The Oldest Grey Gneisses and Tonalite-Trondhjemite Granodiorites in the Fennoscandian Shield: ID-TIMS and SHRIMP Data
T. B. BayanovaEvgeniy KunakkuzinPavel SerovEkaterina SteshenkoE. S. BorisenkoА. Н. ЛарионовО. М. Turkina
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Genesis of the oldest continental crust retains a marked trace in the Earth’s evolution over its 4.5 Ga history. Despite ample isotope data on the role of the continental crust in the Earth’s evolution, there has been much debate on the origin of grey gneisses and tonalite-trondhjemite-granodiorites (TTG). Precise U-Pb (ID-TIMS) and SHRIMP data on single zircon for paragneisses and TTG (3158.2 ± 8.2 Ma) have indicated the Central-Kola and Belomorian (White Sea) megablocks of the Fennoscandian Shield to be 3.16 Ga and 3.70 Ga, respectively. The newly obtained ages of zircon from these megablocks indicate the origin of the discrete continental crust to be 3.16 and 3.70 Ga. It is close to the Nordsim zircon data on the Siurua TTG (Finland), which are 3.45 and 3.73 Ga in the core. The new summarized data on the Earth’s oldest rocks (basement and continental crust) indicate the younger age of the rocks in the Fennoscandian Shield as compared to those in Australia (Kronendonk et al., 2019).Keywords:
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We report zircon oxygen isotope ratios and reconnaissance Ti‐in‐zircon concentrations, guided by cathodoluminescence image studies, for detrital zircons up to 4.34 Ga from the Narryer Gneiss Complex of Western Australia. Zircon oxygen isotope results bolster the view that some Hadean (>3.85 Ga) zircon source melts were enriched in heavy oxygen, a sensitive proxy for melt contamination by sediments altered in liquid water. Zircon crystallization temperatures calculated from Ti concentration in pre‐3.8 Ga zircons yield values around 680°C in all cases except for one lower value in a 4.0 Ga grain. Elevated zircon δ 18 O values reported here and elsewhere, combined with low minimum‐melt crystallization temperatures, and analysis of zircon/melt partitioning of rare earth elements (REEs) provide mutually consistent lines of evidence that the Hadean Earth supported an evolved rock cycle which included formation of granitic water‐saturated melts, extensive continental crust, hydrosphere‐lithosphere interactions, and sediment recycling within the first 150 million years of planet formation.
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Due to a lack of rock samples from the Hadean Eon, the Hadean zircons have become an important means of understanding the Earth's earliest history. This study reports the occurrence of a Hadean detrital zircon with a concordia U–Pb age of 4081 ± 71 Ma and 207Pb/206Pb age of 4087 ± 31 Ma from the Paleoproterozoic metasedimentary rocks in the Jiao–Liao–Ji Belt (JLJB) in the North China Craton. The analyzed zircon grain exhibits low luminescence and striped absorption and has relatively high Th/U ratio (0.37), all suggesting an igneous origin. It is euhedral with length/width ratios of 3:2, implying a short distance of transportation from its source. The Hadean age is ~ 570 million years older than the oldest zircon previously identified in the JLJB. This further demonstrates the existence of a Hadean continental crustal remnant in the North China Craton. In addition, to our knowledge, it is the first case of a Hadean zircon being recognized in the Paleoproterozoic sediments on Earth. The documentation of a 4.09 Ga detrital zircon not only provides a geochronological record of the oldest known crustal materials in the JLJB, but also identifies the geological environment for further exploration for the Hadean zircons or even the Hadean rocks.
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While the earliest history of many planetary bodies within the inner Solar System is dominated by intense bombardment, this record is missing from Earth due to active tectonics and erosion. Where-as rocks from the earliest history of Earth are absent, mineral relics, such as ancient detrital zircon concentrated in sediments within the Jack Hills, Narryer, Illara and Maynard Hills greenstone belts of the Yilgarn Craton in Western Australia preserve a record of this time.Shock in zircon: During shock deformation, resulting from hyper-velocity impact, zircon can be modified in crystallographically-controlled ways. This includes the development of planar and subplanar low-angle grain boundaries, the formation of mechanical twins, transformation to the high pressure polymorph reidite, development of polycrystalline microtexture, and dissociation to its dioxide constituents SiO2 and ZrO2.
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Zircon is the preeminent chronometer of deep time on Earth, informing models of crustal growth and providing our only direct window into the Hadean Eon. However, the quantity of zircon crystallised per unit mass of magma is highly variable, complicating interpretation of the terrestrial zircon record. Here we combine zircon saturation simulations with a dataset of ∼52,000 igneous whole rock geochemical analyses to quantify secular variation in relative zircon abundance throughout Earth history. We find dramatically increasing zircon abundance per mass of magma through geologic time, suggesting that the zircon record underestimates past crustal volume even if preservation bias is eliminated. Similarly, zircons were even less likely to crystallise from low-silica magmas in early Earth history than they are today, together suggesting that the observed Hadean zircon record may require a larger volume of generally felsic Hadean crust than previously expected.
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