Southwestern South China Block Research Articles

Early Paleozoic crustal anatexis during Wuyi-Yunkai orogenesis: Insights from zircon of Fuhuling migmatites in the Yunkai region, South China

Abstract Crustal anatexis is an important process in the tectonic evolution of many orogenic systems. In the Wuyi-Yunkai orogen in the South China block, the duration of partial melting and its relationship with orogenesis are poorly constrained. In this study, we present a multifaceted approach to determine the timing of anatexis and unravel the petrogenesis of Fuhuling migmatites in the Yunkai region of the southwestern South China block. Geochemical analyses indicate that the migmatites have (meta-)sedimentary protoliths. The absence of anhydrous peritectic minerals but the presence of microstructural and outcrop-scale indicators of partial melting suggest that the Fuhuling migmatites experienced fluid-present melting. Complex zoning and variable trace element concentrations in newly formed zircons in migmatites reflect their evolutionary histories during partial melting. Anatectic melt was present at Fuhuling in the Yunkai region from ca. 449–427 Ma during early Paleozoic Wuyi-Yunkai orogenesis. The wide variety of morphologies observed in the Fuhuling migmatites implies that migmatites in the Yunkai region experienced incipient partial melting, melt segregation, and melt migration. Combining new and previous results, we argue that the Yunkai region experienced two stages of crustal anatexis during the early Paleozoic, which may have been triggered by crustal thickening followed by rapid exhumation and orogenic collapse during the intra-plate Wuyi-Yunkai orogeny in the South China block.

Polyphase deformation of the Youjiang fold-and-thrust belt during the Mesozoic: Implications for the tectonic transition of the South China block

The South China block (SCB) experienced the tectonic transition from the Paleo-Tethys to the Paleo-Pacific tectonic domains during the Mesozoic, but the transition process is hotly debated. The Youjiang fold-and-thrust belt (YFTB), in the interior of the Youjiang Basin in the southwestern SCB, is located in a junction of these two tectonic domains and thus witnessed their tectonic evolution. It also separates the northern thin-skinned structures from the southern thick-skinned structures in the basin. Therefore, the YFTB is an intriguing window into the Mesozoic evolution in the southwestern SCB and the Mesozoic tectonic transition of the SCB. In this study, we conduct structural analysis at the middle of the YFTB and discuss the Mesozoic tectonic transition of the SCB. Four phases of deformation are identified in the YFTB during the Mesozoic. The first phase of deformation (D1) is characterized by a series of conjugate joints, NW–SE trending thrust faults and folds resulting from NE–SW shortening that was related to the collision between the SCB and Indochina block. The second phase of deformation (D2) is manifested by the pre-existing NW–SE striking thrust faults transformed to normal faults and the Late Triassic mafic magmatism. Our structural observations, combined with previous geochronological data for mafic dykes, suggest the study area experienced post-orogenic extension in the Late Triassic. The third phase of deformation (D3), accompanied with the westward subduction of the Paleo-Pacific oceanic plate, is represented by a series of conjugate joints and NE–SW-striking faults formed by NW–SE compression. As a result of the continuous subduction of the Paleo-Pacific plate and its subsequent slab rollback, the fourth phase of deformation (D4) is featured with normal faults, magmatic-hydrothermal activities, and regional mineralization, which are associated with the nearly E–W extension setting. Our study results indicate that, in the Early Mesozoic, the Youjiang Basin was dominated by the Paleo-Tethys domain and then transitioned to the Paleo-Pacific domain. Together with the magmatic lull between the Triassic and the Early-Middle Jurassic identified in the SCB, we propose that the tectonic transition process manifested in the YFTB more likely initiated in the Early Jurassic.

Neoproterozoic lithospheric extension related to break-up of the Rodinia supercontinent: Constraints from a newly-identified granite porphyry in southeastern Yunnan, China

Although the southeastern Yunnan region preserves an important record of the evolution of the South China block (SCB), its Precambrian geology is not well understood due to poor exposure. The newly-discovered Laojunshan granite porphyry (LNGP) is the only Precambrian unmetamorphosed granitoid unit exposed in the southwestern SCB. Here, we present integrated studies of the LNGP including whole-rock major and trace-element geochemistry and Nd isotopes, together with zircon U-Pb chronology and Lu-Hf isotopes by LA-ICP-MS. Zircon U-Pb dating results for two granite porphyry dykes yielded ages of 811.2 ± 7.0 Ma and 830.3 ± 5.2 Ma. The studied samples have high total alkalis (6.46–9.68 wt%), SiO2 (69.44–75.6 wt%), P2O5 (0.11–0.33 wt%) and Al2O3 (10.44–14.39 wt%) but low CaO (0.09–1.29 wt%) contents; these characteristics together with high Al saturation indices (1.1–1.7), indicate the LNGP is of strongly peraluminous S-type granite affinity. The LNGP is characterized by Rb-Th-U-K enrichments and Ba-Sr-Nb-Ti-Ta depletions, and right-sloping REE patterns with obvious negative Eu anomalies (Eu/Eu* = 0.2–0.4). Major and trace elements show good correlations with SiO2. Diagrams of Th vs Th/Nd, La vs La/Sm and Zr vs Zr/Sm indicate that fractional crystallization was an important process in the genesis of the LNGP. The Ba vs Eu, Sr vs Eu and Ba vs Sr correlations and high Rb/Sr (8.2–74) and Ca/Sr (12–614) ratios demonstrate that magmatic evolution involved extensive fractionation of plagioclase and K-feldspar. The initial zircon Hf isotopic composition of the porphyry varies widely (εHf(t) = −10.1 to +6.7), with corresponding two-stage Hf model ages of 1291–2331 Ma; whereas whole-rock Nd isotopes are relatively uniform (εNd(t) = −5.8 to −4.6), with two-stage Nd model ages of 1886–2130 Ma. The variable zircon Hf isotopic compositions suggest a heterogenous source for the LNGP, containing juvenile components (with higher εHf(t) values) and ancient reworked crustal materials (with lower εHf(t) values). Geochemical data, including Rb/Sr, Rb/Ba, Al2O3/(MgO + Fe2O3T), CaO/(MgO + Fe2O3T) and (Na2O + K2O)/(Fe2O3T + MgO + TiO2) ratios and (Na2O + K2O + Fe2O3T + MgO + TiO2) content suggests a meta-pelite/greywacke source. Early arc-continent or continent–continent collision may produce mixtures of juvenile sediments formed by erosion of with ancient crustal materials that were strongly folded and deeply buried in the studied area. The LNGP has medium to high magmatic temperatures (761–875 °C) and low oxygen fugacity (ΔFMQ −5.4 to ΔFMQ −2.8). The LNGP was emplaced at middle-upper crustal levels (pressure < 5kbar) in an extensional setting analogous to granitoid associated with the Neoproterozoic age rift-related Nanhua and Kangdian magmatic belts that bound the periphery of the Yangtze block. These belts are interpreted to be the result of the break-up of the Rodinia supercontinent during the period 850 to 700 Ma. We similarly interpret the emplacement of the LNGP, which occurred within an extensional setting, to be a response to the break-up of the Rodinia supercontinent.

Provenance shift during Early‐Middle Triassic and its response to the palaeogeographic and tectonic evolution of the southwestern South China Block

To constrain the relationship between the plate boundary dynamics and the basin sedimentation in the southwestern South China Block (SCB), we undertook detrital zircon U–Pb–Hf isotope compositions of the Lower‐Middle Triassic turbidite system in the Youjiang Basin and biotite 40Ar/39Ar thermochronology of the adjacent Lingshan granite. The detrital zircons from Lower Triassic clastic rocks are dominated by age peaks at ~450 Ma and ~1,000 Ma and minor peaks at ~250–260 Ma with negative εHf(t) values (−6.4 to −10.1). In contrast, the Middle Triassic group contains more volcanic fragments and displays a different age peak at ~240–287 Ma with εHf(t) between −11.9 and 13.2 in addition to the former two peaks. These results suggest a potential provenance associated with juvenile and volcanic components in the adjacent region during the Middle Triassic. A summary of the Triassic tectonothermal history suggests that the eastern Youjiang Basin and Yunkai Terrane were uplifted throughout the Triassic, while the northeastern Indochina Block on the west of the SCB was uplifted mainly during the Middle Triassic. Combined with the palaeocurrent reconstruction, we infer that the turbidite system was filled primarily from east to west during the Early Triassic, mainly controlled by the Palaeo‐Pacific plate subduction. Subsequently, the Middle Triassic turbidite system involved juvenile and volcanic components from the eastern SCB and northeastern Indochina Block in addition to the Early Triassic provenance, as a result of dual impact from the Palaeo‐Pacific plate subduction and Indochina Block collision.

3D Quantitative Metallogenic Prediction of Indium-Rich Ore Bodies in the Dulong Sn-Zn Polymetallic Deposit, Yunnan Province, SW China

The southwestern South China Block is one of the most important Sn polymetallic ore districts in the world, of which the Dulong Sn-Zn polymetallic deposit, closely related to Late Cretaceous granitic magmatism, contains 0.4 Mt Sn, 5.0 Mt Zn, 0.2 Mt Pb, and 7 Kt In, and is one of the largest Sn-Zn polymetallic deposits in this region. In this paper, on the basis of a 3D model of ore bodies established by the cut-off grade of the main ore-forming elements, the In grades were estimated by the ordinary Kriging method and the In-rich cells were extracted. The 3D models of strata, faults, granites, and granite porphyries in the mining area were established and assigned the attributes to the cells, which built buffer zones representing the influence space of the geological factors. The weight of evidence and artificial neural network methods were used to quantitatively evaluate the contribution of each geological factor to mineralization. The results show that the Neoproterozoic Xinzhai Formation (Pt3x), fault (F1), and Silurian granites (S3L) have considerable control effects on the occurrence of In-rich ore bodies. The metallogenic predictions according to the spatial coupling relationship of each geological factor in 3D space were carried out, and then the 3D-space-prospecting target areas of In-rich ore bodies were delineated. In addition, the early geological maps and data information of the mining area were comprehensively integrated in 3D space. The feasibility of 3D quantitative metallogenic prediction based on the deposit model was explored by comparing the two methods, and then, the 3D-space prospecting target area was delineated. The ROC curve evaluation shows that the results of two methods have indicative value for prospecting. The modeling results may support its use for future deep prospecting and exploitation of the Dulong and other similar deposits.

Crust and upper mantle structure beneath Southwest China and its implications for Mesozoic multistage gold deposits

The Youjiang Basin (YB) on the southwestern margin of the Yangtze Craton (YC) contains >200 Triassic or Cretaceous gold deposits, but deep dynamic processes contributing to large-scale gold accumulation remain unknown. Based on a south–north trending linear broadband seismic array, we obtained the crust and mantle structure beneath the YB and southwestern YC by P-wave and S-wave receiver functions and ambient noise tomography. Our results show a remarkable lithospheric boundary beneath the northern margin of the YB, which separates the YB with felsic thin crust and thin mantle lithosphere from the YC with both a thick crust and thick mantle lithosphere. A new structural image shows that the southern end of the YC Moho dips southward and overlaps with the YB Moho. A 5-km-thick high-velocity layer is also found at a depth of 10 km beneath the northern YB. Combining this information with previous tectonic and geological results, we reveal the process of Mesozoic lithospheric evolution in the southwestern South China Block. We infer that both the pre-Mesozoic weak crust and mantle lithosphere and the intense intracontinental orogenesis in the Early-Middle Triassic contributed to the formation of first-stage Au deposits in the YB. Lithospheric delamination in the Early Cretaceous thinned the lithosphere of YB rather than YC, forming a steep lithosphere–asthenosphere boundary beneath the transition zone between YB and YC, which favored upwelling of Au-rich asthenosphere below the thick cratonic root to the bottom of the crust in the northern YB and contributed to the second-stage Au deposits in the YB.

Petrogenesis of the Jurassic Guiping Complex in the Southwestern South China Block: Insights into the Subduction Processes of the Paleo-Pacific Slab

Late Jurassic NE-trending A-type granitoids are widespread in the Shihang belt, South China, though their petrogenesis and geodynamic settings remain controversial. The Guiping complex is located on the southwest margin of the Shihang belt. In this study, the petrography, major and trace element geochemistry, whole-rock Sr-Nd isotopes, and zircon U-Pb geochronology of the Guiping complex were investigated. The Guiping complex is composed of the Fenghuangling and Xishan plutons; both plutons yielded zircon U-Pb ages of ca. 160 Ma. The Fenghuangling pluton has low SiO2 content of 54.26% to 60.31%, whereas the Xishan pluton exhibits high SiO2 content of 65.19% to 71.18%. Both of them are metaluminous and belong to the high-K calc-alkaline series and are enriched in large-ion lithophile elements (LILEs) such as Rb, Th, U, and Pb. The Fenghuangling and Xishan plutons showed enrichment in light rare earth elements (LREEs) and high-field strength elements (Nb, Ta, Zr, and Hf) and depletion in heavy rare earth elements (HREEs). Marked Nb and Ta negative anomalies were not observed. Due to the high contents of Zr + Ce + Nb + Y and high Ga/Al ratios, all the samples belonged to the group of A-type granites. The Fenghuangling and Xishan plutons had low ISr (mainly in the range of 0.7046–0.7058) and high εNd(t) (−0.60 to 1.94) values, though obviously different from those of the Precambrian basement in South China. Furthermore, they lie between the ocean island basalt (OIB) of the asthenosphere and the arc basaltic rocks of the enriched lithospheric mantle. Therefore, we proposed that the basaltic parental magma of the Guiping complex originated from partial melting of the enriched lithospheric mantle, which was metasomatized by asthenosphere-related OIB-type basaltic magma. Mafic microgranular enclaves in the Xishan pluton displayed positive Nb and Ta anomalies, which is consistent with OIB-type basalts. The enclaves also had similar Sr-Nd isotopic compositions to the Xishan pluton. That indicated that the enclaves were probably formed by mixing of the OIB-type basaltic magma and the Xishan pluton. In conclusion, the formation of the Late Jurassic NE-trending A-type granite belt was attributed to back-arc extension as a result of the rollback of the Paleo-Pacific Plate.

The Cretaceous igneous rocks in southeastern Guangxi and their implication for tectonic environment in southwestern South China Block

Abstract Southeastern Guangxi is located in the southwestern South China Block and to the northwest of the South China Sea (SCS), with abundant records of the Cretaceous magmatism. A detailed study of igneous rocks will contribute to a better understanding of the late Mesozoic tectonic environment. Zircon U–Pb dating yields ages of 93.37 ± 0.43 Ma for Yulin andesites and 107.6 ± 1.2 Ma for Luchuan granites. Yulin andesites are hornblende andesites, of which w(MgO) is between 7.72% and 8.42%, and Mg# is between 66.7 and 68.0, belonging to high magnesian andesites (HMAs) from peridotite sources. Luchuan granites are medium- to fine-grained monzogranites. Monzogranites and clastoporphyritic lava are high-K calc-alkaline series and metaluminum to weakly peraluminous series, which belong to the I-type granites. Those are enriched in Rb, Th, K and LREEs and depleted in Nb, Ta, P and Eu, showing the geochemical characteristics related to subduction. Unlike the contemporary “bimodal igneous rock assemblages” in Zhejiang and Fujian, the intermediate-acid magmatites in the southeastern Guangxi imply the compressive tectonic environment. The assemblage of HMAs and adakitic rocks indicates that the southwestern South China Block was under the Neo-Tethyan subduction during Cretaceous, and slab melting contributed to the magma in this area.

Cretaceous exhumation history of the southwestern South China Block: Constraints from fission‐track thermochronology

To detect the exhumation history of the southwestern South China Block (SCB), we performed fission track (FT) analysis on zircon and apatite grains in granitoid and sedimentary samples. From east to west, the study area includes the Yunkai Massif, Lingshan granitic belt, Shiwandashan Basin, and eastern margin of the Youjiang Basin. The granite sample of the Yunkai Massif has two peaks of 75 and 92 Ma for zircon FT dating. The Lingshan granitic belt samples yield zircon FT central ages from 135 to 132 Ma. The Permian bauxite samples of the Youjiang Basin yield zircon FT central ages from 109 to 93 Ma and display age peaks at ~97–93 Ma. The Yunkai Massif granite sample has two peaks at 74 and 104 Ma for apatite FT dating, while the Lingshan granitic belt sample has two peaks at 81 and 111 Ma. The cooling paths of the Yunkai Massif sample started with an Early Cretaceous (~120–100 Ma) cooling event that brought the granitoids into the apatite partial annealing zone (PAZ). Through comparative studies, we suggest that the southwestern SCB underwent three periods of crustal uplift during ~135–132, ~111–92, and ~ 81–74 Ma, respectively, which is consistent with the regional stratigraphic unconformity and crustal extension. Only the Yunkai Massif and Lingshan granitic belt were uplifted during ~135–132 Ma. During ~111–92 Ma, the southwestern SCB experienced crustal uplift, and the basal conglomerate began to deposit in the Shiwandashan Basin. Late Cretaceous uplifted occurred during ~81–74 Ma, and the Yunkai Massif was uplifted throughout the Cretaceous. From 97 to 92 Ma, the Permian bauxite deposits in the Youjiang Basin were uplifted, which provided the basis of the later transformation to Quaternary bauxite.

Insights into post-orogenic extension and opening of the Palaeo-Tethys Ocean recorded by an Early Devonian core complex in South China

The Palaeo-Tethys Ocean formed during the Devonian as a consequence of blocks separating from northern Gondwana. The record of the Palaeo-Tethys Ocean stretches across Eurasia from the European Alps eastward to the southwestern South China block (SCB). Tectonic domes of the SCB record the contraction, extension, decompression, and partial melting of deep crust that occurred as a result of detachment faulting and tectonic evolution of the Palaeo-Tethys Ocean. This study investigates post-orogenic (Kwangsian) extension and opening of the Palaeo-Tethys Ocean as recorded by the Motianling core complex in the southwestern SCB, based on structural analysis and 40Ar/39Ar geochronology. The Motianling core complex in the southernmost part of the Jiangnan fold belt comprises the Sanfang gneissic granite surrounded by low-grade metasedimentary rocks. The core complex records at least three deformation (D) events: pre-D1, D1, and D2. The pre-D1 event produced large-scale NE-striking folds with axial surface cleavages and was coeval with emplacement of the Sanfang batholith. D1 deformation is represented by a top-to-the-NW ductile–brittle normal faulting system with mylonitic foliation and was coincident with greenschist-facies metamorphism, indicating that the core complex formed during NW–SE extension. D2 deformation is dominated by Cenozoic NW–SE-trending normal faults. The timing of D1 deformation and subsequent cooling was constrained by 40Ar/39Ar geochronology. Syn-kinematic muscovite yields an 40Ar/39Ar age of 417 ± 2.3 Ma (1σ) and suggests that the dome was initially developed in the Early Devonian through ductile shear zones. Magmatic biotite from the granitic mylonite yields a weighted mean 40Ar/39Ar age of 363.0 ± 3.1 Ma. Therefore, the Motianling core complex records at least two major stages of tectonic evolution: (1) Kwangsian (Caledonian) orogenic collapse and subsequent first-stage cooling at ∼417 to 363 Ma; and (2) Cenozoic regional extension and final cooling at ca. 45 Ma, as inferred from published apatite fission track data. The present results, combined with previous studies, indicate that the D1 deformation event resulted in the formation and exhumation of the Motianling core complex in a continental rifted margin during the opening of the Palaeo-Tethys Ocean.

Depositional Age, Provenance Characteristics and Tectonic Setting of the Ailaoshan Group in the Southwestern South China Block

The depositional and metamorphic ages and provenances of the Ailaoshan (ALS) Group in the Ailaoshan‐Red River (ALS‐RR) shear zone, southwestern South China Block (SCB), were investigated to constrain the tectonic history of the southwestern SCB. In this study, we use petrology, geochemical analysis, zircon cathodoluminescence imaging and U‐Pb geochronology to analyse samples of quartzite, garnet‐bearing two‐mica schist and metapelite. The age spectra of detrital zircon grains from these metasediments show two dominant age peaks at 550–424 Ma and 876–730 Ma and two subordinate peaks at 970–955 Ma and ∼2450 Ma. The youngest peak, corresponding to the early Palaeozoic, accounts for more than 20% of the total dates and constrains the deposition of the ALS Group to the Palaeozoic rather than the Palaeoproterozoic as traditionally thought. Moreover, two peaks of metamorphic ages corresponding to the Permo‐Triassic and Cenozoic were also identified, and these ages document the tectonothermal events associated with the Indosinian collision between the Indochina Block and the SCB and the Himalayan collision between the Indian and Asian plates. Geochemical data suggest that the provenances of the ALS Group were dominated by continental arc and recycled metasedimentary rocks. The comparison of probability density distribution plots of the detrital zircon U‐Pb age data indicates that the Neoproterozoic detritus in the ALS Group was probably derived from the arc‐related Neoproterozoic intrusive bodies in the northwestern and southwestern SCB. Furthermore, the early Palaeozoic detritus might have been sourced from eroded early Palaeozoic strata and magmatic plutons in Cathaysia and volcanic rocks in the western Indochina Block.

Petrogenesis and metallogenic implications of Late Cretaceous I- and S-type granites in Dachang–Kunlunguan ore belt, southwestern South China Block

The southwestern part of the South China Block is one of the major Sn, W and Cu metallogenic provinces related to Mesozoic granitoid suites. However, the genetic relationship between the magmatic petrogenesis and metallogeny is not well-constrained. Here we evaluate the petrogenesis of Late Cretaceous granites and their link to regional metallogeny from the Dachang–Kunlunguan ore belt in the eastern part of the province. This belt includes biotite granites formed at ca. 98–97 Ma associated with Cu–W deposit in the Kunlunguan area, and the alkali-feldspar granites emplaced at ca. 92 Ma responsible for Dachang Sn mineralization. The Kunlunguan biotite granites have low P2O5 (0.12–0.18 wt%), CIPW normative corundum contents (0.28–1.99 wt%) and ASI values (1.00 to 1.14), consistent with the features of I-type granites. In contrast, the Dachang alkali-feldspar granites are strongly peraluminous (ASI = 1.29–1.44) with high P2O5 (0.25–0.34 wt%) and CIPW normative corundum contents (3.75–4.91 wt%), comparable to S-type granites. Zircon grains from the Kunlunguan and Dachang granites show different εHf(t) values ranging from −11.0 to −1.2 and from −6.5 to −2.1, respectively. Based on the geochemical and Hf isotope features, the parent magmas of the Kunlunguan granites are inferred to have been dominantly derived from mixed melts of Paleo- to Mesoproterozoic metaigneous and metasedimentary sources with minor mantle components, whereas Dachang granites were mainly formed through partial melting of Mesoproterozoic metasedimentary rocks. The Dachang S-type granites display high values of DI and FeOT/MgO, low Tzr and ratios of La/Yb, Zr/Hf and Nb/Ta and strongly negative Sr, Ba and Eu anomalies, suggesting that they are highly fractionated granites; whereas the Kunlunguan I-type granites have witnessed less fractionation. The Dachang granites underwent fractionation of K-feldspar, plagioclase, biotite, monazite and allanite with magmatic-hydrothermal interaction indicated by Nb/Ta < 5. The Kunlunguan granites have high zircon Ce/Ce* ratios (1–400, average 70), which resemble those of typical Cu ore-bearing granitoids in Myanmar, indicating relatively high oxygen fugacity and explaining the associated formation of Cu-W deposit. The highly fractionated Dachang S-type granites show low oxygen fugacity (mostly < 40), similar with that of the typical Sn ore-bearing granitoids, which favored the Sn mineralization.

Geochronology of early Mesozoic diabase units in southwestern China: metallogenic and tectonic implications

AbstractTwo phases of diabase-sill-forming magmatism are recorded within the Badu anticline where magmas were emplaced into upper Palaeozoic carbonates and clastic rocks of the Youjiang fold-and-thrust belt in the SW South China Block, China. Zircons from these diabase units yield weighted mean U–Pb ages of 249.2±2.0 Ma and 187.1±3.3 Ma, and magmatic oxygen fugacity values from ‒20 to ‒6 (average of ‒12, equating to FMQ +5) and ‒20 to ‒10 (average of ‒15, equating to FMQ +2), respectively. These data indicate that the sills were emplaced during Early Triassic and Early Jurassic times. The discovery ofc. 250 Ma mafic magmatism in this area was probably related to post-flood-basalt extension associated with the Emeishan mantle plume or rollback of the subducting Palaeo-Tethys slab. Thec. 190 Ma diabase sills indicate that the southwestern South China Block records Early Jurassic mafic magmatism and lithospheric extension that was likely associated with a transition from post-collisional to within-plate tectonic regimes. The emplacement of diabase intrusions at depth may have driven hydrothermal systems, enabling the mobilization of elements from sedimentary rocks and causing the formation of a giant epigenetic metallogenic domain. The results indicate that high-oxygen-fugacity materials within basement rocks caused crustal contamination of the magmas, contributing to the wide range of oxygen fugacity conditions recorded by the Au-bearing Badu diabase. In addition, data from inherited xenocrystic zircons within the Badu diabase and detrital zircons from basement rocks suggest that the Neoproterozoic Jiangshao suture extends to the south of the Badu anticline.

Enhanced terrestrial input into Paleoproterozoic to Mesoproterozoic carbonates in the southwestern South China Block during the fragmentation of the Columbia supercontinent

Bulk rock elemental, C, O and Sr isotopic data for carbonates of the Palaeoproterozoic to Mesoproterozoic Luoxue and Luzhijiang formations from the southwestern margin of the South China Block are studied to infer aspects of the provenance and depositional environment. Elemental tracers demonstrate that the upper Luoxue and Luzhijiang carbonates were not significantly affected by crustal contamination and diagenetic/metamorphic overprinting, thus preserving their primary chemical signature, which is also consistent with the preservation of primary sedimentary structures and textures. In contrast, C, O and Sr isotopes of the lower Luoxue carbonates appear to have been variably altered by ore-forming fluids. All the Luoxue carbonates lack enrichment of Y, corresponding to chondritic Y/Ho ratios of 25–31, in contrast to modern seawater and marine carbonates. Shale-normalized REE + Y patterns for the Luoxue carbonates show pronounced enrichment in middle REE, but lack strong elemental anomalies (La, Gd, Y). Based on petrographic and sedimentological characteristics, the Luoxue Formation was probably formed in a shallow water, epicontinental basin with restricted but variable exchange to the open ocean, and dominant terrigenous input. The Luzhijiang carbonates, on the other hand, are pure chemical sediments devoid of detrital material, and shale (PAAS) normalized REE + Y distributions exhibit positive La, Gd, and Y (Y/Ho: 39–47) anomalies, which are typical features of marine waters throughout the Archean and Proterozoic periods. 87Sr/86Sr values in both the upper Luoxue and Luzhijiang carbonates are significantly higher than coeval seawater, indicative of enhanced contribution from weathered continental sources. This increased influx from terrestrial sources very likely corresponds to the break-up of the hypothesized Columbia/Nuna supercontinent, as the Luoxue and Luzhijiang carbonates were deposited in a rift-related basin associated with supercontinental fragmentation.

A genetic link between Late Cretaceous granitic magmatism and Sn mineralization in the southwestern South China Block: A case study of the Dulong Sn-dominant polymetallic deposit

The southwestern South China Block, composed of southwestern China and northeastern Vietnam, is one of the most important Sn-polymetallic ore districts in the world. The Dulong skarn Sn-polymetallic deposit, which is closely related to Late Cretaceous granitic magmatism, contains 5.0 mt Zn, 0.4 mt Sn, 0.2 mt Pb, and 7 kt In, and is one of the largest Sn-polymetallic deposits in the region. Sn mineralization occurs mainly as cassiterite at the contact zone between the Laojunshan granitic pluton and carbonates of the Cambrian Tianpeng Formation. The Laojunshan granitic pluton consists of three: medium- to coarse-grained two-mica monzonitic granite, medium- to fine-grained two-mica monzonitic granite, and granite porphyry, with zircon U–Pb ages of 87.5 ± 0.6 Ma, 85.3 ± 1.1 Ma, and 82.8 ± 1.6 Ma, respectively. Zircon grains from these three phases show εHf(t) values from −9.5 to −2.5, typical of a mantle origin. All samples exhibit strongly peraluminous and high-K calc-alkaline affinities, and show trace element patterns characterized by enrichment in Rb, Ta, La, Zr, and Hf, and depletion in Ba, Nb, and Ti, typical of S-type granites. Three main alteration/mineralization stages are recognized within the mineralized contact zones: (1) skarn stage; (2) sulfide mineralization stage; and (3) supergene stage. Cassiterite grains from the skarn alteration stage yield a 207Pb/206Pb–238U/206Pb concordia age of 89.4 ± 1.4 Ma, suggesting that mineralization was associated with the Laojunshan granitic pluton. Regionally, the Sn-polymetallic mineralization in the southwestern South China Block took place within a narrow time span from 97 to 79 Ma. The causative granitic magmas can be produced by partial melting of the Paleoproterozoic–Mesoproterozoic pelitic basement rocks during large-scale lithospheric thinning and asthenospheric upwelling.

Provenance and paleogeography of the Mesozoic strata in the Muang Xai Basin, northern Laos: petrology, whole-rock geochemistry, and U–Pb geochronology constraints

The Muang Xai Basin, located in northern Laos, is associated with the Simao, Vientiane, and Khorat Basins. The paleogeographic link of these basins has not been investigated in great detail; thus, the investigation presented in this study is a comprehensive analysis of petrology, whole-rock geochemistry, and detrital zircon U–Pb chronology used to characterize the provenance of the Muang Xai Basin. Results suggest that the sedimentary source includes felsic rocks from an active continental margin or continental arc with minor amounts of recycled passive continental margin sediments. Sandstones of the Muang Xai Basin contain detrital zircons with varying U–Pb peak ages. The youngest age peak of all the zircons is 103 Ma, which limits the age of the Mesozoic strata to the Late Cretaceous. Detrital zircon U–Pb and trace element data, combined with geochemical result, reveal that the pre-Ordovician zircons were derived from recycled sediments of the Yangtze Block, which are originally sourced from the Qinling Orogenic belt. This provenance is shared with coeval sediments in the Simao and Khorat Basins, while magmatic rocks of the Ailaoshan, Truong Son Belt, and Lincang terrane are responsible for zircons of 416–466 and 219–308 Ma in age. Zircons of 101–110 and 149–175 Ma in age were sourced from magmatic rocks of the southwestern South China Block and northern Vietnam. These provenance results suggest that sediments flow into the Khorat red beds was likely from the Great Simao Basin and northern Vietnam, and not directly from the Yangtze Block.

Transformation from Permian to Quaternary bauxite in southwestern South China Block driven by superimposed orogeny: A case study from Sanhe ore deposit

Permian karstic bauxite and its Quaternary derivative, in western Guangxi, southwestern, South China Block, possess a total tonnage greater than 0.5 billion tons. The primary late Permian karstic bauxite formed in reduced environment in the background of Tethyan accretionary orogenesis. And as one consequence of Cenozoic convergence of the Indian and Eurasia continents, the primary orebody was uplifted, eroded and re-sedimented within Quaternary laterite. The geochemical variation and its controls during the ore transformation from Permian to Quaternary remain poorly understood. Quaternary ore blocks comprise an inner zone of fresh ore, and then it gradually transited through a middle zone to a margin with extensive weathering. One such bauxite block was selected and further subdivided into twenty-three samples for geochemical and mineralogical analysis. The inner and middle zones contain similar mineralogical compositions, dominated by diaspore and amesite, with minor illite, anatase, goethite, pyrite, zircon, and rutile. The margin is composed of diaspore, with small amounts of amesite, boehmite, illite, goethite, anatase, kaolinite, zircon, rutile, and barite. Bauxite in all three zones is composed of mainly Al, Si, Fe, and Ti, and high contents of Zr, Cr, Li, F, S, Zn, V, Sr, Nb, Ba, and REE. Variations in Fe2+ and Fe3+ between the three zones were observed. The elements Si, Al, Fe2+, Mg, Ba, Cr, F, Li, Ni, Zn, and REE decrease from the core of the ore block outwards, corresponding to an increase in S and Fe3+. Depletions in Si, Al, Fe2+, Mg, Ba, and Cr were caused by the dissolution of amesite. Most of the Al and Si in amesite were lost during the weathering, and minor retained to form kaolinite. Depletions in Li, Ni, and Zn resulted from changes in the depositional environment between the late Permian and Quaternary. Dissolution of REE-bearing fluorocarbonates resulted in depletions of REE and F. The enrichment of Fe3+ and S was related to the precipitation of goethite, hematite, and barite in an oxidizing environment, while local enrichment of Ce resulted from the redox change of Ce3+→Ce4+ under the same condition. This shows that the chemical composition of laterite enwrapping the bauxite also took part in Quaternary bauxite transformation. This study shows that the elements migrations during bauxite transformation were influenced by multiple independent factors except for the elemental attributes.

Palaeomagnetic results from Palaeogene red beds of the Chuan-Dian Fragment, southeastern margin of the Tibetan Plateau: implications for the displacement on the Xianshuihe–Xiaojiang fault systems

ABSTRACTThe mechanism of deformation associated with the Cenozoic collision of India with Asia along the eastern boundary remains a poorly understood aspect of the tectonic evolution of the southwestern South China Block (SCB). Consequently, we carried out a palaeomagnetic investigation of Palaeogene red beds of the Dayao area of Yunnan Province in order to contribute to understanding the Palaeogene evolution of the SCB. A characteristic higher temperature magnetic component (HTC), with an unblocking temperature from 660°C to 680°C, was determined by principal component analysis (PCA), and positive fold tests indicated that the remanence was a primary magnetization. The mean direction of the HTC from the Dayao area is Ds = 27.8° Is = 33.1° κ = 64.8, α95 = 4.3° after tilt correction. Compared with other palaeomagnetic results from the SCB, our data suggest that the central part of the Chuan–Dian Fragment (CDF) experienced approximately 16.3 ± 4.7° clockwise rotation with respect to East Asia. Rotation of the CDF occurred along the left-lateral Xianshuihe–Xiaojiang Fault Systems (XSF-XJF), which exhibit an arc-shaped curve centred on the Eastern Himalayan Syntaxis. The XSF-XJF was approximated by a circle centred on a Euler pole at Lat. = 26.5° N, Lon. = 97.2° E (α95 = 0.2°), based on 11 reference points selected from the fault system. The clockwise rotation of the CDF resulted in left-lateral shearing along the XSF-XJF system, with a left-lateral displacement of ~200 km. The nature of diverse intense local deformation along the Xianshuihe-Xiaojiang left-lateral strike-slip fault systems is also discussed.

Petrogenesis of the early Paleozoic strongly peraluminous granites in the Western South China Block and its tectonic implications

Strongly peraluminous (SP) granites have A/CNK (molecular Al2O3/(CaO+Na2O+K2O)) ratios >1.1, indicating a predominant origin from the partial melting of metasedimentary rocks. However, an increasing number of studies have documented that mantle-derived magmas can also be involved in the petrogenesis of some SP granites. This is the case in the Dulong batholith in the southeastern Yunnan province, southwestern South China Block, which is typically composed of SP granitic rocks with high A/CNK values (>1.1). Zircon U–Pb dating of four samples from this batholith yielded consistent crystallization ages of ca. 430 Ma, synchronous with the widespread late-orogenic magmatism (including S- and I-type granites and mafic igneous rocks) in the Wuyi–Yunkai orogen, South China Block (SCB). All the granites have fractionated REE patterns ((La/Yb)N=1.28–12.3) and conspicuous negative Eu anomalies (Eu∗/Eu=0.07–0.43) with a similar depletion in HFSE (Nb, Zr, Hf), P, Ba and Sr, suggesting that these granitic magmas had a dominantly crustal source, likely the Neoproterozoic metasedimentary rocks (i.e., the Danzhou or Banxi groups) that are dominated by pelitic rocks with minor interlayered siltstones in the western Yangtze Block. However, their zircon Hf isotopic results also reveal an important input of the mantle-derived melts into their parental magma. Taken together, their geochemical and isotopic compositions reflect a derivation by magma mixing between the crust- and mantle-derived (∼10% in volume) magmas. Their chemical variations resulted from the fractionation process during the emplacement from the magma chamber after magmatic mixing.In conjunction with regional data within the SCB, it is most likely that the magmatism younger than 435 Ma was generated in a post-collisional extensional regime related to the partial delamination of an overthickened lithospheric mantle root without lower crust.