UPb Ages Research Articles

In-situ U[sbnd]Pb geochronology of vesuvianite in skarn deposits

The timing of hydrothermal activity is a key factor in any predictive genetic model of skarn mineralization, but direct dating is often hindered by the absence of suitable minerals to date. Vesuvianite, a common mineral in skarn deposits, can incorporate U and has a high Pb closure temperature (>650 °C), and consequently may be suitable for dating for skarn deposits. Vesuvianite collected from nine Chinese skarn deposits, including the Jinchuantang SnBi, Shizhuyuan W–Sn–Mo–Bi polymetallic, Jiepai CuW, Dashunlong Sn, Xianghualing Sn, Furong Sn, and Huangshaping W–Sn–Mo–Cu polymetallic deposits in Nanling Range (Southern China), and the Kaerqueka and Saishitang Cu deposits in East Kunlun Orogen (Northwestern China), have high U concentrations (median 66.8 ppm for 460 LA-ICP-MS analysis spots). The flat and stable time-resolved signal spectra for U during LA-ICP-MS analysis, combined with the absence of nanoscale mineral inclusions, porosities, or lattice defects in the JCT vesuvianite, with high and consistent U concentrations, revealed by the HAADF-STEM images, and the marked positive relationships between U and REE collectively suggest that the U incorporation into vesuvianite was primarily controlled by replacement of Ca2+ in eight-fold coordination, confirming that vesuvianite may be a suitable UPb geochronometer. The JCT vesuvianite is characterized by high and consistent U concentrations (260–526 ppm, median 307 ppm) and homogeneous internal texture and elemental distribution, and yields an ID-TIMS UPb concordia age (2σ, 153.9 ± 0.2 Ma, MSWD = 1.6, n = 11), demonstrating that it can be taken as a potential LA-ICP-MS UPb reference material, which can be freely distributed to other laboratories. Using the JCT vesuvianite as a UPb reference material, the Kaerqueka, Saishitang, Jiepai, Dashunlong, Xianghualing, Furong, and Shizhuyuan vesuvianite yielded well-defined LA-ICP-MS UPb ages (239.0–152.1 Ma, 2σ), consistent with previously published age constraints of mineralization and cogenetic igneous rocks. Vesuvianite from the Huangshaping W–Sn–Mo–Cu polymetallic deposit, yielded UPb ages of 157.3 ± 1.4 Ma (2σ, MSWD = 1.8) and 161.0 ± 1.3 Ma (2σ, MSWD = 1.9), providing reliable constraints on the overprinting sequence of the skarn W–Sn–Mo and Cu mineralization events at Huangshaping, which was not previously detectable using other mineralization chronometers. Combined, these ages suggest that vesuvianite is a robust and reliable geochronometer, and offers a new tool for dating skarn deposits.

Homogenization of zircon Hf isotopes during late-stage granite crystallization

Zircon Hf isotope compositions provide valuable constraints on granite petrogenesis and the evolution of continental crust. However, the causes of variations in zircon Hf isotope ratios during magma evolution are poorly understood. Here we compiled zircon UPb ages and LuHf isotope compositions of the Permian-Triassic granites in South China to demonstrate that Hf isotope variability is common in the early stages of granitic magma evolution, but Hf isotope ratios are homogenized during the final stages of granite crystallization. Late-grown zircons have relatively high concentrations of U and have εHf(t) values approximating the median values of multiple analyses for the low-U zircon in the same samples. Hafnium isotope variations measured in low-U zircon could be related to incremental assembly of multiple isotopically distinct magma batches derived from partial melting of heterogeneous crustal rocks; these discrete magma batches may have remained isolated from each other during segregation and extraction in the source and during ascent through the crust. After emplacement, these magma batches may have experienced variable degrees of mixing resulting in homogenization of Hf isotopes in late crystallized zircon. Coupled with whole-rock geochemical data of the Permian-Triassic granites, we argue that the Hf isotope homogenization is related to crystallization of zircon from late-stage magmas or precipitation of hydrothermal zircon from exsolved hydrothermal fluids during the final stages of magma solidification. Hafnium isotope homogenization in late-stage melts/fluids may have been driven by an increased concentration of fluorine and the efficient diffusion of hafnium-fluoride complexes through silicic melt. These results provide a deeper understanding of the mechanisms that contribute to Hf isotope heterogeneity in zircon during granite crystallization.

Pleistocene northward thrusting of the Danghe Nanshan: Implications for the growth of the Qilian Shan, northeastern Tibetan Plateau

Understanding the tectonic history of the Qilian Shan provides significant insights into the growth pattern of the northern Tibetan Plateau. Due to the limitation of methodology and lack of detailed geological information over remote mountain ranges, there has been less information on the Pleistocene evolution of the southern Qilian Shan. Existing models for the Qilian Shan growth also fail to specify whether the southern Qilian Shan grew northward or southward during the late Cenozoic. Here we present provenance data on fieldwork paleocurrents, petrography, heavy mineral analysis, and zircon UPb ages from the Danghe valley between Danghe Nanshan and Yema Nanshan, southern Qilian Shan. Results show a ‘seesaw-like’ change of provenance from Yema Nanshan to the north during the early Pleistocene to Danghe Nanshan to the south, which suggests the northward thrusting of Danghe Nanshan. Integrated with previous studies, our inference of a Quaternary rapid exhumation of the southern Qilian Shan reinforces the argument of northward propagation of the southern Qilian Shan during the Pleistocene and suggests a complex growth pattern of the Qilian Shan, northeastern Tibetan Plateau.

Petrogenesis of early cretaceous intermediate to felsic rocks in Shanghai, South China: Magmatic response to Paleo-Pacific plate subduction

Voluminous magmatism and extensional deformation are the prominent late Mesozoic tectonic features in East Asia, which recorded the evolutionary history of the continental marginal lithosphere. Due to the thick covering of Quaternary and Pliocene sediments in the region, the Early Cretaceous magmatism in northeastern South China is not well constrained. In this study, we collected samples from the boreholes in Shanghai, East China and provided geochronological, elemental and isotopic analyses of andesite and granodiorite. Zircon UPb age results indicate that andesites and granodiorites were formed at ca. 135 to 102 Ma. The andesites exhibit moderate MgO contents, slightly radiogenic Sr (87Sr/86Sr(t) = 0.706149 to 0.706170) and variable unradiogenic Nd and Hf isotopic compositions (εNd(t) = −5.32 to −5.12; εHf(t) = −10.8 to −0.6). The granodiorites exhibit arc-like trace-element patterns with high Sr/Y ratios, as well as slightly radiogenic initial Sr (87Sr/86Sr(t) = 0.706754 to 0.706790) and moderately unradiogenic Nd and Hf isotopic compositions (εNd(t) = −6.54 to −6.27; εHf(t) = − 4.0 to −6.7). Both andesites and granodiorites exhibit similar initial Pb isotopic compositions (andesites: 206Pb/204Pb = 18.232–18.259, 207Pb/204Pb = 15.547–15.548, and 208Pb/204Pb = 38.338–38.350; granodiorites: 206Pb/204Pb = 17.930–17.946, 207Pb/204Pb = 15.542–15.543, and 208Pb/204Pb (38.256–38.293). The results recommend that the ca.130 Ma andesites were produced by the partial melting of an enriched lithospheric mantle, which was previously metasomatized with sediments in the source region in a continental arc setting of the Paleo-Pacific subduction. The ca.102 Ma adakitic granodiorites probably originated from partial melting of the lower crust with the addition of mantle-derived magmas. With the consolidation of previous studies, we argue that the flat slab subduction of the paleo-Pacific Plate led to the formation of these intermediate to felsic magmatism.

(Mis)Identification of magmatic and exhumation ages by detrital zircon U[sbnd]Pb and He double dating: A case study from the Bergell-Gonfolite system (European Alps)

Reliable interpretation of detrital thermochronometric datasets requires correct attribution of these ages as either the record of exhumational cooling or the record of post-magmatic cooling independent of tectonic or erosional exhumation. A classic approach for identifying magmatic cooling ages is through double dating leveraging paired high- and low-temperature geo/thermochronologic systems, which should yield, within error, indistinguishable ages from the same grain. On the contrary, low-temperature thermochronometric ages that are younger than their corresponding crystallization ages are mostly invariably interpreted to record exhumation. Here, we test this last assumption by applying a detrital zircon UPb and (UTh)/He double-dating approach to a well-constrained source-to-sink system in the southern European Alps, archiving the progressive unroofing of the Bergell-Novate volcanic-plutonic complex and associated country rocks. We depth-profile UPb dated unpolished detrital zircon grains and performed (UTh)/He analysis on non-volcanic/plutonic grains. Of the double-dated grains, 40% yielded (UTh)/He ages overlapped in age with Bergell-Novate magmatism (32–30 and 27–24 Ma). Despite yielding (UTh)/He ages younger than their corresponding UPb ages, these ages record post-magmatic cooling within the contact aureole and not exhumation. Our findings indicate that only a fraction of the grains yielding magmatic He ages can be identified by double dating, and that the assumption that all the remaining grains constrain exhumation can be potentially misleading. Finally, we present improved criteria for the interpretation of detrital zircon thermochronometric double-dating results and conclude that many previous interpretations based on a classic double-dating approach should be reconsidered in syn-magmatic orogenic systems.

New constraints on volcanism during Ordovician-Silurian transition: Insights from marine bentonites in northern Yili Block (NW China)

The Yili Block is located at the Chinese western Tianshan and is a constituent of the Kazakhstan microcontinent. It played an important role in the evolution of the SW Central Asian Orogenic Belt (CAOB). The Yili Block underwent the Paleozoic orogeny during the subduction and closure of the Junggar Ocean. Late Paleozoic arc-type magmatic rocks have been widely recorded in the Yili Block; however, it remains poorly constrained when and how the Junggar Ocean started to subduct beneath the Yili Block. In this study, we report new results of zircon LA-ICP-MS UPb dating and LuHf isotopic analysis on a series of bentonite beds in tightly folded upper Ordovician siliceous black shales of the Guozigou Section in the northern Yili Block. The occurrence of bentonite layers and associated sulfides within graptolite-bearing siliceous black shales indicates multiple marine volcanic eruptions. Magmatic zircons separated from seven representative bentonite samples yielded consistent UPb ages ranging from 449.5 ± 2.4 Ma to 444.8 ± 2.0 Ma (Katian). In situ zircon LuHf isotopic analyses show mostly positive εHf(t) values (−0.01 and 1.31 to 11.7) and Neoproterozoic single-stage Hf model ages (TDM1 = 0.6– 1.0 Ga). A linear correlation between the εHf(t) and TDM1 suggests that these zircons likely crystallized from a common magma originated from a depleted mantle source with variable involvement of Precambrian continental crust. Our new zircon ages and Hf isotopic data are comparable with those of the Middle-Late Ordovician continental arc-type magmatic rocks in the nearby Wenquan area, both indicating that the subduction of the Junggar Ocean beneath the northern Yili Block started in the Middle-Late Ordovician, and are therefore important for early Paleozoic paleogeographic and tectonic reconstruction of the Paleo-Asian Ocean domain. In addition, the Late Ordovician marine bentonites and black shales coincide with the worldwide intensive volcanic activities and related climate/environment changes during late Katian to Hirnantian. Thus, these new data also provide further arguments for possible causes of the global climate change and mass extinction during the Ordovician-Silurian transition.

Mobilization and fractionation of HFSE and REE by high fluorine fluid of magmatic origin during the alteration of amphibolite

High field strength elements (HFSE) and rare earth elements (REE) are normally immobile in hydrothermal fluids whereas experimental studies indicate some anionic ligands can significantly enhance their solubility in hydrothermal fluids. However, the role of fluorine (F) in hydrothermal fluids revealed by experiments and natural samples is different or even contradictory, especially in the mechanism of elements mobilization and fractionation. Here, we report a field-based geochemical study of amphibolites with extremely high Nb/Ta ratios (up to 98.7) and enriched HREE concentrations. The Paleoproterozoic amphibolites are intruded by a granitic dike with zircon UPb age of 132 ± 3 Ma. Felsic veinlets are observed in thin-sections of the amphibolite. Biotites with high F contents occur as aggregates along the boundary between the felsic veinlets and amphiboles, which indicates they are formed by reaction between amphibole and the granitic melt. The amphiboles have core-mantle-rim structure with high F actinolitic mantle surrounding low F magnesio-hastingsitic core. Some actinolites intergrow with biotites, titanites and magnetites. These evidences indicate that the actinolites are produced by hydrothermal metasomatism or interface-coupled dissolution–precipitation. The metasomatism occurred at 0.3–0.9 kbar and 500–650 °C within the hornfels facies. Hydrothermal titanites from amphibolites yield a lower intercept age of 131 ± 4.3 Ma, which is consistent with the crystallization age of the granitic dike. These observations indicate that the fluids are derived from fractional crystallization of a felsic magma for diking, and the released fluids are channeled along the interface between dike and amphibolite. The metasomatic minerals show high F and no Cl contents, indicating the hydrothermal fluids are high in fluorine (0.04–0.12 wt%) and low in chlorine. The F contents of minerals negatively correlate with HFSE, REE contents and (La/Yb)N ratios, but positively with Nb/Ta ratios. These reveal the progressive leaching and fractionation of HFSE and REE by the hydrothermal fluids. Mass balance estimates that most trace elements lose 20–60% during the hydrothermal alternation of amphibole. The calculation results also reveal that more Ta and LREE are lost than Nb and HREE, indicating Ta and LREE are more soluble than Nb and HREE, respectively, in the hydrothermal fluids. All these observations suggest that HFSE and REE are not only mobilized but also fractionated because of different degrees of mobilization by fluoride-bearing fluids.

Reworking of continental crust on northeastern North China Craton: Evidence from geochronology and geochemistry of Early Cretaceous granitic rocks

This paper reports new whole–rock major– and trace–element and zircon UPb and HfO isotopic data for Early Cretaceous granitic rocks on Liaodong Peninsula, NE China, with the aim of constraining continental reworking in the northeastern North China Craton. On the basis of SiO2, MgO, and Sr contents and Sr/Y values, the rocks can be divided into three groups. The low–Mg adakitic rocks show zircon UPb ages of (134–120 Ma), and high SiO2 (67.11–71.14 wt%) and low MgO (0.53–0.62 wt%) contents, and high Sr (531–1108 ppm) contents, Sr/Y (29.3–132) ratios, with enriched Sr–Nd–Hf–O isotopic compositions, indicating an origin of thickened lower mafic crust. The high–Mg adakitic rocks show ages of (126–114 Ma) and are characterized by low SiO2 (53.11–63.88 wt%) and high MgO (2.07–8.93 wt%) contents, and high Sr (391–1041 ppm) contents, Sr/Y (23.9–64.1) ratios, with relatively depleted Sr–Nd–Hf–O isotopic compositions, suggesting that they were derived from partial melting of delaminated lower mafic crust. The normal granitic rocks show ages of (127–121 Ma) and high SiO2 (71.65–77.22 wt%) and low MgO (0.10–0.51 wt%) and Sr (12.1–309 ppm) contents, Sr/Y (0.89–24.5) ratios, with relatively depleted Sr–Nd–Hf–O isotopic compositions, suggesting an origin from lower mafic crust with normal crustal thickness. These constraints, combined with the regional occurrence of A–type granites, bimodal igneous rocks, and metamorphic core complexes, as well as the regional distribution of Mesozoic granitic rocks displaying a younging trend from southeast to northwest for the Jurassic rocks but a reverse trend for the Cretaceous rocks, recorded the roll–back of the Paleo–Pacific plate towards the Eurasian continental margin.

Early Cretaceous subduction initiation of the proto-Caribbean plate: geochronological and geochemical evidence from gabbros of the Moa-Baracoa ophiolitic massif, Eastern Cuba

Ophiolites in Cuba are fragments of either the proto-Caribbean or Caribbean plate, however, neither the time of initial subduction nor the mechanism by which it was induced are well constrained. The Moa Baracoa ophiolitic massif, eastern Cuba, consists chiefly of harzburgite, layered gabbro, gabbro sills and dykes, and lavas. The gabbro sills and dykes yielded zircon UPb ages of 136.7 ± 1.8 Ma and 122.2 ± 0.5 Ma, respectively; in contrast, zircons from the layered gabbros yielded a wide range of ages (158 to 2556 Ma). The layered gabbros mostly show high MgO, Al2O3, and CaO, coupled with low TiO2, Na2O + K2O and incompatible elements, suggesting a cumulate origin. Their crystallization sequence (plagioclase before clinopyroxene), low plagioclase An values (69.8–73.5), and clinopyroxene Mg# values (80.4–81.6), are similar to those of mid-ocean ridge gabbros, however, modeled equilibrated melts based on clinopyroxene compositions have a forearc basalt geochemical affinity. In the gabbro sills, crystallization of clinopyroxene before plagioclase, high plagioclase An values (83.7–88.5), high clinopyroxene Mg# values (85.1–89.5), and the presence of magmatic amphibole require hydrous magmas from a supra-subduction zone. The gabbro dykes have moderate plagioclase An values (76.1–80.5) and clinopyroxene Mg# values (83.2–88.1), and modeled melts of the gabbro sills and dykes also show subduction-related geochemical features. High positive whole-rock εNd(t) (+8.48 to +10.81) and low (87Sr/86Sr)i ratios (0.702500–0.703645), coupled with high positive εHf(t)(zircon) (+12.0 to +17.7) suggest that all the Moa-Baracoa gabbros were derived from a depleted source. However, strongly heterogenous δ18O(zircon) values (0.41–6.32‰) in the gabbro sills and dykes indicate addition of a subduction-related fluid. Our data suggest that the Moa-Baracoa ophiolitic massif originated in a forearc environment in the early Cretaceous during subduction initiation. Combined with the regional geological background, we propose that subduction of the proto-Caribbean plate was initiated before 137 Ma. Thus, the zircon in the layered gabbros (δ18O = 6.28 to 8.39‰, εHf(t) = −1.94 to −9.10) must be detrital grains derived from subducted crustal material. The youngest zircon in these rocks implies that some of the crustal material may have been recycled in as little as 20 Myr.

Compositional signatures of ophiolitic rocks from the Dongco massif: Novel insights into the evolution of the central Tibetan Meso-Tethyan oceanic plateau

The role of the Meso-Tethyan oceanic plateau in the growth of the Tibetan Plateau is debated. In this paper, we report coexistence of large-scale mid-ocean ridge basalt (MORB)-like and enormous ocean island basalt (OIB)-type mafic rocks from the Dongco ophiolitic rocks of the Bangong–Nujiang Suture Zone. The MORB-like rocks host tholeiitic geochemical characterizes, and have supra-subduction zone affinity such as enrichments in large ion lithophile elements (LILEs), depletions in high field strength elements, high εNd(t) values (+6.5 to +9.5) and low initial 87Sr/86Sr ratios (0.7033–0.7050). The OIB-type rocks have an alkaline affinity and are characterized by enrichments in LILEs and light rare earth elements, strongly negative Sr anomalies, variable εNd(t) values of −3.7 to +7.3, and low initial 87Sr/86Sr ratios ranging from 0.7034 to 0.7050. Metamorphic zircons from amphibolites in the sole of the ophiolitic rocks yielded UPb ages of 104–100 Ma. Detrital zircons from the upper Cretaceous sandstone of the Abushan Formation (K2ab) yielded young UPb ages of 100–86 Ma. Considering the regional geology, we suggest that the Dongco MORB-like rocks were derived from a depleted mantle source and formed from an intra-oceanic subduction-related setting, representing remnants of normal oceanic crust, whereas the OIB-type rocks were derived from an enriched mantle source representing remnants of a Meso-Tethyan oceanic plateau. Our preferred hypothesis is that the Meso-Tethyan oceanic plateau experienced a phase of accretion at 104–100 Ma, which probably increased local subduction erosion of the upper plate and providing provenance for the deposition of the K2ab sandstone. The maximum depositional ages of K2ab sandstone constraint the minimum ages for closure of the Bangong–Nujiang Tethys Ocean most likely at 100–86 Ma.

S-type like granites and felsic volcanic rocks in the Mahabad area, NW Iran: Late Neoproterozoic extensional tectonics follow collision on the northern boundary of Gondwana

The Late Neoproterozoic basement is exposed in the Mahabad area of northwest Iran, and it is classified into three main groups. The first is metamorphic rocks that include gneiss, schist, phyllite, and slate. The second is sedimentary rocks, which comprise shale, sandstone, and dolomite and the third is igneous rocks including granite, rhyolite, felsic tuff, and ignimbrite, which are the subject of this research. Zircon UPb ages for three igneous samples show that crystallization occurred at 558–552 Ma in the late Neoproterozoic (Ediacaran). The felsic magmatic rocks have SiO2 contents of 64.3 to 78.9 wt%, Al2O3 contents of 12.0 to 16.8 wt%, low TiO2 contents (0.08 to 0.69 wt%), and large variations in K2O (0.05 to 5.46 wt%) and Na2O (1.17 to 6.70 wt%). The felsic magmatic rocks are enriched in light rare earth elements (REEs) such as La and Sm, have flat heavy REE patterns, and strong negative Eu anomalies. They are mainly weakly to strongly peraluminous and calc-alkalic to alkali-calcic rocks. A wide range of initial 87Sr/86Sr(556Ma) ratios (0.7056 to 0.7203) and negative εNd(t) values (−6.0 to −2.9) do not show clear trends of mixing/contamination among mantle, slab fluids, and/or altered oceanic crust. The Mahabad magmatic rocks were produced mainly by partial melting of graywacke, psammite, and shale at high temperatures (~800 °C), based on the zircon saturation index. S-type like granites usually form in syn- to post-collisional systems, but the Mahabad felsic rocks, which have almost the same ages as the associated sedimentary rocks, were generated in an extensional regime according to geochemical discrimination diagrams. This can be explained as follows: sediments were deeply buried during the late Neoproterozoic amalgamation of microcontinents at a subduction zone along the northern boundary of Gondwana, and then erosion of the roof and/or thinning owing to extension led to upwelling of the asthenosphere and intrusion of mafic magma, which was responsible for partially melting the buried metasedimentary rocks to generate S-type like granites and felsic volcanic rocks.

Ta-Nb mineralization in the shallow-level highly-evolved P-poor Shihuiyao granite, Northeast China

The Shihuiyao granite district in the southern Great Xing'an Range, Northeast China, is a newly discovered area of rare-metal-bearing granites (RMG). The latest Jurassic Shihuiyao granitic pluton, emplaced into the Lower Permian Linxi Formation, comprises porphyritic granite, leucogranite, and amazonite-bearing pegmatite, all enriched in rare metals (mainly Ta, Nb, Rb). The Shihuiyao granites are peraluminous (A/CNK = 1.02–1.27), characterized by high contents of SiO2 (69.4–78.0 wt%), Al2O3 (12.7–16.9 wt%) and total alkalis (6.8–12.8 wt%), enriched in Ta, Nb, Rb, Cs, and Sn, but depleted in Ba and Sr. They have strong negative Eu anomalies (Eu/Eu⁎ = 0.02–0.42) and show the M-type REE tetrad effect. Low Nb/Ta (<4) and Zr/Hf (<8) values indicate high degree of magmatic fractionation. Whole-rock and mineral compositions variations indicate that the Shihuiyao granites are a typical low pressure and P-poor RMG, which experienced intensive magmatic fractionation followed by intensive hydrothermal reworking. The narrow range of δ7Li values (−0.86‰ to 0.88‰) and Rayleigh distillation modeling suggest that the porphyritic granite and leucogranite represent products of the same magma source, but evolved independently at depth. LA-ICP-MS UPb dating of monazite from the porphyritic granite and leucogranite yield lower-intercept ages of 150.2 ± 0.6 Ma and 147.0 ± 1.7 Ma, respectively. The 40Ar39Ar plateau ages of mica from leucogranite (146.3 ± 1.1 Ma) and an ore-bearing quartz vein (145.6 ± 1.0 Ma), and the UPb age of columbite (146.7 ± 4.7 Ma) further identify this as a latest Jurassic magmatic-hydrothermal mineralization event in NE China. Rapid crystallization and chemical quenching of magma resulted in neutralization of the acid fluoride solution, and a loss of the fluoride ligand during fluid evolution. This further triggered the subsequent crystallization of NbTa oxides. As revealed from geochemical and textural characteristics of the columbite group minerals, the high-degree of fractional crystallization and Ta-rich hydrosilicate fluid metasomatism are the main factors leading to the enrichment of TaNb within Shihuiyao RMG. In general, we conclude that rare metal-hosted magmatism took place in two episodes in NE China, namely, (i) Latest Jurassic to Early Cretaceous (150–130 Ma) peraluminous to metaluminous Ta-Nb-Sn-W RMG, and (ii) Early Cretaceous (130–120 Ma) alkaline Nb-Ta-Be-REE-Zr RMG, which highlight the prospecting fertility for Nb-Ta-REE (SnW) mineralization in NE China.

Mineralogical and geochemical characteristics of tonsteins from the Middle Jurassic Yan'an Formation, Ordos Basin, North China

Although the Jurassic features a considerable accumulation of coal worldwide, only a few tonsteins within coal seams in this coal-forming period have been investigated. This paper presents the detailed mineralogical and geochemical compositions of six tonstein layers from the Middle Jurassic Yan'an Formation, Ordos Basin, North China. Based on the typical evidence of the distinct field characteristics occurring as paler thin beds with lateral continuity; conchoidal and flint-like fracture planes; sharp contacts with adjacent coals; and the mineralogical compositions, such as vermicular kaolinite, angular quartz, and euhedral zircon crystals with a single UPb age peak, the intra-seam clay beds of the Yan'an Formation were determined to have been mainly derived from a significant pyroclastic source. However, rounded quartz, muscovite, and detrital zircon grains in the tonsteins reflect minor contribution of terrigenous debris from sediment source regions. Moreover, a line of geochemical features including low TiO2/Al2O3 values, Al2O3/TiO2-Nb/Yb, and Al2O3/TiO2-Zr/TiO2 diagrams; low REY concentrations with high fractionation between light REE and heavy REY (REY, rare earth elements and Y; REE, rare earth elements); and pronounced negative Eu anomalies, suggest that the tonsteins have a felsic (rhyolitic) magma origin. A systematic geochemical analysis of enclosing coals of the tonsteins was also conducted to determine the leaching behavior of the trace elements from the tonsteins into the enclosing coals. The results show that Th, U, Nb, Ta, Zr, and Hf tend to be enriched in the overlying and underlying coals and have an enrichment width of up to 30 cm. Similarly, the elevated concentrations of Zn, Rb, Cs, Pb, La, Ce, Pr, and Nd only occur in a relatively narrow zone above and below the tonsteins. The width of the enrichment in the enclosing coals may be mainly related to the properties of the tonsteins, e.g., thickness and original volcanogenic material, the characters of elements including mobility and concentration, and the nature of coals, such as coal rank.

Petrogenesis of the granite related to the Baishaziling Sn deposit, Dayishan ore field, Southern China

The Baishaziling greisen-type tin deposit is located in the Dayishan ore field, Nanling Range, Southern China. In this study, for the first time, we present both zircon and cassiterite UPb dating, whole-rock elements, zircon LuHf and apatite Nd isotopic compositions to better constrain the petrogenesis of granite and its genetic link with Sn mineralization. Zircon UPb ages of fine-grained granite and coarse-grained granite are 154 ± 1.8 Ma and 153 ± 2.1 Ma, respectively, which are consistent with the cassiterite UPb dating of 154 ± 5.4 Ma, implying genetic relationship between the Baishaziling granite and tin metallogenesis. The Baishaziling granites exhibit high SiO2, K2O + Na2O, Zr + Nb + Ce + Y contents, low P2O5 and Sr contents, and high ratios of Ga/Al, TFeO/(TFeO + MgO), implying A-type granite affinity with characteristics of high-K calc-alkaline and weakly peraluminous. The zircon εHf(t) values and apatite εNd(t) values of the granite vary from −4.46 to −1.81 and −8.37 to −7.10, with two-stage Hf and Nd model ages of 1.40 to 1.50 Ga and 1.52 to 1.64 Ga, indicating that they were generated by the partial melt of the Proterozoic basement with the involvement of mantle magma. In addition, formation of Dayishan granite was likely associated with an intraplate extensional setting caused by the subduction of the Palaeo-Pacific plate. The Baishaziling reduced granites have high stannum and boron contents, which are in favor of the tin mineralization.

Geochronology, geochemistry and petrology of the oligocene magmatism in SE segment of the UDMB, Iran

Despite diverse geochronological-geochemical studies on Cenozoic igneous rocks from the SE segment of the Urumieh-Dokhtar Magmatic Belt (UDMB) of Iran, the nature of the Oligocene magmatic rocks from the farthermost end of the SE segment- where it is linked to the Makran magmatic belt- has been ignored due to the difficulty of access. In this study, we focus on syn-collisional mafic to felsic igneous rocks of calc-alkaline and high-K calc-alkaline affinities from the SE segment of the Urumieh-Dokhtar Magmatic Belt (UDMB) near Nagisun, south of Bam. The Nagisun rocks have low Sr/Y and La(n)/Yb(n), similar to igneous rocks from typical arcs. Zircon UPb ages show comparable ages for plutonic (~ 34–25 Ma) and volcanic (~34–27 Ma) rocks. The εHf(t) values for zircons from plutonic rocks range from −0.3 to +12.8, whereas the εHf(t) values for the volcanic rocks vary from −2.6 to +13. Modelling of trace elements compositions using Nagisan basaltic samples indicate that an 87:2:11 mixture of the depleted MORB mantle, subducting (trench)-sediments and altered oceanic crust with 5% aggregated fractional melting closely matches the trace-element abundances of the Nagisun basaltic rocks. Indeed, the modelling of Sr and Nd isotopic data emphasizes that the Nagisun magmatic rocks could be products of bulk mixing between a depleted MORB mantle and/or a mixed, fertilized mantle with the Cadomian lower and upper continental crust. Furthermore, our compiled data display that the magmatism in the SE segment of the UDMB changed through time from normal calc-alkaline magmatism to adakitic magmatism at ~20 Ma, after the collision with Arabia began ca 27 Ma.

Carboniferous to Early Permian magmatism in the Uliastai continental margin (Inner Mongolia) and its correlation with the tectonic evolution of the Hegenshan Ocean

This study presents a comprehensive analysis of zircon U–Pb–Hf isotopic and whole-rock geochemical data of Carboniferous–Early Permian felsic igneous rocks from the Uliastai continental margin (UCM), southeastern Central Asian Orogenic Belt, to constrain the amalgamation of the Inner Mongolia–Daxing'an Orogenic Belt (IMDOB) in the late Paleozoic. Zircon laser ablation inductively coupled plasma mass spectrometry UPb ages of monzogranite, granite porphyry, felsic volcanic rock, and alkali feldspar granite reveal three stages of magmatism in the Early Carboniferous (ca. 336–320 Ma), Late Carboniferous (ca. 311–307 Ma), and Early Permian (ca. 298 Ma). The Early Carboniferous igneous rocks, including monzogranites and later granite porphyries, exhibit affinity with the high-K calc-alkaline highly fractionated I-type granite, and are depleted in high-field-strength elements and enriched in large-ion lithophile elements. Their zircon εHf(t) values of the Early Carboniferous rocks range from +4.6 to +9.1, indicating that they were generated by partial melting of the juvenile crustal materials in a north-dipping subduction-related environment along with the opening of the Hegenshan Ocean. The Late Carboniferous felsic volcanic rocks are highly fractionated high-K calc-alkaline to alkali-calcic I- and A-type granitoids, with relatively strong peraluminous affinities. Considering the zircon εHf(t) values (+6.2 to +10.6) and relatively low Rb/Ba (0.71–3.57) and Rb/Sr (4.18–8.12) ratios, they were likely derived from a juvenile crust and are comparable to partial melts of clay-poor but plagioclase-rich metaigneous rocks. Notably, the A-type granitoids are gradually increasing in the UCM after 311 Ma, and these magmatic rocks have markedly increased zircon saturation temperatures and (K2O + Na2O)/CaO ratios. These changes may result from the slab break-off of the Hegenshan Ocean, and the Late Carboniferous felsic volcanic rocks were formed in a post-collisional setting. The Early Permian alkali feldspar granites are characterized by typical A-type granite geochemistry, with high Ga/Al ratios; low MgO, Cr, Co, and Ni contents; and high zircon saturation temperatures. These rocks have positive εHf(t) values of +10.2 to +12.7 and record an intracontinental extension setting. In combination with the regional geology, the Late Carboniferous post-collisional magmatic rocks suggest that a change from a subduction-related continental margin to a post-collisional tectonic regime might have occurred in the UCM before 311 Ma.

Pre-Late Eocene position of the Lüchun-Jinping microblock in western Yangtze Craton: Constraints from Eocene-Oligocene lamprophyres in southeastern Tibet

The tectono-magmatic history of the Lüchun-Jinping microblock and its possible affinity with the Yangtze Craton are important elements for the reconstruction of Cenozoic plate tectonics in southeastern Tibet. In order to constrain the affinity and decipher the pre-Cenozoic paleopositon of the Lüchun-Jinping microblock, we focused on the petrogenesis of Eocene-Oligocene lamprophyres in the Lüchun-Jinping microblock. The lamprophyres yield zircon UPb ages of 34.7–33.3 Ma and exhibit potassic-ultrapotassic features with elevated K2O/Na2O (1.4–4.0) ratios. They are characterized by high concentrations of compatible elements (e.g., Cr = 187–692 ppm, Ni = 31–218 ppm), large-ion-lithophile elements and light rare-earth elements enrichment, high-field-strength elements depletion, and high radiogenic isotopic values, i.e. (87Sr/86Sr)i = 0.7063–0.7078 and εNd(t) = −3.9 to −2.4. Combined with the low Nb/U ratios, these features suggest that the lithospheric mantle source was metasomatized by subduction-related fluids beneath the Lüchun-Jinping microblock. The relatively high Rb/Sr ratios and high heavy rare-earth element contents indicate that these lamprophyres were derived from partial melting of a phlogopite-bearing lherzolite within the spinel stability field. The parental magmas have experienced fractional crystallization of olivine and clinopyroxene during emplacement. Comprehensive comparisons between the lamprophyres from the Lüchun-Jinping microblock and the potassic-ultrapotassic mafic rocks from the western Yangtze Craton indicate that the Lüchun-Jinping microblock can be regarded as a dismembered part of the western Yangtze Craton due to continental extrusion and Cenozoic sinistral displacement. The compositional trends of the potassic-ultrapotassic mafic rocks suggest that the palaeogeographic position of the Lüchun-Jinping microblock was near the Dali area (west of the Binchuan) and close to the Jinshajiang suture zone before the Cenozoic.

Testing the advantages of simultaneous in-situ Sm[sbnd]Nd, U[sbnd]Pb and elemental analysis of igneous monazite for petrochronological studies. An example from the late Archean, Penzance granite, Western Australia

Monazite has the ability to incorporate a broad range of trace elements (TE), including SmNd, in addition to being a reliable UPb geochronometer. Nonetheless, many current isotopic tracer studies mostly focus on laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS) Hf-in-zircon or whole rock isotope dilution SmNd. The conditions necessary to produce reliable Nd-isotopic data from whole rock analyses (i.e. absence of weathering and alteration) restricts the number of samples suitable for this type of study. Conversely, accessory phases such as monazite, apatite, allanite, epidote and titanite are more resistant to weathering and alteration and have a greater chance of preserving their original Nd-isotopic signature.The Archean Penzance granite, in the Yilgarn Craton, Western Australia, is a relatively unweathered high field strength elements (HFSE)-enriched granite that is associated with the Teutonic Bore volcanic-hosted massive sulfide (VHMS) Camp. Here we compare newly acquired LA split-stream (SS)-MC-ICPMS analyses in monazite from the Penzance granite to previously collected whole rock Nd-isotopes, SHRIMP ages in zircon and monazite and Hf-in-zircon of a specific outcrop. The ƐNd0 of monazite from the Penzance granite have a weighted mean value of +1.6 ± 0.2 both for the values calculated from the SHRIMP single population age and from individual LASS dates. This value is the same within uncertainty to the whole-rock ƐNd0 value of +1.99 ± 0.39, whereas ƐHf0 show similar juvenile values with mean value of +2.2 ± 0.5 (2 SE).These findings complement previous studies that argue that monazite has the potential to provide reliable time constrained isotopic and chemical data, which are comparable to, and at times superior than, Hf-in-zircon and whole-rock Nd-isotopes. Additionally, because monazite can record TE signatures that can be used to retrieve petrological information, this study proposes a LASS-MC-ICPMS routine that achieves simultaneous SmNd isotopes, U/Th-Pb dating and TE analyses in monazite, allowing for further petrological information to be obtained from a single analytical routine. This study also assesses the precision and accuracy in UPb ages in monazite reference materials, when additional masses are measured in the same routine. The application of the routine presented here facilitates the use of monazite as isotopic tracer in petrological and sedimentary studies.

U-Pb isotopic dating of cassiterite: Development of reference materials and in situ applications by LA-SF-ICP-MS

Cassiterite, the economically most important tin mineral, typically has moderate U and variable common Pb contents, making it amenable for UPb dating. Cassiterite has extremely low Th/U ratios (Th/U < 0.01) and its 208Pb is dominantly common Pb. This is particularly helpful as there is significant interference of tungsten oxides on 202Hg and 204Pb. The feasibility of the 208Pb correction procedure is discussed in detail. The 208Pb corrected LA-SF-ICP-MS data are in good agreement with intercept ages in the Tera-Wasserburg diagram and 207Pb corrected ages.Twelve cassiterite samples were investigated using the ID-TIMS and LA-SF-ICP-MS methods. The ID-TIMS results of Pit-AB, Rond-A, RG-114, BB#7 and 19GX cassiterite are reported for the first time in this study. RG-114, BB#7 and 19GX cassiterite have very low common Pb contents and are recommended for use as primary reference materials for in situ cassiterite. Pit-AB, Rond-A and Yankee cassiterite contain a small amount of common Pb, produce reliable and consistent ages and are suitable as primary reference materials. The remaining five cassiterite samples (Kard, Zinnwald, Els, XBD-W and Y724) were only investigated using the LA-SF-ICP-MS method and produce ages consistent with published age data from the host rocks associated with the tin deposits and with published UPb ages of cassiterite from the same deposits. We present an ID-TIMS UPb of 154.3 ± 0.7 Ma for the commonly used cassiterite reference material AY-4. This age differs from previously reported ID-TIMS ages. This age discrepancy is caused by different initial common Pb compositions rather than age heterogeneity.

Matrix effects and improved calibration procedures for SIMS titanite U[sbnd]Pb dating

Titanite is a U-bearing accessory mineral that can serve as a geochronometer. Compared with zircon, titanite is more likely to contain multi-stage fluid-induced growth. Thus, individual titanite grains may yield complex UPb ages that reveal the entire geological history of the rock. Secondary ion mass spectrometry (SIMS), which has high sensitivity and spatial resolution, is a good choice for UPb titanite analyses. A titanite UPb standard named YQ82 was developed with 206Pb/238U age of 1837.6 ± 1.0 Ma (2 standard error (SE), n = 4) and 207Pb/206Pb age of 1845. 0 ± 1.1 Ma (2SE, n = 4), as determined herein using isotope dilution isotope ratio mass spectrometry (ID-IRMS). However, UPb dating of YQ82 via SIMS yielded a bias of up to 14% compared to the ID-IRMS value, indicating a matrix effect. To correct for this matrix effect, six UPb titanite dating reference materials (RMs) were analyzed. Based on the correlations between the age biases and the Fe contents of the six RMs, a calibration protocol was established using the index of the Fe content (FeO+/IP, intensity of primary beam) and a power-law relationship between the calibrated Pb+/U+ and UO+/U+. Using this calibration protocol, the UPb ages decreased the age bias from approximately ±14% to ±1.4%. To test the protocol, a titanite sample (BMB108) with a low Fe content (Fe2O3, 0.4 wt%) was calibrated using MKED1 (Fe2O3, 1.5 wt%) as the RM. Without the calibration, the apparent SIMS 206Pb/238U age was 10% younger than the thermal ionization mass spectrometry (TIMS) 206Pb/238U age (1890 ± 4 Ma). Using the calibration method, the apparent SIMS UPb age was 1898 ± 14 Ma (2SE, n = 19), which is consistent with the TIMS age. This study shows that a considerable matrix effect related to Fe content is present in the SIMS titanite UPb dating results. The calibration protocol developed herein improved the SIMS titanite UPb dating accuracy, yielding a more accurate thermal history of the samples.