Linxi Formation Research Articles

Timing of the final closure of the Paleo-Asian Ocean: Evidence from sedimentary and volcanic rocks of the Linxi Formation in southeastern Inner Mongolia, China

Timing of the final closure of the Paleo-Asian Ocean: Evidence from sedimentary and volcanic rocks of the Linxi Formation in southeastern Inner Mongolia, China

Geochemical characteristics and hydrocarbon potential of source rocks of the Upper Permian Linxi Formation in the Kundu-Taohaiyingzi area, NE China

Geochemical characteristics and hydrocarbon potential of source rocks of the Upper Permian Linxi Formation in the Kundu-Taohaiyingzi area, NE China

Volcanic Age and Geochemistry of the Permian Linxi Formation in Northeast China: Implications for the Tectonic Evolution of the Paleo-Asian Ocean

Abstract The tectonic evolution of the Paleo-Asian Ocean (PAO) has been well studied, including its gradual narrowing and closure by subduction. However, aspects of the tectonic evolution of the oceanic domain remain unclear, including the exact timing and nature of the closure. The Central Asian Orogenic Belt (CAOB) was formed by the closure of the PAO and, therefore, contains information about the tectonic evolution of the oceanic domain. Here, we report a study of the petrology, geochronology, and geochemistry of the Taohaiyingzi section of the Permian Linxi Formation in Alukhorqin Banner (Northeast China) in the central part of the CAOB. A newly discovered andesitic tuff from the lower part of the Linxi Formation yields a weighted mean 206Pb/238U age of 262.2 ± 1.1 Ma (n = 87), indicating that the lower part of the Linxi Formation of the Taohaiyingzi section was deposited during the late Guadalupian. Provenance weathering indicators show that the sedimentary rocks of the Linxi Formation are of low maturity. Element geochemical characteristics indicate that the Linxi Formation clastic rocks were derived from eroded magmatic rocks that formed in a continental arc setting and were deposited close to the arc in a continental arc basin environment. The active margin setting was generated by the subduction of the paleo-Asian oceanic plate beneath the Xilinhot–Songliao block. The inferred palaeosalinity of the sedimentary environment changed gradually from brackish to fresh water, suggesting the end of oceanic plate subduction during the late Guadalupian, and the closure of the PAO during or after the Lopingian.

Late-Permian subduction-to-collision transition and closure of Paleo-Asian Ocean in eastern Central Asian Orogenic Belt: Evidence from borehole cores in the Songliao Basin, Northeast China

The process and timing of the closure of the Paleo-Asian Ocean remain uncertain. The upper Permian Linxi Formation in the Songliao–Xilinhot block provides a key sedimentary record of the closure of the eastern Paleo-Asian Ocean and the associated orogenesis. Here we present a seismic reflection profile and geochronological, paleontological, and geochemical data from 600 m of borehole cores (2022–1394 m depth) from the Songliao Basin to constrain the timing, provenance, and tectonic background of the Linxi Formation. Detrital zircons from the Linxi Formation yield a maximum depositional age of 257.4 ± 4.1 Ma. The formation was intruded by a syenite porphyry dike that crystallized at 245.6 ± 2.7 Ma. Furthermore, the paleontological assemblage indicates an upper Permian stratigraphic age. Borehole samples yield fingerprints of major oxides that are similar to clastic rocks in continental arc basins. Their Rb contents are relatively high (92 ppm on average), Rb/Sr (0.02–2.86) and Th/U (1.12–6.30) ratios are low, and (La/Yb)N and Eu/Eu* ratios are 3.85–8.13 and 0.54–0.98, respectively. These geochemical characteristics indicate a source of continental magma arc. Aided by discrimination diagrams, we propose that the Linxi Formation was deposited within a continental arc or on a continental margin during the closure of the Paleo-Asian Ocean. The Linxi Formation was deposited in a water body that experienced a gradual reduction in salinity, transitioning from a marine environment at the bottom to a transitional marine-terrestrial environment at the top. The water body also became gradually shallower and deposited sediment in the tectonic setting of continental arc. These changes indicate that there was a transition from subduction to collision during the Late Permian.

Insights into the formation of Shihuiyao Ta-Nb deposit in southern Great Xing'an Range, NE China: Evidence from chronology and fluid inclusion

Shihuiyao is a typical granite-type Ta-Nb deposit in the southern Great Xing'an Range (SGXR), Northeast China. The ore field is comprised of several granitic intrusions that were emplaced into the Early Permian Linxi Formation during the Late Jurassic (ca. 145 ∼ 150 Ma). The main Ta-Nb mineralization (stage 1–2) is found within the leucogranite, with minor identified in the porphyritic granite in the deposit. Four distinct stages in the metallogenic process can be identified: weakly mineralized magmatic stage (stage 1), strongly mineralized magmatic stage (stage 2), post-magmatic hydrothermal stage (stage 3) and low-temperature hydrothermal stage (stage 4). Cassiterite samples collected from stage 2 were dated to be 147 ± 3.5 Ma and 147.7 ± 1.9 Ma, providing evidence for the latest Jurassic Ta-Nb metallogenic event at Shihuiyao. To acquire a more profound comprehension of the properties and behaviors of the fluids, fluid inclusions within quartz, albite, amazonite and fluorite were analyzed among all stages. The results showed that the Shihuiyao fluid inclusions were relatively homogeneous and predominantly comprised liquid-rich inclusions, with occasional occurrences of CO2-type, vapor-rich type, and solid-type inclusions. From stage 1 to stage 4, the temperature of the ore-forming fluids decreased gradually, while the salinity showed increasing during stage 4, which indicates a possible contribution from surrounding strata sourced fluids. Raman analysis of the inclusions at each stage revealed that the vapor components were primarily H2O, CO2, and CH4, with additional N2 and CH4 appearing in the late stage, pointing to mixing between the metallogenic fluids and the surrounding strata source materials. Notably, the simultaneous occurrence of liquid-rich and solid-type inclusions under low pressure conditions (0.6 ∼ 1.5 km) in the early ore-forming stage (stages 1–2) suggests a fluid boiling process. We argue that the fluid boiling and rapid chemical quenching of the granitic melt play a crucial role in changing the physicochemical conditions of the ore-forming fluids, ultimately resulting in the enrichment and precipitation of niobium and tantalum. The fluid mixing and intense water–rock interaction in the late stages (stages 3–4) may have also contributed to the minor mineralization.Drawing upon our investigations utilizing petrographic features and composition analysis of individual fluid inclusions, we have prognosticated on the Sn mineralization potential within the Shihuiyao deposit. Our results indicate the Shihuiyao granites are comparable to the tin granites worldwide. Genesis of cassiterites is linked to both magmatic and hydrothermal processes, exsolution from tin-rich fluids (up to 233 ppm) and interaction with country rocks attribute to the probable economic tin mineralization. Consequently, the Shihuiyao deposit has been noteworthy prospectively for further tin mineralization.

Evolution of the Paleo–Asian Ocean in south‐eastern Inner Mongolia, China: Evidence from zircon U–Pb ages, Hf isotopes data and specific deformation of the Linxi and Sunjiafen formations

The crystallization age of the Sunjiafen Formation volcanic rocks is 253.9 ± 1.1 Ma. Geochemical data show that these volcanic rocks belong to the shoshonitic and metaluminous series and have high Sr and low Y contents (461–582 and 5.60–6.74 ppm, respectively), MgO = 1.32–2.65 wt%, and Na2O/K2O = 0.81–1.40. In MgO−SiO2, Cr−SiO2, Sr/Y−Y and (La/Yb)N−YbN diagrams, data for the samples plot in the adakite and “adakite related to thickened lower crust” fields. As such, we suggest that the Sunjiafen Formation C−type adakitic andesites formed in a compressional tectonic setting. The youngest age (youngest peak age) of two sandstone samples from the Linxi Formation are 251 Ma (257 Ma) and 237 Ma (254 Ma). Detrital zircons with ages of >1500 Ma have εHf(t) values of −5.87 to +6.31. A comparison of the age spectra and εHf(t) values of the detrital zircons in the Linxi Formation with those of igneous zircons from the Central Asian Orogenic Belt and North China Craton indicates the formation was derived from both the belt and the craton. We identified “Jura–type Folds” that formed in peripheral foreland basin in the Linxi Formation, and along with plant fossil fragments (Paracalamites frigidus). These and previously published results indicate the Paleo–Asian Ocean closed along the Xar Moron–Changchun Suture zone during the Late Permian. The study area was in a continuous compressional setting during the Late Permian and formed a peripheral foreland basin that was related to continent–continent collision.

Petrogenesis and Tectonic Implications of the Ore-Associated Intrusions in Bayanbaolege Ag Polymetallic Deposit, Inner Mongolia, NE China

The large Bayanbaolege Ag polymetallic ore deposit is located in the Tuquan-Linxi Fe (Sn)-Cu-Pb-Zn-Ag-Nb (Ta) polymetallic metallogenic belt, which is an important part of the Great Xing’an Range metallogenic province, northeast China. The sulfide–quartz vein-type orebodies in the deposit are mainly hosted in the Cretaceous granodiorite porphyry and Late Permian Linxi formation. The U-Pb dating of the zircon from the post-ore diorite porphyrite yields an age of 124.8 ± 1.1 Ma, which constrains the mineralization time at the Early Cretaceous. The Sr-Nd isotope values (87Sr/86Sr)i = 0.708576~0.710536; εNd (t) = −0.51~+0.69; the Hf isotope values 176Hf/177Hf = 0.2827278~0.2830095, the εHf (t) = +3.1~+11.2, TDM2 = 615~1341 Ma of the metallogenic granodiorite porphyry. The Hf isotope values 176Hf/177Hf = 0.2828596~0.2829451, and the εHf (t) = +5.7~+8.8 of the diorite porphyrite, TDM2 = 827~1108 Ma, indicating that the ore-forming materials were the possible involvement of heterogeneous juvenile sources including moderately depleted mantle and newly underplated lower crust. The major and trace elements (including REEs) implied that these intrusions are the I-type granite and linked intimately to the westward subduction of the Paleo-Pacific Ocean plate. From these whole-rock major and trace elements and zircon U-Pb ages, as well as Sr-Nd-Hf isotope data, we conclude that the ore-associated I-type granites in the Bayanbaolege deposit formed in an extensional tectonic setting of the Early Cretaceous, and are compactly related to the retreat of the Paleo-Pacific Ocean subducted plate linked intimately to the westward subduction of the Paleo-Pacific Ocean plate rather than the closure of the Mongol–Okhotsk Ocean. Furthermore, by integrating geological background work and previous research work, implying the mineralization age of the Bayanbaolege deposit should have been formed in the 125–130 Ma.

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

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

Organic geochemical characteristics and gas prospectivity of Permian source rocks in western margin of Songliao Basin, northeastern China

The Permian in western margin of Songliao Basin is a strategic potential field of oil and gas resources in northeastern China, but the systematic researches since few boreholes were drilled up to Permian strata, as well as there is a lack of seismic and well-logging data. This paper presents the lithological composition of the Permian Formations on the basis of regional geological survey in western margin of Songliao Basin. A total of 60 mudstones and shales were collected to evaluate the organic geochemical and petrophysical characteristics and gas prospectivity of Permian source rocks. The Permian is composed of Shoushangou, Dashizai, Zhesi and Linxi Formation, which represent large sets of mudstones and shales, which are mainly deposited in a weakly to strongly reducing marine environment. The primary organic matter precursors in Permian were lower aquatic organisms such as zooplankton and algae, while a few samples of Linxi Formation indicate mixed sources of lower aquatic organisms and terrigenous higher plants. The Permian gas–prone source rocks are at over–mature stage with vitrinite reflectance Ro% = 2.01%–3.14% and mainly classified as fair to good quality and type II–III kerogens, and generally have a tricyclic terpane distribution pattern of C21>C23>C20, but Shoushangou Formation and Linxi Formation have subordinate patterns of C21>C20>C23 and C23>C21>C20, respectively. The residual organic matters in Permian source rocks have CH4 yields up to 24.34 mg/g∙TOC at EASY%Ro = 4.45%, which indicate the possibility of secondary gas generation at Ro% = 2.0%–3.5%. The pores and fractures are developed in Permian source rocks with widths of 5.28–18.64 μm, and mean values of porosity and permeability are 2.17% and 0.0043 × 10−3 μm2, respectively. The source–reservoir–cap rock assemblages in Carboniferous–Permian and Mesozoic sedimentary sequences are included in situ and near–source accumulation in Permian, and long–distance migration and accumulation in underlying Carboniferous and overlying Triassic, Jurassic and Cretaceous. The Permian system can be regarded as a potential gas exploration target in the western margin of Songliao Basin.

Zircon U–Pb ages and geochemical characteristics of granites in the Yaoertu deposit: Implications for Pb–Zn–Ag mineralization in the southern Great Xing’an Range, NE China

Abstract The newly discovered Yaoertu Cu–Pb–Zn–Ag deposit in the southern Great Xing’an Range (SGXR), NE China, is spatially associated with late Yanshanian intrusions. The Cu mineralization occurs within a fine-grained granite, whereas the Pb–Zn–Ag orebodies are developed within country rocks of the Linxi Formation, and are spatially associated with monzogranite or fine-grained granite. The monzogranite and fine-grained Cu-bearing granite have zircon ages of 137.1 ± 0.4 and 137.4 ± 0.5 Ma, indicating that mineralization occurred in the Early Cretaceous, consistent with the formation of extensive skarn or hydrothermal Pb–Zn–Ag deposits in the SGXR. The monzogranite is enriched in SiO2 (69.91–67.87 wt%) and Na2O + K2O (8.43–7.92 wt%), contains 15.20–14.85 wt% Al2O3, and has a mean A/CNK value of 1.01, indicating the monzogranites are weakly-peraluminous high-K calc-alkaline granites. The fine-grained granites contain 77.18–75.14 wt% SiO2, 5.44–4.73 wt% K2O, 13.14–12.16 wt% Al2O3, 0.62–0.52 wt% CaO, and belong to the high-K calc-alkaline series. The monzogranite is enriched in light rare earth elements (REEs), exhibits fractionation of light over heavy REEs (LREE/HREE = 10.58–9.78), and has Eu/Eu* values of 0.74–0.61, whereas the fine-grained granite has LREE/HREE of 8.34–7.07 and more marked negative Eu anomalies (Eu/Eu* = 0.17–0.11). Zircon δ18O values in the monzogranite range from 6.61‰ to 6.87‰, and from 6.67‰ to 7.14‰ in the fine-grained Cu-bearing granite. The chemical composition and zircon Hf–O isotopic data suggest that the Early Cretaceous granites are formed by mixing between the juvenile crust and mantle-derived materials. Post-orogenic extensional settings related to the closure of the Mongol–Okhotsk Ocean resulted in extensive magmatism and large-scale mineralization.

SHRIMP U-Pb dating of detrital zircons from the Permian sandstones along the southern and northern margins of Xar Moron River, central inner Mogolia: Implications for provenance and the tectonic evolution of the eastern segment of the Central Asian Orogenic Belt

The Xar Moron River fault zone, located in the eastern segment of the Central Asian Orogenic Belt (CAOB), represents the intensely debated final collision zone of the Siberian Craton (SC) and North China Craton (NCC). To determine the tectonic evolution of the eastern segment of the CAOB, we undertook petrography and zircon U-Pb dating of the Huanggangliang and Linxi formations in the Wufendi and Xingfuzhilu areas along the Xar Moron River. Petrographic analysis of Permian sandstones revealed a close relationship between the sedimentary and orogenic sources suggesting short transport distances. A sample from the Huanggangliang Formation yielded detrital zircon U-Pb ages ranging from 2653 Ma to 265 Ma, with three age populations: at 2653 to 2443 Ma, 1935 to 1764 Ma, and 482 to 265 Ma, whereas samples from the Linxi Formation yielded detrital zircon U-Pb ages ranging from 3363 Ma to 257 Ma, with four age populations: at 2705 to 2403 Ma, 2011 to 1203 Ma, 571 to 375 Ma, and 356 to 257 Ma. The age spectrum differences of sandstones on both banks indicate that the Xar Moron River fault zone is the final collision zone of the eastern segment of the CAOB. The sandstone of Huanggangliang Formation yielded a weighted mean age of 265.7 ± 1.5 Ma, suggesting that the main deposition of the Huanggangliang Formation was during the Middle Permian. In addition, a comparison of the youngest age in the sedimentary rocks with U-Pb ages obtained for pyroclastic rock implies that the Linxi Formation formed in the late Permian. The results of our study support the view that the final closure of the eastern segment of Paleo-Asian Ocean (PAO) occurred during late Permian to earliest Triassic times.

Corrigendum to “Distribution of Trace Elements, Sr-C Isotopes, and Sedimentary Characteristics as Paleoenvironmental Indicator of the Late Permian Linxi Formation in the Linxi Area, Eastern Inner Mongolia”

The Late Permian on the periphery of the Songliao Basin, eastern Inner Mongolia, is an important hydrocarbon source rock system. Its sedimentary environment plays an important role in the evaluation of hydrocarbon prospects in the area. Unfortunately, until now, the interpretation of the sedimentary environment of this area has been controversial. We investigated the Late Permian sedimentary environment by studying the sedimentary characteristics and geochemistry. Based on these investigations, we conclude that the Linxi Formation is mainly composed of clastic sediments, interbedded with limestone lenses, with bioherm limestone at the top of the formation. Inner-layer marine fossils (calcium algae, bryozoans, and sponges) and freshwater and blackish water microfossils (bivalves) are all present, indicative of a typical shallow water sedimentary environment with an open and concussion background. In terms of geochemistry, the formation is relatively light rare Earth enriched, with significant positive Eu anomaly, slight positive La and Y anomaly, weak positive Gd anomaly, and lack of Ce anomaly. The average B/Ga ratio of the mudstone is greater than 3.3, and the average Sr/Ba ratio of the limestone is greater than 1.0. The range of the 87Sr/86Sr ratio is from 0.707285 to 0.707953. The range of δ13C values is from −4.0‰ to 2.4‰. The sediment assemblages, rare Earth elements, trace elements, and 87Sr/86Sr and C isotopes of the formation indicate that the Linxi Formation formed in a marine sedimentary environment and occasional marine-terrestrial transitional facies. The formation can be further divided into littoral facies, neritic facies, bathyal facies, and delta front.

Distribution of Trace Elements, Sr-C Isotopes, and Sedimentary Characteristics as Paleoenvironmental Indicator of the Late Permian Linxi Formation in the Linxi Area, Eastern Inner Mongolia

The Late Permian on the periphery of the Songliao Basin, eastern Inner Mongolia, is an important hydrocarbon source rock system. Its sedimentary environment plays an important role in the evaluation of hydrocarbon prospects in the area. Unfortunately, until now, the interpretation of the sedimentary environment of this area has been controversial. We investigated the Late Permian sedimentary environment by studying the sedimentary characteristics and geochemistry. Based on these investigations, we conclude that the Linxi Formation is mainly composed of clastic sediments, interbedded with limestone lenses, with bioherm limestone at the top of the formation. Inner-layer marine fossils (calcium algae, bryozoans, and sponges) and freshwater and blackish water microfossils (bivalves) are all present, indicative of a typical shallow water sedimentary environment with an open and concussion background. In terms of geochemistry, the formation is relatively light rare Earth enriched, with significant positive Eu anomaly, slight positive La and Y anomaly, weak positive Gd anomaly, and lack of Ce anomaly. The average B/Ga ratio of the mudstone is greater than 3.3, and the average Sr/Ba ratio of the limestone is greater than 1.0. The range of the 87Sr/86Sr ratio is from 0.707285 to 0.707953. The range of δ13C values is from −4.0‰ to 2.4‰. The sediment assemblages, rare Earth elements, trace elements, and 87Sr/86Sr and C isotopes of the formation indicate that the Linxi Formation formed in a marine sedimentary environment and occasional marine-terrestrial transitional facies. The formation can be further divided into littoral facies, neritic facies, bathyal facies, and delta front.

Sedimentary response to the non-synchronous closure of Paleo-Asian Ocean: evidence from U-Pb ages and Hf isotopic compositions of detrital zircons from the Linxi Formation, NE China

ABSTRACT The Linxi Formation is widely deposited in eastern Inner Mongolia (NE China), especially along northern parts of the Solonker-Xar Moron Suture (SXMS) in the eastern Central Asian Orogenic Belt, yet its depositional age and provenance remain ambiguous. We studied the petrography, geochronology, and Hf isotopes of two sandstone samples from this formation, and estimated the depositional age and the provenance of the Linxi Formation, and further constrained the evolution of the Paleo-Asian Ocean (PAO). Based on detailed field observations and measurements, we. Zircons from rocks of the Linxi Formation are mainly composed of detrital zircons of magmatic origin. The determined ages of detrital zircons range from 233 ± 2 Ma to 1365 ± 14 Ma, and indicate that the provenance of the Linxi Formation experienced at least four tectonic-thermal events between 1365 and 233 Ma: Neoproterozoic (ca. 1365–543 Ma), Early Palaeozoic (ca. 512–426 Ma), Late Palaeozoic (ca. 418–251 Ma), and Early Mesozoic (ca. 249–233 Ma). The youngest detrital zircon, with a U-Pb age of 233 ± 2 Ma, defines the maximum depositional age of the Linxi Formation. The εHf (t) values of dated zircons from the sandstone samples have a large range (−7.8–15.0). The four dominant age groups and the range in εHf (t) values are widely found in the Xing’an Block and Songnen-Zhangguangcai Range, which imply that the source of the Linxi Formation had an affinity with northeastern China. These data together with previous geochronological data (from west to east along the SXMS) on the crystallization and depositional ages of detrital zircon suggest that the Linxi Formation was deposited in different tectonic settings: convergent, collisional, and extensional. This study provides further evidence and constraints for the non-synchronous closure of the PAO.

Provenance, tectonic setting and mineralization significance in the Linxi Formation, eastern Inner Mongolia, NE China

The strata of the Linxi Formation (LXF) occupy an extensive area of eastern Inner Mongolia, NE China and have close spatial and genetic relationships with important Mesozoic Sn–Cu–Pb–Zn–Ag veins and porphyry polymetallic deposits (ores). The rock types of the LXF include sandstone, siltstone and shale. Major and trace element (including rare earth element) data for rocks from the LXF in the Hanmiao area indicate that these rocks have a greywacke protolith derived from intermediate to acidic arc igneous rocks and were deposited during intensive tectonic activity. Laser ablation inductively coupled plasma mass spectrometry U–Pb dating of detrital zircons from sandstones in the LXF yields ages of 985 ± 10 to 242 ± 2 Ma. The youngest population of ages shows a peak at c. 258 Ma, suggesting that the main period of deposition of the LXF was in the late Permian. These ages, together with the youngest zircon age ( c. 242 Ma), suggest that the deposition of the LXF mainly occurred during the late Permian and extended to the early Triassic. According to the results of the analysis of 44 sandstone and siltstone samples, ore elements (e.g. W, Sn, Ag and Mo) are significantly enriched in the LXF in this region. Combined with the fact that increasing numbers of polymetallic deposits have been discovered within the LXF strata in the southern Great Xing'an Range, we conclude that the strata of the LXF may be the source bed of the ore and there is potential for ore prospecting in the polymetallic deposits of the LXF in the Great Xing'an Range.

Ufadendron Elongatum sp. nov., an Angaran Lycopsid from the Upper Permian of Inner Mongolia, China

A new species Ufadendron elongatum sp. nov., attributed to the family Tomiodendraceae Naugolnykh, is represented by two well-preserved stems in the collection under study. This new species was recently discovered from the Upper Permian Linxi Formation in the Jalaid locality, the Inner Mongolia Autonomous region, China. The genus Ufadendron is characterized as having the long fusiform leaf cushions, with small and rounded leaf scar containing a central point-like scar which is situated at the upper part of leaf cushion; the infrafoliar bladder of fusiform shape positioned in the middle part of leaf cushion; the wings and heel well-developed in the lateral parts and the lower part of leaf cushion, respectively. The new species is different from the type species U. ufaense (Naugolnykh 2014) collected from the Lower Permian of the Cis- Urals, western limits of Angaraland, in the elongated leaf cushion and in the well-pronounced heel. It should be noted, that a vascular bundle (conductive strand) occupied the middle part of the central point-like scar. So far, only 5 genera of Angaran elements among lycopsids have been discovered in the region geographically belonging to Angaran Realm (Phytogeoprovince) in China. The new species U. elongatum not only enlarges our knowledge on the taxonomy of Tomiodendraceae lycopsids, and also provides an opportunity to understand the difference between Angaran and Cathaysian floras in paleoclimatic context.

The Triassic Bilugangan deposit: Geological constrains on the genesis of one of the oldest Mo deposits in Inner Mongolia, China

The large Bilugangan porphyry Mo deposit is one of the oldest Mo deposit in eastern part of the Central Asian Orogen in Inner Mongolia. The orogen is currently regarded as the most important Phanerozoic region with widespread crustal growth between the Siberian Craton to the north, and North China Block to the south. The Bilugangan Mo mineralisation is hosted by a porphyritic monzogranite and along the contact with the Late Permian Linxi Formation. The mineralisation is disseminated in places and includes various types of hydrothermal veins. The mineralising stages of the deposit includes the pre-ore quartz–K-feldspar(–biotite) pegmatite succeeded by quartz–K-feldspar–molybdenite–muscovite–sericite(–pyrite) veins, quartz–molybdenite–chalcopyrite–perthite–muscovite–sericite(–pyrite–sphalerite), and post-ore quartz–fluorite–calcite (–sericite–muscovite–) veins. Seven molybdenite samples from the mineralisation yield a Re-Os Mo weighted mean date of 238 ± 1 Ma, which is the same, within error, as the Re-Os isochron age of 238 ± 2 Ma, and the ca. 240 Ma age of the porphyritic monzogranite. This is the oldest porphyry Mo – type deposit in the eastern part of the orogen, and the only large –sized Mo deposit in the region, which shows that the east CAO is prospective for Triassic mineralisation. Thirty-three samples of sulfide from the deposit have a narrow δ34SVCDT(‰) range of 1.5 to 4.3‰, indicating that the sulfur the sulfur has primarily a magmatic source. The sulfides from the Mo-bearing veins have relatively concentrated Pb isotopic compositions with 206Pb/204Pb ratios between 18.295 and 19.576, 207Pb/204Pb ratios between 15.535 and 15.662, and 208Pb/204Pb ratios between 38.066 and 38.653. These values are consistent with those of the initial Pb isotope ratios for whole rock samples from the porphyritic monzogranite and hornfels. Fifteen gangue quartz samples from various veins define a range of δ18Ofluid values from 0.5 to 5.2‰ with δDfluid values ranging from −115 to −60‰, indicating that the onset of the mineralising fluid was generated from in-situ degassing of a magmatic source, followed by an input of meteoric water during the late ore-forming stage. It is proposed that the deposit is a collision-related Dabie-type deposit, which is related to the closure of the Paleo-Asian Ocean between the North China Block and Siberian Craton.

Mapping electrical structures in the southern Great Khingan Range, north-east China, through two-dimensional magnetotelluric sounding

Magnetotelluric (MT) measurements were performed in the southern Great Khingan Range from 2012 to 2014 to reveal the tectonic framework of the Jarud rift basin, and to identify the strata distribution of the Permian Linxi Formation. The dark mudstones in the Linxi Formation have shown the potential of shale gas. The study area is geographically situated in the Jarud Banner and Ar Horqin Banner, Inner Mongolia, China. It is covered predominantly with Cretaceous–Jurassic igneous rocks except for the small south-eastern part. It is tectonically located in the southern Great Khingan Range on the western periphery of the Songliao Basin, north of the Xar Moron Fault.MT data were acquired on six north–west and five north–east profiles in the study area. After geoelectric dimensionality analysis, strike analysis and 2D inversion, the resulting resistivity models revealed numerous large faults, part of which formed the borders of the Jarud Basin. In contrast to the original extent of the Jurassic Jarud Basin, the new borders should extend further to the north. According to well logging data and geological information, two Paleozoic depressions inside the Jarud Basin, Hunnitu depression and Gadasu depression were discovered and inferred to possibly contain Linxi strata. Meanwhile, the Mesozoic Xiretu depression outside of the Jarud Basin was discovered and inferred to possibly contain Cretaceous hydrocarbon-bearing strata. In particular, the Gadasu depression, with an area of ~500 km2, contains reasonably thick conductive sediments exceeding 4 km in depth, which were inferred to be dark mudstones possibly pertaining to shale gas. Overall, the MT work will be helpful for the assessment of potential shale gas.

Molecular Biomarker Characteristics of the Linxi Formation Source Rocks in the Middle‐Western Region of Inner Mongolia: New evidence for late‐stage tectonic evolution of the Paleo‐Asian Ocean

Molecular Biomarker Characteristics of the Linxi Formation Source Rocks in the Middle‐Western Region of Inner Mongolia: New evidence for late‐stage tectonic evolution of the Paleo‐Asian Ocean

Exploration Prospects of Shale Gas Resources in the Upper Permian Linxi Formation in the Suolun-Linxi Area, NE China

The commercial exploitation of shale gas in the United States has changed the world’s energy structure. At present, many countries are increasing their commercial investments in this field, including China. On the basis of systematic analyses of laboratory test results, and drilling and outcrops investigation data in the Suolun-Linxi area, northeast of China, shale gas forming conditions and exploration prospects of the Upper Permian Linxi Formation are studied from such properties as the source rock sedimentary characteristics, the gas concentration of organic-rich shales, and the organic geochemical and reservoir characteristics. Studies indicate that the Upper Permian Linxi Formation has thick black shale (ranging from 50 to 485), excessive total organic carbon (TOC) content (ranging from 0.15% to 3.02%), a high thermal maturity with an average Ro value of 2.6% (ranging from 2.03% to 4.33%), and a high brittle mineral content with an average value of 71.2% (ranging from 58.5% to 97.5%). Moreover, abund...