Characterization of the Lower Silurian Longmaxi marine shale in Changning area in the south Sichuan Basin, China

On the basis of the core description, thin sections, and geochemistry data, we characterized the Lower Silurian Longmaxi marine shale in the Changning area in the south Sichuan Basin. Ten lithofacies were identified in the Lower Silurian Longmaxi marine shale: (a) massive silty shale (Lithofacies A); (b) laminated siliceous calcareous shale (Lithofacies B); (c) calcareous shale (Lithofacies C); (d) silty dolomitic shale (Lithofacies D); (e) argillaceous siltstone (Lithofacies E); (f) siltstone (Lithofacies F); (g) laminated silty shale (Lithofacies G); (h) silty calcareous shale (Lithofacies H); (i) argillaceous lime shale (marl) (Lithofacies I); and (j) bentonite (Lithofacies J). Obvious heterogeneity occurred in different lithofacies in terms of mineralogy, total organic carbon (TOC) content, porosity, permeability, pore surface area, total pore volume, and total gas content. Among these lithofacies, Lithofacies B has the highest porosity (5.4%), pore surface area (12.79 m2/g), total pore volume (0.015 cc/g), TOC content (4.37%), total gas content (3.4 m3/ton), and relatively high brittle minerals, suggesting great shale gas potential. Lithofacies A, C, and D have a relatively high brittle minerals, porosity, pore surface area, total pore volume, TOC content, and total gas content, which means that these 3 lithofacies show relatively good potential for shale gas. The rest of the lithofacies have low brittle minerals content, porosity, pore surface area, total pore volume, TOC content, and total gas content, representing poor shale gas potential.

Rare earth elements geochemistry characteristics and their geological implications of lacustrine oil shale from Chang 7 oil layer in southern Ordos Basin, China

Located in the middle‐western area of China, Ordos Basin is a large continental basin with vast petroleum resources. Oil shale of Chang 7 oil layer from Triassic Yanchang Formation represents the typical lacustrine oil shale in China. Oil shale samples were collected from Chang 7 oil layer to study the characteristics of elements geochemistry. The rare earth elements (REEs) concentration of oil shale samples varies from 138.51 to 206.36 μg/g with an average of 162.64 μg/g, close to the average REEs content of the North American shale Composite and slightly higher than the upper continental crust. The chondrite‐normalized and North American shale composite‐normalized REEs distribution patterns explain that oil shale samples, together with silty mudstone samples, may have been derived from a similar terrigenous source. The oil shale deposited mainly in early diagenetic stage B period and the paleoclimate condition was warm and humid with moderate chemical weathering. The whole sedimentary rate of oil shale was low. The source rocks of oil shale were mainly from the felsic rocks and deposited in the continental island arc tectonic setting. The total REE of oil shale samples show positive correlations with Al, Si, K, and ash yield concentration and negative correlations with oil yield, Fe, P, and total sulfur concentration, illustrating that REEs concentration in clay minerals is more than that in organic matter. In addition, the light REEs and heavy REEs are both present in clay minerals and controlled by land‐derived detritus.

Sequence stratigraphy and depositional system analysis in Paleogene Nanpu Sag: Sequence model and sediment partition

In this study, we use 3D seismic, well‐log, and core data to investigate the sequence stratigraphy and facies associations of the Paleogene in the Nanpu Sag, Bohai Bay Basin, East China. Three second‐order sequences—SQEs3‐SQEs2, SQEs1, and SQEd—and 12 third‐order sequences are identified. Fan‐delta, braid‐delta, sublacustrine fan, and lacustrine systems are identified. Four types of tectonic‐induced sequence models are distinguished, which consist of fault‐cliff type, fault‐slope type, axial ramp type, and multiple fault‐terrace type. Sequence architecture of third‐order sequences is dominated by thin lowstand systems tracts with thick transgressive systems tracts (TST) and highstand systems tracts (HST). High‐resolution sediment‐partition pattern analysis shows that a large volume of sediments can be delivered into prodelta/deep‐lacustrine environments during transgressive periods and lake‐level highstand. This study also shows that sand/strata ratios in fan‐delta/braid‐delta front of systems tracts scale have a negative correlation with the subsidence rate, which is demonstrated by the finding that higher subsidence rates correspond to lower sand/strata ratios. This result suggests that subsidence rate has a significant influence on high‐frequency sequences. The occurrence of the high volume of sand‐rich deposits developed in TST and HST and relatively high sand/strata ratios within these sequence units reveal that TST and HST could also have a high hydrocarbon potential. The strongly asymmetrical accommodation generated by an early rapid tectonic subsidence and late tectonic quiescence results in sequence architecture variations between passive‐margin basins and rift basins. Different subsidence rates among different structural belts result in sequence architecture variations within different sequence models.

The genesis of the Xujiagou copper deposit, Mian‐Lue‐Ning area of Shaanxi Province, NW China: Constraints from mineral chemistry and in situ Pb isotope composition

The Mian‐Lue‐Ning area, bounded by the Hanjiang fault zone in the south and the Mian‐Lue suture in the north, is one of the most important polymetallic mineralization concentration regions in the Qinling orogenic belt. The Xujiagou copper deposit is representative in this area with chalcopyrite and pyrrhotite as the main ore minerals, subordinate sphalerite, and pyrite. However, the source of the ore‐forming materials and the genesis of the Xujiagou copper deposit are still in a controversy due to the lack of detailed study on the mineral composition. In this study, the electron probe micro‐analyzer (EPMA) analysis shows that the pyrrhotite in the Xujiagou copper deposit is mostly monoclinic pyrrhotite of low‐temperature phase, indicating that the metallogenic temperature should be below 304 °C. The percent of FeS molecules in sphalerite is 8.16–12.1%, indicating that the metallogenic temperature is 230–258 °C, which is mesothermal mineralization. The in situ Pb isotope composition of chalcopyrite (206Pb/204Pb = 19.087–20.861; 207Pb/204Pb = 15.533–16.062; 208Pb/204Pb = 38.121–39.391) and pyrrhotite (206Pb/204Pb = 20.241–20.664; 207Pb/204Pb = 15.7–15.996; 208Pb/204Pb = 38.659–39.372) in the Xujiagou copper deposit was analysed by fLA‐MC‐ICPMS, and it indicates that the ore‐forming materials were mainly sourced from the upper crust. Moreover, the contents and ratios of Co and Ni in pyrrhotite (Ni: 500–1600 ppm, Co < 630 ppm, Co/Ni = 0.03–1.71) suggest that the ore‐forming material of the Xujiagou copper deposit could be extracted from the spilite of the Guojiagou Formation, and the ore minerals inherited the geochemical characteristics of the spilite. Combined with previous studies of the Tongchang copper deposit in the Mian‐Lue‐Ning area, it is proposed that the Xujiagou and Tongchang copper deposits have similar ore‐forming materials source and evolution processes, and their formations may be related to the breakup event of the Rodinia supercontinent. However, the relatively lower 207Pb/204Pb and 206Pb/204Pb ratios of the Tongchang chalcopyrite and pyrite than those of the Xujiagou chalcopyrite and pyrrhotite indicate that the Tongchang copper deposit suffered more intense influence of the lower crust materials due to the emplacement of the Tongchang diorite.

The genesis of the Xujiagou copper deposit, Mian-Lue-Ning area of Shaanxi Province, NW China: Constraints from mineral chemistry and in situ Pb isotope composition

The Mian-Lue-Ning area, bounded by the Hanjiang fault zone in the south and the Mian-Lue suture in the north, is one of the most important polymetallic mineralization concentration regions in the Qinling orogenic belt. The Xujiagou copper deposit is representative in this area with chalcopyrite and pyrrhotite as the main ore minerals, subordinate sphalerite, and pyrite. However, the source of the ore-forming materials and the genesis of the Xujiagou copper deposit are still in a controversy due to the lack of detailed study on the mineral composition. In this study, the electron probe micro-analyzer (EPMA) analysis shows that the pyrrhotite in the Xujiagou copper deposit is mostly monoclinic pyrrhotite of low-temperature phase, indicating that the metallogenic temperature should be below 304 °C. The percent of FeS molecules in sphalerite is 8.16–12.1%, indicating that the metallogenic temperature is 230–258 °C, which is mesothermal mineralization. The in situ Pb isotope composition of chalcopyrite (206Pb/204Pb = 19.087–20.861; 207Pb/204Pb = 15.533–16.062; 208Pb/204Pb = 38.121–39.391) and pyrrhotite (206Pb/204Pb = 20.241–20.664; 207Pb/204Pb = 15.7–15.996; 208Pb/204Pb = 38.659–39.372) in the Xujiagou copper deposit was analysed by fLA-MC-ICPMS, and it indicates that the ore-forming materials were mainly sourced from the upper crust. Moreover, the contents and ratios of Co and Ni in pyrrhotite (Ni: 500–1600 ppm, Co < 630 ppm, Co/Ni = 0.03–1.71) suggest that the ore-forming material of the Xujiagou copper deposit could be extracted from the spilite of the Guojiagou Formation, and the ore minerals inherited the geochemical characteristics of the spilite. Combined with previous studies of the Tongchang copper deposit in the Mian-Lue-Ning area, it is proposed that the Xujiagou and Tongchang copper deposits have similar ore-forming materials source and evolution processes, and their formations may be related to the breakup event of the Rodinia supercontinent. However, the relatively lower 207Pb/204Pb and 206Pb/204Pb ratios of the Tongchang chalcopyrite and pyrite than those of the Xujiagou chalcopyrite and pyrrhotite indicate that the Tongchang copper deposit suffered more intense influence of the lower crust materials due to the emplacement of the Tongchang diorite.

Early Paleozoic Tarim Orocline: Insights from paleogeography and tectonic evolution in the Tarim Basin

The Tarim Basin, situated between the Paleo‐Asian tectonic domain and the Proto‐Tethys tectonic domain, has undergone a complex tectonic evolutionary process under various geodynamic conditions and has provided abundant information about the dynamic evolution of the peripheral orogens. From the Sinian to Early Ordovician, the Tarim–Qaidam Block was under a regional extensional setting. A carbonate platform developed widely in the basin, flanked by a passive continental margin setting. As the extension enhanced, the spreading of the Proto‐Tethys Ocean was accompanied by subduction and collision, which resulted in a nearly linear subduction zone. Since the Late Cambrian, a continuous oblique subduction and induced sinistral shearing produced an initial bending of the Proto‐Tethys subduction zone, leading to rotation and bend of the original Tarim–Qaidam Block and internal long striped subaqueous paleo‐uplift. During the Middle and Late Ordovician, the Altyn–Qilian Orogen underwent continent–continent collision and continental crust deep subduction. The buckling of the subduction system enhanced under the sinistral transpressional field, resulted in the bending of the Altyn–Qilian Orogen to an S‐shape. The subaqueous uplift areas in the basin were further elevated, rotated, and eroded to show an S‐shape distribution and an unconformity developed between the Silurian sediments and underlying strata. The extent of the carbonate platform decreased, around which developed a high‐energy facies in a platform margin and formed important oil–gas reservoirs. In the Silurian to Middle Devonian, the Tarim Basin evolved into a “two uplifts with a depression” framework under the dual‐directional compressional field. Near the end of the Late Devonian, the S‐shape distribution pattern of paleo‐uplifts was covered by the Upper Devonian and Carboniferous sequence in an angular unconformity. Moreover, the planar distribution of thrust faults with strike‐slip property around the paleo‐uplifts also showed an S‐shape. Accompanying small‐scale strike‐slip faults presented as the radial axial cleavages of plunging vertical folds around a vertical hinge, which indicated that the pre‐existing thrust faults bent around a vertical axis resulting from late strike‐slip faults, demonstrating a coupling between the Tarim Basin and orocline of the Early Paleozoic orogens in the Proto‐Tethys Ocean, called the Tarim Orocline.

Provenance of Middle to Late Triassic sedimentary rocks in the Zoige Depression in the NE part of the Songpan–Ganzi Flysch Basin: Petrography, heavy minerals, and zircon U–Pb geochronology

The Zoige Depression is an important depocenter zone within the northeast of the Songpan–Ganzi Flysch Basin, which is bounded by the South China, North China, and Qiangtang blocks, and also forms the northeastern margin of Tibetan Plateau. This paper discusses the provenance of Middle to Late Triassic sedimentary rocks in the Zoige Depression using petrography, heavy mineral assemblages, and zircon U–Pb geochronology. The results demonstrate that the detritus is derived from multiple source regions. Four distinct parent rocks can be distinguished based on the heavy mineral assemblages and lithic fragments: Provenance 1 predominantly comprises intermediate–acidic volcanic rocks; Provenance 2 includes high‐grade metamorphic rocks; Provenance 3 contains a mixture of various detrital components; and Provenance 4 primarily consists of mafic volcanic rocks. The different U–Pb ages of the zircons from the Middle to Late Triassic ranging from 260 to 280, 429–480, 792–974, and 1,800–2,500 Ma represent distinct source regions, which are comparable to the 4 provenances mentioned above: Provenance 1 (260–280 Ma), Eastern Kunlun Orogen; Provenance 2 (429–480 and/or 1,800–2,500 Ma), Qinling orogeny (mainly in North Qinling); Provenance 3 (1,800–2,500 Ma), the North China Block; and Provenance 4 (792–974 Ma), the Yangtze Block. Overall, the detritus in the Middle Triassic (Ladinian, T2zg) primarily originates from the Eastern Kunlun Orogen and North Qinling. During the Late Triassic (Early Carnian, T3z), the southern margin of the North China Block was likely transported westward to the basin by a river network between the North China and South China blocks flowing through the Qinling region, because of the predominance of the detrital zircon age ranging from 1,800 to 2,500 Ma and the occurrence of quartz sandstone with visible enlargement texture. Since the Late Triassic (Middle Carnian, T3zh), great changes have occurred in the source terrains, such as the absence of sources of the Eastern Kunlun Orogen and North China Block and the predominance of Yangtze Block. This drastic change can be explained by the Triassic collision between the South China and North China blocks, and the clockwise rotation of the South China Block progressively closed the basin and uplifted the Qinling orogeny.

In situ LA‐ICP‐MS trace element analysis of magnetite from the late Neoarchean Gongchangling BIFs, NE China: Constraints on the genesis of high‐grade iron ore

The Precambrian banded iron formations (BIFs) not only relate to the evolution of life, ocean, and atmosphere but also provide important reserves of iron around the world. The Gongchangling iron ore deposit located in the Anshan‐Benxi area of Liaoning Province, China, is oxide facies Algoma‐type BIFs, and the Gongchangling No.2 mining area is famous for the production of high‐grade iron ore in China. Magnetite is the main ore mineral in the Gongchangling iron ore deposit, and the magnetite mainly exhibits three modes of occurrence: BIFs (without actinolite), actinolite‐bearing BIFs, and high‐grade iron ore. Trace elemental compositions of the magnetite with different occurrences of the Gongchangling iron ore deposit were obtained by laser ablation inductively coupled plasma mass spectrometry to constrain the genesis of the high‐grade iron ore. The magnetite from actinolite‐bearing BIFs shows relatively lower contents of Mg, Al, Mn, and Zn compared to the magnetite from BIFs (without actinolite), suggesting that coexisting minerals have played an important role in the trace element concentration in magnetite. The magnetite from high‐grade iron ore has lower contents of Ti and V and higher contents of Al and Mn than counterpart from BIFs (with/without actinolite), indicating that the high‐grade iron ore may be reformed by high temperature metamorphic hydrothermal fluid. The staurolite‐garnet‐biotite schist is the wall‐rock of high‐grade iron ore, and the garnet‐biotite geothermometry is used to evaluate the metamorphic temperature of 593 ± 17 °C. It is proposed that the metamorphic hydrothermal fluid produced during regional metamorphism reformed BIFs to generate high‐grade iron ore.

In situ LA-ICP-MS trace element analysis of magnetite from the late Neoarchean Gongchangling BIFs, NE China: Constraints on the genesis of high-grade iron ore

The Precambrian banded iron formations (BIFs) not only relate to the evolution of life, ocean, and atmosphere but also provide important reserves of iron around the world. The Gongchangling iron ore deposit located in the Anshan-Benxi area of Liaoning Province, China, is oxide facies Algoma-type BIFs, and the Gongchangling No.2 mining area is famous for the production of high-grade iron ore in China. Magnetite is the main ore mineral in the Gongchangling iron ore deposit, and the magnetite mainly exhibits three modes of occurrence: BIFs (without actinolite), actinolite-bearing BIFs, and high-grade iron ore. Trace elemental compositions of the magnetite with different occurrences of the Gongchangling iron ore deposit were obtained by laser ablation inductively coupled plasma mass spectrometry to constrain the genesis of the high-grade iron ore. The magnetite from actinolite-bearing BIFs shows relatively lower contents of Mg, Al, Mn, and Zn compared to the magnetite from BIFs (without actinolite), suggesting that coexisting minerals have played an important role in the trace element concentration in magnetite. The magnetite from high-grade iron ore has lower contents of Ti and V and higher contents of Al and Mn than counterpart from BIFs (with/without actinolite), indicating that the high-grade iron ore may be reformed by high temperature metamorphic hydrothermal fluid. The staurolite-garnet-biotite schist is the wall-rock of high-grade iron ore, and the garnet-biotite geothermometry is used to evaluate the metamorphic temperature of 593 ± 17 °C. It is proposed that the metamorphic hydrothermal fluid produced during regional metamorphism reformed BIFs to generate high-grade iron ore.

Dynamic processes and mechanisms for collision to post‐orogenic extension in the Western Dabie Orogen: Insights from numerical modeling

Post‐orogenic extension is the last stage of a tectonic process towards the end of each Wilson cycle in both ancient and modern continental collisional zones, but their dynamics, controlling factors, and roles for the post‐orogenic extension processes remain debatable. We present the first two‐dimensional thermo‐mechanical numerical model of double sutures inspired by the Western Dabie Orogen to investigate the dynamics of post‐orogenic extension. Our results indicate that the pro‐continent re‐subduction can delay or stop regional extensional deformation and subsequent voluminous magmatic activity caused by slab breakoff in the collision zone. After the end of the collision stage, the re‐subduction slab under the residual lithosphere in the model with lower convergence, as a strong unit, can maintain the gravitational balance of the whole post‐orogen system, whether crust eclogitization is considered or not. If the oceanic crust and micro‐continental crust eclogitization are applied to the model with medium convergence, the lower crust and the residual lithospheric mantle in the collision zone will be completely removed by the gravitational instability and upwelling of asthenospheric mantle. The lithospheric architecture of this model with the medium convergence and crust eclogitization is very similar to the present‐day tomography. However, in the model with higher convergence, lithosphere delamination can occur only by upwelling of asthenosphere, whether crustal eclogitization is considered or not. On the basis of a comparison of numerical results with field data from the Western Dabie Orogen, we suggest that the dynamics of post‐orogenic extension in the Western Dabie Orogen is controlled by re‐subduction of the North China Block and crustal eclogitization after high pressure/ultrahigh pressure collision. Thus, the observed regional extensional deformation and voluminous magmatic activity in the Western Dabie Orogen can be self‐consistently explained by interactions between the two sutures and crustal eclogitization.