Search

Highlights: [•] TTG in the interior of the Dunhuang blocks was formed at ∼2.7–2.6Ga. [•] Tarim experienced two-stage (∼2.7–2.6 and ∼2.5Ga) intrusion of Archean TTG. [•] Tarim was characterized by episodic generation of juvenile continental crust. [•] Paleoproterozoic and Paleozoic tectonothermal events were prevail in the Tarim craton. [•] Paleoarchean continental crust as old as ∼3.4Ga may have existed in the Dunhuang block [Copyright &y& Elsevier]

• Negative Ce anomaly in the Gaoyuzhuang and Hongshuizhuang formations provides the evidence of transient oxygenation of the Mesoproterozoic ocean. • The proposed Ce anomaly depth model provide a quantitative constrain the surface environment of the Yanshan Basin. • Transient oxygenations could have provided vital niches for the evolution of eukaryotes. Predominantly anoxic conditions with a heterogeneous stratified water column are widely accepted for the Mesoproterozoic ocean. However, transient and episodic oxygenation during the Mesoproterozoic has been recently hypothesized, which may have been an important trigger for eukaryote evolution. In this study, we analyzed the Ce anomalies of the Gaoyuzhuang and Hongshuizhuang formations in North China, to investigate the Earth's surface environment during the Mesoproterozoic. Our data of negative Ce anomalies in the Gaoyuzhuang and Hongshuizhuang formations, reaching 0.73 and 0.84, respectively, suggest transient oxygenations of the surface environment. We applied a Ce N /Ce N * depth model to quantify these transient oxygenations and the results suggest that the atmospheric oxygen level would have increased by 0.01–0.03% present atmospheric level (PAL), 0.02–0.05% PAL, and 0.1–0.4% PAL, with the respective initial levels of 0.05% PAL, 0.1% PAL, and 1% PAL, in the Gaoyuzhuang and Hongshuizhuang formations. Our results suggest that although oxygen levels in the Mesoproterozoic were low, transient oxygenation may have occurred in the surface ocean, which would have provided vital niches for the evolution of eukaryotes. [ABSTRACT FROM AUTHOR]

Abstract: A combined study of zircon U–Pb and Lu–Hf isotopes and whole rock major and trace elements and Sr–Nd isotopes has been conducted for 10 tonalitic–trondhjemitic–granodioritic (TTG) and granitic gneisses from the Kongling terrain, the only known Archean microcontinent in the Yangtze craton, South China. The results reveal a significant magmatic event at ∼2.6–2.7Ga, in addition to the previously reported ∼2.9Ga and ∼3.2–3.3Ga magmatism. The ∼2.6–2.7Ga rocks show relatively high REE (530–1074ppm), apparently negative Eu anomaly (Eu/Eu*=0.22–0.35), low #Mg (19.51–22.63) and low LaN/YbN (10.3–24.2). Besides, they have high K-feldspar proportion and relatively evaluated (K2O+Na2O)/CaO, TFeO/MgO, Zr, Nb, Ce and Y contents. Their 10,000×Ga/Al ratios range between 3.00 and 3.54. All these features suggest that the protoliths of these gneisses are A-type granites. Most of the ∼2.6–2.7Ga zircon grains have ɛ Hf(t) values >0 (up to 7.93, close to the depleted mantle value). This clearly indicates a considerably higher proportion of new crustal components in the ∼2.6–2.7Ga granitoids compared to the ∼3.2–3.3Ga and ∼2.9Ga TTGs. Our results support the conclusion of worldwide studies of igneous and detrital zircons that age peaks at 2.65–2.76Ga represent increases in the volume of juvenile continental crust. The present study also confirms the existence of the two older magmatic events in the Kongling terrain. Both whole rock ɛ Nd(t) values (−3.74 to 1.59) and the zircon ɛ Hf(t) values (−11.18 to 3.55) for the ∼2.9Ga TTG and the Hf isotopes of ∼3.2–3.3Ga igneous zircons (−7.37 to 3.12) are chondritic or subchondritic, suggesting that they were mainly generated by reworking of older rocks with a small amount of new crustal additions. [Copyright &y& Elsevier]

• Crustal xenoliths trapped by Guojialing granodiorites display rather low Au contents. • The Precambrian metamorphic outcrop rocks also show low Au contents. • The metal contents are consistent with the metamorphic devolatilization model. • The Precambrian basement was not the main Au source for Jiaodong giant gold deposits. The Jiaodong gold deposits, eastern China define a giant gold province in the Precambrian high-grade metamorphic crustal basement. Given the early metamorphism, the crustal basement would have been so depleted in gold that cannot be the main source for the later Mesozoic gold deposits. However, previously reported gold contents from crustal rocks were variably high (e.g., >5–10 ppb) and often interpreted to reflect the contribution of crustal basement. In this study, after careful field sampling, we obtained high-precision Au and S contents (with Cu and Ag for many samples) to solve the controversial issue. The Au contents were determined by internal standardization to platinum which was obtained from isotope dilution via high-sensitivity Element XR, and/or another independent method by GFAAS after polyurethane foam pretreatment. These methods achieved low procedural blanks (<0.02 ppb) and reliable analyses of low-Au samples. The analyzed metamorphic crustal rocks are from surface outcrops (n = 30) and crustal xenoliths (n = 10) in the Jiaobei terrane, where hosts the predominant Jiaodong gold resources (>85% of 5000 t). The Archean and Paleoproterozoic metamorphic outcrop rocks were sampled far from gold deposits and faults with the minimum effect of ore-forming fluids, and they display overall low Au contents (0.20 ± 0.18 ppb and 0.47 ± 0.29 ppb, respectively). More importantly, the crustal xenoliths trapped in pre-mineralization Guojialing granodiorites were not affected by hydrothermal overprint and better reflect the pristine features of the metamorphic basement. Although the crustal xenoliths contain variable sulfides with a wide range of Cu, Ag and S contents, they show rather low Au contents (0.18 ± 0.15 ppb). These results indicate the strong depletion of Au contents in the Precambrian crustal basement, consistent with the metamorphic devolatilization model. The Precambrian metamorphic crustal basement thus was rather difficult as the main gold source for the Jiaodong gold deposits, suggesting alternative sources. [ABSTRACT FROM AUTHOR]

• The Fe speciation data help bridge the global data gap of Fe species gap during 1500–1400 Ma. • The global Fe speciation data suggest underestimated oxic depositional conditions during the Mesoproterozoic. • Oxic environment could have provided suitable habitats for complex eukaryotes and their subsequent evolution. The Mesoproterozoic (1600–1000 million years ago, Ma) is thought to have experienced important changes in ocean chemistry and eukaryote evolution. Our understanding of global redox conditions remains in its infancy, and redox data derived from the Mesoproterozoic thus far indicate heterogeneity and complexity in the realm of ocean redox, which hampers our understanding of the potential significance of variations in ocean redox on eukaryote evolution. In this study, we report analyses of Fe species and redox-sensitive trace metals of U and Mo from the Jixian Group, including the Wumishan, Hongshuizhuang and Tieling formations, as well as the Xiamaling Formation, in the Yanshan Basin in North China. The Fe speciation data from the Wumishan, Hongshuizhuang, and Tieling formations first reported in this study help bridge the global data gap of Fe species data during 1500–1400 Ma. Our data suggest likely oxic bottom water conditions for the Wumishan Formation and ferruginous bottom water conditions for the Tieling Formation. The Fe species data can distinguish ferruginous conditions for the upper Hongshuizhuang from sulfidic depositional conditions for the lower-middle Hongshuizhuang Formation. Based on Fe species and trace metal data, redox conditions likely varied from anoxic conditions with transient euxinia in the lower-middle Xiamaling Formation to oxic conditions in the upper Xiamaling Formation. The new Fe speciation data from the Yanshan Basin, when combined with previously published data, show a surprisingly high proportion of middle Proterozoic samples with very low highly reactive to total iron ratios. This suggests either a relatively large extent of oxic seafloor, or non-uniformitarian iron cycling. Resolving this discrepancy will be key to understanding the extent of suitable habitats for complex eukaryotes. [ABSTRACT FROM AUTHOR]

• Rutile xenocrysts were captured from the basement by the Yangyuan carbonatites. • The Zr-in-rutile thermometer records formation of the rutiles at 743–932 °C. • Cr and Nb contents and Hf isotopes in rutiles support pelitic granulite protoliths. • U-Pb rutile cooling ages peak at 1810 ± 4 Ma, 1735 ± 5 Ma, and 1673 ± 8 Ma. • In combination with dating results from other systems yields episodic cooling path. The amalgamation of the North China Craton along the Trans-North China Orogen at ~1.85 Ga has been widely discussed, but its thermal evolution after amalgamation is relatively poorly constrained. This study presents U-Pb ages, trace element compositions, and Hf isotopes of rutile xenocrysts carried by the Yangyuan Cenozoic carbonatites, revealing an episodic cooling history of the Trans-North China Orogen. The Zr-in-rutile thermometer records formation of the rutiles under granulite facies at 743–932 °C. Cr and Nb concentrations in rutiles support a pelitic protolith, which is consistent with 176Hf/177Hf ratios that are close to zircons from pelitic granulites in the orogen. U-Pb ages of the rutiles are cooling ages, for which three populations can be clearly delineated. In combination with dating results from other systems, three episodes of cooling can be identified: [1] active cooling after the ~1.85 Ga peak metamorphism, with a cooling rate increasing from 7.9 °C/Ma in the period 1850–1810 Ma to 15.3 °C/Ma at 1810–1792 Ma; [2] active cooling after mantle upwelling at ~1.78 Ga, with cooling rate increasing from 8.7 °C/Ma at 1780–1735 Ma to 17.2 °C/Ma between 1735 and 1719 Ma; [3] passive cooling after a rifting event at ~1.68 Ga, with cooling rate decreasing from 20.6 °C/Ma at 1680–1673 Ma to 6.9 °C/Ma in the period 1673–1633 Ma. The first two episodes correspond to exhumation-related cooling, while the last episode resulted from the rift-to-drift transition of the North China Craton due to increasing distance from the heat source during break-up of the Columbia supercontinent. [ABSTRACT FROM AUTHOR]