Precambrian Research最新文献

The recognition of unconformities is important for determining the long-term evolution of orogenic belts. We document an unconformity between Tonian and Cryogenian strata of the Nanhua Basin of South China. This stratigraphic break has previously been termed the Xuefeng Movement and is developed within the Yangtze Block and western Cathaysia Block. Deformation characteristics and detrital zircon U-Pb geochronology from rock units underlying and overlying the unconformity (i.e., the Niuguping and Gucheng formations, respectively) from the Anhua region of the northern Hunan Province in the southeastern Yangtze Block are documented. The Niuguping Formation displays a set of top-to-northwest imbricate thrust faults and related fault-propagation folds, NEE-plunging subparallel crenulation lineations, a foliation, and shows widespread greenschist facies metamorphism. In contrast, the overlying Gucheng Formation exhibits no obvious stratal deformation (apart from a mild inclination of bedding) and is unmetamorphosed. Detrital zircon age spectra of the samples from the Niuguping Formation are characterized by unimodal age peaks of ca. 750–730 Ma, while those of the samples from the Gucheng Formation yield a multimodal age distribution, with a few syn-sedimentary detrital zircons (<700 Ma), and much older zircons with age peaks of ca. 800 Ma, 2.1–1.9 Ga, and 2.6–2.3 Ga. Combining this data with previous geochronology indicates deposition of the Niuguping and Gucheng formations in the late Tonian (725–720 Ma) and Cryogenian (690–660 Ma), respectively. The time gap across the unconformity is some 30 Ma (720–690 Ma). The unconformity and associated structural and metamorphic break extend across the Yangtze Block and western Cathaysia Block, defining a major regional orogenic event. The significantly different detrital zircon spectra of strata across the unconformity suggest a dramatic change in the source due to the Xuefeng Orogeny in the Yangtze Block. Zircon U-Pb-Hf isotopic evidence indicates that detritus of the Niuguping Formation below the unconformity was derived from syn-depositional magmatic activity in the northern Yangtze Block, while a significant increase of Archean to Paleoproterozoic ages for the Gucheng Formation, suggests that the Xuefeng Orogeny resulted in crustal thickening and denudation in the northern Yangtze Block. These changes in provenance, along with changing deformational and metamorphic patterns, indicate evolution of the Nanhua Basin from a compressional to an extensional back-arc basin, which we infer is related to tectonic switching from an advancing to a retreating subduction system in the northern Yangtze Block at the time of the unconformity.

The Paraguay Belt and Brasília Orogen, the western and eastern segments of the Tocantins Province, respectively, recorded the interactions among the Amazonian, São Francisco-Congo Craton, and Paranapanema Block during the amalgamation of Western Gondwana. The Paraguay Belt joins the Brasília Orogen in the Transbrasiliano Lineament, which separates metasedimentary rocks to the west from metavolcano-sedimentary rocks to the east. This study focuses on four geological units cropping out in Brazil, between the Mato Grosso State southeastern region and the Goiás State western region: the Nova Xavantina Metavolcano-sedimentary Sequence (NXS), the Cuiabá Group (CG), the Barra do Garças-Coxim Unit (BGCU), a newly identified register of foreland sedimentation, and the Bom Jardim de Goiás Metavolcano-sedimentary Sequence (BJGS). Whole-rock elemental geochemistry and Nd-Sr isotopes, along with zircon U-Pb-Hf-O systematics, provide insights into the evolution of this domain.

Metavolcanic rocks from the NXS suggest that it represents a back-arc extensional tectonic environment with oceanic spreading (zircon age of ca. 710 Ma with δ¹⁸O of 4.8 ‰ and chondritic εHf_(t) formed along the southeastern edge of the Amazonian Craton. Metarenites preserve bimodal sources of ca. 0.85 Ga and ca. 1.90 Ga, while lithic metarenites exhibit Mesoproterozoic zircon grains suggesting sources from both the Amazonian Craton and the Paranapanema Block. The maximum depositional age of the lithic metarenites is ca. 715 Ma, coinciding with the age of the volcanic peak. The CG is interpreted as passive margin deposits through the erosion of depleted Mesoproterozoic (1.50–1.45 Ga and 1.23–1.17 Ga) and more evolved Paleoproterozoic (1.97–1.77 Ga) rocks from the Amazonian Craton.

Metasedimentary rocks from the BGCU present age peaks of 800–600 Ma, recording the input of juvenile material into the basin as revealed by zircon grains of ca. 716 Ma and ca. 810 Ma with δ¹⁸O of 4.7–4.8 ‰ and εHf_(t) of + 7. These findings indicate Goiás Magmatic Arc juvenile sources and disclose the evolution of the unit towards a continental arc, as suggested by more evolved zircon grains younger than ca. 700 Ma. Gneisses of ca. 690–655 Ma occur as basement rocks of the BGCU and suggest prolongation of the Goiás Magmatic Arc westward within the Transbrasiliano corridor.

The causes of early metazoan diversification during the Ediacaran-Cambrian transition interval are controversial, partly because the global correlation of the Ediacaran-Cambrian boundary remains problematic due to a lack of unambiguous stratigraphic markers. Here we report geochemical data from two fossiliferous sections (Majada de Andaluz and Vía Verde) that straddle the Ediacaran-Cambrian interval in the Central Iberian Zone, Spain. Carbon isotope chemostratigraphy reveals a rise to a low positive δ¹³C_carb plateau at the base of the Majada section, which is associated with the first appearance of Cloudina and a pristine Sr isotope value of 0.708512, suggesting that the lower Villarta Formation correlates with the terminal Ediacaran strata in China and Namibia. A prominent negative δ¹³C_carb excursion is also revealed from the lower member of the Villarta Formation at Majada, likely corresponding to the global basal Cambrian carbon isotope excursion (BACE). Nitrogen isotope data from the correlative Vía Verde section reveal a similar trend to that found in coeval strata of South China, and indicate a change from predominantly anaerobic to aerobic nitrogen cycling in Central Iberia. By correlating sections in which both the BACE excursion and Treptichnus pedum are recognized, it appears that T. pedum is exclusively found above the BACE, which postdates the last appearance of Cloudina and a negative δ¹³C_carb plateau. Further studies of sections with both fossil and isotopic control will help to constrain the timing and causation of Ediacaran-Cambrian bioradiations as well as their relationship to global carbon cycle perturbations.

Recognizing the deposits of linear megadunes in ancient aeolian successions is challenging when the original bedform topography is not preserved. The present study provides a detailed analysis of the depositional architecture of an ∼ 80 m-thick interval of the Mesoproterozoic Galho do Miguel Formation (SE Brazil) to reconstruct the original morphologies of linear megadunes and to understand their dynamics. The depositional architecture reveals compound cross-bedded sets, which transition laterally into thick low-angle cross-stratified sets and planar-parallel sandstone beds. The depositional features indicate the presence of complex megadunes separated by adjoining wide dry inter-megadune areas, which were themselves flooded seasonally by a water table that rose above the depositional surface. Linear megadunes experienced two phases of development: vertical accretion and subsequent episode of lateral migration. During the vertical accretion phase, high sediment supply, typical of Precambrian aeolian systems, and the convergence of winds from two distinct directions promoted high rates of megadune growth, bedform elongation, and the formation of superimposed dunes. The lateral migration phase was characterised by a component of lateral bedform shift relative to the main along-crest sediment transport direction; this prevented preservation of a bimodal pattern of dune cross-strata azimuths. The accumulation of these deposits occurred via a combination of megadune climbing that occurred with progressive rise of the water-table level. The water-table rise hindered cannibalization of the lower parts of the migrating megadunes, allowing the accumulation and subsequent preservation of megadune sandstone packages. The rise of the water table was also responsible for the accumulation of thick dry inter-megadune strata. The unusual thickness of linear megadune and inter-megadune deposits of the Galho do Miguel Formation, as compared with Phanerozoic examples, are attributed to the interplay between the barren conditions of the early Earth, the morphodynamics of the linear megadunes, and the seasonal impact of the water table at the accumulation surface. Without a water-table control, opportunities for megadune accumulation and preservation were likely limited in Precambrian ergs.

Detrital zircons from different lithostratigraphic units of Supersequence 1 across the Neoproterozoic northwest Officer and Yeneena basins were investigated using LA-ICPMS U−Pb and Lu − Hf isotope analyses to understand the source of detritus and potential links to regional tectono-magmatic events, as well as the crustal evolution of the sedimentary sources terranes. All data show that Proterozoic detritus dominate the age spectra of samples from the Yeneena Basin and northwest Officer Basin, with signatures varying in the heights of peaks rather than the presence or absence of age groups in different formations. The Archean and Proterozoic ages overlap with known magmatic events in the Pilbara Craton, Fortescue Basin, Rudall Province, Capricorn Orogen, Musgrave Province, Madura Province, and the Coompana Province suggesting these terranes are possible sources of the detritus. The detrital zircon population of the samples have main age peaks in the range 1300 to 1000 Ma, suggesting magmatic rocks of the Rudall and Musgrave provinces are the main sources of detrital zircons, which can be differentiated by Hf signature. The distal Musgrave Province persistently dominates the provenance of detritus in the northwestern Officer and Yeneena basins, while detritus from the proximal Rudall Province contributes to a lesser extent. This suggests that the distal Musgrave Province remained emergent during the evolution of the northwestern Officer Basin and the Yeneena Basin, whereas the proximal Rudall Province potentially became buried by the accumulating sedimentary rocks and contributed less detrital zircons. The 833.6 ± 1.4 Ma maximum depositional age obtained in this study for the sedimentary rocks of the northwestern Officer and Yeneena basins point to the approximate time of the start of basins development. The extensional event in this age period is related to emplacement and extrusion of the ca. 830 Ma Willouran Large Igneous Province (LIP). The Duke gabbro sills observed in the Yeneena Basin are part of this LIP, which extends from the Adelaide Rift Complex, through the Musgrave Province, central–northwest Officer Basin and into the Yeneena Basin. The sediment source terranes suggest a southeast–northwest oriented depositional system between central and Western Australia, possibly because of intracontinental rifting related to widespread mafic–ultramafic magmatic events that could include the earlier Ngaanyatjarra Rift formed by the 1090–1040 Ma Warakurna LIP as well as the ca. 830 Ma Willouran LIP, synchronous with the breakup of Rodinia in the mid-Neoproterozoic.

The Central Asian orogenic belt (CAOB) is one of the largest Phanerozoic accretionary orogens in Earth’s history with myriad metal deposits. Mesoproterozoic basements are sporadically preserved along the southern CAOB. Their tectonic affinity remains controversial, which hampers us from better understanding the basic architecture and Phanerozoic tectonic evolution of the CAOB. This study identified Mesoproterozoic sedimentary rocks in the northeastern Altyn Tagh, southeast of the Tarim craton, from which detrital zircons have remarkable U-Pb age peaks at ca. 1.7 and 1.4 Ga and highly variable ε_Hf(t) values from −9 to + 14, resembling those from the southern CAOB. Based on provenance tracing and comparison of available geological records, the Mesoproterozoic basements in the southern CAOB show an affinity with the Rondonian-San Ignacio Province in western Amazonia associated with the peripheral accretion of the Nuna supercontinent and were subsequently fragmented and dispersed over 3000 km in distance. Predominately juvenile 1.8–1.3 Ga magmatic rocks with a scarcity of ca. 1.4 Ga ones in southern Laurentia and southwestern Baltica preclude a possible link. Likewise, a paucity of ca. 1.4 Ga zircons and incomparable isotope signatures in North China, India, and Australia challenge their affinities with the CAOB’s basements. This work focuses on the tectonic attribution of the Mesoproterozoic basements in the southern CAOB and highlights the energetic Mesoproterozoic era, during which the accretionary orogeny was intensely operative along the periphery of the Nuna supercontinent.

It has been generally agreed that changes in Earth surface systems are the key factors to trigger the Great Oxidation Event (GOE) at about 2.4 Ga. In the present study, we selected banded iron formation (BIF) samples that formed at ∼2.54 Ga (before the onset of the GOE) and ∼2.28 Ga (after the onset of the GOE) from the North China Craton (NCC) for Li isotope investigations, which represent an effective tool for studying continental silicate weathering processes, to reveal possible changes in Earth’s surface systems before and after the onset of the GOE. Using the Li isotope fractionation coefficients between iron oxide and fluid first obtained by our synthetic experiments, we demonstrate that the Li isotopic composition (δ⁷Li values) of ancient seawater increased from ∼+10.5 ‰ at 2.54 Ga to ∼+15 ‰ at 2.28 Ga. Monte Carlo simulations based on mass balance show that changes in the riverine Li isotopic composition are the main reason for the variations in the δ⁷Li values of seawater during this period. In view of the present data, enhanced erosion rates caused by uplifted mountains and increased secondary mineral formation in floodplains are the main reasons for this riverine change. The close coupling of δ⁷Li values with the ⁸⁷Sr/⁸⁶Sr values of seawater, εHf values and δ¹⁸O values of zircon and preserved orogeny length over time indicate the assembly of supercratons before GOE initiation. The assembly of small and scattered landmasses could be the cause of mountain uplift, and thus enhanced erosion rates bring more nutrients to the ocean through collisional orogenic processes accompanied by these convergences, promoting aerobic photosynthesis, e.g., the participation of cyanobacteria, disrupting the balance between oxygen production and consumption, and finally triggering the GOE.

Rutile provides crucial insights into the evolution of plate tectonics and variations in tectonomagmatic modes over Earth’s history primarily because rutile predominantly tracks higher-pressure metamorphism (>1.0–1.2 GPa). Comparison of rutile age and compositional data with diverse proxies tracks the presence of low temperature/pressure (T/P) metamorphic conditions, enabling the tracing of plate tectonics onset and evolution. We present a new compilation of rutile U-Pb ages that lag zircon ages by approximately 50 Myr, potentially indicating cooling ages of metamorphic belts. The earliest rutile age peak (ca. 1.8 Ga) aligns with the oldest evidence for low T/P metamorphic conditions. This coincides with a shift in the T/P distribution of rutile whereas the T/P of Archean rutile forms a tight normal distribution. In the Paleoproterozoic, the rutile T/P distribution is skewed to lower T/P, following the broader pattern of the full metamorphic record. The degree of lower T/P skew increases through time until the lower T/P population dominates the rutile record from the Neoproterozoic to the present. These data further highlight shifts in dominant metamorphic modes that can be tied to the evolution of the tectonic system, with early Archean metamorphism being associated with plume-driven vertical tectonics or hot, shallow subduction and post-Archean metamorphism being associated with progressive less plume-driven vertical tectonics and colder, steeper subduction. Although the frequency of rutile is lower during the Mesoproterozoic, the rutile T/P during this time follows the progressive skewing pattern from the Paleoproterozoic to the present and is likely inconsistent with a phase of interrupted tectonics characterized by stagnant lid.

The Dharwar Craton (DC) in southern India is one of the classic Archean terrains, preserving an evolutionary history from the Paleoarchean to Mesoarchean. The present study attempts to understand the origin and evolution of mafic (amphibolites) and ultramafic (harzburgite) rocks from the Holenarsipur Greenstone Belt (HGB), to decipher the evolution of the cratonic core of the Western Dharwar Craton (WDC). Relict olivine and orthopyroxene pseudomorphs and Neodymium isotopic compositions reveal that harzburgites from the HGB represent a preserved section of the early Archean mantle, re-fertilized by fluid/rock interaction during serpentinization. The ultramafic and mafic rocks from the southern and northern parts of the belt exhibit coherent whole-rock geochemical and Nd isotopic compositions. Ultramafic rocks were derived from a depleted mantle source (ƐNd_(t=3300)= +0.65 to +8.70) while, the amphibolites were generated by partial melting of an enriched mantle source (ƐNd_(t=3300) = -4.16 to +0.92). The ultramafic rocks with fore-arc affinity may signify the prevalence of a subduction setting during the early stages of the first accretionary event in the WDC. The compositionally uniform mafic rocks from the HGB with oceanic plateau affinity affirm a plume environment. Our study establishes the prevalence and transition from vertical to horizontal tectonics during the Paleoarchean-Mesoarchean evolution of the WDC.