Pb Ages Of Detrital Zircons Research Articles

Palaeoenvironmental analysis of Archaean siliciclastic sedimentary rocks in the west–central Jack Hills belt, Western Australia with new constraints on ages and correlations

Abstract: Zircons with ages between 3.05 and 4.4 Ga and hosted in conglomerates and associated sandstones in the Jack Hills belt of Western Australia have been the subject of numerous studies but none, to date, has been concerned with understanding the depositional environments of the sedimentary rocks. Intact, 250 and 320 m thick stratigraphical sections include conglomerates and sandstones that are equivalent to those at the W74 discovery site that host the ancient zircons. Facies associations are compatible with a prograding fan delta depositional setting in which alluvial fans were sourced by proximal highlands, were built directly into a lake or sea and were characterized by high gradients and high flow velocities. Locally, the succession is capped by a 20–25 m thick interval of cross-bedded quartz arenite–gritstone with a similar U–Pb detrital zircon age spectrum to the W74 discovery site, suggesting that this facies association is of comparable age and genetically related to the underlying facies associations. The quartz arenite–gritstone facies association is interpreted to record transgressive tidal reworking of the fan delta. Similar sedimentary units, including areally extensive quartz arenite successions, are widely developed elsewhere in Western Australia and on other continents, indicating that stable continental crust was widespread by 3.0 Ga.

Chemostratigraphy of Mesoproterozoic and Neoproterozoic carbonates of the Nico Pérez Terrane, Río de la Plata Craton, Uruguay

Increasing evidence shows that Mesoproterozoic rocks are widespread in the Río de la Plata Craton. Carbon and strontium isotope analyses were carried out for three different, carbonate-bearing successions in the southern Nico Pérez Terrane. The Parque UTE Group is erected, comprising (from base to top) the mainly volcanogenic Cañada Espinillo Formation, the dolomitic Mina Valencia Formation and the mixed carbonate-siliciclastic Cerro del Mástil Formation. A δ 13C curve was obtained for carbonates of the Parque UTE Group, which is characterized by a plateau at +1 to +1.6‰ V-PDB, bracketed between two negative excursions (−1.8‰ V-PDB at the base and −3.3‰ V-PDB at the top). These values are consistent with a Mesoproterozoic depositional age for the unit, as indicated by U–Pb ages of synsedimentary volcanics and gabbros of 1429 ± 21 and 1492 ± 4 Ma, respectively. The Mataojo Formation and the Marcos de los Reyes Formation were previously thought to be of similar age. The chemostratigraphic data, however, show that there is a hiatus of at least 700 Myr between these units, and that neither is correlative of the Parque UTE Group. Dolomitic marbles of the Mataojo Formation yielded a δ 13C curve that varies within a narrow range around 0‰ V-PDB (−0.6 to +0.4‰). This, along with the youngest U–Pb detrital zircon ages of 1802 Ma constrains deposition between 1.8 and 1.5 Ga (late Palaeoproterozoic–early Mesoproterozoic). In contrast with the latter unit, the Marco de los Reyes Formation yielded δ 13C values defining moderate to high-amplitude δ 13C excursions between +4.4‰ V-PDB and −3.2‰ V-PDB. Corresponding 87Sr/ 86Sr values range between 0.7070 and 0.7080. Both C and Sr chemostratigraphy point to a later Neoproterozoic, possibly Ediacaran age for the Marco de los Reyes Formation. It is here proposed that modest 13C seawater enrichment and low-amplitude δ 13C excursions, following a long period of isotopic invariance around 0‰ V-PDB, began already around 1.50–1.45 Ga. Thereafter, the amplitude of δ 13C excursions increased gradually until the late Neoproterozoic. One important conclusion emerging from the data presented in this paper is that Mesoproterozoic rocks are common in the Río de la Plata Craton, and probably fringe it at both its western and eastern side. This suggests a rather central position of the craton within Rodinia, as also suggested by other lines of evidence, such as detrital zircon ages and a widespread thermal overprint at 1.25 Ga.

Provenance of Ordovician clastic sequences of the San Rafael Block (Central Argentina), with emphasis on the Ponón Trehué Formation

The Ordovician Ponón Trehué Formation is the only early Palaeozoic sedimentary sequence known to record a primary contact with the Grenvillian-age basement of the Argentinean Cuyania terrane, in its southwards extension named the San Rafael block. Petrographic and geochemical data indicate contributions from a dominantly upper continental crustal component and a subordinated depleted component. Nd isotopes indicate ε Nd of − 4.6, ƒ Sm/Nd − 0.36 and T DM 1.47 Ga in average. Pb-isotope ratios display average values for 206Pb/ 204Pb, 207Pb/ 204Pb, and 208Pb/ 204Pb of 19.15, 15.69 and 38.94 respectively. U–Pb detrital zircon ages from the Ponón Trehué Formation cluster around values of 1.2 Ga, indicating a main derivation from a local basement source (Cerro La Ventana Formation). The Upper Ordovician Pavón Formation records a younger episode of clastic sedimentation within the San Rafael block, and it shows a more complex detrital zircon age population (peaks at 1.1 and 1.4 Ga as well as Palaeoproterozoic and Neoproterozoic detrital grains). Detailed comparison between the two Ordovician clastic units indicates a shift with time in provenance from localized basement to more regional sources. Middle to early Upper Ordovician age is inferred for accretion of the Cuyania terrane to the proto-Andean margin of Gondwana.

The Pacific Gondwana margin in the late Neoproterozoic–early Paleozoic: Detrital zircon U–Pb ages from metasediments in northwest Argentina reveal their maximum age, provenance and tectonic setting

U–Pb detrital zircon ages are reported from Puncoviscana Formation (late Neoproterozoic–Early Cambrian) and Mesón Group (Late Cambrian) greywackes of northwest Argentina, to constrain provenance and depositional environment. The new data are combined with previously-published detrital zircon ages, to show that Puncoviscana Formation age patterns contain two broad groups: late Mesoproterozoic–early Neoproterozoic (1150–850 Ma) and late Neoproterozoic–Early Cambrian (650–520 Ma); with their relative proportions varying inversely with youngest component age. The 1150–850 Ma age components are dominant in greywackes with oldest late Neoproterozoic components > 600 Ma. The former diminish considerably when late Neoproteozoic components become dominant and younger, to 520 Ma. A northernmost greywacke sample from Purmamarca, Jujuy, is distinctive: whilst its zircon age pattern partly resembles other Puncoviscana Formation samples, it contains no Cambrian–late Neoproterozoic ages, the youngest ages being early Neoproterozoic. This may reflect an early, Neoproterozoic, passive-margin depocentre for the Formation, or an older (early Neoproterozoic) succession within it, which may predate the Brasiliano orogeny in Brazil. The youngest age components, c. 520 Ma, in a greywacke from Rancagua (Cachi, Salta province), dominate an almost unimodal pattern suggestive of contemporary volcanic sources at a late Early Cambrian depocentre. Detrital zircon age patterns of the Mesón Group (Lizoite Formation) have major Cambrian–latest Neoproterozoic components resembling those of the Puncoviscana Formation, but its Mesoproterozoic component is diminished, and there are no significant age components of this age. Small youngest components at c. 500 Ma suggest a maximum Late Cambrian stratigraphic age. The Puncoviscana Formation detrital zircon patterns suggest a provenance in a continental hinterland having a stabilised, extensive late Mesoproterozoic orogen (with minor Paleoproterozoic and Archean precursors), and a more variable late Neoproterozoic orogen containing an evolving sequence of less extensive subcomponents. A direct relationship with the Brazilian Shield is suggested; with sediment supplies originating within active-margin orogens of the interior and collisional orogens at the suture between African and South American cratons, but ultimate deposition in passive-margin environments of western Gondwanaland.

Detrital zircon, detrital titanite and igneous clast U–Pb geochronology and basement–cover relationships of the Colonsay Group, SW Scotland: Laurentian provenance and correlation with the Neoproterozoic Dalradian Supergroup

A multidisciplinary provenance study, including in situ U–Pb dating of detrital titanite, was undertaken on the enigmatic low-grade metasediments of the Colonsay Group, SW Scotland to determine their source and assess previous correlations with major Neoproterozoic sedimentary sequences within the Scottish Caledonides. Sm–Nd model ages ( t DM) of the Colonsay Group range from 1664 to 2140 Ma, suggesting a Palaeoproterozoic source. Laser Ablation ICPMS U–Pb detrital zircon ages from six Colonsay Group formations are consistent with sources within Laurentia with predominant input from a ca. 1780 Ma source (Rhinns Complex) and some Grenvillian ( ca. 1.3–0.95 Ga), Pinwarian ( ca. 1.51–1.45 Ga), Labradorian ( ca. 1.71–1.62 Ga) and Ketilidian ( ca. 1.9–1.75 Ga) detritus. Archaean (> ca. 2.5 Ga) input is minimal. Clasts from the basal Octofad Sandstone Formation indicate input from ca. 1795 Ma and ca. 1400 Ma sources (U–Pb SIMS zircon ages). U–Pb (SIMS) analyses of detrital titanite record Grenville metamorphic events in the source terranes and suggest a slightly younger maximum depositional age of ca. 942 Ma compared with the youngest detrital zircon age of ca. 1025 Ma. The data support the interpretation that the Octofad Sandstone Formation is part of the Colonsay Group and that the Colonsay Group rests unconformably on the Rhinns Complex on Islay. Deposition of the Colonsay Group is, therefore, tied to the margins of Laurentia, possibly in the foreland to the Grenville Belt, as the Rhinns Complex does not record any Grenville deformation. The data are consistent with deposition of the Colonsay Group in a foreland basin related to the Rigolet phase ( ca. 1000–980 Ma) of the Grenville Orogeny. On the basis of detrital zircon U–Pb ages the Colonsay Group best correlates with the lowermost Dalradian Grampian Group of Scotland. This correlation has significant implications for Dalradian basin dynamics, timing of Dalradian deposition and timing of Dalradian deformation. In addition, the unconformable contact between the Colonsay Group and the Rhinns Complex on Islay would make it one of the few places where the basement to the Dalradian can be observed.

Provenance of Neoproterozoic and early Paleozoic siliciclastic rocks of the Teplá-Barrandian unit (Bohemian Massif): Evidence from U–Pb detrital zircon ages

New LA-ICP-MS U–Pb detrital zircon ages from Ediacaran and Paleozoic siliciclastic rocks are used to constrain provenance and paleogeographic affinities of the Teplá-Barrandian unit (TBU) in the centre of the Bohemian Massif (Central Europe, Czech Republic). The samples taken span the period from ≤ 635 Ma to ∼ 385 Ma and permit recognition of provenance changes that reflect changes in geotectonic regime. Detrital zircon age spectra of two Ediacaran, one Lower Cambrian and three Upper Ordovician samples resemble the ages known from the NW African proportion of Gondwana, particularly the Trans-Saharan belt, while three rocks from higher Lower Cambrian to Lowermost Ordovician strata contain detritus that may have been derived exclusively from local sources. The age spectrum of the Devonian rock is a combination of the NW Gondwanan and local features. These new findings in combination with a wide range of published data are in agreement with a Neoproterozoic subduction-related setting at the margin of Gondwana followed by a Cambrian/Early Ordovician rifting stage and an Ordovician passive margin setting. Furthermore the data are in favour of a position of the TBU at the Gondwanan margin throughout pre-Variscan times.

Comment on “Detrital U–Pb zircon dating of lower Ordovician syn-arc-continent collision conglomerates in the Irish Caledonides” by Peter D. Clift, Andrew Carter, Amy E. Draut, Hoang Van Long, David M. Chew, Hans A. Schouten, Tectonophysics 479 (2009), 165–174 (doi:10.1016/j.tecto.2008.07.018)

Clift et al. [Clift, P.D., Carter, A., Draut, A.E., Van Long, H., Chew, D.M. & Schouten, H.A., 2009.Detrital U–Pb zircon dating of lower Ordovician syn-arc-continent collision conglomerates in the Irish Caledonides. Tectonophysics, 479, 165–174] present U–Pb detrital zircon age data for one sample from each of three formations of the Ordovician South Mayo Trough sedimentary sequence and interpret these in the context of a ‘Taiwan model’ of arc-continent collision on the Laurentian margin of Iapetus. Their interpretation relies on an overly simplistic treatment of their data and does not take cognisance of other data relevant to the relative time of deposition of the sampled horizons within the orogenic history and the evolving availability of sediment source rocks.

LA ICP MS U–Pb ages of detrital zircons from Russia largest rivers: Implications for major granitoid events in Eurasia and global episodes of supercontinent formation

The paper presents LA ICP MS U–Pb age data on detrital zircons from sands of major Russian rivers: the Don, Volga, Ob’, Yenisey and Amur. The obtained data are discussed in terms of major episodes of granitic magmatism, which are recorded in the continental blocks that form the modern Eurasian continent. Results are compared with published igneous and detrital zircon age data obtained from parental and sedimentary rocks of the river basins under consideration and worldwide. The U–Pb age results allowed us to confirm (i) the episodic character of continent formation; (ii) the Neoarchean global magmatic event and, possibly, formation of the Kenorland supercontinent; (iii) the global episode of crust formation at 2.0–1.8 Ga, which formed the Columbia supercontinent; (iv) the breakup of Columbia at 1.3–1.2 Ga; (v) the major period of Phanerozoic crustal growth in Central Asia which is likely to be a result of the Altaid orogeny. On the other hand, our data did not unambiguously confirm previous idea about the global character of the Grenvillian and Pan-African orogenies as a result of the assembly of Rodinia and Gondwana, respectively. The “Rodinia” peak is not observed in the histograms of the Ob’, Yenisey and Amur, whereas the “Gondwana” signature is not obvious in the histograms of the Don, Volga and Amur. Of special interest are the 2.7–2.5 Ga and 2.0–1.7 Ga peaks in the Ob’ zircon age spectrum in spite of the absence of so far identified Archean and Paleoproterozoic parental rocks in the Ob’ catchment area. The obtained age spectra were joined into three groups based on statistics: Baltica (Don and Volga), Siberia (Ob’ and Yenisey) and East Asia (Amur). For future reconstructions we suggest to include all the available results on Don and Volga rivers into a North America-Baltica Group corresponding to Laurentia-derived continental blocks, and to consider a Siberia Group (Siberian Craton; Ob’, Yenisey, Lena, Indigirka rivers) and an East Asia Group (North and South China cratonic blocks; Amur, Yellow, Yangtze and Mekong rivers).

A buried Palaeoproterozoic spreading ridge in the northern Nagssugtoqidian orogen, West Greenland

Abstract Ophiolitic rocks on two small island groups in the northern Nagssugtoqidian orogen, West Greenland, shed new light on the eastern Laurentian Palaeoproterozoic plate-tectonic collage. The islands expose amphibolite-facies (520–550 °C, 2.6–3.0 kbar) tholeiitic pillow lava, black chloritic shale, manganiferous banded iron formation (BIF), podded chert, jasper, graded andalusite–staurolite schist with numerous sills, and terrigenous sandstone, an association that occurs today in oceanic spreading zones undergoing burial in forearc trenches. Deformation is weak, with upright folds preserving original younging. U–Pb detrital zircon ages indicate derivation from Archaean, 1890 Ma, and 1850 Ma sources. Occurring north of the Archaean Aasiaat domain that partly escaped Nagssugtoqidian deformation, the sequence pinpoints a new SSE-dipping Palaeoproterozoic subduction zone that links up in the NE with the Disko Bugt suture at the Nagssugtoqidian–Rinkian boundary. This solves an enigma of the previous plate-tectonic model, in which the Arfersiorfik magmatic arc in the central Nagssugtoqidian orogen was located within its own suture zone, and helps to explain why the Nagssugtoqidian–Rinkian system is so broad. It also explains previously recognized geochemical and Pb-isotopic compositional breaks in orthogneisses in the Kangaatsiaq region, and greatly simplifies correlation with adjacent eastern Canadian orogens in Baffin Island and Labrador.

U–Pb ages of detrital zircons from the Basal allochthonous units of NW Iberia: Provenance and paleoposition on the northern margin of Gondwana during the Neoproterozoic and Paleozoic

LA-ICP-MS U–Pb ages of detrital zircons from eight siliciclastic samples from the Basal units of the Variscan allochthonous complexes of NW Iberia are used to establish the maximum depositional age and provenance of two tectonically-stacked metasedimentary sequences deposited on the outermost margin of Gondwana, and subsequently involved in the Rheic Ocean suture. The maximum depositional ages for the two sequences is latest Neoproterozoic and latest Cambrian, respectively. The age spectra are also used to discuss the paleoposition of the NW Iberian basement on the continental margin of Gondwana prior to the opening of the Rheic Ocean, which is tentatively placed in northern Africa, between the West African and Saharan cratons. Based on similarities and differences with age data from the NW Iberian autochthon and other allochthonous terranes involved in the Rheic suture, the relative proportions of Mesoproterozoic zircons in both assemblages are proposed as markers of proximity to the eastern part of the West African craton during late Neoproterozoic and late Cambrian. The geodynamic processes that took place along this part of Gondwana during the late Neoproterozoic, late Cambrian and Early Ordovician are discussed in the light of the LA-ICP-MS results, as well as the sedimentological record, magmatic evolution and plate tectonic setting of NW Iberia. These processes are linked to late Neoproterozoic and Cambro-Ordovician subduction events beneath the northern Gondwanan margin.

Plate tectonic significance of Middle Cambrian and Ordovician siliciclastic rocks of the Bavarian Facies, Armorican Terrane Assemblage, Germany — U–Pb and Hf isotope evidence from detrital zircons

We have examined the provenance and tectonic setting of the Middle Cambrian and Ordovician siliciclastic deposits and associated volcanic rocks of the Bavarian Facies, Franconia, Germany, in the Saxothuringian Zone of the Variscan orogen. The units were deposited on the North African rifted margin of Gondwana and represent deep-water equivalents of the shallow marine mature sandstone successions of the areally extensive Thuringian Facies. U–Pb ages of detrital zircons of the Middle Cambrian Wildensteiner Formation, the Middle Ordovician Plattensandstone of the Randschiefer Series, and the Upper Ordovician Döbra sandstone fall into four distinct age groups: 2.4 to 1.8 Ga (15%), 0.75 to 0.54 Ga (55%) and 0.54 to 0.44 Ga (14%); minor abundances (4%) occur around 1 Ga. This age distribution is consistent with a northern Gondwanan derivation, mainly from the Cadomian continental magmatic arc in northern Africa. The c. 2 Ga ages indicate a provenance from Eburnean or equivalent sources on the West Africa craton and in northeast Africa and Arabia. The scarcity of grains of Kibaran age (c. 1 Ga) is characteristic of a derivation from metamorphic and magmatic sources on the Arabian–Nubian Shield, rather than from distant major Kibaran age terrains. The youngest group mainly reflects Late Cambrian to Ordovician rift magmatism widespread in the northern Gondwanan Cadomian terranes. The Hf isotopic compositions of selected dated zircons at the time of their crystallization ( ε Hf( t) ; T = 3.5–0.47 Ga) vary between − 30 and + 7. Zircons with positive ε Hf( t ) values are almost exclusively restricted to the age group between 0.5 and 0.9 Ga. The Hf isotope data suggest that magmatism associated with the Cadomian continental magmatic arc and post-Cadomian (late Cambrian-Ordovician) marginal rifts in northern Gondwana mainly involved mixing of juvenile magmas with Paleoproterozoic crust.

Use of U–Pb ages of detrital zircons to infer maximum depositional ages of strata: A test against a Colorado Plateau Mesozoic database

We test the research strategy of using youngest U–Pb ages of detrital zircons to constrain the maximum depositional ages of strata containing the zircon grains by comparing U–Pb ages of detrital zircons in 58 samples of Mesozoic sandstone from the Colorado Plateau and adjacent areas with depositional ages known independently from biostratigraphy. Our analysis confirms the validity of the research strategy but indicates that results vary somewhat depending upon how youngest grain age is specified. We use four alternate measures of youngest age which vary from least to most statistically robust as follows: (a) youngest single grain age, (b) youngest graphical age peak controlled by more than one grain age; (c) mean age of the youngest two or more grains that overlap in age at 1 σ, (d) mean age of the youngest three or more grains that overlap in age at 2 σ. We also calculated the “youngest detrital zircon age” generated by Isoplot 2008 but do not recommend that model age as a youngest-age measure. In general, the youngest-age measures based on multiple grain ages are more consistently compatible with depositional ages, but the youngest single grain ages are compatible with depositional age for > 90% of samples, and lie within 5 Ma of depositional age for ~60% of samples. Selected minor discrepancies between youngest grain age and depositional age may reflect stratigraphic miscorrelations rather than errors in U–Pb geochronology.

Insights into North American Paleogeography and Paleotectonics from U–Pb ages of detrital zircons in Mesozoic strata of the Colorado Plateau, USA

Individual U–Pb ages for 5,655 detrital zircons (DZ) in 61 sandstone samples from Mesozoic strata of the Colorado Plateau and nearby areas provide insights into paleogeographic relations across the interior of North America and the paleotectonic evolution of North American continental margins. Pre-Mesozoic DZ grains derived either directly, or ultimately through sediment recycling, from distant sources in eastern North America are more abundant than DZ grains derived from the nearby Cordilleran magmatic arc of western North America. Sediment dispersal patterns included Triassic fluvial transport of detritus westward from the Ouachita orogen uplifted along the northern flank of rift highlands precursor to the oceanic Gulf of Mexico, Jurassic eolian transport southward into widespread ergs from deflation of floodplains of transcontinental paleorivers with headwaters in pre-Atlantic Appalachian highlands, and Jurassic-Cretaceous recycling of eolianite DZ from retroarc Sevier thrust sheets and from sedimentary cover of the Mogollon paleohighlands flanking the Border rift system.

U–Pb detrital zircon ages and Sm–Nd isotopic features in low-grade metasedimentary rocks of the Famatina belt: implications for late Neoproterozoic–early Palaeozoic evolution of the proto-Andean margin of Gondwana

Abstract: The Famatina belt, Central Andes, is part of an ancient accretionary margin built along Western Gondwana in the early Palaeozoic. U–Pb ion microprobe analysis of detrital zircons and Sm–Nd whole-rock analysis of two early Palaeozoic low-grade metasedimentary units record the early evolution of this region. Detrital zircons in the Negro Peinado and Achavil formations have ages ranging from Palaeoproterozoic to Cambrian, consistent with derivation from Gondwanan sources. T DM ages suggest that the sedimentary rocks were derived from a composite source area, which separated from the mantle during the Palaeoproterozoic ( c . 1.8–1.6 Ga). Constraints from the youngest detrital grains indicate accumulation in a Mid- to Late Cambrian foreland basin adjacent to the inboard Pampean orogenic tract. The dominance of Cambrian ages in the Negro Peinado Formation suggests derivation principally from the eastern Pampean belt whereas the dominance of late Neoproterozoic ages in the Achavil Formation suggests that input from the Pampean belt was overwhelmed by older sources. The paucity of Palaeoproterozoic ages argues against direct input from older areas such as the Río de la Plata craton. The predominance of Meso- and Neoproterozoic ages over older sources suggests that a Brasiliano-age magmatic arc developed on a Mesoproterozoic basement, probably a southern extension of the Arequipa–Antofalla massif.

Late Neoproterozoic passive margin of East Gondwana: Geochemical constraints from the Anakie Inlier, central Queensland, Australia

Development of the East Gondwana passive margin and when it occurred are constrained by the composition of low-grade mafic schists and U–Pb ages of detrital zircons in psammitic schists from the Bathampton Metamorphics in the Anakie Inlier of central Queensland. These rocks show considerable variation in light lithophile elements due to post-magmatic processes. They have flat heavy rare earth element patterns, low-TiO 2 (<2 wt.%) contents and their immobile element Ti, V, Y, La, Nb, Th and Zr values, indicate that they have an N-MORB-like magmatic affinity. However, they differ from N-MORB in that they show light rare earth depleted patterns and lower incompatible trace element contents. Their relative low abundance and association with metasediments suggest they formed in a magma-poor rifted margin setting. They are associated with psammitic rocks with detrital zircon ages indicating probable deposition in the late Neoproterozoic at ca 600 Ma. A magma-poor rifted margin in northeastern Australia differs from the volcanic passive setting that occurred in southeastern Australia at this time. These findings support development of the East Gondwana margin at 600 Ma that may have been related to rifting of a microcontinent off East Gondwana well after the breakup of Rodinia at ca 750 Ma.

Detrital zircon ages of Neoproterozoic sedimentary successions in Uruguay and Argentina: Insights into the geological evolution of the Río de la Plata Craton

Although the Río de la Plata craton is exposed over a relatively large area in Uruguay, eastern Argentina, and southernmost Brazil, its geological evolution is poorly known because of great geological complexity and paucity of data. We report here U–Pb ages of detrital zircons from samples of five Neoproterozoic sandstone units deposited on the Río de la Plata Craton (RPC) in Uruguay and Argentina. The zircon ages provide definitive evidence of a great range of ages of rocks composing the craton—ages that would not have been known without the geochronology of detrital zircons. In turn, these new ages allow for a much fuller understanding of the nature and history of the craton. Quartz arenites of the Piedras de Afilar Formation show typical Transamazonian ages with peaks at 2.00–2.07, 1.87 and 1.78 Ga. However, the most important zircon population in the sample analyzed is Mesoproterozoic with maxima at 1.49, 1.35, 1.24 and 1.0 Ga. Archean zircons are subordinate. On the other hand, zircons recovered from two sandstone levels in the Arroyo del Soldado Group (Yerbal and Cerros San Francisco formations) are mostly Archean in age with maxima at 3.2 and 2.72–2.78 Ga. Palaeoproterozoic zircons are also prominent in this unit with peaks at 2.45 and 2.19, of which the latter is a typical Transamazonian age. Two samples from the Sierras Bayas Group in Tandilia (Argentina) show different age spectra. Sandstones of the Villa Mónica Formation have a unimodal zircon population of Transamazonian age with a peak at 2.15 Ga. Sandstones of the Cerro Largo Formation are characterized by a dominant Transamazonian zircon population with peaks at 2.15, 2.0 and 1.78, but also display important Archean-earliest Palaeoproterozoic (3.33, 2.99, 2.7, 2.46 Ga) and Mesoproterozoic peaks (1.55, 1.23 and 1.05 Ga). The abundance of Mesoproterozoic detrital zircons is surprising, given the limited outcrop area of Mesoproterozoic units in the RPC. Thus, the Mesoproterozoic orogenic event responsible for the generation of the dextral Sarandí del Yí megashear is much more important than previously assumed. A proto-Andean, Mesoproterozoic belt is suggested as the source of the Mesoproterozoic detritus. Archean rocks of the RPC crop out only in the Nico Pérez Terrane in Uruguay. The presence of Archean zircons in sandstones of the Sierras Bayas Group in Argentina suggests that the Nico Pérez Terrane was much closer to Tandilia than it is today. This is in accordance with the proposed sinistral reactivation of the Sarandí del Yí Shear Zone in the Cambrian, which resulted from tangential collision of the Cuchilla Dionisio-Pelotas Terrane. Finally, the conspicuous absence of Neoproterozoic zircons confirm other lines of evidence suggesting that the Neoproterozoic strata were deposited on a stable continental margin opening to the east and south. The Neoproterozoic basins had obviously no contribution whatsoever from Brasiliano-Pan African belts, supporting the idea of Cambrian terrane accretion as the mechanism responsible for the present configuration of the Río de la Plata Craton and the final amalgamation of Gondwana.

High-pressure metamorphic rocks from Tongbaishan, central China: U–Pb and 40Ar/ 39Ar age constraints on the provenance of protoliths and timing of metamorphism

The Tongbaishan area is situated at the junction between the Paleozoic North Qinling orogen and the Mesozoic South Qinling–Dabie–Sulu orogen. The area contains an important geological section for understanding the assembly of the Sino-Korean and Yangtze cratons. Two eclogite zones were recently recognized in this area. The peak metamorphic conditions are estimated to be 530–610 °C and 17–20 kbar for the northern eclogite zone and 460–560 °C and 13–19 kbar for the southern eclogite zone. SHRIMP U–Pb analyses on zircons of eclogite, metagabbro and orthogneiss indicate that their protoliths were formed during the Neoproterozoic, and experienced high-pressure (HP) recrystallization during the Permian–Triassic period. Therefore, the Tongbaishan area belongs to the western extension of the Dabieshan HP/UHP terrane. In addition, U–Pb ages of detrital zircons from quartzite, paragneiss and schist show three age populations of 2.49 Ga, 1.93 Ga and 1.85–1.82 Ga, suggesting that extensive Paleoproterozoic orogeny or tectonothermal events took place in the northern margin of the Yangtze craton. The SHRIMP dating and complementary 40Ar/ 39Ar dating of muscovite from quartzofeldspathic gneiss and quartzite reveal that the crustal rocks were subducted to great depths and underwent HP eclogite facies metamorphism at ca. 255 Ma, slightly earlier than the timing of deep subduction of the UHP slice. The HP rocks were later exhumed to mid- to upper-crustal levels and suffered from retrograde recrystallization at ca. 238 Ma. The dating also discloses a still later phase of deformation and recrystallization at ca. 215 Ma. The available age data throughout the Tongbai–Dabie–Sulu orogen reflect diachronous exhumation from ca. 238 Ma for the HP slice to ca. 213 Ma for the UHP slice. Such a phenomenon could be explained by successive subduction and exhumation of the broken slices of the subducting Yangtze slab.

Kimberlite-sourced bentonite, its paleoenvironment and implications for the Late Cretaceous K14 kimberlite cluster, northern AlbertaThis article is one of a selection of papers published in this Special Issue on the themeGeology of northeastern British Columbia and northwestern Alberta: diamonds, shallow gas, gravel, and glaciers.

Kimberlite-sourced bentonite layers have been discovered in Late Cretaceous sedimentary drill cores located within the Buffalo Head Hills kimberlite field, north-central Alberta. Two bentonites are unambiguously differentiated from “common” intermediate to felsic volcanic-derived Alberta bentonite by having similar whole-rock geochemical composition to ultramafic rocks from the Buffalo Head Hills kimberlite field and worldwide crater-facies kimberlite. The results demonstrate that the geochemical analysis of bentonite can provide a quick, cost-effective means of testing for low-volume kimberlite volcanism. The kimberlite-sourced bentonite is associated with a cluster of Buffalo Head Hills kimberlites known as the K14 complex. The K14A kimberlite occurrence has a previously reported206Pb/238U perovskite emplacement age of 86.8 ± 2.1 Ma. This age is compatible with early or mid-Cenomanian (∼99–96 Ma) and mid-Coniacian to Santonian (∼88–84 Ma) palynological results for mudstone underlying and bracketing the K14A and K14B kimberlites, respectively. Conversely, biostratigraphically significant palynomorphs in host rock cores bracketing the K14C kimberlite indicate emplacement during the Cenomanian or Turonian (∼96–92 Ma). U–Pb detrital zircon ages from syndepositional kimberlite-sourced bentonite directly adjacent to the K14C kimberlite contains a high frequency of young (∼99–92 Ma) zircon with a best maximum206Pb/238U depositional age of 91.7 ± 2.9 Ma. Thus, this innovative approach may provide evidence of multiple episodes of kimberlite emplacement within or near the K14 complex.

U–Pb detrital zircon ages in synorogenic deposits of the NW Iberian Massif (Variscan belt): interplay of Devonian–Carboniferous sedimentation and thrust tectonics

Detrital zircons from Devonian and Carboniferous synorogenic flysch deposits occurring in an imbricate stack have been dated by laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) to: (1) obtain a maximum depositional age to constrain the maximum age limit for thrusting of exotic terranes in the NW Iberian Massif; (2) correlate the zircon age populations with published ages in nearby units to establish their possible source areas. The maximum depositional ages are Late Devonian for rocks high in the structural nappe pile (Gimonde Formation), in accordance with palynomorph dating, and around the Devonian–Carboniferous boundary for structurally lower samples (San Vitero Formation). Used in conjunction with previously published ages, the new ages are interpreted in terms of the advance of the thrust system responsible for the emplacement of exotic terranes upon the Iberian autochthon during the Variscan collision. Early Variscan zircon population ages indicate the exotic terranes as the source of synorogenic sediments, whereas their scarcity suggests derivation from the Iberian autochthon. One of the samples analysed lacks Variscan detrital zircons; this feature, together with the absence of an Early Palaeozoic zircon age population, puts into question its synorogenic character and suggests that the sample may be representative of the preorogenic parautochthon.

West Gondwana amalgamation based on detrital zircon ages from Neoproterozoic Ribeira and Dom Feliciano belts of South America and comparison with coeval sequences from SW Africa

Abstract Neoproterozoic–Cambrian amalgamation of West Gondwana involved the collision of several terranes of older crust that are now in eastern South America and western Africa. U–Pb (SHRIMP) detrital zircon ages from representative metasedimentary units of the Ribeira and Dom Feliciano belts (South America) and Gariep and Damara belts (Africa) provide constraints on the possible sediment source areas across probable suture zones. Ribeira detrital zircons are Palaeoproterozoic and Archaean. For the Dom Feliciano Belt, a contribution of Meso- and Neoproterozoic zircons is present, which definitely indicate Neoproterozoic sedimentation. It is proposed that the inflow of material to the Ribeira basin was essentially derived from the Paranapanema and Rio de la Plata cratons, whereas for the Damara and Gariep–Rocha belts source areas were from the Namaqua Belt. The Dom Feliciano Belt received sediments from the South American side and to a lesser degree from African sources. These results highlight the differences in the detrital zircon signatures across a proposed West Gondwanan suture, with those in the west being derived from distinctive South American basement sources and those in the east from distinctive African sources.