Pb Age Distributions Research Articles

Provenance records of the North Jiangsu Basin, East China: Zircon U–Pb geochronology and geochemistry from the Paleogene Dainan Formation in the Gaoyou Sag

Detailed zircon U–Pb dating and whole-rock geochemical analyses were carried out on the sedimentary rocks of the Paleogene Dainan Formation from Gaoyou Sag in the North Jiangsu Basin, East China. Whole-rock rare earth element characteristics suggest that the provenance was mainly from the Late Proterozoic low-grade metamorphic felsic rocks in the Dabie–Sulu orogenic belt, with the parent rocks probably being the I-type high-potassium granite gneiss. Cathodoluminescence images indicate that most of the detrital zircons are originally magmatic. A few zircons show overgrowths, indicating multiple-episode tectonic events. The U–Pb age distribution patterns of the detrital zircons suggest four main magmatic episodes in the provenance: Late Archean–Early Proterozoic (2450–2600Ma), Early Proterozoic (1700–1900Ma), Late Proterozoic (700–850Ma), and Late Paleozoic–Mesozoic (100–300Ma). These zircon U–Pb age and whole-rock geochemical results suggest that the sediments of the Dainan Formation were mainly sourced from the recycled orogenic belts within and/or around the North Jiangsu Basin, including the basement of the Yangtze Block, the Neoproterozoic rocks in the Dabie–Sulu orogenic belt, and the Mesozoic igneous rocks in the south part of Zhangbaling Uplift.

Provenance from zircon U–Pb age distributions in crustally contaminated granitoids

The basement of sedimentary basins is often entirely covered by a potentially multi-stage basin fill and therefore removed from direct observation and sampling. Melts intruding through the basin stratigraphy at a subsequent stage in the geological evolution of a region may assimilate significant volumes of country rocks. This component may be preserved in the intrusive body either as xenoliths or it may be reflected only by the age spectrum of incorporated zircons. Here we present the case of an Ordovician calc-alkaline intrusive belt in NW Argentina named the “Faja Eruptiva de la Puna Oriental” (Faja Eruptiva), which in the course of intrusion sampled the unexposed and unknown basement of the Ordovician basin in this region, and parts of the basin stratigraphy. We present new LA–ICP-MS U–Pb ages on zircons from 9 granodiorites and granites of the Faja Eruptiva.The main part of the Faja Eruptiva intruded c. 445Ma in the Late Ordovician. The zircon ages obtained from the intrusive rocks have a large spread between 2683.5±21.6 and 440.0±4.9Ma and reflect the underlying crust and may be interpreted in several ways. The inherited zircons may have been derived from the oldest known unit in the region, the thick siliciclastic turbidite successions of the upper Neoproterozoic–lower Cambrian Puncoviscana Formation, which is inferred to represent the basement of the NW Argentina. The basement to the Puncoviscana Formation is not known. Alternatively, the inherited zircons may reflect the geochronological structure of the entire unexposed Early Paleozoic crust underlying this region of which the Puncoviscana Formation was only one component. This crust likely contained rocks pertaining to and detritus derived from earlier orogenic cycles of the southwestern Amazonia craton, including sources of Early Meso- and Paleoproterozoic age.Detritus derived, in turn, from the Faja Eruptiva intrusive belt reflects the origin of the granitoids as well as the inherited geochronological and isotope geochemical structure of either the basement and/or distant sources having supplied material to the basement rocks. If unrecognized, sediment formed from such granitoid sources may erroneously be used to infer the exposure of, and direct detrital contributions from, a variety of older source rocks in fact not directly involved in the studied source–sink system.

Ediacaran–Cambrian paleogeography and geodynamic setting of the Central Iberian Zone: Constraints from coupled U–Pb–Hf isotopes of detrital zircons

The combination of U–Pb geochronology and Hf isotopes in detrital zircons represents a valuable tool in the characterization of sediment sources and paleographic reconstructions. This analytical methodology has been applied to seven Neoproterozoic to Lower Cambrian metasedimentary rocks from the northern and southern domains of the Central Iberian Zone (CIZ), which represent part of the Cadomian basement in the Iberian Massif. The samples collected in the southern domain (belonging to the Schist–Greywacke Complex) yield Ediacaran maximum depositional ages (582–550Ma), whereas samples from northern areas range from 588 to 536Ma, respectively in the Talavera and the Guadarrama sectors. A relatively large fraction of zircon grains gave anomalously young ages (∼500–320Ma), probably due to post-depositional lead loss induced by thermal overprints. Zircon U–Pb age distribution provide similar broad age ranges in all cases: abundant Ediacaran–Late Cryogenian zircons (540–700Ma), and less abundant ages in the following ranges: 700–850Ma, 880–1100Ma, 1900–2250Ma and 2400–2800Ma. However, the southern CIZ samples display an important juvenile input at ca. 1000–1075Ma, 800Ma and in the range 700–570Ma, which contrasts with the mostly recycled nature of the northern CIZ zircons. The Ediacaran juvenile signatures can be derived from the nearby Cadomian arc, as sedimentation likely occurred in a back-arc setting, but Early Cryogenian and Stenian juvenile peaks are more characteristic of basement rocks from NE Africa, so the CIZ was likely situated near the Saharan Metacraton and the Arabian–Nubian Shield during Late Ediacaran. On the other hand, the predominance of evolved components in the younger northern CIZ sediments implies important geodynamic modifications in northern Gondwana at the Ediacaran–Cambrian boundary. The differences between southern and northern CIZ metasedimentary rocks can be interpreted as a transition from a back-arc scenario to development of new basins after the arc collision against the main continent. In this context, the arc signature would decline and the influence of hinterland more mature regions would increase.

Spatial U–Pb age distribution of plutonic rocks in the central Abukuma Plateau, northeastern Japan Arc

Spatial U–Pb age distribution of plutonic rocks in the central Abukuma Plateau, northeastern Japan Arc

Detrital zircon U–Pb age constraints on Cretaceous sedimentary rocks of Lingshan Island and implications for tectonic evolution of Eastern Shandong, North China

In order to clarify sedimentary environment and tectonic history of the Sulu orogenic belt, we date zircons from four sandstones and one interbedded rhyolite collected on Lingshan Island in East Shandong, North China. U–Pb age distributions of detrital zircon populations from the newly named Lingshandao Formation resemble that of overlying fluvial sedimentary rocks. Age distributions exhibit four main populations of Paleoproterozoic (2.5–2.3Ga), Mesoproterozoic (2.1–1.9Ga), Neoproterozoic (850–700Ma) and Early Cretaceous (138–121Ma) age, accompanied by sporadic occurrences of Archean, Paleozoic and early Mesozoic aged grains. Data indicate a depositional age of ∼121Ma for the Lingshandao Formation, which constrains the maximum depositional age of overlying fluvial sedimentary rocks. The Lingshandao Formation thus more likely correlates to the uppermost Laiyang Group of the Jiaolai basin. Proterozoic age populations indicate a mixture of material derived from the North China (NCB) and Yangtze Block (YZB) basement along with protolith material from the HP–UHP metamorphic units of the Sulu orogenic belt. Proterozoic age populations differ from those of the roughly coeval Jiaolai basin, which represents a terrestrial depositional environment to the north of the Sulu terrane. The YZB appears to be a significant source of detrital zircons, consistent with field observations.

Precambrian tectonic attribution and evolution of the Songliao terrane revealed by zircon xenocrysts from Cenozoic alkali basalts, Xilinhot region, NE China

U–Pb dating and Hf isotopic analyses were carried out on xenocrystic zircons from the Cenozoic alkali basalts in the Xilinhot region, NE China, to constrain the nature and evolution of the underlying Songliao terrane, and its role in the assembly of the Central Asian Orogenic Belt. Our data show a wide range of U–Pb age distributions, with Precambrian clusters of ca. 2500Ma, 2400–2200Ma, 2000–1700Ma, 1500Ma, 1000–800Ma, 700–500Ma, and Phanerozoic peaks at 494Ma, 411Ma, 312Ma, 263Ma, 214Ma, 140Ma and 44Ma. The Phanerozoic age peaks are well-recognized in supracrustal rocks in the Songliao terrane or near the sample location. Although most Phanerozoic zircons have positive ɛHf(t) values, which is consistent with many locations throughout the Central Asian Orogenic Belt, some Phanerozoic ones have negative ɛHf(t) values and Tcrust up to ca. 2.5Ga, indicating the involvement of old crustal components during the Phanerozoic thermal events. More importantly, zircons with 207Pb/206Pb ages between ca. 2500Ma and 1700Ma have Tcrust of 2.5–3.5Ga, revealing the possible occurrence of unexposed Archean crustal remnants beneath the Songliao terrane, where no known supracrustal rocks are older than Neoproterozoic. Integrating the U–Pb and Hf isotopic results, we interpret the Songliao terrane as ancient in nature and with a complex crustal evolution history, including the addition of juvenile materials at ∼2.5Ga, reworking of pre-existing crustal components at 2.4–2.2Ga, 2.0–1.8Ga, 1.8–1.7Ga, 1.0Ga, 0.7–0.6Ga, and massive crustal growth during the Phanerozoic. Comparing these data with the Precambrian zircon age distributions of other cratons or micro-continents, we suggest the Songliao terrane shows an affinity to both Monglia and NE Gondwana.

U–Pb age and Lu–Hf isotopic data of detrital zircons from the Neoproterozoic Damara Sequence: Implications for Congo and Kalahari before Gondwana

The proximity of the Congo and Kalahari cratons during the Neoproterozoic breakup of the supercontinent Rodinia and during subsequent assembly of Gondwana is unclear. Neoproterozoic metasedimentary rocks from the rifted margins of Congo and Kalahari in the Damara Orogen yield distinctive detrital zircon U–Pb age distributions that correspond to the ages of prominent crustal components within the respective cratons. The most abundant zircons from Neoproterozoic strata deposited on the Congo margin give ages of 1150–1000 and 800–600Ma, whereas, the most abundant zircons from the Kalahari margin strata range from 1350 to 1100Ma. A 1350–1200Ma detrital zircon population in the Kalahari margin strata is absent in the Damara–Congo strata. A prominent c. 1050–1000Ma detrital zircon age population from Damara–Congo strata is nearly absent from the Damara–Kalahari strata, even though orogenic events of this age are found on both cratons. Damara strata on the Kalahari margin also lack detrital zircons with U–Pb ages of 900–600Ma. The differences in detrital zircon age distributions are robust when comparing strata of the same age on both cratons, and remains so, even when younger, deeper water facies are excluded, which could have been biased by other sediment sources. These data suggest that the Congo and Kalahari cratons were not proximal in Rodinia, and did not establish their current relative positions until the end of the Neoproterozoic when they were sutured together during the collisional orogenies that formed Gondwana.

Bias in detrital zircon geochronology and thermochronometry

Detrital studies that utilize zircon U–Pb geochronology and fission-track (FT) thermochronometry are subject to a range of potential sources of bias that should be properly evaluated and minimized. Some of them are common to any single-grain mineral analysis (e.g., variable bedrock mineral fertility, hydraulic sorting during transport, selective grain loss during sample processing), whereas others are intrinsic to zircon, and are related to radiation damage and age discordance. In this article, we quantify the impact of intrinsic bias on detrital studies thanks to the analysis of modern detritus shed from the European Alps, and illustrate the general implications on geological interpretations.We show that detrital zircon U–Pb age distributions based on statistically robust datasets are highly reproducible and representative of the parent bedrock ages in the catchment. Arbitrary or selective removal of discordant grain ages can be minimized by using the Kolmogorov–Smirnov test to identify an appropriate cutoff level. Loss of metamict (α-damaged) zircon has a minor impact on data representativeness, and is mainly controlled by regional metamorphism rather than by mechanical abrasion during river transport. Zircon FT grain-age distributions were found to have poor reproducibility, although age spectra are consistent with bedrock data. However, unlike the U–Pb datasets, U-rich zircon grains (>1000ppm) are systematically missed, and undatable grains may exceed 50%. We identify two major sources of distribution bias specific to zircon FT datasets: (i) sediment sources dominated by U-rich zircon grains are markedly underrepresented in the detrital record, because such grains often have uncountable high densities of fission tracks (“U concentration bias”); (ii) sediment sources that shed zircon grains with high levels of α-damage are underrepresented, because these grains are lost during chemical etching for FT revelation (“etching bias”). In the case of multimethod dating on the same grains (e.g., FT and U–Pb double dating), bias affecting detrital zircon FT dating propagates to the entire dataset. These effects may not impact on exhumation-rate studies that utilize the youngest grain ages (i.e., lag-time approach). However, they represent a limiting factor for conventional provenance studies, and generally preclude application of zircon FT dating to sediment budget calculations.

Genetic linkage between the Yellow River, the Mu Us desert and the Chinese Loess Plateau

Arid and semi arid northern China holds some of the world's most significant sand sea and loess deposits. In particular, arguably the most important late Cenozoic wind-blown dust archives on land are exposed on the Chinese Loess Plateau. The origin of this loess-forming dust and its relationship to adjacent sand seas is unclear and has been the subject of considerable debate. Polarization of opinion over the sources of loess also reflects uncertainty over its relationship to large river systems and to the sources of proximal desert sands. It is critical to resolve this in order to elucidate the origins of sand seas, to determine the activity of past dust emitting regions and to fully exploit loess climate archives. Here we combine zircon U–Pb, fission-track and double dating with heavy mineral analysis to test the role of proximal deserts and rivers in contributing dust to the Loess Plateau. We focus on the Mu Us desert to test hypotheses over its sediment sources and because previous studies have often presented contrasting interpretations over its importance as a loess source. Spatial complexity of zircon ages and heavy mineral assemblages in Mu Us sand rules out significant aeolian mixing and shows that grains originating in northern Tibet dominate in the western Mu Us, with local sources dominating in the east of the desert. The western Mu Us far-travelled grains are shown to be delivered by the Yellow River and associated systems. Crucially, the western Mu Us grains and Yellow River grains show U–Pb age distributions and heavy mineral assemblages virtually identical to those of the Quaternary loess. Thus, our results demonstrate that the Yellow River and associated systems transports large quantities of sediment from northern Tibet to the Mu Us desert and further suggest that the river contributes a significant volume of material to the Loess Plateau. This provides the first evidence of a genetic link between the Yellow River and formation of the Chinese Loess Plateau and suggests a greater role for fluvial activity in past dust and desert sand sea formation.

Provenance of Pleistocene Rhine River Middle Terrace sands between the Swiss–German border and Cologne based on U–Pb detrital zircon ages

Abstract Detrital zircon U–Pb age distributions derived from samples representing ancient or relatively young large-scale continental drainage networks are commonly taken to reflect the geochronological evolution of the tapped continental area. Here, we present detrital zircon U–Pb ages and associated heavy mineral data from Pleistocene Rhine River Middle Terrace sands and equivalents between the Swiss–German border and Cologne in order to test the commonly assumed Alpine provenance of the material. Samples from eight localities were analyzed for their heavy mineral assemblages. Detrital zircon U–Pb ages were determined by laser ablation inductively coupled mass spectrometry on selected samples from five locations along the Rhine River. The zircon age populations of all samples show a similar distribution, their main peaks being between 300 and 500 Ma. Minor age populations are recognized at 570 and 1,070 Ma. The 300–400 Ma maximum reflects the Variscan basement drained by or recycled into the Rhine River and its tributaries. The 400–500 Ma peak with predominantly Early Silurian ages points to Baltica or to the mid-German crystalline rise as original sources. One distinct peak at c. 570 Ma probably represents input from Cadomian terranes. The Precambrian U–Pb ages are compatible with derivation from sources in Baltica and in northern Gondwana. The heavy mineral populations of Middle Terrace sands and equivalents are characterized to a variable extend by garnet, epidote, and green hornblende. This association is often referred to as the Alpine spectrum and is considered to be indicative of an Alpine provenance. However, hornblende, epidote, and garnet are dominant heavy minerals of collisional orogens in general and may also be derived from Variscan and Caledonian units or from intermittent storage units. A remarkable feature of the detrital zircon age distribution in the Rhine River sediments from the Swiss–German border to Cologne is the absence of ages younger than 200 Ma and in particular of any ages reflecting the Alpine orogeny between c. 100 and 35 Ma. Sediments from rivers draining the equally collisional Himalaya orogen contain detrital zircons as young as 20 Ma. Our results question the assumption that Pleistocene Rhine River sediments were directly derived from the Alps. The lag time between the formation and deposition age of the youngest zircon in the studied Pleistocene Rhine River deposits is 200 Ma. Together with the absence of Alpine zircon ages, this stresses that detrital zircon age data from ancient sedimentary units found in poorly understood tectonic or paleogeographic settings need to be interpreted with great care, one could miss an entire orogenic cycle.

Detrital record of Indosinian mountain building in SW China: Provenance of the Middle Triassic turbidites in the Youjiang Basin

Sandstone petrology, geochemistry, and detrital zircon U–Pb ages and trace elements are integrated to investigate the provenance of Middle Triassic strata from the Youjiang Basin, SW China. Variations in abundance of volcanic lithic fragments and ratios of Th/U, Zr/Sc and Th/Sc suggest that sandstones from the southwestern portion of the basin display less sedimentary recycling and have a lower felsic average source composition than time-equivalent strata from the central, eastern and northern portions of the basin. All sandstones exhibit an overall similar detrital zircon U–Pb age distribution pattern with major age groups at 1200–900Ma, 480–420Ma and 300–240Ma together with some Devonian–Carboniferous and Neoproterozoic grains. Combined with the compiled paleocurrents, the detrital zircon ages suggest multiple-sourced components. Pre-Devonian detrital zircons were likely derived from the recycled late Neoproterozoic–early Paleozoic clastic rocks and from magmatic and metamorphic rocks related to the early Paleozoic orogeny in the southeastern segment of the South China Craton. Late Paleozoic–Triassic detrital zircons together with the presence of well preserved volcanic lithic fragments were considered to indicate first-cycled detrital input from a subduction–collisional mountain belt to the southwest of the craton. These provenance data argue for an Indosinian mountain building event following the closure of a Paleo-Tethys branch ocean rather than for a thermotectonic reactivation event as recently proposed for the Indosinian orogeny in the southwestern China. In association with this orogeny the Youjiang Basin evolved into a foreland basin.

Cosmogenic nuclide burial ages and provenance of Late Cenozoic deposits in the Sichuan Basin: Implications for Early Quaternary glaciations in east Tibet

Collision between the Indian and the Eurasian plates since the early Cenozoic produces one of the world’s most remarkable continental escarpments between the Tibetan Plateau and the adjacent Sichuan Basin. Yet Tertiary sediments are rare in the Sichuan Basin; the oldest preserved Late Cenozoic deposits called Dayi conglomerates directly overlie the Cretaceous or Jurassic red beds. Using cosmogenic 10Be and 26Al burial dating, we obtain deposition ages of ∼2.0 Ma and catchment erosion rates of ∼400 mm/ka for the Dayi conglomerates. Zircon U–Pb age distributions suggest derivation of these conglomerates from the Songpan-Ganzi flysch, the Pengguan complex and Late Permian and Triassic granite plutons in the headwater regions of the Min Jiang (Jiang, a Chinese term, means river). The formation of the poorly-sorted, sub-angular to sub-rounded and tens-centimeter-sized deposits in the western margin of the Sichuan Basin, after long distance transportation, is best explained by glacial activity ∼2.0 Ma ago in east Tibet.

Cosmogenic nuclide burial ages and provenance of the Xigeda paleo-lake: Implications for evolution of the Middle Yangtze River

Hundred-meter-thick lacustrine sediments are widespread along the Middle Yangtze River. The distribution of these sediments suggests that an event blocked the river, forming a lake stretching 160 km from east to west and 110 km north to south, with a depth of ≥ 500 m. Using the cosmogenic nuclides 10Be and 26Al we have dated the time of burial of the lacustrine sediments and fluvial gravels beneath the sediments. Our results indicate a deposition age for these sediments between 1.34 and 1.58 Ma. Studies of zircon U–Pb age distributions within lacustrine sediments and fluvial sands related to the paleo-lake formation show diverse provenances from the upper Yangtze River and Yalong River, a major tributary of the Yangtze River. These deposits contain geologic fingerprints that allow identification of the source region of the lacustrine deposits and fluvial sands. Our results support the reversal of the Middle Yangtze River before the paleo-lake formation. We hypothesize that the geologic event that dammed the originally southward flow was the lateral movement of Chenghai fault in the Dali fault system initiated in early Quaternary. Water eventually cut through the spillway and flowed to the east.

Construction of detrital mineral populations: insights from mixing of U–Pb zircon ages in Himalayan rivers

ABSTRACTFission‐track, U–Pb and Pb–Pb analyses of detrital heavy mineral populations in depositional basins and modern river sediments are widely used to infer the exhumational history of mountain belts. However, relatively few studies address the underlying assumption that detrital mineral populations provide an accurate representation of their entire source region. Implicit in this assumption is the idea that all units have equal potential to contribute heavy minerals in proportion to their exposure area in the source region. In reality, the detrital mineral population may be biased by variable concentrations of minerals in bedrock and differential erosion rates within the source region. This study evaluates the relative importance of these two variables by using mixing of U–Pb zircon ages to trace zircon populations from source units, through the fluvial system, and into the foreland.The first part of the study focuses on the Marsyandi drainage in central Nepal, using tributaries that drain single formations to define the U–Pb age distributions of individual units and using trunk river samples to evaluate the relative contributions from each lithology. Observed mixing proportions are compared with proportions predicted by a simple model incorporating lithologic exposure area and zircon concentration. The relative erosion rates that account for the discrepancy between the observed and predicted mixing proportions are then modelled and compared with independent erosional proxies. The study also compares U–Pb age distributions from four adjacent drainages spanning ∼250 km along the Himalayan front using the Kolmogorov–Smirnov statistic and statistical estimates of the proportion of zircon derived from each upstream lithology. Results show that, along this broad swath of rugged mountains, the U–Pb age distributions are remarkably similar, thereby allowing data from more localized sources to be extrapolated along strike.

Comparison of detrital zircon age distributions by kernel functional estimation

A method is presented for assessing and statistically testing the similarity of detrital zircon U–Pb age distributions frequently used in provenance analysis, correlation and tectonic reconstructions. The method accounts for intrinsic measurement uncertainties by constructing kernel functional estimates of each set of age data that compensate for different degrees of measurement error by the application of varying levels of smoothing. The dissimilarity between these estimates can be quantified and provides a meaningful comparison between age distributions. A Monte Carlo permutation algorithm is used to test for equality between age distributions by grouping two sets of age data and then resampling the age distributions. The techniques are demonstrated with both synthetic and real data. Synthetic data illustrate the behaviour of the algorithms with data containing varying age modes and measurement uncertainties. Real data from Tasmania and southeastern Australia illustrates both the broad correlation with previous similarity assessment techniques and the applicability of the described methods to provenance, regional correlation and tectonic reconstruction.