Paleo-Pacific Plate Research Articles

Change in the direction of Early Cretaceous tectonic extension in eastern North China Craton as the result of Paleo-Pacific/Eurasian plate interaction

How the subduction direction of the Paleo-Pacific plate beneath the Eurasian plate changes in the Early Cretaceous remains highly controversial due to the disappearance of the subducted oceanic plate. Intraplate deformation structures in the east Asian continent, however, provide excellent opportunities for reconstructing paleostress fields in continental interior in relation to the Paleo-Pacific/Eurasian plate interaction. Anisotropy of magnetic susceptibility (AMS), geological, and geochronological analyses of post-kinematic mafic dykes intruding the detachment fault zone of the Wulian metamorphic core complex (WL MCC) in Jiaodong Peninsula exemplify emplacement of mantle-sourced dykes in a WNW–ESE (301°–121°) oriented tectonic extensional setting at ca. 120 Ma. In combination with the results from our previous kinematic analysis of the MCC, a ca. 21° clockwise change in the direction of intraplate extension is obtained for early (135–122 Ma) extensional exhumation of the MCC to late (122–108 Ma) emplacement of the dykes. Such a change is suggested to be related to the variation in subduction direction of the Paleo-Pacific plate beneath the Eurasian plate, from westward (pre-122 Ma) to west-northwestward (post-122 Ma).

Genetic link between concealed granite and tin mineralization in the Yuling tin deposit, Nanling Range, South China: Constraints from zircon and cassiterite U-Pb dating, geochemistry, and Lu-Hf isotopes

The Nanling Range in South China is a world-renowned tin (Sn)-tungsten (W) metallogenic belt. The Yuling deposit is a representative Sn deposit newly discovered in the Nanling Range in recent years. The mineralization of Sn and accompanied Pb-Zn(-Sb) is controlled by faults and hosted in shallow metamorphic sandstone of the Cambrian, with concealed granitic intrusion in the deep. Here, we report for the first time precise in-situ U-Pb dating of zircon and cassiterite, petrogeochemistry, zircon trace element and Hf isotope data from the Yuling Sn deposit to clarify the genetic link between Sn mineralization and the concealed biotite granite. The U-Pb ages of the concealed granite and Sn mineralization are 155.1 ± 0.7 Ma and 154.4 ± 1.4 Ma, respectively, indicating that Sn mineralization occurred simultaneously with Late Jurassic granitic magmatism. Combined with the geological observation that Sn mineralization developed locally near the roof of concealed granite, further attests to the close genetic link between the Yuling Sn deposit and the deep-seated concealed granite. The zircon εHf(t) values range from −5.14 to −1.37 (mean = −2.87), and the two-stage Hf mode ages (TDM2) range from 1.29 to 1.52 Ga (mean = 1.38 Ga), indicating that the source rocks of the Yuling granite primarily originated from the remelting of ancient crustal rocks in the Mesoproterozoic of South China, with the involvement of some juvenile mantle-derived components. The Yuling granite exhibits high content of SiO2, K2O, K2O + Na2O, low content of MgO, CaO, Ba, Sr, Ni, Cr, relatively flat REE patterns and significant negative Eu anomalies. Petrographic and whole-rock geochemical data show that the Yuling granite has an A2-type granite affinity and formed in an intraplate extensional setting of intense crust-mantle interaction similar to that of numerous A2-type granites associated with Sn-W mineralization during the early Yanshanian in the Nanling Range. It may be related to the extension and thinning of the continental lithosphere and asthenosphere upwelling induced by northwestward subduction of the paleo-Pacific plate. Moreover, the factors controlling the Sn mineralization potential of granitic magma were evaluated using zircon trace elements and petrogeochemical data. These findings indicate that the Yuling granite crystallized from higher temperature, lower water content, F-rich, and highly-fractionated reduced granitic magma. Such granitic magma is conducive to generating Sn-rich fluids and has high Sn mineralization potential.

Genesis and Tectonic Implications of Early Cretaceous Granites in the Haobugao Area, Southern Great Xing’an Range: Insights from Zircon U–Pb Geochronology, Hf Isotopic Composition, and Petrochemistry

In the Huanggangliang–Ganzhuermiao metallogenic belt in the southern Great Xing’an Range, the Haobugao Pb–Zn deposit is the most widespread skarn-type polymetallic deposit. The observed mineralization processes in this area are closely associated with both magmatic and tectonic activity. The zircon U–Pb ages of two granitoid phases are 134.0 ± 0.6 Ma and 133.4 ± 0.9 Ma (Early Cretaceous). High SiO2 content (average mass fractions of 77.98 wt.% and 73.25 wt.%), high alkalinity (average mass fractions of 6.19 wt.% and 8.78 wt.%), and low CaO levels (average mass fractions of 0.16 wt.% and 0.12 wt.%) are characteristic of these rocks. They are also enriched in high-field-strength elements (HFSEs) (Th, U, Ta, Zr, Hf, etc.) and depleted in large ion lithophile elements (LILEs) (Ba, Sr, etc.). Furthermore, the Nb/Ta ratios (7.80~8.82, 10.00~10.83) point to a crustal origin of the magma. The zircon Hf isotopic compositions suggest that the melting of young crust derived from Meso-Neoproterozoic and Neoproterozoic depleted mantle gave rise to the magma in these granite porphyries. These rocks formed in an extensional environment driven by the subduction and retreat of the Paleo-Pacific plate during the Early Cretaceous.

Geochronological and geochemical constraints on the multistage magmatic processes of the Lianhuashan batholith, South China: Implications for the petrogenesis and polymetallic mineralization

The Lianhuashan batholith, which is a composite pluton located in the middle part of a world-class giant tungsten (W) polymetallic belt of South China, hosts a huge reserve of more than 50000 t of W. Despite this great economic significance, the petrogenesis of each phases of the Lianhuashan batholith, as well as its temporal and genetic relationships to the W polymetallic mineralization, is still unclear. To address these questions, we conducted a comprehensive study for the Lianhuashan batholith, including LA-ICP-MS zircon U-Pb dating, whole-rock geochemical and in-situ mineral trace element analyses. Different from previous studies, four main granitic phases (G1 to G4) were identified. The biotite granite (G1) formed from 168.6 to 165.3 Ma, has the lowest Si content and high Al and Fe contents, and is significantly depleted in Ba, Sr, Ti, P, and Nb whereas enriched in U, Hf, Zr, and Y contents. In contrast, the two-mica granite (G2) and fine-grained muscovite granite (G3), formed from 162.8 to 160.5 Ma, exhibit similar characteristics including enrichment in La, Y, Hf, Th, U, and depletion in Ba, Sr, and P. The porphyritic granite (G4) is the latest magmatic phase and formed at 158.3 Ma. It is characterized by high K, Si, Th, U, Zr, Hf contents but low Al, Fe, Sr, P, and Ti contents. These features support that four main phases of the Lianhuashan batholith belong to S-type granites that experienced significant fractional crystallization, and display reduced and low temperature features. Combined with previously published studies, we suggest that the Lianhuashan batholith formed in an intraplate extensional setting triggered by the high-angle rollback subduction of the paleo-Pacific plate. The three early phases (i.e., G1–G3) are in turn more oxidized and are responsible for the transition from Sn- to W-dominated mineralization. In contrast, the G4 granite, characterized by lower oxygen fugacity, is resposible for Pb-Zn-Ag-U polymetallic mineralization.

Formation and Tectonic Evolution of Ophiolites in the Sabah Area (Borneo, SE Asia)

Zircon U-Pb dating, rock geochemistry, Sr-Nd-Pb, and zircon Hf isotope analyses were conducted on the ultrabasic and basic rocks of ophiolites in the Sabah area (Borneo, SE Asia). The zircon U-Pb ages of ultrabasic and basic rocks range from 248 to 244 Ma, indicating that the ophiolites already existed in the early Triassic. The rare earth elements of basic rocks in Central Sabah show N-MORB-type characteristics and E-MORB-type characteristics in the northwest and southeast. The εNd(t) values of basic rocks range from 3.66 to 8.73, and the εHf(t) values of zircon in ultrabasic rocks are between −10.2 and −6.1. Trace element analysis shows that the magmatic source was influenced by melts and fluids from the subducting plate of the Paleo-Tethys Ocean. The tectonic evolution of the Sabah area can be traced back to the Early Triassic. At that time, the fast subduction of the Paleo-Tethys Ocean plate and the retreating of the Paleo-Pacific plate resulted in the upwelling of mantle material in relatively small extensional settings, leading to the formation of the ophiolites. From the Jurassic to the Early Cretaceous, the Paleo-Pacific plate was intensely subducted, and the ophiolite intrusion in the Sabah area moved to the continental crust of South China or the Sundaland margin as fore-arc ophiolites. From the Late Cretaceous to the Miocene, with the expansion of the Proto-South China Sea and South China Sea oceanic crust, the ophiolites in the Sabah area drifted southward with microplate fragments and sutured with East Borneo.

Constraints of in-situ elemental compositions and U–Pb ages of cassiterite on the origin of the Cretaceous Gejiu and Dulong tin deposits, SW China: Implications for the linkage of tin belts in SE Asia

The western Yunnan tin belts in SW China and the southern Myanmar tin belt in SE Asia are traditionally thought to be part of the SE Asian tin metallogenic province. However, the tectonic setting of the Youjiang tin belt in SW China and its genetic relationship to the other tin belts are poorly documented. In this belt, the giant Gejiu and Dulong tin deposits are typical of skarn-type deposits and both contain contact and distal skarn orebodies. Tin ores of distal orebodies in these two deposits are composed of cassiterite, sulfide and calcite. In both deposits, most cassiterite grains from these ores are rich in Fe and W and depleted in Nb and Ta. They have 238U/206Pb ages between 81.4 and 84.1 Ma, similar to hosting granitic plutons. Two generations of cassiterite with distinctly different microtextures and trace elemental compositions can be recognized. The first generation (Cst-I) is compositionally and texturally homogeneous, but the second generation (Cst-II) displays distinctly oscillatory zoning. Cst-II contains Nb, W, and U lower, and Fe, Ta, Zr and chalcophile elements (Ga, Ge, In, and Sb) higher than Cst-I. These textural and compositional variations reflect the potential involvement of meteoric water in the magmatic-hydrothermal system. In this system, ore-forming fluids were evolved to more alkaline and oxidized to facilitate the precipitation of cassiterite. Our study indicates that these two deposits in the Youjiang tin belt have mineralization styles similar to those in the adjacent western Yunnan and southern Myanmar tin belts and all these tin belts belong to the SE Asian metallogenic provinces. A westward-younging trend of tin mineralization in these three belts can be explained by the rollback of the subducted Neo-Tethyan oceanic slab. This setting is different from the one related to the subduction of the Paleo-Pacific plate.

Light δD apatites reveal deep origin water in North China Craton intracontinental granites and basalts

Water is essential to the formation of intracontinental granites, but its origin remains elusive. Here we address this scientific problem by analyzing D/H isotopes of apatites, hydrous minerals in Jurassic and Early Cretaceous granites and basalts from eastern North China Craton, where water was previously interpreted as derived from subducting slab. Results reveal extremely low δD values in pristine Early Cretaceous granitic (−203‰ to −127‰) and basaltic (−197‰ to −107‰) apatites, contrasting with relatively high δD values (−137‰ to −47‰) in Jurassic granites. Given the depth-dependent D/H isotopic fractionation during slab dehydration and high-water contents in coeval primitive mafic magmas, the Early Cretaceous magma water is attributed to the stagnant slab within the mantle transition zone. Secular change in the depth of water aligns with steepening of subducting Paleo-Pacific plate from Jurassic to Early Cretaceous, demonstrating the potential of apatite H isotopes in tracing water origin in granites and basalts.

Causal sources of the significant high-amplitude magnetic anomaly zone in the northern south China Sea continental margin

The magnetic anomaly map shows an obvious NE-SW trending high-amplitude magnetic anomaly zone (NSCSMA) in the northern South China Sea (SCS) continental margin that extends from southwest Taiwan to the area at approximately 114°E and 20°N. The origin of this high-amplitude magnetic anomaly zone is important for understanding the magmatism, margin rifting, and tectonic evolution of the SCS. To gain insight into this issue, in this study, wavelet spectrum analysis, 2D magnetic modeling, and compact inversion were used to identify the causal source of the NSCSMA. Combining wavelet spectrum analysis with magnetic modeling results, we find that the depths of the major magnetic sources are not consistent along the strike of the NSCSMA (∼15 km in the east and ∼25 km in the west). The compact inversion results along five profiles across the NSCSMA also revealed two kinds of geometries (quasi-dike in the east and dome-like in the west) of the causal sources of the NSCSMA with different depths. Based on our findings and previous paleo-subduction model, we propose that the major source of the NSCSMA is serpentinized upper mantle materials associated with paleo-Pacific plate subduction.

Remnants and fragments of the subducted paleo-Pacific plate: Constraints from geochemistry and geophysics

Remnants and fragments of the subducted paleo-Pacific plate: Constraints from geochemistry and geophysics

Seismic Reflection Imaging Reveals Relict Subduction Zone of the Paleo-Pacific Plate in the Northeastern South China Sea

The South China Sea (SCS) is widely accepted as an active margin that is associated with the subduction of the Paleo-Pacific plate during the Mesozoic. However, the exact location of the subduction or suture zone remains unclear. Understanding the location of the subduction zone is crucial for comprehending the Mesozoic tectonic evolution of the South China Block and the Cenozoic rifting process of the SCS. To clarify the position of the subduction zone and the influences of preexisting structures, we used a multichannel seismic reflection profile to investigate the crustal architecture. The seismic profile reveals a crustal “crocodile” structure that is interpreted as relict subduction in the Chaoshan Depression and a set of south-dipping crust-mantle reflectors related to the initial rifting in the continent–ocean transition (COT) zone. The results indicate that a Mesozoic subduction zone is located at the northeastern margin of the SCS and that preexisting structures (subduction-related structures) facilitated the rifting process. Combined with previous studies on the oceanic plateau collision-accretionary zone of the northern SCS and the Mesozoic accretionary zone in Palawan of the southern SCS, we infer that a section of the suture zone of the Paleo-Pacific plate subduction is preserved at the northeastern SCS margin and that the rifting of the SCS may have initiated at the suture zone.

Jurassic tectonic regime transition of the southwestern Yangtze Block, SW China: evidence from sedimentary provenance analysis

The Lufeng Basin (LFB) in central Yunnan is located on the southwestern margin of the Yangtze Block in the eastern segment of the Tethys tectonic domain, where the Paleo-Tethys and the Paleo-Pacific tectonic domains converge. Jurassic red beds in this basin record the closure of the Paleo-Tethys Ocean and key information of the tectonic transition driven by the spreading of the Paleo-Pacific plate. We present sedimentary provenance analysis on these Jurassic red beds based on heavy mineral assemblages and detrital zircon geochronology. The results indicate that the strata in this basin were deposited after 214 Ma, and the Early Jurassic Lufeng Formation (Lufeng Fm.) mainly sourced from the west of the LFB with minor contributions from the western Yunnan. In contrast, the sedimentary provenance of the Middle Jurassic Chuanjie Formation (Chuanjie Fm.) and Laoluocun Formation (Laoluocun Fm.) shifted to the proximal ancient basement surrounding the basin and the east of the LFB. The regional unconformity between the Early and the Middle Jurassic may have resulted from the subduction of the Paleo-Pacific plate, jointly influenced by two tectonic domains. The sedimentary provenance shift in the Middle Jurassic may be related to the local uplift of the southwestern Yangtze Block caused by the subduction of the Paleo-Pacific plate. The tectonic regime transition associated with this highest peak of local uplift occurred at ∼170 Ma.

Co-existence of magmatic- and metamorphic-hydrothermal gold deposits in the northern margin of the North China Craton and their genetic link to Paleo-Pacific plate subduction: Insights from in-situ analyses of sulfur isotopes and trace elements

Mid-Mesozoic gold deposits, whose metallogeny was coincident with subduction of the Paleo-Pacific plate, are widely distributed in the northern margin of the North China Craton (NCC). The genesis of these deposits is currently debated with magmatic- and metamorphic-hydrothermal models. In-situ analyses of sulfur isotopes and trace elements provide an opportunity to clarify such a debate. Here, the two techniques were used to determine the trace elements and sulfur isotopes of sulfides from three mid-Mesozoic gold deposits (Jiapigou, Jinchangyu, and Jinchanggouliang) in the northern margin of NCC. The Co/Ni ratios of pyrites suggest that Jiapigou and Jinchanggouliang deposits are of magmatic-hydrothermal origin, whereas the Jinchangyu deposits receive ore-forming materials mainly from crustal-hydrothermal fluids. In addition, gold-bearing sulfides display positive (5.60 ± 1.87 ‰, SD), near-zero (−0.31 ± 1.27 ‰, SD) and negative δ34SV-CDT values (−3.84 ± 2.97 ‰, SD) in the Jiapigou, Jinchangyu, and Jinchanggouliang, respectively. The results suggest that mid-Mesozoic gold deposits in the northern margin of NCC have variable sources of sulfur and metals. The Jiapigou deposit, representative of the arc settings, received sulfur and metals mainly from the subducting ocean crust. The Jinchanggouliang deposit, representative of cratonic destruction settings, received sulfur and metals mainly from the metasomastized mantle. The Jinchangyu deposit, representative of the back-arc settings, received sulfur and metals mainly from the metamorphic basement. We infer that subduction of the Paleo-Pacific plate in the mid-Mesozoic not only led to direct metallogeny of magmatic-hydrothermal gold deposits at arc settings and cratonic destruction settings, but also triggered the release of ore-forming fluids from the basement at back-arc, favoring metallogeny of crustal-hydrothermal gold deposits. Thus, mid-Mesozoic gold deposits in the northern margin of NCC cannot be exclusively explained by a single model.

Cenozoic deformation of the Weihe Graben in central China: Insights from Analogue modeling

The Weihe Graben is located in the composite area of the Tethys-Himalayan tectonic and circum-pacific tectonic domains and is an essential structural feature related to Cenozoic intracontinental deformation. However, two different hypotheses, sinistral strike-slip, and extensional mechanism, are proposed for the origin of this graben. In this study, we designed crust-scale analog models based on fault kinematics to better understand the deformation and dynamic mechanisms of the Weihe Graben. Fault kinematics shows early sinistral strike-slip and NW-SE extensional deformation and late NE-SW, S-N, and NW-SE extensional deformation. Our experimental results reveal that Model C1 (sinistral strike-slip + NW-SE extension) is the most similar to the geological structure of the Weihe Graben in the Paleogene. Furthermore, the NE-SW extension results in two larger subsidence areas in the northeast and southwest of the basin, similar to the formation of the two depressions during the Miocene. The S-N extension causes the basin to expand southward and northward, consistent with the sedimentary characteristics of Weihe Graben in the Pliocene. The NW-SE extension leads to relatively strong fault activity in the northwest and southeast of the basin, resembling the active fault characteristics since the Quaternary. Therefore, we propose that the Weihe Graben formed by a strike-slip and extensional combined mechanism in the Eocene-Oligocene, influenced by the far-field effects of the Indian-Asian collision and crust-mantle processes triggered by the stagnant paleo-Pacific plate. Subsequently, during the Neogene-Quaternary, the graben’s development shifted towards an extensional regime under the influence of the eastward extrusion of the Tibetan Plateau.

Molybdenum isotopic evidence for the initiation of a big mantle wedge beneath eastern Asia

The subduction of the Paleo-Pacific plate and the formation of a big mantle wedge have influenced the tectonic-magmatic evolution of eastern Asia. However, the timing and mechanism of big mantle wedge formation remain obscure. Here we report molybdenum (Mo) isotope compositions of Mesozoic-Cenozoic mafic rocks in eastern China, which provide constraints on the geodynamics of Paleo-Pacific subduction. The >125 Ma rocks have high δ98Mo values of −0.27‰ to 0.17‰ and arc-like features, whereas the <125 Ma rocks have low δ98Mo values of −0.67‰ to −0.04‰ and ocean-island basalt (OIB)-like features. The mantle sources of these two types of rocks contain subducted Paleo-Pacific slab components at sub-arc and mantle transition zone depths, respectively. Therefore, the Paleo-Pacific subduction involves shallow subduction yielding a small mantle wedge in the early stage and deep subduction yielding a big mantle wedge in the late stage. The dramatic change in Mo isotopes reveals that the big mantle wedge beneath eastern Asia was initiated at ∼125 Ma.

Zircon U-Pb Dating, Geochemistry, Lu-Hf Isotope Characteristics, and Geological Significance of Volcanic Rocks in Zhenghe Fozi Mountain National Geopark, Fujian, China

Fozi Mountain National Geopark is located in Zhenghe County in the northern region of Fujian Province, where the volcanic rocks of the Zhaixia Formation of the Shimaoshan Group are exposed. Zircon U-Pb dating and geochemical analysis were carried out to constrain its age and tectonic environment. The results show that three zircon U-Pb dating samples have attained ages of 99.2 ± 1.0 Ma, 99.6 ± 0.8 Ma, and 99.7 ± 2.0 Ma. Volcanic rocks in the core scenic area of Fozi Mountain were formed during the Late Cretaceous period. Elemental analysis showed that these volcanic rocks were dominated by the shoshonite series. They include gray dacite porphyry, grayish-white breccia tuff, volcanic agglomerate, and gray tuffaceous sandstone. These rocks were characterized by high silicon, high alkali content, and rich potassium levels. Lu-Hf isotope analysis of zircons revealed that their εHf(t) values varied from −8.7 to −6.8. The corresponding TDM2 values were primarily distributed in the range of 1.71 Ga to 1.59 Ga. These findings indicated that the magma primarily originated from the partial melting of the Mesoproterozoic crystalline basement, accompanied by a small number of mantle-derived materials. Tectonic environment analysis indicated that these rocks were formed in the post-orogenic intraplate extensional environment, which was associated with the back-arc extension or lithospheric thinning caused by the subduction of the paleo-Pacific plate beneath the Eurasian plate. The formation of these volcanic rocks was attributed to post-orogenic magmatism.

Early Cretaceous fayalite-, ferrosilite-, and biotite-bearing rhyolitic porphyries in the Baishuizhai area, South China: Formation by fractional crystallization in the shallow crust

Fayalite- and ferrosilite-bearing felsic igneous rocks are a distinctive category of A-type granitic magmas, and the origins of these rocks have proved controversial. In addition, the shallow-crust high-silica rhyolitic magmas responsible for such rocks are considered to have higher viscosities than their plutonic equivalents, which casts doubt on whether fractional crystallization could have occurred during the evolution of these high-silica melts. Here, we report a study of earliest Early Cretaceous (145–142 Ma) fayalite- and ferrosilite-bearing rhyolitic porphyries (FFBRPs) and closely associated biotite-bearing rhyolitic porphyries (BBRPs) from the Baishuizhai area, Guangzhou, Guangdong Province, South China. The Baishuizhai suite provides an excellent opportunity to establish the magmatic origins and evolutionary processes responsible for the FFBRPs. Rocks from the Baishuizhai suite have SiO2 contents of 73.2–77.7 wt%, Na2O + K2O contents of 7.83–9.42 wt%, and Fe2O3T contents of 1.52–2.97 wt%. They have A-type granite features including mineral assemblages (euhedral fayalite and ferrosilite in addition to anhedral amphibole and biotite) and geochemical characteristics [e.g., high 10,000 × Ga/Al values (3.7–4.7), high FeOT/MgO ratios (13.0–34.8), and high Zr (245–514 ppm) and Nb (56.4–96.5 ppm) contents]. The rocks have slightly enriched in situ plagioclase Sr [(87Sr/86Sr)i = 0.7070–0.7076] and whole-rock Nd [εNd(t) = −2.3 to −2.0] compositions and depleted zircon Hf isotopic compositions [εHf(t) = −0.9 to +7.0] relative to chondrite, as well as higher zircon δ18O values (6.3‰–7.8‰) and intermediate in situ plagioclase Pb isotopic compositions [(206Pb/204Pb)i = 18.368–18.727, (207Pb/204Pb)i = 15.488–15.673, and (208Pb/204Pb)i = 38.049–38.801] compared with depleted mantle. We infer that the primary magmas of this rock suite were formed by mixing of mantle-derived magmas with subordinate (20%–50%) metasedimentary-rock-derived magmas. Fayalite and ferrosilite in the FFBRPs were most likely crystallized from dry, hot, and reduced magma in a lower (<16.5 km depth) chamber. The lower magma gradually evolved during ascent to a water-enriched magma in an intermediate chamber at a shallow depth (3–4 km), forming the FFBRPs (elevation: 129–168 m). Finally, the residual melts migrated upward and resided in an upper (<3 km depth) chamber, forming the BBRPs (elevation: 196–824 m). According to the established temporal–spatial distribution of Early Cretaceous A-type granites and adakitic rocks in South China, we suggest that the Paleo-Pacific plate underwent diachronous rollback, starting during the earliest Early Cretaceous. This rollback enhanced the degree of crust–mantle interaction in the Guangzhou area.

Two episodes of mineralization in the Baolun high-grade lode gold deposit, Hainan Island, South China: Insight from LA-ICP-MS analysis of hydrothermal rutile

The Baolun is the largest gold deposit on Hainan Island, South China, with proven reserves of over 80 t Au at an average grade of 10.3 g/t. Two economic types of gold mineralization coexist in the long-lived fault system: 1) Au-bearing sulfide quartz veins (Va), dominated by pyrite, pyrrhotite, and arsenopyrite; and 2) Au-Bi assemblage quartz veins (Vb), dominated by bismuthinite, maldonite, and native bismuth. The Baolun district has a long history of tectonic reactivation, involving crustal compression and extension, magmatic activity, multiple episodes of fluid flow, and hydrothermal mineralization. Consequently, there exists no generally accepted model for the precise timing and source of the Baolun gold deposits. In this study, two different rutile grains (Rt-1 and Rt-2) were recognized separately in the Va veins and Vb veins based on paragenetic sequence. In situ U-Pb dating indicates that Rt-1 and Rt-2 yielded two group ages of 248.3 ± 2.8 Ma (MSWD = 0.8) and 78.2 ± 8.4 Ma (MSWD = 1.4), respectively. The Rt-1 contains 632.0–6051.2 ppm W, 459.1–2169.1 ppm Zr, 1215.0–8267.5 ppm Nb, 107.3–1229.6 ppm Ta, 19.0–79.2 ppm Hf, 160.2–311.8 ppm Sn, 1371.6–3500.7 ppm V, 325.3–3813.9 ppm Cr, 0.6–22.7 ppm Sb, 0.1–38.8 ppm Y, with Ca below the detection limit. The Rt-2 contains 704.9–4390.2 ppm W, 1.8–168.5 ppm Zr, 1264.6–4062.9 ppm Nb, 108.1–453.2 ppm Ta, 0.3–6.9 ppm Hf, 6.5–32.1 ppm Sn, 14.3–311.0 ppm V, 29.4–167.1 ppm Cr, 0.6–27.3 ppm Sb, 56.3–38.8 ppm Y, and 548.7–2750.3 ppm Ca. Obviously, LA-ICP-MS trace element analysis revealed significant discrepancies in composition between Rt-1 and Rt-2, suggesting that they precipitated from distinct fluid systems. Combining mineral studies, field observations, structural analysis, and geologic evolution, the results of this study indicate that the Baolun gold deposit formed during two episodes of mineralization. The Early Triassic gold mineralization is clearly classified as an orogenic type. The ore-forming materials were liberated by the deformation and metamorphism during the Indosinian orogeny of South China, which was triggered by the closure of the Paleotethys Ocean. The Late Cretaceous Au-Bi mineralization belongs to a magmatic-hydrothermal origin, which was deposited under an extension setting associated with the post-subduction of the paleo-Pacific plate. The new mineralization model proposed provides significant implications for future exploration strategies.

Contrasting Tectonomagmatic Conditions for Coexisting Iron Oxide-Apatite Deposits and Porphyry and Skarn Cu ± Au Deposits in the Middle-Lower Yangtze River Metallogenic Belt, China

Abstract Porphyry Cu ± Mo ± Au and iron oxide-apatite (IOA) deposits rarely occur in spatial and temporal proximity in Phanerozoic arc-related settings, and the formation of these mineral deposit types in an evolving arc setting remains poorly understood. Specifically, the roles of magma composition and the tectonic regime remain the subject of some debate. Here, we systematically estimated the P-T-fO2 conditions and H2O-S-Cl contents for dioritic to granodioritic source magmas for porphyry and skarn Cu ± Au (150–135 Ma) and IOA deposits (~130 Ma) that formed in transpressional and transtensional settings in the Middle-Lower Yangtze River metallogenic belt, China. Our estimates show that, compared to IOA deposits, the porphyry- and skarn-related magmas were relatively felsic, cooler, and more hydrous. These geochemical features are consistent with the tectonic transition from subduction to slab rollback of the paleo-Pacific plate in the East Asia continental margin at &amp;lt;135 Ma and concomitant crustal extension and steepening of the regional geothermal gradient. Apatite data reveal that the silicate melts associated with the porphyry and skarn Cu ± Au and IOA deposits had comparable predegassed S concentrations (~0.13 ± 0.06 wt % vs. ~0.16 ± 0.09 wt % on average), but that IOA-related melts contained higher predegassed Cl/H2O ratios (~0.11 ± 0.03 vs. ~0.04 ± 0.03 for porphyry- and skarn-related magmas) that decreased by one order of magnitude after magmatic degassing. Magmatic fO2 estimated using zircon and amphibole, reported in log units relative to the fayalite-magnetite-quartz (FMQ) redox buffer, gradually increased during cooling of the porphyry- and skarn-related magmas (ΔFMQ +0.7 to +2.5) at 950° to 800°C and decreased to ΔFMQ +1 at 700°C owing to fractionation of Fe2+-rich minerals and subsequent S degassing, respectively. In contrast, the magmatic fO2 values for the IOA-related source magmas varied significantly from ΔFMQ –1.5 to ΔFMQ +2.5 but generally show an increasing trend with cooling from 970° to 700°C that probably resulted from variable degrees of evaporite assimilation, fractionation of Fe2+-rich minerals, and Cl degassing. These results are consistent with the hypothesis that Cl enrichment of the IOA-related source magmas played a determinant role in their formation. We propose that the porphyry and skarn Cu ± Au deposits in the Middle-Lower Yangtze River metallogenic belt formed in a transpressional setting in response to paleo-Pacific flat-slab subduction that favored storage and evolution of S-rich hydrous ore-forming magmas at variable crustal levels. A subsequent extensional setting formed due to slab rollback, leading to rapid degassing of Cl-rich IOA-related magmas. For the latter scenario, assimilation of evaporite by mafic to intermediate magmas would lead to an enrichment of Cl in the predegassed magmas and subsequent exsolution of hypersaline magmatic-hydrothermal fluid enriched in Fe as FeCl2. This Fe-rich ore fluid efficiently transported Fe to the apical parts of the magma bodies and overlying extensional normal faults where IOA mineralization was localized. The concomitant loss of S, H2O, and Cu with Cl by volcanic outgassing may have inhibited sulfide mineralization at lower temperatures.

Mesozoic intra-arc basin records the tectonic transition from the Paleo-Asian Ocean to the Paleo-Pacific Ocean in northeastern Eurasia

Abstract Since the Paleozoic, the tectonic evolution of northeastern Eurasia has been dominated by the Paleo-Asian Ocean and the Paleo-Pacific Ocean tectonic domains. However, the spatiotemporal framework and the timing of tectonic transition between these two oceanic domains remain enigmatic. To address this issue, we report petrological, geochronological, and geochemical data for eight sandstone samples deposited along the convergent margin between the Northeast China terranes and the North China craton in central Jilin Province, China. The results show that these sandstones are immature graywackes with a maximum depositional age of Early Triassic (248 ± 1 Ma), and their sediments were largely derived from coeval magmatic rocks in a juvenile continental arc. According to our new results and previous studies, we identified a sedimentary basin (most likely an intra-arc or forearc basin) intimately associated with one or more continental arcs along the northeastern edge of the North China craton, and we suggest that the southwestward subduction of the Jilin-Heilongjiang Ocean in the early Mesozoic accounts for this continental arc setting. There is a distinct temporal gap between the closure of the Paleo-Asian Ocean (ca. 260 Ma) and the onset of Paleo-Pacific plate subduction (234–220 Ma), which is essentially coeval with the southwestward subduction of the Jilin-Heilongjiang Ocean between 256 Ma and 239 Ma, meaning the latter is a key link that marks the transition between these two tectonic domains.

Multiscale structures of crust-mantle beneath the South China block and their geodynamic implication

The South China block is an ideal location in which to study the impact of paleo-Pacific plate subduction on the upper mantle and crustal structures of the overriding plate, as well as the mechanism of lithospheric thinning. By integrating data from permanent seismic stations of the China Seismic Network and a dense seismic array at Xuefeng Mountain, this study employed P- and S-wave receiver functions (RFs) to image multiscale structures of crust-mantle across the South China block, thereby providing constraints for a comprehensive understanding of its tectonic evolution. The main findings are as follows: (1) The thickness of lithosphere and crust gradually decreases from northwest (NW) to southeast (SE). (2) Beneath the Xuefeng Mountain uplift zone, the P-to-S conversion from the 660 km discontinuity arrives earlier than that predicted from a one-dimensional model, other weak discontinuities can be traced in the upper mantle, including the lithosphere-asthenosphere boundary, the crust becomes thinner, and the Conrad discontinuity becomes deeper. Referring to previous results from tomography and geological reconstructions, we deduce that lithospheric delamination might have occurred beneath the Xuefeng Mountain uplift zone, and the delaminated lithosphere penetrated the upper mantle and reached the mantle transition zone.