Concordant Ages Research Articles

Geochronology and Geochemistry of Archean TTG and Tremolite Schist Xenoliths in Yemadong Complex: Evidence for ≥3.0 Ga Archean Continental Crust in Kongling High-Grade Metamorphic Terrane, Yangtze Craton, China

The origin and significance of the tonalite–trondhjemite–granodiorite (TTG) units and the familiar metabasite xenoliths they host in the Yangtze Craton, China, remain controversial, and resolving these issues is important if we are to understand the evolution of the early Yangtze Craton. We focused on biotite–tremolite schist xenoliths in the Archean TTG units of the Kongling high-grade metamorphic terrane, and U–Pb dating of their zircons yielded 207Pb/206Pb ages of ca. 3.00 Ga, which provides a minimum age for the formation of the pre-metamorphic basic igneous rock. The host TTGs and late intrusive granitic dikes yield three groups of upper intercept ages at 2.87–2.88, 2.91–2.94, and 3.07 Ga, and a concordant age at 2.94 Ga, which suggest that the Yangtze continental nucleus underwent three important metamorphic–magmatic events in the Mesoarchean at ca. 3.00, 2.94, and 2.87 Ga. The biotite–tremolite schists have high ratios of K2O/Na2O and high contents of CaO, Cr, and Ni, thus showing the characteristics of high-K calc-alkaline island-arc volcanic rocks (basalt–andesite) that form by the partial melting of subducted oceanic crust. The data also provide further proof that a Mesoarchean metamorphic basement exists in the Yangtze Plate. Derivation of the magmatic protoliths of the biotite–tremolite schist enclaves from an oceanic crust during slab subduction, and the presence of these xenoliths within the TTG suite, indicate the existence of the initiation of plate tectonics during the Mesoarchean (≤2.94 Ga).

First Results of Dating Detrital Zircons from the Late Precambrian Quartzite–Schist Sequences of the Chu Block (Southern Kazakhstan)

U-Pb geochronological studies of detrital zircons from quartzite-schist sequences of the Akbastau Formations of the Chu block (northwest of the Chu-Kendyktas terrane, Southern Kazakhstan) have been provided. The concordant ages of detrital zircons are predominantly within the intervals of 1672-2115 Ma with peaks at 1697, 1780, 1857 and 2066 Ma. Individual zircon grains display ages of 2291-2332 Ma with peaks at 2303 and 2322 Ma. Neoarchean ages 2608-2747 with peak at 2681 Ma characterize another significant zircon population. The lower limit of deposition for the Akbastau Formations of the Chu block, corresponding to the youngest statistically significant zircon population, is estimated at 1,7 billion years.

The tectonic evolution of the Changchun-Yanji suture zone: Constraints of zircon U-Pb ages of the Yantongshan accretionary complex (NE China)

The Changchun-Yanji suture zone is composed of a sequence of high-pressure metamorphic rocks. It lies to the southern margin of the Jilin-Heilongjiang high-pressure metamorphic belt and separates the Jiamusi-Khanka Block to the north from the North China Craton. In this study, to understand the nature of the Changchun-Yanji suture zone, we studied four samples in diverse lithology (i.e., two chlorite schists, one felsic dike sample, and one piemontite schist) from the Yantongshan Complex. These rocks have been intensely deformed without preservation of original sedimentary features. Integrated evidence from high-pressure metamorphic minerals and zircon U-Pb age results suggests that most of the Paleozoic strata in this region might be part of a tectonic mélange. Phengite in the piemontite schist contains high Si (≥3.304 based on 11 oxygens), indicative of high-pressure metamorphism. The piemontite schist contains 6 zircon age populations from 2505 ± 40 to 231 ± 10 Ma. The 22 youngest zircon grains define a weighted mean age of 237 ± 5 Ma, placing an upper limit on the depositional time of the protolith of the piemontite schist. The felsic dike intruded into the metasedimentary rocks gave concordant ages with a weighted mean age of 217 ± 5 Ma, providing a minimum emplacement age for the Yantongshan Complex. We conclude from these data that the final collision between the Jiamusi-Khanka Block and North China Craton along the Changchun-Yanji suture zone occurred before the Late Triassic. Moreover, this closure time is at least 30 Myr later than closure along the Solonker-Xar Moron suture in the late Permian. As such, we consider that the Changchun-Yanji suture zone was not related to amalgamation of the Siberian and North China cratons, but was the result of the subduction of the Paleo-Pacific Plate.

Origin and Age Determination of the Neotethys Meliata Basin Ophiolite Fragments in the Late Jurassic–Early Cretaceous Accretionary Wedge Mélange (Inner Western Carpathians, Slovakia)

This study reports the Neotethyan Meliata Basin ophiolite fragments in the Late Jurassic–Early Cretaceous accretionary wedge mélange in the southern part of the Inner Western Carpathians (IWC). Here we present new lithostratigraphical, petrographical, geochemical, and geochronological data obtained from the mélange blocks used to reconstruct the Meliaticum paleotectonic zones in a tentative evolutionary model of this accretionary wedge. The Dobšiná mélange block continental margin carbonatic and siliciclastic sediments have calc-alkaline basalt intercalations. The basalt Concordia age dated to 245.5 ± 3.3 Ma by U–Pb SIMS on zircon most likely indicates the pre-oceanic advanced early Middle Triassic continental rifting stage. The evolving marginal oceanic crust is composed of Middle to Upper Triassic cherty shales to radiolarites. The detrital zircon U–Pb SIMS Concordia ages of 247 ± 4 Ma and 243 ± 4 Ma from a cherty shale, and the xenocryst zircon population Concordia age of 266 ± 3 Ma from a 0.5 m thick “normal” mid-ocean ridge (N-MOR) basalt layer in this cherty shale reveal the connection of the oceanic basin to the adjacent rifting continental margin. The chertified reddish limestone transition to radiolarite indicates syn-rift basin deepening. Upwards, regular alternating N-MOR basalts and radiolarites are often disturbed by peperite breccia horizons. The Nd isotope values of these basalts (εNd240 = 7–8) are consistent with their chondrite normalized rare earth element (REE) patterns and indicate a depleted mantle source. The Triassic ophiolitic suite also comprises rare ocean island (OI) basalts (εNd240 = 5) and serpentinized subduction unrelated peridotites. The Middle to Late Jurassic shortening and southward intra-oceanic and continental margin subduction at approximately 170–150 Ma enhanced the formation of the trench-like Jurassic flysch succession which preceded the closure of the Meliata Basin. The flysch sediments form a mélange matrix of olistolithic unsubducted, obducted, and MP–HP/LT metamorphosed exhumed blocks of the Triassic to Lower Jurassic successions. Blocks of peridotites, rodingites, blueschists, greenschists, rare amphibolites, deep-water shaly sediments and shallow- to deep-water carbonates are typical members of the mélange. The Meliatic accretionary wedge mélange nappe outliers were incorporated in the IWC orogenic wedge in the late Early Cretaceous according to metamorphic rutile U–Pb SIMS ages of 100 ± 10 Ma determined from a Jaklovce metabasalt.

U–Pb ages of rare rutile inclusions in diamond indicate entrapment synchronous with kimberlite formation

The timing of diamond crystallization is generally inferred from radiometric dating of individual or multiple mineral inclusions in diamond, where ages are derived from isochrons or isotopic evolution models. Resulting ages often significantly predate the emplacement ages of host kimberlites, but age information from both approaches can be ambiguous due to long-lived mantle heterogeneities where mixing can mimic isochrons or in-situ aging. Direct dating of accessory mineral inclusions in diamonds, by contrast, has rarely been attempted because of the scarcity of such inclusions, requiring careful high grading of large amounts of diamond concentrates. Here, we report secondary ionization mass spectrometry (SIMS) U–Pb ages from a suite of rare rutile inclusions which were extracted from diamond in an eclogitic paragenesis from the Mir kimberlite pipe, Siberia. One population of rutile inclusions shows diamond crystal-shapes and was petrographically identified as completely enclosed in diamond prior to host burning for inclusion extraction. These rutile inclusions were screened for U abundances (0.24–23 μg/g U) and those with the highest U contents were dated, yielding a uniform U–Pb concordia age of 375.5 ± 7.0 Ma (error 95% confidence; mean square of weighted deviates MSWD = 0.74; number of spots n = 19). This age is nearly coeval with, but significantly (at the 95% confidence level) older than combined U–Pb rutile ages for rutile intergrown with polycrystalline fibrous diamond (362.9 ± 9.5 Ma; MSWD = 0.96; n = 7) and rutile from the eclogitic xenolith matrix (369 ± 16 Ma; MSWD = 0.30; n = 5). Rutile ages are consistent with published ages for kimberlite emplacement (ca. 360 Ma), but much younger than Re–Os sulfide isochron ages between ca. 913 Ma and 2.1 Ga for groups of individual inclusions in diamond from the same location. When assuming the oldest Re–Os sulfide age as the timing of rutile entrapment, diffusive Pb-loss from rutile inclusions over ca. 2 Ga would require diffusivities of Pb in diamond of >10−22 m2/s, as well as Pb partitioning coefficients between diamond and rutile at near or greater unity. Alternatively, if Pb is immobile in diamond, ancient rutile ages could have been reset due to processes other than volume diffusion through diamond (e.g., due to fluid migration along cracks). However, in this scenario it remains difficult to explain why all rutile inclusions dated here would have been located along cracks, whereas sulfide inclusions investigated in other studies were not. Another explanation is that rutile inclusions were entrapped at the time of diamond formation, and that this event is recorded by U–Pb rutile ages, whereas sulfides predate diamond crystallization and their Re–Os systematics were incompletely reset. Trace element heterogeneity in rutile suggests that they originated in chemically different environments, and thus rutile likely predates diamond formation. However, due to rapid diffusion of Pb in rutile, radiogenic Pb accumulated only after entrapment in diamond (for inclusions) or eruptive quenching (for intergrown or matrix rutile). Overall, rutile geochronology indicates that at least some diamond from the Mir pipe formed briefly (within the ca. 20 Ma age difference between U–Pb rutile ages for inclusions and intergrown/matrix rutile) before kimberlite eruption, supporting models that link diamond formation with carbon-rich precursors of kimberlite magmas.

Geochronology of an Apollo 16 Clast Provides Evidence for a Basin-Forming Impact 4.3 Billion Years Ago.

We examined lithic breccias from the Apollo sample collection in order to identify ferroan anorthosite samples suitable for geochronology, and better define the age relationships between rocks of the lunar highlands. Clast 3A is a previously unstudied noritic anorthosite from Apollo 16 lithic breccia 60016 with textural evidence of slow subsolidus recrystallization. We estimate a cooling rate of ~10 °C/Myr and calculate a pyroxene solvus temperature of 1,100–1,000 °C. Pyroxene exsolution lamellae (1–3 μm) indicate that the last stage of cooling was rapid at ~0.2 °C/year, typical of rates observed in thick ejecta blankets. We calculate concordant ages from the 147Sm‐143Nd, 146Sm‐142Nd, Rb‐Sr, and Ar‐Ar isotopic systems of 4,302 ± 28, 4,296 + 39/−53, 4,275 ± 38, and 4,311 ± 31 Ma, respectively, with a weighted average of 4,304 ± 12 Ma. The closure temperature of the Sm‐Nd system is ~855 ± 14 °C, whereas the closure temperature of the Ar‐Ar system is 275 ± 25 °C. Cooling from 855 to 275 °C at 10 °C/Myr should result in an age difference between the two isotopic systems of ~60 Myr. The concordant Sm‐Nd, Rb‐Sr, and Ar‐Ar ages imply that they record the time the rock was excavated by a large impact from the midcrust. The ages clearly predate various late accretion scenarios in which an uptick in impacts at 3.8 Ga is preceded by a period of relative quiescence between 4.4 and ~4.1 Ga, and instead are consistent with decreasing accretion rates following the formation of the Moon.

Ages and sources of detrital zircons from the Early Mesozoic metasedimentary rocks of the Un’ya-Bom terrane of the Mongol-Okhotsk fold belt: results of U-Th-Pb and Lu-Hf isotope studies

The U-Pb and Lu-Hf isotope studies have been performed to investigate detrital zircons from metaterrigenous deposits (Nel, Kurnal and Amkan f ormations) of the Un’ya-Bom terrane of the eastern part of the Mongol-Okhotsk fold belt. T he concordant age s of the youngest zircon in the metasiltstone of the studied formations are as follows: 213±3.6 Ma – N el , 194±4.4 Ma – K urnal , and 202±2.5 Ma – A mkan. The new data suggest that a previously assumed (Middle Jurassic) age of the A mkan f ormation is uncertain . Most likely, the Kurnal and Amkan formations are of the same ( E arly Jurassic) age. In any case, a relationship between these formation s needs to be clarified . I n co mbination with the data from previous studies of the Tukuringra t errane, our study results show that the E arly Mesozoic sedimentary rocks in the structure of the eastern part of the Mongol-Okhotsk fold belt are more abundant than it is currently assumed . Our study results suggest that during the accumulation of the Nel, Kurnal and Amkan formation s , the materials were supplied from different provinces, especially from the Amur s uperterrane ( i.e. from the south , in modern coordinates) , as well as from the fram e of the North Asian craton ( i.e from the north , in modern coordinates), although the contribution of the craton source was insignificant.

Provenance of the Buritirama Formation reveals the Paleoproterozoic assembly of the Bacajá and Carajás blocks (Amazon Craton) and the chronocorrelation of Mn-deposits in the Transamazonian/Birimian system of northern Brazil/West Africa

The Mn-bearing Buritirama Formation along the homonymous ridge consists of a 40 km long by ca. 3 km wide NW-SE trending unit that limits the northern border of the Archean Carajás domain with the Paleoproterozoic Bacajá domain in the southeastern Amazon Craton. The Buritirama Formation type section is arranged in four imbricated thrusts that individualize three main stratigraphic units: Lower Unit, Intermediate Unit and Upper Unit. Detrital U-Pb zircons ages from the Lower Unit and Upper Unit record the prevalence of Neoarchean populations, suggesting the Carajás block as the main sedimentary source for the Buritirama basin. However, minor Paleoproterozoic populations attributed to the Bacajá block are also recognized, defining a maximum depositional age at ca. 2186 Ma. The anorogenic Buritirama metagranite occurs at the local basement, represents a probable precursor stage of the Buritirama rift and yielded a Concordia age of 2549 ± 5.9 Ma (U-Pb).Supported by these new geochronological data, a framework model for the evolution of the Buritirama basin is proposed. Contrary to previous interpretations, the basin was active until at least the Rhyacian, representing a platformal setting fringing the northern margin of the Carajás block. A regional metamorphic event (ca. 2.06 Ga) records the closure of the basin during the Transamazonian Orogeny and the amalgamation of the Bacajá and Carajás blocks. The depositional age bracketed in between 2.3 and 2.1 Ga links the Buritirama Mn deposit (southeastern Amazon Craton) with those of Serra do Navio (Amapá Block, northeastern Amazon Craton) and Nsuta (West African Craton), suggesting: (i) widespread chronocorrelation of Mn-bearing sequences in the continental-scale Rhyacian Transamazonian/Birimian orogenic system, and (ii) formation of the primary carbonate ores under the influence of a major global manganese deposition episode.

New U-Pb geochronological data on the age of volcanic-plutonic assemblage of Oloy belt, Alazey-Oloy fold system (West Chukotka)

New U-Pb (SIMS and LA-ICP-MS) geochronological data for rocks of Egdygkych complex of hypabyssal intrusions, Nichan and Vukney plutons, and felsic volcanic rocks and tuffs from host strata of Oloy volcanic belt of Alasey-Oloy fold system are obtained. Concordant ages of Egdygkych complex rocks correspond to Early Cretaceous (Berriasian-Valanginian), those for host strata, to the end of Late Jurassic (Tithonian) - beginning of Early Cretaceous (Berriasian). New U-Pb geochronological data allow confidently to distinguish uniform volcanic-plutonic assemblage of Late Jurassic (Tithonian) - Early Cretaceous (Berriasian-Valanginian) age of Oloy volcanic belt. Obtained data more definitely determine age limits of Au-Mo-Cu mineralization, associated with contacts between rocks of Egdydkych complex and host volcanic-sedimentary rocks or contacts of separated intrusive phases.

Shallow reworking of magmatic zircon grains of latest Neoproterozoic (Timanian) age in serpentinite of the Voykar Massif, Polar Urals: new constraints from U-Pb isotopic data, and first trace elements and Lu-Hf isotopic data

ABSTRACTThe Voykar Massif of the Polar Urals in Russia consists of an ultramafic complex (mantle section) in the northwest, followed by a late Cambrian to Silurian mafic complex (intra-oceanic primitive island arc) and early Devonian intrusive rocks of an evolved island arc to the southeast. These complexes represent tectonic nappes thrust over the East European continental margin during the late Palaeozoic Uralian Orogeny. LA-ICP-MS U-Pb dating of zircon grains (n = 42) from an antigorite-serpentinite lens within the mafic complex yielded a Concordia age of 542 ± 2 Ma with an age range of 549–527 Ma. Additionally, few grains contain inherited domains with ages between ~990 and 3277 Ma. Hafnium isotopic data of the main age group show 176Hf/177Hft from 0.28242 to 028249 and εHft ranging from +1.9 to −6.5. The evolved Hf isotope data and the trace-element composition of the zircon grains point to an involvement of a continental crustal component in the parental magma. The zircon grains originate from igneous rocks formed during the Timanian Orogeny that affected the East European margin in the latest Neoproterozoic. During the Timanian or Uralian Orogeny, the magmatic zircons were eroded and shallowly recycled into the serpentinised mantle above the subduction zone. Finally, Uralian thrusting led to juxtaposition and imbrication of the zircon-bearing serpentinite and intra-oceanic volcanic rocks of the mafic complex.

U-Pb geochronology of garnet from Daschkesan skarn deposit (Lesser Caucasus)

We present new U-Pb ID-TIMS data for garnet from the Daschkesan skarn deposit. The uranium content ranges from 8 to 8,5 ppm. Garnet is characterized by low common Pb (Pbc/Pbt = 0,01-0,17). The concordia age is 146±3 Ma (MSWD = 0,87). Our results demonstrate the reliability of calcium garnet as a mineral for the precise timing of skarn formation.

Modelling U-Pb discordance in the Acasta Gneiss: Implications for fluid–rock interaction in Earth's oldest dated crust

The U–Pb isotopic system in zircon is the tool of choice to interrogate high-temperature geological processes, yet this system has potential to investigate lower temperature fluid–rock interaction as well. In some cases, removal of radiogenic Pb is incomplete, potentially allowing regression of discordant U–Pb data on a concordia diagram to determine both the age of crystallization and the timing of fluid-driven isotopic disturbance. However, in rocks preserving more complex histories, simple regression is not effective at resolving multiple Pb loss events. Here, we use a ‘concordant–discordant comparison’ (CDC) test to establish the times of U–Pb disturbance in the Acasta Gneiss Complex (AGC), Canada. AGC c. 4.03 to c. 3.40 Ga orthogneisses experienced a long and complex post-crystallization history, for which U–Pb zircon data reflects not only the heterogeneous nature of the rock, but also the varying degrees and duration of crustal reworking that inevitably involved open system processes. The CDC test calculates the similarity between the concordant age structure and a modelled age structure, the latter inferred from discordant analyses, over a wide range of potential disturbance times. Our analysis reveals concordant zircon components implying new growth and/or recrystallization at 3992 ± 5, 3501 ± 6, 3442 ± 5 and 3126 ± 6 Ma. In addition, we establish episodes of radiogenic-Pb loss driven by fluid–rock interaction at 3150 ± 50 Ma, and probably at 2875 ± 50 Ma and c. 2590 Ma. These Pb-loss episodes correlate with previously recognised events recording growth of zircon rims during metamorphism, granite emplacement, and unroofing. Pb-loss within the AGC shows an antithetic relationship in different samples that are in close geographic proximity. We suggest that zircon alteration and associated new growth effectively rendered those grains that underwent Pb-loss at a particular time less susceptible to alteration during the next episode of fluid interaction.

Constraints on the timing of Fe–(Cu) metallogenesis in the eastern Aqishan–Yamansu–Shaquanzi metallogenic belt, Eastern Tianshan, NW China

The Aqishan–Yamansu–Shaquanzi belt is a well-known Fe–Cu–Au metallogenic belt located in the Central Asian Orogenic Belt, NW China. Numerous Fe-rich ores as well as polymetallic mineralization (including Cu, Pb, Zn, and Co) and related magmatic–hydrothermal alteration are hosted in volcanic rocks, especially those in the eastern part of this metallogenic belt. The complex geology of the belt and the divergent geochronological data obtained by different investigators and analytical methods have led to contrasting opinions on the deposit types and metallogenesis of the belt. The present study focuses on the Shaquanzi, Yamansu, and Heifengshan Fe–(Cu) polymetallic deposits in the eastern part of the belt. These deposits display Fe-dominated metallogenesis related to volcanic–sedimentary exhalative processes and polymetallic metallogenesis related to volcanic–hydrothermal metasomatism and skarnization caused by subsequent intrusion processes. The Shaquanzi, Yamansu, and Heifengshan deposits have undergone multiple periods of metallogenesis and contain late Fe–Cu–Pb–Mo, Fe–Cu–Zn–(Co), and Fe–Cu mineralization, respectively. Using zircon U–Pb dating by laser-ablation–multi-collector–inductively coupled plasma–mass spectrometry, two rhyolite samples (at the base and lower-middle part of the Tugutubulak Formation) and a basaltic andesite and a diorite dyke in the Shaquanzi ore district yield concordant ages of 327.9 ± 2.6 Ma (mean square of weighted deviates or MSWD = 0.015), 327.8 ± 1.4 Ma (MSWD = 1.03), 285.9 ± 2.0 Ma (MSWD = 0.81), and 276.3 ± 1.4 Ma (MSWD = 0.21), respectively. Basaltic andesite from the Upper Yamansu Formation in the Yamansu deposit area yields a concordant age of 327.6 ± 6.4 Ma (MSWD = 0.54), whereas rhyolite, diorite dyke, and granite stock from the Heifengshan deposit give concordant ages of 285.7 ± 1.3 Ma (MSWD = 0.01), 273.7 ± 1.6 Ma (MSWD = 1.2), and 274.4 ± 2.5 Ma (MSWD = 0.20), respectively. Hydrothermal molybdenite in the Shaquanzi deposit yields a weighted mean Re–Os age of 314.6 ± 1.7 Ma (MSWD = 0.29) and an isochron age of 316 ± 6 Ma (MSWD = 0.46). A collation of previously reported ages and our new age data show that extensive magmatism occurred during the Carboniferous–early Permian (ca. 350–270 Ma) in the Aqishan–Yamansu–Shaquanzi belt, with a peak period between ca. 335 and ca. 305 Ma. The Yamansu Formation and Tugutubulak Formation volcanic rocks formed at ca. 348–334 Ma (early Carboniferous) and between ca. 328 and ca. 303 Ma (late early Carboniferous to late Carboniferous), respectively. In the eastern part of the belt, Fe-dominated mineralization resulting from volcanic–sedimentary exhalative processes (e.g., ca. 334, ca. 322, and ca. 286 Ma) and polymetallic mineralization resulting from volcanic–hydrothermal metasomatism (e.g., ca. 334–323 Ma), as well as skarnization and magmtic-hydrothermal caused by intrusions occurred at different stages (e.g., ca. 322–316, ca. 299, and ca. 274 Ma), leading to the superimposition of polymetallic mineralization between ca. 334 and ca. 273 Ma.

Age and provenance of the Chaung Magyi Group, Yeywa Dome, Myanmar, based on U-Pb dating of detrital zircons

The unfossiliferous Chaung Magyi Group of the Northern Shan Plateau of Myanmar, comprises metasedimentary rocks, in particular greywacke, slates, phyllites and schists, of inferred Proterozoic age that unconformably underlie the late Cambrian Pangyun Formation. To address the problem of the youngest stratigraphic extent of the group, four samples of the Chaung Magyi Group from the Yeywa Dome, East of Kyaukse, Mandalay Division, were collected for U-Pb dating of detrital zircons. Based on the youngest detrital zircons present in sandstone samples from the western Yeywa Dome area, at least the upper part of the Chaung Magyi Group is of latest Proterozoic age and possibly extends to the late Cambrian (~500 Ma). Comparison of detrital zircon age-frequency histogram distributions show only slight differences in distributions of detrital zircons in the Cambrian of NW Thailand and the Chaung Magyi Group samples (Sibumasu Terrane), with a concentration of ages between 1300 Ma and 820 Ma, with two major peaks at 1086 Ma and 1022 Ma. A younger Cryogenian age peak at 647 Ma is also observed. A spread of older concordant ages ranges from 3093 Ma to 1406 Ma. The minimum depositional age for the Chaung Magyi Group of the west of the Bawdwin Mine (982 Ma) based on ages of cross-cutting dykes (reported in Mitchell, 2018), and the maximum depositional age for Yeywa Dome (~600–500 Ma) based on detrital zircons suggest the age difference (~>400 my) is too great for the Chaung Magyi Group to be considered a single group, the dyke ages require re-investigation.

Petrogenesis and geodynamic implications of Neoproterozoic gabbro-diorites, adakitic granites, and A-type granites in the southwestern margin of the Yangtze Block, South China

The occurrence of A-type granites following adakitic granites can provide unique insights into the tectonic evolution of subduction zone. This paper presents a comprehensive study of zircon U-Pb-Hf isotopes and whole-rock geochemistry on the Neoproterozoic gabbro-diorites, biotite granites, and K-feldspar granites in the southwestern margin of the Yangtze Block, South China. Zircon U-Pb dating of three distinctive intrusions yield concordant ages of 810.4 ± 2.0 Ma for gabbro-diorites, 800.3 ± 2.1 Ma for biotite granites, and 749.1 ± 2.0 Ma for K-feldspar granites. The gabbro-diorites are sodic and calc-alkaline rocks with low SiO2 (52.62–53.87 wt%), medium MgO (2.67–3.41 wt%), high Fe2O3T (7.18–7.49 wt%) and CaO (5.68–7.50 wt%) contents. They display high Th/Zr and Rb/Y ratios but low Nb/Zr and Nb/Y ratios. Together with their positive whole-rock εNd(t) (+1.0 to +1.5) and zircon εHf(t) values (+3.66 to +8.18), we suggest that these gabbro-diorites were derived from the partial melting of depleted lithospheric mantle modified by subduction-related fluids. The biotite granites display relatively high Sr contents (335–395 ppm), Sr/Y ratios (38.9–54.3), low Y (7.04–9.71 ppm) and Yb contents (0.78–1.08 ppm), resembling the adakitic granites. Their low Mg# (36–41) values, Cr (2.94–3.59 ppm) and Ni (1.32–1.55 ppm) contents as well as positive εNd(t) (+0.5 to +0.6) and εHf(t) values (+1.62 to +8.07) indicate that they were formed by partial melting of thickened juvenile mafic lower crust, probably within the existence of garnet. The K-feldspar granites have high SiO2 (76.61–77.14 wt%) and alkalis (Na2O + K2O = 8.55–9.69 wt%) contents, high 10,000 * Ga/Al (2.56–2.80) ratios and differentiation index (95.3–96.9), indicating the affinity to highly fractionated A-type granites. We suggest that these A-type granites were mainly generated by the partial melting of felsic crustal rocks under low pressure condition and subsequently extensive fractional crystallization is significant. Taking into account previous studies from the western Yangtze Block, South China, we propose that the gabbro-diorites, adakitic granites and A-type granites in this study were formed in persistent subduction process during the Neoproterozoic.

U-Pb zircon and Hf composition of granite Song Ma block

The granite of the Song Ma block mainly consists of two types of granite: biotite granite and hornblende-biotite granite. Biotite granites have the percent of plagioclase (35– 45%), K-feldspar (25–35%), quartz (~20%) and biotite (~10%). Biotite-hornblende granite with the content of plagioclase (40–50%), Kfeldspar (10–15%), hornblende (5–10%) and biotite (5%). Zircon crystals were selected from the granite of Song Ma block are V0741, V0856 and V1006 samples with the LA-ICPMS U-Pb analyses gave concordant ages concentrated at 257±4Ma, 262±3Ma and 241±6Ma (weighted mean). Those ages are older than the results of the previous research. The mineral assemblages and geochemical characteristics show the typical of I-type granites. The results of Hf isotope composition analysis give the value of εHf(t) from +7.3 to +13.9, which is proven the sources of the granite Song Ma block similar to the granite of Phan Si Pan zone, NW Viet Nam during the period from late Permian to early Triassic.

An extensive K-bentonite as an indicator of a super-eruption in northern Iberia 477 My ago

Zircon and monazite ID-TIMS U-Pb dating of four Lower Ordovician altered ash-fall tuff beds (K-Bentonites) in NW Iberia provided coetaneous ages of 477.5±1, 477±1.3 Ma, 477.2±1.1 Ma and 477.3±1 Ma, with a pooled concordia age of 477.2±0.74 Ma. A conservative estimation of the volume and mass of the studied K-bentonite beds (using data from the Cantabrian Zone) returns a minimum volume for the preserved deposits of ca. 37.5 km3 (Volcanic Explosivity Index - VEI = 6, Colossal). When considering other putative equivalent beds in other parts of Iberia and neighbouring realms the volume of ejecta associated to this event would make it reach the Supervolcanic-Apocalyptic status (VEI=8, >1000 km3). Contrary to most cases of this kind of gargantuan eruption events, the studied magmatic event took place in relation to continental margin extension and thinning and not to plate convergence. We speculate that a geochronologically coincident large caldera event observed in the geological record of NW Iberia could be ground zero of this super-eruption.

Late Jurassic I-type rhyolites from the Duobuza region, north-central Tibet: evidence for the occurrence of juvenile lower crust and crustal growth in the Southern Qiangtang Terrane

ABSTRACT Previous studies of the Southern Qiangtang (SQ) Terrane in Tibet have shown significant growth of continental crust during the late Mesozoic (ca. 170–120 Ma) through underplating of mantle-derived melts beneath ancient lower crust. However, the mechanism of crustal growth remains defective during the Late Jurassic. In this contribution, we report new zircon U–Pb ages, whole-rock major and trace element concentrations, and zircon Hf isotopic compositions of Late Jurassic rhyolites from the Duobuza area in the SQ Terrane. Zircon LA–ICP–MS U–Pb dating yields concordant ages and weighted mean 206Pb/238U ages of 151–149 Ma, indicating that the rhyolites were erupted during the Late Jurassic. The rhyolite samples yield high SiO2 (69.1–73.4 wt.%) and Na2O (3.82–6.49 wt.%) concentrations, high values of the differentiation index (95.4–98.5), moderate Al2O3 (13.1–15.5 wt.%) and K2O (2.55–3.26 wt.%) concentrations, and relatively low TiO2 (0.34–0.44 wt.%), MgO (0.14–0.67 wt.%), and MnO (0.01–0.10 wt.%) concentrations and Mg# values (8.9–33.5). They are enriched in Rb, Th, Ba, and Pb, depleted in Nb, Ta, Sr, Ti, and P, and yield negative Eu anomalies. These characteristics suggest that the Duobuza rhyolites represent medium- to high-K calc-alkaline, highly fractionated I-type granitoids. Variations in major and trace element concentrations indicate that the rhyolites underwent significant fractionation of Fe–Ti oxides, plagioclase, K-feldspar, hornblende, biotite, and apatite. The samples yield positive zircon εHf(t) values (+0.08 to +10.9) and young TCD M model ages (816–386 Ma), indicating that they were generated through partial melting of juvenile lower crust of the SQ Terrane. These data are distinct from those of Late Jurassic intermediate–felsic plutons in the SQ Terrane that generally yield negative εHf(t) values and ancient TC DM model ages. We suggest that the occurrence of the Duobuza rhyolites is the significant witness. The presence of juvenile lower crust beneath the SQ Terrane during the Late Jurassic (ca. 150 Ma) rather than during the Early Cretaceous (ca. 120Ma). The transition from ancient to juvenile lower crust has already began in the SQ Terrane during the Late Jurassic.

Early Paleozoic zircon ages of the Higher Himalayan Gneisses of the Everest region and their Pan-African/Proto-Himalayan orogenic signature

The U-Pb analysis of zircons from two independent leucosome bodies belonging to the paragneiss of the Higher Himalayan Crystallines Sequence (HHCS) in the Everest region of eastern Nepal Himalaya was carried out using laser ablation-multi collector inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) at the University of Arizona, Tucson, USA. The analysis of zircons from sample 07EVT3 forms a discordia with upper and lower intercepts at 478±25 and 21.5±4.1 Ma with concordant ages of 488.5±9.2 and 20.9±0.9 Ma for cores and rims, respectively. Similarly, the analysis of zircons from sample U1206 forms a discordia with upper and lower intercepts at 515±20 and 34.8±2.7 Ma, and provides concordant ages of 463.9±10.9 and 24.6±0.6 Ma for cores and rims, respectively. No inherited zircon grains with older ages were found indicating that almost all these zircons must have formed along with the leucotomies during the ca. 500 Ma metamorphism of the protoliths. The high U/Th ratio, i.e. average 11.0 for zircons from sample 07EVT3 and 253.6 for sample U1206 also support a metamorphic origin of the zircons. The occurrences of zircons in the ca. 500Ma leucosomes in the HHCS strongly points that rocks in the Himalayan area had undergone to a high-grade metamorphism during the late Pan-African time. We call this metamorphism as the Proto himalayan metamorphism. More studies along this line will help to better understand and constrain the Pan-African orogenic history of the Proto-Himalayan Orogen within the Peri-Gondwana Orogenicterrains.

Petrology, geochronology, and geophysical characterization of Mesoproterozoic rocks in central Illinois, USA

The Precambrian basement rocks of the Eastern Granite-Rhyolite Province (EGRP) in central Illinois (midcontinent region of North America) exhibit a complex history of early volcanism, granite emplacement, and intrusion of mafic rocks. A comprehensive suite of dedicated petrographic analyses, geophysical logs, and drill core from four basement-penetrating wells, two-dimensional and three-dimensional seismic reflection data, and U-Pb age data from the Illinois Basin–Decatur Project (IBDP) and Illinois Carbon Capture Storage (ICCS) Project site provide new constraints for interpreting the Precambrian basement of the Illinois Basin. These new data reveal the basement to be compositionally and structurally complex, having typical EGRP felsic volcanic rocks intruded by the first reported gabbro in the Precambrian basement in Illinois. Zircons (n ​= ​29) from rhyolite give a U-Pb weighted mean average age of 1467 ​± ​9 ​Ma. Zircons (n ​= ​3) from a gabbro dike that intrudes the rhyolite yield a concordia age of 1073 ​± ​12 ​Ma, which corresponds to Grenville-age extension and represents the first Grenville-age rock in Illinois and in the EGRP. A high-resolution three-dimensional seismic reflection volume, coincident with the four wells, provides a context for interpreting the petrological data and implies a high degree of heterogeneity for basement rocks at the IBDP–ICCS site, as also shown by the drill cores. The occurrence of Grenville-age gabbro is related to a prominent bowl-like structure observed on local two-dimensional seismic reflection profiles and the three-dimensional volume that is interpreted as a deep-seated mafic sill complex. Furthermore, heterogeneities such as the brecciated EGRP rhyolite and later gabbro intrusion observed in the basement lithology at the IBDP–ICCS may reflect previously unknown distal elements of the 1.1 ​Ga Midcontinent Rift in the EGRP and more likely Grenville-age extension.