Growth of the Archean sialic crust as revealed by zircon in the TTGs in eastern Finland

U–Pb age determinations on zircon from granitoids in the Archean of eastern Finland were conducted using the SIMS, LA-ICP-MS and TIMS techniques, with an emphasis on low-HREE granitoids. The oldest rocks in the Fennoscandian Shield are 3.4–3.5 Ga. Several samples were collected close to these rocks, but none of the samples were as old, indicating that the oldest rocks are only small, possibly allochthonous fragments in the Neoarchean bedrock. Some tonalite–trondhjemite–granodiorite (TTG) samples yielded homogeneous 2.72–2.73 Ga zircon populations, and in these samples, the initial εNd was also close to the depleted mantle (DM) values. However, several granitoid samples with a main zircon population of 2.7–2.8 Ga had 2.9–3.2 Ga grains or inherited cores, and in some samples, all grains were of 2.9–3.0 Ga. In these samples, the εNd value was also close to zero or slightly negative. These features suggest that apart from the juvenile Neoarchean magmas, the abundance of reworked 2.9 Ga material is considerable in the Archean crust, which developed during successive juvenile magmatic inputs that melted and assimilated the older sialic crust. The low- HREE geochemical character of granitoids has no correlation with their age, with the low-HREE granitoids yielding an age span of 2.72–2.98 Ga.

Detrital zircon populations of the eastern Laurentian margin in the Appalachians

Newly compiled U-Pb detrital zircon data from eight geographic domains along the eastern Laurentian margin from Newfoundland (Canada) to Alabama (United States) show a highly consistent signature along strike, with only minor local variations. The Precambrian signature is characterized by a small ca. 2.7 Ga population and a major ca. 1.9–0.9 Ga population that peaks at ca. 1.2–1.0 Ga. Detrital zircon populations are from Laurentian Archean crust (ca. 2.7 Ga population), Paleoproterozoic orogens (ca. 1.9–1.6 Ga), the Granite-Rhyolite Province (ca. 1.5–1.4 Ga), and the Elzevir terrane and Grenville Province (ca. 1.3–0.9 Ga). The Mesoproterozoic populations vary in size depending on proximity to the ca. 1.5–1.4 Ga Granite-Rhyolite Province, the ca. 1245–1225 Ma Elzevir terrane, and the ca. 1.2–0.9 Ga Grenville Province. A middle Ordovician zircon population varies in size along strike depending on input from the Taconic orogenic belt, but it is strongest in the northern Appalachians. Because of the general along-strike consistency in detrital zircon age populations, the compilation of all 7534 concordant U-Pb detrital zircon data can be used in future U-Pb detrital zircon studies as an indicator for eastern Laurentian margin sources.

Lithostratigraphic and structural data from Hardangervidda, southern Norway supporting extended interaction between Avalonia and Baltica

Lithostratigraphic and structural data from the Early Paleozoic Hardangervidda Group in southern Norway indicate that SW Baltica was affected several hundred kilometres NE of the inferred collision zone with Avalonia (Thor Suture). The first sign of plate interaction was the deposition of a 50–60 m-thick quartz arenite (Holberg Formation) of Floian/Dapingian age in an otherwise mud- and carbonate-dominated shelf. An overlying 5–6 m-thick marble unit of Dapingian–Darriwilian age (‘Orthoceratite Limestone’) marks a change into greenish-grey, calcareous phyllite, locally with beds of impure marble and poorly sorted metasandstone (Solnut Formation).A series of décollement folds (D2 structures) with axes orientated NW–SE (cross-folds) which superimposed a thin-skinned fold thrust with a NE–SW trend (D1 structures) are interpreted as having developed during the progressive underthrusting of Baltica underneath Avalonia. Support for this model is seen in detrital zircon populations: with the Holberg and underlying formations having a Baltican signature, in contrast to the overlying Solnut Formation with a peri-Gondwana signature, including a distinct Late Neoproterozoic zircon population. It is further speculated that the c. 471–458 Ma Garborg eclogite and surrounding paragneisses in the Stavanger area are related to the suture zone between Baltica and Avalonia rather than being related to the Iapetus Ocean and Laurentia, as generally thought.

Zircons from a Pegmatite Cutting Eclogite (Gridino, Belomorian Mobile Belt): U-Pb-O and Trace Element Constraints on Eclogite Metamorphism and Fluid Activity

This report presents new data on U-Pb geochronology, oxygen isotopes, and trace element composition of zircon from a pegmatite vein crosscutting an eclogite boudin on Stolbikha Island, Gridino area, Belomorian mobile belt (BMB). The zircon grains occur as two distinct populations. The predominant population is pegmatitic and shows dark cathodoluminescence (CL); about a third of this population contains inherited cores. The second zircon population is typical of granulite and exhibits a well-defined sectorial (mosaic) zoning in CL. Both the inherited cores and sectorial in CL zircons appear to have been captured from metabasites as xenocrysts during the pegmatite vein formation. A U-Pb age of 1890 ± 2 Ma for the main zircon population is interpreted as the age of the pegmatite injection. This value is close to the age threshold for the BMB eclogites (~1.9 Ga) and unambiguously defines the upper age limit for the eclogite metamorphism. The pegmatite formation is thus related to partial melting events that occurred during the retrograde amphibolite-facies metamorphism shortly after the eclogitization. A U-Pb date of 2743 ± 10 Ma obtained for the sectorial in CL zircons is considered as the age of the granulite-facies metamorphism established previously within the BMB. The values of δ18O in the zircon populations overlap in a broad range, i.e., δ18O in the pegmatitic zircons varies from 6.1‰ to 8.3‰, inherited cores show a generally higher δ18O of 6.7–8.8‰, and in the captured granulitic zircons δ18O is 6.2–7.9‰. As a result of fluid attack during the final stage of the pegmatite vein formation, the composition of the pegmatitic zircons in terms of non-formula elements (REE, Y, Ca, Sr, Ti) has become anomalous, with the content of these elements having been increased by more than tenfold in the alteration zones. Our data provide new constraints on the timing of eclogite metamorphism within the BMB and show that the late-stage pegmatite-related fluids exerted a very pronounced influence on trace element abundances in zircon, yet had no significant impact on the isotopic composition of oxygen.

First results of dating detrital zircons from the late precambrian quartzite-schist sequences of Chu block (Southern Kazakhstan)

U-Pb geochronological studies of detrital zircons from quartzite-schist sequences of the Akbastau Formations of the Chu block (northwest of the Chu-Kendyktas terrane, Southern Kazakhstan) have been provided. The concordant ages of detrital zircons are predominantly within the intervals of 1672-2115 Ma with peaks at 1697, 1780, 1857 and 2066 Ma. Individual zircon grains display ages of 2291-2332 Ma with peaks at 2303 and 2322 Ma. Neoarchean ages 2608-2747 with peak at 2681 Ma characterize another significant zircon population. The lower limit of deposition for the Akbastau Formations of the Chu block, corresponding to the youngest statistically significant zircon population, is estimated at 1,7 billion years.

A Yilgarn seed to the Pilbara Craton (Australia)? Evidence from inherited zircons

Knowledge of the age and compositional architecture of Archean cratonic lithosphere is critical for models of geodynamics and continental growth on early Earth, but can be difficult to unravel from the exposed geology. We report the occurrence of numerous >3.7 Ga zircon crystals in 3.45 Ga rhyolites of the eastern Pilbara Craton (Western Australia), which preserve evidence for an Eoarchean meta-igneous component in the deep Pilbara crust. This inherited zircon population shares similar and distinctive age and Hf-O isotope characteristics with the oldest gneissic components of the Yilgarn Craton ∼500 km farther south, suggesting a common ca. 3.75 Ga felsic crustal nucleus to these two Archean granite-greenstone terranes. We infer a pivotal role for such ‘seeds’ in facilitating the growth and persistence of Archean continental lithosphere.

Laurentian origin of the Cuyania suspect terrane, western Argentina, confirmed by Hf isotopes in zircon

The proto-Andean margin of Argentina consists of several suspect terranes, the origins of which are disputed. The Cuyania (greater Precordillera) suspect terrane was originally interpreted to be of southeast Laurentian affinity, but more recently a southwestern Gondwanan provenance has been argued. Both potential source regions comprise Mesoproterozoic rocks, but we show they are isotopically distinct, using previously published zircon Lu-Hf data. Detrital zircon εHf data from southwestern Gondwana (Namaqua-Natal belt) show no correlation with new zircon U-Pb and Lu-Hf data from Cuyania, suggesting that Gondwana was not the source of these sediments. Rather, detrital zircons from Cambrian strata in Cuyania yield Mesoproterozoic zircons with depleted εHf that correlate to the Grenville margin of Laurentia, and a ca. 535 Ma zircon population sourced directly from rift-related rocks of the Ouachita Embayment, thus recording rifting and drifting of Cuyania from Laurentia. By contrast, zircons from Middle to Late Ordovician strata of Cuyania record a larger range of εHf values, correlated with Western Sierras Pampeanas Mesoproterozoic basement inliers of Argentina. These synorogenic clastic deposits record the Ordovician arrival of Cuyania at the proto-Andean margin of Gondwana. The new data require the terrane boundaries of Cuyania to be redefined, thereby excluding Western Sierras Pampeanas basement inliers. The results verify the Laurentian microcontinent model for the origin of Cuyania.

Stepwise chemical abrasion ID-TIMS-TEA of microfractured Hadean zircon

The Hadean Jack Hills zircons represent the oldest known terrestrial material, providing a unique and truly direct record of Hadean Earth history. This zircon population has been extensively studied via high spatial resolution, high throughput in situ isotopic and elemental analysis techniques such as secondary ionization mass spectrometry (SIMS), but not by comparatively destructive, high-temporal-precision (

O GRUPO IBIÁ (FAIXA BRASÍLIA MERIDIONAL): EVIDÊNCIAS ISOTÓPICAS SM-ND E U-PB DE BACIA COLISIONAL TIPO FLYSCH

O Grupo Ibiá inclui as formações Cubatão e Rio Verde. A primeira, composta por metaconglomerado suportado pela matriz, com intercalações de metapelito e quartzito, ocorre em lentes esparsas sobre discordância erosiva no topo do Grupo Canastra. Datações U-Pb em grãos detríticos de zircão deste conglomerado indicam idade máxima de sedimentação em ca. 1190 Ma, e mostram o mesmo amplo espectro de valores dados pelo Grupo Canastra. A Formação Rio Verde, composta de clorita-muscovita xisto com lentes de quartzito, repousa sobre as unidades antes referidas. Grãos de zircão do xisto Rio Verde fornecem idades dominantemente menores que 1000 Ma, com a média da moda dos menores valores em ca. 639 Ma. Esta população de zircão consiste de grãos mal arredondados a euédricos. Os dados analíticos, incluindo Sm-Nd (Nd(640 Ma) de -0,1 a 0,5 Ma e idades-modelo em 1,2 Ga) e a composição mineralógica do xisto Rio Verde sugerem protólitos provenientes de fontes ricas em rochas ígneas intermediárias a máficas, tais como arcos magmáticos e ofiolitos. Em conclusão, o Grupo Ibiá representaria bacia colisional (flysch), relacionada a frentes de empurrão da Faixa Brasília Meridional.Palavras-chave: geocronologia U-Pb, bacia orogênica, Grupo Ibiá, Faixa BrasíliaABSTRACT: THE IBIÁ GROUP (SOUTHERN BRASÍLIA BELT): ISOTOPIC SM-ND AND U-PB EVIDENCE FOR A COLLISIONAL FLYSCH-TYPE BASIN. The Ibiá Group includes the Cubatão and Rio Verde formations. The first consists of matrix-supported metaconglomerate with intercalations of metapelite and quartzite, occurring in sparse lenses on the top of the Canastra Group. U-Pb ages of detrital zircon grains from the Cubatão conglomerate and Canastra quartzite show similar wide age spectra and youngest values around 1190 Ma. The Rio Verde Formation, consisting of chlorite-muscovite schist and quartzite lenses, overlies the Cubatão Formation and Canastra Group. U-Pb data from zircon grains of the Rio Verde schist show a great dominance of ages younger than 1000 Ma, with a mean age of the youngest values around 639 Ma. The younger zircon population shows poorly rounded to euhedral zircon crystals, some of them of volcanic origin. Lithochemical data, including Sm-Nd isotopic data (Nd(640 Ma) from -0.1 to 0.5 and Tdm model ages around 1.2 Ga), together with the mineralogical composition suggest provenance from intermediate to mafic igneous sources for the Rio Verde sediments. In conclusion, the Ibiá Group is related to a collisional (flysch) basin associated with thrust fronts along the Southern Brasília Belt.Keywords: U-Pb geochronology, orogenic basin, flysch, Ibiá Group, Brasília Belt