Search

Zircon is not a common accessory mineral of banded iron formation (BIF), but it occurs in all BIF samples from the Kostomuksha Greenstone Belt (KGB) in the Karelian Craton of the Fennoscandian Shield. The Kostomuksha BIF is of Algoma type and is part of three variably old rock associations: BIF-1 is part of a sedimentary sequence in Mesoarchean (2.87–2.84 Ga) basaltic komatiites with dacites, BIF-2 is located in Meso-Neoarchean (2.8–2.79 Ga) felsic volcanics, and BIF-3 is part of a Neoarchean (2.76–2.74 Ga) greywacke unit with felsic volcanics. Analysis of zircon has revealed grains with cores and several generations of rims. The U-Th-Pb systems of zircon indicate that its isotopic age varies considerably from 2.98 to 1.89 Ga, forming up to 4 age clusters in each sample. Most samples contain single inherited grains. The bulk of zircon in BIF is younger than the host rocks and is interpreted as metamorphic. Most of metamorphic zircons display an oscillatory zoning, and their Th/U ratio varies from 0.01 to 4.7, i.e. their origin is hard to interpret reliably, based on the above characteristics. Paleoproterozoic (1.89–1.85 Ga) zircons from BIF-1 and 3, occurring as rims and individual grains, are related to local tectono-thermal processes simultaneous with the formation of the Svecofennian orogen and were described for the central Karelian Craton for the first time. Thus, both inherited and metamorphic zircons have been revealed in Meso-Neoarchean BIF metamorphosed repeatedly in the Neoarchean-Paleoproterozoic under up to amphibolite-facies conditions. All metamorphic generations of zircons from BIF are correlated with the tectono-thermal events in the Karelian Craton, including the effect of the Svecofennian orogeny. [ABSTRACT FROM AUTHOR]

Two episodes of late Neoarchean-early Paleoproterozoic and late Paleoproterozoic garnet-bearing leucogranite, named the Baotou and Jining–Liangcheng garnet-bearing leucogranite are recognized from the Khondalite Belt in the North China Craton. Geochemical studies show that garnet-bearing leucogranite has high Al2O3 content and FeOT/MgO ratio, with a large variation in the CaO content and K2O/Na2O ratio. Additionally, the garnet-bearing leucogranite is enriched in light rare earth elements and large ion lithophile elements and depleted in Nb, Ta, P, and Ti. However, there are some variations in Eu anomalies, as indicated by the Eu-enriched and Eu-depleted patterns. The sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb dating of the Baotou garnet-bearing leucogranite and Jining–Liangcheng garnet-bearing leucogranite show that their anatectic ages were 2.35–2.43 Ga and 1.90–1.92 Ga, respectively, with zircon εHf (t) values of −0.01–3.93 and −4.36–1.99, and two-stage model ages (TDM2) of 2.76–3.02 Ga and 2.57–2.83 Ga, respectively. U-Th-Pb SHRIMP analyses on zircon reveal ages of ca. 2.35–2.47 Ga for the Baotou metapelite gneiss and ca. 1.90–1.91 Ga for the Jining–Liangcheng metapelite gneiss, which represent the ages of tectonic metamorphic events related to the anatexis. Our results, combined with geological and petrographical observations, geochemistry, and geochronology, suggest that (1) Garnet-bearing leucogranites formed as a result of anatexis of metapelite gneisses; (2) They are mixtures of leucosomes, residual minerals and mantle-derived materials; (3) Two episodes of garnet-bearing leucogranites have been identified; (4) Both two episodes of garnet-bearing leucogranites corresponded to late Neoarchean-early Paleoproterozoic and late Palaeoproterozoic tectonothermal events, respectively. [ABSTRACT FROM AUTHOR]

Serious errors and inconsistencies in the article undermine many of its interpretations to the point that principal conclusions are not valid. Much dependence is placed on the maximum depositional age (Dmax) of sandstone units based on zircon analysis of 10 samples, but calculation of those Dmax values is flawed, and their use confuses maximum with actual depositional ages and makes age distinctions finer than the resolution of the data. Conclusions that are compromised include the concept of a westward/downward-younging tectonostratigraphic stack of accretionary units, comparison of mapped sandstones with other California Coast Ranges sandstones, comparison of timing of events recorded in the map area with regional events, identification of mélange only in a narrow band in the study area, and detrital provenance of some of the sandstone units. [ABSTRACT FROM AUTHOR]

The means and timing of amalgamation of the Harlik arc to the Yemaquan arc remain controversial. To address these polemic relations, we report new field data and zircon U-Pb analyses of early Palaeozoic strata in the Harlik Mountains. Field investigations reveal that the Huangcaopo Group in northern Harlik are composed mainly of clastic rocks, while their counterparts in central Harlik (Harlik arc) are dominantly volcanic and pyroclastic rocks. Weighted mean ages of the youngest group of samples from northern Harlik are 455 ± 9 Ma and 456 ± 7 Ma, whereas their counterparts in central Harlik have a weighted mean age of 446 ± 4 Ma, indicating that parts of the Huangcaopo Group in central Harlik should be re-defined as the early Silurian Hongliuxia Formation. The youngest grain from the Dananhu Formation in central Harlik is 421 ± 3 Ma, the time of deposition in early Devonian. Age spectra of the Huangcaopo Group in northern Harlik have only two peaks at 488 Ma and 923 Ma, demonstrating affinity with the Tuva-Mongolian microcontinent, a correlation supported by the occurrence of Silurian Tuvaella brachiopod fauna in those strata. In contrast, multiple peaks at 494 Ma, 813 Ma, 1485 Ma, 1872 Ma and 2469 Ma in the Hongliuxia and Dananhu Formations in central Harlik have neither an affinity with the Tuva-Mongolian nor with the Central Tianshan microcontinent, which suggests that the Harlik arc probably evolved independently as a Japan-type arc in the early Palaeozoic. The above disparate relations, combined with the fact that the Silurian Tuvaella brachiopod fauna occurs throughout the Kelameili ophiolite belt but not in the Harlik arc, provide robust evidence that the contrasting provenance of the Yemaquan and Harlik arcs is an expression of a cryptic suture in northern Harlik rather than along the Kelameili ophiolite belt to the north. [ABSTRACT FROM AUTHOR]

Sedimentary successions and igneous rocks exposed in Eastern Sayan provide an important geological record on the tectonic evolution of the southwestern margin of the Siberian Craton during the Neoproterozoic. Here we present sedimentological and detrital zircon provenance data from key stratigraphic units, complemented by a new U-Pb zircon age of 718 ± 6 Ma from the mafic Nersa magmatic complex. Sedimentological and provenance data from the lower part of the stratigraphic succession (Karagasy Group) show that these rocks were deposited in a passive margin setting after 750 Ma and before the emplacement of the Nersa magmatic complex. The upper part of the stratigraphic succession (Oselok Group) consists of Cryogenian and Ediacaran glaciogenic deposits and associated cap carbonates, overlain by mid to late Ediacaran sequences of dominantly shallow-marine and fluvial siliciclastic rocks. Provenance data indicate that rocks from the Tuva-Mongolia and Zavkhan terranes provided a significant input of detrital material into the sedimentary basin that was the source of the glacial deposits. Orogenic processes during the late Ediacaran (ca 570–540 Ma) in Eastern Sayan were likely associated with the accretion of the Tuva-Mongolia terrane, as indicated by the detrital zircon data. [ABSTRACT FROM AUTHOR]

To better constrain the petrogenesis and tectonic setting of the granitic plutons in the Guaizihu region, NW China, this study presents a new analysis of zircon U–Pb ages, whole-rock geochemistry and Hf isotopes of syenogranites. The syenogranites are medium- to coarse-grained, with moderate to high SiO2 (72.55–74.27 wt%) and Al2O3 (13.02–13.75 wt%) contents and moderate total alkalis (Na2O + K2O, 7.58–7.98 wt%), which indicate that the syenogranites exhibit calc-alkaline and peraluminous characteristics. The low Ce, 10 000 Ga/Al, and A/CNK values and the negative correlation of SiO2 and P2O5 confirm that the syenogranites are of I-type origin. The whole-rock geochemistry demonstrates that the magma was generated by partial melting and experienced fractional crystallisation. Specifically, the correlations of Rb with Ba and Sr and the weak negative Eu anomalies indicate that the plagioclase has undergone fractional crystallisation. In addition, Eu anomalies and La/Sm–La relations show that both fractional crystallisation and partial melting contributed to the magmatic process. The positive εHf(t) values (0.11–9.78); enrichments in Rb, Th and other LILEs; depletions in Nb, Sr and other HFSEs; and the ratios of geochemical elements indicate that the syenogranites were generated from the partial melting of juvenile crust. The granites are inferred to have formed in a post-collisional extensional environment. Additionally, the U–Pb zircon ages (284.6 ± 0.84 Ma and 269.4 ± 1.2 Ma) of syenogranites suggest that final closure of the Paleo-Asian Ocean occurred before 285 Ma. Granites in the Guaizihu region are dated at 284.6 ± 0.84 Ma and 269.4 ± 1.2 Ma, and the Paleo-Asian Ocean, where the Engger Us Ophiolite Belt is located, closed before 285 Ma. Granites are medium- to coarse-grained and highly fractionated I-type syenogranites. Granites have been derived from magma generated by the partial melting of juvenile crust followed by fractional crystallisation. Granites were formed in an intraplate extensional environment related to a post-collisional event. [ABSTRACT FROM AUTHOR]

Nineteen ichnotaxa, together with algal and invertebrate remains, and various pseudo-traces and sedimentary structures are described from the Torneträsk Formation exposed near Lake Torneträsk, Lapland, Sweden, representing a marked increase in the diversity of biotic traces recorded from this unit. The "lower siltstone" interval of the Torneträsk Formation contains mostly simple pascichnia, fodinichnia and domichnia burrows and trails of low-energy shoreface to intertidal settings. The assemblage has very few forms characteristic of high-energy, soft-sediment, foreshore or upper shoreface environments (representative of the Skolithos ichnofacies). Uranium-lead (U-Pb) LA-ICPMS analysis of zircon from a thin claystone layer within the "lower siltstone" interval yielded a maximum depositional age of 584 ± 13 Ma, mid-Ediacaran. Most of the zircon is represented by rounded detrital grains that yield dates between 3.3 and 1.0 Ga. Although the age of the basal sandstone-dominated interval of the Torneträsk Formation remains elusive owing to the absence of fossils, the ichnofossil suite from the overlying "lower siltstone" interval lacks deep arthropod trackways, such as Rusophycus and Cruziana, and is suggestive of a very early (Terreneuvian, possibly Fortunian) Cambrian age. The ichnofauna is otherwise similar to early Cambrian trace fossil assemblages from other parts of Baltica, regions further south in modern Europe, and from Greenland. [ABSTRACT FROM AUTHOR]