Detrital Zircon Suites Research Articles

Retrieving meaningful information from detrital zircon in Palaeoproterozoic sedimentary rocks: Provenance, timing of deposition, metamorphism and alteration of zircon in sandstones of the Pretoria Group in the Transvaal Basin, South Africa

Abstract The Palaeoproterozoic sandstones and quartzites of the Pretoria Group (Transvaal Supergroup) in the Transvaal Basin of South Africa are important markers for regional correlations and dating of events of global importance (e.g., the Great Oxidation Event). The succession has few independent age markers, and much of the discussion about the time of deposition and the source of material of these rocks has been based on data from detrital zircon suites. The clastic sedimentary rocks of the Pretoria Group contain detrital zircon grains ranging from the Mesoarchaean to ages that are near-contemporaneous to, and even younger than the overlying and crosscutting igneous rocks of the Bushveld Complex. We show that the U-Pb age and Lu-Hf isotope distributions of the detrital zircon population in the Pretoria Group are the result of three different types of processes, acting successively: (1) Crystallisation in the igneous or metamorphic protosource rock (i.e., the rock where the zircon originally crystallised), (2) Metamorphic and hydrothermal resetting of the U-Pb chronometer induced by emplacement and crystallisation of the 2 055 Ma Bushveld Complex, and (3) Late, low-temperature processes (e.g., weathering). Critical age markers of maximum ages of deposition obtained after excluding effects of (2) and (3) are the 2 200 Ma Magaliesberg Formation (outside of the Bushveld aureole) and the 2 080 to 2 100 Ma Lakenvalei Formation. The Leeuwpoort Formation is a worst-case example, containing both young (<2 200 Ma) unmodified detrital zircon and hydrothermally altered zircon in the same age range. The two can only be distinguished from trace element analyses. Age distributions of Archaean and early Palaeoproterozoic zircon age fractions overlap with detrital zircon age suites in lower (i.e., pre-Timeball Hill Formation) parts of the Transvaal Supergroup, suggesting recycling within the basin or from the basin margin. Overlaps in 2 200 to 2 350 Ma zircon ages with those of volcanogenic zircon in the Timeball Hill Formation again suggest recycling. The origin of 2 080 to 2 150 Ma zircon is uncertain, but neither poorly constrained sources in the Kaapvaal Craton (e.g., Okwa Basement Complex) nor recycling of volcanogenic material from post-Magaliesberg formations can be ruled out.

A juvenile Paleozoic ocean floor origin for eastern Stikinia, Canadian Cordillera

Abstract The Cordillera of Canada and Alaska is a type example of an accretionary orogen, but the origin of some terranes remains contentious (e.g., Stikinia of British Columbia and Yukon, Canada). Presented herein are igneous and detrital zircon U/Pb-Hf and trace-element data, as well as the first radio larian ages from the Asitka Group, the basement to eastern Stikinia. The data are used to evaluate the role of juvenile and ancient crust in the evolution of Stikinia and the tectonic environment of magmatism. Two rhyolites are dated by U-Pb zircon at 288.64 ± 0.21 Ma and 293.89 ± 0.31 Ma, with εHf(t) = +10. Red chert contains radiolarians that are correlated with P. scalprata m. rhombothoracata + Ruzhencevispongus uralicus assemblages (Artinskian–Kungurian). Detrital zircon U/Pb-Hf from a rare Asitka Group sandstone have a mode at ca. 320 Ma and εHf(t) +10 to +16; the detrital zircon suite includes five Paleoproterozoic zircons (~5% of the population). Detrital zircons from a stratigraphically over lying Hazelton Group (Telkwa Formation) volcanic sandstone indicate deposition at ca. 196 Ma with zircon εHf(t) that are on a crustal evolution line anchored from the Asitka Group. Zircon trace-element data indicate that the Carboniferous detrital zircons formed in an ocean arc environment. The Proterozoic detrital zircons were derived from a peripheral landmass, but there is no zircon εHf(t) evidence that such a land-mass played any role in the magmatic evolution of eastern Stikinia. The data support that eastern Stikinia formed on Paleozoic ocean floor during the Carboniferous to early Permian. Consistent with previous fossil modeling, zircon statistical comparisons demonstrate that Stikinia and Wrangellia were related terranes during the Carboniferous to Permian, and they evolved separately from Yukon-Tanana terrane and cratonic North America.

Eoarchean to Neoarchean crustal evolution of the Western Dharwar Craton, southern India: Clues from U-Pb-Hf isotope composition of detrital zircon

In this study, we use the U-Pb-Hf isotope composition of detrital zircon in metasedimentary rocks from the Western Dharwar Craton to identify older crusts, their nature, source, timing of extraction, and reworking. The samples come from the Holenarsipur region in the central and Sargur region in the southern part of the craton and belong to both the Sargur and the Bababudan Groups. Both detrital and metamorphic domains were identified in the zircon. The metamorphic overgrowths correspond to two age populations at 2536–2516 Ma, and 2478–2460 Ma. Rutile grains provide either concordant 2453 Ma, or discordant younger ages suggestive of thorough re-equilibration during metamorphism. Detrital zircon grains define notable clusters at 3685–3638 Ma, 3573–3524 Ma, 3471–3411 Ma, 3373–3341 Ma, 3277–3249 Ma, 3172–3148 Ma, 3084–2996 Ma, and 2659–2647 Ma which correspond to major episodes of granitoid crust formation in the craton. Based on detrital and metamorphic zircon populations, deposition of both the older Sargur Group and the basal unit of the younger Bababudan Group in the Holenarsipur region were broadly contemporaneous after c. 3160 Ma, while the sediments of the purported Sargur Group in the Sargur region were deposited after c. 2650 Ma. The new age data requires revision of the stratigraphic positions of the Sargur-type and Dharwar-type greenstone belt successions. The detrital zircon suites have both radiogenic and crust-like Hf isotopic compositions. However, the majority have radiogenic compositions that were acquired during major events of juvenile crust extraction between c. 3850 Ma and 3250 Ma. Strongly unradiogenic ɛHf(t) of some Paleoarchean detrital zircon require protoliths extracted from the mantle in the Hadean, but which persisted for a long period and contributed to granitoid formation in the Paleoarchean and the Mesoarchean. Mixing of juvenile magmas with preexisting crustal components also played a significant role in the petrogenesis of the granitoids. A distinct shift in the ɛHf(t) of the detrital zircon from chondritic to positive value at c. 3600 Ma is noted, reflecting a marked increase in the contribution of strongly depleted mantle reservoir to the source of the post-c. 3600 Ma granitoids.

Tracking chemical alteration in magmatic zircon using rare earth element abundances

Trace elements in magmatic zircon provide a wealth of petrogenetic information about host magmas. However, they are susceptible to alteration through post-magmatic interaction with hydrothermal fluids. Trace element analyses can also be biased by the inadvertent inclusion of exotic materials, such as mineral or glass inclusions, in analyzed volumes of zircon. In order to screen out samples with altered chemical signatures, zircons with high, flat light rare earth element (LREE) patterns are typically considered to be altered. However, visual selection of such patterns is qualitative and does not address ambiguous cases. The light rare earth element index (LREE-I = Dy/Nd + Dy/Sm) provides an approach for quantitative screening for aqueous alteration and contamination of zircon by exotic materials and was used to assess secondary processes in the greenschist facies Jack Hills detrital zircon suite. However, in addition to aqueous alteration, the LREE-I is also sensitive to melt compositional evolution, and its applicability to alteration in settings other than the Jack Hills quartzite is thus far undetermined. We investigate igneous zircon populations from a variety of geologic settings that show evidence for alteration by contact metamorphism during magma intrusion and deuteric fluid interactions during pluton crystallization. In suites with a high proportion of texturally altered zircons, low LREE-I values are common and this parameter correlates well with other contamination indicators (based on the observed secondary phases deposited in the zircons during fluid flow). Filtering zircon trace element compositions based on the LREE-I appears to remove the majority of chemically altered zircon, in many cases revealing previously obscured magmatic signals.

Erosional and depositional history of the Atlantic passive margin as recorded in detrital zircon fission-track ages and lithic detritus in Atlantic Coastal plain sediments

Comparison of fission-track (FT) ages of detrital zircons recovered from Atlantic Coastal Plain sediments to FT ages of zircons from bedrock in source terranes in the Appalachians provides a key to understanding the provenance of the sediments and, in turn, the erosional and depositional history of the Atlantic passive margin. In Appalachian source terranes, the oldest zircon fission-track (ZFT) ages from bedrock in the western Appalachians (defined for this paper as the Appalachian Plateau, Valley and Ridge, and far western Blue Ridge) are notably older than the oldest ages from bedrock in the eastern Appalachians (Piedmont and main part of the Blue Ridge). The age difference is seen both in ZFT sample ages and in individual zircon grain ages and reflects differences in the thermotectonic history of the rocks. In the east, ZFT data indicate that the rocks cooled from temperatures high enough to partially or totally reset ZFT ages during the Paleozoic and (or) Mesozoic. The majority of the rocks are interpreted to have cooled through the ZFT closure temperature (∼235 °C) at various times during the late Paleozoic Alleghanian orogeny. In contrast, most of the rocks sampled in the western Appalachians have never been heated to temperatures high enough to totally reset their ZFT ages. Reflecting their contrasting thermotectonic histories, nearly 80 percent of the sampled western rocks yield one or more zircon grains with very old FT ages, in excess of 800 Ma; zircon grains yielding FT ages this old have not been found in rocks in the Piedmont and main part of the Blue Ridge. The ZFT data suggest that the asymmetry of zircon ages of exposed bedrock in the eastern and western Appalachians was in evidence by no later than the Early Cretaceous and probably by the Late Triassic. Detrital zircon suites from sands collected in the Atlantic Coastal Plain provide a record of detritus eroded from source terranes in the Appalachians during the Mesozoic and Cenozoic. In Virginia and Maryland, sands of Early Cretaceous through late early Oligocene age do not yield any old zircons comparable in age to the old zircons found in bedrock in the western Appalachians. Very old zircons yielding FT ages >800 Ma are only encountered in Coastal Plain sands of middle early Miocene and younger age. Miocene and younger fluvial-deltaic deposits associated with the major mid-Atlantic Coastal Plain rivers that now head in the western Appalachians (the Hudson, Delaware, Susquehanna, Potomac, James, and Roanoke) contain abundant clasts of fossiliferous chert and quartzite and other distinctive rock types derived from Paleozoic rocks of the western Appalachians. These distinctive clasts have not been reported in older Coastal Plain sediments. The ZFT and lithic detritus data indicate that the drainage divide for one or more east-flowing mid-Atlantic rivers migrated west into the western Appalachians, and the river(s) began transporting western Appalachian detritus to the Atlantic Coastal Plain, sometime between the late early Oligocene and middle early Miocene. By no later than late middle Miocene most if not all of the major rivers that now head west of the Blue Ridge were transporting western Appalachian detritus to the Coastal Plain. Prior to the drainage divide migrating into the western Appalachians, the ZFT data are consistent with the dominant source of Atlantic Coastal Plain sediments being detritus from the Piedmont and main part of the Blue Ridge, with possible input from distant volcanic sources. The ZFT data suggest that the rapid increase in the rate of siliciclastic sediment accumulation in middle Atlantic margin offshore basins that peaked in the middle Miocene and produced almost 30 percent of the total volume of post-rift siliciclastic sediments in the offshore basins began in the early Miocene when Atlantic river(s) gained access to the relatively easily eroded Paleozoic sedimentary rocks of the western Appalachians.

Provenance and temporal constraints of the Early Cambrian Maotianshan Shale, Yunnan Province, China

The Cambrian Maotianshan Shale in Yunnan Province, China contains the well-preserved soft-body fossils of the Chengjiang Biota. The high quality preservation of the non-mineralizing biota (soft tissues and whole carcasses) shows regional and temporal differences, suggesting that paleogeography and local environmental conditions might have contributed to the taphonomy of these fossils. In this paper we present new results from petrographic, geochemical and detrital zircon analyses, and provide a new interpretation about the provenance of the Maotianshan Shale, as well as add to the understanding of the paleogeography of the South China Block during the Cambrian Stage 3. Results from petrographic analysis indicate that the provenance of the Maotianshan Shale is a recycled orogen overall, bordering the western and southwestern margin on the Yangtze Block. The most likely source of the terrigenous material is an exhumed area extending from the Kangdian paleoland to the southeast, paralleling the Song Ma fault zone. Minor regional differences in geochemical and petrographic proxies between the northwestern Jianshan/Ercai area and the southeastern Maotianshan/Xiaolantian area suggest influence of local sources. Sediments of the southeastern province are less mature and samples include minor elements commonly associated with mafic sources. Sediments from the northwestern province are more mature, largely lack mafic components and are enriched in Zr and Hf. The major population of the Maotianshan Shale detrital zircons group at ~800Ma. This crystallization age matches well with the age of a widely spread felsic volcanic and intrusive event associated with the Neoproterozoic Kangdian rift, suggesting that these igneous rocks are most likely a major provenance for the Maotianshan sediments. The youngest zircon population yields consistent Concordia ages of ~520Ma, representing a maximum age constraint on the timing of deposition of the Maotianshan Shale. The zircon crystals of the ~520Ma populations are euhedral with magmatic zoning, indicative of short-distance transport. Volcanic activity along the Song Ma suture zone is a potential source for the ~520Ma detrital zircon suite.

Interaction between the Central Asian Orogenic Belt (CAOB) and the Siberian craton as recorded by detrital zircon suites from Transbaikalia

This study was carried out to determine the initial age and time-span of interaction of two major crustal provinces of Asia: the Siberian craton and the Central Asian Orogenic Belt (CAOB). Single grain U–Pb ages of detrital zircon suites from the Neoproterozoic passive margin strata of the Siberian craton were used to determine the first appearance of extraregional source regions. Twelve Neoproterozoic clastic rocks from the Baikal-Patom margin of Siberia and one sample from the volcaniclastic Padrinsky Group that was deposited unconformably above accreted CAOB crust were analyzed. In total, 1152 grains were analyzed, 703 were used for provenance interpretation. Stratigraphically lower strata from the Siberian margin yield Archean – Paleoproterozoic detrital zircon ages, which are similar to, and probably derived from the Siberian Precambrian craton. A few extra-regional Mesoproterozoic grains are also present.The provenance shift happens in the upper portion of the section and is marked by the influx of extra-regional zircons. The youngest included zircons (610Ma) constrain the age of deposition and provide evidence for the timing of initial interaction between CAOB and Siberia in this region. Neoproterozoic zircons also dominate the overlying sedimentary units of the CAOB, suggesting continuing convergence. Coeval volcaniclastic unit deposited across the accreted CAOB terrane has a similar U–Pb detrital zircon age distribution, strengthening the provenance links between the two sides of the orogen.Analysis of the local tectonics suggests that the beginning of accretion might have started even before the first appearance of Neoproterozoic zircon: during the development of a regional unconformity, capped by 635Ma (?) “Snowball Earth” tillites of Dzhemkukan Formation. The absence of Neoproterozoic zircons in Dzhemkukan Formation is probably explained by the absence of development of an extremely thickened crust.Our data are in good agreement with data collected from other Neoproterozoic sedimentary basins of southern Siberian craton, including Cisbaikalia and Bodaibo Synclinorium.

U-Pb zircon geochronology of Roxbury Conglomerate, Boston Basin, Massachusetts: Tectono-stratigraphic implications for Avalonia in and beyond SE New England

High-precision CA-TIMS ²⁰⁶Pb/²³⁸U zircon dates from sandstone and Brighton igneous rocks associated with Roxbury Conglomerate in the Boston Basin, eastern Massachusetts provide constraints on the age and tectonic significance of these deposits. Detrital zircon suites from Roxbury-related sandstones representing, in ascending order, the Franklin Park Member (proposed name), the Brookline Member and the Squantum Member establish closely comparable maximum depositional ages of 595.14 ± 0.90, 598.87 ± 0.71 and 596.39 ± 0.79 Ma, respectively. The youngest of these is the best maximum age estimate of the conglomerate. Brighton dacite near the base of the Brookline Member and amygdaloidal andesite near the top yield respective crystallization ages of 584.19 ± 0.70 Ma and 585.37 ± 0.72 Ma. These virtually identical dates support previous interpretations of these particular units as shallow intrusions and thus represent minimum ages of associated conglomerate. The Roxbury-Brighton sequence is traditionally shown as inter-fingering northward with ≤570 Ma mudstone of the Cambridge “Argillite,” but the age range of the conglomerate makes this impossible. Conglomerate lacking quartzite clasts typical of the Roxbury proper and associated with 593.19 ± 0.73 Ma rhyolite on the south side of the basin is re-assigned to the Lynn-Mattapan Volcanic Complex which rests unconformably on Dedham Granite in basement exposed west and north of the basin as well. Calc-alkaline geochemistry and ages of the 609 to 584 Ma Dedham-Lynn-Mattapan-Brighton assemblage link it with 630 to 580 Ma arc sequences in other northern Appalachian Avalonian terranes. Roxbury Conglomerate accumulated in fault-bounded intra-arc basins near the end of this magmatic cycle.

The detrital zircon record: Supercontinents, parallel evolution—Or coincidence?

The interpretation of detrital zircon data is facing some important and potentially damaging inconsistencies that are commonly overlooked: combined U–Pb and Lu–Hf data from detrital zircon in sedimentary rocks have been used successfully to study global processes such as the extraction, growth and preservation of continental crust, suggesting generally similar patterns of evolution in different continents. Yet at the same time, such data are increasingly used to identify the source-rocks of individual clastic deposits, based on the implicit assumption that material from different source terranes carry distinct provenance signatures.For example, Precambrian detrital zircon age-fractions from late Mesoproterozoic to Recent sediments in Laurentia show patterns of U–Pb ages and initial Hf isotope composition indistinguishable from that of zircon from granitoids in Fennoscandia. This is perhaps not too surprising, as Fennoscandia and Laurentia have been near neighbours in three supercontinents (Nuna, Rodinia, Pangea). Transport and homogenization of clastic material in intracontinental basins and along a common continental margin would smooth out differences, and the two neighbouring continents may have closely parallel histories of internal growth.A stronger reason for concern is that Precambrian age fractions in detrital zircon suites from areas as distant as Australia and South Africa also show distribution patterns that cannot be distinguished with confidence from sources in Fennoscandia. These continents have not been near neighbours in post-Archaean supercontinents. Since exchange of detritus between such distant blocks is improbable, one is left with the alternative explanations of pronounced parallel evolution (perhaps related to the assembly of supercontinents in the geological past), or pure coincidence.Whatever the explanation: exchange, parallelism or coincidence, observations such as these challenge the main assumptions underlying the use of zircon as an indicator of sedimentary provenance: That a given age and initial Hf isotopic pattern of a population of detrital zircons can be unequivocally related to a specific first-generation source.

Baltica in the Cordillera?

U-Pb ages of detrital zircon suites from Paleozoic strata in the Arctic Alaska–Chukotka terrane (AAC), Alexander terrane, northern Sierra terrane, and eastern Klamath terrane of the North American Cordillera suggest an exotic Gondwana or Baltic origin. We evaluate these hypotheses with U-Pb ages of detrital zircon suites from Cambrian–Devonian strata of northern Baltica. Precambrian zircon populations (ca. 0.8–3.0 Ga) from Baltica compare well with similar age detritus in the AAC and Cordilleran terranes, but the amount and age of younger Neoproterozoic and Ordovician–Silurian components are variable. The AAC shares its stratigraphy with Baltica and has the most similar detrital zircon suites. Closing the Arctic places the AAC against the Lomonosov Ridge and the edge of Baltica in pre-Cretaceous time. After the Caledonian orogeny and before the Ural Mountains formed, the Baltica, AAC, and Cordilleran margins shared a Devonian–Carboniferous rift history and became along-strike portions of a Carboniferous–Permian continental margin. This rifting event might have been responsible for the initial separation of Baltica and Caledonian affinity terranes from this margin.

Stratigraphy and U-Pb detrital zircon geochronology of Wrangel Island, Russia: Implications for Arctic paleogeography

Wrangel Island represents a small but unique exposure of Neoproterozoic basement and its upper Paleozoic and Mesozoic cover within the mostly unexplored East Siberian Shelf. Its geology is critical for testing the continuity of stratigraphic units and structures across the Chukchi Sea from Alaska to Arctic Russia, for evaluating the hydrocarbon potential of this offshore region, and for constraining paleogeography and plate reconstructions of the Arctic. Upper Paleozoic platform carbonates and shales on Wrangel likely match those of the Chukchi Shelf and adjacent North Slope of Alaska (e.g., Sherwood et al., 2002), but Triassic basinal turbidites contrast with Alaska's thin shelfal units. Detrital zircon suites from upper Paleozoic strata on Wrangel reveal that local basement-derived detritus (500–800 Ma) decreases up section, replaced by 900–2000-Ma zircon populations compatible with a Baltic shield provenance. Cambrian–Ordovician–Silurian zircons (420–490 Ma) are present in lesser abundance in most samples and are inferred to have been derived from the Arctic part of the Caledonide belt. Triassic detrital zircon suites contrast with those from underlying strata: Precambrian zircons have less of an age range (1700–2000 Ma), and Devonian and younger (400 Ma) zircons are much more abundant. This change reflects breakup of the carbonate platform during Permian–Triassic rifting, with zircon age populations in Triassic strata compatible with sediment sources in the Urals, Taimyr, and Siberia. Detrital zircon data suggest that Wrangel Island, Chukotka, and northern Alaska (the Arctic Alaska-Chukotka microplate) restore against the Lomonosov Ridge upon closure of the Amerasia Basin and to the edge of the Barents Shelf after closing the Eurasia Basin. The detrital zircon data thus suggest that the Barents Shelf lays close to the paleo-Pacific margin in the early Mesozoic and that subduction-driven tectonics may have been a greater factor in the evolution of the Amerasia Basin of the Arctic than previously suspected.

Geochronological, geochemical and isotopic study of detrital zircon suites from late Neoproterozoic clastic strata along the NE margin of the East European Craton: Implications for plate tectonic models

U/Pb dating, REE geochemistry and Lu/Hf isotopic studies utilizing LA-ICPMS on detrital zircon suites from Neoproterozoic siliciclastic rocks from the northeast periphery of the East European Craton (sandstones of Djejim Formation, Djejim-Parma Hills, Southern Timan Ridge and sandstones of the Engane-Pe Formation, Northern Engane-Pe uplift, Polar Urals) are used to assess the provenance of sediments and test tectonic models for the late Precambrian assembly of continents. The data support the conclusion that Neoproterozoic complexes of the Timan–Pechora region (TPR) are composed mainly of sedimentary rocks ( SW Pre-Uralides–Timanides) eroded from Baltica and deposited along the passive margin of Baltica. However, late Precambrian–Early Cambrian volcanic-sedimentary and volcanic rocks, granitoids, and rare ophiolites of the TPR ( NE Pre-Uralides–Timanides ) comprise more juvenile material developed some distance from Baltica. Important differences exist between the U/Pb ages and Lu/Hf isotopic systematics of zircons from rocks of the NE Pre-Uralides–Timanides and the Neoproterozoic complexes of the Peri-Gondwanan terranes and do not support a Peri-Gondwanan origin for the NE Pre-Uralides–Timanides . In our interpretation, the SW Pre-Uralides–Timanides were deposited in the Neoproterozoic along the passive Timanian–Uralian margin of Baltica, but the NE Pre-Uralides–Timanides were formed along the active (Bolshezemel) margin of a paleocontinent called Arctida and were caught in the collision zone between the two paleocontinents, Arctida and Baltica.

High abundance of early Archaean grains and the age distribution of detrital zircons in a sillimanite-bearing quartzite from Mt Narryer, Western Australia

SHRIMP U–Pb analyses are reported for a detrital zircon population from a sample of sillimanite-bearing quartzite from the Narryer sedimentary succession in the Narryer Terrane of the northwestern Yilgarn Craton. The detrital zircons define two distinctive age groups, an older group from 4000 Ga to 4280 Ma and a younger group from 3750 to 3250 Ma. The abundance of older group zircons of about 12% far exceeds the abundance of about 2% reported in the first discovery of ancient zircons in a quartzite from the Narryer metasediments, and is equivalent to the abundance of >3900 Ma zircons in metaconglomerate sample W74 from the Jack Hills, confirmed by new measurements reported in this paper. Most analyses of the Narryer and the Jack Hills detrital zircon populations are discordant. The Jack Hills zircon analyses are dominated by strong recent Pb loss whereas the Narryer zircon analyses have had a more complex history and have experienced at least one Pb loss event, possibly associated with the high-grade metamorphism at ca. 2700 Ma, and a further disturbance of the U–Pb systems during relatively recent times. Although the number of analyses is limited and many of the zircon analyses are discordant, the age distributions of the older (>3900 Ma) zircons from the Narryer and Jack Hills samples are different, suggesting a complex provenance for the ancient zircons. The distribution of ages in the younger population of Mt Narryer zircons is similar to that reported for zircons from the surrounding Meeberrie gneiss, supporting previous suggestions that zircons from the gneisses or their precursors were a major contributor to the detrital zircon suite. The younger zircon population from Jack Hills sample (W74), lacks the strong age peak from 3600 to 3750 Ma present in the Narryer zircon population, and conversely the strong zircon age group at ca. 3350–3500 Ma in the Jack Hills population is only weakly represented in the Narryer zircon population. The age distributions for the Narryer and the Jack Hills zircon populations are taken as benchmarks for comparing zircon populations from quartzite occurrences elsewhere in the Yilgarn Craton.

New insights into Arctic paleogeography and tectonics from U‐Pb detrital zircon geochronology

To test existing models for the formation of the Amerasian Basin, detrital zircon suites from 12 samples of Triassic sandstone from the circum‐Arctic region were dated by laser ablation‐inductively coupled plasma‐mass spectrometry (ICP‐MS). The northern Verkhoyansk (NE Russia) has Permo‐Carboniferous (265–320 Ma) and Cambro‐Silurian (410–505 Ma) zircon populations derived via river systems from the active Baikal Mountain region along the southern Siberian craton. Chukotka, Wrangel Island (Russia), and the Lisburne Hills (western Alaska) also have Permo‐Carboniferous (280–330 Ma) and late Precambrian‐Silurian (420–580 Ma) zircons in addition to Permo‐Triassic (235–265 Ma), Devonian (340–390 Ma), and late Precambrian (1000–1300 Ma) zircons. These ages suggest at least partial derivation from the Taimyr, Siberian Trap, and/or east Urals regions of Arctic Russia. The northerly derived Ivishak Formation (Sadlerochit Mountains, Alaska) and Pat Bay Formation (Sverdrup Basin, Canada) are dominated by Cambrian–latest Precambrian (500–600 Ma) and 445–490 Ma zircons. Permo‐Carboniferous and Permo‐Triassic zircons are absent. The Bjorne Formation (Sverdrup Basin), derived from the south, differs from other samples studied with mostly 1130–1240 Ma and older Precambrian zircons in addition to 430–470 Ma zircons. The most popular plate tectonic model for the origin of the Amerasian Basin involves counterclockwise rotation of the Arctic Alaska–Chukotka microplate away from the Canadian Arctic margin. The detrital zircon data suggest that the Chukotka part of the microplate originated closer to the Taimyr and Verkhoyansk, east of the Polar Urals of Russia, and not from the Canadian Arctic.

On the scarcity of >3900 Ma detrital zircons in ≥3500 Ma metasediments

Constant volume models for the continental crust require a flux of crustal material back into the mantle (recycling), equal in volume to that of the juvenile igneous suites added to the continental crust throughout time. In growth of crustal volume models, there is not equilibrium between the volume of juvenile crustal additions and any recycling (destruction) of crust. By establishing the proportion of >3900 Ma detrital zircons in early Archaean sediments it might be possible to constrain the relative importance of crustal growth and recycling. Gneiss complexes in western Greenland, northern Labrador and northeastern China contain rare ≥3500 Ma detrital metasediments. In sediments deposited between 3500 and 3600 Ma, ≥3900 Ma zircons have not been detected in a suite of 117 detrital grains. Based on statistical considerations, at the 95% confidence level any ‘missed’ ≥3900 Ma component forms <3% of this suite. Likewise, >3900 Ma detrital grains do not occur amongst 54 detrital grains from (even rarer) 3700–3800 Ma sediments, arguing with 95% certainty that any ‘missed’ ≥3900 Ma component forms <5% of this most ancient suite. If the age spectra of these detrital zircon suites are representative of the complexes in which they reside, then constant volume (recycling=new additions) models require that by 3500 Ma, >97% of >3900 Ma crust was destroyed by recycling. Such an extremely high recycling rate (≈25% of the crust 100 Ma −1) is hard to reconcile with the diversity of initial Nd and Sr isotopic ratios of well preserved early Archaean granitoid suites in the same complexes, because significant average crustal residence times are required to permit the radiogenic isotopic systems to evolve. The most likely interpretation of the detrital zircon record in the Greenland, Labrador and China sediments is that in their provenance areas the volume of continental crust was small at 3900 Ma, and that it grew significantly during the early Archaean. If the measured ≥3500 Ma detrital sediment suites are globally representative, they support growth models for the continental crust in the early Archaean, rather than models involving recycling of a voluminous >3900 Ma sialic crust. Because of its global coverage and the dating of thousands of grains, the age spectra for detrital zircons from 3000–3200 Ma sediments provides a more reliable impression of crustal ages. However, as they were deposited 700–900 Ma after 3900 Ma, the globally small proportion of ≥3900 Ma detrital grains in them (from Jack Hills, Mt. Narryer and Wyoming) can be accommodated in both crustal growth and moderate recycling models.

Thermal evolution of the central Halls Creek Orogen, northern Australia

The Halls Creek Orogen in northern Australia records the Palaeoproterozoic collision of the Kimberley Craton with the North Australian Craton. Integrated structural, metamorphic and geochronological studies of the Tickalara Metamorphics show that this involved a protracted episode of high‐temperature, low‐pressure metamorphism associated with intense and prolonged mafic and felsic intrusive activity in the interval ca 1850–1820 Ma. Tectonothermal development of the region commenced with an inferred mantle perturbation event, probably at ca 1880 Ma. This resulted in the generation of mafic magmas in the upper mantle or lower crust, while upper crustal extension preceded the rapid deposition of the Tickalara sedimentary protoliths. An older age limit for these rocks is provided by a psammopelitic gneiss from the Tickalara Metamorphics, which yield a 207Pb/206Pb SHRIMP age of 1867 ± 4 Ma for the youngest detrital zircon suite. Voluminous layered mafic intrusives were emplaced in the middle crust at ca 1860–1855 Ma, prior to the attainment of lower granulite facies peak metamorphic conditions in the middle crust. Locally preserved layer‐parallel D1 foliations that were developed during prograde metamorphism were pervasively overprinted by the dominant regional S2 gneissosity coincident with peak metamorphism. Overgrowths on zircons record a metamorphic 207Pb/206Pb age of 1845 ± 4 Ma. The S2 fabric is folded around tight folds and cut by ductile shear zones associated with D3 (ca 1830 Ma), and all pre‐existing structures are folded around large‐scale, open F4 folds (ca 1820 Ma). Construction of a temperature‐time path for the mid‐crustal section exposed in the central Halls Creek Orogen, based on detailed SHRIMP zircon data, key field relationships and petrological evidence, suggests the existence of one protracted thermal event (>400–500°C for 25–30 million years) encompassing two deformation phases. Protoliths to the Tickalara Metamorphics were relatively cold (∼350°C) when intruded by the Fletcher Creek Granite at ca 1850 Ma, but were subsequently heated rapidly to 700–800°C during peak metamorphism at ca 1845 Ma. Repeated injection of mafic magmas caused multiple remelting of the metasedimentary wall rocks, with mappable increases in leucosome volume that show a strong spatial relationship to these intrusives. This mafic igneous activity prolonged the elevated geotherm and ensured that the rocks remained very hot (≥650°C) for at least 10 million years. The Mabel Downs Tonalite was emplaced during amphibolite facies metamorphism, with intrusion commencing at ca 1835 Ma. Its compositional heterogeneity, and the presence of mutual cross‐cutting relations between ductile shear zones and multiple injections of mingled magma suggest that it was emplaced syn‐D3. Broad‐scale folding attributable to F4 was accompanied by widespread intrusion of granitoids, and F4 fold limbs are truncated by large, mostly brittle retrograde S4 shear zones.

Heavy-Mineral Analysis and Typology of Detrital Zircons: A New Approach to Provenance Study (Saxothuringian Flysch, Germany)

ABSTRACT Heavy-mineral analyses of Famennian and Visean Saxothuringian graywackes yielded stable, zircon-dominated spectra. The typology of the detrital zircons was examined using Pupin's typology method in order to test the application of that method to detrital zircons. The results of heavy-mineral and zircon-typology analyses allow subdivision of the autochthonous Saxothuringian flysch (Thuringian Facies) into three groups (Famennian flysch of the Erbendorf Paleozoic; older part (Lehestener Schichten) and younger part (Teuschnitzer Schichten) of the Carboniferous flysch). The allochthonous Saxothuringian flysch (Bavarian Facies) can be correlated with the older part of the Carboniferous flysch. Within the flysch of the Thuringian Facies, the proportions of reddish zircons decrease from the Famennian to the youngest Visean flysch sediments. Both the Famennian graywackes and the youngest units of the Saxothuringian flysch are dominated by euhedral zircons but can be distinguished by the presence of different zircon types. The older part of the Carboniferous flysch is dominated by rounded zircons. From the methodological standpoint, our study demonstrates that zircon typology is a useful tool to characterize detrital zircon suites. It allows refinement of subdivisions based upon stable heavy-mineral spectra. In combination with data from previous studies, our results permit us to outline the development of the provenance area of the Saxothuringian flysch.

Detrital Zircon Link Between Headwaters and Terminus of the Upper Triassic Chinle-Dockum Paleoriver System

New detrital-zircon geochronologic data reveal that a through-going paleoriver connected Texas with Nevada in Late Triassic time. Sandstone from the Upper Triassic Santa Rosa Sandstone (Dockum Group) from northwestern Texas contains a detrital zircon suite nearly identical to that found in western Nevada in the Upper Triassic Osobb Formation (Auld Lang Syne Group, correlative with the Chinle Formation). The Santa Rosa Sandstone was derived in large part from the eroded Cambrian core of the Amarillo-Wichita uplift, as evidenced by abundant zircons with ages of 515 to 525 million years. Other zircon grains in the sandstone are Permian, Devonian, Proterozoic, and Archean in age and, with the exception of the Archean grain, are also matched by the population in the Nevada strata.

Precambrian U-Pb zircon ages from Western Galicia(NW Spain)

Abstract Zircon and monazite U-Pb ages are reported from two orthogneiss samples and one paragneiss from western Galicia. The three investigated suites of zircons reveal upper intercept ages of 2.2–2.5 Ga and lower intercept ages of about 470 Ma. The discordia pattern of the paragneiss is interpreted as resulting from lead loss of a detrital zircon suite of Early Proterozoic age: the provenance age of a