Yangtze Craton Research Articles

Geochronological constraints on the formation and evolution of the Huangling basement in the Yangtze craton, South China

The Huangling basement has the largest area and highest metamorphic grade of exposed Archean and Paleoproterozoic rocks in the Yangtze craton basement. Hence, it provides an excellent opportunity for examining the formation and evolution of the Yangtze craton. The Huangling basement consists mainly of the Kongling complex, with a chronotectonic framework established through the geochronological study of amphibolites, khondalites, ultramafic rocks, and diabase dikes. Our results indicate that magmatic events in the Huangling basement are divided into four phases; the first phase is represented by the emplacement of the intermediate–basic volcanic rocks (3.2–3.0 Ga) that formed a typical Archean greenstone belt. The greenstone belt was subsequently intruded by tonalite–trondhjemite–granodiorite rocks (3.0–2.9 Ga) and A-type granite (2.7–2.6 Ga) constituting a typical Archean granite–greenstone terrane. The granite–greenstone terrane was transformed into an Archean continental nucleus during the first metamorphism (2.6–2.5 Ga), resulting in an unconformity between Archean and Proterozoic strata. Stable cratonization and sedimentation are considered to have lasted for 500 Ma (2.5–2.05 Ga), during which sedimentary rocks with a thickness of 2500 m were deposited. Later, the sedimentary rocks were metamorphosed to form the khondalite series during the second metamorphic event at 2.05–1.9 Ga, which caused an angular unconformity between Paleoproterozoic and Mesoproterozoic strata. The Paleoproterozoic high-grade terrane and Archean continental nucleus simultaneously merged to form the Huangling basement. The Kongling complex was intruded by A-type K-feldspar granite and diabase dikes, which were emplaced at 1.87–1.85 Ga and had not undergone deformation and metamorphism, indicating that the formation and evolution of the Kongling complex and the Huangling basement ended before 1.87–1.85 Ga.

Geology and pyrite sulfur isotopes of the Suoluogou gold deposit: Implication for crustal continuum model of orogenic gold deposit in northwestern margin of Yangtze Craton, SW China

Since the crustal continuum model was proposed mainly based on the variation of mineralization and alteration features of hypozonal, mesozonal, to epizonal deposits, genetic relationship between the different types in one ore belt still requires research. This issue was addressed by the research on the Suoluogou mesozonal orogenic gold deposit and its comparison with the hypozonal orogenic gold deposit in northwestern margin of Yangtze craton. Gold mineralization in Suoluogou is controlled by the bends of roughly EW-trending brittle fault developed in the Late Triassic strata, and it is associated with quartz, sericite, chlorite, carbonate, and sulfide alteration. Pyrite and arsenopyrite are the predominant sulfide mineral, with minor amounts of chalcopyrite, sphalerite, and galena. Four paragenetic stages are divided based on crosscutting relationship and mineral assemblage, including the pre-mineralization stage (stage Ⅰ), the hydrothermal quartz-sulfide stage (stage Ⅱ) and carbonate-quartz-sulfide stage (stage III), and the post-mineralization carbonate-pyrite stage (stage Ⅳ). The δ34S values for pyrite in stage Ⅱ vary from 6.01% to 8.59‰, and those in stage III are from 0.96% to 4.62‰. The decrease of δ34S value from stage Ⅱ to III is explained to the result of the isotope fractionation during separation of oxidised sulfur species in the brecciated space. The sulfide δ34S values in the initial gold mineralization stage of the Suoluogou deposit are similar to those of regional hypozonal orogenic deposit with derivation from metasomatized mantle in the craton margin. This similarity suggests that the mesozonal and hypozonal gold deposits formed in different depths of a crustal continuum model for the deposits sharing similar subcrustal source.

Thermal evolution of the Upper Yangtze Craton: Secular cooling and short-lived thermal perturbations

The Upper Yangtze Craton (UYC) has stayed in a long-term stabilization state. Its thermal history can be portioned into two parts: secular evolutionary trend and short-lived thermal perturbations. The former is simulated by a two-dimensional forward transient thermal model, and the latter is discussed by reviewing previous studies. The numerical modeling indicates the UYC tended to be cooling as it adjusted to the asynchronous rates of change in the internal radiogenic heat production and in the convecting mantle temperature below the cratonic root. The average cooling rate of the UYC has been ~58 °C/Ga since the end of Archean, faster than that of the convecting mantle. The surface heat flow presents a decreasing trend from ~82 mW/m2 at 2.5 Ga to ~53 mW/m2 at present-day, which constructs the long-term evolutionary path. The thermal perturbations derived from the occasional tectono-thermal events, such as mantle plume activity and regional lithosphere extension, serves as paroxysmal factors. The thermal effects of the Emeishan mantle plume were very strong, yielding to the maximum heat flow anomaly of ~100 mW/m2 at the Late Permian. In contrast, the influences of regional lithosphere extension during both the early Paleozoic period and the Early Permian-Middle Triassic period were rather weak, resulting in a surface heat flow increase of <10 mW/m2. These thermal anomalies disappeared after a time period depending on the thermal relaxation without altering the stability of the UYC, and then the thermal evolution of craton returned to its original cooling route. The thermal history of the UYC, the overprinting of the secular cooling and the short-lived perturbations, helps better understand the craton evolution and provide the fundamental thermal data for petroliferous-basin analysis.

Late Neoproterozoic mafic sills in the Suizhou area of the South Qinling block: constraints for the tectonic evolution of the northern margin of the Yangtze craton

ABSTRACT Late Neoproterozoic gabbroic-diabasic sills are widely distributed in the eastern part of the South Qinling block. Here, a comprehensive geochronological and geochemical study of these mafic sills has been conducted. LA-ICP-MS zircon U-Pb results indicate that the gabbroic-diabasic sills were formed at 632 ± 2 Ma. The sills consist mainly of tholeiitic basaltic rocks with relatively low TiO2 contents (0.81–1.07 wt.%) and high Fe2O3 T/MgO ratios. These rocks display relatively flat to moderate light rare earth elements (LREE) enrichment patterns and positive Eu anomalies (Eu/Eu* = 1.15–1.41). On the primitive mantle normalized spider diagram, the samples show E-MORB-like geochemical features. All of the studied samples display similar Sm-Nd isotopic compositions with negative εNd (t) values ranging from −6.2 to −5.5. Most clinopyroxenes from the mafic sills have relatively low Al/Ti ratios similar to those of typical rift-related cumulates. Therefore, Whole-rock and mineral geochemical features of the studied samples suggest that the mafic sills might have been derived from partial melting of a metasomatised subcontinental lithospheric mantle source in a continental rift setting. It can be advocated that the break-up of the Rodinia supercontinent in the northern part of the Yangtze craton might have been a long-lasting process. By comparison of the coherent Neoproterozoic magmatic events in the northern Yangtze craton and the Tarim block, it can be suggested that the Yangtze craton might have been adjacent to the Tarim block in the reconstruction model of the Rodinia supercontinent.

Metamorphic evolution and 40Ar/39Ar geochronology of the Wuguan complex, eastern Qinling area, China: Implications for the late Paleozoic tectonic evolution of the Qinling orogen

The late Paleozoic tectonic evolution of the Qinling orogen is poorly constrained. The Wuguan Complex, which lies between the North Qinling and South Qinling tectonic belts, represents a late Paleozoic metamorphic belt of the Qinling orogen. In this contribution, we report new metamorphic pressure–temperature (P–T) constraints and 40Ar/39Ar ages of garnet amphibolites and garnet-bearing metapelites from the Wuguan Complex. Textural observations and mineral compositions, especially garnets with two stages of growth, indicate the Wuguan Complex experienced two distinct metamorphic events. Conventional geothermobarometry and phase equilibria modelling indicate that the earlier metamorphism had peak P–T conditions of 660–725 °C and 8.0–12.0 kbar with a clockwise P–T path, whereas the later metamorphic overprint records P–T conditions of 560–610 °C and 7.5–9.0 kbar. Hornblendes from amphibolites yield 40Ar/39Ar ages of ~306–299 Ma, whereas muscovites from garnet-bearing metapelites yield 40Ar/39Ar ages of ~252–251 Ma. The two groups of 40Ar/39Ar ages correspond to the cooling time of the two distinct metamorphic events, respectively. Combining with previously published zircon UPb ages from the Wuguan Complex and two groups of metamorphic ages from the Guishan Complex and the Xiongdian eclogite belt, we infer that the earlier metamorphism probably occurred at the hangingwall of a subduction zone during Carboniferous subduction of oceanic crust. The later metamorphic overprint might have been related to the initial collision between the Sino-Korean and Yangtze cratons during the late Permain.

High-resolution 3-D crustal shear-wave velocity model reveals structural and seismicity segmentation of the central-southern Tanlu Fault zone, eastern China

The Early-Mesozoic collision between the North and South China Cratons resulted in different tectonic and geologic structures including the Tanlu Fault zone. One way to understand the geodynamic processes beneath these structures is by studying the velocity structures and spatial distribution of earthquakes in the region. We performed ambient noise tomography for the region around the central-southern Tanlu Fault zone using the vertical component of continuous ambient noise data obtained in central East China. Empirical Green's functions were retrieved from ambient noise cross-correlation functions, from which we extracted the Rayleigh wave phase velocity dispersion curves and performed quality control on the dispersion data. The direct surface wave tomography method was used to invert all phase velocity dispersion data for the 3-D shear-wave velocity model in the crust and uppermost mantle. Our results show velocity segmentations with high-velocity structures existing beneath orogenic belts and uplifts within the first 10 km depths which correspond to the High-Pressure/Ultra-High Pressure Metamorphic rocks in the area. The existence of high-velocity structures beneath orogenic belts at shallow depths implies that the exhumation of eclogites is confined to the uppermost crust. Generally, fault zones in this area are characterized by a low velocity from 0 to 5 km and relatively high velocity in the lower part of the upper crust and the middle crust. The spatial distribution of earthquakes along Tanlu Fault reveals two seismicity zones: the southern segment and the Suqian-Tancheng segment with a low-velocity transitional zone at Suqian, which suggests that the two segments respond differently to stresses. Regions of dense earthquake distribution correspond to the transition zones of high to low shear-wave velocities, which could be as a result of instability in the upper mantle and variation in compositions of crustal materials.

The age and geochemistry of the mid-Cretaceous volcanic rocks in the Jinan Basin: Implications for the mid-Cretaceous tectonic environments of the Korean Peninsula and Northeast Asia

Most of the mid to late Cretaceous igneous rocks in the southeastern margin of the Northeast Asian continent are interpreted to be formed due to upwelling of asthenospheric mantle caused by slab rollback and associated back-arc extension. However, the tectonic environments for the mid-Cretaceous igneous rocks in the Korean Peninsula is uncertain. The Jinan Basin is one of the Cretaceous pull-part basins in the southern Korean Peninsula. The mid-Cretaceous volcanic rocks occur in association with the Jinan Basin. In this study, we investigated whole-rock major and trace element geochemistry and Sr-Nd-Pb isotope compositions of the mid-Cretaceous volcanic rocks in the Jinan Basin to understand their petrogenesis and tectonic environments. Also, U-Pb zircon age of sedimentary and volcanic rocks in the Jinan Basin was determined to constrain the age of sedimentation and igneous activities. The results show that sedimentation started at least from 97.7 Ma and continued until 89.5 Ma. The sedimentary sequences were intermittently extruded by rhyolitic and andesitic magma 90–89 Ma, and later intruded by basaltic trachyandesite 84 Ma. The volcanic rocks in the Jinan Basin show geochemical features of alkaline to high-K calc-alkaline magma series with strong enrichment in large ion lithophile elements and light rare earth elements relative to high field strength elements. The mafic and intermediate volcanic rocks are distinguished from the typical continental arc magmas by enrichment in incompatible elements such as Y and Zr with high Zr/Y, suggesting that they originated from an enriched source. The basaltic trachyandesites and andesite have Sr isotope ratios (87Sr/86Sri) of 0.708769–0.709484 and Nd isotope ratios (143Nd/144Ndi) of 0.511782–0.511886, which indicate that they formed from mantle which was contaminated with crustal component in their petrogenesis. The Pb isotope data (206Pb/204Pb = 17.572–18.158, 207Pb/204Pb = 15.535–15.640 and 208Pb/204Pb = 38.366–38.921) suggest that the crustal component should have experienced a prolonged evolution with a low μ value. The Mg# (46–57) and Cr contents (40–180 ppm) in the mafic to intermediate rocks higher than typical crustal melts at a given range of SiO2, showing that they evolved from a mantle-derived primary magma. We attribute the crust-like geochemical and isotopic signature of the Jinan mafic and intermediate igneous rocks to the partial melting of enriched lithospheric mantle metasomatized by subduction components and subsequent crustal assimilation during magma ascent and pooling at the mid-crustal depth of ~9–10 km. The geochemical and isotopic features of the Jinan volcanic rocks are similar to those of the mid-Cretaceous basaltic volcanic rocks in the Yeongdong and Gyeongsang Basins, South Korea and the southeastern margin of South China Craton (SCC). This may suggest that the partial melting of the enriched lithospheric mantle, induced by slab rollback of the paleo-Pacific plate and upwelling of the asthenospheric mantle, was widespread underneath the southeastern Korean Peninsular as well as the southeastern margin of SCC.

Discovery of Intracratonic Rift in the Upper Yangtze and Its Control Effect on the Formation of the Anyue Giant Gas Field

Abstract —According to drilling and seismic data, the Late Sinian–Early Cambrian intracratonic rift was found in the Deyang–Anyue area of the Upper Yangtze craton. This rift is controlled by a tensional fault and extends in the N–NW direction with a N–S length of 320 km and an E–W width of 50–300 km. After three stages of the rift evolution, i.e., the formation stage, development stage, and dying stage, a favorable near-source accumulation assemblage formed. The research results indicate that: (1) the sedimentation stage of the Late Sinian Dengying Formation is the rift formation stage, during which trough-basin facies sedimentation is developed in the rift, while platform marginal-facies mounds and shoals are developed on both sides, controlling the formation and distribution of high-quality reservoirs in the Dengying Formation; (2) the sedimentation stage of the Early Cambrian Meishucun–Qiongzhusi Formation is the rift development stage, during which deep-water continental-shelf facies argillaceous rocks accumulated in the rift with a thickness of 500–1000 m, indicating the sedimentation of high-quality source rocks; (3) the sedimentation stage of the Canglangpu Formation is the rift dying stage, terminating the evolution history of the intracratonic rift by gap filling. The intracratonic rift is a key factor for the formation of the Anyue giant gas field, where the high-quality source rocks provide abundant gases for the giant gas field. A regional lateral sealing occurred during the rapid rise of the western paleouplift in Central Sichuan during the Late Yanshan–Himalayan period, favorable for the preservation of the Gaoshiti–Moxi giant gas field on the eastern wing of the rift. The intracratonic rift and its role in giant gas accumulation provide a significant reference for deep oil and gas exploration in paleocratonic basins.

Baddeleyite U-Pb geochronology and geochemistry of Late Paleoproterozoic mafic dykes from the Kongling complex of the northern Yangtze block, South China

Baddeleyite LA-ICP-MS U-Pb geochronology, major and trace element geochemistry and Nd-Pb isotope analysis of Late Paleoproterozoic mafic dykes from the northern Yangtze block, South China craton are used to investigate their petrogenesis and tectonic implications. The dykes mainly consist of fine-grained dolerite. Baddeleyite U-Pb dating indicates that the dolerites formed at 1866 ± 21 Ma. These rocks are alkaline in composition with high TiO2 contents (2.21–3.17 wt%) and display LREE enrichments ((La/Yb)N = 6.0–6.4) with slight positive Eu anomalies (Eu/Eu* = 1.06–1.14). On primitive mantle normalized multi-element diagrams they generally show convex upward curves with weak negative Nb-Ta anomalies. All of the samples have similar Nd-Pb isotopic compositions with negative εNd(t) values ranging from −4.3 to −3.2, (206Pb/204Pb)i from 14.91 to 15.31, (207Pb/204Pb)i from 14.99 to 15.08, and (208Pb/204Pb)i from 34.75 to 35.58. Geochemical features such as EM1-like isotopic compositions suggest the dolerites were derived from partial melting of metasomatised subcontinental lithospheric mantle. The mafic dykes are inferred to have formed in a post-orogenic extensional setting. We suggest that the northern Yangtze block experienced a tectonic switch from convergence to extension at ~1.87 Ga. A comparison of Paleoproterozoic magmatic, metamorphic, and sedimentary events in the Yangtze block with the southern Siberian craton and northern Laurentia supports a Paleoproterozoic position of the Yangtze block adjacent to southern Siberia and northern Laurentia within the Columbia supercontinent.

A new crustal shear-velocity model in Southwest China from joint seismological inversion and its implications for regional crustal dynamics

SUMMARY Southwest (SW) China is located in a transition site from the active Tibetan Plateau to the stable Yangtze craton, which has complicated tectonic deformation and severe seismic hazards. We combine data from ambient noise, teleseismic body and surface waves, and petroleum wells to better constrain the crustal shear-velocity structure in SW China. We jointly invert the Rayleigh wave dispersion (5–40 s period), Rayleigh wave ZH ratio (20–60 s period), and P-wave receiver function for 114 permanent stations with a stepwise linearized joint inversion method. Compared to previous tomography results, we observe higher shear velocity in the sedimentary rocks within the Sichuan Basin, which is consistent with sonic logging measurements. Our model reveals widespread low-velocity zones in the mid-lower crust, and their boundaries correlate well with major fault systems. Between two main mid-crustal low-velocity channels, a prominent high-velocity region surrounded by earthquakes is observed in the inner zone of the Emeishan large igneous province (ELIP) and around the Anninghe-Zemuhe fault zone. These observations are comparable to regional tomography results using very dense arrays. Based on the results, we suggest that mid-lower crustal ductile flow and upper-crustal rigid fault movement play equally important roles in controlling the regional deformation styles and earthquake distribution in SW China. Our results also resolve thick crust-mantle transition zones beneath the eastern Tibetan Plateau and the inner zone of the ELIP due to ‘top-down’ and ‘bottom-up’ crust-mantle interactions, respectively. Our new model can serve as a reference crustal model of future high resolution model construction in SW China.

Remobilization of metasomatized mantle lithosphere: a new model for the Jiaodong gold province, eastern China

The ~ 120 Ma Jiaodong province in the North China Craton, composed of abundant Mesozoic granites that intrude Precambrian metamorphic basement, is bordered by the Sulu orogenic belt, which formed through the collision between the North China and Yangtze cratons in the Triassic. Insights on the geodynamic setting and source of gold and fluids in the Jiaodong gold province are based on a detailed study of the Sanshandao deposit. The δ13CPDB and δ18O values of hydrothermal calcite from this deposit range from − 4.3 to − 6.5‰ and 11.4 to 15.1‰, respectively, which are compatible with a mantle source. Pre-ore mafic dykes in the deposit show typical arc-like geochemical features, suggesting that the mantle source was metasomatized prior to the basic magmatism. The δ34S values of gold-related pyrite from Sanshandao and other large deposits in the province range from 10.9 to 11.5‰ and are higher than those of the country rocks, but consistent with the signature of orogenic gold deposits sourced from a Neoproterozoic sedimentary reservoir. These high positive δ34S values are interpreted to be inherited from auriferous Neoproterozoic sedimentary rocks of the northern Yangtze Craton, which were subducted into the mantle lithosphere beneath the North China Craton during the formation of eclogite in the adjacent Sulu orogenic belt in the Triassic. The syn-ore Lower Cretaceous mafic dykes display oceanic island basalt-like geochemical features, suggesting that asthenosphere upwelling triggered the release of gold and sulfur from an enriched and fertilized mantle lithosphere, contributing to the auriferous fluid which formed the widespread gold mineralization in the Jiaodong province at ca. 120 Ma.

P and S Wave Tomography Beneath the SE Tibetan Plateau: Evidence for Lithospheric Delamination

AbstractThe SE Tibetan Plateau is located between the eastern Himalayan syntaxis and the stable Yangtze craton; its tectonic evolution is important for understanding the plateau expansion. However, interactions between the plateau and its adjacent subduction zone and craton are still unclear. In this study, we determine updated high‐resolution P and S wave tomographic models of the crust and upper mantle beneath the SE Tibetan Plateau. We confirmed the existence of a high‐velocity layer in the upper mantle down to 200‐km depth beneath the Sichuan basin, the core of the Yangtze craton, representing the stable cratonic root. Another high‐velocity body is located beneath Burma, reflecting the subducting Indian slab in the upper mantle and possibly the remnant Burman slab in the mantle transition zone. A strong low‐velocity column exists above the high‐velocity body in the transition zone and reaches the Moho under the Tengchong volcano. In addition, we find high‐velocity fragments in the upper mantle beneath the Yangtze craton and northern Indochina, which may reflect delaminated lithosphere beneath the SE Tibetan Plateau. The lithospheric delamination may be closely related to mantle flow extruded from the high plateau and possible thermal upwelling from the lower mantle. Our results provide new insight into the deep interactions between the Tibetan plateau and its surrounding blocks near the SE plateau margin.

Mercury evidence of intense volcanic effects on land during the Permian-Triassic transition

Abstract The Siberian Traps large igneous province (STLIP) was the likely trigger for the ca. 252 Ma latest Permian mass extinction (LPME), but direct evidence for global volcanic effects on land remains rare. Here, we used mercury (Hg) enrichments, a proxy for ancient volcanic activity, to assess volcanic inputs to two terrestrial Permian-Triassic boundary (PTB) sections that were separated by thousands of kilometers and represent different latitudinal zones—the peri-equatorial Lubei section (South China craton) and the high-latitude (40–60°N) Dalongkou section (Junggar terrane). Both sections exhibit strong Hg enrichment within a discrete (≤40 m) stratigraphic window representing the LPME. At Lubei, this interval is also characterized by negative mass-independent fractionation (MIF) of odd Hg isotopes, consistent with massive volcanogenic and/or terrestrial Hg inputs. These findings are significant in documenting Hg spikes and negative MIF excursions near the PTB in terrestrial sections for the first time, providing evidence of the global influence of the STLIP, as well as in demonstrating at high stratigraphic resolution its synchronicity with the PTB negative carbon-isotope excursion (CIE), supporting a common global cause for these anomalies.

The multiple granitic magmatism in the giant Huayangchuan uranium polymetallic ore district: Implications for tectonic evolution of the southern margin of North China Craton in the Qinling Orogen

The giant Huayangchuan U–Nb–Pb deposit in the Qinling Orogen is a newly verified carbonatite-hosted deposit in the southern margin of the North China Craton (NCC), central China. Granitic magmatism is extensively developed in the Huayangchuan deposit area, but their ages and petrogenesis are not well constrained. The exposed granitic rocks are mainly biotite monzogranite porphyry, granite pegmatite, granodiorite, and biotite monzogranite with zircon U–Pb ages of 1808 ± 10 Ma, 1807 ± 14 Ma, 233 ± 1.4 Ma, and 132 ± 0.6 Ma, respectively. The Paleoproterozoic biotite monzogranite porphyry belongs to shoshonite and metaluminous series, showing enrichment of LREE and LILE and depletion of HREE and HFSE, with high Zr + Y + Nb + Ce values and Ga/Al ratios, which are consistent with A-type granite. Whereas, the contemporaneous granite pegmatite dykes with weak mineralization are also cala-alkaline to shoshonite and peraluminous series, enriched in Rb, Ba, and LREE, and depleted in Nb, Ta, Ti and HREE. The shoshonite and weakly peraluminous Triassic granodiorite is slightly enriched in LREE with flat HREE patterns, enriched in Ba and Sr and depleted in Nb, Ta, P, and Ti, with similar geochemical characteristics to adakite-like rocks. The Early Cretaceous biotite monzogranite is characterized by LREE enrichment and flat HREE patterns, belonging to shoshonite and metaluminous to weakly peraluminous I-type granite, with U and LILE enrichment, and HFSE-depleted. The high initial 87Sr/86Sr ratios and enriched Nd (εNd(t) = −17.5 to −17.1) and Hf (εHf(t) = −33.2 to −14.6) isotopes reveal that the Huayangchuan granitic rocks are obviously sourced from crustal-derived magmas. Combined with regional geology and this study, we proposed that: (1) the Paleoproterozoic biotite monzogranite porphyry and granite pegmatite were generated from ancient lower crust during post-collisional extension setting; (2) the granodiorite was likely sourced from partial melting of thickened lower crust with pelagic sediments materials addition during the Triassic although the mineralization-related carbonatite having similar age with granodiorite may derive from mantle; and (3) the Early Cretaceous biotite monzogranite was mainly sourced from the partial melting of lower crust induced by the underplating of the mafic magma. We suggest that the first phase of magma occurred in the Huayangchuan district during the Paleoproterozoic following the amalgamation of the Eastern and Western blocks along the Trans-North China Orogen. Since the Mesozoic, the ongoing northward subduction of the Yangtze Craton (YZC) resulted in the crust thickening, developing the Triassic granitic magmatism and major mineralization associated with carbonatite. The widespread Cretaceous granite and deposit in the southern margin of NCC indicate that intracontinental extension and lithospheric thinning occurred in response to the tectonic regime transition from NS-trending to EW-trending subduction.

Basement evolution of the Sulu orogenic belt: Constraints on zircon U–Pb ages and trace elements from the Weihai gneisses

Early Paleoproterozoic rocks occur sporadically in the Sulu ultrahigh‐pressure (UHP) belt. The tectonic affinity and evolution of these rocks are still disputed. Two magmatic events (Early Paleoproterozoic and Neoproterozoic) and two metamorphic events (Late Paleoproterozoic and Triassic) were revealed by the zircon U–Pb ages and trace elements from the Weihai gneisses, which provide a key clue to understand the evolution of the Sulu orogen. The Early Paleoproterozoic zircons occur as residual zircons or zircon‐inherited cores in the two gneisses. They generally show oscillation zoning and enriched HREE patterns. The weighted mean age of the upper intercept age of these zircons is ~2.46 Ga, suggesting that the protolith of the ancient gneisses formed at ~2.46 Ga. The Late Paleoproterozoic zircons occur as metamorphic zircon core or mantle domains in the two gneisses and a quartzite. They generally show planar zoning or no zoning, with relative lower Th/U ratios. These zircons yield metamorphic ages of 1.8–1.84 Ga, which are consistent with the age of the Paleoproterozoic ultrahigh‐temperature metamorphism of the northern Sulu orogenic belt. The Neoproterozoic zircons occur as magmatic cores or mantles in the zircon grains of the two gneisses and their wall rocks. They show typical oscillatory zoning and enriched HREE patterns, yielding magmatic ages of ca. 740–787 Ma. The Neoproterozoic igneous rocks are the dominant basement rocks in the Sulu orogenic belt. Triassic UHP metamorphism has been well studied in the Sulu orogenic belt. Two Triassic age groups were also recognized here: 228–229 and 208 Ma, respectively. The Triassic zircons occur as metamorphic rims of the two ancient gneisses and their wall rocks. These two age ranges correspond to the time of subduction and exhumation of the the South China Craton (SCC) during the Triassic orogeny. This study provides solid evidence that the Weihai ancient gneisses have a strong affinity with the Yangtze Block. The protoliths of them are Paleoproterozoic (ca. 2.46 Ga) magmatic rocks. They were subsequently suffered from ca. 1.8‐ to 1.84‐Ga metamorphism, new crustal materials overgrowth during ca. 740‐ to 787‐Ma magmatism, and then experienced Triassic continental deep subduction and exhumation process to build the Sulu orogenic belt.

Petrogenesis and geochronology of Paleoproterozoic magmatic rocks in the Kongling complex: Evidence for a collisional orogenic event in the Yangtze craton

The early Precambrian crustal evolution of the Yangtze craton, especially the Paleoproterozoic orogenic event, has drawn great attention. The early Precambrian magmatism in the Archean-Palaeoproterozoic basement Kongling complex is critical for understanding the early Precambrian tectonic evolution of the Yangtze craton. We report the presence of two distinct groups of Paleoproterozoic felsic rocks in the Kongling complex, providing new insights into the Paleoproterozoic orogenic event in the northern Yangtze craton. Group 1 is composed mainly of deformed or weakly-deformed monzogranite, syenite, and quartz monzonite, and formed at 2.0–1.95 Ga. They have high Sr/Y (60–71) and La/Ybcn (32–464) ratios and are inferred to have equilibrated with garnet-rich residues, suggesting that they have been generated by partial melting of thickened continental crust. Group 2 is represented mainly by undeformed to weakly-deformed K-feldspar granitic rocks, with minor rhyolites, and was emplaced at 1.87–1.84 Ga. In contrast to Group 1, Group 2 rocks display flat HREE patterns and large negative Eu, Sr and Ti anomalies, with low Sr/Y (<10) and La/Ybcn (16–21) ratios. Group 2 rocks have a typical A-type affinity with high Ga/Al ratios (2.83–3.82), and high crystallization temperatures (850–950 °C). Group 2 rocks are interpreted to have been derived from relatively shallow partial melting of thin continental crust during post-orogenic collapse, which marked lithospheric extension of the Yangtze craton in the late Paleoproterozoic (ca. 1.85 Ga).Zircons from Group 1 rocks have εHf (t) values ranging from −9.4 to −14.9, whereas zircons from Group 2 rocks have lower εHf (t) values ranging from −14.2 to −20.6. Both groups have similar two-stage Hf model ages ranging between 3.2 and 3.6 Ga. The production of the two groups of granitoids in the Huangling dome is ascribed to the reworking (dehydration melting) of Archean rocks in the Paleoproterozoic. Widespread Paleoproterozoic magmatism and metamorphism indicate that the Yangtze craton experienced an orogenic event at ca. 2.0 Ga and subsequent orogenic collapse caused by lithospheric extension at ca. 1.85 Ga.

Detailed structure of the lithosphere and deep dynamics beneath the Three River belt and western edge of the Yangtze craton

Detailed structure of the lithosphere and deep dynamics beneath the Three River belt and western edge of the Yangtze craton

Inverting Rayleigh surface wave velocities for crustal thickness in eastern Tibet and the western Yangtze craton based on deep learning neural networks

Abstract. Crustal thickness is an important factor affecting lithospheric structure and deep geodynamics. In this paper, a deep learning neural network based on a stacked sparse auto-encoder is proposed for the inversion of crustal thickness in eastern Tibet and the western Yangtze craton. First, with the phase velocity of the Rayleigh surface wave as input and the theoretical crustal thickness as output, 12 deep-sSAE neural networks are constructed, which are trained by 380 000 and tested by 120 000 theoretical models. We then invert the observed phase velocities through these 12 neural networks. According to the test error and misfit of other crustal thickness models, the optimal crustal thickness model is selected as the crustal thickness of the study area. Compared with other ways to detect crustal thickness such as seismic wave reflection and receiver function, we adopt a new way for inversion of earth model parameters, and realize that a deep learning neural network based on data driven with the highly non-linear mapping ability can be widely used by geophysicists, and our result has good agreement with high-resolution crustal thickness models. Compared with other methods, our experimental results based on a deep learning neural network and a new Rayleigh wave phase velocity model reveal some details: there is a northward-dipping Moho gradient zone in the Qiangtang block and a relatively shallow north-west–south-east oriented crust at the Songpan–Ganzi block. Crustal thickness around Xi'an and the Ordos basin is shallow, about 35 km. The change in crustal thickness in the Sichuan–Yunnan block is sharp, where crustal thickness is 60 km north-west and 35 km south-east. We conclude that the deep learning neural network is a promising, efficient, and believable geophysical inversion tool.

Three‐Dimensional Model of the Lithospheric Structure Under the Eastern Tibetan Plateau: Implications for the Active Tectonics and Seismic Hazards

AbstractDetails of lithospheric structures in three‐dimensions (3‐D) are key to understanding the dynamics of crustal deformation and earthquakes in active orogenic systems. In this study, we develop a 3‐D model of the eastern Tibetan Plateau using the Skua‐Gocad software based on the latest Rayleigh wave tomography. We perform a quantitative modeling workflow to map the details of the Moho discontinuities and the high‐velocity anomalies. Then, we integrate the topography, major active faults, large earthquakes, and crustal Poisson's ratios to analyze the relationship between the shallow and deep structures of this region. Our study shows that the Moho is generally coupled with the topography. A steep Moho ramp exists under the plateau margin and its surface projection intersects the Longmen Shan (LMS) at a low oblique angle (~22°). Active deformation and large earthquakes are related to the steep Moho ramp under the plateau margin. Moreover, based on a 3‐D model of the Poisson's ratio perturbations, we find that deformation across the central LMS is characterized as a crocodile‐type wedge in the crust and lithospheric mantle, due to the resistance of the Yangtze craton and isostatic rebound. We suggest that a tectonic wedging model that contains both the upper‐crust thrusting and lower‐crustal thickening contribute to the LMS orogeny. Three‐dimensional visualization of the lithospheric structure is well suited to reveal the different levels of deformation and their relationships. Quantitative modeling methods provide effective constraints on the active blocks in the eastern Tibetan Plateau.

Ancient deep roots for Mesozoic world-class gold deposits in the north China craton: An integrated genetic perspective

The North China Craton (NCC) hosts some of the world-class gold deposits that formed more than 2 billion years after the major orogenic cycles and cratonization. The diverse models for the genesis of these deposits remain equivocal, and mostly focused on the craton margin examples, although synchronous deposits formed in the interior domains. Here we adopt an integrated geological and geophysical perspective to evaluate the possible factors that contributed to the formation of the major gold deposits in the NCC. In the Archean tectonic framework of the NCC, the locations of the major gold deposits fall within or adjacent to greenstone belts or the margins of micro-continents. In the Paleoproterozoic framework, they are markedly aligned along two major collisional sutures – the Trans North China Orogen and the Jiao-Liao-Ji Belt. Since the Mesozoic intrusions hosting these deposits do not carry adequate signals for the source of gold, we explore the deep roots based on available geophysical data. We show that the gold deposits are preferentially distributed above zones of uplifted MOHO and shallow LAB corresponding to thinned crust and eroded sub-lithospheric mantle, and that the mineralization is located above regions of high heat flow representing mantle upwelling. The NCC was at the center of a multi-convergent regime during the Mesozoic which intensely churned the mantle and significantly enriched it. The geophysical data on Moho and LAB upwarp from the centre towards east of the craton is more consistent with paleo-Pacific slab subduction from the east exerting the dominant control on lithospheric thinning. Based on these results, and together with an evaluation of the geochemical and isotopic features of the Mesozoic magmatic intrusions hosting the gold mineralization, we propose a genetic model that invokes reworking of ancient Au archives preserved in the lower crust and metasomatised upper mantle and which were generated through multiple subduction, underplating and cumulation events associated with cratonization of the NCC as well as the subduction-collision of Yangtze Craton with the NCC. The heat and material input along zones of heterogeneously thinned lithosphere from a rising turbulent mantle triggered by Mesozoic convergent margins surrounding the craton aided in reworking the deep roots of the ancient Au reservoirs, leading to the major gold metallogeny along craton margins as well as in the interior of the NCC.