Western Yangtze Block Research Articles

Deep seismic reflection insights into syn-Rodinian crustal recycling

Owing to most tectonic events in Precambrian have been modified by intensive post-Precambrian overprinting, the question how exactly crustal recycling operated on earlier Earth remains an open one. Here, we report a SE-striking, 175 km-long deep seismic reflection profile that traverses the Sichuan basin, China. The image shows a well-preserved syn-Rodinian orogen resting beneath flat lying sedimentary cover of the basin. Relative to the counterparts of modern Earth, this orogen shows a structurally complex Moho geometry composed of multiple layers of basaltic sills, exhibiting extensive crustal extraction. The outlined crustal architecture further depicts that growth of the upper plate arose dominantly through tectonothermal processes, while coeval ductile underthrusting and overthrusting drove contemporaneous crustal shortening of the lower plate. Together with accretionary stacking above, these processes coevally generated a crustal thickness to ~ 30 km. This well-preserved syn-Rodinian orogen provides direct evidence for understanding contemporaneous continental recycling that assembled the Rodinia supercontinent. Plain language summaryNear-vertical, deep seismic reflection profiling is an effective technique for mapping complicated subsurface structures. In this study, the obtained high-resolution deep seismic reflection image captured a non-modified syn-Rodinian orogenic belt resting beneath flay lying sedimentary cover of the Sichuan basin, western Yangtze block, China. Structural interpretation and associated tectonic implications tell us: (1) the Yangtze block was actually a tectonic collage of microcontinents accreted during Neoproterozoic Rodinian assembly, (2) contemporaneous crustal recycling is featured with extensive crustal extraction, and (3) crustal growth was dominantly driven by tectonothermal events in the upper plate of the convergent zone and crustal shortening in a way of coeval underthrusting and overthrusting in the lower plate, a scenario that has shown no big difference with the counterparts of modern Earth. This well-preserved Neoproterozoic orogenic belt provides direct evidence for helping understand the continental recycling that assembled the Rodinia Supercontinent.

Nature and Evolution of Pre‐Neoproterozoic Continental Crust in South China: A Review and Tectonic Implications

AbstractSouth China as an amalgamation of the Yangtze and Cathaysia blocks is composed of Archean to Mesoproterozoic basement overlain by Neoproterozoic and younger cover. Both the constituent Yangtze and Cathaysia blocks contain well‐preserved Neoproterozoic rocks that have been extensively studied in terms of the age and tectonic nature, but less is known about their earlier crustal history due to the incomplete rock record. Recent efforts in investigating the yet survived crustal nature based on isotopic and elemental signatures preserved in igneous and sedimentary rocks have steadily improved our knowledge about the pre‐Neoproterozoic continental crustal evolution in South China. In this paper, we summarize the up‐to‐date pre‐Neoproterozoic records, including petrological, geochronological, geochemical and geophysical data, across South China, and discuss its spatiotemporal patterns of the pre‐Neoproterozoic crust and the relevant tectonic events. While the xenocrystic/inherited and detrital zircon records suggest widespread Archean (mainly ca. 2.5 Ga) crustal components within both the Yangtze and Cathaysia blocks, exposed Archean rocks are only limited to isolated crustal provinces in the Yangtze Block. These Archean rocks are dominated by TTGs (tonalite‐trondhjemite‐granodiorite) with varied ages (3.3–2.5 Ga) and zircon Hf isotopes, indicating a compositionally heterogeneous nature of the Archean Yangtze Block and, by inference, the development of multiple ancient terranes. The early Paleoproterozoic (2.4–2.2 Ga) tectonomagmatic events characterize the western Yangtze Block and are supportive of an east‐west subdivision of the Yangtze basement, whereas the late Paleoproterozoic (2.1–1.7 Ga) orogeneses may have affected a larger area covering both the western and eastern parts of the Yangtze Block, and also the Cathaysia Block. The eastern Yangtze Block with generally northeastward‐younging late Paleoproterozoic magmatism and metamorphism likely experienced a prolonged 2.05–1.75 Ga orogenic process welding the various Archean proto‐continents, consistent with the documentation of a buried late Paleoproterozoic orogenic belt imaged by deep seismic profiling from its central part and of a slightly older ophiolitic mélange in the northern part. The Cathaysia Block was probably involved in a short‐lived 1.9–1.8 Ga orogenic event. The two orogeneses overlapped in time and may have contributed to the cratonization of a possible unified South China, and are referred to be linked with the assembly of the Nuna Supercontinent. The subsequent late Paleoproterozoic to early Mesoproterozoic rift successions and intrusions (1.7–1.5 Ga) in the southwestern Yangtze Block, and the ca. 1.43 Ga rifting in Hainan Island of the Cathaysia Block could be responses to the Nuna break‐up. Late Mesoproterozoic (1.2–1.0 Ga) magmatism of varied age and nature in different localities of the Yangtze Block is reflective of a complex tectonic process in the context of the assembly of the Rodinia Supercontinent. Similar‐aged metamorphism (1.3–1.0 Ga) is recorded in Hainan Island, reflecting the Grenvillian continental collision during the Rodinia assembly, but further studies are necessary to better constrain the late Mesoproterozoic tectonic framework of South China.

Neoproterozoic metasomatized mantle beneath the western Yangtze Block, South China: Evidence from whole-rock geochemistry and zircon U-Pb-Hf isotopes of mafic rocks

Mafic rocks contain important information about the nature of the mantle source and tectonic settings. Here we present results of whole-rock geochemistry and zircon U-Pb-Hf isotopes for the Early Neoproterozoic mafic rocks (Ganyuhe hornblende gabbros and Guandaoshan gabbro-diorites) in the western Yangtze Block, South China, in order to constrain the petrogenesis and the mantle metasomatism processes under subduction setting. LA-ICP-MS zircon U-Pb dating results display concordant crystallization ages of 857.4 ± 2.0 Ma for the hornblende gabbros and 856.4 ± 2.8 Ma for the gabbro-diorites, respectively. These mafic rocks possess low SiO2 contents (45.5–53.3 wt%) as well as high MgO contents (3.27–9.20 wt%) and Mg# (48.6–62.6) values, and show characteristics of arc-like trace element patterns with enriched Rb, Ba, Sr, K, and Pb but depleted Nb, Ta, Zr, Hf, and Ti. The prominently positive whole-rock εNd(t) (+4.9 to + 5.8) and zircon εHf(t) (+8.48 to + 20.6) values suggest a depleted lithospheric mantle source. We therefore infer that the mafic rocks investigated here were derived from an amphibole-bearing spinel peridotite source that was affected by metasomatism between the overlying mantle wedge and subduction-related fluids. In conjunction with the compiled data of Neoproterozoic mafic rocks from the western Yangtze Block, we propose that there exist Neoproterozoic metasomatized mantle sources beneath the western Yangtze Block during the subduction process in response to the evolution of the supercontinent Rodinia. The Neoproterozoic metasomatic agents in mantle sources include the slab fluids, sediment melts, and oceanic slab melts.

The Mojiawan I-type granite of the Kangding Complex in the western Yangtze Block: new constraint on the Neoproterozoic magmatism and tectonic evolution of South China

ABSTRACT Studies of Neoproterozoic granites near the western margin of the Yangtze Block provide new evidence on the mechanism leading to the Neoproterozoic break-up of the Rodinia Supercontinent. This contribution focusses on the structural geology, geochemistry, zircon U-Pb geochronology, and Hf-O isotope systematics of the poorly documented Mojiawan Monzogranite in the Kangding Complex, which gives us better constraints on the break-up of the supercontinent. Zircon U-Pb data show that two samples of the Mojiawan Monzogranite were emplaced during 797 ± 13 Ma and 785 ± 11 Ma (ca. 791 Ma). The granitic samples are Na-rich, calc-alkaline, peraluminous with A/CNK values of 1.13 and 1.25, and their zircons have positive εHf(t) values between +4.28 and +9.79 with crustal model ages TDM2 of 1424 to 1079 Ma, indicative of a juvenile source. The granitic samples also have low Rb/Sr and Rb/Ba ratios and high δ18O values of 5.42‰–8.97‰, consistent with derivation from the partial melting of the lower to middle crust. Combined with the geochemistry of other granitic plutons along the western margin of the Yangtze Block, we propose that the region records rifting at ca. 800 Ma due to the action of a mantle super-plume leading to the break-up of the Rodinia Supercontinent.

Detrital Zircon U-Pb-Hf Isotopes of Middle Neoproterozoic Sedimentary Rocks in the Altyn Tagh Orogen, Southeastern Tarim: Insights for a Tarim-South China-North India Connection in the Periphery of Rodinia

Abstract The location of the Tarim craton during the assembly and breakup of the Rodinia supercontinent remains enigmatic, with some models advocating a Tarim-Australia connection and others a location at the heart of the unified Rodinia supercontinent between Australia and Laurentia. In this study, our new zircon U-Pb dating results suggest that middle Neoproterozoic sedimentary rocks in the Altyn Tagh orogen of the southeastern Tarim craton were deposited between ca. 880 and 760 Ma in a rifting-related setting slightly prior to the breakup of Rodinia at ca. 750 Ma. A compilation of existing Neoproterozoic geological records also indicates that the Altyn Tagh orogen of the southeastern Tarim craton underwent collision at ca. 1.0-0.9 Ga and rifting at ca. 850-600 Ma related to the assembly and breakup of Rodinia. Furthermore, in order to establish the paleoposition of the Tarim craton with respect to Rodinia, available detrital zircon U-Pb ages and Hf isotopes from Meso- to Neoproterozoic sedimentary rocks were compiled. Comparable detrital zircon ages (at ca. 0.9, 1.3-1.1, and 1.7 Ga) and Hf isotopes indicate a close linkage among rocks of the southeastern Tarim craton, Cathaysia, and North India but exclude a northern or western Australian affinity. In addition, detrital zircons from the northern Tarim craton exhibit a prominent age peak at ca. 830 Ma with minor spectra at ca. 1.9 and 2.5 Ga but lack Mesoproterozoic ages, comparable to the northern and western Yangtze block. Together with comparable geological responses to the assembly and breakup of the Rodinia supercontinent, we offer a new perspective of the location of the Tarim craton between South China and North India in the periphery of Rodinia.

Development of contrasting folding styles in the western Yangtze block, South China: Insights from numerical modeling

Two contrasting folding styles are well developed along both sides of the Qiyueshan fault, South China. The western part is characterized by chevron anticlines and box synclines in the Eastern Sichuan domain, while the eastern part features typical chevron synclines and box anticlines in the western Hunan-Hubei domain. These contrasting structures are important in better understanding the intracontinental orogeny around the eastern Sichuan Basin. In this study, we conducted a series of 2-D numerical simulations using the finite difference method. The model results indicate that the key parameters include stratigraphic competence, number and distribution of intermediate weak layers, and thickness of basal weak layer. Among them, the thickness of basal weak layer plays the most important role in shaping the two distinct structural styles. In the models with thin basal weak layer (<1 km), high-quality chevron anticlines are developed with the presence of multiple intermediate weak layers. These folds are mainly characterized by fault-propagation folds or fault-bend folds. The basal weak layer corresponds to the relatively thin Cambrian evaporite layer in the Eastern Sichuan domain. In contrast, in the models with thick basal weak layer (>8 km), a series of coherent chevron synclines are developed, and mainly form décollement folds. These characteristics are similar to the structural styles revealed by seismic profiles in the western Hunan-Hubei domain. This weak layer may represent the accumulation of mechanically weak rocks (e.g., slate, schist and phyllite) in Neoproterozoic Lengjiaxi and Tieshajie Groups in the western Hunan-Hubei domain, which is also consistent with the geophysical observations (e.g., weak seismic reflection, high electrical conductivity, and Bouguer gravity low anomaly). Both the differential thickness of basal weak layer along the “flat-ramp-flat” décollement and the overlying multiple intermediate weak layers contribute to the formation of the contrasting folding styles around the eastern Sichuan Basin.

Provenance and tectonic setting of the Neoproterozoic meta-sedimentary rocks at southeastern Tibetan Plateau: Implications for the tectonic affinity of Yidun terrane

The Yidun terrane is located between the Qiangtang terrane and Songpan-Ganzi Fold Belt and its tectonic affinity with other adjacent terrane is of significance to understand the tectonic evolution of southeastern Tibetan Plateau. In this study, we present integrated bulk geochemical data, detrital zircon U-Pb ages and Hf isotopes of meta-sedimentary rocks from the Qiasi Group, which represents the oldest Precambrian basement of the Yidun terrane, to constrain the depositional age and provenance of the meta-sedimentary rocks as well as the tectonic affinity of the terrane. Zircon U-Pb dating results show that detrital zircons from three samples have almost identical age spectra with two major age clusters of 800–780 Ma and 850–820 Ma at age peaks of ca. 790 Ma and ca. 827 Ma, and one minor age cluster of 910–860 Ma. The youngest group of detrital zircons reveals that the maximum depositional age of the Qiasi Group was 779–777 Ma. Low SiO2/Al2O3 (2.9–4.1), CIA (59–47), and PIA (63–47) values indicate that the sediments could be derived mainly from the low mature sources that had undergone weak to mildly moderate degree of chemical weathering. Bulk geochemical and zircon U-Pb-Hf isotopic data suggest that the provenance was composed mainly of Neoproterozoic intermediate to felsic igneous rocks. In combination with the published geological data and detrital zircon U-Pb age spectra on the western Yangtze Block, and the Kunlun-Qilian-Qaidam Block, we propose that the Neoproterozoic basement of the Yidun terrane has a close tectonic affinity with the former one.

Genesis of ca. 850–835 Ma high-Mg# diorites in the western Yangtze Block, South China: Implications for mantle metasomatism under the subduction process

High-Mg# (molar 100 × Mg/(Mg + Fe)) diorites can provide significant insights on mantle metasomatism under a subduction zone. Here we investigate the genesis of the Neoproterozoic Shuilu high-Mg# diorites in the western Yangtze Block to evaluate the role of subduction-related fluids and sediment melts acting on mantle sources during the subduction process. Zircon U–Pb dating results display new weighted mean 206Pb/238U ages of 850.1 ± 1.7 Ma, 840.9 ± 2.4 Ma, and 836.6 ± 1.9 Ma for these high-Mg# diorites. They are metaluminous and calc-alkaline rocks, and characterized by moderate SiO2 contents (57.08–61.12 wt%) and high MgO contents (3.36–4.30 wt%) and Mg# values (56–60). The relatively low initial 87Sr/86Sr ratios (0.703406 to 0.704157) and highly positive whole-rock εNd(t) (+3.26 to +4.26) and zircon εHf(t) values (+8.43 to +13.6) imply that these rocks were predominantly sourced from depleted lithospheric mantle. These high-Mg# diorites also show enrichment of light rare earth elements and large ion lithophile elements (e.g., Rb, Ba, K, and Sr) as well as depletion of high field strength elements (e.g., Nb, Ta, Zr, and Hf), resembling a typical arc magma affinity. The highly variable Ba contents and Rb/Y, Th/Ce, Th/Sm, Ba/La, and Th/Yb ratios indicate significant incorporation of subduction-related fluids and sediment-derived melts into the primary mantle source. We therefore propose that the ca. 850–835 Ma high-Mg# diorites investigated in this study were formed by partial melting of a metasomatized mantle source enriched by subduction fluids and sediment melts. Our new data, in conjunction with numerous studies of metasomatized mantle magmatism from the western Yangtze Block, suggest that the Neoproterozoic mantle sources beneath the western Yangtze Block were metasomatized by subduction-related compositions involving slab fluids, sediment melts, and oceanic slab melts during the subduction process.

The ~1.97 Ga dioritic block in the Hong’an Terrane, central China: syn-collisional alkaline magmatism at the northern margin of the Yangtze Block

It is uncertain whether the Yangtze Block was part of the Paleo- to Mesoproterozoic Columbia supercontinent and, if so, where it was located within the supercontinent. Our analysis of a dioritic intrusion near Tiepu village in the southern Hong’an Terrane of central China provides new constraints on the Paleoproterozoic tectonic evolution of the Yangtze Block. Three representative samples from the Tiepu diorite yield zircon U–Pb ages within analytical error of ~1970 Ma. The diorites have narrow ranges of SiO2 (53.5–57.7 wt%) and MgO (2.97–4.48 wt%). High contents of alkalis (K2O + Na2O = 6.70–10.2 wt%) and total rare-earth elements (∑REE = 570–803 ppm) suggest an affinity to intermediate alkaline rock. These rocks are generally characterized by arc-like trace-element patterns such as enrichment in large-ion lithophile elements and light REEs, and depletion in high-field-strength elements and heavy REEs. Zircons from the diorites display negative εHf(t) values (−12.5 to −3.1), and whole-rock samples show negative εNd(t) values (−6.53 to − 4.08). These isotopic characteristics are consistent with older crustal sources. Combining the new data and previous results from Paleoproterozoic rocks in the northern Yangtze Block, we propose a tectonic transition from pre- to post-collisional settings at ~2.0–1.95 Ga. The Tiepu alkaline diorite is interpreted as a syn-collisional magmatic intrusion that formed shortly after the cessation of oceanic plate subduction at ~2.0 Ga. The Tiepu alkaline diorite thus records the final tectonic amalgamation of the eastern and western Yangtze Block microcontinents during the assembly of Columbia.

Basin-orogen patterns and the late Triassic foreland basin conversion process in the western Yangtze Block, China

Due to the formation of the Qinling and Sanjiang Orogenic Belts along the northern and southwestern margins, the western Yangtze Block experienced a complex basin-orogen tectonic transformation, forming a unique pattern for a basin-orogen system in the late Triassic that should be recorded by sediments in the Sichuan Basin. This paper combines with regional tectonics, field observations, and borehole and seismic exploration data to establish the prototypes of the basins and tectonic frameworks of basin-orogen from Member 1 (M1) to 6 (M6) of the Xujiahe Formation during the late Triassic in the western Yangtze Block. From stage M1 to M2, a set of deltaic deposits formed and extended parallel to the Qinling Orogenic Belt, indicating that the start of the scissor-type collision in the northern margin of the western Yangtze Block led to the development of the northern Sichuan Basin and Songpan-Ganzi peripheral foreland basin. Meanwhile, a set of deltaic deposits, which occur in the Southeastern Sichuan Basin, were controlled by the uplift of the Jiangnan-Xuefeng Tectonic Belt, transforming into a denuded zone at that time. A set of deltaic deposits developed in the southwestern part of the western Yangtze Block, indicating the beginning of the initial stage of the related foreland basin in this area at stage M2. From stage M3, the intensive activity of orogenic belts surrounding the western Yangtze Block shaped the tectono-sedimentary features along the margin of the western Yangtze Block. The successive development of delta plain and alluvial fan deposits and their westward and eastward expansions respectively in the north and southwest margins of the western Yangtze Block reflect the uplift of the Qinling and Sanjiang Orogenic Belts; while the formation of a set of deltaic deposits in the southeastern part of the western Yangtze Block results from the persistent uplift of the Jiangnan-Xuefeng Tectonic Belt. During stages M5 to M6, the lacustrine-swamp deposits began expansion and became the main sedimentary facies in the western Yangtze Block. Southward and northward migrations of the aforementioned two separated foreland basins, respectively controlled by Qinling and Sanjiang Orogenic Belts, caused these two separated foreland basins to connect with each other and to form a single unified complex foreland basin that was controlled by these two orogenic belts. These results suggest a two-stage foreland basin evolution mode for the western Yangtze Block during the late Triassic. At stage I (M1 to M4), two separated foreland basins developed in the northern and southwestern margins of the western Yangtze Block, controlled respectively by the Qinling and Sanjiang Orogenic Belts. During Stage II (M5 to M6), these two separated foreland basins connected with each other to form a single composite foreland basin related to those two orogenic belts.

Sources of the Laoxiongdong carbonate-hosted Pb–Zn deposit in Southwest China: constraints from S–Pb–Zn isotopic compositions

The Sichuan–Yunnan–Guizhou (SYG) Pb–Zn mineral province, which has significant Pb–Zn repositories in China, is situated in the western Yangtze Block. Despite decades of research, the Pb–Zn source of deposits is still disputed between the basement rocks, sedimentary wall rocks, or the Emeishan flood basalts. The newly-discovered Laoxiongdong Pb–Zn deposit is hosted by the Late Ediacaran Dengying Formation in the SYG province. Moreover, the Laoxiongdong orebodies lie between regional deep faults and close to the Proterozoic basement and Emeishan basalts. Hence, this deposit represents a good case study on the ore-forming material source for the regional Pb–Zn mineralization. The Laoxiongdong Pb–Zn ores are massive, vein, or disseminated and have mainly sphalerite, galena, pyrite, quartz, and calcite. In this paper, we present new sulfide S–Pb–Zn isotope data of the deposit. The sulfide δ34SCDT values (+ 11.8 to + 16.5 ‰) suggest that the reduced sulfur was mainly sourced from evaporites in the Late Ediacaran–Cambrian sequences. Pb isotopic ratios (206Pb/204Pb = 18.004–18.107, 207Pb/204Pb = 15.652–15.667, and 208Pb/204Pb = 38.037–38.248) suggest that the lead metal was primarily originated from the basement rocks. The sphalerite δ66Zn values (+ 0.16 to + 0.37 ‰) are also highly similar (within error) to those of basement rocks (+ 0.10 to + 0.34 ‰), suggesting a basement-rocks zinc source with minor contributions from the carbonate host rocks and Emeishan flood basalts. The narrow sphalerite Zn isotopic range (0.21 ‰) also indicates that the Zn isotopic fractionation between the sphalerite and initial fluid was limited during the sphalerite ore precipitation. Therefore, we propose that both the Late Ediacaran Dengying Formation rocks and Proterozoic basement rocks were important ore-forming material source for the Laoxiongdong deposit, whereas the Emeishan flood basalts represent only a minor ore-material source.

New insights into the evolution of Mississippi Valley-Type hydrothermal system: A case study of the Wusihe Pb-Zn deposit, South China, using quartz in-situ trace elements and sulfides in situ S-Pb isotopes

AbstractUnraveling the evolution of Mississippi Valley-type (MVT) hydrothermal system is crucial for understanding ore genesis and exploration. In this paper, we take the Wusihe Pb-Zn deposit in the western Yangtze Block (South China) as a case study, using detailed ore deposit geology, quartz in situ trace elements, and sulfides in situ S-Pb isotopes, to propose a new integrated model for the evolution of MVT hydrothermal system. Four hydrothermal stages were identified in the Wusihe ore district: (I) lamellar pyrite-sphalerite; (II) disseminated, stock-work, and brecciated sphalerite-galena; (III) massive galena, and (IV) veined calcite-bitumen. Within the most representative stage (stage II), Al concentrations in quartz (Q) increase from 8.46–354 ppm (mean 134 ppm) of Q1 to 171–3049 ppm (mean 1062 ppm) of Q2, and then decrease to 3.18–149 ppm (mean 25.4 ppm) of Q3. This trend indicates the role of acid-producing processes that resulted from sulfide precipitation and acid consumption by carbonate buffering. The occurrence of authigenic non-altered K-feldspar provides further evidence that the ore-forming fluids were weakly acidic with pH values of &amp;gt; ~5.5. Moreover, new bulk δ34S values of sulfides (+1.8 to +14.3‰) are overall lower than those previously reported (+7.1 to +20.9‰), implying that in addition to thermochemical sulfate reduction (TSR), bacterial sulfate reduction (BSR) may play an important role in the formation of S2–. In situ δ34S values show a larger range (–4.3 to +26.6‰), and significantly, varies within single grains (up to +12.3‰), suggesting mixing of two isotopically distinct S2– end-members produced by TSR and BSR. The diagenetic and hydrothermal early phase (stage I) sulfides were formed within a nearly closed system of BSR, whereas the formation of late phase (stage II and stage III) sulfides was caused by the input of hydrothermal fluids that promoted TSR. New galena in situ Pb isotopic ratios (206Pb/204Pb = 18.02–18.19, 207Pb/204Pb = 15.66–15.69, and 208Pb/204Pb = 38.14–38.39) suggest that the sources of mineralizing metals in the Wusihe deposit are mainly Proterozoic basement rocks. Hence, a multi-process model (i.e., basin-mountain coupling, fluid mixing, local sulfate reduction, in situ acid-producing and involvement of black shales and carbonate sequences) was responsible for the formation of the Wusihe deposit, while S2– was produced by both TSR and BSR, providing new insights into the evolution of MVT hydrothermal system.

Petrogenesis and geochemical diversity of Late Mesoproterozoic S-type granites in the western Yangtze Block, South China: Co-entrainment of peritectic selective phases and accessory minerals

Deciphering the geochemical diversity of S-type granites is crucial for obtaining more profound insight into their petrogenesis. We therefore undertook an integrated study of whole-rock geochemistry, Sr-Nd isotopes, and zircon U-Pb-Hf isotopes for newly recognized Late Mesoproterozoic S-type granites, including two-mica-, biotite-, garnet-bearing two-mica granites, from the western Yangtze Block, South China. The crystallization ages of these granites are ca. 1040 Ma (weighted mean 206Pb/238U age = 1036.4 ± 4.5 Ma to 1042.2 ± 1.1 Ma). They are peraluminous to strongly peraluminous (A/CNK = molar ratio of Al2O3/(CaO + Na2O + K2O) = 1.02–1.67), high-K calc-alkaline rocks, and display high concentrations of normative-corundum (0.54–7.04 wt%) as well as positive correlations of A/CNK and FeOT + MgO values, which are characteristics of S-type granites. These S-type granites are characterized by enriched in Rb, Th, K, and Pb, depleted in Ba, Sr, Ti, and Eu, with negative whole-rock εNd(t) (−0.3 to −6.8) and predominantly negative zircon εHf(t) values (−8.09 to +5.70), indicating the affinity of middle-upper crustal trends and a heterogeneous metasedimentary source. Compared with the geochemical diversity of S-type granites around the world, the variably negative εNd(t) values as well as positive and negative εHf(t) values of our S-type granites may be caused by source heterogeneity and disequilibrium melting processes. More importantly, similar to typical more mafic S-type granites from the Cape Granite Suite (South Africa) and north Queensland (Australia), the high and variable FeOT + MgO contents (2.21–6.64 wt%) are significantly attributed to coupled co-entrainment of peritectic and accessory minerals (e.g., garnet, ilmenite, zircon, and monazite), evidenced by positive relationships between FeOT + MgO and TiO2, CaO, Zr, Th, Hf, Y, Yb, light rare earth elements (LREEs). In conjunction with the existence of coeval rift-related A-type igneous rocks, depleted asthenosphere-derived mafic rocks, marine sedimentary sequences, and subsequent extensive Neoproterozoic arc magmatism along the western Yangtze Block, we infer that the ca. 1040 Ma S-type granites studied here were produced in a continental rifting basin that was transformed into a compression setting during the Neoproterozoic.

Does Neoproterozoic Nam Co formation in Northwest Vietnam belong to South China or Indochina?

The Nam Co belt is located to the northeast of Song Ma fault in Northwestern Vietnam. The nature of the Precambrian basement rocks in this area and its tectonic attribute have still been controversial. In this study, zircon U-Pb-Hf isotopic compositions and whole rock geochemical compositions of meta-sedimentary rocks from the Nam Co Formation in this belt have been integrated to reveal the components and tectonic setting of the formation of the basement rocks in the Nam Co belt. Zircon U-Pb dating results show that the Nam Co Formation is of middle-late Neoproterozoic age. The lower Nam Co Formation probably formed between 821 and 726 Ma, and the upper Formation was deposited later than 726–721 Ma. The sedimentary rocks from the lower and upper Nam Co formations are characterized by numerous Neoproterozoic (900–800 Ma) detrital materials, but the upper part contains more Meso- to Paleoproterozoic sediments than the lower part. Detrital zircon age distribution and Hf-isotope compositions of the Nam Co Formation are similar to those of Neoproterozoic sedimentary rocks in the southern Yangtze Block. Both zircon U-Pb-Hf isotopic and whole rock geochemical data suggest that the source materials of the Neoproterozoic Nam Co Formation were dominated by felsic rocks with minor mafic materials, and the detrital materials were mainly derived from the Jiangnan Orogenic Belt in the southern Yangtze Block, with minor contribution from the Panxi belt in the western Yangtze Block, Ailaoshan-Song Hong and Phan Si Pan belts. The lower Nam Co Formation contains more sediments derived from the Panxi belt than the upper part. The middle-late Neoproterozoic Nam Co Formation probably was deposited in a continental extensional basin, and the sedimentation environment in later period (<726–721 Ma) was more stable. Integration of our data with other lines of evidence suggests that the Nam Co Formation belongs to the basement of the Yangtze Block, and the Nam Co belt was originally located in the southwest corner of the Yangtze Block. This further suggests that the Song Ma fault probably is the boundary between the South China and Indochina blocks.

Provenance investigation for the Cambrian–Ordovician strata from the northern margin of the western Yangtze Block: implications for locating the South China Block in Gondwana

AbstractWe report new U–Pb isotopic data for detrital zircons from Cambrian–Ordovician strata on the northern margin of the western Yangtze Block, which together with published U–Pb isotopic data for coeval strata in the South China Block, provide critical constraints on the provenance of these sediments and further shed light on the early Palaeozoic position of the South China Block in the context of Gondwana. Detrital zircons in this study yield four major age peaks in the early Palaeoproterozoic, early Neoproterozoic, middle Neoproterozoic and late Neoproterozoic – early Palaeozoic. The dominant age population of 900–700 Ma matches well with magmatic ages from the nearby Panxi–Hannan Belt, which indicates that Cambrian–Ordovician sedimentary rocks in the western Yangtze Block were mainly of local derivation. However, compilations of detrital zircon ages for the Cambrian–Ordovician strata from the Cathaysia Block and the eastern Yangtze Block show that both blocks are dominated by late Mesoproterozoic- and early Neoproterozoic-aged detrital zircons, which suggests a remarkable exotic input with typical Gondwana signatures. According to the integrated detrital zircon age spectra of the Cambrian–Ordovician sedimentary rocks from the entire South China Block and palaeocurrent data, the South China Block should have been linked with North India and Western Australia within East Gondwana. Specifically, the Cathaysia Block was located adjacent to Western Australia, while the Yangtze Block was connected with North India.

Detrital zircon provenance and palaeogeographic implications of the Ediacaran Shigu Group in the Zhongza Terrane, SW China

ABSTRACT The palaeogeographic positions of South China in the Rodinia and Gondwana supercontinents have long been controversial. We report petrological, detrital zircon U-Pb geochronological, trace element geochemical and Lu-Hf isotopic compositions for metasedimentary rocks from the Shigu Group in the Zhongza Terrane, SW China. The Shigu Group has been regarded as the Precambrian basement of the Zhongza Terrane, and comprises mainly a suite of medium-/low-grade metamorphosed terrigenous clastic rocks. The youngest zircon age population of two metasedimentary rock samples from the bottom Shigu Group yields a weighted average age of 633 ± 9 Ma, indicative of a late Neoproterozoic (ca. 630 Ma) maximum deposition age. The youngest detrital zircon age of a metasedimentary rock from the upper Shigu Group is 597 ± 7 Ma, it may suggesting a considerably later deposition. Detrital zircon age distribution patterns of the Shigu Group metasedimentary rocks are similar to those of coeval sedimentary rocks in the western Yangtze Block. According to Hf isotope compositions and zircon morphology, the Neoproterozoic Shigu Group detritus may have sourced from Neoproterozoic magmatic rocks in the western Yangtze Block, whilst the pre-Neoproterozoic zircons might originate from recycling of the underlying metasedimentary rocks. Our results indicate that the basement of the Zhongza Terrane was a continuation of the Yangtze Block, which implies that Precambrian Yangtze basement rocks were present beyond the current boundary of the Block. In the late Neoproterozoic (ca. 630–570 Ma), the western Yangtze Block may have been facing the Proto-Tethys and received detritus mainly from the local Neoproterozoic magmatic rocks. During the late Neoproterozoic-earliest Palaeozoic (ca. 570–520 Ma), the seaway closure between the Yangtze (incl. Zhongza) Block and northern India likely created new pathways for delivery of the earliest Neoproterozoic and Pan-African detrital zircons into the lowermost Palaeozoic sequences in South China.

Rb-Sr isotopic age, S-Pb-Sr isotopic compositions and genesis of the ca. 200 Ma Yunluheba Pb-Zn deposit in NW Guizhou Province, SW China

Over 400 carbonate-hosted Pb-Zn deposits in the western Yangtze block constitute the giant Sichuan-Yunnan-Guizhou (SYG) Pb-Zn metallogenic province. The majority of Pb-Zn deposits hosted in Devonian carbonates are characterized by low δ34S values, causing debates on their ore genesis. To address this issue, the Yunluheba Pb-Zn deposit is chosen as a case study. Ore bodies of this deposit are hosted in middle Devonian carbonates. Ore minerals are sphalerite, galena and pyrite. Sphalerite Rb-Sr dating yields an isochron age of 206.2 ± 4.9 Ma, consistent with ages of most Pb-Zn deposits in the SYG province, arguing for a regional intensive Pb-Zn mineralization event during the late Indosinian orogeny. Sulfides have Pb isotopic compositions that form a linear trend above the average crustal Pb growth curve in the 207Pb/204Pb vs. 206Pb/204Pb plot. Moreover, Pb isotopic ratios are consistent with the Devonian to Permian carbonates’ age-corrected Pb isotopic ratios, suggesting mixed crustal Pb sources with the majority of Pb derived from country rocks. Combined with initial 87Sr/86Sr ratios of sphalerite, approximately equal to 87Sr/86Sr200Ma ratios of the D-P carbonates, ore-forming metals are considered to have been derived mainly from ore-hosting rocks. Sulfides’ δ34SCDT values range mainly from −2.0‰ to 2.9‰, interpreted to be a result of mixing by syn-sedimentary pyrite originated biogenic S and thermochemically reduced S. We conclude that the Yunluheba deposit and other Pb-Zn deposits in the SYG province should belong to Mississippi Valley-type deposits.

Neoproterozoic peraluminous granites in the western margin of the Yangtze Block, South China: Implications for the reworking of mature continental crust

Significant widespread melting of the mantle and juvenile mafic lower crust is known to have occurred along the western margin of the Yangtze Block during the Neoproterozoic, but melting of the mature continental crust remains poorly understood. Peraluminous granites can provide vital insights on the reworking of mature continental crustal materials. We present zircon U-Pb-Hf isotopic, whole-rock geochemical, and Sr-Nd isotopic data from the Neoproterozoic peraluminous granites in the western Yangtze Block, South China. Zircon U-Pb dating displays concordant crystallization ages of ca. 840 Ma for the Kuanyu granites and ca. 835 Ma for the Cida granites. These peraluminous granites have high SiO2 (66.9–75.6 wt%), K2O (4.61–7.29 wt%) contents, as well as high K2O/Na2O (1.44–3.25) and A/CNK (molar ratio of Al2O3/(CaO + Na2O + K2O)) (1.04–1.18) ratios. The samples are enriched in Rb, K, Th, U, and Pb, and depleted in Nb, Ta, Ba, Sr, and Ti, indicating a middle-upper crustal affinity. They are also characterized by high (87Sr/86Sr)i (0.7099–0.7217) and negative εNd(t) values (−5.1 to −2.9), which resemble the isotopic features of an evolved continental crust source. Furthermore, these peraluminous granites possess variable CaO/Na2O (0.09–0.65) and Al2O3/TiO2 (25.3–88.4) ratios, moderate Rb/Ba (1.68–3.86) and Rb/Sr (0.32–0.85) ratios, as well as high molar Al2O3/(MgO + FeOT) (2.04–5.23) and low molar CaO/(MgO + FeOT) (0.15–0.48), implying that they predominantly originate from heterogeneous metasedimentary sources (metapelites + metagreywackes). Scarce evidence of hybridization processes between crust- and mantle-derived components indicates that their heterogeneous zircon Hf isotopic compositions (εHf(t) = −7.75 to +3.31) may be caused by disequilibrium partial melting of heterogeneous metasedimentary sources, similar to peraluminous granites from the Altai area and Jiangnan orogen in China. In combination with previously reported results, we suggest that the Kuanyu and Cida peraluminous granites represent melting of mature crustal material in an evolved middle-upper crust source during the early stages of the Neoproterozoic subduction process. The western margin of the Yangtze Block underwent not only melting of the juvenile mafic lower crust but also reworking of the mature continental crust during the Neoproterozoic.

Petrogenesis and geodynamic implications of Neoproterozoic gabbro-diorites, adakitic granites, and A-type granites in the southwestern margin of the Yangtze Block, South China

The occurrence of A-type granites following adakitic granites can provide unique insights into the tectonic evolution of subduction zone. This paper presents a comprehensive study of zircon U-Pb-Hf isotopes and whole-rock geochemistry on the Neoproterozoic gabbro-diorites, biotite granites, and K-feldspar granites in the southwestern margin of the Yangtze Block, South China. Zircon U-Pb dating of three distinctive intrusions yield concordant ages of 810.4 ± 2.0 Ma for gabbro-diorites, 800.3 ± 2.1 Ma for biotite granites, and 749.1 ± 2.0 Ma for K-feldspar granites. The gabbro-diorites are sodic and calc-alkaline rocks with low SiO2 (52.62–53.87 wt%), medium MgO (2.67–3.41 wt%), high Fe2O3T (7.18–7.49 wt%) and CaO (5.68–7.50 wt%) contents. They display high Th/Zr and Rb/Y ratios but low Nb/Zr and Nb/Y ratios. Together with their positive whole-rock εNd(t) (+1.0 to +1.5) and zircon εHf(t) values (+3.66 to +8.18), we suggest that these gabbro-diorites were derived from the partial melting of depleted lithospheric mantle modified by subduction-related fluids. The biotite granites display relatively high Sr contents (335–395 ppm), Sr/Y ratios (38.9–54.3), low Y (7.04–9.71 ppm) and Yb contents (0.78–1.08 ppm), resembling the adakitic granites. Their low Mg# (36–41) values, Cr (2.94–3.59 ppm) and Ni (1.32–1.55 ppm) contents as well as positive εNd(t) (+0.5 to +0.6) and εHf(t) values (+1.62 to +8.07) indicate that they were formed by partial melting of thickened juvenile mafic lower crust, probably within the existence of garnet. The K-feldspar granites have high SiO2 (76.61–77.14 wt%) and alkalis (Na2O + K2O = 8.55–9.69 wt%) contents, high 10,000 * Ga/Al (2.56–2.80) ratios and differentiation index (95.3–96.9), indicating the affinity to highly fractionated A-type granites. We suggest that these A-type granites were mainly generated by the partial melting of felsic crustal rocks under low pressure condition and subsequently extensive fractional crystallization is significant. Taking into account previous studies from the western Yangtze Block, South China, we propose that the gabbro-diorites, adakitic granites and A-type granites in this study were formed in persistent subduction process during the Neoproterozoic.

Early Neoproterozoic syncollisional S-type granites from the western Yangtze Block, South China: Implications for final closure of the back-arc basin

Early Neoproterozoic syncollisional S-type granites from the western Yangtze Block, South China: Implications for final closure of the back-arc basin