Search

Hadean (≥4.0 Ga) zircon grains provide the only direct record of the first half-billion years of Earth’s history. Determining accurate and precise crystallization ages of these ancient zircons is a prerequisite for any interpretation of crustal evolution, surface environment, and geodynamics on the early Earth, but this may be compromised by mobilization of radiogenic Pb due to subsequent thermal overprinting. Here we report a detrital zircon from the Jack Hills (Western Australia) with 4486–4425 Ma concordant ion microprobe ages that yield a concordia age of 4463 ± 17 Ma (2σ), the oldest zircon age recorded from Earth. However, scanning ion imaging reveals that this >4.4 Ga apparent age resulted from incorporation of micrometer-scale patches of unsupported radiogenic Pb with extremely high 207Pb/206Pb ratios and >4.5 Ga 207Pb/206Pb ages. Isotopic modeling demonstrates that these patches likely resulted from redistribution of radiogenic Pb in a ca. 4.3 Ga zircon during a ca. 3.8 Ga or older event. This highlights that even a concordia age can be spurious and should be carefully evaluated before being interpreted as the crystallization age of ancient zircon.

Mafic dyke swarms and continental flood basalts record crucial information on mantle dynamics, crustal palaeostress, and supercontinent cycle. However, these mafic rocks are relatively difficult to date using zircon U–Pb methods because autogenous magmatic zircons are rare or absent in the rapidly cooled mafic magmas and most zircons extracted from these rocks are likely captured from their wall rocks. One good example of this dilemma is the Precambrian mafic dykes and layered basites in the Aksu–Wushi area, NW Tarim Craton, from which abundant zircon U–Pb data have yielded age spectra similar to those of the detrital zircons from the associated (meta-)sedimentary rocks. In this paper, we performed the first 40Ar/39Ar dating on separated mineral grains from the mafic dykes (plagioclase and amphibole) and layered basites (plagioclase and pyroxene) in this area. Amphiboles from a mafic dyke cross-cutting the blueschist-bearing Aksu Group yielded a 40Ar/39Ar plateau age of 744.5 ± 2.8 Ma, whereas pyroxenes from a layered basite within the Sugetbrak Formation yielded a 40Ar/39Ar plateau age of 521.0 ± 2.1 Ma. The 40Ar/39Ar plateau age of 744.5 ± 2.8 was interpreted as the crystallization age of the studied mafic dykes and provides a minimum constraint for the third phase of deformation of the blueschist terrane. The mini-plateau age of 521.0 ± 2.1 Ma was interpreted as a minimum crystallization age for basite of the Sugetbrak Formation. We also conducted geochemical analyses on layered basites of the Sugetbrak Formation from the Xiaoenbrak Formation, and the results indicated that these layered basites shared a uniform geochemical characteristic. Combining geochronological and geochemical analyses, we propose that the basites of the Sugetbrak Formation probably recorded a rifting event on the north-western margin of the Tarim Craton during Early Cambrian.

Eoarchean [3.6 to 4.0 billion years ago (Ga)] tonalite-trondhjemite-granodiorite (TTG) is the major component of Earth’s oldest remnant continental crust, thereby holding the key to understanding how continental crust originated and when plate tectonics started in the early Earth. TTGs are mostly generated by partial melting of hydrated mafic rocks at different depths, but whether this requires subduction remains enigmatic. Recent studies show that most Archean TTGs formed at relatively low pressures (≤1.5 GPa) and do not require subduction. We report a suite of newly discovered Eoarchean tonalitic gneisses dated at ~3.7 Ga from the Tarim Craton, northwestern China. These rocks are probably the oldest high-pressure TTGs so far documented worldwide. Thermodynamic and trace element modeling demonstrates that the parent magma may have been generated by water-fluxed partial melting of moderately enriched arc-like basalts at 1.8 to 1.9 GPa and 800° to 830°C, indicating an apparent geothermal gradient (400° to 450°C GPa−1) typical for hot subduction zones. They also locally record geochemical evidence for magma interaction with a mantle wedge. Accordingly, we propose that these high-pressure TTGs were generated by partial melting of a subducted proto-arc during arc accretion. Our model implies that modern-style plate tectonics was operative, at least locally, at ~3.7 Ga and was responsible for generating some of the oldest continental nuclei.

The blueschist-bearing Aksu Group and the associated rocks in the Aksu–Wushi area play a crucial role in understanding Neoproterozoic tectonic evolution of the Tarim Craton. However, the age and tectonic setting of the Aksu Group, and its relationship with the Qiaoenbrak Formation, remain controversial. Previous studies were mainly focused on the well-known Aksu Group from the Aksu area, but little attention has been paid to the equivalent rocks from the Wushi area. In this paper, we present the first detrital zircon LA-ICP-MS U–Pb ages for the Aksu Group metasedimentary rocks from the Wushi area, and we also measure U–Pb ages for detrital zircons from the Qiaoenbrak Formation to figure out its affinity to the Aksu Group. Our data show that the protolith of the metasedimentary rocks of the Aksu Group from the Wushi area was deposited between ca. 790 and 761 Ma, which was earlier than that from the Aksu area ( > ca. 750 Ma). The Qiaoenbrak Formation was deposited between ca. 780 and 744 Ma, probably contemporaneous with or earlier than the metasedimentary rocks of the Aksu Group. Geochemical data for the metasedimentary rocks of the Aksu Group from the Wushi area suggest that they were deposited in an arc-related setting. Combining the detrital zircon age spectra and geochemistry analyses, we propose that the sedimentary rocks of the Aksu Group and the Qiaoenbrak Formation were probably deposited in a similar tectonic setting related to an active continental margin in the northern margin of the Tarim Craton. The Aksu blueschists and the associated Neoproterozoic arc were probably related to the circum-Rodinia subduction system, rather than assembly of the Rodinia or Gondwana in the interior of the supercontinents.

Widespread Paleoproterozoic supracrustal rocks in the Trans-North China Orogen (TNCO) contain important information for the tectonic evolution of the North China Craton (NCC). Here, we report new in situ LA-ICP-MS detrital zircon U-Pb ages, Lu-Hf isotopic data, and whole-rock geochemical data for the Yejishan Group in the Lüliang Complex. The group is a well-preserved volcanic-sedimentary assemblage and can be subdivided into the Qingyangshuwan, Bailongshan, and Chengdaogou formations from bottom up. The Bailongshan Formation consists dominantly of meta-basalts interbedded with several layers of clastic rocks, and the other two formations are composed exclusively of sedimentary rocks. In this study, samples of fourteen clastic rocks and thirteen basalts from the Yejishan Group were collected for measurements based on detailed fieldwork. Detrital zircon U-Pb dating of six clastic samples from the three formations display similar age distributions and give youngest ages between ca. 1870 and 1988 Ma. The youngest age peak of 1870 Ma obtained from the Qingyangshuwan Formation is regarded as the maximum depositional age of the Yejishan Group for the first time. The ca. 1779 Ma mafic dyke intruding into the Yejishan Group constrains the depositional age between 1870 and 1779 Ma. Clastic interlayer in the Bailongshan basalts has a youngest age peak of ca. 1988 Ma, suggesting that the basalts should erupt after 1988 Ma. The time constraint of the Bailongshan basalts in this study disapproves the previous view that the Bailongshan Formation should be removed from the Yejishan Group and assigned to the Lüliang Group. According to the geochemical features, the Yejishan clastic rocks had a chemical immature source mixed of felsic and basaltic magmatic rocks with weak weathering, and suggestive of an active tectonic setting. The basalts show variable LREE enrichment, prominent Nb-Ta, Ti, P anomalies, probably derived from a subcontinental lithospheric mantle metasomatized by sub