Zircon Ages Research Articles

Diagenetic effect on reservoir quality of siliciclastic and volcaniclastic sandstones from a Paleogene volcanic rifted margin, East Greenland

ABSTRACT Siliciclastic and volcaniclastic sediments in a volcanic rifted-margin succession may experience a complex diagenetic history during burial that can have a large impact on sandstone reservoir properties. To understand such changes, variations in initial sediment composition and succeeding diagenetic changes have been studied for a Paleogene outcrop analogue in the Kangerlussuaq area, East Greenland. The nature of the mafic volcanics-bearing succession, which consists of intra-volcanic sandstones, accommodated over quartz-rich pre-volcanic fluvial sandstones, are comparable to the settings of recently discovered hydrocarbon-producing sandstones in the Faroe–Shetland Basin on the conjugate Atlantic margin. Our petrographic and provenance investigations of the pre- and intra-volcanic sandstones are supported by geochemical and X-ray diffraction analyses. The intra-volcanic sandstones were deposited in shallow marine environments with mixed siliciclastic and volcaniclastic input, the latter rich in felsic to mafic volcanic rock fragments and feldspar grains. Similar zircon age distributions of pre- and intra-volcanic sandstones support a continued supply from the same siliciclastic sediment source after the onset of volcanism. Variations in initial detrital grain and pore-fluid (fresh to marine) compositions resulted in different diagenetic changes in the pre- and intra-volcanic sandstones. However, where siliciclastic sandstones were overlain by volcaniclastic rocks rather than massive lava flows, the diagenetic changes resemble those of the intra-volcanic sandstones. The cementing phases are typically quartz, illite (probably illitized kaolinite), and rare anatase in the pre-volcanic sandstones. Chlorite, calcite, zeolite/feldspar, opal/quartz, and titanite are characteristic authigenic phases in the intra-volcanic sandstones. Precipitation of different minerals in the pre- and intra-volcanic sandstones show that the detrital composition (and to a lesser extent depositional environment) played a major role during early and late diagenesis after deep burial (up to 6–8 km). Inter-eruptive siliciclastic units may prove to form highly valuable reservoirs when they are composed of mixed siliciclastic and volcaniclastic material. In the stratigraphically youngest intra-volcanic sandstones and pre-volcanic sandstones overlain by hyaloclastite or tuff, there is a high potential for preservation of interparticle porosity during burial (< 5 km) due to early chlorite rims and the generation of secondary porosity after the dissolution of early zeolite cement.

Buried Paleoproterozoic orogen of the East European Craton: Age and origin of the Vyatka terrane

We studied the composition, isotopic geochemistry and zircon U–Pb dating of Paleoproterozoic supracrustal rocks of the Uni Formation and granitoids of the Talitsky Complex of the Vyatka terrane located on the northeastern margin of the East European Craton. The obtained data were used to constrain the age, origin and tectonic setting of the Vyatka terrane, which is thought to be a part of the Paleoproterozoic Vyatka–Kama orogenic belt.Most of the gneisses and schists of the Uni Formation have greywacke protoliths, while metatuffites of andesite to dacite compositions are subordinate. All of these rocks have similar suprasubduction geochemical characteristics and highly radiogenic Nd isotopic compositions. The age of magmatic zircons from the felsic metatuffites is 2.08 Ga, and individual detrital grains from these rocks date back to ca. 2.12 Ga. Detrital zircons from the metasedimentary rocks have well-preserved crystals and multimodal U–Pb age distributions peaking at 2.08, 2.10, and 2.14 Ga. Some zircons from migmatized gneisses have 2.05-Ga-old rims, corresponding to the age of the Talitsky granitoids. The obtained data suggest that the Vyatka terrane could be considered an accretionary complex of the Vyatka–Kama orogen. The Uni Formation was formed by the accretion of sediments derived from an eroding Paleoproterozoic magmatic arcs (2.08–2.14 Ga). The completion of the accretionary tectonics was marked by the intrusion of the ca. 2.05-Ga-aged post-tectonic Talitsky granitoids.The Vyatka terrane accretionary complex contains chronologic counterparts of the West Sarmatian and Volga–Don orogens. This complements the model of a simultaneous amalgamation of Archean blocks of the Volgo–Sarmatia dating back to 2.2–2.0 Ga. Data from the Vyatka terrane confirm the model of the superposition of the East European and Siberian cratons in the Paleoproterozoic Columbia/Nuna supercontinent reconstruction. The Vyatka–Kama and Batomga belts may have been parts of a single extended Paleoproterozoic orogen that combined the Archean Volgo–Uralia and Aldan composite terranes.

The Harz Mountains (Germany) – Cadomia meets Avalonia and Baltica: U–Pb ages of detrital and magmatic zircon as a key for the decoding of Pangaea's central suture

Abstract U–Pb ages of detrital ( n = 2391) and magmatic ( n = 170) zircon grains from the Harz Mountains were obtained by LA-ICP-MS for provenance studies and absolute age dating. Results point to a complete closure of the Rheic Ocean at c. 419 Ma. A narrow Rhenish Seaway then re-opened in Emsian to mid-Devonian time ( c. 390–400 Ma). Devonian sedimentary rocks of the Harz Mountains were deposited on the northwestern (Rheno-Hercynian) and on the southeastern (Saxo-Thuringian) margins of the Rhenish Seaway. A new U–Pb zircon age from a plagiogranite (329 ± 2 Ma) within a harzburgite makes the existence of oceanic lithosphere in the Rhenish Seaway probable. The Rhenish Seaway was completely closed by Serpukhovian time ( c. 328 Ma). Existence of a terrane in the seaway is not supported by the new data. Provenance studies and spatial arrangement allow reconstruction of the thin- to thick-skinned obduction style of the Harz Mountains onto the southeastern margin of East Avalonia (Rheno-Hercynian Zone) during the Variscan orogeny. Detrital zircon populations define Rheno-Hercynian and Saxo-Thuringian nappes. Intrusion of the granitoid plutons of the Harz Mountains occurred in a time window of c. 300 to 295 Myr and constrained the termination of Variscan deformation.

Long-lived shallow crustal magma reservoirs beneath Ulleung Island volcano (South Korea) and their rejuvenation recorded in zircon

The microanalytical data obtained from zircons collected on Ulleung Island, an emergent peak of a Quaternary alkaline magma system in the southwestern East Sea (Sea of Japan) back-arc basin, provide new insights into the origin of magma, magmatic processes, and the trigger of explosive eruptions. The U-Th ages of zircons extracted from a pumice layer formed during the intense Plinian eruption at ∼11 ka, known as the Ulleung-Oki tephra, show a wide range spanning the pre-eruption equilibrium state (>350 ka) up to the eruption event itself. Zircon grains derived from syenite and gabbro fragments entrained within the tephra yield weighted mean U-Th and U-Pb ages of 34.7 ± 2.3 ka and 1378 ± 22 ka, respectively. Under cathodoluminescence, zircons from the syenite reveal dark, featureless, or oscillatory-zoned cores, containing numerous inclusions of britholite-like Y-rich phases and pyrochlore. These cores are partially or entirely replaced by inclusion-free, brighter domains, suggesting that in situ dissolution and precipitation processes significantly influenced the transformation of zircon. The consistently low δ18O values (5.4–2.6‰) of zircons from Middle Pleistocene trachyte lavas, the Ulleung-Oki tephra, and lithic fragments indicate the persistent presence of shallow crustal magma reservoirs beneath Ulleung Island. These reservoirs have undergone substantial assimilation of hydrothermally altered rocks, with assimilation rates typically around 10–30% and reaching a maximum of approximately 50%. Zircon core-to-rim variations in oxygen isotopic composition, along with textural observations, suggest the rejuvenation of magma reservoirs through the injection of new melt, which likely served as a trigger for the explosive eruptions at Ulleung Island. The similarity in the range of εHf values between the core and rim (+7.4 to −4.3) implies the Hf isotopic consistency of the recharge melt.

U-Pb geochronology and petrography of Neoproterozoic to early Cambrian volcanic rocks in basement crustal terranes beneath the deep-water Gulf of Mexico

Abstract Though the Gulf of Mexico has been studied for more than a century, the lithology, age, and origin of the basement crustal terranes remain poorly understood. New U-Pb zircon ages of a volcanic sample in the DeSoto Canyon 535 #1 Raptor well were obtained by in situ laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) and resulted in a crystallization age of 509 ± 23 Ma, indicating the crustal terrane beneath the northeastern Gulf of Mexico is likely of peri-Gondwanan affinity. This basement may be associated with the felsic volcanism related to Neoproterozoic to Cambrian subduction along the supercontinent’s western margin. This subduction-related volcanism likely represents the last pulses of magmatic activity before the west coast of Gondwana transitioned into a passive margin ca. 500 Ma. The data presented here for the DeSoto Canyon 535 #1 Raptor trachyte represent the oldest radiometrically dated evidence of volcanism in the present-day deep-water Gulf of Mexico.

Precursors to a continental-arc ignimbrite flare-up: Early central volcanoes of the San Juan Mountains, Colorado, USA

Abstract Our newly acquired and recently published map, geochronologic, and compositional data for early intermediate-composition central volcanoes in the northeastern San Juan Mountains provide insights about the broad magmatic precursors to the large continental-arc ignimbrite flare-up in the mid-Cenozoic Southern Rocky Mountain volcanic field (SRMVF). Initial volcanism migrated from central Colorado to northern New Mexico ca. 40–29 Ma, as part of a more regional trend of southward-progressing mid-Cenozoic magmatism in the U.S. segment of the North American Cordillera. Within the San Juan locus, which represents the largest preserved erosional remnant of the SRMVF and site of most intense eruptive activity, new 40Ar/39Ar and U-Pb zircon ages show that eruptions at many individual edifices began nearly concurrently, at ca. 35 Ma, with peak activity at 34–32 Ma. Broadly similar precursor effusive volcanism characterizes other major loci of continental-arc ignimbrite magmatism along the western American cordilleras, but none of these sites records early volcanism as voluminous, spatially widespread, well exposed, or compositionally diverse as the San Juan locus in Colorado. Early San Juan volcanism was larger in volume than the later ignimbrites, constituting about two thirds the total erupted. Early lava and breccias are as much as 700–900 m thick where exposed along eroded flanks of the San Juan Mountains; drill holes have penetrated sections as thick as 2600 m. The early volcanoes were dominantly andesitic, with lesser dacite and minor rhyolite. Such volcanism is widely interpreted as initiated by basaltic magma from the mantle, but mid-Cenozoic basalt is almost nonexistent at the San Juan locus—an absence inferred to be due to extensive lower-crustal assimilation and fractionation. The early volcanic rocks are calc-alkaline and typical of high-K continental-arc volcanism; they become modestly more alkalic and enriched in trace elements such as light rare earth elements, Zr, Nb, and Th from the San Juan locus northeastward into central Colorado. Such variations may reflect synmagmatic crustal thickening and deeper levels of primary magma generation concurrent with assembly of upper-crustal magma bodies that could support large ignimbrite eruptions. Substantial uncertainties remain for growth histories of the early volcanoes, however, because of unexposed lower parts of edifices, eroded upper parts, and limited availability of mineral phases that could be dated reliably. Although the early volcanoes are widely distributed within the SRMVF, many are clustered at sites of subsequent ignimbrite calderas. The precursor edifices are inferred to record incubation stages in construction of overall translithospheric batholithic-scale magmatic systems. Prolonged processes of incremental magma generation, accumulation, fractionation, and solidification intermittently generated sufficient liquid to erupt large ignimbrites. Maturation of focused eruptions and intrusions was prolonged, 5 m.y. or more, prior to the culminating ignimbrite at some centers in the San Juan Mountains. Some large-volume ignimbrites and related calderas, including the ~5000 km3 Fish Canyon Tuff and associated La Garita caldera, formed as much as several million years later than peak growth of associated precursor volcanoes, recording a sustained interval of diminished eruptive activity as the magma reservoir increased in volume and evolved to more silicic compositions capable of supporting a subsequent large ignimbrite eruption. Dike configurations at early volcanoes that were active nearly concurrently in the SRMVF vary from symmetrically radial to more parallel trends. The contrasting dike geometries are inferred to record possible multiple fluctuations from compressive to weakly extensional regional stress, concurrent with destabilization of the prior flat-slab plate configuration that triggered mid-Cenozoic ignimbrite flare-ups along the Cordilleran margin of the North American plate. These apparent fluctuations in regional stress preceded development of substantial extensional strain in the Southern Rocky Mountain region; outflow ignimbrite sheets of the SRMVF spread across subsequent horst-and-graben structures of the Rio Grande rift without complementary thickness variations.

Reconstruction of microcontinents during the assembly of Rodinia: A case study from the Central Tianshan Block

There are multiple Precambrian microcontinents within the Central Asian Orogenic Belt (CAOB), and the paleogeographic locations of those in the southwest CAOB (e.g. Central Tianshan Block, Yili Block, Aktau-Mointy terrane, Erementau-Niyaz terrane and Kokchetav terrane) remain controversial. The Central Tianshan Block is one of the representative Precambrian microcontinents in the southwest CAOB. It can be divided into the western Central Tianshan Block and eastern Central Tianshan Block. To investigate their relationships and paleogeographic locations, we collected metasedimentary rocks from the Kekesu Group in western Central Tianshan Block. The results of zircon LA–ICP–MS U-Pb dating, combining with previously obtained magmatic ages, suggest that the Kekesu Group was deposited during the latest Mesoproterozoic to earliest Neoproterozoic (ca. 1010–969 Ma). Detrital zircon populations of Kekesu Group were dominated by 2.0–0.9 Ga grains with an age peak at 1474 Ma. The Archean to end-Paleoproterozoic zircon grains had variable εHf(t) values from +9.7 to −8.7, and most of the Mesoproterozoic zircons had positive εHf(t) values (+13.9 to +2.4). The Kekesu Group in western Central Tianshan Block has similar zircon age populations and εHf(t) values to those in eastern Central Tianshan Block. In combination with magmatism, these data suggest that western and eastern Central Tianshan Block had similar tectonic affinities and locations during the earliest Neoproterozoic. We compiled 4566 concordant detrital zircon U-Pb age data of Mesoproterozoic to early Neoproterozoic strata from microcontinents in the southwest CAOB and potential sources. The zircon age probability density plots, kernel density estimates and multidimensional scaling plot suggest that the Central Tianshan and Yili blocks had similar sources mainly derived from the Musgrave Province in central Australia in the latest Mesoproterozoic to early Neoproterozoic, and were possibly located near Australia during assembly of Rodinia. The sources for the Erementau-Niyaz and Kokchetav terranes were probably mainly within the northern Laurentia in the latest Mesoproterozoic to earliest Neoproterozoic.

Overlapping events in the Acoculco geothermal system, Puebla, Mexico

The Acoculco geothermal system records two major hydrothermal events (prograde skarn overlapped by retrograde/”epithermal” to geothermal stages) in association with pre-calderic to calderic events in the eponymous complex, which belongs to the Trans-Mexican Volcanic Belt. The prograde skarn is associated with a hornblende granite (hereby dated at 1.01 ± 0.01 Ma; U-Pb zircon age), whose apex is found at a ∼1600 m depth, and later andesite dykes and sills. Mineralizing fluids in this stage have salinities of fluid inclusions that range between 9.5 wt.% NaCl equiv. and 33 wt.% NaCl and temperatures of homogenization (Th) up to 470 °C that record various process of mineral formation, such as boiling, depressurization, conductive cooling, isothermal mixing, and dilution. The retrograde stage is represented by early “fossil” brines with salinities 1.4 and 7.3 wt.% NaCl equiv. and Th up to 303 °C, and present-day gasothermal fluids that attain stabilized temperatures up to ∼300 °C at a depth of almost 2000 m. Th of early brines are consistently higher than stabilized temperatures at most of the depths that were surveyed, the isotherms that correspond to 250° and 200 °C dropped ca. 500 m from the early retrograde to the gasothermal stage. These features support the hypothesis that the Acoculco geothermal system has entered its senile stage. Despite that, upon a calculated geothermal gradient at ∼150 °C/km for both Th of the early retrograde stage and stabilized temperatures in the EAC-1 well, it is not foreseen a rapid waning of the thermal anomaly and its ultimate cause.Besides the early Ca-silicate assemblages that constitute the skarn, the architecture of alteration assemblages also records the overlapping of shallow on deep hypogene assemblages. The deep hypogene assemblages respond to alkaline to moderately acidic fluids that correspond to the early retrograde (or “epithermal”) stage: propylitic, phyllic and argillic (kaolinite + nontronite + montmorillonite), from bottom to top. Contrastingly, the shallow hypogene assemblages respond to dominantly acidic fluids, and consist of advanced argillic assemblages that may grade upwards into argillic (kaolinite + illite-smectite) or silicic (vuggy or massive to brecciated silica). The shallow hypogene character of advanced argillic assemblages, as due to steam-heated ground environments (as opposed to deep hypogene or magmatic-hydrothermal assemblages), is deduced from (1) the overall tabular shape of their distribution, (2) the mineralogical association and characteristics of rhombohedral alunite within it, and (3) its relationship with overlying silicic alteration that, in turn, do not possess likely characteristics of sinters. Shallow hypogene advanced argillic assemblages are associated with the present-day gasothermal activity due to sealing that was possibly promoted during the early retrograde/”epithermal” activity.

U–Pb ages from felsic rocks of the External Ligurian sedimentary mélange (Northern Apennine, Italy): tracing the pre-Jurassic history of the hyperextended Adria continental margin

The External Ligurian Units are regarded as representative of the ocean continent transition between the Ligure‒Piemontese oceanic basin and the hyperextended Adria continental margin. The remnants of this transition are preserved as slide blocks embedded in a sedimentary mélange. This mélange sedimented during the Santonian to Campanian in the rear of the accretionary wedge that developed during the east-dipping subduction of the oceanic lithosphere below Adria. The main characteristic of this mélange is the occurrence of slide blocks of subcontinental mantle, lower continental crust, granitoids, gabbro, basalt and sedimentary rocks in a sedimentary matrix. In this paper, we provide petrographic observations coupled with U‒Pb zircon ages and δ 18 O zircon isotopes of the slide blocks of felsic granulites and granitoids. U‒Pb data indicate ages ranging from Neoproterozoic to Late Triassic. δ 18 O isotopes help to define the source of the magma of granitoids and felsic granulites, which was the continental crust. Overall, the collected data indicate that the slide blocks from sedimentary mélanges can be regarded as true archives that are able to provide useful constraints about the geodynamic history of the source area of these deposits. Supplementary material: The dataset related to the U–Pb and ∂ 18 O analysisis available at https://doi.org/10.6084/m9.figshare.c.6922535 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Geochemical characteristic of rocks and isotope dating of zircon from the andesite dyke of the rudyanskaya sequence (D2 rd) of the Middle Urals Eastern zone

Relevance. In the basin of the r. Pishma subvolcanic formations, localized in the fields of spreading of the Middle Devonian Rudyanskaya sequence, are presented by different on composition small bodies, sills and dykes, forming with volcanogenic rocks of the Rudyanskaya sequence a basalt-andesite-rhyolite complex, the formation of which is associated with volcanic activity, that took place in Middle Devonian in the environments close to the modern island arcs. Their isotope dating and geochemical characteristic are rather relevant for specification of the composition, volume and paleogeodynamic environment of the vulcanite formation. In the paper new data are presented on geochemistry of andesites of the dyke from the Diviy Kamen rock, considered as part of the Rudyanskaya sequence, as well as the age of zircon from them. Methods. Chemical analyses of vulcanites have been performed in the Centre of collective usage “Geoanalyst” of the Institute of Geology and Geochemistry of the UB of RAS (Ekaterinburg) by X-ray fluorescent method and ICP–MS method. Zircons were studied with scanning electron microscope JSM-6390L of the Jeol firm and with electrone-probe microanalyser Cameca SX100. The data on the U and Pb isotopes, as well as on elements-impurities in zircon grains were obtained by mass-spectrometry method with inductively coupled plasma with laser ablation (LA–ICP–MS) at the quadrupole ISP–MS NexION 300S with the attachment for laser ablation LA NWR 213. Purpose of researches. Getting new data on geochemical composition and age of the andesites of dyke in the section along the r. Pishma. Results. The andesite dyke intersects medium clastic unstratified tuffs and dipply occurring fragment of the lava flow of porphyry dacite. Andesites present themselves porphyry rocks with 20–30% of plagioclase phenocrysts. At the normalized many-component diagram the Ta–Nb minimum is displayed. For zircon grains are characteristic differentiated spectra, demonstrating the growth of compositions from light to heavy REE, positive Ce and negative Eu anomalies, typical of the zircon of magmatic genesis. For the first time geochronological study of zircons from andesites has been maded by method of LA–ICP–MS. 4 age groups of zircons, Ma: 390–387, 369–362, 337–345, 327–329 were revealed. Conclusions. The data obtained testify that andesites were formed under suprasubduction environments, and magmatic stage of rock formation corresponds to the upper Devonian varieties of zircons. Early Carboniferous datings can be associated with the thermal impact caused by the intrusion of a later dolerite dyke.

The first U–Pb age for shocked zircon from the Mien impact structure, Sweden, and implications for metamictization‐induced zircon texture formed during impact events

AbstractShocked zircon from impactites from the Mien impact structure, Sweden, has been investigated with the aim to date the impact event and correlate the degree of U–Pb age resetting with shock‐related microtextures. In situ U–Pb spot isotope analyses of granular and microporous–granular zircon grains from the impact melt rocks give an age of 120.0 ± 1.0 Ma. This essentially confirms the previous best estimate age of 122.4 ± 2.3 Ma, while also increasing precision on the Mien impact age. U–Pb isotope mapping shows that radiation damage likely explains the similar U–Pb age reset associated with different shock‐related microtextures. Microporous and some of the granular and microporous–granular domains yield higher U concentrations along with younger 238U/206Pb dates. Lower U contents with older 238U/206Pb dates are predominately observed in pristine domains. Due to the U‐decay, the zircon lattice is damaged, a process through which Pb can be lost. This would result in younger 238U/206Pb dates, as observed for the high U domains. As the zircon crystal lattices were locally weakened, metamictization possibly facilitated the development of microporous and granular textures during the impact event. Analyses of unshocked Mien zircon confirm that radiation damage already existed before impact. Lead loss from granular domains occurred through recrystallization and from microporous domains through Pb leaching by hydrothermal fluids. In addition, our study demonstrates the utility of combined U–Pb isotope mapping and spot analysis in unraveling the link between U–Pb resetting and shock‐related microtextures, the formation of which was in this case likely promoted by pre‐existing radiation damage.

Middle Devonian, late Carboniferous, and Triassic magmatic flare-ups in eastern Armorica (Sakarya Zone, Turkey) as revealed by detrital zircon U-Pb-Hf isotopic data

Abstract The Sakarya Zone of northern Turkey contains a well-preserved Early–Middle Jurassic and Late Cretaceous submarine magmatic arc constructed over pre-Jurassic bedrocks that are considered to be the eastward extension of the Armorican Terrane Assemblage in Europe. In this study, we present U-Pb-Hf isotopic data from the detrital zircons of middle Permian and Lower Jurassic sandstones to reveal episodes of Paleozoic–early Mesozoic magmatic flare-ups. Detrital zircon ages, together with data from the literature, define three major age groups at 400–380 Ma, 326–310 Ma, and 250–230 Ma, which indicates three distinct magmatic flare-ups. In addition, there are minor age clusters at 460–430 Ma and 215–195 Ma. Initial εHf values of the detrital zircons indicate significant juvenile input during the Triassic flare-up, the involvement of significantly reworked crustal material during the late Carboniferous magmatic flare-up, and both juvenile and reworked crustal material during the Middle Devonian magmatic flare-up. Within the pre-Jurassic continental basement rocks of the Sakarya Zone, the late Carboniferous igneous rocks are well documented and most voluminous, and the Middle Devonian rocks are known locally, while the Triassic igneous rocks—apart from those in Triassic accretionary complexes—are hardly known. Because the Sakarya Zone is a Gondwana-derived continental block that was later involved in the Variscan and Alpine orogenies, these magmatic flare-ups cannot be explained by subduction-related processes along a single subduction zone. We propose that the Sakarya Zone rifted from the northern margin of Gondwana during the Late Ordovician–Silurian, the Devonian magmatic flare-up (400–380 Ma) was related to the southward subduction of the Rheic Ocean beneath the Sakarya Zone during its northward drift, the late Carboniferous magmatic flare-up (326–310 Ma) occurred following the collision of the Sakarya Zone with Laurussia, and the Triassic flare-up (250–230 Ma) resulted from northward subduction of the Tethys Ocean beneath the Sakarya Zone. Comparison with data from the literature shows that the Triassic and late Carboniferous magmatic flare-ups are also characteristic features of neighboring Armorican domains, such as the Balkans and the Caucasus; however, the Middle Devonian flare-up appears to be restricted to the Sakarya Zone. Along with the late Carboniferous flare-up, the Late Ordovician–Silurian flare-up, which is locally recorded in the Sakarya Zone, is typical of the Armorican Terrane Assemblage as a whole.

Neogene faulting, basin development, and relief generation in the southern Klamath Mountains (USA)

Abstract Development and evaluation of models for tectonic evolution in the Cascadia forearc require understanding of along-strike heterogeneity of strain distribution, uplift, and upper-plate characteristics. Here, we investigated the Neogene geologic record of the Klamath Mountains province in southernmost Cascadia and obtained apatite (U-Th)/He (AHe) thermochronology of Mesozoic plutons, Neogene graben sediment thickness, detrital zircon records from Neogene grabens, gravity and magnetic data, and kinematic analysis of faults. We documented three aspects of Neogene tectonics: early Miocene and younger rock exhumation, development of topographic relief sufficient to isolate Neogene graben-filling sediments from sources outside of the Klamath Mountains, and initiation of mid-Miocene or younger right-lateral and reverse faulting. Key findings are: (1) 10 new apatite AHe mean cooling ages from the Canyon Creek and Granite Peak plutons in the Trinity Alps range from 24.7 ± 2.1 Ma to 15.7 ± 2.1 Ma. Inverse thermal modeling of these data and published apatite fission-track ages indicate the most rapid rock cooling between ca. 25 and 15 Ma. One new AHe mean cooling age (26.7 ± 3.2 Ma) from the Ironside Mountain batholith 40 km west of the Trinity Alps, combined with previously published AHe ages, suggests geographically widespread latest Oligocene to Miocene cooling in the southern Klamath Mountains province. (2) AHe ages of 39.4 ± 5.1 Ma on the downthrown side and 22.7 ± 3.0 Ma on the upthrown side of the Browns Meadow fault suggest early Miocene to younger fault activity. (3) U-Pb detrital zircon ages (n = 862) and Lu-Hf isotope geochemistry from Miocene Weaverville Formation sediments in the Weaverville, Lowden Ranch, Hayfork, and Hyampom grabens south and southwest of the Trinity Alps can be traced to entirely Klamath Mountains sources; they suggest the south-central Klamath Mountains had, by the middle Miocene, sufficient relief to isolate these grabens from more distal sediment sources. (4) Two Miocene detrital zircon U-Pb ages of 10.6 ± 0.4 Ma and 16.7 ± 0.2 Ma from the Lowden Ranch graben show that the maximum depositional age of the upper Weaverville Formation here is younger than previously recognized. (5) A prominent steep-sided negative gravity anomaly associated with the Hayfork graben shows that both the north and south margins are fault-controlled, and inversion of gravity data suggests basin fill is between 1 km and 1.9 km thick. Abrupt elevation changes of basin fill-to-bedrock contacts reported in well logs record E-side-up and right-lateral faulting at the eastern end of the Hayfork graben. A NE-striking gravity gradient separates the main graben on the west from a narrower, thinner basin to the east, supporting this interpretation. (6) Of fset of both the base of the Weaverville Formation and the cataclasite-capped La Grange fault surface by a fault on the southwest margin of the Weaverville basin documents 200 m of reverse and 1500 m of right-lateral strike-slip motion on this structure, here named the Democrat Gulch fault; folded and steeply dipping strata adjacent to the fault confirm that faulting postdated deposition of the Weaverville Formation. Based on these findings, we suggest that Miocene rock cooling recorded by AHe ages, accompanying graben formation, and development of topographic relief record early to middle Miocene initiation of underplating or “subcretion” in the southern Cascadia subduction zone beneath the southern Klamath Mountains.

Landscape inversion episodes in SE China during the Mesozoic−early Cenozoic: Constrained by trace-element contents, Nd isotope geochemistry, and detrital zircon U-Pb geochronology of sedimentary basins

The tectonics and landscape of SE China experienced significant changes throughout the Mesozoic and early Cenozoic, largely in response to variations in the slab dynamics of the paleo-Pacific plate, which was subducting beneath continental Asia. We investigated the Mesozoic Yong’an basin in western Fujian Province of SE China in comparison to the sedimentary records of coeval basins in the region to document how its clastic sediment types and their provenance varied through time during the Mesozoic and what regional geologic processes may have controlled these variations. The average εNd value of samples from the Middle Jurassic Zhangping Formation is −16.6, and its detrital zircons are dominated by 1800 Ma and 2000 Ma grains, sourced from the northern Wuyishan Mountains. These mountains underwent significant rock and surface uplift by the Middle Jurassic and became the main source of clastic sediments in SE China. The Lower Cretaceous Bantou Formation contains pyroclastic rocks and represents fluvial-lacustrine deposits with εNd values of −14.8 to −12.4 and abundant 160−120 Ma detrital zircons, sourced from Late Jurassic granitoid rocks, which were widely exposed at the surface in SE China by this time. The upper Lower and lower Upper Cretaceous Shaxian Formation contains coarse-grained and poorly sorted sandstones-conglomerates with volcanic and granitic rock fragments, and it rests unconformably on the Bantou Formation. The Shaxian Formation represents fluvial- to alluvial-fan deposits, and its formation marks the timing of a rapid uplift of the paleo−Coastal Mountains. The Upper Cretaceous Chong’an Formation (>2000 m thick) contains abundant volcanic and granitic rock clasts and represents alluvial-fan and fluvial deposits. The average εNd values of the Shaxian and Chong’an Formations range between −9.3 and −7.5, and their most abundant detrital zircon ages are between 120 Ma and 80 Ma. By the end of the Late Cretaceous, the paleo−Coastal Mountains constituted a nearly 4-km-high magmatic belt, with much of SE China situated in its rain shadow at a lower elevation to the north. Eocene−Oligocene sedimentary basin rocks in Taiwan have an average εNd value of −10.9 and abundant Phanerozoic detrital zircons. The sediment source for these rocks was the paleo−Coastal Mountains. The Miocene basinal strata in Taiwan have more negative εNd values (−13.0) and contain Jurassic−Cretaceous as well as abundant Paleoproterozoic and Neoproterozoic zircons, indicating that the Wuyishan Mountains were again the main sediment source later in the Cenozoic. Denudation rates in the SE margin of South China were high (0.12−0.10 km/yr) during the Cretaceous (140−60 Ma), while they were very low in SW China and in the interior of South China during the same period. These differences confirm the existence of high coastal mountains in SE China until the Late Cretaceous. Denudation rates in eastern South China, particularly the coastal areas, were very low (0.06−0.02 km/yr) during the late Cenozoic (30−0 Ma), whereas they were the fastest (0.14−0.16 km/yr) in the northern Nanling belt and the Yangtze block farther inland to the north, indicating the surface elevation became higher in the western part of South China but lower in its eastern part in the late Cenozoic. This dynamic landscape evolution of SE China through multiple and major shifts throughout the Mesozoic and early Cenozoic was driven by the subducting slab dynamics and the tectonics of the Tibetan Plateau.

Provenance and tectonic settings of the lower Permian siliciclastic rocks in central Inner Mongolia: Implications for the evolution of the Paleo-Asian Ocean

The fine siliciclastic rocks of the Early Permian Shoushangou Formation in West Ujimqin Banner, central Inner Mongolia are a hot spot for Late Paleozoic oil and gas exploration and hold key implications for the tectonic evolution during the closure of the Paleo-Asian Ocean (PAO). Therefore, the sedimentary characteristics of the Shoushangou Formation in West Ujimqin Banner, along with the geochemical and Sm–Nd isotopic characteristics of the clastic rock samples in Well MXD1, were investigated to privde additional constraints on its provenance and tectonic background.The mudstones contain large proportions of quartz, feldspars and igneous debris, and the CIAcorr (0.52–0.66), the A–CN–K plot and the lack of Ce anomalies in REE show a weakly weathered provenance and the chemically unaltered parent rocks, suggestive of the proximal deposition and rapid burial. Slip-roll beddings, gravity flow, Bouma sequences and the (trace) fossil assemblages in the profile indicate that the Shoushangou Formation is mainly a shallow- (semi-)abyssal submarine fan and the lithological information indicates the presence of microfaces including main and lesser water supply channels, interchannels, lobes, natural levees. Furthermore, a narrow continental shelf above the submarine fan and the concentrated zircon age population of the Shoushangou formation suggest that the tectonic background is most likely a retroarc basin.The Al2O3/TiO2 ratio (19–24, average 22), the multivariable principal element source identification diagram and the plots of Th/Sc vs. Zr/Sc and Co/Th vs. La/Sc indicate that the source is mainly felsic, with a minor proportion of intermediate rocks. Trace elemental characteristics (e.g., Th, Zr, Sc, Co, Nd, and REE) deomonstrate that the tectonic setting of the Shoushangou Formation is confined by continental arcs.The whole rock Sm–Nd isotopic spectra of the Carboniferous and Permian strata indicate that the Baolidao arc is the main source of central Inner Mongolia. Additionally, the comparison of the Nd model ages and the zircon U–Pb ages of the Baolidao arc and other entities in the Central Asian Orogenic Belt (CAOB) is in support of a prolonged subduction of the PAO since ∼510 Ma, suggesting that the Baolidao arc was developed from the southern continental arc of the original South Mongolia microcontinent. Accordingly, it is suggested that the Shoushangou Formation was deposited in a retro-arc basin which was strongly influenced by the steep subduction of the PAO and the results collectively suggest that the PAO terminated no earlier than the middle to late Permian.

Early development of the Patagonian fold and thrust belt and the onset of foreland sedimentation in the Austral-Magallanes basin

Retroarc basins register the onset of compressional exhumation during the Cretaceous in the Andes, evidenced by sequences showing sedimentary provenance from the orogen. We present U–Pb detrital zircon geochronology from the Austral-Magallanes basin in the Southern Patagonian Andes (∼47–48°SL), where the foreland stage began in the Early Cretaceous. The littoral sandstones from basal foreland sequences of the Río Belgrano Formation yield ∼123 Ma maximum depositional ages, confirming previous studies that the basin infill is early Aptian. The detrital zircon ages showed two main clusters of 129-115 Ma and 151 -140 Ma, that are derived from the coetaneously exhumed Andean arc and earlier Jurassic magmatic sources. The zircons which overlap with ages of Upper Jurassic Andean synrift volcanics, confirms orogenic exhumation coeval with sedimentation. A sequence of retroarc volcanism dated between 119 and 97 Ma overlies the basal foreland littoral-continental deposits, and it is followed by a Late Cretaceous-Paleocene hiatus. Further analysis of detrital zircons ages distribution and geological considerations, indicates that the studied northern depocenter was a source region of sedimentary supply after its Cenomanian exhumation.

Geochronology, geochemistry, and tectonic setting of Ordovician metavolcanic rocks in the Liberty–Orrington belt, Maine: implications for the evolution of peri-Gondwanan arcs in the northern Appalachians

The Yarmouth Island Formation of the East Harpswell Group (EHG) is an assemblage of metamorphosed volcanic and sedimentary rocks exposed in Casco Bay, Maine. Although previously interpreted to have been deposited ca. 445 Ma, two new U–Pb zircon ages from metavolcanic rocks in the unit indicate prolonged magmatism between ca. 469–474 Ma and correlation with adjacent rocks of the more regionally extensive Casco Bay Group (CBG) in the Liberty–Orrington belt. New detrital zircon results from metasedimentary rocks in the EHG support this correlation; the age spectra indicate a peri-Gondwanan sedimentary source, consistent with earlier published results from the CBG. Detailed geochemical studies of the metavolcanic rocks in the Yarmouth Island Formation indicate a wide range of sub-alkaline compositions (SiO2 = 47.9–77.5 wt.%), flat chondrite-normalized rare earth element distributions, and a prominent negative Nb anomaly in normalized spider plots. Tectonic discrimination plots suggest eruption in a volcanic arc setting with continental crustal influence. This tectonic setting, when combined with the co-existence of marine sedimentary rocks, suggests a transitional setting between that of subduction and subsidence in a marine setting, perhaps due to the onset of back-arc rifting. Similarities in age, lithologic character, sediment provenance, and volcanic rock geochemistry indicate a strong correlation between these rocks in the Liberty–Orrington belt and those along strike in the southern portion of the Miramichi belt of eastern Maine and southern New Brunswick. These findings confirm the preservation of >500 km long, ca. 470 Ma, arc/back-arc terrane within the Ganderia superterrane in the northern Appalachians.

A review of depositional ages of the metasedimentary rocks in the units of the eastern Circum-Rhodope belt, Bulgaria-Greece

The Circum-Rhodope belt is a major tectonic unit in the Alpine orogen of northern Aegean region, which consists of metasedimentary rocks and unmetamorphosed arc-related Jurassic Evros ophiolite. Here, we review the biostratigraphic and detrital zircon ages available for the distinct units of the eastern Circum-Rhodope belt in Bulgaria and Greece. Both, biostratigraphic and radiometric ages consistently reveal a Middle Triassic to latest Late Jurassic timing of the sedimentation. Most importantly, the continuous depositional record sketches sedimentary sources from the continental margin (Rhodope) and the Jurassic arc system (Evros ophiolite) as documented mostly by the detrital zircons in clastic successions.

Nature of the Two Episodes of Paleoproterozoic Magmatism (2495 Ma and 2190 Ma) in the Trans-North China Orogen, North China, with Implications for the Tectonic Evolution

Abstract Understanding the temporal and spatial evolution of the North China Craton (NCC) basement, formed by amalgamation, is a crucial issue in global geosciences. The Huozhou complex is situated at the core of the Trans-North China Orogen (TNCO) in the NCC and comprises a considerable number of Palaeoproterozoic granitic gneisses, providing valuable insights into the tectonic evolution of the TNCO. In this study, comprehensive field geological surveys, petrology, chronology, geochemistry, and Hf isotope analysis were conducted to investigate the genesis and tectonic context of the Xingtangsi and Zhengnangou granitic gneisses and elucidate the TNCO’s tectonic evolution. The Xingtangsi granite gneiss yielded a magmatic zircon age of 2495±34 Ma, implying its Palaeo-proterozoic or Archean origin, as previously suggested. Its protolith was I-type peraluminous granite, primarily generated through the partial melting of pre-existing continental crust materials with a small quantity of mantle-derived magma. The Zhengnangou granitic gneiss’s protolith was A-type granite, and its magmatic zircon age was 2,190 ±11 Ma, indicating its Palaeoproterozoic origin rather than Archean. TDM1(Ma) for the Zhengnangou granitic gneiss ranged from 2,424 to 2,498 Ma, TDM2(Ma) varied from 2563 to 2684 Ma, and the εHf(t) value ranged from 1.3 to 3.3. These results suggest that it was primarily derived from newly formed crustal materials without any mantle-derived addition. Integrating our data with the literature, the ∼2.5 Ga magmatic activity in the Huozhou area may have formed in the tectonic setting of the continental arc, and ∼2.2 Ga A-type granite may have formed in a post-collisional extensional environment.

High‐frequency temporal variability of provenance signal in the submarine fan with the narrow shelf: Insights from sediment delivery and formation of late Triassic Zhuoni fan in the northeastern Paleo‐Tethys Ocean

AbstractThe submarine fan with a narrow shelf is usually reactive to environmental signal propagation; however, source‐to‐sink functioning can be further complicated by several allogenic forcings. Here, we document the high‐frequency provenance variations and different sediment delivery models recorded in the late Triassic Zhuoni fan developed in the northeastern Paleo‐Tethys Ocean, mainly based on process‐based sedimentological and provenance study of the Panyuan section in the West Qinling area in the northeastern margin of Tibetan Plateau. High‐, low‐density turbidites, hybrid event beds and hyperpycnites are distributed in the lobe‐dominated submarine fan succession. Field sedimentological evidence from surrounding outcrops suggests that shelf‐edge failure was the main cause of most high‐ and low‐density turbidites with the overall absence of submarine slides or slumps, whereas the narrow shelf configuration together with late Triassic humid pulses is favourable for the occurrence of flood‐related hyperpycnites in the Zhuoni fan. Detrital zircon grains (N = 6; n = 123–272) generally have Palaeozoic‐Mesozoic ages (ca. 350–250 Ma and 500–400 Ma) and Neoarchean‐Paleoproterozoic ages (ca. 2100–1750 Ma and 2600–2400 Ma), but they can be further categized into three age groups due to different proportions of Precambrian age populations. The results demonstrate that the potential source areas may include the South and North Qinling Orogenic Belt, Qilian Orogenic Belt, different segments of North China Craton and the tectonic junction area between the Qinling and Qilian Orogenic Belts. The temporal changes in provenance signals, which are reflected by both the detrital zircon age spectra and heavy mineral assemblages, indicate different contributions of those sources in response to sea‐level fluctuation. It could thus give rise to temporal variations between reactive and buffered source‐to‐sink sediment delivery models of the Zhuoni fan, despite the overall narrow shelf configuration. The development of the lowstand Zhuoni fan was directly related to extrabasinal hyperpycnal delivery from the river mouth and its high‐frequency provenance variability recorded different efficiencies of signal transfer through the onshore catchment with significantly influence of temporal storage, fluvial rejuvenation or even regional climate variability. The highstand submarine fan was thought to be formed by shelf‐edge failure with sediment buffering in the shelf region, which was associated with a strong magnitude of provenance mixing. Our work provides a new perspective for deciphering the different origins of deep‐water sediment delivery in response to high‐frequency sea‐level and climate changes.