Identification of facies-controlled eogenetic karstification in the Upper Cretaceous of the Halfaya oilfield and its impact on reservoir capacity

Penecontemporaneous dissolution has been considered as the dominant origin of reservoir formation, whereas epigenetic dissolution has also played certain roles in the improvement of reservoir capacity in the Upper Cretaceous Mishrif Formation in the Middle East. Here, we report a possible new reservoir origin based on a case study in the Halfaya oilfield of Iraq, that is, facies-controlled eogenetic karstification. The most representative evidence is that early selective dissolution is not only present in relative high-energy categories of rocks but also in low-energy ones in the cored intervals. Meanwhile, the occurrence frequency of karstification varies among different environments and associated lithologies. In grainstones and packstones, spongy-like dissolution pores and irregular karst channels are widely developed, with fillings and massive plastic breccias. In contrast, in wackestones, lots of high-angle karst channels are developed based on biological burrows, and the karst is more frequently observed in the shoal environment than in the low-energy environments. These characteristics are indication of eogenetic karstification. The model of this karstification can be attributed to a multiple superimposition of short-term exposure during the penecontemporaneous stage and the medium-term exposure after the shallow burial stage. The karst has impact on reservoir capacity, according to which 3 areas are divided in the increasing order, namely, the tight bedrock area, spongy-like dissolution pores area, and karst channels and vugs-filled area, among which the karst channels and vugs-filled area is the most favourable for oil accumulation. This understanding might be general to the Mishrif reservoirs in the entire Middle East.

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Depositional settings and changing composition of the Jambi palaeoflora within the Permian Mengkarang Formation (Sumatra, Indonesia)

The Merangin River section in Sumatra exposes the Permian Mengkarang Formation. This is composed of eight intervals showing upwards fining and thinning of volcanic tuffs and volcaniclastic sedimentary rocks, overlain by their reworked alluvial products. Isotopic age evaluation of the top and the base of the Merangin section indicates an average duration of 630,000 years (from 296.77 ± 0.04 to 296.14 ± 0.09 Ma). Extrapolation of the eight intervals onto neighbouring tributaries by using earlier geological studies and the strike of the beds allows for the integration of the data assembled in recent expeditions, and those from 1925, leading to the lithostratigraphic assignment of more than 2,000 palaeobotanical specimens. The compilation of all assembled palaeobotanical data indicates there is a change in composition from a palaeoflora dominated by Cordaites, ferns, or club mosses to one in which seed ferns were dominant. These changes, coupled to eustatic sea-level fluctuations, indicate a climatic origin for this transition and extend palaeofloral trends perceived earlier in Far Western low latitudes to the Far Eastern Palaeotethys.

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Geochemistry of high-Nb basalt-andesite in the Erguna Massif (NE China) and implications for the early Cretaceous back-arc extension

This paper presents new zircon laser ablation–inductively coupled plasma–mass spectrometry U–Pb age and whole-rock geochemical data for the high-Nb basalts-andesites (HNBs) from the Meiletu Formation in NE China. In comparison with common arc basalts and Nb-enriched basalts, the Meiletu HNBs (140 Ma) show high TiO2 (1.7–3.2 wt.%), P2O5 (0.7–1.7 wt.%), Nb (14.6–34.0 ppm), and Zr (314–600 ppm) contents, and high (Nb/Th)PM (0.26–0.91), (Nb/La)PM (0.22–0.36), Nb/U (8.2–24.8) ratios. They show strongly light rare earth element (REE) enriched chondrite-normalized REE patterns with unusually high total REE contents [(TREE)cn = 978–1802]. Unlike adakite-associated HNBs derived from a mantle wedge metasomatized by adakites, the Meiletu HNBs were derived from low degree (80 km). The Meiletu HNBs were generated in an intracontinental back-arc extension setting. Combined with previous geological observations, it is argued that Early Cretaceous tectonics of the Erguna Massif is dominated by the Mongol–Okhotsk oceanic slab-rollback and back-arc extension.

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Constraining the denudation process in the eastern Sichuan Basin, China using low-temperature thermochronology and vitrinite reflectance data

The temperature history of samples and maximum palaeogeothermal profiles of boreholes were reconstructed based on low-temperature thermochronology and vitrinite reflectance data, and the results provide limits for the timescale and amount of uplift–denudation of the eastern Sichuan Basin. The thermal history showed that the uplifting and cooling of eastern Sichuan Basin began around the Late Cretaceous (approximately 100–80 Ma). The region had experienced a continuous cooling process from the Late Cretaceous until the present, with the geothermal gradient decreasing from 32–36 °C/km to 20–23 °C/km. The amount of denudation at the Puguang region in north-eastern Sichuan was approximately 2.3 km, whereas that at south-eastern Sichuan was 1.9 km, and the erosion thickness in the eastern Sichuan fold belt that was revealed via the field samples is 2.3 ± 0.3–2.6 ± 0.3 km. The north-eastern Sichuan experienced sustained cooling with inconspicuous fluctuations, whereas the thrust belt and the south-eastern Sichuan Basin presented 2–4 stages with different cooling rates. It may indicate that the eastern Sichuan fold belt experienced a complex structural evolution, characterized by episodic upliftings and deformations since Late Cretaceous, while a different and gentle deformation took place in the northeastern Sichuan Basin.

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Geochemistry and 40Ar/39Ar geochronology of the Nandurbar-Dhule mafic dyke swarm: Dyke-sill-flow correlations and stratigraphic development across the Deccan flood basalt province

The ENE-WSW-trending Nandurbar-Dhule swarm is the best developed tholeiitic dyke swarm in the Deccan Traps. We obtained 40Ar/39Ar ages of 67.06 ± 0.60, 67.49 ± 0.89, and 63.43 ± 0.48 Ma (2σ internal errors) on three of its largest dykes (36–79 km long), indicating that swarm emplacement spanned ≥2.5 million years under regional crustal extension. Our Sr-Nd-Pb isotopic data, combined with previously available geochemical data, identify multiple magma injections in some dykes and also identify probable feeder dykes of some lavas in the lower Western Ghats sequence and in Saurashtra, each ~200 km away. Several dykes are compositionally distinct from hitherto analysed lavas; >50% of the analysed Nandurbar-Dhule dykes are isotopically like the Mahabaleshwar and Panhala formations of the upper Western Ghats sequence, covering a very narrow isotopic range, but have the distinctive chemical signatures of the high-TiO2 Kolhapur Unit of the southernmost Western Ghats. These dykes thus possess a unique combination of isotopic and chemical characteristics not hitherto known in Deccan tholeiites, cross-combining features of different eruptive units in the Wai Subgroup of the Western Ghats. This new, “Nandurbar-type” chemical-isotopic flavour is however frequently observed in dykes, sills, and lavas in the Pachmarhi, Shahdol, and Mandla areas 450–600 km to the east, and in Deccan-age dykes cutting through the Early Cretaceous Rajmahal Traps of eastern India. Varied geochemical evidence indicates that the northern and northeastern Deccan lava stratigraphy (such as the Pavagadh section and the Pachmarhi-Shahdol-Jabalpur-Mandla areas) is largely independent of the Western Ghats lava stratigraphy.

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Geochemistry and zircon U–Pb ages of basic rocks in the Kekebieketi mining area from Fuyun (Xinjiang, China) and their geological significance

The Kekebieketi mining area is located on the southern margin of the Altai Mountains in Northern Xinjiang and is at the junction of the Siberia and Kazakhstan-Junggar plates. Faults and magmatic rocks are well developed, and the main stratum is the Middle Devonian Kaxiweng Formation. The Kekebieketi-Kalatongke-Xibodu basic to ultrabasic mixed rock strip is an appropriate area for mineral resources and a good target to study the crustal and mantle evolution. This study explored the tectonic age and setting of basic rocks in the Kekebieketi mining area and provides a basis for the study of basic rocks and discrimination of the wider formation. The basic rocks were mainly gabbro, pyroxenolite, basic subvolcano rock, and diabase. They have all been altered to different degrees through amphibolization, sub-amphibolization, actinolitization, chloritization, epidotization, zoisitization, uralitization, and serpentinization. Similar rare earth element compositions of the different rock types suggest that they are homologous. Enrichment of large ion lithophile elements (Cs, Rb, Sr, and U) and depletion of high field strength elements (Nb, Ta, Zr, and Hf) showed that the basic rocks have the characteristics of island arc magmatic rocks. A single zircon U–Pb age of 321 ± 2.5 Ma was obtained from a hornblende gabbro. Based on the petrological and geochemical features, we concluded that the basic rocks in the study area were formed during the post-collision period in East Junggar. The parental magma is mainly composed of metasomatic mantle wedge material and upwelling asthenosphere, which is why it showed geochemical characteristics of subduction.

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Sr–Nd isotopes of Early and Late Carboniferous volcanic rocks in Yining Massif (Xinjiang, NW China): Implications for petrogenesis and tectonic evolution of Western Tianshan

The Yining Massif, sandwiched between the Junggar and Tarim plates, has received much attention because of its special position and complex tectonic evolution, as well as a significant breakthrough in geological prospecting. Moreover, a lot of studies have focused on two suites of volcanic rocks of the Early Carboniferous Dahalajunshan and Late Carboniferous Yishijilike formations in the Yining Massif. Recently, an increasing number of results have shown that two suites of volcanic rocks have obvious differences in lithochemistry and geochemistry and might have formed in an arc and intracontinental rift settings, respectively. Furthermore, the latest results of Sr–Nd isotopes indicate that the initial (87Sr/86Sr)i ratio is less than 0.7119 for all samples in the two formations. But the (87Sr/86Sr)i of andesites in the Dahalajunshan Formation is obviously higher than that of the Yishijilike Formation. However, those of dacites and rhyolites are just the opposite. The (143Nd/144Nd)i of all kinds of rocks from the Dahalajunshan Formation is less than 0.5126, and that of basalts in Yishijilike Formation is also less than 0.5126. However, the results of the andesites and rhyolites are higher than 0.5126. In short, the parameters of (87Sr/86Sr)i, (143Nd/144Nd)i, εNd(t), TDM (Ga), and εSr(t) are different obviously between the two suites, showing progressive change from basic to acid rocks in the two formations (gradual increase and decrease), but the changed direction is just the opposite. Consequently, it illustrates that there is a significant distinction in composition and petrogenesis between the Dahalajunshan and Yishijilike formations. The Dahalajunshan Formation volcanic rocks might be derived from an enriched mantle, whereas the volcanic rocks in the Yishijilike Formation are related to the evolution of continental crust. In other words, the differentiation of Sr–Nd isotopes provides a reference information for studying the petrogenesis and tectonic setting of the two suites of volcanic rocks.

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Hybodont dentition from the Upper Jurassic of Monte Nerone Pelagic Carbonate Platform (Umbria-Marche Apennine, Italy) and its ecological implications

Here we describe a number of articulated teeth of a hybodont shark from Upper Jurassic deposits of the Monte Nerone Pelagic Carbonate Platform, in the Umbria-Marche-Sabina Palaeogeographic Domain (Northern Apennines, Central Italy). The material has been referred to as Asteracanthus cf. A. magnus, a quite well-known taxon already reported from Middle to Upper Jurassic deposits of Europe. Teeth indicate an extreme crushing feeding behaviour, suggesting as putative prey both infaunal and epifaunal hard-shelled invertebrates dwelling the sea-floor, such as large bivalves, brachiopods, gastropods as well as vagile crustaceans. The finding represents, to date, the first formal report of hybodont shark in the Umbria-Marche-Sabina Domain, throwing further light on the ecology within Pelagic Carbonate Platform settings, and on the occurrence of Asteracanthus in the Late Jurassic of the Western Tethys.

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Triassic alkaline magmatism and mineralization in the Xiong’ershan area, East Qinling, China

The Qinling orogenic belt is a complex subduction–accretion–collision orogen that welded the North China Craton and the Yangtze Craton during the final continental collision in the Triassic. The Xiong’ershan area, located in east Qinling, exposes a typical Triassic syenite pluton and several contemporaneous Mo, Au, and Cu deposits. The aegirine–augite syenites and syenites from the Mogou pluton are characterized by alkalic to peralkalic (total alkali Na2O + K2O = 13.95–14.63 wt.%, CaO = 0.06–2.87 wt.%), and shoshonitic features (K2O = 11.86–14.34 wt.%). Zircon LA–ICP–MS U–Pb dating of the aegirine–augite syenite and syenite yield emplacement ages of 232.5 ± 0.6 and 221.8 ± 0.7 Ma, indicating multiple pulses of magmatism. Evidence from zircon Hf isotopes; occurrence of mafic microgranular enclaves; heterogeneous peralkaline composition; and wide ranges of MgO, Ni, and other trace elements suggest that the parental magma was mainly sourced from partial melting of Archean to Paleoproterozoic crustal sources, mixed with juvenile mantle-derived mafic magmas. The Mogou pluton was probably emplaced in the tectonic transition from syn-collision to post-collision settings and accompanying slab break-off process, from the commencement of collision at approximately 245 Ma and post-collisional extension at approximately 210 Ma. Gold, molybdenum, and copper deposits formed during the interval of 255–208 Ma, and the close temporal and spatial relationship between these Triassic polymetallic deposits and the Mogou alkaline pluton invokes a genetic linkage. The heat source for magmatism and related metallogeny is correlated to a hot upwelling asthenospheric mantle that caused partial melting of the Archean to Paleoproterozoic crustal basement, resulting in magma mixing between the two end-members.

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Petrogenesis of two stages of Cretaceous granites in south-west Fujian Province: Implications for the tectonic transition of South-east China

Zircon U–Pb data indicate that Cretaceous magmatism in Fujian Province occurred in two pulses, with the Xiyang, Yangshan, Wuci, Dehua, Juzhou, and Xiandu plutons emplaced between 147 ± 1 and 126 ± 1 Ma and the Hutou and Guangqiao plutons emplaced at 109 ± 1 Ma. Combining these previously published ages with the new data obtained during this study indicates that the magmatism in the study area occurred in two distinct stages at 145–125 and 110–100 Ma, both have I-type characteristics. The earlier I-type granites are characterized by relatively high concentrations of SiO2 (mean = 75.73 wt.%) and low concentrations of MgO (mean of 0.21 wt.%) and are metaluminous to weakly peraluminous (mean A/CNK = 1.03). They contain relatively high total rare-earth element (ΣREE) concentrations (mean of 139.3 ppm) with well-developed Eu anomalies and are relatively enriched in the large-ion lithophile elements (LILE) and the light REE (LREE) but depleted in Ba, Sr, P, Ti, Nb, and Ta. In contrast, the younger I-type granites contain lower concentrations of SiO2 (mean of 67.63 wt.%), higher concentrations of MgO (mean of 1.47 wt.%), and are metaluminous to weakly peraluminous (mean A/CNK = 0.96). They also have moderate REE values (mean of 150.26 ppm) without significant Eu anomalies and are relatively enriched in LREE, Rb, Th, U, K, Sr, and Pb and depleted in Nb, Ta, P, Ti, Y, and Yb. The two-stage Nd model ages (T2DM) of Cretaceous granites are 1.46 to 1.88 Ga (mean of 1.57 Ga) and 1.16–1.60 Ga (mean of 1.36 Ga), respectively. The two-stage Hf model ages (T2DM) of Cretaceous granites are 1.19–1.82 Ga (mean of 1.54 Ga) and 0.46–1.20 Ga (mean of 0.91 Ga), respectively. It is likely that the earlier I-type granites were derived from magmas generated by the partial melting of Mesoproterozoic metabasaltic material, whereas the later I-type granites were derived from magmas generated by partial melting of a mixed lithology containing Mesoproterozoic to Neoproterozoic metabasaltic to metatonalitic rocks that were underplated by medium- to high-K basaltic magmas. A geological comparison between south-western Fujian Province and adjacent areas indicates that the earlier I-type granites were emplaced in a compressional environment associated with the subduction of the paleo-Pacific Plate at 145–125 Ma. In contrast, the later I-type plutons are associated with intraplate extension accompanied by the post-subduction slab roll-back of the paleo-Pacific Plate. The two magmatic stages represent a transition from compressional to intraplate extensional tectonics between 125 and 110 Ma, with mantle-derived material playing a crucial role in the formation of the I-type granites.

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Hydrothermal activity in the fourth member of the Triassic Leikoupo Formation in the Yuanba area, northeast Sichuan, SW China

Large quantities of saddle dolomite, dolomite, calcite, celestite, fluorite, barite, quartz, and pyrite occur as fracture fillings in the fourth member of the Middle Triassic Leikoupo Formation in the Yuanba area, northeast Sichuan. They were found either as single mineral or mineral assemblages. The properties, origin, and evolution of the fluids from which these fracture fillings were precipitated were studied using a variety of methods, including polarized light microscopy, cathodoluminescence microscopy, fluid inclusions, and stable isotope analysis. Homogenization temperatures of the liquid–gas inclusions trapped within the dolomite ranged from 112 to 138 °C. Based on this temperature range and the oxygen isotope data, the δ18O values of the fluids from which the dolomite precipitated were determined to be between −5‰ and 7‰ (Standard Mean Ocean Water). The study suggests that the fracture-filling dolomite, ubiquitous in the fourth member of the Middle Triassic Leikoupo Formation in the study area, was produced from high-temperature, high-salinity brines. These brines were interpreted to have originated from the mixtures of hot brines trapped in rock formations and hydrothermal fluids from deep sources and to have become enriched in strontium during diagenesis.

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Geochemical characterization, petrogenesis, and emplacement tectonics of Paleoproterozoic high-Ti and low-Ti mafic intrusive rocks from the western Arunachal Himalaya, northeastern India and their possible relation to the ~1.9 Ga LIP event of the Indian shield

Paleoproterozoic mafic intrusive rocks from the western Arunachal Himalaya have been studied for understanding their petrogenesis and tectonic environment, in which they were emplaced, with the help of their bulk-rock major, trace, and rare-earth elements (REE) compositions. These mafic intrusive rocks, mostly small bodies of dykes, sills, and lenses, are exposed in the Higher as well as the Lesser Himalaya sequences. Geochemically, 2 suites of rocks are identified and grouped into high-titanium and low-titanium mafic intrusive rocks. High-titanium mafic intrusive (HTMI) samples have comparatively low Mg number (Mg#) and high high-field strength element contents in comparison to low-titanium mafic intrusive (LTMI) rocks. Multielements and REE patterns for these two identified groups are distinct. The LTMI and HTMI were originally spatially separated at the time of their emplacement and believed that probably they have been juxtaposed during the Himalayan orogeny by crustal shortening through development of imbricated thrusts. It is perceived that the HTMI and LTMI have different petrogenetic histories and are derived from two different melts; however, they are postulated to be parts of the same igneous province. Geochemical characteristics do not support any serious effect of crustal contamination; however, interaction with metasomatized lithospheric mantle cannot be discounted. Trace element petrogenetic modelling suggests derivation of the LTMI samples from a parental magma generated in spinel stability field (thin lithosphere) through a ~20–25% melting of a lherzolite mantle source, whereas the HTMI samples were derived from ~5–10% melting of a similar mantle source but from garnet stability field (thick lithosphere). Widespread Paleoproterozoic mafic events in the Indian shield such as Bastanar dyke swarm in the Bastar and the Hampi dyke swarm (also includes Pulivendla sills in the Cuddapah Basin) in the eastern Dharwar Craton together with mafic intrusive and extrusive rocks in the Himalayan region suggest presence of a large igneous province at ~1.9 Ga.

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The timing of oceanic anoxic events in the Cretaceous succession of Cauvery Basin: Constraints from 40Ar/39Ar ages of glauconite in the Karai Shale Formation

This study presents 40Ar/39Ar ages of autochthonous glauconites from the lower segment of the onshore Karai Shale Formation of the Cauvery Basin that constrain its age from 100.3 ± 0.7 to 92.6 ± 0.6 Ma. The 40Ar/39Ar plateau ages of the 3 glauconite samples are consistent with the existing biostratigraphic age of Late Albian to Middle Turonian of the Karai Shale Formation. These ages are significant because of the occurrences of oceanic anoxic event (OAE)1d and oceanic anoxic event (OAE)2 in contemporaneous offshore deposits, the Sattapadi Shale and Bhuvanagiri Formation, respectively. Although in the onshore deposits, the glauconite ages mark condensed deposits of the transgressive Karai Shale. As onshore glauconitic deposits may be transitional to organic-rich black shale deposits offshore in modern and ancient sedimentary settings, the 40Ar/39Ar ages precisely constrains the timing of Cretaceous oceanic anoxic events reported in the Cauvery Basin.

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Different magma sources and evolutions of white and black pumice from the middle Okinawa Trough: Evidence from major, trace elements and Sr–Nd–Pb isotopes

The Okinawa Trough (OT) is a back-arc, initial marginal sea basin that is located behind the Ryukyu arc–trench system. Pumice, including both white and black pumice, is the most widely distributed magmatic rock in the OT. Here, the major and trace element concentrations and Sr–Nd–Pb isotopic compositions of the white and black pumice from the middle OT (MOT) are presented and combined with previously published elemental data for volcanic rocks from the MOT. Their different magma sources and magmatic evolutions are discussed, and their different magma chamber structures are provided in this paper. These results demonstrate that the white pumice has two magma sources: One was produced by the crystallization-based differentiation of basaltic magma, and the other was produced by the assimilation-fractional crystallization (AFC; with less than 5% contamination) of basaltic magma. The black pumice also includes two magma sources: One is similar to that of the first white pumice, whereas the other was produced by basaltic magma that experienced more than 20% upper crustal contamination. There are two different structure magma chambers for white and black pumice in the MOT crust: The deeper one generated no contaminated pumice, whereas the shallow one generated contaminated black pumice. The contamination of the white pumice (<5%) may have occurred during the eruption of magma on the seafloor, rather than in the magma chamber. Different magma sources and evolutions of white and black pumice imply the complexity of magmatism in the Okinawa Trough.

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Study on the potential of cultivated land quality improvement based on a geological detector

The restrictive factors of cultivated land are key to the improvement of cultivated land quality, scientific implementation of the land consolidation projects, and the efficiency of remediation. On the basis of the provincial plots of cultivated land quality in Shaanxi Province, this paper analysed the improvement potential of cultivated land quality from the perspective of restrictive factors. First, the potential exponential model was used to determine the distribution of various combinations of restrictive factors at the provincial scale. Second, a geological detector was used to determine the influences of different combinations of restrictive factors on cultivated land quality. Finally, through the investigation of cultivated land consolidation projects that have been implemented in the study area, the improvement potential level of different combinations of restrictive factors was determined. The degree of influence of the single restrictive factor or combinations of restrictive factors on the quality of cultivated land was improved, and the difference of the quality of cultivated land in different index areas could be revealed as well. The results showed that there were 12 single-factor restrictions and 34 double-factor restrictions. The area under single-factor restrictions reached 76.77% of the total land. The quality of cultivated land in the southern and central areas of Shaanxi Province was relatively good. The quality of cultivated land in the northern region was under significant influence of restrictive factors whereas that in southern and middle areas was less affected. From the perspective of improvement potential of restrictive factors, Shaanxi was relatively low with huge internal diversity, whereas the improvement potential in northern Shaanxi had huge advantage.

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Garnet zoning in kyanite-bearing eclogite from Golfo Aranci: New data on the early prograde P–T evolution in NE Sardinia, Italy

Kyanite-bearing eclogites from the Migmatite Complex of Golfo Aranci area, NE Sardinia, have been investigated using microstructural analyses and thermodynamic modelling in order to define their early metamorphic evolution and P–T path. The eclogites underwent a clockwise P–T path recorded by the different composition of garnet core, mantle, and rim. P–T pseudosection modelling allowed to estimate P–T conditions in the range T = 580–630 °C and P = 1.5–2.00 GPa for the formation of garnet core, between T = 620–690 °C and P = 2.0–2.3 GPa for garnet mantle, and within T = 650–700 °C and P = 1.4–2.1 GPa for garnet rim. The first part of the P–T path is characterized by a prograde increase in P and T in the eclogite facies that occurred between the growth of the garnet core and mantle. After the peak pressure, recorded by the garnet mantle composition, the rocks underwent moderate temperature increase and significant pressure decrease until they reach the peak temperature near to the eclogite/upper granulite-facies transition, recorded by the garnet rim composition. The P–T trajectory proposed for the early stage of the metamorphic evolution of the kyanite-bearing eclogites is similar to already existing observations from the Sardinian Low- to Medium-Grade Metamorphic Complex and introduce new constraints on their prograde eclogite-facies evolution. The role of the new prograde path is then discussed in the light of the Variscan orogen.

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Geochemistry and a metallogenic model for Nb–Ta-bearing granitic pegmatites from the northern Qaidam Basin

The Palaeoproterozoic Dakendaba Group and the Cambrian–Ordovician Tanjianshan Group occur on the northern margin of the Qaidam Basin. Their fracture structure is well developed, and intermediate-acid rocks are widely distributed, providing an ideal setting for pegmatite mineralization. Field investigation has identified 200 pegmatite veins, of which more than 40 have various degrees of Nb and Ta mineralization. These pegmatites can be classified into four types, based on distribution, mineral assemblages, and a genesis sequence of ① microcline, ② microcline–albite, ③ albite, and ④ muscovite–albite. Pegmatite types ③ and ④ are typically mineralized, with Nb and Ta occurring as niobite and columbite–tantalite. These pegmatites have high silica and alkali content and are Al-rich. The aluminium saturation index (A/CNK) is 1.36–1.60, and (A/NK) is 1.45–1.93. Their ∑REE values are 2.61–23.95 × 10−6. Other REE ratios have values of (La/Yb)N = 2.02–8.04, (La/Sm)N = 0.47–3.20, and (Gd/Yb)N = 1.34–4.93. Their REE distribution pattern is right-inclined, with slight LREE enrichment. Negative Eu anomalies are apparent in types ① and ④ but not in types ② and ③. High field strength elements, such as Nb, Ta, Zr, and Hf, have low contents, with Y depletion. Concentrations of REE are generally low, although Sr and Ba are slightly enriched. Thus, pegmatites of this area have low Ca, K, and Al but high Na and Si and are rich in rare metal elements and poor in REEs. This suggests liquid immiscibility was involved in their metallogenic evolution process.

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Petrogenesis of rhyolite at Kalatage in the Eastern Tianshan, Northwest China: Evidences from geochemistry, zircon U–Pb geochronology, and Hf isotopes

The Central Asian Orogenic Belt was one of the most important sites for juvenile crustal growth during the Phanerozoic worldwide, and Eastern Tianshan, Northwest China, located in southern Central Asian Orogenic Belt, is one of the key areas for unravelling the accretionary processes and continental growth. Zircon U–Pb geochronological, Hf isotopic, and whole-rock geochemical analyses are reported for the Upper Carboniferous Qishan Formation rhyolites from the Kalatage area in the middle of the Harlik–Dananhu arc, Eastern Tianshan, to investigate its petrogenesis and geodynamic setting. Zircon U–Pb ages obtained by laser-ablation inductively coupled mass spectrometry (LA-ICP-MS) indicated that zircon crystallization age of the rhyolite was 299.1 ± 2.1 Ma. The rhyolites are classified as subalkaline and high-K calc-alkaline series with A/CNK values mainly lower than 1.10. The REE patterns exhibit right inclined curves with negative Eu anomalies, and the trace element spider diagrams show depletions in Nb, Ta, and Eu, which is consistent with the geochemical characteristics of the island arc calc-alkaline magma suffered fractional crystallization. In situ zircons Hf isotopic analyses yielded positive initial εHf(t) values ranging from 8.0 to 11.9 and the two-stage Hf isotope crustal model ages (TDMC) of 554 to 807 Ma. It indicated that the rhyolite was derived from remelting of juvenile crust. The geochemical data for the rhyolites indicate that they were probably generated in a suprasubduction zone setting. It is proposed that the North Tianshan oceanic crust subducted northward beneath the Harlik–Dananhu arc during the Late Upper Carboniferous, and the rhyolites were derived from remelting of juvenile crust and generated in a suprasubduction zone setting.

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Geoheritages in the Qinling Orogenic Belt of China: Features and comparative analyses

The Qinling Orogenic Belt is a typical composite continental orogenic belt in China. The Qinling Mountains is not only a natural demarcation that separates China into a southern part and a northern part, in terms of Qinling Orogenic Belt’s geography, climate, organisms, and river systems, but also forms a cultural boundary between the Yellow River Civilization to the north and the Yangtze River Civilization to the south. The Xi’an Qinling Zhongnanshan World Geopark is situated in the major portion of the collision zone between the South China and North China plates, which is also a typical section of the Qinling Orogenic Belt. It has been the focus of great attention for its long history of geological evolution, intensive tectonism, various rock types, unique strata, frequent magmatism, and abundant geological heritage sites. This paper clarifies the regional geological background of the Qinling Zhongnanshan World Geopark in 3 aspects, that is, the regional geological setting, regional evolution, and regional strata. With an integrated analysis and classification, the paper describes the main features of geological relics and geological remains of scientific significance in the geopark, on the basis of an investigation and systematic analysis for the geological relic resources of the world geopark. Compared with the geosites in other world geoparks, the sites in the Zhongnanshan area are particularly distinctive, even unique.

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Origin and influence of a Late Mesozoic multistage I- and A-type granitic complex in northern Fujian Province, South China

The Late Mesozoic Pucheng granitic complex in northern Fujian Province is composed of the Chengbu and Shipi peralkaline A-type granitoids, the Pucheng A-type granitoid, and the Yongxing I-type granitoid. The Chengbu and Shipi peralkaline granitoids were dated at 160 and 130 Ma, respectively, and are explained by the melting of meta-igneous rocks with TDM2 of 1.70 Ga and 2.01 to 2.05 Ga. The Pucheng A-type granitoid was emplaced at 110 and 102 Ma and is divided into two groups. Group 1 comprises the extremely felsic A-type granite (SiO2 > 73 wt.%), high FeOT/MgO (>16), and low Ga/Al ratios and Zr + Nb + Ce + Y content. Group 2 contains a less-evolved A-type granite with high Zr + Nb + Ce + Y content. Both groups can be explained by dehydration melting of meta-igneous rocks at low pressure. The diversity between the two groups was caused by different physicochemical environments. The Yongxing I-type granite, which is abundant in mafic microgranular enclaves, was emplaced at 106 ± 1 Ma. It can be explained by the mingling of mantle-derived and crustal-derived magmas. Our data along with previously published data demonstrate that the northern Fujian Province was under an extensional environment from 160 to 100 Ma. This was caused by the NW-trending subduction of the palaeo-Pacific Plate. The Jurassic peralkaline granitoids were probably generated in a rift environment as a result of the reactivation of pre-existing faults caused by the initial subduction of the palaeo-Pacific Plate. The Early Cretaceous peralkaline granitoids may represent a rift environment as a tectonic response from low-angle subduction to an increasing subduction dip angle. The 100 Ma granitoids derive from an intraplate extensional environment caused by the roll-back of the palaeo-Pacific Plate.

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Linking the Qinling Orogeny with the Chang 7 shale (Triassic Yanchang Formation) deposition: Evidence from major, trace, and rare earth element geochemistry

The lacustrine organic-rich shale of Triassic Chang 7 Member is an unconventional shale oil reservoir that is a major hydrocarbon target in the Ordos Basin, northern China. This study uses major, trace, and rare earth element (REE) abundance documented through a 115-m-thick core obtained from the western part of the Ordos Basin to assess dominant controls on temporal variations in total organic carbon in the Chang 7 shale. Our results suggest that total organic carbon trends in the Chang 7 can be directly linked with the tectonic and magmatic evolution of Qinling orogen. The Chang 73 and Chang 72 submembers of the lower part of the Chang 7 were likely deposited in association with heighted tectonic and magmatic activity of the Qinling Orogeny. Sediments derived from these submembers display light REE and trace element compositions similar to the Tianshui rhyolites suggesting deposition contemporaneous with magmatic and/or volcanic activity of the Qinling Orogeny. Higher nutrient concentrations introduced by volcanic eruptions would have stimulated primary productivity in the surface water enhancing the organic carbon flux to the lake bottom. In addition, higher rates of mountain building in the Qinling Orogeny may have been responsible for increasing subsidence rate of the southern Ordos Basin, also favourable for the establishment of anoxic bottom-water conditions. In contrast, lower light REE and trace element abundances of the Chang 71 submember at the top of the Chang 7 suggest deposition during a tectonically and magmatic quiescent period of the Qinling Orogeny. The diminished volcanic flux could have lowered the rate of primary productivity. Moreover, a reduced rate of mountain building would have decreased the rate of foredeep subsidence favouring shallower water and oxic/suboxic bottom-water conditions. Thus, the hydrography of the lake during accumulation of the Chang 71 was not conducive for organic carbon accumulation. Together, the correlation of the Qinling mountain building and magmatism and deposition of the Chang 7 shale provide evidence for the role of tectonism in the accumulation of organic carbon enriched sediment.

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Detrital zircon U–Pb ages, Hf isotopic constraints, and trace element analysis of Upper Cretaceous–Neogene sedimentary units in the Western Nepal Himalaya: Implications for provenance changes and India–Asia collision

The present study integrates detrital zircon U–Pb–Hf isotopic analysis from 13 sandstone samples from an Upper Cretaceous–Miocene sedimentary sequence in Nepal Himalaya to determine their provenance. These sequences constrain a shifting of provenance from south to north. The U–Pb ages from Upper Cretaceous–Palaeocene strata (Amile Formation) mainly cluster between ~1,860 and 1,400 Ma with a peak at ~1,630 Ma and an absence of grains younger than the Palaeoproterozoic (1,400 Ma) age. The detritus yielded positive detrital zircon ƐHf (t) values (as high as +10). However, the detrital zircon U–Pb ages from Eocene–Miocene sequence cluster at ~500–650, ~700–900, ~1,600–1,850, and ~2,500 Ma, and in addition, they have both positive and negative ƐHf (t) values (+11 to −25). This finding further elucidates that the detritus in the Amile Formation was entirely sourced from India, which changed following the time of the Bhainskati Formation deposition, to a mixture of both Asian and Indian affinities (the Himalayan region). This change in source region marks the possible time of the India–Asia collision during this transition phase, that is, Late Palaeocene–Earliest Eocene.

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Enigmatic nodule-bearing rocks in the mafic to ultramafic Diana Mills pluton, Piedmont Province, Virginia

The Silurian Diana Mills pluton is a metamorphosed mafic–ultramafic body in the Piedmont Province of Virginia. Major rock types in the pluton include metadiorite and chlorite-amphibole rock, with lesser amounts of hornblendite. However, the most visually striking rocks, found at 4 locations, consist of tan-weathering nodules in a dark green matrix. The purpose of this study was to investigate the origin of these nodular rocks. All rocks show evidence of greenschist-facies metamorphism, but relict igneous textures are preserved locally. Apart from the nodules, Ca-amphibole is ubiquitous in all rock types. Nodules are dominated by serpentine or talc (+magnetite ± chromite), and many show a thin (≤5 mm) radially oriented shell of serpentine against adjacent matrix. The nodules are ultramafic (Mg# ~80–90, up to7,563 ppm Cr, up to 2,038 ppm Ni), and their normative mineralogy is dominated by olivine and orthopyroxene. Thus, they represent metamorphosed harzburgites and pyroxenites. Matrix minerals are dominantly amphibole + chlorite, along with variable amounts of talc + magnetite. Some matrix samples are chlorite-rich, probably reflecting metasomatic reaction with nodules (i.e., they are small-scale “blackwalls”). Matrix samples are also ultramafic (high Mg#, Cr, and Ni). The matrix of the nodule-bearing outcrops is essentially the same as other chlorite-amphibole rocks elsewhere in the pluton. We consider these rocks to represent emplacement of an original hornblende peridotite crystal mush (a mixture of crystals and hydrous melt), which locally carried harzburgitic nodules. The nodules most likely represent the earliest-formed cumulates from the Diana Mills parent magma.

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Genesis of the Xishadegai Mo deposit in Inner Mongolia, North China: Constraints from geology, geochronology, fluid inclusion, and isotopic compositions

The Xishadegai Mo deposit is a medium-sized deposit located in the northern margin of the North China Craton. The Mo mineralization is structurally controlled, and spatially and temporally related to the Xishadegai felsic intrusive rocks. Ore bodies mainly occur as quartz veins/veinlets in altered granitic rocks associated with potassic, phyllic, argillic, and fluorite alterations. The ore-forming process can be divided into 3 stages: Stage I K-feldspar-quartz ± molybdenite, Stage II quartz-pyrite-molybdenite-muscovite ± fluorite, and Stage III quartz-fluorite ± muscovite. Four types of fluid inclusions were distinguished in smoky grey and dark grey quartz of the main-ore stage (II), including two-phase aqueous inclusions, CO2-H2O inclusions, daughter mineral-bearing multiphase inclusions, and minor vapour aqueous inclusions. The fluid inclusions in smoky grey and dark grey quartz are homogenized at temperatures of 195–350 °C and 191–291 °C, respectively, with calculated salinities of 3.9–11.1% NaCleq and 31.5–33.0% NaCleq, respectively. The ore-forming fluids belong to a H2O-CO2-NaCl system characterized by abundant CO2, moderate to high temperature, and low to high salinity. The δ18OH2O and δD values of ore-stage quartz vary from −0.2‰ to 0.9‰ and from −120‰ to −104‰, respectively, indicating that the ore-forming fluids were evolved from magmatic water and gradually mixed with significant amounts of meteoric water. Sulphur and lead isotopic compositions indicate that the ore materials were mainly derived from magmatic sources. Zircon LA-ICP-MS U–Pb dating on the mineralized porphyritic moyite yielded a weighted mean age of 235.1 ± 2.0 Ma, corresponding to the Triassic postcollisional setting following the closure of the Paleo-Asian Ocean between the Siberian Plate and the North China Craton. The εHf(t) values and TDM2 ages range from −15.0 to −12.8 and from 2.2 to 2.1 Ga, respectively, suggesting that the Xishadegai granite was mainly generated by melting of Paleoproterozoic crustal components. Collectively, evidence from geology, fluid inclusion, H-O-S-Pb isotopes, and geochronology suggests that the Xishadegai deposit could be classified as a magmatic–hydrothermal vein Mo deposit. Phase separation (immiscibility and boiling) was the most likely mechanism for ore deposition.

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Formation age and geodynamic setting of the Neoproterozoic Shalong iron formation in the Central Tianshan, NW China: Constraints from zircon U–Pb dating, geochemistry, and Hf–Nd isotopes of the host rocks

The recently discovered Shalong iron deposit in the Central Tianshan with a reserve of 14 Mt of iron at 25–30 wt.% Fe shows the typical iron formation geological characteristics: The host rocks are mainly metamorphosed volcanic rocks including quartz hornblende schist, mica quartz schist, and plagioclase amphibolite; the iron orebodies are stratiform and bedded, indicative of strata-controlled deposits; metallic minerals are dominated by magnetite with haematite and ilmenite in minor quantities, whereas gangue minerals are quartz, amphibole, chloritoid, and plagioclase; and ore textures are banded and laminated. Here, we present LA-ICP-MS zircon U–Pb ages, whole-rock geochemistry, Sm–Nd isotope, and zircon Lu–Hf isotope analyses on the host rocks in order to constrain the timing and geodynamic setting of the deposit. The geochemical characteristics, Sm–Nd isotope, and zircon Lu–Hf isotope compositions of the host rocks indicate that their protoliths were likely basic and felsic volcanic rocks. The bimodal kind rock types, the juvenile Nd–Hf isotopic characters, and the within-plate trace element geochemical features of the host rocks together indicate that these rocks formed in a rift-like extensional setting. Zircon U–Pb dating of the host rock interlayer suggests that the Shalong iron formation was formed in the Neoproterozoic (ca. 760 Ma). Combined with geological characteristics, close association with meta-volcanic rocks, absence of glacial deposits, and within-plate geochemical and juvenile isotopic signatures of the host rocks, the Shalong iron deposit is interpreted to be an Algoma-type Neoproterozoic iron formation. The formation of this deposit can be linked to rift-like volcanic activities related to the breakup of the Rodinia supercontinent. Our results in conjunction with previous studies suggest that the Central Tianshan should be related to the breakup of Rodinia, as indicated by the Neoproterozoic rift-related igneous events of the Central Tianshan.

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