Detrital Zircon Samples Research Articles

The Mount Distin assemblage: Neoproterozoic metasedimentary rocks in the Nome Complex of Seward Peninsula, Alaska, USA

Proterozoic to Paleozoic rocks involved in Jurassic and earliest Cretaceous (Brookian) orogenesis and subsequently extended, metamorphosed at high temperature and intruded by Cretaceous plutons, are widespread on the Seward Peninsula, Alaska, USA. These blueschist and greenschist facies metasedimentary and metaigneous rocks make up the highly deformed Nome Complex. We describe and bracket the age of a distinct Precambrian succession in the Nome Complex, the Mount Distin assemblage (informal). The Mount Distin assemblage consists of marble, impure marble, calc-schist, quartz mica schist, and lesser quartzite that was originally a shelfal succession. Metamafic bodies, originally dikes and/or sills, are minor but common components. Three new U-Pb detrital zircon (DZ) samples are characterized by numerous peaks between 1.5 Ga and 1.0 Ga, with minor ages of 1000−900 Ma and 2000−1500 Ma and only 1−2 grains 3.0−2.5 Ga, indicating maximum depositional ages of 1000−900 Ma. A new U-Pb zircon age for an orthogneiss lens in marble is 661 ± 3 Ma like others previously dated in the Nome Complex. Orthogneiss ages provide an upper age limit for the Mount Distin assemblage of ca. 687 Ma, the oldest reported date. Zircons in three other orthogneiss bodies are interpreted as inherited as they have a range of U-Pb ages like those in their country rocks. The Mount Distin assemblage DZ ages are like those from schist-bearing carbonate sequences in the Central and Schist belts of the Brooks Range and inherited zircon populations in Neoproterozoic plutons on Wrangel Island, Russia. The discovery and documentation of the Mount Distin assemblage explains the previously reported enigmatic lack of country rocks to Neoproterozoic orthogneiss bodies, establishes the continuity of older rocks across the entire extent of the Arctic Alaska−Chukotka microplate, and further confirms this microplate’s Baltica affinities.

Using detrital zircons to constrain the late Paleozoic tectonic evolution of the adjacent tectonic domains of the Tarim Craton, NW China

Late Paleozoic tectonic evolution of adjacent tectonic domains of the Tarim Craton is vital for understanding the basin-mountain coupling and dynamical mechanisms of the Paleo-Asian and Tethys tectonic systems as well as hydrocarbon exploration and development. U−Pb dating and Lu-Hf isotope analyses of detrital zircons of the Late Paleozoic sedimentary successions from three sections across the Tarim basin enable comprehensive identification of provenance changes and reconstruction of the tectonic evolutionary history of the Tarim Basin in the late Paleozoic. A total of 248, 499, and 470 valid ages were obtained for detrital zircon samples from the Kuqa Section in the northern margin of the Tarim Basin; the Xiaohaizi Section in the platform area; and the Aitegou Section in the southwestern margin of the Tarim Basin, respectively. Each age interval corresponds to the geotectonic thermal events of the opening–subduction–closing of the surrounding oceans and the collage/collision of terranes/island arcs. Based on detailed age spectral analysis and other evidence, we propose a tectonic evolutionary model that encompasses the northern and southwestern margins of the Tarim Basin. The South Tianshan Ocean closed completely in the late Carboniferous, resulting in the collision of the Tarim Basin with the Yili–Middle Tianshan Terranes no later than 315 Ma. In the southwestern part of the Tarim Basin, the Tianshuihai Terrane began to approach in the late Carboniferous, gradually amalgamated with the southwestern margin of the Tarim Basin, and finally wholly collided with the Tarim Basin at the end of the Permian, making the Tarim Basin an entirely terrestrial basin. The southwestern Tarim Basin was not an open passive continental margin in the Carboniferous–Permian but a retro-arc basin.

Minimizing the effects of Pb loss in detrital and igneous U–Pb zircon geochronology by CA-LA-ICP-MS

Abstract. Detrital zircon geochronology by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) is a widely used tool for determining maximum depositional ages and sediment provenance, as well as reconstructing sediment routing pathways. Although the accuracy and precision of U–Pb geochronology measurements have improved over the past 2 decades, Pb loss continues to impact the ability to resolve zircon age populations by biasing affected zircon toward younger apparent ages. Chemical abrasion (CA) has been shown to reduce or eliminate the effects of Pb loss in zircon U–Pb geochronology but has yet to be widely applied to large-n detrital zircon analyses. Here, we assess the efficacy of the chemical abrasion treatment on zircon prior to analysis by LA-ICP-MS and discuss the advantages and limitations of this technique in relation to detrital zircon geochronology. We show that (i) CA does not systematically bias LA-ICP-MS U–Pb dates for 13 reference materials that span a wide variety of crystallization dates and U concentrations, (ii) CA-LA-ICP-MS U–Pb zircon geochronology can reduce or eliminate Pb loss in samples that have experienced significant radiation damage, and (iii) bulk CA prior to detrital zircon U–Pb geochronology by LA-ICP-MS improves the resolution of age populations defined by 206Pb/238U dates (Neoproterozoic and younger) and increases the percentage of concordant analyses in age populations defined by 207Pb/206Pb dates (Mesoproterozoic and older). The selective dissolution of zircon that has experienced high degrees of radiation damage suggests that some detrital zircon age populations could be destroyed or have their abundance significantly modified during this process. However, we did not identify this effect in either of the detrital zircon samples that were analyzed as part of this study. We conclude that pre-treatment of detrital zircon by bulk CA may be useful for applications that require increased resolution of detrital zircon populations and increased confidence that 206Pb/238U dates are unaffected by Pb loss.

Closing of the east Palaeo‐Tethys Ocean: Source–sink records from Middle Triassic sediments, Hainan Island, China

During the Late Palaeozoic to Early Mesozoic, the eastern segment of the Palaeo–Tethys Ocean closed, resulting in the collision and amalgamation of the South and North Hainan Islands. However, the exact location of the collision and amalgamation remains controversial. The main collision locations are the Baisha, Jiusuo–Lingshui and Changjiang–Qionghai faults. The U–Pb age of detrital zircons was used to determine the main source area of the Middle Triassic basin in the eastern part of Hainan Island and the closed position of the eastern section of the Palaeo‐Tethys Ocean. The U–Pb dating of detrital zircons revealed that the Middle Triassic Lingwen Formation contains four age groups: 235–258, 335–446, 766–986 and >1000 Ma. Notably, rock masses aged 248–252 Ma that were formed in a post‐orogenic tension setting have been identified in the western region of Hainan Island, whereas no igneous rocks from this period have been discovered in the eastern region. The detrital zircon samples from the Lingwen Formation revealed a peak at 249 ± 2 Ma U–Pb age, which is important evidence indicating that the Changjiang–Qionghai Fault is the product of collision between the southern and northern Hainan terranes. Affected by the bipolar subduction of the eastern segment of the Palaeo‐Tethys Ocean, the collision and amalgamation of the South and North Hainan terranes occurred, and experienced a post‐collision (extensional) process at ca. 249 ± 2 Ma.

The First Detrital Zircon Data on the Northwestern Precambrian Yenisei Ridge: Identification of the Continental–Arc Kiselikha Terrane

Northwestern segment of the Precambrian Yenisei Ridge contains ophiolite and is known in the literature as the Isakovka terrane or Isakovka domain. We suggest dividing it into two belts: the Kiselikha one (western) and Torzhikha (eastern), which differed in geodynamic regime during the late Neoproterozoic (~750–600 Ma). It is believed that the Kiselikha belt is composed mostly of volcanic rocks erupted at island arc setting in the second half of the Neoproterozoic, and that collision of this arc with Siberian continent formed the Yenisei Ridge orogen. This idea has not been sufficiently confirmed by geological and geochronological data. Dating of four detrital zircon samples extracted from sedimentary and volcanic-sedimentary rocks in the southern part of the belt revealed that the sampled strata belong to three different Precambrian levels: the Mesoproterozoic, the mid Neoproterozoic (800–750 Ma), and the end of the Neoproterozoic (620–600 Ma). Thus the authorized stratigraphic layout of the belt as well as its proposed island-arc origin require revision. By this paper we announce the identification of the Kiselikha terrane, which was a part of active margin of the Siberia paleocontinent at the beginning of the Neoproterozoic. Approximately in the middle of the Neoproterozoic, this block was rifted off Siberia and further evolved as a microcontinent bounded by an active margin from the outer side.

Late Paleozoic sedimentary and tectonic evolution of the Linxi basin in the eastern Inner Mongolia, eastern Central Asian Orogenic Belt

The eastern Central Asian Orogenic Belt (CAOB) encompasses a vast region from northeast China to Inner Mongolia, where there were many Late Paleozoic sedimentary basins that can provide key constraints to tectonic evolution of the eastern CAOB. Analysis of sedimentary sequence and environment of eight measured sections from the Linxi basin, Inner Mongolia indicates that there were shallow marine clastic-carbonate sedimentary facies, delta and littoral facies and terrestrial facies during the Early, Middle and Late Permian, respectively, and no oceanic sedimentary facies was found. Sedimentary sequence from the initial clastic-carbonate rocks to marine volcanic rocks reveals a transgression process during 300–270 Ma, while clastic and carbonate rocks with conglomerates and plant fossils represent a regression process during 270–250 Ma. According to the detrital zircon (DZ) age distribution of twenty-two samples (1610 zircons), the northern and southern provenances of the Permian sedimentary strata have been recognized, respectively. These DZ samples express the dispersed polymodal age populations and are attributed to a divergent dominated tectonic setting, implying the Linxi basin is an intracontinental extension basin during the Permian. This is consistent with an extensional process revealed by the syn-depositional deformed structures, coarse clastic sediments and olistostrome successions in the Linxi basin during the Early to Middle Permian and also similar to the speculation that there was an extension setting based on studies of volcanic rocks and intrusions in the Linxi basin. Combined with previous studies on the West Ujimqin basin, the Late Carboniferous to Permian tectonic frame of the eastern Inner Mongolia expresses extensional tectonics characterized by alternating combination of basins and uplifts superimposed on a pre-existing Early-Middle Paleozoic orogenic lithosphere and Precambrian basement, rather than a single ocean basin of Paleo Asian Ocean (PAO), which provides constraints for understanding sedimentary and tectonic evolution of the eastern CAOB during the Late Paleozoic.

Large detrital zircon data set investigation and provenance mapping: Local versus regional and continental sediment sources before, during, and after Ancestral Rocky Mountain deformation

The increasing number and size of detrital geochronology data sets offer new opportunities for increased accuracy and resolution of sediment routing models. However, the new opportunities come coupled with challenges in large data integration and visualization. We address these challenges by outlining two novel approaches that aid in analyzing and interpreting large detrital geochronology data sets: (1) combination of bottom-up and top-down detrital zircon source modeling, and (2) sediment provenance mapping. Combining source-modeling methods provides guidance in identifying empirical detrital zircon sources and determining source proportions. Provenance mapping integrates source proportions from modeling results and complimentary geologic data (e.g., paleocurrents, paleogeography, and stratal thickness maps) to extrapolate provenance information through areas with sparse or ambiguous data, thus mitigating issues of data distribution heterogeneity. Sediment provenance maps also provide a synoptic view of data that, along with detrital zircon source modeling, aids in circumventing lengthy descriptions of individual age modes for data sets containing hundreds of samples, which can obscure underlying trends in the data. We apply this approach to late Paleozoic−early Mesozoic strata, using 329 published and new U-Pb detrital zircon samples, and document five sediment-routing episodes in the core zone of intraplate deformation in western Laurentia (i.e., the Ancestral Rocky Mountains (ARM)). The transitions between these episodes are defined by changes in sediment source distribution, which are illustrated by provenance maps that show (1) the degree and extent of ARM basin isolation from transcontinental sediment sources and (2) ARM-driven changes in transcontinental sediment routing systems. We map possible sediment pathways of distally derived sediment around the ARM core, illustrating that ARM uplifts diverted transcontinental systems around areas of intense intraplate deformation. Further, the evolution of sediment routing in western Laurentia before, during, and after ARM deformation provides an example of the interaction between transcontinental sediment routing and intraplate deformation.

Detrital zircon age signatures of the basal Cambrian sandstone unit in North China: implications for drainage divides during global Sauk transgression and separation from Gondwanaland

ABSTRACT The basal Cambrian sandstone unit in the North China craton (NCC) is an example of globally widespread siliciclastic succession that resides on the Great Unconformity and deposited on a hypothesized low-lying peneplain during the Cambrian global eustatic sea-level rise. Detrital zircon age signatures from this distinct sequence enable recognition of the ancient drainage system of the NCC in deep time and track its potential linkage with the Gondwana landmass. LA-ICP-MS U–Pb dating of the fossil-calibrated basal Cambrian (Series 2) detrital zircon samples from seven measured sections reveal marked spatial changes in their age signatures that can be divided into three distinct types. The first is generally characterized by the bimodal age populations with broad peaks at ~1.85 Ga and ~2.5 Ga that correlate with the Archean to Paleoproterozoic basement inboard of the NCC. The second is featured by multimodal distribution with diagnostic Neoproterozoic peaks that correspond to subregional magmatic record. The third also shows multiple-zircon age populations, but yields significant crystallization ages close to the early Cambrian age. Comparing our new data with existing age spectra for the Cambrian strata across the NCC and the northern Gondwana demonstrates that separate drainage systems did exist in the peneplained basement during global Cambrian transgression and the basal Cambrian unit in the NCC was not a part of the far-travelled sand sheet across the northern margin of Gondwana. The most suitable source for Cambrian-aged grains constrained by paleogeographic restoration is the arc terrane developed along the northern margin of the NCC as a result of subduction of the Paleo-Asian oceanic plate. Our new continental-scale detrital zircon provenance signatures in the basal Cambrian unit suggest that the NCC should be considered a discrete continental block separated by the Proto-Tethys Ocean in the Cambrian, rather than an integral part of the northern Gondwana.

Understanding ancient tectonic settings through detrital zircon analysis

The fertility and heterogeneity of source rocks play important roles in determining the age distribution of detrital zircon populations that accumulate in sedimentary basins. Detrital zircon samples in active convergent tectonic settings are typified by a dominance of well-defined young age modes associated with an arc/magmatic orogenic belt, while passive margins and extensional settings more commonly express dispersed polymodal age populations reflecting more extensive, geologically heterogeneous catchments and greater sediment recycling. Two newly defined metrics relating to (i) the temporal difference between the 10th and 50th percentiles of a detrital zircon age population, and (ii) a modified χ2-distribution analysis of age population modality and dispersion, provide a powerful means of distinguishing between geologically active convergent systems and divergent/passive margin basins. Importantly, this tool does not require knowledge of depositional age. This “age distribution fingerprint” is assessed in three distinct Cenozoic to Proterozoic case-studies to demonstrate the tectonic resolution of the approach and potential in refining views of basin evolution. The age distribution fingerprint offers the ability to characterize sediment routing histories and investigate tectonic settings since the onset of subduction related plate tectonics, even where stratigraphic constraints are lacking.

Ages and tectonic attributes of Late Palaeozoic stratigraphic units in the Tarbagatay, West Junggar (NW China): revisited

ABSTRACT As the youngest single age of detrital zircons is used as the maximum depositional age, the ages and tectonic attributes of the Late Palaeozoic stratigraphic units in the Tarbagatay area, West Junggar, NW China, become very confusing. An improved statistical approach indicates that 5, 6 and 7 concordant zircons are required for the maximum depositional age at 95% confidence level when the quantity of total concordant zircon in a sample does not exceed 50, 140 and 432, respectively, but the defined maximum depositional age is valid only when it is compatible with geological facts. Otherwise, it is still uncertain if the age is valid. This approach is applied to resolve a chronological chaos in the Late Palaeozoic stratigraphic units in the Tarbagatay area. As a result, the maximum depositional ages obtained from the detrital zircon samples in literature and this study are at 95% confidence level, most of which are well compatible with the fossils in the units, intrusive relationships of plutons to country rocks and ages of interbedded volcanic rocks. The valid maximum depositional ages and other geological facts definitely show the presence of the Devonian, Early Carboniferous and Early Permian stratigraphic units in the Tarbagatay area. The Late Devonian-Early Carboniferous marine-dominated units occurred in the Zharma-Saur arc and its transition to a back-arc basin to the south during the southward subduction of the Irtysh-Zaisan Ocean, whereas the Permian terrestrial unit was formed in a post-collisional setting after the closure of the Irtysh-Zaisan Ocean in the Late Carboniferous.

Cambrian and earliest Ordovician fauna and geology of the Sông Đà and adjacent terranes in Việt Nam (Vietnam)

AbstractLater Cambrian and earliest Ordovician trilobites and brachiopods spanning eight horizons from five localities within the Sông Mã, Hàm Rồng and Đông Sơn formations of the Thanh Hóa province of Việt Nam, constrain the age and faunal affinities of rocks within the Sông Đà terrane, one of several suture/fault-bounded units situated between South China to the north and Indochina to the south. ‘Ghost-like’ preservation in dolomite coupled with tectonic deformation leaves many of the fossils poorly preserved, and poor exposure precludes collecting within continuously exposed stratigraphic successions. Cambrian carbonate facies pass conformably into Lower Ordovician carbonate-rich strata that also include minor siliciclastic facies, and the recovered fauna spans several uppermost Cambrian and Lower Ordovician biozones. The fauna is of equatorial Gondwanan affinity, and comparable to that from South China, North China, Sibumasu and Australia. A new species of Miaolingian ‘ptychopariid’ trilobite,Kaotaia xuanensis, is described. Detrital zircon samples from Cambrian–Ordovician rocks of the North Việt Nam and Sông Đà terranes, and from Palaeozoic samples from the Trường Sơn sector of Indochina immediately to the south, contain a predominance of ages spanning the Neoproterozoic period and have a typical equatorial Gondwanan signature. We associate the Cambrian and Tremadocian of the Sông Đà terrane with areas immediately to the north of it, including the North Việt Nam terrane and the southern parts of Yunnan and Guangxi provinces of China.

The birth of the Alps: Ediacaran to Paleozoic accretionary processes and crustal growth along the northern Gondwana margin

New whole-rock geochemical and coupled U–Pb and Lu–Hf LA-ICP-MS zircon data of metasedimentary rocks of the Austroalpine, South Alpine and Penninic basement domains are presented, to disentangle the pre-Variscan tectonic evolution of the proto-Alps. The studied units seem to record distinct stages of protracted Late Ediacaran to Carboniferous tectonosedimentary processes prior to the Variscan collision. In the case of Austroalpine and South Alpine units, nevertheless, no major differences in terms of provenance are observed, since most detrital zircon samples are characterized by a major Pan-African peak. Their detrital zircon spectra record a provenance from the northeastern Saharan Metacraton and the Sinai basement at the northern Arabian-Nubian Shield, being thus located along the eastern Early Paleozoic northern Gondwana margin, whereas sources located further west are inferred for the Penninic Unit, which might have been placed close to the Moldanubian Unit of the Bohemian Massif. In any case, it is thus clear that the Alpine basement remained in a close position to the Gondwana mainland at least during the Early Paleozoic. The Late Ediacaran to Silurian tectonic evolution, which includes Cadomian and Cenerian tectonometamorphic and magmatic processes, seem thus to record a continuum related to a retreating-mode accretionary orogen, with diachronous back-arc basin opening and possibly discrete compressional/transpressional pulses linked to changes in subduction zone dynamics. On the other hand, it is inferred that the Alpine basement essentially comprises Pan-African metasedimentary and subordinate metaigneous rocks, possibly with very few Early Neoproterozoic relics. This basement was significantly reworked during the protracted Paleozoic orogenic evolution, due to anatexis and/or assimilation by mantle-derived juvenile magmatism.

Provenance analysis of Late Triassic turbidites in the eastern Songpan–Ganzi Flysch Complex: Sedimentary record of tectonic evolution of the eastern Paleo-Tethys Ocean

The Xikang Group represents the main stratigraphic sequence of the Songpan–Ganzi complex (SGC), a flysch basin with a wide distribution and monotonous lithology. Strong tectonic deformation and accumulation of folds have led to destruction of the original sedimentary sequence and sedimentary structure of the strata, such that there exists no reasonable sedimentary model that can explain the massive flysch. Herein, we investigated in detail the sedimentary structures, paleocurrent direction, particle sizes, major and trace elements, and magmatic zircon and detrital zircon U–Pb ages of the typical sections of the Xikang Group in the eastern SGC. Our analysis led to the following findings: 1) The U–Pb zircon ages from the Galazi (215.6 ± 1.1 Ma), Sudi (214.5 ± 1.5 Ma and 221.1 ± 1.5 Ma), and Gulu (221.3 ± 1.2 Ma) plutons, which intrude the Xikang Group, and detrital zircon samples from the uppermost part of the Xinduqiao Fm. indicated the youngest mean age (YC1σ[2+]) of 222.4 ± 2.5 Ma. This in turn indicates a pre-221 Ma depositional age for the Xikang Group. 2) The provenance was derived from the North China Block and Qiangtang Terrane, Qinling–Dabie orogenic belt, South China Block, and East Kunlun–Qaidam, and the construction of the flysch in the SGC belongs to the multi-source or line source provided by the peripheral orogenic belt. 3) Trace elements suggested that the Xikang Group may have been deposited in a sedimentary basin associated with a continental island arc. 4) The sedimentary environment in this area belonged to the distal fan of a submarine, and the new sedimentation age and sedimentation rate indicate that the Carnian pluvial event may have been the trigger mechanism. 5) The formation of the flysch deposition in SGC related to the Paleo-Tethys Ocean underwent a scissor-like closure from east to west.

Crystal Chemistry and Structural Variations for Zircon Samples from Various Localities

This study investigates the variations of structural parameters and chemistry of a partially metamict and seven detrital zircon samples from different localities using single-crystal X-ray diffraction, synchrotron high-resolution powder X-ray diffraction, and electron-probe micro-analysis techniques. The unit-cell parameters for the eight zircon samples vary linearly with increasing unit-cell volume, V. A zircon sample from the Canadian Arctic Islands has the smallest unit-cell parameters, bond distances, ideal stoichiometric composition, unaffected by α-radiation damage, and is chemically pure. A zircon sample from Jemaa, Nigeria has the largest unit-cell parameters because of the effect of α-radiation doses received over a long time (2384 Ma). All the samples show good correlations between Zr and Si apfu (atom per formula unit) versus unit-cell volume, V. The α-radiation doses in the samples are lower than ~3.5 × 1015 α-decay events/mg. Substitutions of other cations at the Zr and Si sites control the variations of the structural parameters. Relatively large unit-cell parameters and bond distances occur because the Zr site accommodates other cations that have larger ionic radii than the Zr atom. Geological age increases the radiation doses in zircon and it is related to V.

Landscape configuration of eastern South China during the late Neoproterozoic: New constraints from sedimentary indices and zircon U-Pb-Hf isotopes of the southeastern margin of the Yangtze Block

The study of the paleogeographic landscapes of South China can contribute to understanding its paleo-position within supercontinents. We conducted a combined study of the Nanhuan-Sinian sedimentary records from the southeastern margin of the Yangtze Block (SMYB) in South China. The results reveal the following. (1) 18 datasets from directional sedimentary structures in the strata of the northern and southern SMYB indicate that the siliciclasts were transported towards the SE-SSE and the NWW-NW-NNW-NNE-NE, respectively. (2) 1670 analyses of 18 zircon samples collected from or near the directional structures have concordant U-Pb ages of 3243–563 Ma. 15 detrital zircon samples have a major peak within the range of 850–730 Ma, and 7 detrital zircon samples have εHf(t) values ranging from − 38.7 to + 13.7, and two-stage modal ages of 4.1–0.9 Ga. These new data reveal three important paleogeographic features of eastern South China during the late Tonian to Ediacaran, as follows. (a) The SMYB was a sedimentary basin (called the “Zhe-Gan-Wan Basin”) with opposing paleotopographic dips on two sides. Its northern part was mainly inclined to the SW-S-SE and belonged to the foreslope of the eastern Yangtze Platform. Conversely, its southern part was mainly inclined to the NW-N-NE and was the submerged part of the eroded area in the eastern Cathaysia Block (ECB) extending to the north. (b) The eroded area (called “Zhe-Min Land”) was probably located within the northeastern ECB and was surrounded by depositional areas on at least three sides. The ECB was likely connected to a Grenville-age orogen in the south direction. (c) The paleogeographic landscape configuration of eastern South China may have comprised five domains, from north to south: the eastern Yangtze Platform, the Zhe-Gan-Wan Basin, the Zhe-Min Land in the northeastern ECB, the Gan-Min-Zhe Basin, and a Grenville-age orogen. According to the landscape configuration, during the supercontinental cycles from Rodinia to Gondwana, the Cathaysia Block was likely located on the margin of the supercontinents, close to North India, West Australia, and East Antarctica.

Along-strike variations in sediment provenance within the Nanaimo basin reveal mechanisms of forearc basin sediment influx events

Abstract The along-strike variability in sediment provenance within the Nanaimo basin is important for understanding the tectonic evolution of North America’s Late Cretaceous Pacific margin, providing context for paleogeographic reconstructions. Here, we provide 35 point-counted sandstone samples and 22 new detrital zircon samples from the Nanaimo basin. These new detrital zircon samples compose a portion of a basin-wide data set (N = 49, n = 10,942) that is leveraged to discern spatio-temporal changes in sediment provenance. Provenance data demonstrates that the majority of Nanaimo basin strata were sourced from regions within and east of the Coast Mountains Batholith, while only the southernmost Nanaimo basin, exposed in the San Juan Islands, was supplied sediment from the North Cascade thrust system. Additionally, near-identical age modes and synchronous changes in detrital zircon facies are used to hypothesize a correlation between the Nanaimo Group and the protolith of the Swakane Gneiss. These observations, along with previously identified events in the Cordillera, are used to define two basin-wide events that affected the Nanaimo basin: the first at 84 Ma and the second at 72 Ma. The first event is correlated to the onset of Kula-Farallon spreading, which affected basin subsidence, introduced Proterozoic detrital zircon to the central and southern Nanaimo basin, and uplifted the North Cascade thrust system. The second basin-wide event, which is speculated to have been driven by increased rates of subduction and obliquity, resulted in localized high-flux events in the arc, increased exhumation of the Cascade Crystalline Core, underplating of the Swakane Gneiss, and coarse-grained sedimentation across the basin. The data presented here provides added context for the evolution of the basin and provides insight into the protracted geodynamics of forearc basins undergoing oblique subduction.

The evolution of submarine slope-channel systems: Timing of incision, bypass, and aggradation in Late Cretaceous Nanaimo Group channel-system strata, British Columbia, Canada

Abstract Submarine channel systems convey terrestrially derived detritus from shallow-marine environments to some of the largest sediment accumulations on Earth, submarine fans. The stratigraphic record of submarine slope channels includes heterogeneous, composite deposits that provide evidence for erosion, sediment bypass, and deposition. However, the timing and duration of these processes is poorly constrained over geologic time scales. We integrate geochronology with detailed stratigraphic characterization to temporally constrain the stratigraphic evolution recorded by horizontally to vertically aligned channel-fill stacking patterns in a Nanaimo Group channel system exposed on Hornby and Denman Islands, British Columbia, Canada. Twelve detrital zircon samples (n = 300/sample) were used to calculate maximum depositional ages, which identified a new age range for the succession from ca. 79 to 63 Ma. We document five phases of submarine-channel evolution over 16.0 ± 1.7 m.y. including: an initial phase dominated by incision, sediment bypass, and limited deposition (phase 1); followed by increasingly shorter and more rapid phases of deposition on the slope by laterally migrating (phase 2) and aggrading channels (phase 3); a long period of deep incision (phase 4); and a final rapid phase of vertical channel aggradation (phase 5). Our results suggest that ∼60% of the evolutionary history of the submarine channel system is captured in an incomplete, poorly preserved record of incision and sediment bypass, which makes up <20% of outcropping stratigraphy. Our findings are applicable to interpreting submarine channel-system evolution in ancient and modern settings worldwide and fundamentally important to understanding long-term sediment dispersal in the deep sea.

Unravelling the Neoproterozoic accretionary history of Oman, using an array of isotopic systems in zircon

The Omani basement is a critical window into the Neoproterozoic tectonic geography of the Mozambique Ocean and the amalgamation of Gondwana. It is located east of the juvenile arc terranes of the Arabian–Nubian Shield, yet the relationship between the two is not well understood. Magmatic and detrital zircon samples were analysed from four main basement terranes: Mirbat, the Huqf, Jebel Ja'alan and Al Jobah. The U–Pb (zircon) ages from igneous samples give ages between 838 and 765 Ma. The Lu–Hf and oxygen isotopes show that the zircons are juvenile (ɛ Hf ( t ) values 10.5–8.7), with low δ 18 O values ( c. 2–6‰). Detritus from sediments in Mirbat, the southernmost basement outcrop, preserves a main age peak at c. 786 Ma, with Lu–Hf between +6.05 and +12.64. Lu–Hf isotopic data for Proterozoic detrital zircons from Jebel Ja'alan suggest that this region was depositionally connected to a source yielding both evolved Tonian and Neoarchean to Paleoproterozoic detritus. These sources could correlate to the terranes in India and South China, which record similarly low δ 18 O values. These data support a model in which Oman formed in its own tectonic domain, accreting onto the Neoproterozoic active margin of a continent. Supplementary material : All U–Pb, hafnium and oxygen data, and figures including all weighted average plots and initial 176 Hf/ 177 Hf v. 206 Pb/ 238 U age to aid in identifying Pb-loss in the data are available at https://doi.org/10.6084/m9.figshare.c.4682363

A multidisciplinary approach to sediment provenance analysis of the late Silurian–Devonian Lower Old Red Sandstone succession, northern Midland Valley Basin, Scotland

Sediment provenance analysis is important in reconstructing chronological and spatial relationships between source area erosion/exhumation and sediment deposition in adjacent basins. Here we provide new provenance data from conglomerate clast populations, sandstone petrography, heavy mineral assemblages and U–Pb detrital zircon geochronology from the 9 km-thick, Siluro-Devonian Lower Old Red Sandstone (LORS), northern Midland Valley Basin (MVB), Scotland. The MVB developed in the foreland to the Caledonian Orogeny and comprises a mixed succession of conglomerate and pebbly sandstone interbedded with volcanic lithologies. Analysis of 554 samples (137 petrographical thin sections across 22 formations/members; 390 conglomerate clast recordings across 7 locations; 17 heavy mineral and 10 detrital zircon samples) from the entire succession shows a proximal, consistent source or sources located to the east or NE. These results indicate that sediment derived directly from areas of Scandian uplift of Norwegian lithologies is unlikely to have made a significant contribution to the LORS of the northern MVB and the provenance instead lies more proximally in both the neighbouring Dalradian Supergroup metasedimentary rocks and other contemporaneous lithologies. The coarse-grained nature of the sedimentary rocks suggests consistent and continued uplift of these lithologies throughout deposition of the LORS. Integration of these provenance techniques allows robust interpretation of basin fill from a tectonically rejuvenating source in the Caledonian foreland. Supplementary material: MVB LORS palaeocurrent data; List of sample locations is available at https://doi.org/10.6084/m9.figshare.c.4661966

Pre-Mesozoic Crimea as a continuation of the Dobrogea platform: insights from detrital zircons in Upper Jurassic conglomerates, Mountainous Crimea

U–Pb dating, Hf-isotope, and trace-element studies on two detrital zircon samples from sandstone interlayers in the Upper Jurassic conglomerates of the Southern coast of the Mountainous Crimea provide new information on the primary crystalline complexes from which those conglomerates were sourced. The U–Pb age spectra of studied zircons suggest that they were most likely sourced from the (meta)sedimentary complexes of the Eastern and Western Pontides blocks and the Dobrogea platform. In particular, a close similarity of the Precambrian age spectra with the detrital zircons from Late Neoproterozoic–Late Paleozoic (meta)sedimentary complexes of the Dobrogea block provides strong supporting evidence for the affinity between the Pre-Mesozoic basement of the Crimea and the Dobrogea platform. The zircons in the first sample were recycled through Dobrogea sedimentary complexes and originated from terranes with Amazonia affinities, while zircons in the second sample were recycled through the Taurides and originated from terranes related to northeastern Africa and Arabia. The strong similarity of the Precambrian parts of the age spectra of the Dobrogea complexes and the sample K15-007 suggests a resemblance of the Crimea’s Pre-Mesozoic foundation and the Dobrogea platform. Initial analytical data are provided in Electronic Supplementary Materials A (ESM A). Descriptions of measurement parameters, methodologies, and constants used to process primary analytical data and some processing results are reported in ESM B (Figs. B1–B8). Schemes of locations within Balkans–Anatolia–Black Sea–Caucasus region the crystalline complexes with Jurassic, Triassic, Permian–Carboniferous, as well as Late Neoproterozoic–Cambrian and Ordovician–Devonian ages are in ESM C (Figs. C1, C3–C5).