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As the Neoproterozoic suture zone between the Yangtze and Cathaysia blocks, the Jiangnan orogen is both a research hotspot and controversial issue. In particular, its formation process and tectonic polarity have been extensively investigated in recent years, leading to three kinds of geological interpretations, including southeastward subduction of the Yangtze block, northwestward subduction of the Cathaysia block and divergent double subduction. However, none of these interpretations can completely reconcile the abundant geological and geophysical data. Therefore, we performed a magnetotelluric profile across the middle Jiangnan orogen and western Cathaysia Block, and employed a new 3D electrical resistivity model. In contrast to previous resistivity models applied across eastern and western Jiangnan, our model revealed a thick lithosphere of middle Jiangnan with high resistivity and a steep low-resistivity boundary on the east. This structure indicates that the middle section of block convergence can be interpreted as a transitional region of polarity conversion rather than oceanic or continental subduction. Hence, we suggest that previous geological interpretations involving the subduction hypothesis are applicable to the convergence process in South China, but are only appropriate for local sections with different characteristics. Combined with previous observations, we propose a segmented accretion model, including southeastward subduction of the oceanic slab in eastern Jiangnan, northwestward subduction of the Cathaysia block in the west, and transitional accretion in the middle. Graphical Abstract

The Jiangnan Orogen, located at the southeastern margin of the Yangtze Block, South China, records the complete history of assembly and evolution between the Yangtze and Cathaysia blocks. Neoproterozoic sedimentary sequences are widespread in the Jiangnan Orogen, with a regional angular unconformity separating the Sibao and Danzhou groups. U-Pb geochronology and Lu-Hf isotope analyses were carried out on detrital zircons across this unconformity and also the associated granites in the western Jiangnan Orogen in this study. Our new results, combined with previous data, indicate that the Sibao and Danzhou groups were deposited at 860-832 and 803-764 Ma, respectively. Thus, this unconformity was constrained to be 832-803 Ma. Detrital zircons from the Sibao and Danzhou groups have generally similar age populations, with three major peaks at 2000-1550, 1000-880, and 850-830 (760) Ma and one minor at 2600-2400 Ma. The most important generation of juvenile crust appears to have occurred at 2600-2400 Ma. Both recycling of ancient crustal materials and addition of juvenile mantle components took place in the time intervals of 2000-1550 and 1000-880 Ma. Detrital zircons in the age population of 850-830 (760) Ma were probably derived from proximal magmatic rocks in the western Jiangnan Orogen, reflecting fast erosion of newly formed igneous rocks. The Sibao and equivalents were deformed and intruded by granites posterior to the deposition of the Sibao and equivalent sequences after 832 Ma but before deposition of the Danzhou and equivalent units before 803 Ma. The pre-Danzhou deformation marked a collision along the southeast margin of the Yangtze Craton. The regional angular unconformity sealed the Jiangnan Orogeny, and thus the collision between the Yangtze and Cathaysia blocks was during 832-803 Ma. The Danzhou and equivalent units unconformably overlying the Sibao Group record the onset of rifting along the southeastern Yangtze margin beginning around 803 Ma.

Fifteen sandstone samples taken from pre-Cretaceous strata of the Yangtze Block are analysed to constrain the evolution of the South China Block, especially the assembly between the Yangtze and Cathaysia blocks. The results show that the maximum depositional age of the Neoproterozoic Lengjiaxi Group adjacent to the Cathaysia Block is c. 830 Ma, differing from that of the Kunyang and Dahongshan groups (> 960 Ma) on the southwestern margin of the Yangtze Block. The detrital zircons from Palaeozoic samples from the Yangtze Block have similar age populations to those in the Cathaysia Block, and they may originate from the Cathaysia Block according to palaeogeographic, palaeocurrent and former research data. The detrital zircons of Middle-Upper Jurassic sandstones in the southwestern and central Yangtze Block yield dominant age populations at 2.0-1.7 Ga and subordinate groups of 2.6-2.4 Ga, 0.8-0.7 Ga and 0.6-0.4 Ga. The Upper Triassic strata may be derived from the southern Yangtze and North China blocks due to the collisions between the Indosina, South China and North China blocks, whereas the Jurassic sediments may be partly derived from uplift and erosion of the Jiangnan Orogen due to an intracontinental orogeny induced by Pacific subduction towards the Eurasia Plate. The detrital age spectra and provenance data for basement in the South China Block are analysed and compared with each other. The South China Block has affinity with Australia not only in the Columbia supercontinent but also in the Rodinia supercontinent. We infer the existence of an ancient orogen under the western Jiangnan Orogen, which may have occurred during the Columbia age, earlier than the Sibao orogeny. This is supported by seismic profile proof from the SinoProbe.

利用背景噪声方法,对布设在华南块体及其周边区域数个流动地震台阵中共 704个地震台站的连续数据进行分析,反演得到华南岩石圈的三维 S波速度结构.结果表明,地表附近 S波速度结构与华南块体的地表构造高度相关,低速异常区集中在四川盆地和江汉盆地等沉积层较厚的区域.江南造山带的中上地壳呈现明显的高速异常,该高速异常即为华夏块体与扬子块体的分界区域.华夏块体中地壳低速异常区与该区域广泛分布的中生代花岗岩对应较好.华南上地幔岩石圈的东部与西部存在显著的速度差异,西部呈现显著的高速异常,反映西部四川盆地底部稳固的克拉通结构;上地幔的低速异常揭示华南东部岩石圈因太平洋板块俯冲的影响而明显减薄(<70 km).推测华夏块体以琼州海峡为中心的上地幔显著低速区可能对应海南地幔柱的热物质上涌.

The South China craton was formed by collision of the Yangtze and Cathaysia blocks during the Neoproterozoic Jiangnan orogeny (also termed the Jingnin or Sibao orogeny in Chinese literature). Basement rocks within the Yangtze block consist mainly of Proterozoic sediments of the Lengjiaxi and Banxi groups. U-Pb ages of detrital zircons obtained by the laser ablation inductively coupled plasma mass spectrometry dating technique imply that the deposition of the Lengjiaxi Group continued until the Neoproterozoic. The youngest detrital zircons suggest a minimum deposition age of ∼830 Ma for the Lengjiaxi Group and an initial deposition age of 785±12 Ma for the Banxi Group, indicating a temporal hiatus of 48±13 Ma between these Neoproterozoic sedimentary rocks distributed in northwestern Hunan Province, South China craton. Detrital zircons from both the Lengjiaxi and Banxi groups have a wide range of εHf(t) values from -12 to 14.2 and a continuous Nd and Hf model age spectrum from ∼820 Ma to 2200 Ma. Model ages of many detrital zircon grains reach up to ca. 2.9-3.5 Ga, indicating that both juvenile mantle material and ancient crust provided sedimentary detritus. This is also consistent with the Nd isotopic signature of sedimentary rocks recorded in the Lengjiaxi Group, suggesting a back-arc tectonic setting. The Banxi Group has slightly enriched Nd isotopic signatures relative to the Lengjiaxi Group, implying a higher percentage of old continental material in the sedimentary source. Combined with previously published data, new results help us to reconstruct the Neoproterozoic tectonic evolution of the South China craton.

The debate of assembly time between the Yangtze and Cathaysia blocks has aroused the disputation on the formation mechanism of Jiangnan orogen, South China. Therefore widespread magmatism of 830-750 Ma in South China is interpreted as the product of either plume or arc magmatism, which results in distinctive depositional background in the Neoproterozoic. Granite gravel located at the unconformity between the Banxi Group and Fanjingshan Group of the western Jiangnan orogen was collected, which gave a new age limit to the deposition of the Banxi Group and Nanhu rift. Zircons from the granite gravel crystallized yield a weighted mean 2~6pb/238U age of 789~11 Ma, which probably represents the end of the Jiangnan orogeny and marks the onset of Nanhua rift. These zircons have negative εHf（t） of -2.1 to -6.0, with TDM of 1.38-1.52 Ga and Tcrust of 1.81-1.98 Ga indicating an old continental crust origin for the granite.

Most, if not all, orogenic gold deposits form during the late stages of deformation. In some cases, there is barren pre-mineralization hydrothermal alteration which may be part of a single progressive hydrothermal event or a temporally distinct event relative to deposition of gold. At the Huangjindong orogenic gold deposit in the Jiangnan Orogen of southern China, there are two phases of vein generation, the earlier of which is largely barren. Structural analysis, petrography, mineralogy, and in situ laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) gold mapping of different generations of gold-bearing sulfides reveal the relationship between different phases of hydrothermal alteration and quartz veins. Pre-existing barren and brittle quartz veins, which are controlled by tight folds and thrust faults, provide favorable competent hosts for later overprinting by gold-bearing ore-bearing fluids that infiltrated during ongoing reverse-sinistral fault movement. These overpressured fluids caused hydrofracturing to complete brecciation of segments of the earlier quartz veins and associated pre-existing breccias, with deposition of auriferous quartz-sulfide veins and breccia cements or matrices within them. Although fluid inclusion data are non-definitive, fluid pressure fluctuations are interpreted to have resulted in phase separation within the ore fluid which was primarily responsible for destabilization of gold complexes and deposition of native gold with other invisible gold-bearing ore-related sulfides formed by sulfidation reactions, in hydrofractures and breccia matrices. Thus, competent barren quartz veins in fold/thrust belts may locally host superimposed gold mineralization and provide favorable targets for gold resources.

The remobilization, migration, precipitation, and enrichment of ore-forming elements are closely related to structures. Therefore, detailed regional and ore-field structural analyses are critical for determining the genesis of a mineral deposit. The Jiangnan Orogenic Belt (JOB) is an important gold polymetallic metallogenic belt in South China, which is characterized by multiple periods of gold mineralization in the Paleozoic and Mesozoic. However, the genesis of these gold polymetallic deposits is still not well understood due to a lack of systematic research on the regional geology, ore-controlling structures and metallogenic mechanism. In this study, a detailed structural survey at the surface and in the subsurface tunnels was conducted on the Woxi Au-Sb-W deposit, the genesis of which is relatively controversial among the gold polymetallic deposits in the JOB due to poor structural constraints. In addition, a wolframite U-Pb dating was carried out to further constrain the relationship between structures and mineralization. Based on the results of these studies, together with those from previous studies, it is proposed that the Woxi deposit and surrounding areas likely underwent six periods of regional deformation, which are constrained in time and geodynamic setting. Furthermore, we present a systematic discussion on the roles of ore-controlling structures in the transportation, distribution, and deposition of ore-forming elements and localization of orebodies. According to the wolframite dating results, structural analyses, and previous data, we propose that the Woxi Au-Sb-W deposit was formed in two stages during the Yanshanian: a W (wolframite)-Au mineralization stage at ca. 140 Ma and an Au-Sb-W (scheelite) mineralization stage at <130 Ma. These mineralizing events are interpreted to have a tight relationship with tectonic reactivation, and the ore-forming fluids were derived from deep sources, including those of magmatic or metamorphic origins. The Woxi deposit can therefore be classified as an “intracontinental reactivation-type”, and the mineralization is related to lithospheric extension caused by plate retreat, retention, and delamination following the cessation of westward subduction of the Paleo-Pacific Plate beneath the East Asian continent.

Jiuhua Mountain, situated in the eastern segment of the Jiangnan Orogenic Belt, is characterized by extensive granite masses, providing an ideal setting for investigating the exhumation history of the region. This study presents the first zircon (U-Th)/He thermochronological investigation utilizing an age-elevation approach for Jiuhua Mountain. Zircon (U-Th)/He analyses of six bedrock samples yielded consistent ages ranging from 89 to 74 Ma. This finding aligns with the results of thermal history simulations (90-70 Ma), indicating significant exhumation in the Jiuhua Mountain region during the late Cretaceous. Comparative analysis with adjacent orogenic belts (e.g., the Mufu and Dabie Mountains) reveals a consistent pattern: low-temperature thermochronological ages are younger than the U-Pb zircon ages. This indicates a widespread tectonic exhumation and erosion phase affecting multiple regions within the northern part of the South China Block. The rapid exhumation observed during the late Cretaceous is primarily attributed to extensional tectonics, driven by the rollback of the Paleo-Pacific slab and accompanied by thermal upwelling of the asthenosphere beneath the South China Block. These findings establish crucial temporal constraints for the exhumation history of the eastern Jiangnan Orogenic Belt, significantly enhancing our understanding of its poorly constrained Cretaceous tectonic evolution.

The nature and evolution of the Late Mesozoic lithospheric mantle of the South China Block (SCB) and the influence of subduction of the (paleo-) Pacific plate on it are still highly controversial. A volcanic pipe containing plentiful peridotite xenoliths was newly identified in the eastern Jiangnan orogenic belt of the SCB, which provides us with the possibility to solve the above-mentioned disputes. The host olivine basalts yield a whole-rock K–Ar age of 73 Ma, thus indicating Late Cretaceous magmatic activity. The peridotite xenoliths are all spinel lherzolites, with Mg# in olivine ranging from 87.6 to 89.0, Al 2 O 3 in ortho- and clinopyroxene ranging, respectively, from 3.93 wt% to 4.97 wt% and from 5.30 wt% to 7.00 wt%, and Cr# in spinel ranging from 10.3 to 13.7, all of which are characteristics of a fertile lithospheric mantle. The clinopyroxenes from the peridotite xenoliths show light/heavy rare earth element (L/HREE) fractionation (i.e. (La/Yb) N  = 1.69–3.91), which is different from the LREE-depleted patterns of Middle-Late Jurassic peridotite xenoliths. Besides, the 87 Sr/ 86 Sr(t) and ε Nd (t) values of clinopyroxene vary from 0.7044 to 0.7052 and from 0.6 to 2.8, respectively, which corresponds to a very weakly depleted mantle source. Collectively, the mineral chemistry of peridotite xenoliths and the trace elemental and Sr–Nd isotopic compositions of clinopyroxene both suggest that the lithospheric mantle beneath the eastern Jiangnan orogenic belt underwent a low degree of partial melting (i.e. 1%–3% batch melting or 2%–3% fractional melting), followed by silicate melt metasomatism, the metasomatic agent being derived from a subducted oceanic plate. Combined with previous studies on peridotite xenoliths of different ages, we outline a complete Phanerozoic lithospheric mantle evolution of the SCB and propose that the subduction of the (paleo-) Pacific plate contributed both material and energy to the modification of the lithospheric mantle since the Cretaceous (and possibly until the Eocene).