High Field Strength Elements Research Articles

Geochemistry and geochronology of Beiya giant Au polymetallic deposit: Implications for multistage ore-fluid pulses in a skarn system

Garnet is one of the most typical minerals in skarn deposits, and its texture and composition is a proxy for the changing physicochemical conditions. Thus, mineralogical and geochemical characterization of different garnet types can provide important information on the nature and evolution of ore-forming fluids. The Beiya gold-polymetallic deposit is China’s largest skarn-type Au deposit with Au reserves of >380 t. Although several studies have addressed the skarn formation mechanism and nature of fluid evolution, a systematic study on the changes in the composition and texture of garnet from wall rocks to ore-related intrusion has not yet been attempted. In this work, we carry out a systematic investigation along a horizonal profile in the open pit of the Beiya deposit. Our results show that the garnet in the profile belongs to grossular-andradite solid solution series with all grains showing well-developed fine oscillatory zoning. The garnets are of two types: the first type (Grt I) formed within the endoskarn, is reddish to red with euhedral-subhedral megacryst structure (>10 mm) and has altered surfaces. The second type (Grt II) occur in the proximal exoskarn, is red to brown–red, euhedral-subhedral, with medium- to fine- grained structure (0.3–4 mm) and displaying oscillatory zoning. Unlike Grt I, hydrous minerals occur in the pores among Grt II garnets, such as calcite, quartz, epidote and chlorite. Also, calcite-chlorite vein, quartz vein and quartz-chalcopyrite-pyrite vein cuts the Grt II garnets. Grt I is dominantly grossular (Adr14.88-43.58Grs55.25-82.87), depleted in the large ion lithophile elements (LILE), relatively enriched in high field strength elements (HFSEs) and enriched in heavy rare earth elements (HREEs) with negative Eu anomalies. Grt II is mostly andradite (Adr62.18-98.91Grs0-36.22), depleted in the LILE and HFSE, enriched in light rare earth elements (LREEs) with slightly positive Eu anomalies. Here we selected Grt II for in-situ LA-ICP-MS U-Pb dating, since Grt I contains negligible uranium. The results yielded a lower intercept 206Pb/238U age of 35.3 ± 1.8 Ma (MSWD = 0.68, n = 23). The calculated oxygen isotopic compositions of hydrothermal fluids in equilibrium with two types of garnets range from 6.46 ‰ to 8.56 ‰ (average 7.89 ‰), which are within the range of values for magmatic fluids. We infer that the physicochemical conditions of the hydrothermal fluid changed from relatively reduced, nearly neutral, and low water/rock ratios to slightly oxidized, slightly acidic, and high water/rock ratios. The chlorine content in the fluid increased during garnet growth. The fluctuating compositions of garnets in the skarn suggest multistage ore-fluid activities and the fluid evolution provide insights into the ore-forming process.

Exploring the orogenic nature of the Cariris Velhos event (ca. 1000-920 Ma) of the Borborema Province (NE Brazil)

The Cariris Velhos event (ca. 1000-920 Ma) is a significant marker in the early Neoproterozoic lithospheric record of the Borborema Province in NE Brazil. Despite its importance, the nature of this event remains unresolved having been linked to either continental rifting or accretionary-collisional orogeny. In this study, we support a compressive setting for the Cariris Velhos event based on compiled geochemical and isotopic data (Sm–Nd and Lu–Hf) from metagranitic rocks in the Alto Pajeú Terrane (central Borborema) and in the Pernambuco-Alagoas Super Terrane, as well as in the Riacho do Pontal and Sergipano fold belts (Southern Borborema). Geochemistry of these rocks share similarities with calcic to alkali-calcic and ferroan magmas, along with some Cordilleran magnesian members. They also correspond to meta-to peraluminous I- and S-type granitic rocks. Trace element distribution on spider diagrams reveals negative anomalies of HFSE (high field strength elements) such as Nb and Ta, which are typically associated with subduction-related settings. Additionally, chondrite normalized REE (rare-earth element) diagrams indicate mixing of both crustal and mantle material. Tectonic discriminant diagrams using elements with low mobility supports the hypothesis of a continental margin subduction-related setting. Calculated εNd and εHf values vary from negative to slightly positive, suggesting contributions from juvenile inputs and reworking of Meso-Paleoproterozoic crust, as indicated by the associated TDM model ages. Although the nature of the Cariris Velhos event requires further investigation, we suggest that, at least partially, it corresponded to an orogenic episode that might have occurred concurrently with continental break-up, similar to modern continental arc-retro arc foreland settings.

In situ U-Pb isotopic dating method on titanite, and application to determine REE-Fe-Cu mineralization age of the Sin Quyen deposit, Lao Cai province

Titanite (CaTiSiO5) is an important common accessory mineral in hydrothermal deposits, with appreciable amounts of U and Th incorporated into their structures for age dating. Titanite crystals are usually larger than zircon and monazite and may have zoning in texture and geochemistry which represent different parageneses likely related to multiple hydrothermal or mineralization events. In line with the development of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) that has high sensitivity and spatial resolution, in-situ U-Pb isotopic dating method on titanite has become a powerful tool to investigate the ages of different zones from a mineral. Titanite typically accommodates a significant amount of Rare Earth Elements (REEs) and High Field Strength Elements (HFSEs) and the titanite generated from different origins exhibits distinct trace element geochemistry. Therefore, the trace element geochemistry of titanite can serve as an indicator of its formation environment. Titanite occurred in different mineralization stages of the Sin Quyen deposit, making it a suitable mineral for investigating the timing of mineralization. In this paper, we present the U-Pb age of titanite from different stages of the Sin Quyen deposit used to constrain the timing and origin of such events. The hydrothermal titanites are dated by in situ LA-ICP-MS technique and have U-Pb ages of 873±12 Ma and 844±12 Ma which indicate two mineralization/hydrothermal events at the same time, respectively. These ages from the Sin Quyen deposit are well correlated with regional tectono-thermal events during the long-lived Neoproterozoic subduction in the western Yangtze Block.

Petrogenesis of the Wushi carbonatites in the northwestern Tarim Basin: Implications to deep carbon recycling

Carbonatites are rarely exposed on the Earth's surface, and most of them are supposed to have been mantle-derived. However, in the most recent years, carbonatites with pronounced crustal signatures had also been recognized. Although carbonatites have attracted the attention of many researchers, their genesis remains controversial. In this study, we present a systematic C–O–Sr–Nd–Mg isotopic investigation on the carbonatites exposed in Wushi County located at the northwestern margin of the Tarim Basin to determine their petrogenesis. The Wushi carbonatites, containing the xenoliths of wall rocks, intruded into Precambrian sandstones as dykes. Carbonatites predominantly comprise calcite and accessory minerals, such as dolomite, apatite, mica and quartz, and thus are classified as calciocarbonatites. Trace element geochemistry indicates that the total rare earth element (REE) concentrations in Wushi carbonatites range from 13.6 to 57.3 ppm, with an average of 30.9 ppm. The chondrite-normalized REE patterns of the test samples indicate that light rare earth elements (LREE) in the samples have been enriched relative to their heavy rare earth elements (HREE) and that the LREE/HREE ratios vary from 6.0 to 9.1. The patterns also show negative Eu anomalies (δEu = 0.65–0.75) and weak negative Ce anomalies (δCe = 0.75–0.99). The primitive mantle-normalized trace element spider diagrams display the enrichment of large ion lithophile elements (LILE), such as Pb, Sr and Ba, and the strong depletion of high field strength elements (HFSE), such as Nb, Ta, Zr, Hf and Ti. The δ18OV-SMOW (15.9‰–18.2‰) and δ13CV-PDB (−2.97‰–3.36‰) values are more consistent with those of crust-derived carbonatites than with mantle-derived carbonatites. The values of δ18OV-SMOW and δ13CV-PDB are consistent with the high values of 87Sr/86Sr (0.7096–0.7110) and low values of 143Nd/144Nd (0.5118–0.5120). Typically, δ26Mg of the carbonatites (−1.76‰ to −1.27‰) is much lighter than the δ26Mg of the mantle (−0.25‰ ± 0.04‰) but heavier than the δ26Mg of sedimentary carbonates (−2.72‰ to −2.15‰) obtained from the neighbouring Western Tianshan orogenic belt. The light Mg isotope anomalies cannot be explained using secondary alteration, continental crustal contamination, kinetic isotope fractionation and fractional crystallization. Thus, these anomalies could be due to the isotope exchange reactions occurring between the oceanic crust and sedimentary carbonates during subduction. In this study, we propose a two-stage model to explain Wushi carbonatite formation based on its isotope characteristics. During the early stage of carbonatite formation, sedimentary carbonate rocks located along the northern margin of the Tarim Craton and oceanic crust were subducted into the deep mantle. Isotope exchange and carbonation occurred during subduction to form carbonated eclogite. In the second stage of carbonatite formation, the heat produced by the Permian mantle plume in the Tarim Basin melted the carbonated eclogite to produce carbonatite melt. Thus, the petrogenesis of the Wushi carbonatites revealed by the presence of multiple isotopes indicates that the carbonatite produced by carbonated eclogite in the deep mantle would exhibit typical crustal isotopic characteristics and provide new insights into deep carbon recycling.

The Paleocene high-silica granites and their associated diorite enclaves in central Lhasa subterrane, Xizang, China: Insights to crust-mantle mixing and crustal reworking

The history of crustal growth in the western Central Lhasa (CL) subterranean during India-Eurasian collision remains unclear. The crucial problem is lack of suitable and ideal magmatism recorded information of the early stages of continental collision. Here, we report the newly discovered high-silica granites (K-feldspar granite with aplite granite) hosting diorite enclaves from the Geji area in western CL subterrane. This suite of magmatic rocks is of great significance to reveal the crustal growth of CL during the India-Eurasia collision. Zircon U-Pb dating results show that the K-feldspar granite, aplite granite, and diorite enclaves were formed at 57.2–57.9 Ma, 58.7 Ma and 58.2 Ma, respectively. Petrology, mineralogy, and mineral chemistry of the plagioclase from the diorite enclave indicates that it was generated by magma mixing between mafic and felsic magmas. The mafic melt was sourced from partial melting of mantle wedge and metasomatized by subduction fluid and sediment melt. This has been supported by the positive zircon εHf(t) values (+0.6 to + 4.9), enrichment of large ion lithophile elements (Rb, K, Th, and U) and depletion of high field strength elements (Nb and Ta), and the high Th/Ce ratios of diorite enclaves. The K-feldspar granite also has positive zircon εHf(t) values (+1.3 to +6.2), arc magma geochemical features, and a lower crust Nb/Ta ratio (8.3). These indicate that the K-feldspar granite was originated from the juvenile lower crust. In conclusion, we think that the crust-mantle magma mixing has a substantial impact on the crustal growth of the western CL subterrane during the subduction slab roll-back of the Paleocene Neo-Tethys Ocean when paired with other research and the context of regional tectonic development.

The search for plausible economic mineral deposits in the central parts of Tanzania; insight from stream sediment geochemistry, multivariate statistics and geostatistics

Exploration success relies heavily on the data obtained, but, significantly on the type of analytical methods deployed and the interpretation reached. A poorly analyzed data may obscure the true reflectivity of the data, and thus, compromised the decision made. A combined data processing approach of descriptive statistics, enrichment-depletion data normalization, geospatial elemental distribution, and stacked overlayed comparison of elements have been used in this study. The prime purpose was to demonstrate potential elemental anomalies, and predict areas of higher degree of confidence for subsequent exploration and mineral resource evaluation. One-hundred and sixty-six stream sediment samples from the Dodoma Region of the Tanzania Craton have been examined; to reveal potential elements or mineral commodity that warrant further exploration. Forty-three elements of target were examined, as this craton is globally known for its rich earth mineral commodity. Our result indicates an enrichment of transition metals (TMs) (Cu, Ni, Co, Cr, Mn and Zn), High Field Strength Elements (Y, Th, U, Zr, Nb, Hf, Ta and Pb), Large Ion Lithophile Elements (Ba and Rb) and Rare-Earth Elements (La and Ce), Platinum Group Element (Pd and Pt) and other metals (Au, As, Bi, W, Mo and Li).Obtained results point to a likely poly-metallic sources and processes; as the underlain geology is marked largely by pegmatite and migmatites, and moderate proportion of fine clastic sedimentary rocks, and minor volcanic rocks mostly to the northern domain. Theoretically, the Large Ion Lithophile Elements (LILEs), Rare-Earth Elements (REEs) and Platinum Group Elements (PGEs) are associated with felsic rocks or variable stages of plutonic granitization. Although, the TMs are often associated with mafic-ultramafic rocks, the linkage of such metals with organic-rich shales been reported elsewhere. These rocks may equally contribute to the occurrence of other metals as stated in this paper. Its intriguing to note a strong positive correlation of Li with TMs, possibility of Li control by mafic minerals in pegmatite bodies. This work proposes a polymetallic enrichment controlled by the area geology. To suggest an alluvial mining potential of the above elements in the area, resource evaluation is a requirement. The geospatial maps reveal areas worth focusing for subsequent exploration. The adopted geostatistical methods and other approach utilized in this research are effective, indicative of handling bulk exploration data for decision and subsequent exploration.

High-precision zirconium isotope analysis of Pacific seawater reveals large mass-dependent fractionations in the ocean

Zirconium (Zr) stable isotopes recently emerged as potential tracers of magmatic processes and, as a result, their behavior in high-temperature environments have been the focus of extensive characterization. In contrast, few studies have focused on Zr behavior and isotopic fractionation in low temperature or aqueous environments. Here, we describe a new analytical routine for highly precise and accurate analysis of Zr isotopes of water samples, using a combination of double-spike and iron co-precipitation methods. To assess the impact of potential systematic biases a series of experiments were conducted on natural and synthetic water samples. Our results show that the spike-to-sample ratio, matrix composition, and high field-strength element (HFSE) concentration have negligible effects on measured seawater Zr isotopic compositions, and that the Fe co-precipitation method used yields accurate and precise Zr isotope data. We thus apply this method to natural seawater samples collected from a water column profile in the Pacific Ocean off the coast of California, with depths ranging from 5 to 711 m. We find that the natural seawater samples are highly fractionated relative to solid-Earth values and display marked variability in δ94/90Zr as a function of depth, ranging from ∼ +0.650 ‰ near the surface, to + 1.530 ‰ near the profile bottom, with an analytical uncertainty of ± ∼0.045 ‰ (2 SE, external reproducibility). The δ94/90Zr value of seawater is much higher than that of Earth’s mantle and continental crust, which has a δ94/90Zr value near zero, indicating the presence of processes in the hydrosphere capable of inducing large mass-dependent fractionation. Furthermore, the seawater δ94/90Zr value exhibits systematic variations with respect to water depth and salinity, suggesting that Zr isotopic compositions may be sensitive to seawater chemical properties and source highlighting its potential utility as a tracer of biogeochemical processes within the ocean.

Subduction initiation of Paleo-Pacific Ocean and final closure of Paleo-Asian Ocean: constraints from Triassic granitoids in Jiamusi Block, NE China

ABSTRACT The process of transformation and superposition from the Paleo-Asian Ocean to the Paleo-Pacific Ocean tectonic systems have been a critical scientific issue. However, the final closure of the Paleo-Asian Ocean and the initial subduction time of the Paleo-Pacific Ocean, together with the conversion between them in Northeast China, still remain controversial. The Triassic granites in the Jiamusi Block are located in the key position of Northeast China, recording the conversion process of two tectonic domains. Through systematic geochronology and geochemical studies, it can provide important constraints for revealing the superposition and conversion mechanism of the Paleo-Asian Ocean and the Paleo-Pacific Ocean and the early subduction history of the Paleo-Pacific Ocean plate. In this study, the granites in Jiamusi Block were formed in the Early–Middle Triassic (250–245 Ma), characterized by high Si, rich alkali, poor Mg, and are obviously enriched in light rare earth elements and large ion lithophile elements, depleting in high field strength elements such as Nb, Ta, and Ti. They have typical characteristics of continental magmatic arc, deriving from partial melting of amphibolite facies and garnet amphibolite facies mafic lower crust. Combined with zircon Hf isotope results, zircon εHf(t) values are −6.52 to + 9.79, and the two-stage Hf model ages are 0.6–1.7 Ga, indicating that the magma should be derived from a relative younger crustal source, and some ancient materials may be added. Moreover, based on the spatial-temporal framework and Hf isotope characteristics of magmatism in Northeast China, we believe that the Jiamusi Block belonged to an exotic block. Meanwhile, combined with geochronological and geochemical data of igneous rocks on the Jiamusi Block, we suggest that the evolution of Jiamusi Block was dominated by the Paleo-Pacific Ocean rather than Paleo-Asian Ocean during the Triassic, which means the subduction of Paleo-Pacific Ocean plate initiated at least in Early Triassic.

Petrogenesis, geochronology and thermochronology of Oligocene to Miocene Western Anatolia granitoid plutons in Turkey

Western Anatolia (WA) hosts numerous I-and S-type plutons with variable compositions. This paper presents new comprehensive geology, petrography, bulk-rock and mineral chemistry, SrNd isotope, zircon UPb, biotite and hornblende Ar/Ar and first apatite UTh / He age data for seven plutons (Baklan, Evciler, Eybek, Ilıca-Şamlı, Kozak, Uludağ, Yenice) in WA. Western Anatolia granitoid plutons (WAGPs) are mainly granodiorite, with minor monzonite, diorite, quartz monzonite and granite. The dominant minerals are K-feldspar (usually orthoclase and rarely microcline), plagioclase (albite, oligoclase), amphibole (hornblende), biotite, quartz, and accessory minerals are titanite, zircon, apatite, and opaques. The crystallization temperatures are relatively uniform (640-740 °C) but pressures range from those typical of the upper crust to those typical of the mid crust. The plutons are characterized by moderate to high SiO2 (58.1–73.7 wt%), K2O (2.1–6.9 wt%), and relatively low P2O5 (0.03–0.60 wt%) and most are I-type and metaluminous, except for the Uludağ pluton that is S-type. They are enriched in large ion lithophile elements (LILEs e.g. U, Th, Rb, and K), light rare earth elements (LREEs) and Pb, and depleted in heavy rare earth elements (HREEs), high field strength elements (HFSE, e.g. Nb, Ti) and Sr, Ba and P relative to other LILEs. The plutons display small Eu anomalies (Eu/Eu*: 0.17–1.15) due to probaly suppression of feldspar fractionation/or partial melting of lower crust. They have relatively high initial 87Sr/86Sr = 0.705775–0.708357 and low initial 143Nd/144Nd = 0.512290–0.512581. The LA-ICP-MS 206Pb238U age analyses of zircon in addition to 40Ar/39Ar dating of hornblendes and biotites suggest that the plutons were emplaced between 29.4 Ma and 19.8 Ma and cooled with a relatively high rate (∼363 °C/Ma). These ages correlate with the collision between the Sakarya Zone and Anatolide–Tauride Platform in Rupelian to Burdigalian time. The (UTh)/He apatite ages range from 19.8 Ma to 7.6 Ma, for the WA granitoid plutons and indicates that the crustal terrains hosting the seven plutons had complex exhumation histories within their arc environment. Consequently, these ages constrain the crystallization, cooling and exumation times of the Oligocene to Miocene plutons in a post-collisional setting on the northern margin of Gondwana.

Petrogenesis and tectonic implication of the Triassic syenitic complexes in the eastern Qinling orogen, central China

Alkaline igneous rocks usually provide significant information on the mantle-derived components, magmatic processes and crust–mantle interaction due to their deep source. The Songxian syenite, located in the Xiong'er terrane of the Qinling Orogen, is one of the most typical alkaline intrusions in the Early Mesozoic. Geochemical, geochronological and in situ Sr–Nd–Hf isotopic analyses were conducted on the alkali-feldspar syenite and aegirine-augite syenite samples collected from the Songxian syenite, in order to reveal the magma sources, petrogenesis and possible geodynamic implications. All the rock samples are enriched in alkali (Na2O + K2O = 11.74 to 15.14 wt%), light rare earth elements and large ion lithophile elements, and depleted in high field strength elements. Zircon LA–ICP–MS U–Pb dating of medium- and fine-grained alkali-feldspar syenites yield emplacement ages of 235.5 ± 0.9 Ma and 232.7 ± 3.3 Ma, respectively. In addition, the aegirine-augite syenite generally developed as dyke or vein cutting through the two types of alkali-feldspar syenite in the field, indicating multiple pulses of magmatism. In-situ Sr-Nd isotopic analyses on magmatic apatite yielded (87Sr/86Sr)i ratios from 0.707959 to 0.710414 and εNd(t) from −19.3 to −13.9 for alkali-feldspar syenite, while the analyses for aegirine-augite syenite yielded (87Sr/86Sr)i ratios from 0.717948 to 0.721470 and εNd(t) from −25.5 to −24.5. The difference suggests that the Songxian syenite may be generated from multi-stage magmatic episodes. The first magmatic incident mainly originated from the partial melting of the enriched lithospheric mantle, mixed with the lower crust-derived melts. The second mixing magma might involve more proportion of Archean to Paleoproterozoic crust-derived materials. During the Early Mesozoic, the Mian-Lue oceanic slab subducted northward to the asthenosphere mantle and induced asthenosphere upwelling. Under the tectonic setting, the mixture of enriched mantle- and crust-derived melts occurred to form the Songxian syenite.

Late Cryogenian Circum-Rodinia Syn-Subduction Extension: Insights from Highly Fractionated S-Type and A-Type Granitoids in the Northern Tarim Craton

Late Cryogenian–Ediacaran magmatism represents the latest Precambrian tectonothermal event in the Tarim Craton. However, its geodynamic setting and geological significance are controversial. Here, we report the geochronology, whole-rock geochemistry, and Sr-Nd-Hf isotopic compositions of newly identified late Cryogenian A-type and highly fractionated S-type granites from two locations in the northern Tarim Craton. LA-ICP-MS zircon U-Pb analyses yield ages of 642 ± 7 Ma for a syenogranite and 643 ± 4.5 Ma for a mylonitized granite. The syenogranite is weakly peraluminous and shows an A-type granite affinity, as indicated by its high K2O + Na2O contents (8.35–8.64 wt.%), high field strength elements (Zr + Nb + Ce + Y = 435.8 − 463.4 × 10−6), Ga/Al ratios (2.79−2.83), and zircon saturation temperatures (829–844 °C). In contrast, the mylonitized granite contains Al-oversaturated minerals (e.g., garnet) and has high a differentiation index (DI = of 98.9–99.4), with lower zircon saturation temperatures (786–792 °C); the samples display high SiO2 contents (72.99–74.00 wt.%) and A/CNK values (1.16–1.17) and low Nb/Ta and Zr/Hf ratios and are enriched in Rb and depleted in Ba, Sr, which all point to a highly fractionated S-type granite affinity. The granites are characterized by elevated large-ion lithosphere elements (LILEs) and flat high-field-strength elements (HFSEs) patterns, with deep Nb and Ta troughs and pronounced negative Eu anomalies (Eu/Eu* = 0.17–0.38). They show apparently negative εNd(t) values (−10.1 to −9.8 and −6.8 to −7.9, respectively) and εHf(t) values (−9.66 to −1.77 and −33.5 to −1.3, respectively) with Paleoproterozoic crustal model ages, indicating that they were mainly generated by the partial melting of mature crustal materials with a minor contribution from a mantle-derived magmatic source. By integrating with previously published geological, sedimentological, and structural data, we suggest that the granites formed due to a high-temperature gradient in a syn-subduction extensional setting that was probably induced by northward slab rollback of the Paleo-Asian Oceanic lithosphere. Our new data highlight an upper-plate extension in the northern Tarim Craton that constitutes the northern periphery of the Rodinia supercontinent. The linear distribution of late Cryogenian magmatic rocks provides critical evidence for the orogen strike extension of the terminal suture between the Tarim Craton and southwestern Altaids.

Influence of micro-scale factors in weathering and elements mobility: Evidence from a comparative study of granite and basalt weathering profiles across India

In an attempt to constrain the micro-scale factors during weathering, large number of samples from four weathering profiles developed on granite and basalt rocks under different climatic conditions from western (Gujarat, GJ) and central (Bundelkhand, BK) India are reported in this study and compared these profiles with profiles from north eastern (Arunachal Pradesh, Lesser Himalaya, AP) India. In granitic profiles, the chemical index of alteration (CIA) values ranges from 50 to 85 whereas 42–90 CIA values are observed for basalt profiles. This highlights the significance of climate conditions (i.e., precipitation) over lithology in controlling weathering advancement. The results of the mass-transfer coefficient (τ) of major oxides significantly vary among the three granite and basalt profiles. The trace elements distribution in weathering profiles seems to be strongly controlled by the degree of weathering and formation of secondary minerals. Regardless of the nature of parent rocks, HFSEs (high field strength elements) tend to be enriched in the upper portion of all the weathering profiles; whereas REEs (rare earth elements) display variable behavior in each profile. The principal component analysis (PCA) reveals that three components are accounted for 89.14% of the variance. PC1 is attributed to weathering intensity and formation of secondary minerals, PC2 can be related to the lithology of parent material, and PC3 might be a location-specific factors. The present study shows that the geochemical characteristics of weathering profiles cannot be explained by the influence of a single weathering agent rather, there seems to be a significant role of local (or micro-scale) factors that controls the weathering and soil formation.

Late Mesozoic tectonic affinity of the Tengchong Block and southward extension of the Bangong-Nujiang Suture: constraints from Early Cretaceous granitic rocks in western Yunnan, SW China

ABSTRACT The southward extension of the Bangong-Nujiang Suture and the late Mesozoic tectonic affinity of the Tengchong Block have been subject to debate, and the Early Cretaceous magmatism in the eastern Tengchong Block provides a crucial window to address these issues. This paper reports comprehensive petrographic, geochemical, geochronological, and isotopic data of Early Cretaceous granitic rocks from the eastern Tengchong Block. Results show that these granitic rocks consist of monzogranites and granodiorites, with zircon U-Pb ages of 120.4–113.9 Ma. These granitic rocks are characterized by enrichments in large ion lithophile elements (e.g. Rb, U, K) and light rare earth elements, but depleted in high field strength elements (e.g. Nb, Ta, P, Ti), and have negative apatite εNd(t) values (−10.4 to −6.7) and negative zircon εHf(t) values (−11.7 to −1.2). The molar [Al2O3/(CaO + K2O + Na2O)] (A/CNK) values and whole-rock zircon saturated temperatures of the studied monzogranites of 1.04–1.09 and 714–799°C, respectively, indicate that they have I-type granite affinity. The presence of biotite indicative of I-type granite and the observation that P2O5 decrease with increasing SiO2 further indicated that the monzogranites have affinities of I-type granites. The petrography, geochemical and isotopic signatures of the studied monzogranites indicate that they originated from partial melting of mafic lower crustal rocks. The Na-rich granodiorites have elevated Na2O/K2O ratios and Na2O contents of 1.19–2.04 and 3.12–3.47 wt.%, respectively. These granodiorites also have relatively high Mg# values and the transition metal element Cr of 53.3–54.5 and up to 100 ppm, respectively. According to these isotopic and geochemical features and the occurrence of magnesiohornblende, we propose that the Na-rich granodiorites were derived from partial melting of ancient basaltic lower crust, with certain inputs of mantle materials in the magma source. Considering these results as well as published data, we finally propose that the Tengchong Block was likely the southeastern extension of the Lhasa Block and that these Early Cretaceous granitic rocks were formed in the setting of the volcanic arc due to westward subduction of the Bangong-Nujiang Tethys oceanic lithosphere beneath the Tengchong Block.

The origin and significance of euhedral apatite crystals on conodonts

Crystal overgrowth on fossil remains is well-documented in the literature. Attention has specifically focused on bioapatite (i.e., an apatite of biochemical origin regardless of post-mortem changes) configurations, in order to decipher any possible relation to fossilization/diagenesis. This study investigates the Rare Earth Element (REE) and other High-Field-Strength Element (HFSE) composition of euhedral crystals formed on the surface of conodont elements compared with that of crystal-free surfaces. Euhedral crystals are by definition crystals characterized by sharp faces, developing solids that, for apatite, assume the form of hexagonal prisms, reflecting its crystal symmetry. Late Ordovician (Amorphognathus ordovicicus Zone) conodonts from two localities in Sardinia and the Carnic Alps (Italy) are herein investigated. Conodont elements reveal the occurrence of smooth surfaces and surfaces partially covered with euhedral crystals. Since euhedral crystals did not reasonably grow during the organism's lifetime, the REE and HFSE analysis can provide important insights into the crystal growth process. The experimental results indicated a substantial contribution of diagenetic imprinting for all the analyzed material, although more evident on euhedral crystals that are significantly enriched in middle and, subordinately, in heavy REE with respect to smooth surfaces. The positive correlations between La + Th vs log[ΣREE] and Ce + Th vs log[ΣREE] could support the hypothesis that the neoformed euhedral crystals grew also by depleting the pristine bioapatite of the conodont elements. Nevertheless, the occurrence of two types of apatite cannot be ruled out: euhedral crystals as neoformed products of diagenetic processes and smooth surfaces as remains of the pristine conodont bioapatite after diagenesis.

Intra−Neo-Tethyan subduction initiation inferred from the Indawgyi mafic rocks in the Central Ophiolite Belt, Myanmar

Geological evidence has demonstrated the presence of an intra−Neo-Tethyan subduction system during the Cretaceous. However, when and how this intra-oceanic subduction was initiated, especially for the eastern Neo-Tethys, are still not well constrained. Here we present geochemical and geochronological analyses of the Indawgyi mafic rocks from the Central Ophiolite Belt in the West Burma Block (Myanmar), which record early forearc spreading during the intra−Neo-Tethyan subduction initiation. Zircon U-Pb ages of gabbros indicate the ophiolitic crust formation at ca. 120 Ma. Gabbros show mid-oceanic-ridge basalt−like rare earth element patterns and depleted Sr-Nd-Pb isotopic compositions with negative anomalies of high field strength elements (e.g., Nb, Ta, Zr, and Hf), similar to forearc basalt characteristics. Basalts show more slab-derived component signatures than the gabbros and represent mantle wedge magmas most likely formed between forearc spreading and arc maturation. These data, together with regional geological records and geophysical observations, suggest that the Indawgyi gabbros were derived from an intra−Neo-Tethyan forearc setting during the early stage of subduction initiation. Considering the timing of supra-subduction zone ophiolites and metamorphic sole in the Indo-Burma Range, we propose that spontaneous subduction initiation and sinking of the eastern Neo-Tethyan lithosphere during the Early Cretaceous (ca. 120 Ma) led to formation of the Indawgyi forearc crust, whereas subsequent mature subduction resulted in the Middle Cretaceous (ca. 108‒90 Ma) arc magmatism in the West Burma Block. These findings confirm the double-subduction model of the Neo-Tethys Ocean and shed new light on the intra−Neo-Tethyan subduction initiation.

High-field-strength-element enriched arc rocks witness subducted mélange recycling in upwelling asthenosphere

Arc rocks, as the most common product of plate subduction factory, are generally characterized by high field strength element (HFSE) depletions. However, HFSE-enriched arc rocks are also identified, and their origin is still controversial. We conduct combined radiogenic and stable isotope analyses on two ocean island basalt (OIB)-associated HFSE-enriched mafic intrusions in the Yunnan-Burma region. Median incompatible trace element compositions argue for a binary mixing between OIB-like asthenosphere dominated mantle source and a complex arc-like component. The high radiogenic Pb [(206Pb/204Pb)i = 17.99–18.85, (207Pb/204Pb)i = 15.53–15.71 and (208Pb/204Pb)i = 38.16–39.20] and heavy Mg (δ26Mg = −0.115‰ to 0.357‰) isotopes could only be inherited from subducted marine sediments. Altered oceanic crust (often abbreviated to AOC) is also involved in the source as evidenced by light Li isotopes (δ7Li = −2.54‰ to −1.31‰). Therefore, we argue that this arc-like component is inherited from subducted mélange (i.e., bulk marine sediment and AOC) recycling, rather than the more common melt and/or fluid metasomatism. In summary, the OIB associated HFSEs-enriched arc rocks are products of and witnessed the subducted mélange recycling and its interaction with the upwelling asthenosphere.

Zircon U–Pb ages and O–Hf isotopes of Quaternary trachytes from the East Sea: Implications for the genesis of low-δ18O magmas

Quaternary intraplate magmatism formed several volcanic islands and seamounts, including Dokdo (DD), Ulleungdo (UD), Simheungtack (ST), Anyongbok, and Isabu in the southwest of the East Sea back-arc basin. In this study, we present whole-rock geochemical, zircon U–Pb age, and in situ O–Hf isotope data for the submerged volcanic rocks from DD, UD, and ST to provide new insights into the eruption timing of these volcanoes and constrain the magma evolution processes. All samples used in this study were trachytes and exhibited ferroan, alkalic, and metaluminous to weakly peraluminous characteristics. They showed light rare earth element (REE)-enriched patterns with (La/Yb)N ratios of 25.3–31.7 and mostly negative Eu anomalies in a chondrite-normalized REE plot. In addition, they were enriched in large-ion lithophile elements and high field strength elements; they exhibited positive Pb anomalies and strongly negative Ba, Sr, P, and Ti anomalies. The zircons yielded a weighted-mean 206Pb/238U age of 2.61, 0.348–0.704, and 2.76–2.94 Ma for the DD, UD, and ST trachytes, respectively. All zircons exhibited lower δ18O values than normal depleted mantle values, regardless of the crystallization age and spatial distribution of volcanoes. The δ18O values showed no correlation with U contents or Th/U ratios, indicating that the low δ18O signatures were of primary magmatic origin. The Hf isotopic compositions of the zircons were relatively heterogeneous but predominately characterized by positive εHf values. Binary O–Hf mixing modeling revealed that low-δ18O rocks with positive εHf values from the UD and ST volcanoes were derived from a hybrid source of recycled juvenile crustal materials with low-δ18O and positive εHf signatures and an enriched mantle source with normal δ18O and negative εHf values. The juvenile oceanic crust in the source was likely metasomatized by seawater at high temperatures prior to melting. In contrast, the felsic magma that formed the DD volcanoes may have assimilated with regional basement rocks (Triassic–Jurassic granitoids), resulting in increased δ18O values and decreased εHf values relative to those of the UD and ST volcanoes. Our study highlights the significant contribution of recycled oceanic crust materials to the generation of the Quaternary magmas.

Amphibole peridotite and hornblendite of the Schrisheimite Hill intrusion of the Revdinskiy massif (Ural Platinum Belt)

Research subject. We studied the concentric-zonal massif (≈400 × 400 m) of amphibole-rich ultramafic rocks composed of amphibole peridotite (schriesheimite and amphibole wehrlite) with olivinite schlieren, pyroxenite and hornblendite located among gabbro at the Schrisheimite Hill in the Revdinskiy massif of the Ural Platinum Belt. The scope is characteristic of both of amphibole peridotites, which are extremely rare in the massifs of the Ural Platinum Belt, and the associated amphibole-rich ultramafic rocks. Aim. To expand the understanding of aqueous ultramafic magmatism in Ural Platinum Belt massifs. Materials and methods. To determine the age, we selected monofractions of high-alumina amphibole (pargasite-magnesiohastingsite) from two samples. Amphibole is not significantly altered by the regional metamorphism. 40Ar/39Ar dating was carried out according to the method described in (Travin et al., 2009; Yudin et al., 2021). The argon isotope composition was measured using a Micromass 5400 mass spectrometer (IGM SB RAS). Results. Amphibole peridotite has a hypidiomorphic (subhedral) or poikilitic texture. It is typical of igneous rocks, which indicates the crystallization of amphibole peridotite was by H2O-saturated magmas. Schlieren segregations of fine-grained schriesheimites and fine-grained olivinites are among coarse-grained schriesheimites. Amphibole peridotite, pyroxenite and hornblendite are deficient in high field strength elements (HFSE) and enriched in large ion lithophile elements (LILE) (with contrast concentration of Cs and Rb), which is typical of supra-subduction igneous formations. We obtained a 40Ar/39Ar age of 437.2 ± 6.7 Ma for an amphibole from hornblendite, which differs significantly from the ages of hornblendite determined earlier. Conclusions. Our new data indicate that hornblendite dikes in different UPB massifs formed over a fairly long time interval from the Early Silurian to the Early Devonian. These data expand modern ideas about the timing and nature of ultramafic water magmatism in the massifs of the Ural Platinum Belt.

Petrogenesis and metallogenic implications of Eocene–Oligocene magmatism in the Yulong porphyry copper belt, eastern Tibet: a review and analysis of geochronological, geochemical, and Sr–Nd–Pb–Hf isotopic data

The Eocene Yulong porphyry copper belt (YPCB) in eastern Tibet contains more than 10 Mt of Cu metal, associated with subvolcanic to volcanic porphyry systems intruding Cenozoic sedimentary basins. Geochemical data analysis shows that all samples belonged to high-K calc-alkaline and shoshonite series with metaluminous–peraluminous characteristics, light rare earth elements and large ion lithophile elements enrichment, high field strength elements depletion, and share geochemical similarities to adakitic rocks. We have established a spatiotemporal evolution framework through geochronological data analysis, combined with calculated bulk zirconium saturation temperature, calculated crustal thickness and geophysical evidence. We identified three magmatic processes triggered by asthenospheric upwelling and thermal erosion of thickened crust: (1) ultrapotassic magma derived from metasomatized mantle; (2) coeval ultrapotassic melt that promoted the partial melting of eroded lower crust and the generation of high-K adakitic felsic melt; high-K and Mg# adakitic fertile magma was formed by interaction between the eroded sulfide-rich juvenile lower crust-derived melt and mantle peridotite; (3) high-K adakitic fertile magma was derived from sulfide-rich juvenile lower crust, triggered by injecting hydrous ultrapotassic magma into thinned lower crust. The difference between deposit size in the north/south section of Yulong belt may be caused by the temperature of magmatic source and the volume of coeval potassic–ultrapotassic magmatism. Our analysis of the YPCB suggests that magmatism after 175 Ma within Intermontane arc complex of the Canadian Cordillera has high potential for porphyry deposits.

An early Paleozoic accumulation-foundering cycle of ultramafic cumulates in the Harlik arc and its implications for continental crustal growth in the Altaids

Lower crustal ultramafic cumulates that are normally denser than the ambient mantle peridotites are prone to founder back into the convecting mantle, prompting the formation of continental Moho and andesitic crust at subduction zones. However, foundering events of ultramafic cumulates have not been widely reported in island arcs, including the Altaids, one of the largest accretionary orogens. The Altaids involved prominent continental crustal growth by the development of an archipelago in the Phanerozoic. But the scarcity of foundering events of ultramafic cumulates hinders a better understanding of the formation mechanism of andesitic crust in the Altaids. In this study, we report two dioritic plutons in the Harlik arc, southern Altaids, trying to identify the accumulation-foundering cycles of dense ultramafic cumulates. LA-ICP-MS dating revealed that the Miaoergou and Banfanggou plutons in the Harlik arc were intruded at ca. 445 ± 6 Ma and 423 ± 4 Ma, respectively. The homogeneous Miaoergou pluton is mainly composed of quartz diorites, they have a high- to medium-K calc-alkaline composition, and show enrichments in light rare earth elements (LREE) and large ion lithophile elements (LILE), but depletions in high field strength elements (HFSE, especially Nb and Ta) and heavy rare earth elements (HREE). They also have juvenile isotopic signatures (εNd(t) = +3.3 − +3.7, initial 87Sr/86Sr = 0.70399–0.70452), which together suggest that they were derived from fractional crystallization of the coeval hydrous basaltic magmas in an arc setting. The heterogenous Banfanggou pluton comprises a spectrum of lithologies from hornblende gabbro to diorite. The hornblende gabbros and diorites have a high- to medium-K calc-alkaline composition, and show enrichments in LREE and LILE, but depletions in HREE and HFSE. They all have juvenile isotopic compositions with εNd(t) ranging from +3.5 to +4.2, and (87Sr/86Sr)i from 0.70384 to 0.7047, respectively, indicating that their parental magmas were intruded in a subduction-related setting, and underwent fractional crystallization of clinopyroxene, plagioclase, amphibole and quartz. Both pulses of hydrous basaltic magmatism involved open-system conditions, with the development of mafic microgranular enclaves (MMEs) in the diorites and their mingling. In a synthesis of the published data, we found that the Harlik arc was continuously affected by subduction in the early Paleozoic, through extensive arc magmatism from ca. 452 to 440 Ma, the Harlik arc matured with the accumulation of thick ultramafic cumulates in its lower crust. The foundering of these ultramafic cumulates occurred later, which led to strong extension and related A-type granitic magmatism at ca. 436 Ma. This accumulation-foundering cycle of ultramafic cumulates facilitated the formation of andesitic composition and continental Moho in the Harlik arc, and contributed to the significant continental crustal growth in the Altaids.