Detrital Zircon Grains Research Articles

Syn-collisional detrital zircon source evolution in the northern Moroccan Variscides

U–Pb dating and Hf isotopic analyses of detrital zircon grains from Cambrian-Devonian strata in the Moroccan Mesetas were undertaken in order to constrain its Paleozoic paleogeographic evolution. In this work, we analyzed 12 Late Devonian-Late Carboniferous samples of syn-collisional detrital rocks from the Western and Eastern Moroccan Mesetas. All our samples present significant Ediacaran (ca. 620 Ma) and Rhyacian (ca. 2.1 Ga) detrital zircon populations, suggesting that the West African Craton remained the main source of sediments for northern Morocco at least until the Late Carboniferous. Locally, a Stenian-Tonian (ca. 1.0 Ga) detrital zircon population is also present, probably fed from intermittent and distant source areas located in NE Africa (e.g. Sahara Metacraton). The collisional evolution started with the approach of an Avalonian promontory to the northern Gondwana continental margin (latest Devonian - earliest Carboniferous), after the closure of the Rheic Ocean. This process entailed the former subduction of the Rheic oceanic lithosphere underneath the Avalonian continental terrane and the formation of a magmatic arc in the upper plate. In this scenario, the first syn-collisional sediments (Tournaisian; Tiflet and Debdou-Mekkam areas) are characterized by a Devonian detrital zircon population (ca. 370 Ma), presumably derived from the magmatic arc, and an increasing number of Mesoproterozoic dates, putatively also sourced from the continental crust of the Avalonian terrane. After the initial collision, only the Visean samples located in areas close to the exotic terrane (e.g. Ben Slimane area) displayed a minor Avalonian component. Finally, the Late Carboniferous samples from the Jerada area recorded an important Middle Carboniferous (ca. 330 Ma) detrital zircon population, probably sourced from Variscan granitoids emplaced in the Eastern Moroccan Meseta and attesting to crustal thickening and subsequent thermal maturation of the Gondwana continental crust in this area.

Recalibrating Rodinian rifting in the northwestern United States

Abstract A lack of precise age constraints for Neoproterozoic strata in the northwestern United States (Washington State), including the Buffalo Hump Formation (BHF), has resulted in conflicting interpretations of Rodinia amalgamation and breakup processes. Previous detrital zircon (DZ) studies identified a youngest ca. 1.1 Ga DZ age population in the BHF, interpreted to reflect mostly first-cycle sourcing of unidentified but proximal magmatic rocks intruded during the amalgamation of Rodinia at ca. 1.0 Ga. Alternatively, the ca. 1.1 Ga DZ population has been suggested to represent a distal source with deposition occurring during the early phases of Rodinia rifting, more than 250 m.y. after zircon crystallization. We combined conventional laser-ablation split-stream analyses of U-Pb/Lu-Hf isotopes in zircon with a method of rapid (8 s per spot) U-Pb analysis to evaluate these opposing models. Our study of ∼2000 DZ grains from the BHF identified for the first time a minor (∼1%) yet significant ca. 760 Ma population, which constrains the maximum depositional age. This new geochronology implies that the BHF records early rift deposition during the breakup of Rodinia and correlates with sedimentary rocks found in other late Tonian basins of southwestern Laurentia.

Provenance of Carboniferous sandstones in the central and southern parts of the Pennine Basin, UK: evidence from detrital zircon ages

New U–Pb isotopic analyses of detrital zircon grains from Namurian and Westphalian sandstones in the central and southern parts of the Pennine Basin are combined with published analyses from the same region, to assess existing views on the nature and location of the source areas that supplied the clastic sediment. The study confirms that most sandstones were derived from distant areas to the north, west and south, and that a local source, in the Wales–Brabant High, also supplied sediment at times. The northern sources are thought to lie mainly in Laurentia (East Greenland), with some input from Baltica (Norway). Most sandstones entering from the west are thought to have been supplied from areas of Avalonian basement, with some components recycled from sediments that were themselves derived from the Caledonian belt that lay to the north. An exception is the Clifton Rock: its source is thought to lie entirely in the Irish Caledonides or in southern Scotland. Sandstones supplied from the south contain significant numbers of late Devonian and Carboniferous grains, indicating the inclusion of material eroded from the active Variscan orogenic belt in Europe. Northern provenance prevailed during Namurian and early Langsettian times, then alternated with western supply until the late Langsettian. Western input then dominated through most of Duckmantian times, until superseded in the late Duckmantian by supply from the Variscan orogen. The Woolley Edge Rock, now shown to be an isolated member of the northern group, entered the area just before the earliest sandstone of Variscan origin. Supplementary material: U–Pb data of detrital zircon is available at https://doi.org/10.6084/m9.figshare.c.5174702

RECOGNIZING PORPHYRY COPPER POTENTIAL FROM TILL ZIRCON COMPOSITION: A CASE STUDY FROM THE HIGHLAND VALLEY PORPHYRY DISTRICT, SOUTH-CENTRAL BRITISH COLUMBIA

Abstract The detrital zircons in tills overlying the Guichon Creek batholith, British Columbia, Canada, have trace element concentrations and ages similar to those of zircons from the bedrock samples from which they are interpreted to have been sourced. Rocks from the core of the batholith that host porphyry copper mineralization have distinct zircon compositions relative to the distal, barren margin. We analyzed 296 zircons separated from 12 subglacial till samples to obtain U-Pb ages and trace element compositions. Laser ablation U-Pb ages of the detrital zircons overlap within error with chemical abrasion-thermal ionization mass spectrometry U-Pb ages of the Late Triassic Guichon Creek batholith and confirm that the detrital zircons are likely derived from the batholith. The youngest intrusions of the batholith produced the Highland Valley Copper porphyry deposits and contain distinctive zircons with elevated Eu/EuN* >0.4 attributed to high magmatic water contents and oxidation states, indicating higher porphyry copper potential. Zircon from till samples adjacent to and 9 km down-ice from the mineralized centers have mean Eu/EuN* >0.4, which are indicative of potential porphyry copper mineralization. Detrital zircon grains from more distal up- and down-ice locations (10–15 km) have zircon Eu/EuN* mean values of 0.26 to 0.37, reflecting background values. We conclude that detrital zircon compositions in glacial sediments transported several kilometers can be used to establish the regional potential for porphyry copper mineralization.

Geochemistry and detrital zircon geochronology of the Lower Jurassic clastic rocks of the northern Qiangtang Basin, northern Tibet: Implications for palaeoenvironment, provenance, and tectonic setting

The Lower Jurassic clastic rocks in the Qiangtang Basin, northern Tibet, are generally regarded as the early sediments of the new Mesozoic Qiangtang Basin (Late Triassic to Early Cretaceous). However, little attention has been paid to the palaeoenvironment, provenance, and tectonic setting of this unit. The QZ‐16 well, located in the east of the northern Qiangtang Basin, firstly recovered the Lower Jurassic strata (Quemo Co Formation) in the Qiangtang Basin. In this paper, the whole‐rock major‐trace elemental characteristics of 34 mudstone samples and detrital zircon geochronology of one sandstone, collected from the Quemo Co Formation of the QZ‐16 well, are reported in order to infer the palaeoenvironment, provenance, and tectonic setting. Quemo Co mudstones are dominated by SiO2 and Al2O3, while Fe2O3, CaO, K2O, and MgO are the second most abundant oxides. They are generally characterized by high REE concentrations, enrichment in light rare earth elements (LREEs), marked negative Eu and Ce anomalies. The REE contents of the mudstones are mainly controlled by terrigenous detrital minerals input. The Lower Jurassic mudstones have relatively high chemical index of alteration (79.1–83.6) values, indicating moderate to high degree of chemical weathering conditions of the source area. Sr/Ba and Rb/K ratios reflect that the marine transgression impact was limited and the mudstones were generally deposited in a brackish water environment. Major, trace, and REE contents indicate that the mudstones were mainly derived from intermediate igneous rocks with subordinate felsic igneous rocks. Prominent age peaks at ca. 200 ~ 400 Ma, 700 ~ 970 Ma, 1,700 ~ 1,900 Ma, and 2,100 ~ 2,700 Ma of the detrital zircon grains suggest that the Lower Jurassic clastic rocks were derived from the Central Qiangtang and Songpan–Ganzi complex. Moreover, tectonic discrimination diagrams based on major and trace elements show that the source of the Lower Jurassic sediments were most likely derived from a collision tectonic setting.

Non‐metamorphosed autochthonous Kuncha‐Naudanda‐Heklang Formations and their differences from those of the Kuncha nappe: A multichronological approach

AbstractNon‐metamorphosed, autochthonous Lesser Himalayan sediments (LHS), which are correlated to the Kuncha and Naudanda Formations, were found in a narrow belt between the Main Boundary Thrust and the Lesser Himalayan Thrust at the base of the Kuncha nappe in southeastern Nepal. The autochthonous Naudanda Formation is comprised of cross‐bedded and rippled orthoquartzite and yielded a maximum depositional age of 1795.1 Ma ±5.1 Ma using detrital zircons. Low‐grade metamorphosed quartzite in the Kuncha nappe yielded a maximum depositional age of 1867.4 Ma ±3.4 Ma, although it is totally recrystallized. These ages and age distribution patterns of detrital zircon grains indicate that the meta‐quartzite of the nappe is originally Naudanda Formation. A zircon fission‐track age of the autochthonous Naudanda Formation shows partially annealed age of 864 Ma ±56 Ma, in contrast, that of the Kuncha nappe shows a totally annealed age of 11.9 Ma ±1.6 Ma. These results suggest that the autochthonous LHS have never undergone metamorphism during the Himalayan orogeny. We also discovered a non‐metamorphosed Heklang Formation that rests on the Naudanda Formation, and designated it as a sub‐type section on the basis of detailed lithostratigraphic study. It is characterized by black and light green slate with dolerite sills and ill‐sorted quartzose sandstone, and correlated to the metamorphosed Dandagaon Phyllites in the Kathmandu area. Non‐metamorphosed autochthonous formations distributed to the south of the nappe front suggest that they escaped from thermal metamorphism by hot nappe.

Mesoarchean (ultra)-high temperature and high-pressure metamorphism along a microblock suture: Evidence from Earth's oldest khondalites in southern India

Early continent-building processes on Earth are challenging to investigate, particularly since juvenile felsic crust formed during the Early Archean (4.0–3.2 Ga) is rarely preserved. Thus, associated sedimentary records are of fundamental importance, although in many cases these have been metamorphosed and reworked to various degrees since deposition. Here we present new petrological and zircon and monazite U–Pb age data from one of the Earth's oldest ‘khondalite’ (granulite-facies aluminous metapelite) belt, which we define as the Mercara khondalite belt, and associated charnockite and mafic granulite from the Mercara suture, the collision zone welding the Coorg and Western Dharwar Blocks in southern India. Petrologic analyses and phase equilibria modelling of the khondalites and associated charnockite and mafic granulite reveal a clockwise pressure–temperature (P–T) path with a peak temperature of above ca. 900 °C, and pressures up to 12 kbar. Detrital zircon grains in the metasedimentary rocks have magmatic cores with oscillatory zoning and ages up to ca. 3.5 Ga, and metamorphic overgrowths with ages of 3.1–3.0 Ga. Monazite in the khondalites yield identical metamorphic ages in the range of 3.1 to 3.0 Ga. Some of these rocks are overprinted by a younger thermal event at ca. 2.8–2.6 Ga. We correlate the high P–T metamorphism with the subduction-collision history between the northern margin of the Mercara block and the Western Dharwar Craton during the Mesoarchean, which indicates that plate tectonics had been established on Earth by at least ca. 3.1 Ga, in agreement with many independent lines of evidence. The Mesoarchean Mercara khondalite belt signals emergence of continents on the early Earth with active drainage systems leading to the deposition of voluminous detritus. The ca. 3.1 to 3.0 Ga (ultra) high temperature and high pressure metamorphism also coincides with the timing of assembly of the Earth's oldest supercontinent Ur

Mineralization proximal to the final Nuna suture in northeastern Australia

Mineralization along continental suture zones is facilitated through the frequent presence of pathways from fertile mantle source regions to crustal repositories. Due to their inherent rheological weakness, these suture zones are often concealed, which hinders surface-based observations. Here, we use zircon U-Pb and sericite 40Ar/39Ar dating, and whole-rock geochemical data, to investigate the crystallization and mineralization history from a sequence of granites (sensu lato), volcanic rocks and sedimentary rocks from the Au–Ag–Pb–Zn Empress Springs Project in northeast Australia, which are under >50 m of Phanerozoic cover and located near the interpreted ca. 1.6 Ga tectonic boundary between the North Australia Craton (Mount Isa Inlier) and Laurentia (Georgetown Inlier). Zircon U-Pb dating indicates that granite emplacement, volcanic eruptions and dolerite intrusions occurred between 1564 ± 6 and 1546 ± 13 Ma (2σ), corresponding to the 1560–1550 Ma Esmeralda Supersuite and the Croydon Volcanic Group exposed in the western Georgetown Inlier. U-Pb ages from detrital zircon grains in a sedimentary rock revealed a near unimodal ca. 1560 Ma population, likely sourced from the surrounding granitic and volcanic rocks. Sericite 40Ar/39Ar dating yielded disturbed spectra with evidence for incorporation of excess radiogenic Ar, but with a probable Carboniferous to Permian age, potentially dating the timing of gold mineralization. Geochemical similarities point towards an epithermal origin for the Empress Springs Project, consistent with post-orogenic mineralization at either ca. 1560–1550 Ma or 330–250 Ma. It is likely that a series of west-dipping lithospheric-scale faults below the Empress Springs Project, here termed the Empress Suture Zone, demarcate the final suture zone associated with the assembly of Nuna. Reactivation of the Empress Suture Zone ~1.3 b.y. after crystallization with potential contemporaneous gold mineralization attests to its longevity for fluid mobility.

Precambrian crustal evolution of the Tethyan Yunnan, Southwest China: Records in detrital zircons from Paleozoic sedimentary rocks of the Baoshan block

The Baoshan Block in western Yunnan is one of the major micro-continents in the eastern Tethyan orogenic belt, but its provenance and crustal evolution remain enigmatic. Sedimentary rocks prior to an amalgamation of continental blocks recorded key information about crustal evolution and the origin of terranes and can provide vital data for understanding orogenic processes. Here, we present U–Pb ages and Hf isotopic data on detrital zircon and Nd isotopic data on whole rock for Paleozoic sedimentary rocks from the Baoshan Block to constrain their provenance and tectonic history. Crystallization ages of detrital zircon grains illustrate that the magmatic activities of the source areas occurred at ~2.5 Ga, ~1.9 Ga, and ~1.0–0.5 Ga, with a major peak occurring around 0.95 Ga. Several old zircon grains from >3.0 Ga up to ~3.8–3.7 Ga were observed. Zircon Hf isotopic data from one sample suggest that the major crustal growth occurred at ~2.1 Ga. Cambrian to Carboniferous sedimentary rocks showed similar Nd isotopic features with two–stage Nd model ages of approximately 2.1 Ga. The age distribution patterns and isotopic data suggest that detritus material in the Paleozoic sedimentary rocks was mainly derived from continental terrane similar to the Indian craton, implying that the Baoshan Block was likely adjacent to Northwest India before the collision of the Baoshan and Simao blocks. No significant inputs of the juvenile crust were documented in the Nd–Hf isotopic data, appearing to be incompatible with a magmatic arc setting for the Baoshan Block during the Paleozoic. This block might have been situated at the interior of Gondwana before rifting and drifting northwards to the Eurasia continent.

Cordilleran Subduction Initiation: Retroarc Timing and Basinal Response in the Inyo Mountains, Eastern California

Abstract Subduction zones drive plate tectonics on Earth, yet subduction initiation and the related upper plate depositional and structural kinematics remain poorly understood because upper plate records are rare and often strongly overprinted by magmatism and deformation. During the late Paleozoic time, Laurentia’s western margin was truncated by a sinistral strike-slip fault that transformed into a subduction zone. Thick Permian strata in the Inyo Mountains of central-eastern California record this transition. Two basins that were separated by a transpressional antiform contain sedimentary lithofacies that record distinct patterns of shoaling and deepening conditions before and during tectonism associated with subduction initiation. Sandstone petrography and lithofacies analysis show that rocks in a southeastern basin are dominated by carbonate grains derived from adjacent carbonate shelves, whereas sandstones in a northwestern basin are predominantly quartzose with likely derivation from distant ergs or underlying strata. Detrital zircon spectra from all but the youngest strata in both basins are typical of Laurentian continent spectra with prominent peaks that indicate ultimate sources in Appalachia, Grenville, Yavapai/Mazatzal, and the Wyoming or Superior cratons. The first Cordilleran arc-derived detrital zircon grains appear in the uppermost strata of the northwestern basin and record Late Permian (ca. 260 Ma) Cordilleran arc magmatism at this approximate latitude, and a possible source area is suggested by geochemical similarities between these detrital zircons and broadly coeval magmatic zircons in the El Paso Mountains to the southwest. Deformation responsible for basin partitioning is consistent with sinistrally oblique contraction in the earliest Permian time. The data presented from the Inyo Mountains shed more light on the nature of Cordilleran subduction initiation and the upper-crustal response to this transition.

Arenópolis sequence, evolution of a marginal basin in the Neoproterozoic Goiás magmatic arc, central Brazil

The Goiás Magmatic Arc in central South America and the Arabian-Nubian shield record major phases of Tonian subduction during a period marked otherwise by dominant tectonic quiescence. Several basins were formed in convergent tectonic settings and their rock record may enlighten some aspects of the tectonic history of the Rodinia Supercontinent, such as provenance patterns and pathways and subsidence mechanisms. In this study, we present the results of geologic mapping and geochronologic analyses of detrital zircon grains of the Tonian volcanic-sedimentary Córrego do Santo Antônio unit, Arenópolis Sequence, of the Goiás Magmatic Arc, Brasília Belt, central Brazil. It is formed of several mafic-ultramafic rocks exposed along eastwards dipping thrust sheets. These bodies are covered by thin layers of metachert and marble. Between the thrust faults, voluminous mica schist and quartzite packages are exposed. Two samples of siliciclastic metasedimentary rocks from an eastern thrust sheet show multimodal age spectra for detrital zircon grains, with a major peak with ages between 1.9 and 2.1 Ga, and minor peaks of 0.99 Ga, 1.2 Ga, ca. 1.4 Ga, 2.45 Ga, 2.75 Ga and ca. 3.0 Ga. One sample of quartzite from the westernmost thrust sheet presents a similar pattern, but with major peaks of 1.2 Ga, 1.55 Ga, 1.85 Ga, and 2.1 Ga and minor peaks of 0.9 Ga and 2.55 Ga. Another sample of metagraywacke from the westernmost thrust sheet displays a unimodal pattern, with a peak of 893 Ma and a minor contribution of Mesoproterozoic zircon grains. Geologic field data suggest that the Córrego do Santo Antônio unit represents a Tonian trench basin, which might have received low rates of sedimentation. Aeromagnetic data confirm that these rocks occur along the limit of blocks, corroborating formation in a trench basin. Geochronologic data from one sample corroborate this interpretation. However, the U–Pb isotopic data of the other samples indicate that the sedimentation history is quite more complex. We suggest three possibly complementary hypotheses to explain both geologic and geochronologic data: 1) Metasedimentary rocks were formed in distinct tectonic environments and later were tectonically imbricated; 2) Older metasedimentary rocks were the preferred source for the basin; 3) The basin preferentially received sediments from older terrains by longitudinal turbidity currents. We conclude that the basin, which originated the Córrego do Santo Antônio unit had a link with cratonic areas and received sediments from longitudinal currents.

Geology and geochronology of the Jurassic magmatic arc in the Magdalena quadrangle, north-central Sonora, Mexico

This work reports on the geology and U–Pb LA-ICPMS zircon geochronology of a crustal section that is part of the Jurassic magmatic arc in the Magdalena quadrangle of north-central Sonora, Mexico. This rock succession is variably metamorphosed and strained as it was affected by Late Cretaceous shortening, intruded by early Tertiary granitoids, and further exhumed in the lower plate of the early Miocene Magdalena metamorphic core complex. The older and more extensively exposed Jurassic unit is the >3.5 km thick Sierra Guacomea rhyolite that is composed of massive to poorly bedded rhyolite, bedded quartz-phyric rhyolitic ignimbrite and interbedded ash-fall tuffs and quartz-rich sandstone beds. Three rhyolite samples collected at different localities of its outcrops yielded concordia ages of 175.2 ± 0.9, 171.7 ± 0.6, and 171.4 ± 0.7 Ma. The quartz-phyric Rancho La Víbora, Los Vallecitos, and the Agua Caliente rhyolitic domes that are associated with the Sierra Guacomea rhyolite yield concordia ages of 176 ± 0.8, 174.4 ± 0.9 and 173.1 ± 0.8 Ma, respectively. The Rancho Los Pozos unit composed of interbedded rhyolitic ash-fall tuff and flows, sandstone, siltstone and subordinate limestone beds has an estimated thickness of 600 m and yielded a crystallization concordia age of 170.7 ± 0.6 Ma from a rhyolite bed. The porphyritic El Rincón granite that intrudes into the Sierra Guacomea rhyolite yields crystallization ages of 167.43 ± 0.42 and 164.4 ± 0.7 in samples from different localities. The La Jojoba metasandstone that consists of foliated, quartz-rich to arkosic strata of fluvial origin is at least 900 m thick; detrital zircon grains dated from three sandstone samples yielded dominantly Jurassic ages with peaks at 172, 170, and 163.7 Ma, and a combined maximum depositional age of ca. 163 Ma.The younger plutons are the porphyritic El Nopalito granite that has an interpreted crystallization age of 160.8 ± 0.6 Ma, and the leucocratic, two-mica, garnet-bearing La Cebolla granite that yielded a concordia age of 158.1 ± 1 Ma. These granitic intrusions record the waning magmatic pulses of the arc, in the study quadrangle, but their volcanic equivalents were not identified. Inherited zircon grains in the reported volcanic and plutonic units are only of Jurassic age, except by two Proterozoic zircon grains yielded by the El Nopalito granite. The El Salto granite augen gneiss is a xenolith dated at 1071.9 ± 5 Ma that indicates the presence of Grenvillian basement in the study area.Major- and trace-element geochemical data indicate that the volcanic and plutonic units are silica-rich, mostly high K calc-alkaline to shoshonitic rocks associated with a continental margin arc setting. The plutons are mostly peraluminous, and in conjunction with trace element geochemistry data, they suggest crustal assimilation of magmas emplaced in a probably thickened continental crust. Chondrite-normalized REE patterns and primitive mantle-normalized trace element diagrams also suggest partial melting and fractional crystallization processes.The ages obtained indicate that the arc in the study area developed from ca. 176 to 158 Ma, encompassing a 17 m.y. interval of magmatism and associated sedimentation. Regional correlation and geochronologic published data indicate that the arc crustal section of the Magdalena quadrangle is part of the Jurassic magmatic arc that regionally lasted from ca. 190 to 158 Ma.

U-Pb geochronology of detrital and igneous zircon grains from the Águilas Arc in the Internal Betics (SE Spain): Implications for Carboniferous-Permian paleogeography of Pangea

New U–Pb detrital zircon and U–Pb zircon ages of metaigneous rocks in the Águilas Arc (Betic Chain, SE Spain) allow us to determine the maximum depositional ages of the rocks. Within the Nevado-Filábride Complex, a Late Carboniferous depositional age for the Lomo de Bas schists and quartzites, and a Permian-Triassic maximum depositional age for the Tahal Fm are determined. Within the Alpujárride Complex, the maximum depositional age of the Micaschists and Quartzite Fm is Late Carboniferous and the Meta-detrital Fm was deposited in the Early Permian. Furthermore, the maximum depositional age of the Saladilla Fm in the Maláguide Complex is also Early Permian. The age distribution patterns for the Carboniferous rocks of the Nevado-Filábride and Alpujárride complexes are similar to those from the Cantabrian Zone of the Iberian Massif, suggesting deposition in Carboniferous foreland basins located eastwards of the Iberian Massif. However, age patterns in Maláguide and samples from the North-eastern Iberian Peninsula and South France show strong similarities suggesting that it can be located near those areas in the Late Carboniferous times.The samples with Early Permian maximum depositional ages from the three complexes contain more Paleozoic zircon grains relative to the older Carboniferous samples, but have similar age distribution patterns, suggesting that they were deposited in the same basin. Samples from unconformable Middle Miocene sediments have Early Permian youngest zircon populations and age distribution patterns corresponding to a mixing of detrital zircon grains from the Alpujárride and Maláguide complexes. Furthermore, there is no record of any major felsic rocks formation and/or exhumation event after the Early Permian in those two complexes.

Provenance and U–Pb Ages of Detrital Zircons from the Sandstones of the Middle Devonian Asyvvozh Formation, Dzhezhimparma Highland, Southern Timan

Results of U–Pb dating of detrital zircon grains from diamond-bearing sandstones of the Middle Devonian Asyvvozh Formation in the Southern Timan are presented. It was established that the sandstones do not contain zircon grains younger than the Middle Riphean. The datings of 92 zircon grains fall in the interval of 1144 ± 36 to 3090 ± 19 Ma, and the difference between the depositional age of the Middle Devonian sandstones and the age of the youngest zircons is around 750 m.y. The main sources of terrigenous material during the deposition of the Asyvvozh sandstones were the crystalline complexes of the ancient basement of the East European Craton (the central regions of Volga–Uralia) and the complexes of accretionary orogens on the margins of Baltica. The presumably Cambrian kimberlites located within the Komi–Permyak and Sysola arches of the basement of the East European Craton could have been the sources of diamonds.

Provenance of Upper Jurassic to Lower Cretaceous synrift strata in the Terra Nova oil field, Jeanne d’Arc basin, offshore Newfoundland: A new detrital zircon U-Pb-Hf reference frame for the Atlantic Canadian margin

Detrital zircon provenance studies of Terra Nova oil field strata, offshore Newfoundland, were conducted to constrain the depositional age and correlation of synrift sandstone units in the Jeanne d’Arc basin and test continuity with coeval rocks in adjacent regions. Braided fluvial sandstone intervals of the Jeanne d’Arc Formation from delineation wells K-18, C-09, and E-79 yield Mesozoic to Archean detrital zircon U-Pb populations that are consistent across the Terra Nova field. A Tithonian to Berriasian maximum depositional age for the Jeanne d’Arc Formation is proposed from circa 145 Ma detrital zircon grains in each of the nine sandstone samples analyzed. Tithonian to Berriasian detrital zircon grains have chondritic to superchrondritic Hf isotope compositions and indicate that synrift drainage systems carved valleys into the existing landscape during mantle-derived volcanism. Polycyclic Paleozoic to Archean detrital zircon grains were derived from mature, upper Paleozoic sedimentary rocks of the Avalon uplift south-southwest of the Jeanne d’Arc basin and have original provenance from the Newfoundland Appalachians, Canadian shield, and Iberia. Coeval synrift fluvial strata from the frontier Flemish Pass basin yield analogous Tithonian to Berriasian and Paleozoic to Archean detrital zircon populations and suggest potential Late Jurassic to Early Cretaceous continuity with Jeanne d’Arc basin units. Tithonian to Berriasian deltaic strata of the Scotian basin, southwest of the southwest Grand Banks fault zone, lack Mesozoic detrital zircon grains typical of Grand Banks basins and demonstrate that regional faulting and igneous activity controlled Late Jurassic to Early Cretaceous drainage patterns along the nascent Atlantic Canadian margin.

Treptichnus pedum in the Três Marias Formation, south-central Brazil, and its implications for the Ediacaran-Cambrian transition in South America

The index ichnofossil Treptichnus pedum Seilacher 1955 and four types of Microbially Induced Sedimentary Structures (MISS) - variants of wrinkle and elephant skin structures – are the first fossils described from the Três Marias Formation, topmost unit of the Bambuí Group in south-central Brazil. Despite the stratigraphic importance and widespread occurrence of this unit in the São Francisco Craton and adjacent Brasília Fold Belt, its age is still poorly constrained between the Neoproterozoic and Early Paleozoic. This discovery is significant because the first appearance datum (FAD) of T. pedum has been used to define the Ediacaran-Cambrian boundary. Hence, the presence of T. pedum indicates an early Paleozoic age for the uppermost Bambuí Group and means that the Ediacaran-Cambrian limit must lie deeper in the Bambuí Group, somewhere between this level and that of the terminal Ediacaran index-fossil Cloudina sp. in the Sete Lagoas Formation, near the base of the group. Available age determinations on detrital zircon grains and chemostratigraphic correlations are consistent with an Ediacaran to Cambrian age for this group. Detailed paleontological prospection throughout the group may now proceed within a better constrained time frame and hopefully reveal additional fossil evidence of Cambrian as well as Ediacaran life in the Bambuí Group.

Ediacaran and Cambrian rocks on Scatarie Island and nearby Hay Island, Avalonian Mira terrane, Cape Breton Island, Nova Scotia, Canada

Scatarie Island and adjacent Hay Island, located 2 km east of the eastern tip of the Avalonian Mira terrane of southern Cape Breton Island, Nova Scotia, contain a succession of epiclastic and other sedimentary rocks of inferred Ediacaran to Cambrian age. The age assignment was based previously on lithological comparison with the Main-à-Dieu Group and overlying Bengal Road and MacCodrum formations of the Mira River Group. Detrital zircon grains from two sandstone samples from the Bengal Road Formation yielded typical Avalonian detrital zircon spectra with middle to late Neoproterozoic, Meso- to Paleoproterozoic (1300–2200 Ma) and Neoarchean ages. They indicate maximum depositional ages of 532.4 ± 4.2 Ma and 525.4 ± 2.4 Ma from essentially the same stratigraphic level, consistent with the interpretation that the rocks are Cambrian. The Bengal Road Formation also yielded scarce organic-walled microfossils including an acanthomorphic acritarch identified as Polygonium sp., also consistent with Cambrian age. The fine-grained siliciclastic succession on Hay Island, tentatively attributed to the MacCodrum Formation, yielded trace fossils, including Teichichnus isp. and Gyrolithes scintillus, that confirm Cambrian age. The Hay Island Gyrolithes scintillus expands the geographical distribution of this ichnospecies, previously known mainly from the Chapel Island Formation of Newfoundland, and represents a younger occurrence.

Zircon U-Pb geochronology and Nd-Pb isotope geochemistry of Blue Ridge basement in the eastern Great Smoky Mountains, U.S.A.: Implications for the Proterozoic tectonic evolution of the southeastern Laurentian margin

The Mesoproterozoic to Paleozoic history of the southeastern Laurentian margin involved repeated collisional and accretionary tectonomagmatic events that reworked and recycled older continental crust of preceding events. The Great Smoky Mountains Basement Complex (GSMBC) within the southern Appalachian Blue Ridge exposes complexly deformed orthogneiss and paragneiss that preserve a record of Laurentian margin evolution from ca. 1.9 Ga to 450 Ma. The GSMBC consists primarily of: (1) 1.34 to 1.31 Ga (pre-Elzevirian) granodioritic orthogneiss and entrained mafic xenoliths that represent some of the oldest crust in Appalachian Grenville massifs (correlated with pre-Elzevirian crustal components in the Adirondack, Green Mountains, New Jersey Highlands, and French Broad massifs), (2) ca. l.15 to 1.05 Ga augen and granitic orthogneiss produced during Shawinigan and Ottawan phases of Grenville-age magmatism and metamorphism, respectively, and (3) paragneiss derived from protoliths with either Grenville-age (1.1–1.0 Ga) or post-Grenville (Neoproterozoic) depositional ages based on presence/absence of ca. 1.0 Ga metamorphic zircon and 1.9 to 1.1 Ga detrital zircon. All lithologies experienced Taconian metamorphism and variable migmatization. Pre-Ottawan paragneiss exhibits major detrital zircon ages modes at 2.0 to 1.6 and 1.4 to 1.3 Ga that require a component of older Proterozoic crust in the sediment source region. Detrital zircon grains in post-Ottawan paragneiss exhibits the full spectrum of Grenville-age modes that correlate with the five phases of Grenville magmatic/metamorphic events in eastern Laurentia. These paragneiss samples also contain scattered 750 to 600 Ma detrital zircon grains that constrain their maximum depositional age to late Neoproterozoic. The sedimentary protoliths of the latter paragneiss consist largely of detritus from exhumation of all Grenville crustal age components during post-orogenic exhumation and crustal extension leading up to Late Neoproterozoic breakup of Rodinia. Most zircon U-Pb age systematics exhibit variable discordance that can be attributed to disturbance of the U-Pb system and/or new zircon growth during either high-grade Ottawan (ca. 1.04 Ga) or Taconian (ca. 0.46 Ga) regional metamorphism and migmatization. Neodymium TDM model ages for granodioritic orthogneiss and paragneiss range from 1.8 to 1.6 Ga, indicating that the rocks were derived from recycling of Proterozoic crust (that is, they are not juvenile), consistent with the 1.9 to 1.6 Ga detrital zircon grains in pre-Ottawan paragneiss and with 1.8 to 1.7 Ga inherited zircon in some 1.33 Ga orthogneisses and a 1.35 Ga xenolith. Whole rock Pb isotope compositions of GSMBC rocks overlap the field of compositions characteristic of Amazonian crust and of other basement rocks from the south-central Appalachians. The Pb isotopes and geochronology in orthogneiss, mafic xenoliths, and pre-Ottawan paragneiss are consistent with a correlation of the GSMBC with the Mars Hill terrane within the French Broad massif and with the greater Grenvillian south-central Appalachian basement (SCAB) that is considered exotic to Laurentia, and transferred during Rodinian collision prior to ca. 1.2 Ga. Similarities in protolith ages and Pb isotopes point to the Paragua terrane of Amazonia (southwestern Brazil and eastern Bolivia) as a likely match for SCAB. Initial Amazonia-Laurentia collision occurred at ca. 1.35 to 1.32 Ga with final transfer of SCAB to Laurentia occurring after 1.20 Ga within the sinistral oblique collision zone between Laurentia and Amazonia defined by previous workers.

LA-ICPMS U–Pb geochronology of detrital zircon grains from the Coconino, Moenkopi, and Chinle formations in the Petrified Forest National Park (Arizona)

Abstract. Uranium–lead (U–Pb) geochronology was conducted by laser ablation – inductively coupled plasma mass spectrometry (LA-ICPMS) on 7175 detrital zircon grains from 29 samples from the Coconino Sandstone, Moenkopi Formation, and Chinle Formation. These samples were recovered from ∼ 520 m of drill core that was acquired during the Colorado Plateau Coring Project (CPCP), located in Petrified Forest National Park (Arizona). A sample from the lower Permian Coconino Sandstone yields a broad distribution of Proterozoic and Paleozoic ages that are consistent with derivation from the Appalachian and Ouachita orogens, with little input from local basement or Ancestral Rocky Mountain sources. Four samples from the Holbrook Member of the Moenkopi Formation yield a different set of Precambrian and Paleozoic age groups, indicating derivation from the Ouachita orogen, the East Mexico arc, and the Permo-Triassic arc built along the Cordilleran margin. A total of 23 samples from the Chinle Formation contain variable proportions of Proterozoic and Paleozoic zircon grains but are dominated by Late Triassic grains. LA-ICPMS ages of these grains belong to five main groups that correspond to the Mesa Redondo Member, Blue Mesa Member and lower part of the Sonsela Member, upper part of the Sonsela Member, middle part of the Petrified Forest Member, and upper part of the Petrified Forest Member. The ages of pre-Triassic grains also correspond to these chronostratigraphic units and are interpreted to reflect varying contributions from the Appalachian orogen to the east, Ouachita orogen to the southeast, Precambrian basement exposed in the ancestral Mogollon Highlands to the south, East Mexico arc, and Permian–Triassic arc built along the southern Cordilleran margin. Triassic grains in each chronostratigraphic unit also have distinct U and thorium (Th) concentrations, which are interpreted to reflect temporal changes in the chemistry of arc magmatism. Comparison of our LA-ICPMS ages with available chemical abrasion thermal ionization mass spectrometry (CA-TIMS) ages and new magnetostratigraphic data provides new insights into the depositional history of the Chinle Formation, as well as methods utilized to determine depositional ages of fluvial strata. For parts of the Chinle Formation that are dominated by fine-grained clastic strata (e.g., mudstone and siltstone), such as the Blue Mesa Member and Petrified Forest Member, all three chronometers agree (to within ∼ 1 Myr), and robust depositional chronologies have been determined. In contrast, for stratigraphic intervals dominated by coarse-grained clastic strata (e.g., sandstone), such as most of the Sonsela Member, the three chronologic records disagree due to recycling of older zircon grains and variable dilution of syn-depositional-age grains. This results in LA-ICPMS ages that significantly predate deposition and CA-TIMS ages that range between the other two chronometers. These complications challenge attempts to establish a well-defined chronostratigraphic age model for the Chinle Formation.

Stages of Carboniferous-Triassic magmatism in the Black Sea region based on isotope-geochronological study of detrital zircons from jurassic coarse clastic strata of the Mountainous Crimea

The article presents the results of U-Pb isotope dating of detrital zircons from the Jurassic coarse rocks in the apex and the western slope of Mnt. Biyuk-Sinor (the southern wall of the Baidar basin, near the village of Orlinoe). These dates are compared with the detrital zircon dates obtained for sandy rocks from the Upper Jurassic coarse clastic strata composing the slopes of Mnt. Spilia near Balaklava Harbor and Mnt. Southern Demerdzhi near Alushta city, as well as the Middle Jurassic Bitak conglomerates near the village of Strogonovka (suburb of Simferopol city). The comparison shows a high degree of similarity of the averaged age characteristics of the main detrital zircon populations. Sandy rocks of Jurassic coarse clastic strata for zircon dating were sampled in four locations of the Mountaineous Crimea. Based on their dates and a summary set of ages of detrital zircon grains from sandstones of the Southern Coast of Crimea, spanning the stratigraphic interval from the Middle Jurassic to Neogene, we can provide a statistically reliable specification of the Carboniferous-Triassic time interval (360–200 Ma) of magmatic activity within the Black Sea region. This period was bounded in time by the Late Devonian and Early Jurassic relative magmatic lulls. None of the zircon grains of the Carboniferous-Triassic age has revealed Hf-isotopic characteristics indicating any significant contribution of crustal material older than the Mesoproterozoic into the protolith of the parent zircon rocks. Within the Carboniferous-Triassic interval of magmatic activity, three stages are distinguished: (I) 360–315 Ma, (II) 315–270 Ma, and (III) 270–200 Ma. Magmatic stage I (360–315 Ma) is related to the closure of the Reik ocean, which completed after the subducted slab ‘broke off’ into the mantle and was accompanied by the ubiquitously manifested HT-LP metamorphism. Zircon grains of stage I are characterized by peak ages of about 325–340 Ma and the dominance of negative εHf. Magmatic stages II (315–270 Ma) and III (270–200 Ma) correlate with functioning of the Scythian-Pontian volcanic suprasubduction belt. In these magmatic stages, zircon εHf values scatter from weakly negative to substantially positive (referred to the depleted mantle), which is typical for volcanic arcs. Fuzzy separation of stages II and III and strong variability of the peak ages of zircons from the studied samples (which we associate with these stages) can be due both to changes in magmatic activity in different segments of the belt, and to changes in the erosion intensity of crystalline complexes of the belt during the subsequent stages evolution caused by tectonic rearrangements within the Paleo-Tethys ocean and its peri-oceanic structures.