•∼820Ma unconformity separating Sibao and Danzhou groups in South China is revisited.•Sediments across the unconformity have similar detrital zircon age and O–Hf isotopes.•Sedimentary provenance and ...tectonic setting did not change across the unconformity.•The Sibao and Danzhou group sediments were deposited in a continental rift basin.•The unconformity is due to rapid crust uplifting, unroofing and basin subsidence.
Middle Neoproterozoic sedimentary rocks are widespread in the South China Block, especially around the periphery of the Yangtze Block. They are separated into the Sibao and Danzhou groups (and their equivalents) by a “mid-Neoproterozoic angular unconformity” in the south margin of the Yangtze Block. This unconformity, previously interpreted as the evidence of continental collision between the Yangtze and Cathaysia blocks, is crucial for understanding the early-middle Neoproterozoic tectonic regime in the South China Block. We report integrated in situ U–Pb, Hf and O isotopes analyses of detrital zircon grains from sandstones across this unconformity, and use these data to decipher their sedimentary provenances and shed new light on the basin evolution and the tectonic significance of this mid-Neoproterozoic unconformity.
A total of 324 concordant U–Pb zircon analyses are obtained from three sandstone samples, including two from the Yuxi Formation of the Sibao Group and one from the Baizhu Formation of the Danzhou Group that are located below and above the unconformity, respectively. Detrital zircon grains from the lower part of the Yuxi Formation are dominated by a single age population peaked at ca. 830Ma, providing a maximum depositional age for the formation. Zircon grains from the middle part of the Yuxi Formation and the lowest part of the Baizhu Formation have very similar, “multi-peaked” age patterns, with five distinctive peaks at ca. 840Ma, ca. 940Ma, ca. 1620Ma, ca. 1850Ma and ca. 2500Ma, and zircon grains within the youngest age population are of the similar Hf–O isotopic features in the two samples, indicating that sediments across the mid-Neoproterozoic unconformity share similar sedimentary provenances and deposited within the similar tectonic setting. Appearance of abundant low-δ18O magmatic zircon grains from these sediments is coincident with regional intraplate anorogenic magmatism at ca. 850Ma, suggesting that both the Sibao and Danzhou groups were most likely deposited in a continental rift basin. Thus, this study strongly argues against the orogenic origin of the regional mid-Neoproterozoic unconformity between them. Our new data suggest that the mid-Neoproterozoic unconformity is likely attributed to a rapid regional crust uplifting (associated with widespread mafic and granitic magmatism), unroofing and basin subsidence related to the proposed mantle plume activity at ca. 825Ma.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Biomass burning is a significant process in the Earth system, driving ecosystem dynamics and changes in global vegetation, and affecting the carbon cycle and climate. Projections of future fire ...activities require an understanding of the connection between fire history and climate in the past. Polar snow/ice contain long-term records of past climates and fire activity and hold great promise to improve our understanding of wildfire patterns. Here, using ultra-high-performance liquid chromatography-tandem mass spectrometry techniques, we quantified three organic compounds (levoglucosan, vanillic, and syringic acids) released by biomass burning in snow samples collected along a 1250-km transect from the coast to the ice sheet summit Dome A in East Antarctica. Results indicate that these tracers are ubiquitous and have reached the ice sheet summit from the continental emissions in the Southern Hemisphere. These compounds showed high levels close to the coastal areas and decreased to a low level on the Antarctic plateau. The snow samples had similar levoglucosan/vanillic acid (∼45) and levoglucosan/syringic acid ratios (∼243) as aerosols from biomass burning. Multivariate analysis indicates that these compounds were likely derived from the burning of grasses and evergreen broadleaf trees that are widespread in Southern Hemisphere than from evergreen conifers that dominate northern hemisphere fire-prone ecosystems. Snow accumulation rate influenced the levels of these compounds, while coexisting ions had little effect on compound contents in the snow. The low concentrations of levoglucosan at inland sites (mean of 2.7 pg mL−1; versus 3.5 and 3.7 pg mL−1 in coastal and transition zones, respectively) could be associated with the oxidation by OH radicals under sunlight. Our analysis demonstrated that the ubiquity of multiple biomarkers from biomass burning in East Antarctic surface snow can provide baseline concentrations for future studies in Antarctica.
Display omitted
•Biomarkers from biomass burning are ubiquitous in the East Antarctic surface snow.•Organic tracers were likely derived from the burning of grasses and deciduous trees.•Biomarkers reached Antarctica through long-range meridional transportation.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The Macquarie Ridge Complex (MRC) on the Australia‐Pacific plate boundary south of New Zealand is an extinct mid‐ocean ridge that has experienced a complex tectonic history and produced highly ...heterogeneous mid‐ocean ridge basalts (MORBs). When and how seafloor spreading ceased along the proto‐Macquarie mid‐ocean ridge remain elusive, and it is unclear how the mantle source of MORBs is affected by the gradual cessation of seafloor spreading at mid‐ocean ridges. To constrain the tectonic evolution of the MRC, the mantle source variations for MORBs at dying mid‐ocean ridges, and the mechanisms of mantle enrichment and asthenospheric heterogeneities, we report 11 pyroxene, plagioclase, basaltic glass, groundmass, and sericite 40Ar/39Ar and one zircon U‐Pb ages for the MRC MORBs. Our data reveal that basalts from the MRC seamounts were erupted between 25.9 and 1.6 Ma and Macquarie Island at ∼10 Ma. Combined age and plate reconstruction results reveal that the cessation of seafloor spreading at the MRC generally propagated from south to north along the ridge. Basalts produced by the then dying Macquarie mid‐ocean ridge at different times on different seamounts/island show a large variation in isotopic compositions and there is no clear correlation between ages and isotopic ratios. The heterogeneity of mantle source for MORBs from the proto‐Macquarie mid‐ocean ridge suggests that the upper asthenospheric mantle is heterogeneous, and such heterogeneity becomes most obvious at dying mid‐ocean ridges where the degrees of partial melting are low and a large range of melt compositions are produced.
Key Points
New ages of basalts from the Macquarie Ridge Complex (MRC) at the Australia‐Pacific plate boundary are reported
The cessation of seafloor spreading was progressed from south to north along the Macquarie Ridge
Upper mantle heterogeneity becomes obvious at dying mid‐ocean ridges where the melting degrees are low and various melt compositions are produced
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Late Permian to Triassic (“Indosinian-aged”) orogenesis and associated magmatism are widespread in southeastern China, but their causes and significance to the regional geodynamic evolution are ...highly controversial. We report in this study integrated in situ U–Pb, Hf and O isotope analyses of detrital zircons from the Middle to Late Permian sediments in southeastern China, and use these data to decipher the sedimentary provenances, regional magmatic barcode and crustal evolution processes. A total of 409 concordant U–Pb zircon analyses on five sedimentary samples exhibits four major age peaks at ca 1870Ma, ca 445Ma, ca 370Ma and ca 280Ma, one “broad” age group of ca 1180–960Ma, and two subordinate peaks at ca 2530Ma and ca 800Ma. All the Archean-aged detrital zircons are oval in shape with clear abrasive imprints, indicating long-distance transport and/or multi-cycled sedimentary processing from unknown Archean sources. In contrast, detrital zircons of the remaining age groups are mostly euhedral to subhedral crystals, suggesting short-distance transport. Zircon ages of ca 1870Ma, ca 1180–960Ma, ca 800Ma, ca 445Ma and ca 280Ma are coeval with exposed magmatic rocks in the Cathaysia Block of southeastern China. The ubiquitous existence of ca 280Ma detrital zircons in the studied sediments implies a widespread occurrence of Early Permian magmatic rocks in southeastern China coastal region.
In situ Hf and O isotope analyses of detrital zircons reveal periods with growth of juvenile continental crust at ca 1870Ma, ca 1400Ma, ca 1140–940Ma, ca 445Ma and c. 280Ma, alternating with reworking and recycling of continental crust at ca 1200Ma, ca 830Ma and ca 370Ma. The ca 280Ma detrital zircons display coherent, negative correlations between εHf(T) and δ18O values, indicating that their igneous source rocks probably formed with reworking of ancient supracrustal materials by mantle-derived magmas in an active continental margin. This interpreted ca 280Ma magmatic event in an active continental margin is coincident with the remarkable change in regional palaeogeography from a carbonate platform in Carboniferous-earliest Permian time to an elongated uplift along the southeastern coastal regions in mid-Early Permian. Therefore, the early Indosinian tectonism and magmatism in southeastern South China most likely reflect the formation of a continental arc by the subduction of the Paleo-Pacific plate in the Early Permian.
► In situ U–Pb, Hf and O isotope analyses on detrital zircons in the Middle to Late Permian sediments from SE China. ► Deciphering sedimentary provenances, regional magmatic barcode and crustal evolution. ► An Early Permian active continental margin in SE China related to subduction of the Paleo-Pacific plate. ► Alternating growths of juvenile continental crust and reworking and recycling of continental crust in the Cathaysia Block.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Thermal ionization mass spectrometry (TIMS) has shown excellent analytical precision for Sr isotopic ratio analysis, even for small masses of material (0.5–10 ng). However, because of the sensitivity ...limit of TIMS, it is still not possible to obtain high precision 87Sr/86Sr isotope ratios for picogram-level sample sizes (30–100 pg) due to the lack of a highly sensitive emitter. This study is the first to employ a highly sensitive silicotungstic acid emitter to measure Sr isotopes at the picogram-level using TIMS. This emitter produces a 3-fold enhancement in the ionization efficiency of Sr and not only significantly reduces the required sample size but also has good external precision. Analyses of the NIST 987 standard yield an external reproducibility (2 RSD, n = 8) better than ±0.013% even for 30 pg of Sr. It is possible to yield an internal precision (2 RSE) of ±0.003% for 100 pg of sample using the default 1011 Ohm feedback resistors. This method was verified by using a suite of silicate reference materials. Replicate digestions and analyses (n = 8) of the basalt standard BCR-2 (87Sr/86Sr = 0.704998 ± 0.000028, 2 SD) at the 326 ± 30 pg level demonstrates that good external reproducibility is reached on ultratrace level silicate samples. This method has a wide variety of potential applications for samples containing ultralow amounts of Sr in geoscience and archeological studies, such as single grains of mica, sphalerite, and pyrite, single mantle melt inclusions, precious extra-terrestrial materials, and human hair to name just a few.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
Display omitted
•2.4 Ga granitoids of the Quanji Massif show high-K I-type and A2-type affinities.•The magmas were derived from early Paleoproterozoic juvenile crust and ancient crust.•The plate ...tectonics did not shut down during the early Paleoproterozoic.
The global plate tectonic regime in the early Paleoproterozoic period is highly debated. Granitoids bear key information to address such a debate. Petrological, geochemical and geochronological studies are conducted on two post-collisional granitoid plutons in the Quanji Massif, northwestern China, to investigate the tectonic regime during this period. The granitoids are composed of syenogranite, monzogranite, and granodiorite, with minor tonalite, which intruded into the Delingha and Hudesheng regions at ca. 2.39–2.37 Ga. These plutons are high-K I-type granitoids with variable Ga/Al ratios, showing some characteristics of A2-type granitoids. They are characterized by enrichment of LILEs and LREEs and depletion of Sr, P, Ti, and Eu. They show depleted Nd and Hf isotope signatures with whole rock εNd(t) = +0.7 to +4.8 and zircon εHf(t) = −1.0 to +7.8, indicating a juvenile crustal growth event at ca. 2.44–2.37 Ga. Our new results together with other coeval post-collisional granitoids in the Quanji Massif suggest that the protracted post-collisional magmatism at ca. 2.39–2.34 Ga occurred just after a short interval of subduction and generation of juvenile magmas before or around ca. 2.4 Ga. Collectively, the formation of these ca. 2.4–2.3 Ga granitoids in the Quanji Massif may correlate with coeval granitoids in the Tarim and North China cratons, and is broadly coeval with magmatism in several other cratons in the West African and Canadian Shields. Thus, the globally well-documented magmatism at early Paleoproterozoic, or Siderian, provides further information for filling up the age gap of the so-called plate tectonic “shutdown” in the early Paleoproterozoic period worldwide. The geologic record therefore suggests no Siderian shutdown of plate tectonics, but instead, continuous global subduction and generation of juvenile magmas from the Archean through the Paleoproterozoic.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The northern Lau back-arc basin (NLB) lavas display a diverse geochemical nature caused by complex geological processes in this region. Independent component (IC) analysis was applied to investigate ...the nature of mantle sources in the NLB, based on a compiled geochemical data set from the NLB, central-southern Lau Basin (C-SLB), Pacific and Indian Ocean ridges, and Samoan islands. We identified three ICs in the five-dimensional space of Sr-Nd-Pb isotopic ratios, which can account for 96.5% of the isotopic variance. The correlations between the ICs and the incompatible trace elements ratios were further used to examine the origin of these ICs. The first IC (IC1) separates Samoan islands (IC1 < −1) from the other groups, and shows negative correlation with (La/Sm)
N
ratios. The second IC (IC2) discriminates mid-ocean ridge basalts (MORBs) from Pacific (IC2 > 0) and Indian (IC2 < 0) Ocean ridges as well as the C-SLB (IC2 > 0) and NLB (IC2 < 0) lavas. IC2 correlates positively with Ba/Th ratios. The third IC (IC3) distinguishes MORBs (IC3 > 0) and back-arc basin lavas (IC3 < 0), and displays a negative correlation with Th/Nb values. On a regional scale in the NLB, there is a broad increase in IC1, IC2, and IC3 from the north to the south. Only IC3 presents obvious decrease from the west to the east, i.e. with decreasing distance from the arc. The geochemical nature and the statistical properties of these ICs suggest that IC1 is related to an enriched mantle component most likely from the nearby Samoan plume, and the IC2 corresponds to a fluid-rich component from the subducting Pacific slab. The IC3 may represent partial melt of recycled subducted sediment or recycled continental crustal materials. The geographic distribution of three ICs supports that the relatively recent mixing of Samoan plume materials with the subduction-metasomatized back-arc mantle may be responsible for the observed geochemical diversity in NLB lavas.
Full text
Available for:
BFBNIB, GIS, IJS, KISLJ, NUK, PNG, UL, UM, UPUK
•Reported the newly identified Laojunshan granite porphyry from the SW Yangtze block (SE Yunnan).•Zircon U-Pb geochronology of the Laojunshan granite porphyry yielded a Neoproterozoic age of ca. ...811–830 Ma.•The Laojunshan granite porphyry recorded both juvenile crustal growth and ancient crustal reworking.•The Laojunshan granite porphyry is responsible for a lithospheric extension that is related to the Rodinia breakup.
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.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The western margin of the Yangtze Block hosts the giant Upper Yangtze Pb-Zn metallogenic province, with the occurrence of >400 carbonate-hosted Pb-Zn deposits. More than 50% of these deposits are ...hosted in carbonate rocks of late Ediacaran to early Cambrian age. Although they have attracted great attention over the past two decades, it is still unclear why such carbonate sequences host so many Pb-Zn deposits and the role that the country rocks played during mineralization. The newly-discovered Maliping Pb-Zn deposit (~6 Mt @ 4.18 wt% Pb and 9.18 wt% Zn) is hosted in early Cambrian strata composed of carbonate and phosphate rocks, black shales, as well as evaporite sulfates, of which the carbonate rocks are the direct ore-hosting rocks. Evidence from mineralogy and the concentrations of ore-forming metals indicate that the phosphate rocks played an important role in providing geochemical barriers during Pb-Zn ore formation. Homogenization temperatures of the primary fluid inclusions in sphalerite and quartz range from 185 to 282 °C, and their salinities vary from 3.39 to 17.17 wt% NaCl equiv. The REE and C-O isotopes imply that the hydrothermal carbonates were formed under relatively oxidizing conditions and that the wall rocks were involved in the Pb-Zn mineralization through dissolution. Sulfur isotopic compositions (δ34S = +7.60–+31.79‰) of sulfides reveal that S2− originated from evaporite sulfates within the ore-hosting strata, and that the black shales acted as an important reducing agent during thermo-chemical reduction (TSR). Pb isotopic ratios of galena (206Pb/204Pb = 17.856–17.973, 207Pb/204Pb = 15.668–15.689 and 208Pb/204Pb = 37.953–38.101) are similar to those of Proterozoic basement rocks in the region. This implies that the basement could be the key source of mineralizing metals. Hence, we propose that: (i) The favorable lithological combination of early Cambrian phosphate rocks, black shales, carbonates and evaporites, as well as Proterozoic basement in the area, were responsible for controlling the majority of Pb-Zn deposits in the late Ediacaran-early Cambrian carbonate sequences in the western Yangtze Block; and (ii) the Maliping Pb-Zn deposit resulted from a combination of mineralized fluids, various trap structures and favorable lithologies, of which the fluids were epigenetic with low to moderate temperatures and salinities.
Display omitted
•>50% of Pb-Zn deposits in the Upper Yangtze province are hosted in the late Ediacaran to early Cambrian strata.•Th and salinities of the Maliping deposit differs from those of typical MVT deposits.•REE-C-O-S-Pb data suggest a mix origin of ore-forming elements.•Phosphate rocks, black shales, carbonates, evaporites and basements were involved in ore formation.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The Alxa Block in northern China has been traditionally considered as the westernmost part of the Archean North China Craton (NCC). However, recent studies revealed that there are few Archean rocks ...exposed in the Alxa Block, and the Paleoproterozoic geology of this block is different from that of the western part of the NCC. Thus, the tectonic affinity of the Alxa Block to the NCC and/or other Precambrian blocks needs further investigations. In this study, we carry out integrated analyses of in situ zircon U-Pb age and Hf-O isotopes as well as whole-rock geochemistry and Nd isotopes for the Neoproterozoic Dabusushan and Naimumaodao granites from central Alxa Block. Secondary ion mass spectrometry (SIMS) U-Pb zircon dating results indicate that the Naimumaodao and Dabusushan granite plutons were formed at ca. 930 Ma and ca. 910 Ma, respectively. These granites are peraluminous (A/CNK value >1.0), and contain peraluminous minerals such as muscovite and tourmaline, similar to those of S-type granites. They are characterized by high zircon ...O values of ca. 8.2 to 12.1 permil, corresponding to a calculated magmatic ...O value of ca. 10.5 to 14.3 permil, variable zircon ...(t) values of ...6.2 to +3.8 (corresponding to Hf model ages of 2.2 to 1.6 Ga) and whole-rock ...(t) values of ...10.1 to ...4.5 (corresponding to Nd model ages of 2.4-1.9 Ga). The petrological and Nd-Hf-O isotopic study indicated that these granites were most probably generated by remelting of dominant (meta)sedimentary rocks in an orogenesis-related compressional environment. There is a clear contrast in the Precambrian geological evolution, including basement rock age data, Precambrian magmatism and detrital zircon age patterns, between the Alxa Block and the NCC. Furthermore, the new in-situ detrital zircon ages on Neoproterozoic (meta)sedimentary rock suggest that Alxa Block is likely related to the Cathaysia Block of South China during the Neoproterozoic, and amalgamated with the NCC since the Early Paleozoic. Thus, our new data suggest that the Alxa Block is most likely a separated Precambrian terrane from the Western Block of the NCC. (ProQuest: ... denotes formulae/symbols omitted.)
PDF
