Alxa Block Research Articles

Paleozoic-Mesozoic multistage tectonic evolution of the North Qilian Shan revealed by detrital zircon U-Pb and fission-track double dating

Knowledge of the tectonic history of the North Qilian Shan contributes to our understanding of the formation of the Tibetan Plateau. However, when the North Qilian Shan were formed has been controversial. We report new detrital zircon U-Pb (n = 899) and double dating fission track and U-Pb (n = 450) results from nine samples of Lower Cretaceous and Oligocene sandstone units, integrating with published geochronology and thermochronology data in order to reconstruct the evolution history of the North Qilian Shan. Nine samples from this unit show detrital zircon fission track age populations that center at: 636–603, 495, 406–331, 290–266, 247–224, 193–151, and 144–102 Ma. These fission track data are older than their corresponding depositional ages, suggesting that the fission track ages are unreset and record magmatic-related cooling or exhumation stages of the North Qilian Shan since the Late Neoproterozoic. Deconvolved fission track peak ages document major cooling events associated with the opening of the North Qilian Ocean at 636–603 Ma. Late Cambrian peak age (495 Ma) is attributed to the early phase of the subduction of the North Qilian Ocean. The Devonian period exhibited a cooling phase between 406 and 381 Ma, corresponding to the collision event between the Qaidam terrane, Central Qilian Shan, and Alxa Block. The peak age of 331 Ma signifies the cooling event associated with the subduction of the Paleo-Asian Ocean. The peak ages during the Early-Middle Permian period (290–266 Ma) provide evidence of significant exhumation events that occurred during the Late Paleozoic era. These events were linked to the subduction of the Paleo-Tethys Ocean to the south and the Paleo-Asian Ocean to the north. The Triassic peak ages (247–224 Ma) unveil exhumation signals in the North Qilian Shan region that were generated by the far-field effect of the collision between the Qiangtang and Kunlun terranes. During the Jurassic (193–151 Ma) and the Early Cretaceous periods (144–102 Ma), the exhumation of the North Qilian Shan region was a consequence of the subduction of the Neo-Tethys Ocean and the collision between the Lhasa and Qiangtang blocks, respectively. The peak ages of the Xinminpu Formation in the Early Cretaceous (123–121 Ma) are in close proximity to their depositional age (125–105 Ma), indicating that the North Qilian Shan underwent rapid exhumation during this period. The peak ages observed in the Oligocene Baiyanghe Formation (31–24 Ma) from 144 to 102 Ma, indicate that it has been influenced by the recycled sediments originating from the Cretaceous Xinminpu Formation. The North Qilian Shan gradually emerged during the late Paleozoic and Mesozoic eras, providing the foundation for its subsequent reactivation in the Cenozoic era. The multi-stage cooling or exhumation events of North Qilian Shan since Late Neoproterozoic reflect the complex formation process of the northeastern margin of Tibetan Plateau.

Late mesozoic exhumation of silurian – Devonian and permian Ni-Co sulfide deposits in the East Kunlun orogenic belt: Constraints from zircon fission track ages

Ni-Co sulfide deposits of late Silurian – early Devonian and Permian ages hosted within in basic-ultrabasic rock bodies in the East Kunlun orogenic belt have been extensively investigated. Nevertheless, we have only a limited understanding of the history and dynamics of uplift, exhumation, and tectonic deformation of the basic-ultrabasic ore-bearing rock bodies of these Ni-Co sulfide deposits. We used the zircon fission track ages of seven samples obtained from basic-ultrabasic ore-bearing rocks to determine the timing of the exhumation of these Ni-Co sulfide deposits. Combining our results with published data on the timing of orogenesis, cooling events, magmatic activities, and basin infilling in adjacent areas, we conclude the following: 1) The ZFT ages obtained in this study indicate the exhumation during 169.6 ± 5.5–142.2 ± 3.2 Ma; 2) Combined with previous results, our data indicate that the exhumation during the Jurassic – Cretaceous occurred across a large area along the East Kunlun orogenic belt to the Alxa block; 3) The synchroneity of orogeny, magmatic activity, and basin infilling events suggests that the late Mesozoic exhumation was a geomorphological response to the Lhasa-Qiangtang collision driven by the breakup of Gondwanaland.

Crustal-scale architecture and origin of the Haiyuan Arcuate Tectonic Belt, NE Tibet

The Haiyuan Arcuate Tectonic Belt (HATB) in the northeastern Tibetan Plateau features the interactions of three intersecting blocks: the eastern Qilian Shan, the Alxa Block, and the Ordos Block. While the HATB has displayed active responses to the ongoing collision between the Indian and Eurasian plates, the exact process behind the formation of this arcuate belt remains unclear. In pursuit of further insights into this topic and a deeper comprehension of the tectonic responses in NE Tibet, we conducted receiver function calculations using teleseismic waveforms recorded by two seismic short-period dense arrays spanning the western and eastern HATB, respectively, extending into the Alxa and Ordos Blocks. The CCP results in the HATB show major structural features that are different from those of adjacent blocks, mainly characterized by structural discontinuities in the crust due to severe deformation, including bending and uplifting in the lower crust. Together with previous geological studies, the bending interfaces in the lower crust of the HATB illuminate the existence of a crustal-scale tectonic accretionary wedge within the HATB, which originated in the Early Paleozoic. Furthermore, a decoupled deformation is seen within the HATB, with the lower crust undergoing shortening and the upper crust experiencing sequential stepwise thrusting towards the north. These scenarios, coupled with the resistance from the rigid Alxa and Ordos Blocks, lead to the conclusion that the arcuate shape of this belt is influenced by the weak crust of the HATB, which primarily orients the northeast, where the weak lithosphere of the Helan tectonic belt is situated between the Alxa and Ordos Blocks. Meanwhile, the progression of a series of thrusting faults in the upper crust within the HATB extends outward, involving adjacent blocks in plateau's growth.

Newly discovered diamictite and cap carbonate couplet in the southern Alxa Block, northwestern China: Implications for stratigraphic correlation and Marinoan glaciation

Neoproterozoic glaciogenic diamictite and cap carbonate couplets have played a pivotal role in understanding glacial-interglacial cycles and establishing regional stratigraphic correlation. The Alxa Block in northwestern China preserves a sequence of Neoproterozoic diamictites along its southern margin, but the age and origin of the succession remain debatable due to the lack of discovery of cap carbonate. We report newly discovered cap carbonates that overlie the diamictites of the Shaohuotonggou Formation in the Longshoushan region in the southern Alxa Block. Based on detailed geological investigations, we identified massive diamictites, stratified diamictites, and cap carbonates in the lower part of the formation. The presence of ice-rafted dropstones, bullet-shaped and facetted clasts, glacial striations, and relatively low chemical index of alteration values of sedimentary matrix support a glaciogenic origin of the diamictites. The 2- to 2.6-m-thick cap carbonates are mainly composed of thinly laminated microcrystalline dolomites and show sheet cracks, cemented breccias, and tepee-like structures at the basal part of the unit. These features and their consistently negative δ13C values (−5.2‰ to −2.2‰) are characteristic of Marinoan-age cap carbonates (ca. 635 Ma). The quasi-continuous deposition of the massive diamictites, stratified diamictites, and cap carbonates suggests that the formation of this couplet was closely related to the Marinoan glaciation and subsequent deglaciation. We propose a three-stage depositional model for the glaciogenic succession and recommend that the diamictite and cap carbonate couplet in the Alxa Block provides a credible mark of the Cryogenian−Ediacaran boundary for further stratigraphic correlation and investigation.

Successful subduction of oceanic plate after failed attempts in the Late Archean: Petrological and geochemical constraints

When and how plate tectonics started and evolved to the style as we know it today is a fundamental yet highly controversial question. Numerical geodynamic modelling predicts that the transition into plate tectonics in the Archean was episodic with possible alternation between success and failure of subduction. However, direct geological evidence for failed subduction is scarce. While this possibility has been suggested by numerical modelling, it is still lack of geological evidence. Here we present a combined study of zircon U-Pb ages and Hf-O isotopes, as well as whole-rock major and trace elements, for meta-igneous rocks from the Alxa Block in the westmost North China Craton (NCC). Two periods of Late Archean magmatism ca. 2.75 Ga and 2.5 Ga are identified to occur surrounding a pre-3.0 Ga continental nucleus. Geochemical analyses of the ca. 2.75 Ga meta-mafic and meta-felsic igneous rocks show two different modes of tectonic regime. The felsic rocks resemble typical Archean TTG (tonalite-trondhjemite-granodiorite) in composition, with variable εHf(t) values from -2.8 to 11.0 and δ18O values from 4.3 ‰ to 7.9 ‰. Petrogenetic modelling suggests that their magmatic source was the ca. 3.1–2.75 Ga oceanic crust that was hydrothermally altered at different temperatures and then mixed with the older continental crust and partially melted in lower crust in the garnet stability field. This requires tectonic accretion of the oceanic crust to the continental nucleus, signifying an attempted or failed subduction. On the contrary, the meta-mafic rocks exhibit arc-like trace element patterns and calc-alkaline evolution trend, indicating a metasomatic mantle source due to successful subduction of the oceanic slab to subarc depths. Taken together, the present results provide robust constraints on the behavior of oceanic slab at the Archean convergent margin, where successful oceanic subduction would be achieved after several failed attempts. Such failure-success processes of oceanic subduction may be applicable to the whole NCC and other cratons elsewhere in the world, reflecting the gradual maturation of plate tectonics in the Archean, consistent with predictions of numerical modelling.

Lithospheric electrical structure of the Alxa Block, NW China, southern central Asian Orogenic Belt, revealed by 2D inversion of magnetotelluric data

The Alxa Block, situated between the Tarim and North China cratons, is essential to understand the tectonics of northwest China. It has undergone several tectonic cycles, including rifting from northern Gondwana in Neoproterozoic, subduction-accretion of the Paleo-Asian Ocean during Paleozoic, and intracontinental deformation in Mesozoic. Broadband magnetotelluric data covering a frequency range of 320 Hz to ∼ 10000 s was collected along two NW-SE trending lines across major tectonic units of the Alxa region to investigate the deep structures of the Alxa Block. By applying the nonlinear conjugate gradient (NLCG) inversion approach, two-dimensional (2D) electrical resistivity models were obtained at a 100 km depth. These findings indicate that the Zhusileng-Hangwula and Zongnaishan-Shalazhashan tectonic zones have high resistivity in their lower crust and upper mantle, with an overall layered high-low–high resistivity pattern. The upper crust of the Nuru-Langshan tectonic zone is primarily characterized by high resistivity values, whereas the lower crust and upper mantle contain a large-scale low-resistivity (or conductive) zone that is likely the result of partial melting. The large-scale Early–Middle Jurassic nappe structures may be connected to sub-horizontal low-resistivity anomalies in the shallow strata, which are located up to 20 km below the Yagan and Zhusileng–Hangwula tectonic zones. Below the Engger Us fault, two low-resistivity bands are interpreted as the product of bidirectional subduction and final closure of the Paleo-Asian Ocean. The Alxa Block may have subducted southwards beneath the Ordos Block, as suggested by a clear low-resistivity anomaly between the two blocks. Furthermore, partial melting may have happened for high-conductivity bodies in the upper mantle based on the modified Archie formula’s evaluation of their melting degree.

Geologic fingerprints of the Rodinia breakup in the western North China Craton: Constraints from the Early Neoproterozoic (ca. 825–810 Ma) mafic volcanic and intrusive rocks in the Langshan tectonic belt

As an important component of the East Asian continental block, the North China Craton (NCC) is crucial for reconstructing Rodinia in the Neoproterozoic. However, whether the NCC is a component of the Rodinia supercontinent and records its breakup remains enigmatic. Here, we identified Langshan gabbro and mafic dikes with ages of ca. 829–826 Ma and mafic volcanic rocks with ages of 810 ± 3 Ma in the northeastern Alxa Block of the western NCC. The Langshan gabbro shows flat rare earth element (REE) patterns [(La/Yb)N = 1.3–1.5] and slight enrichment in most incompatible trace elements, similar to enriched mid-ocean ridge basalt. The mafic volcanic rocks show the geochemical features of oceanic island basalt, including high total alkaline content (7.86–8.47 wt%), enriched in light REEs (La/YbN = 8.0–9.1), and insignificant Eu anomalies. Together with contemporaneous felsic volcanic rocks identified in previous studies, the mafic volcanic rocks (ca. 810 Ma) define a bimodal association. The mafic intrusive and associated dike swarms from ca. 825–810 Ma along with the coeval Jinchuan ultramafic–mafic intrusions, followed by rifting sedimentary successions, were formed in an extensional rift setting, supporting the geologic fingerprints of the Rodinia breakup in the western NCC. The well-matched mafic magmatic barcodes, similar provenance, and sedimentary successions preserved in the Adelaide superbasin of southeastern Australia and the Langshan rift basin of the western NCC support a possible link between the NCC and Australia during the Early Neoproterozoic.

Neoproterozoic (ca. 830 Ma) carbonatite dykes from Qianlishan in the western North China Craton: Petrogenesis and metallogenic implications

Carbonatite and the associated alkaline intrusive complexes are the primary sources of light rare earth element (REEs). Our field investigations in the western margin of the North China Craton (NCC) have firstly identified over 20 carbonatite dykes and minor coeval carbonate-rich lamprophyre dykes intruding the Paleoproterozoic Qianlishan Group in the southern Qianlishan area. Zircon LA-ICP-MS U-Pb dating on six carbonatite samples yielded crystallization ages ranging from 835.2 ± 3.7 Ma to 828.9 ± 3.8 Ma, indicating emplacement of the carbonatite dykes at ca. 830 Ma during the Neoproterozoic. Geochemical data show that most of the carbonatite dykes are rich in light REEs with total light REEs > 600 ppm to 4932 ppm, and the carbonate-rich lamprophyre dykes are characterized by high Cr (1219–1267 ppm) and Ni concentrations (857–1076 ppm). Petrological and scanning electron microprobe (SEM) analytical results show that the dominant REE-bearing minerals in the carbonatites are monazite, and other REE-bearing minerals are parisite and bastnäsite. Geochemical and carbon–oxygen isotopic results indicate that the ca. 830 Ma carbonatite dykes were produced by low-degree partial melting of a previously metasomatised subcontinental lithospheric mantle with assimilation of crustal material. Emplacement of the ca. 830 Ma carbonatite and carbonate-rich lamprophyre dykes in the southern Qianlishan area and the previously reported ca. 810 Ma mafic dykes in the middle Qianlishan area occurred in a continental rift environment. Simultaneous development of the Neoproterozoic (830 − 810 Ma) continental rifting events in the western margin of the NCC and the eastern margin of the Alxa Block as previously suggested may indicate a connection between the eastern Alxa Block and the western NCC during and before the middle Neoproterozoic.

Differential deformation mechanism of E–W‐trending Weiningbeishan fold‐thrust belt from central Ningxia, NW China

The Weiningbeishan fold‐thrust belt (WFTB), located in the transitional zone of the Tibetan Plateau, Alxa Block and Ordos Basin, is an ancient intracontinental orogenic belt. In this article, structural analysis is conducted to more finely reveal differential deformation between the Devonian and Carboniferous strata in the WFTB. We found that the E–W‐trending open folds are mainly developed in the Upper Devonian Zhongning Formation (D3z), while the E–W‐trending close and tight folds are mainly developed in the Carboniferous strata. The N–S minimum shortening strain of the bottom boundary of Member 2 of the Upper Devonian Zhongning Formation (D3z2) is 21.1%, whereas the N–S minimum shortening strain of the bottom boundary of Member 2 of the Upper Carboniferous Danliangshan Formation (C2d2) is 64.1%. We propose that the E–W‐trending open folds in the Upper Devonian Zhongning Formation (D3z) and E–W‐trending close and tight folds in the Carboniferous strata were formed under the N–S compression during the Middle–Late Triassic. The Zhangyigou and Laoyagou faults are the main tear faults to accommodate differential displacement on the surface between the Upper Devonian Zhongning Formation (D3z) in the east and Carboniferous strata in the middle, which change into a décollement fault in the deep part of the Dafosigou fold area in the middle to accommodate differential deformation between the deep Upper Devonian Zhongning Formation (D3z) and Carboniferous strata. This study enables us to better understand the Mesozoic intracontinental deformation in response to far‐field plate‐boundary convergence in NW China.

Devonian Andean‐type convergence in the southern Dunhuang block (NW China): Petro‐structural, metamorphic P–T and geochronological constraints

AbstractArchean to Palaeoproterozoic basement rocks exposed in the Dunhuang block in NW China were affected by Palaeozoic crustal reworking, as constrained by previous zircon U–Pb geochronological investigations. However, relationships between the Palaeozoic metamorphic ages, P–T evolution and deformational history of the region remain ambiguous. In order to address this issue, P–T–t–D paths of paragneisses from the basement of the Hongliuxia belt in the southern Dunhuang block were investigated. Inclusions in garnet and kyanite from the paragneisses are considered as vestiges of Palaeozoic M1 metamorphism corresponding to initiation of the prograde evolution. The earliest continuous metamorphic fabric is an originally steep N–S striking foliation S2. This fabric was reworked by vertical folds F3 associated with the development of a ubiquitous steep, mainly south‐dipping, E‐W striking axial planar foliation S3. The S2 foliation in paragneisses is mainly associated with Grt–St–Ky–Sil–Bt–Ms–Pl–Qz–Rt assemblages in samples from the western domain and with Grt–Ky–Sil–Bt–Kfs–Pl–Qz–Rt assemblages in samples from the northeastern domain of the Hongliuxia belt. The S3 foliation is associated with Grt–Sil–St–Bt–Ms–Pl–Qz–Ilm assemblages in the western domain and with Grt–Sil–Bt–Ms–Pl–Qz–Kfs–Ilm assemblages in the northeastern domain, followed by growth of chlorite in both domains. Early prograde stage (M1) from 4.0–6.5 kbar and 540–560°C to metamorphic peak (M2a) at 9–10 kbar and ~650–675°C is mainly recorded by paragneisses from the western domain. Subsequent decompression is initially accompanied by heating (M2b) constrained to 6.5–7 kbar and 675–710°C in the western domain, and to 6–6.5 kbar and ~730°C in the northeastern domain, followed by cooling (M3) through 4–6.5 kbar and 550–650°C till late chloritization (late M3). In situ U–Pb dating of monazite combined with monazite trace‐element compositions suggests that prograde evolution (M1) most likely started at c. 406 Ma, peak‐P conditions (M2a) were reached at 400–394 Ma, decompression associated with heating (M2b) took place at 393–391 Ma, and cooling (M3) during exhumation probably lasted from 380 to 354 Ma. The prograde metamorphism probably reflects burial during underthrusting of neighbouring continental basement (the Alxa block or an equivalent) below the Dunhuang block. This event culminated in pure shear thickening (D2a) of the whole supra‐subduction margin followed by minor heating and exhumation (D2b). The D3‐M3 event is interpreted as reflecting exhumation during orthogonal shortening of the system, possibly in response to an independent orogenic cycle. Combined with the available regional data, this study reveals the existence of a complex tectono‐metamorphic evolution for the Dunhuang block characterized by two distinct orogenic phases with (i) the thickening of a previously thinned arc‐back‐arc crust recorded in the northern and central belts at 420–410 Ma in the pro‐wedge side of the active margin (Sanweishan phase), followed by (ii) the 410–390 Ma thickening in the retro‐wedge side (Hongliuxia phase). Such a tectonic evolution of the whole Dunhuang block resembles Andean‐type migration of crustal thickening from the convergent front to hinterlands. The D3‐M3 event, potentially responsible for the juxtaposition of rocks from different geological occurrences and depths, is seemingly independent from this Andean‐type orogenic cycle.

Structure and provenance of the Cretaceous Pingshanhu Basin in the Hexi Corridor: Implications for Mesozoic tectonics in the northern Tibetan Plateau

Abstract The construction of Earth’s largest highland, the Tibetan Plateau, is generally considered to have been generated by the Cenozoic India-Asia collision. However, the extent to which high topography existed prior to the Cenozoic remains unclear. The Hexi Corridor foreland basin of the northern Tibetan Plateau is an ideal region in which to investigate this history, given its widespread exposure of Early Cretaceous sedimentary sequences. In this study, we examined the Early Cretaceous strata in the northern Hexi Corridor to understand the relationships between pre-Cenozoic sedimentation and tectonic deformation and constrain the late Mesozoic tectonic setting of the adjacent Qilian Shan and Alxa blocks bordering the northern Tibetan Plateau. Results of sandstone petrology analyses, paleocurrent observations, and U-Pb geochronology suggest that the oldest Early Cretaceous sediments deposited in the northern Hexi Corridor were sourced from the southern Alxa block during the earliest Cretaceous. By the late Early Cretaceous, Hexi Corridor sediments were sourced from both the southern Alxa block to the north and the Qilian Shan to the south. Sandstone petrologic results indicate that the northern Hexi Corridor experienced a tectonic transition from contraction to extension during the Early Cretaceous. These findings suggest that the northern Tibetan Plateau region was partially uplifted to a high elevation during the late Mesozoic before the India-Asia collision.

Paleoproterozoic ultrahigh‐temperature mafic granulites with a high‐pressure prograde path from the Alxa Block: Implications on the tectonic evolution of the Khondalite Belt, North China Craton

AbstractUltrahigh‐temperature (UHT) granulites, a prominent feature of Paleoproterozoic orogenic belts, preserve a record of geodynamic processes during the Precambrian (Archean–Paleoproterozoic). Quantitative pressure–temperature–time (P–T–t) paths of these UHT granulites can constrain the tectonic processes and metamorphic evolution in such a tectonic regime. Here, UHT mafic granulites with a high‐pressure (HP) prograde path are first reported in the Diebusige Complex in the Alxa Block, western part of the Khondalite Belt (KB), North China Craton (NCC). The detailed petrographic studies show that two mafic granulite samples preserve corona textures around relict garnet or garnet pseudomorphs (completely replaced by plagioclase), and a third mafic granulite sample has a relatively simple mineralogy with a granoblastic‐polygonal texture. These mafic granulites have similar peak (Tmax) assemblages of clinopyroxene + orthopyroxene + plagioclase + ilmenite ± garnet ± amphibole ± quartz + melt. Phase equilibrium modelling and Ti‐in‐amphibole and rare earth element (REE)‐based thermometries all constrain similar peak conditions of ~880–950°C/~8.5–10 kbar implying an ~100°C/kbar apparent geothermal gradient for these mafic granulites. Based on the corona textures or pseudomorphs of garnet and mineral assemblages, we identified a Pmax (~14 kbar) prograde stage before the Tmax stage. Thus, a clockwise P–T path with heating and decompression followed by near‐isobaric cooling (IBC) is recorded from these UHT mafic granulites. In addition, zircon and apatite SHRIMP or LA–ICP–MS U–Pb dating yields an age interval of ~1.81–1.7 Ga, which is interpreted as representing the cooling time from ~900–800°C to ~575°C at the middle‐upper crustal levels (<25 km deep) for these mafic granulites, with an ~1.5–2.5°C/Myr cooling rate. The new P–T–t path of these rocks includes high‐pressure prograde, UHT peak, and slow cooling retrograde processes, which implicates a post‐collisional tectonic setting for UHT metamorphism in the KB and the processes of collision, exhumation, and cooling of the KB.

The Long-Term Tectonism of the Longshou Shan in the Southwest Alxa Block—Constrained by (U-Th)/He Thermochronometric Data

The Long-Term Tectonism of the Longshou Shan in the Southwest Alxa Block—Constrained by (U-Th)/He Thermochronometric Data

Lithospheric deformation revealed by teleseismic phases SKS PKS and SKKS splitting in the NE margin of the Tibetan plateau

The NE margin of the Tibetan plateau influenced by multiple blocks, the regional dynamic model and lithospheric deformation characteristics are still controversial. In this study, 15 years data from permanent broadband seismic stations of the seismic network in the study area were adopted for splitting analyses of teleseismic phases XKS (SKS PKS and SKKS, shortly named XKS) using a grid search method, longer observations provided each station with a large number of clear seismic phases. The results show that the fast wave directions of XKS splitting are oriented toward WNW or NW, with reference to the direction of absolute plate motion, the lithospheric deformation is dominantly driven by the asthenosphere, although there are local variations. On both sides of the Haiyuan fault zone, there is a noticeable variation in lithospheric azimuthal anisotropy, further enhances the possibility that it is an extended boundary of the Tibetan plateau. Rheological anisotropic features on the western side of the Ordos block emphasize the obstruction of the rigid Ordos lithosphere. The crust and mantle seem to be coupled below the Qinling orogen, possibly related to the lateral movement of lithospheric material. However, in the Hexi corridor, there may be layered anisotropy within the lithosphere, which is inferred to relate to the subductions of the Alxa block and the Qilian orogen. The thicker lithosphere on the southern margin of the Alxa block may influence the asthenospheric flow. In addition, anisotropy results at the southeastern edge of the Alxa block and the Yinchuan graben reflect the possibility that the lithosphere may be characterized by a combination of horizontal and vertical movements. These results have greatly improved our understanding of the dynamic models and lithospheric deformation characteristics of the northeastern margin of the Tibetan plateau and adjacent areas.

Provenance of the Upper Carboniferous Yanghugou Formation in the Western Margin of the Ordos Basin, China: Constraints on Paleogeography and Basin Development

The Carboniferous Yanghugou Formation in the western margin of the Ordos Basin exhibits significant potential for oil and gas exploration. However, due to the influence of complex tectonic activities, there are substantial variations in stratigraphic thickness and depositional environments across the formation. The lack of a systematic source–sink comparative study has resulted in an unclear understanding of sediment sources and paleogeographic patterns, impacting the exploration for hydrocarbon accumulations. We conducted a comprehensive study of the source–sink system characteristics and paleogeography in the research area through field outcrop observations and drilling core sampling. By utilizing detrital zircon U–Pb geochronology and geochemistry, paleocurrent directions, lithofacies types, and sedimentary features, we delve into the understanding of the source–sink systems. Four major source–sink regions in the research area were identified: the Alxa, Yinshan, Alxa–Yinshan mixed and Qilian source–sink regions. The Alxa source–sink region formed a transitional delta-barrier-island sedimentary system. The northern part of the Yinshan source–sink region developed a transitional tidal-controlled delta-tidal-flat sedimentary system, while the southern deep-water area developed a shallow marine to semi-deep marine shelf sedimentary systems. The sediments of Alxa–Yinshan mixed source–sink region were deposited in a transitional tidal-controlled delta-tidal-flat barrier-island system. The Qilian source–sink region is characterized by small tidal-controlled delta-barrier-island system. From the analysis of the source–sink systems, it is inferred that the Alxa Block and the North China Craton had already merged before deposition of the late Carboniferous Yanghugou Formation. The delta sand bodies in the Alxa–Yinshan mixed source–sink region have the highest compositional and structural maturity, the best reservoir performance, and the great exploration potential.

Late Cenozoic geomorphic evolution in the northeastern Tibetan Plateau: Constraints on U-Pb age spectra of detrital zircons in the Wuwei Basin, NW China

Basin deposits of the northeastern Tibetan Plateau provide an ideal record for understanding the uplift and the geomorphic evolution in NW China. In this study, we present U-Pb age spectra of detrital zircon from the borehole WW-01 in the Wuwei Basin in the northeastern Tibetan Plateau. The results indicate that: (1) The dominant provenance of the Wuwei Basin was derived from Qilianshan Orogenic Belt (QOB) at 10.34–9.51 Ma, 8.18 Ma, 2.02–0.09 Ma, and the Alxa Block (AB) at 8.69 Ma and 8.14–4.51 Ma. (2) The transitions in the dominant provenance area, at 9.51–8.69 Ma and 8.18–8.14 Ma, were controlled by the closely related to the pre-existing landforms of the basin and its periphery. And the transitions of 8.69–8.18 Ma and 4.51–2.02 Ma were prevailing in the uplift of the northeastern Tibetan Plateau. (3) Provenance analysis, integrated with the sedimentary and structural analysis, show that the initial uplift of the northeastern Tibetan Plateau in the Cenozoic occurred at ca. 8.25 Ma, and the last uplift occurred at ca. 2.58 Ma, corresponding to the geomorphological formation of the northeastern Tibetan Plateau.

Paleoenvironmental, Paleoclimatic, and Tectonic Implications of the Yanghugou Formation in the Western Margin of the Ordos Basin, China: Evidence from Palynology and Elemental Geochemical Characteristics

This study explores the sedimentary environment and tectonic implications of the Upper Carboniferous Yanghugou Formation on the western margin of the Ordos Basin through the analysis of major elements, trace elements, and rare earth elements in 23 mudstone samples. The results indicate moderate chemical weathering conditions, as reflected in the Chemical Alteration Index (CIA) and Component Variability Index (ICV) values. Warm and humid paleoclimatic conditions are suggested by Rb/Sr, Sr/Cu, spore, and pollen fossil samples. The paleosalinity of the water is identified as a transitional phase of a brackish water environment based on Sr/Ba and Th/U ratios. Additionally, V/(V + Ni) and δU values imply a transitional phase of a weak oxidation–reduction environment for the water. In accordance with the La-Th-Sc, Th-Sc-Zr, Th-Co-Zr, (La/Yb)-∑REE, and (La/Th)-Hf discrimination diagrams, it is inferred that the study area is part of a continental marginal tectonic setting. The sediments are primarily derived from an upper-crust felsic source area. The sedimentary period of the Yanghugou Formation in the western margin of the basin is considered a marginal rift basin characterized by north–south compression and western rift extension. This setting is influenced by the active continental margin of the Alxa block and the passive continental margin of the Qilian–north Qinling orogenic belt.

Metamorphic evolution of high-pressure and ultrahigh-temperature granulites from the Alxa Block, North China Craton: Implications for the collision and exhumation of Paleoproterozoic orogenic belts

High-pressure (HP) and ultrahigh-temperature (UHT) granulites with a high geothermal gradient (greater than 500 °C/GPa) are prominent features of Paleoproterozoic orogenic belts and may represent paired metamorphic belts present during the early stages of plate tectonics. Understanding their pressure−temperature−time (P-T-t) paths and metamorphic evolutionary relationships could provide valuable constraints on the tectonic processes of Paleoproterozoic orogenic belts. Here, we describe garnet mafic and clinopyroxene-orthopyroxene (Cpx-Opx) granulites from the Diebusige area of the Alxa Block in the western part of the Khondalite Belt, North China Craton. Through detailed petrographic, phase equilibrium modeling, and Ti-in-amphibole thermometric studies, we obtained the preserved peak mineral assemblages of two types of mafic granulites: garnet + clinopyroxene + amphibole + plagioclase + quartz + ilmenite, and clinopyroxene + orthopyroxene + plagioclase + amphibole + garnet (rare) + ilmenite. The preserved peak P-T conditions were determined to be 850−890 °C/11.4−13.2 kbar (HP granulite-facies) and 950−970 °C/8.2−9.2 kbar (UHT conditions), with thermal gradients of ∼70 °C/kbar (moderate differential temperature/differential pressure, dT/dP) and ∼110 °C/kbar (high dT/dP), respectively. Using sensitive high-resolution ion microprobe U-Pb dating and rare earth element analysis of zircons, we found that the garnet mafic granulite recorded an HP granulite-facies metamorphic age of ca. 1.95 Ga and a retrograde cooling age of ca. 1.8 Ga, while the Cpx-Opx granulite recorded a consistent retrograde cooling age of ca. 1.8 Ga. By combining these results with the metamorphic evolution and timing (ca. 1.92−1.91 Ga) of UHT rocks from the Khondalite Belt, we suggest that the garnet (HP) mafic and Cpx-Opx (UHT) granulites may represent different stages of the same metamorphic event, shedding light on the processes involved in the collision and subsequent exhumation of Paleoproterozoic orogenic belts.

Dispersion of sandy sediments during marine–continental transition: An integrated study from the Late Paleozoic western Ordos Basin

The western Ordos Basin is located at the intersection of multiple tectonic units and is a key area in the restoration of the palaeogeographic pattern of the East Asia region. The stability of the area and well-developed Paleozoic strata in the basin provide ideal conditions for revealing the relationships among the tectonic processes apparent there. However, within the constraints of extensive post-depositional modification, poor-quality seismic data, and a lack of wells, sediment dispersion differences, source attribution, and source-to-sink processes are still unclear. In this study, we integrated sedimentology, mineral, and geochemical data obtained from marine, continental, and marine-continental transition sedimentary environments in the study area in order to reveal the evolution from source-to-sink. The results show that the western Ordos Basin was predominantly supplied by the Alxa block, Yinshan orogenic belt, Qilian orogenic belt, and the North Qinling orogenic belt during the Late Paleozoic era. As a result of the closure of the Paleo Asian Ocean, the Alxa block and Yinshan orogenic belt became the primary source areas to the north, being composed of granite and alkaline basalt and some sedimentary rocks. In the southern area, the Qilian and the Northern Qinling orogenic belts were subjected to the PaleoTethys remnant ocean basin subduction during the Early Permian. Granite and alkaline basalt were the source rocks of the North Qinling orogenic belt, which became the source area during the Shanxi period. The Qilian orogenic belt contains some sedimentary rocks. The continuous uplift of the southern and northern orogenic belts. The tectonic setting of the northern source area transitions from a passive continental margin to an active continental margin with a mixture of continental arc. The tectonic setting of the southern source area transitions from a mixture of passive continental margin and active continental margin to an active continental margin and continental island arc. The climate changes gradually from humid to semi-arid and then to arid, and meanwhile, the sea level shows a decreasing trend, leading to an increasing distance of advancement for each source-to-sink system, and gradually converged in the central and eastern regions of the basin. Tidal and fan delta sandy sediments in the study area were transformed into fluvial delta sandy sediments in the northern two source-to-sink systems. The southern source-to-sink systems gradually changed from scattered sandbarrier and coastal sandy sediment to fluvial sandy sediment after the Shanxi Formation.

Metamorphic Ages and PT Conditions of Amphibolites in the Diebusige and Bayanwulashan Complexes of the Alxa Block, North China Craton

The metamorphism and geological significance of amphibolites in the Diebusige and Bayanwulashan Complexes of the eastern Alxa Block, North China Craton, were poorly understood until now. This study presents the results of petrology, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb analysis, phase equilibrium modeling and geothermobarometry for these rocks. The peak mineral assemblage of clinopyroxene + hornblende + plagioclase + K-feldspar + ilmenite + quartz + melt is inferred for amphibolite sample ALS2164 in the Diebusige Complex. Correspondingly, the peak mineral assemblage of clinopyroxene + hornblende + plagioclase ± K-feldspar + ilmenite + quartz + melt is identified for amphibolite sample ALS2191 in the Bayanwulshan Complex. Phase equilibrium modelling constrained the peak metamorphic condition of amphibolite sample ALS2164 in the Diebusige Complex to be 825–910 °C/7.2–10.8 kbar, which is similar to that (800–870 °C/7.0–10.7 kbar) of amphibolite sample ALS2191 in the Bayanwulashan Complex. Hbl–pl–qz thermobarometry yielded the metamorphic PT conditions of 732–810 °C/3.0–6.7 kbar for these amphibolites, which are consistent with the average temperatures of 763 °C, 768 °C and 780 °C calculated by Ti-zircon thermometry. As a result, phase equilibrium modelling yielded wide PT condition ranges of 800–910 °C/7.0–10.8 kbar, the lower limit of which is consistent with the upper limit of estimates by the hbl–pl–qz thermobarometer. In addition, LA-ICP-MS U–Pb analysis on metamorphic zircons yielded weighted mean 207Pb/206Pb ages of 1901 ± 22–1817 ± 21 Ma, which represent the timing of amphibolite-facies metamorphism. As a whole, the PT estimates display a high geothermal gradient, which is consistent with coeval ultrahigh-temperature metamorphism and associated mantle-derived mafic-ultramafic rocks in the Diebusige Complex. Combing this information with the previously published data from the Diebusige Complex, an extensional setting after continental collision is inferred for the eastern Alxa Block during the late Paleoproterozoic. The HREE enrichment patterns of metamorphic zircons from the amphibolites in this study are in agreement with that these amphibolites formed at relatively shallower crust than the garnet-bearing mafic granulites in the Diebusige Complex.