization temperature of the pyroxene, amphibole, and primary biotite in graphite orbicules are 840–1012 °C, 681–761 °C, and 658–720 °C, respectively, corresponding with their crystallization order. The pressure and depth calculation results of the amphibole, representing those of the magmatism, are 157–220 Mpa and 5.95–8.32 km, respectively. Both amphibole and biotite crystallized in a reducing environment with extremely low oxygen fugacity. The elemental compositions of these silicates indicate that the Huangyangshan pluton experienced significant mixing of mafic mantle-derived magma and felsic crust-derived magma. The cores of graphite orbicules were formed in a relatively earlier magmatic stage, while the granites and their dioritic enclaves were formed in a later magmatic stage. During magmatism, the mixing of mantle-derived basic magma had an important influence on the evolution and differentiation of the melts. According to the coexisting sulfides with graphite and compositional difference of amphibole and biotite in the granites and graphite ores, the graphite mineralization might be triggered by a magma mixing process.

Heterogeneous orogenic lithospheric mantle beneath the North Qaidam orogen: Geochemical evidence from syn-exhumation and post-collisional mafic magmatic rocks

Syn-exhumation and post-collisional mafic magmatic rocks are generally exposed in continental collision orogens. Their geochemical compositions can not only reflect the nature of orogenic lithospheric mantle but also provide a window to trace crust-mantle interaction in continental collision zones. Herein, we present a combined study of whole-rock major-trace elements and Sr-Nd-Hf isotopes as well as zircon U Pb ages and Hf O isotopes for the Paleozoic mafic magmatic rocks from the North Qaidam orogen, northeastern Tibet. Zircon U Pb dating for the rocks yields two groups of ages of 420 ± 8 to 395 ± 2 Ma and 383 ± 5 to 368 ± 3 Ma, corresponding to the syn -exhumation and post-collisional stages, respectively. These mafic magmatic rocks are characterized by arc-like trace element distribution patterns, enriched to depleted radiogenic Sr-Nd-Hf isotope compositions and high zircon δ 18 O values, which are comparable to those of the orthogneiss and continental eclogite in this orogen. This indicates their derivation from the orogenic lithospheric mantle modified by the subducted continental crust during the continental collision. Significant differences in radiogenic isotope compositions between the syn -exhumation and post-collisional mafic magmatic rocks suggest their mantle sources were mainly metasomatized by the orthogneiss- and continental eclogite-derived melts, respectively. Quantitative modelling of trace element compositions shows that about 93.0:6.0:1.0 and 91.7:2.8:5.5 mixture of the mantle peridotite, orthogneiss- and continental eclogite-derived melts with 23% and 28% aggregated fractional melting can closely match the syn -exhumation and post-collisional mafic magmatic rocks, respectively. The reaction of the continental crust-derived melts with mantle peridotite would result in the heterogeneous orogenic lithospheric mantle. Therefore, the syn -exhumation and post-collisional mafic magmatic rocks provide a snapshot of the compositional variations in the orogenic lithospheric mantle and the crust-mantle interaction in the continental collision zone. • Syn-exhumation/post-collisional mafic magmatic rocks in the North Qiadam orogen. • They were derived from a heterogeneous orogenic lithospheric mantle. • The mantle source was metasomatized by melts from the subducted continental crust. • These rocks record crust-mantle interaction in the continental subduction channel.

The effects of source composition and melting conditions on the composition of syn-exhumation granites in collisional orogen

Partial melting of the deeply subducted continental crust during exhumation has been widely recognized recently in collisional orogens. However, it is little known about the effects of source composition and melting conditions on the composition of syn -exhumation granitic rocks. This is investigated by studying zircon and titanite U Pb geochronology and whole-rock geochemistry for syn -exhumation granites from the Weihai area in the Sulu orogen. The U Pb dating of relict zircon cores yields concordant Neoproterozoic and Late Triassic ages, respectively, for the meta-igneous protolith and granulite-facies metamorphism. Magmatic zircon rims give U Pb ages of 207 ± 1 to 214 ± 2 Ma for the formation of Weihai granites, in agreement with magmatic titanite ages of 209 ± 6 to 217 ± 5 Ma in the Late Triassic. The compositional zonation of titanite shows decreases of Nb/Ta and (La/Yb) N from core to rim, indicating the geochemical changes during granitic magmatism. In general, these granites show arc-like trace element distribution patterns and enriched Sr–Nd–Hf isotope compositions with an affinity to deeply subducted crustal slices from the South China Block. Compared with previously reported syn -exhumation granites from Lanshan in the southern Sulu orogen, the Weihai granites in the northeastern Sulu orogen exhibit different Sr-Nd-O isotope compositions. The Weihai granites have relatively low SiO 2 and Na 2 O contents but high Al 2 O 3 and K 2 O contents with high A/CNK and K/Na ratios. They contain high Ba and Sr but low Y with high Sr/Y and (La/Yb) N ratios. Both zircon and whole-rock of the Weihai granites exhibit no significant negative Eu anomalies. Based on the differences in geochemical features and the spatial-temporal distributions, it is inferred that the syn -exhumation granites in the Weihai area are derived from decompressional melting of the lower crustal slice at greater depth, whereas those in the Lanshan area are derived from decompressional melting of the upper crustal slice at relatively shallower depth. These differences indicate that both source compositions and melting conditions have exerted significant influences on the geochemical composition of syn -exhumation granites in the collisional orogen. This has bearing on the crust-mantle interaction in continental subduction channel through metasomatic reaction of the mantle wedge with these granitic melts produced during the exhumation. • The Weihai granites in the northern Sulu orogen indicate syn -exhumation magmatism. • The Weihai granites are characterized by relatively high Sr/Y and (La/Yb) N ratios. • Syn-exhumation granites in the Sulu orogen are distinct from south to north. • Differences in source compositions and melting conditions caused the compositional diversity of granites.