The eruption of large igneous provinces usually has major geodynamic influences on overriding plates. Seamount chains indicate that the drifting direction of the Pacific Plate changed by ~80° in the Early Cretaceous when the Ontong Java Plateau formed. This, however, is not fully consistent with the magnetic anomalies. Here we show that there is an angle of ~25° between the magnetic anomaly lines M0 and 34 of both the Japanese and the Hawaiian lineations, suggesting that the orientations of both spreading ridges changed by roughly the same angle towards the same direction. The configurations of the Shatsky Rise, the Papanin Ridge and the Osbourn Trough suggest that the eruption of the Ontong Java plume head uplifted the southeastern corner of the Pacific Plate, and pushed its east part northward by ~700 km within 2 Ma. Meanwhile, the west part of the Pacific Plate was subducting southwestward underneath the eastern Asian Continent. These two forces together rotated the Pacific Plate anticlockwisely by ca 50°. Consequently, the drifting direction of the Pacific Plate also changed from southwestward to northwestward, which plausibly explains the ca 80° bending of the Shatsky Rise and the Papanin Ridge. The ridge between the Pacific and the Izanagi/Kula plates was pointed towards the ~300° orientation, whereas the Pacific Plate was subducting towards the ~250° orientation before ~125 Ma, and towards ~280° afterward.
The Xuhuai fold thrust belt (XHTB) is a curved structure in the southeastern margin of the North China Craton (NCC) that has attracted great attentions due to its tectonic and petrological characteristics. However, few geophysical studies have focused on the deep structure of this belt. In this study, we carry out a systematic demonstration of the main geophysical features that characterize the XHTB and surrounding areas. The results reveal small negative gravity and magnetic anomalies, thin crust and lithosphere, lower shear velocity and shallower earthquake epicenters relative to other areas of the NCC, collectively indicating a lithospheric-scale rheological anomaly at this belt. The magnetic alignments show a trend similar to that of geological units in southeastern NCC and adjacent areas, although they differ from the SKS-splitting fast polarization directions, except in the Qinling-Dabie orogen where a vertical coherent deformation of the crust and mantle may be involved there. Based on the geophysical data, we propose a detachment-controlled model, which was caused by the different detachment depth/strength, for the formation of XHTB to explain its arcuate shape as well as the magnetic alignments, thus providing new insight into the deep processes of southeastern NCC.
The origin of the Mesozoic high Ba-Sr (HBS) granitic magmatism in the Jiaodong Peninsula remains controversial in petrogenesis models and geodynamic settings. Here, we report zircon U-Pb age, trace element and oxygen isotope compositions, and whole-rock major-trace element and Sr-Nd isotope compositions of the HBS Yashan granodiorite. The zircon U-Pb age of ~118 Ma denotes that the Yashan granodiorite belongs to the Weideshan-stage magmatic activity, which is consistent with the age of Mo mineralization in the Yashan intrusion. The low Sr/Y (48.8-115) and high (La/Yb)N (23.8-50.4) ratios of the Yashan granodiorite are analogous to adakitic features derived from the lower-crust. This is also supported by the whole-rock initial 87Sr/86Sr ratios (0.709 6-0.710 3) and zircon δ18O values (6.79‰-8.03‰). Contemporaneous mantle-derived mafic microgranular enclaves indicate the involvement of the metasomatized lithospheric mantle. The high magma oxygen fugacity of the Yashan intrusion as indicated by high zircon Ce4+/Ce3+ values suggests the involvement of plate subduction. The obviously lower Dy/Yb, La/Yb and Sr/Y ratios of magmatic rocks in the Weideshan-stage than those in the early-stage imply lithospheric thinning of the eastern North China Craton. We propose that the Yashan HBS granodiorite was formed by crust-mantle interactions during slab rollback.
The Early Cretaceous aluminous A-type granites in the Lower Yangtze River belt (LYRB) can provide important insights into the Mesozoic magmatism in eastern China, but their origin remains highly controversial. In this study, radiogenic Ca-Nd isotopic analysis was performed for syenite porphyry and alkali-feldspar granite porphyry of the Yangshan pluton, a typical aluminous A-type granitic intrusion in the LYRB, to constrain its source and geodynamic setting. The results show that εCa(126 Ma), εNd(126 Ma) and K/Casource of the syenite porphyry range from -0.24 to +0.96, -7.2 to -6.0, and 0.31 to 1.26, respectively. The corresponding values for the alkali-feldspar granite porphyry range from 0.26 to 0.84, -8.0 to -6.1, and 0.79 to 1.08, respectively. Binary mixing modeling indicates that they were originated from the same sources with different proportion, namely, a mixing of 50% to 75% Neoproterozoic crust and 50% to 25% asthenospheric mantle. Together with previous works, we propose that the Early Cretaceous subduction of the ridge between the Pacific and Izanagi plates was responsible for the formation of the aluminous A-type granites in the LYRB.
About 45% of tungsten, ~20% of tin, and ~9% of fluorite of known world reserves are associated with Late Mesozoic igneous rocks, Southeast (SE) China. Here we demonstrate that Fogang granite, the largest inland batholith, is mainly of A2-type that is commonly found in post-orogenic settings and experienced plate subduction induced metasomatism. In contrast, the Yajishan syenite and Nankunshan granite intruding the Fogang granite ~20 Ma later are of A1-type formed in intraplate settings. We found that F-rich fluid fractionation, which could make the decline of Ga/Al ratio, total (Nb + Y + Ce + Zr) and Zr concentrations, Nb/Ta and Zr/Hf ratios, leads to chemical variations of a few Fogang granites changing from A2-type to highly fractionated or I- and S-type granitoids. Crystal and F-rich fluid fractionations, as well as crustal contamination most likely derived from the Fogang granite, result in some Nankunshan granites developing from A1-type into A2-type. These late- or post-magmatic processes should be taken into account carefully when discriminating the petrogenetic types of igneous rocks, especially for the A2-type suites. Combining with the distribution of 180–140 Ma A1- and A2-type igneous rocks, rare metal deposits, and fluorite deposits in SE China, we highlight the significant role of slab-released F-rich fluids in formation of A-type suites and subsequent chemical differentiation and rare metal and fluorine mineralization. A model of flat-slab northeastward rollback is thus proposed, in which the subduction front reached somewhere near Fogang and then started to roll back at ~165 Ma. The inland Jurassic granites of SE China represent a unique locality for formation of A-type suites and their associated mineralization. These granites are not anorogenic, but they are the result of slab rollback from a flat slab, founding of that slab at shallow levels, and metasomatism of by F-rich fluids related to slab heating by the asthenosphere.
Early Cretaceous A-type granitic plutons are widely distributed in Shandong Peninsula, which are of great significance to understanding the regional tectonic evolution. Here we report geochemical characteristics, geochronological results, and zircon Hf-O isotope compositions of Wulian granites to reveal its origin and geological significance. Wulian granites share most characteristics of the A2-type granite. Zircon U-Pb LA-ICPMS analysis for Wulian A-type granites yields average age of 116.6 ± 2.1 Ma. Zircon O isotope values range from 4.20‰ to 5.57‰, and these values are marginally identical to or slightly lower than those of mantle zircon (5.3‰ ± 0.3‰). Zircon εHf(t) values (-26.1– -23.5) indicate an enriched source. Integrated zircon Hf-O isotope compositions indicate that the source region of Wulian granites involves the components of ancient oceanic crust. Apatites from Wulian granites have the lower chlorine (0.06 wt.%–0.15 wt.%) and higher fluorine contents (2.11 wt.%–2.48 wt.%) compared with Haiyang pluton, together with their high (La/Gd)N, (La/Yd)N and low (Gd/Yb)N ratios and high oxygen state magma reflected from zircon, signifying that slab derived component was added into the overlying mantle region through fluid/melt. The geochemical characteristics of Wulian granites could be explained by the ridge (between the Izanagi and Pacific plates) subduction model, which began to influence the tectonic evolution of the Xuhuai region and Shandong Peninsula since ~130 ± 5 Ma. The following slab window between two plates plausibly explains regional extension, resulting in hot magma upwelling and the formation of the A-type granite belt. Integrating the Lower Yangtze River belt and Shandong Peninsula A-type granite proves the northward movement of the ridge between the Pacific Plate and Izanagi Plate.
The Qinling Orogen resulted from the collision between the North and South China blocks in the Triassic. Mesozoic granitoids, ranging from the Triassic to the Cretaceous, are widely distributed in this orogen, and they provide excellent clues for understanding the crustal evolution and geodynamic evolution of the orogenic belt. The Triassic belt is mostly located in the South Qinling, whereas the Cretaceous belt is located mostly in the North Qinling. The Taibai complex pluton is located at the conjunction of the two belts. Here we present a data set comprising zircon U-Pb dating and elemental and Sr-Nd isotopic geochemistry for Late Mesozoic granite and microgranular enclaves (MME) exposed in the Taibai complex pluton. The granite and MME yield concordant U-Pb zircon ages of 124 to 118 Ma, indicating that they were products of roughly simultaneous magmatism in the Late Mesozoic. The granite rocks are high-K, calc-alkaline, and weakly peraluminous in compositions, and they are characterized by enrichment in large ion lithophile elements (e.g., Rb, Ba), depletion in high field strength elements (e.g., Nb, Ta, Zr, Ti), and variable Sr/Y ratios of 7.64 to 63.6. Low MgO, Cr, and Ni contents imply that the magma(s) were essentially crust-derived. Both the granite and the MME show relative depletion in Sr-Nd isotopic composition (initial 87Sr/86Sr of 0.704 4 to 0.706 7, initial εNd values of -3.4 to -2.6), suggesting that the magma(s) originated from juvenile crustal rocks. These Sr-Nd isotopic characteristics are significantly different from those of other Late Mesozoic granitoids exposed elsewhere in the Qinling orogenic belt, which formed from much older and enriched sources and with negligible contributions from mantle or juvenile crust. We propose a reworking event of the juvenile crust during the Late Mesozoic that was triggered by the tectonic extension and subsequent asthenospheric upwelling that occurred in eastern China.
We present crustal models for seismic P-waves (Vp), S-waves (Vs) and the Vp/Vs ratio across the southern North China Craton along latitude 36°N. Our results are based on inverse and forward modeling of long-range wide-angle reflection/refraction data. The crust of the southern Ordos Block has high lower crustal velocity (7.0 km/s) and a Moho depth of ~42 km. In contrast, thick sediments and a lower average velocity (compared with the Ordos block) found underneath the Shanxi Graben are likely to be the products of rifting that has occurred since the Cenozoic. Steep Moho dips, exposed basement rock and higher average crustal velocity beneath the Lüliang Mountain and the Taihang Mountain are characteristic of an orogenic belt. The Tanlu fault and the Liaocheng-Lankao fault show sharp crustal velocity variations and a Moho offset. This seismic profile sampled the entire region of the Eastern North China Craton where the cratonic root has been destroyed and the unique crustal structure is correlated with the substantially modified lithosphere. Our crustal seismic velocity model shows a strong correspondence between surface geology, local tectonics and the deep crustal structure.
The Nanling-Xuancheng ore region of Anhui Province is located in the Middle-Lower Yangtze River metallogenic belt. Insufficient exploration and research have been carried out in this newly defined ore district, although the Chating large porphyry Cu-Au deposit and a few middle-sized skarn-type Cu polymetallic deposits have been discovered. In this study, we carried out high-resolution seismic reflection, magnetotelluric, gravity, and magnetic investigations, and constructed the 3D geological structure of the uppermost crust in a depth range of 0–5 km using a comprehensive inversion of the new data constrained by previous deep-drilling data. We hence proposed some new insights to understand the mineralization processes of this district. A system of alternating ridges and valleys is suggested as the major structure pattern, composed of "two-layer structure" of the basins and "three-layer structure" of anticlines. Moreover, a conjugated fault system and its distribution features are revealed in our models, including the Jiangnan fault, Zhouwang fault, and Kunshan thrust nappe. The Jiangnan and Kunshan faults are suggested to have controlled the diagenesis and metallogenesis. Two deep concealed plutons located in Chating and Magushan are found, forming the Mesozoic diorite-felsic intrusions. These intrusions are believed to be the causes of hydrothermal deposits such as the Chating deposit and the Magushan deposit.
The Lhasa terrane records voluminous magmatism related to the subduction of the Neo-Tethyan oceanic lithosphere, the study of which constrains the tectonomagmatic evolution of the region. We report zircon U-Pb ages, whole-rock compositions and Sr-Nd isotopic data from volcanic rocks in the Bangba district within the central Lhasa subterrane to constrain their magmatic source and petrogenesis. Zircon U-Pb dating of two volcanic rock samples yields End Cretaceous ages of 70.0 ± 0.8 and 74.3 ± 1.2 Ma. The rocks have high SiO2 (65.41 wt.%–68.45 wt.%), Al2O3 (16.16 wt.%–16.59 wt.%) and K2O (5.00 wt.%–6.73 wt.%) contents, and low TFe2O3 (2.33 wt.%–2.79 wt.%), MgO (0.64 wt.%–1.44 wt.%) and TiO2 (0.61 wt.%–0.65 wt.%) contents, with aluminium saturation indices (A/CNK) of 0.99–1.06. The major- and trace-element compositions of the rocks show they are metaluminous to slightly peraluminous high-K calc-alkaline trachydacite. The relatively high SiO2 and Sr-Nd isotopic compositions ((87Sr/86Sr)i = 0.722 654, 0.722 038 and 0.725 787 and εNd(t) = -12.27, -12.36 and -6.09, respectively) indicate that the trachydacites formed by partial melting of crustal material. The trachydacites are relatively enriched in light rare earth elements, depleted in heavy rare earth elements, have high (La/Yb)N and (Gd/Yb)N ratios (> 61 and > 6, respectively), and low Y (< 18 ppm) and Yb (< 18 ppm) contents, indicating they most likely formed from partial melting of lower crust in the garnet stability field. Considering the geodynamic setting of the region during this period, partial melting of the ancient Lhasa crust was likely triggered by underplating mafic magmas during rollback of the Neo-Tethyan slab.
Systematical analyses of data from GEOROC and PetDB database show that large amount of Cenozoic andesites occurred in the various oceanic environments such as mid-oceanic ridge, plume- related island and oceanic arc. In this study, we employed the geochemical data of 351 mid-ocean ridge andesites (MORA), 2 539 plume-related andesites (PRA) and 3 488 oceanic arc andesites (OAA) from the database to discuss the relationship between andesite tectonic settings and their geochemical features, thereby making an attempt to construct tectonic discrimination diagrams. Based on the data-driven pattern, all available elements were employed to derive logratios for the possible coordinates, and the overlap-rate calculation was adopted to evaluate the discrimination effect of more than 330 000 prospective diagrams. Finally, four tectonic discrimination diagrams have been successfully established to identify MORA, PRA and OAA, which can be utilized to identify the original settings of andesite with an age range from Cenozoic to Archean a certain extent. Of these diagrams, PRA is mainly distinguished by high LREE/HREE ratio due to enriched mantle source. Whereas, OAA is mainly characterized by high LILE/HFSE ratio, which reveals that fluids derived from subducted slab play an important role in forming oceanic arc andesites. Consequently, the petrogenesis of andesites is closely related to their tectonic settings. However, it should be noted that those andesites formed in both continental and oceanic environments cannot be effectively distinguished using these diagrams. We strongly recommend integrating the discrimination diagrams result with other geological information to reach a comprehensive interpretation of evolution history with those ancient andesites. This paper presents a case study which suggests that data-driven method is a powerful tool for solving geological problems in this 'big data' era.
Basaltic magmas can provide important information about mantle source nature, tectonic settings and tectonic evolution for a given terrain. This paper reports geology, petrography and geochemistry of whole-rock major and trace elements and Nd-Sr isotopes for a suite of garnet amphibolites from southeastern Wulan (Ulan), Quanji Massif, northwestern China. The garnet amphibolites were likely generated from basaltic lavas, associated with both paragneisses and orthogneisses of the lower Delingha Group. The basaltic protolith of these amphibolites can be broadly constrained to be formed at ~2.33 Ga in an extensional setting post-collision. The geochemistry of amphibolites shows subalkaline and highly evolved characteristics. They display high-Fe low-Ti characteristics, with TFeO of 13.1 wt.%–17.9 wt.% and TiO2 of 1.42 wt.%–3.09 wt.% (in most samples TiO2 ≤2.5 wt.%). The chondrite-normalized REE patterns show enrichment of LREE and LILE and the primitive-mantle-normalized incompatible element patterns display negative P, Ti, Nb-Ta and Zr-Hf anomalies. The (87Sr/86Sr)t values of 0.697 8–0.712 3 and εNd(t) values of -2.81–5.08 respond to depleted mantle model ages (TDM) of 2.33–3.30 Ga. These suggest that the precursor magmas of the protolith of the garnet amphibolites were probably derived from the Early Paleoproterozoic depleted sub-continental lithospheric mantle that had been metasomatized by subduction-induced fluids and melts. The precursor basaltic magmas were contaminated by the older crustal components during magma ascending. This post-collisional basaltic magmatic event at ~2.33 Ga in Quanji Massif thus enhanced the subduction shutdown or slowdown tectonic regime both in NW China and coevally with those plate tectonics in some important domains worldwide during the Early Paleoproterozoic.
Compositional changes in successively erupted felsic rocks can be used to infer physical changes in lower crustal conditions and to enhance the understanding of the tectonic regime. This study presents geochronological, geochemical and isotopic data for two Ⅰ-type granitic plutons in the Sonid Left Banner of the Central Asian Orogenic Belt. Our new data, together with compiled Ⅰ-type granitoid data, reveal the presence of magma compositional transition at ~305 Ma in the Baolidao arc-accretion belt. The early stage granitoids (330–305 Ma) are medium-K calc-alkaline with higher Sr/Y ratios. The late stage granitoids (305–270 Ma) are high-K calc-alkaline with lower Sr/Y ratios. The two-stage granitoids have roughly similar predominately positive Sr-Nd-Hf isotope values, but with a decreasing trend from the early to late stages. Geochemical data indicate that the early stage granitoids were generated by dehydration melting of juvenile mafic crust at amphibole-dominated depths. In contrast, the late stage granitoids were produced by dehydration melting of a mixed lithology containing juvenile K-rich mafic lower crust and supracrustal materials at the plagioclase-stable crustal level. We propose that the compositional transition of these granitoids can be linked with different slab behaviors of the northward subducting Paleo-Asian oceanic plate, and also with the back-arc tectonic settings.
The Late Paleozoic volcanic rocks are widespread in the Western Tianshan, and mainly consist of basalt, trachyte, trachy-andesite, andesite and rhyolite. However, the formation timing and tectonic implications of the volcanic rocks remain controversial. To obtain a comprehensive dataset with adequate spatial coverage, we collected the published zircon U-Pb age data of the Late Paleozoic volcanic rocks in the Western Tianshan, and mapped the temporal and spatial distribution of these volcanic rocks. Our compiled dataset indicates that the volcanic rocks in the Western Tianshan were formed in two major stages: Late Silurian–Early Carboniferous and Late Carboniferous–Middle Permian. The southward subduction of the Northern Tianshan Ocean formed a large number of Late Silurian–Early Carboniferous volcanic rocks. The flare-up of Late Carboniferous to Middle Permian magmatism in the NTOB was considered to be the response of the slab-roll back of the northern Tianshan Ocean. Therefore, arc magma assemblage and spatial-temporal distribution are of great significance to identify the dynamic mechanism transformation of subduction zone.
Crustal anatexis in continental subduction zones has great bearing on chemical differentiation of the continental crust at convergent plate boundaries. This was experimentally investigated for ultrahigh-pressure (UHP) metafelsic rocks at 0.5–3.0 GPa and 650–900 ℃. The results show that partial melting begins at about 750 ℃ when pressure drops from 3.0 to 2.0 GPa, corresponding to decompressional exhumation of the deeply subducted continental crust. As the pressure further decreases to 1.0 GPa, the partial melting degree reaches the maximum of ~25% at 900 ℃. Partial melts produced in these experiments are rich in silica and alkali, and poor in iron, manganese and magnesium. As the degree of partial melting increases, the composition of partial melts gradually converges toward homogeneous one. In the absence of free water, the partial melting of metafelsic rocks were triggered by the breakdown of hydrous minerals. At low temperatures of ~750 ℃ at 1.0–2.0 GPa, phengite dehydration melting occurs at first, giving rise to small amounts of felsic melts and peritectic K-feldspar. As the temperature rises up to 850–900 ℃, biotite begins to break down and gives rise to large amounts of felsic melts and peritectic minerals such as garnet, K-feldspar and orthopyroxene. It is noted that peritectic garnet is much different from anatectic garnet crystallized from anatectic melts and metamorphic garnet formed through metamorphic dehydration reaction under subsolidus conditions. The peritectic garnet is characterized not only by anhedral shapes with many multiphase crystal inclusions but also by compositions poor in spessartine and grossular but rich in almandine and pyrope. On the other hand, the anatectic garnets are characterized not only by euhedral shapes with few inclusions but also by compositions rich in grossular and spessartine but poor in almandine and pyrope. These observations provide experimental constraints on the origin of garnets in UHP metamorphic rocks, which have great bearing on understanding of anatectic metamorphism in collisional orogens.
Orthopyroxene is an important constitutive mineral in the crust and the upper mantle. Its thermal properties play a key role in constructing the thermal structure of the crust and the upper mantle. In this study, we developed a new method to synthesize polycrystalline ferrosilite, one end-member of orthopyroxene, via the reaction of FeO + SiO2 → FeSiO3. We found that the P-T condition of 3 GPa and 1 273 K is suitable to synthesize dense ferrosilite samples with low porosity. We employed the transient plane-source method to investigate the thermal conductivity κ and thermal diffusivity D of synthetic ferrosilite at 1 GPa and 293–873 K, of which, κ = 1.786 + 1.048 × 103T-1 – 9.269 × 104T-2 and D = 0.424 + 0.223 × 103T-1 + 1.64 × 104T-2. Our results suggest phonon conduction should be the dominant mechanism at P-T conditions of interest since the thermal conductivity and the thermal diffusivity of ferrosilite both decrease with increasing temperature. The calculated heat capacity of ferrosilite at 1 GPa increases with temperature, which increases with increasing temperature with about 10% per 100 K (< 500 K) and 4% per 100 K (> 500 K). Iron content of an asteroid significantly influences its thermal evolution history and temperature distribution inside. It is expected that the mantle temperature of the Fe-rich asteroid will be higher and the Fe-rich asteroid's cooling history will be longer.
Four carbonate carbon isotope (δ13Ccarb) excursions are recognized in the Ediacaran Doushantuo Formation in South China, the genesis of which remains disputed. Whereas three of these δ13Ccarb excursions possibly record secular biogeochemical variations, the other one, namely Weng'an negative carbonate carbon isotope excursion (WANCE) with an age of ca. 620 Ma occurs mainly within the northern Yangtze Platform. In this study, a SIMS U-Pb age of ca. 620 Ma was documented from continental rift volcanism within the adjacent South Qinling terrane. Its temporal overlap with WANCE suggests a possible causal link. Volcanism-induced seafloor uplift may have prompted DOC oxidation in surficial oxygenated oceans, inducing the occurrence of WANCE.
Although tropical cyclones play a critical role in global climate changes, their long-term variations in the past are not well documented. In this article, a sediment core from the South Yellow Sea was studied in order to reveal the influence of tropical cyclones on depositional processes. Integrating the results of radiocarbon dating and sediment grain-size analysis, we show that the studied sequence was deposited during the Holocene and the sedimentary dynamics were stable and at a relatively low level, with a median grain-size range of 5.3–8.7 μm. It is found that coarse particles were likely transported by highly dynamic depositional events. Based on the findings, a record of paleo-tropical cyclones was derived for the Early Holocene, and several intervals with a reduced influence of tropical cyclones were identified. In addition, it reveals a good agreement between the grain-size results and the changes in solar activity, monsoonal intensity, and the El Niño-Southern Oscillation. Overall, it can be concluded that the influence of tropical cyclones on the sedimentary evolution of the muddy zone of the South Yellow Sea was substantial during the Early Holocene on centennial timescales, and that solar maxima may control the intensity of tropical cyclones via strengthening the walker circulation over the tropical Pacific.
The beginning of the Phanerozoic was marked by abundant phytoplankton, the occurrence of animals and a newly established ecosystem structure, which in turn, resulted in different modes of organic matter (OM) cycling and preservation. In this study, we present an integrated paleontological and geochemical study of the Lower Cambrian Yanjiahe and Shuijingtuo formations in the Three Gorges area, South China. Rock samples were analyzed for major and trace elements, in order to evaluate the marine productivity. Fossil samples were recovered using palynological methods, destructive analytical techniques, SEM analysis and thin section microscopy. The organic-walled microfossils in the investigated sections include marine microphytoplankton, linings of sponge spicules, small shelly fossils and some unnamed sheet-like and net-like microfossils (animal parts). Additionally, fecal structures and coprolites fossils were found. Three intervals in the Luojiacun Section were proposed to discuss the links among microfossil assemblages and their influence on OM cycling and preservation. We found that the marine microphytoplankton was the main source of OM in the investigated sections. The appearance of fecal structures groups and massive appearance of animals in the pelagic zone indicate a higher transfer efficiency of OM aggregates through the water column. Oxygenated conditions in the water-column would have led to a moderate to low preservation of OM, an effect which would have been further enhanced by the potential presence of bioturbators. Overall, our study reveals the contribution of various organisms to the sedimentary OM and highlights the links and feedbacks among microfossil assemblages, OM recycling efficiency and OM preservation in the tipping point of the Phanerozoic "new world".
Continuous exploration has triggered a heated debate on hydrocarbon resource potential in the southern slope zone of the Kuqa foreland basin, and sources of the Mesozoic–Cenozoic oil and gas have become a key problem to be solved in this region. Composition and organic geochemical parameters of crude oil and natural gas from the southern slope zone of the Kuqa foreland basin were illustrated in order to reveal their origin by using a combination of gas chromatograph (GC), gas chromatogram-mass spectrum (GC-MS) and carbon isotope analyses. The characteristics of crude oil, such as low density, viscosity, solidification point and sulfur content, and high wax content, indicate that source of the crude oil is continental. The biomarker compositions of crude oil are characterized by low to medium molecular weight compounds (n-C12 to n-C20), high Pr/Ph ratios (> 1.0), low phytane/n-C18 ratios (0.06–0.54), and predominant regular sterane C29. All biomarker parameters clearly indicate that the crude oil was derived mainly from algae and aquatic plankton and deposited under weak reduction-oxidation environment, and has the characteristics of mixed kerogens. The Cretaceous crude oil was mainly derived from the Triassic lacustrine source rocks, which also contributed to the Paleogene crude oil together with Jurassic coal source rocks. Natural gas is characterized by moderate methane content, high heavy hydrocarbon and nitrogen content, and no hydrogen sulfide. The methane and ethane in Paleogene natural gas are relatively rich in 13C with δ13C1 and δ13C2 values ranging from -37.3‰ to -31.2‰ (mean = -34.25‰) and from -25‰ to -21.3‰ (mean = -23.09‰), respectively, indicating the coal-derived gas from the Middle and Lower Jurassic strata. Hydrocarbon products in the southern slope zone of the Kuqa foreland basin are primarily generated from source rocks in the mature stage. The low-amplitude structural and lithologic traps with the updip pinch-out sand bodies or plugging secondary fault at relatively high tectonic positions are the most favorable areas for discovery and breakthrough in the study area. Results of this study will provide useful information for controlling factors of reservoirs and oil and gas exploration deployment in the southern slope zone of the Kuqa foreland basin.
Using the Global Network of Isotopes in Precipitation (GNIP) method at three stations in the Cis-Ural region (Iskor, Perm, and Kungur), the monitoring results of the isotope composition of atmospheric precipitation conducted in 2016–2019 are reported. Local lines of meteoric waters were obtained for each station and for the Cis-Ural region as a whole. The latitudinal zoning of changes in isotope parameters from north to south in the Cis-Ural region is noted, which is related to meteorological parameters. The northern regions are characterized by isotopically light values compared to the southern regions. Additionally, the correlation relationship between the isotopic compositions of atmospheric precipitation and meteorological parameters at each station is estimated. Correlation analysis data show that about 50%–90% of variations in the isotopic composition of precipitation within the annual cycle are determined by the air temperature. At the same time, the amount of precipitation shows a relationship between the average strength and the isotopic composition of precipitation for a series of observations at Kungur Station; for a series of observations at Iskor and Perm stations, there is no correlation. Differences in the relationship with meteorological data are explained by different sources of moisture introduction to the station and, accordingly, the formation of the isotopic composition of precipitation. The flow of moisture is controlled by the transfer of air masses in different directions, the formation of cyclones and anticyclones. So, for the northern part of the region during the year is characterized by the predominance of cyclones, whereas for the southern-anticyclones.