Search

Whether the Yangtze Block was involved in the global late Mesoproterozoic orogeny remains contentious. The Mesoproterozoic Huodiya Group exposed in the Hannan‐Micangshan Massif on the northwestern margin of the Yangtze Block could provide sufficient evidence to further clarify the regional tectonic affinity. However, quantitative insights into the depositional age, provenance and basin tectonic setting of the Huodiya Group still lack comprehensively constraining. This study presents a combined analysis of zircon U–Pb dating for meta‐sedimentary rocks of the Shangliang Formation of the upper Huodiya Group and intruded gabbroic dykes. Intergraded dating results constrain the late Mesoproterozoic maximum depositional age at ca. 1050 Ma. Ca. 900 Ma formation age for the intruded gabbroic dyke, plus the minimum formation age of the overlying Xixiang Group at ca. 950 Ma, further suggests its deposition should be prior to ca. 950–900 Ma. Concordant detrital zircon ages define pronounced age clusters of ca. 2229–1741 and 2950–2388 Ma and few age populations of ca. 3232–3082 and 1069–1033 Ma, consistent with the tectonothermal events in proximal domains at the northwestern and northern Yangtze Block. Detrital zircon age distribution patterns and cumulative curves, coupled with the stable carbonate platform and shallow‐marine facies sedimentation, suggest a passive continental margin setting for the Huodiya Group on the Yangtze Block margin at the late Mesoproterozoic. Comparative insights into the contemporaneous passive continental margin from north to southwest argue against the existence of a late Mesoproterozoic orogeny along the exterior periphery of the Yangtze Block. [ABSTRACT FROM AUTHOR]

To address the limitations of current dating methods, it is crucial to not only enhance existing techniques like U–Pb zircon dating but also explore alternative tools. This study focuses on three common mineral phases—zircon, apatite, and titanite—in an I-type granite. The goal is to assess their reliability as dating tools and propose improved methods for dating granitic rocks. In the case study of the Mt Stirling pluton within the Mt Buller igneous suite in Southeastern Australia, significant variability in laser ablation U–Pb zircon ages (around 100 million years) was observed. To improve the reliability of zircon age data and reduce non-magmatic-related variabilities, a data filtering protocol is applied. This protocol involves several steps such as trimming zircons with excessive K and Ca, excluding zircons with unusual core–rim age relationships, removing zircons with excessive non-formula elements (Al, Fe, and Mn), identifying hydrothermally altered zircons, and applying a 10% discordance threshold. The filtered Concordia Age (406 ± 1 Ma; mean square weighted deviation (MSWD) = 0.7, n = 80) of the host rock exhibits improved precision and reduced error compared to the unfiltered data (399 ± 2 Ma; MSWD = 9.3, n = 240). The filtered individual dates show less scatter and a mean that is different (i.e. outside statistical uncertainty), noting that their total still spans a considerable time range of ~50 million years, exceeding the individual zircon analytical reproducibility of 2 standard errors (~15 million years of 2 SE). Caution is advised when using the proposed error for the pooled analyses as a definitive precision. Similarly, trace element filtering approaches were applied to apatite and titanite samples from Mt Stirling, two phases that arguably cannot be inherited. For apatite, monitoring Ca and P as well as Zr/Y and Th/U ratios, along with identifying age groupings based on Sr concentrations, was effective in eliminating outliers and enhancing dating precision. In the case of titanite, monitoring Ca and Ti, Sr/Zr and Sr/Th ratios, and Sr/Ca and Zr/Ti ratios successfully enhanced dating precision. Notably, apatite and titanite grains were grouped in distinct Sr concentrations (high-, mid-, and low-Sr), with these groups corresponding to different date groups: high-Sr apatite and high- and low-Sr titanite returned c. 403 Ma, while low-Sr apatite and mid-Sr titanite returned c. 420 and 393 Ma, respectively. The spuriously younger or older dates may indicate an open system and influences from various common-Pb sources. The 403 Ma date coincides with the filtered zircon data, placing further confidence in the coupled approach, and is interpreted here as the igneous intrusion age. Notable is that this age is 25 Myr older than previously reported K–Ar age data, thus far considered to be the age of the intrusion. This study underscores the potential for erroneous zircon dates due to cryptic chemical influences. To enhance the reliability of age interpretation using laser ablation analyses, employing a petrochronological approach using split-stream combined age and trace element data is recommended in addition to the combination of multiple geochronometers. In the case of Mt Buller, it has proven crucial to carefully verify chemical closure of all applied geochronometers by monitoring concomitant trace element concentrations. Applied to other intrusions, petrochronology can play a critical role in obtaining reliable age information, even for igneous rocks that appear pristine. With this, we emphasise the importance of a careful approach towards individual age data interpretation, which can be produced fast and in abundance with modern analytical approaches. [ABSTRACT FROM AUTHOR]

High-pressure and ultra-high-pressure metamorphic terrains display an internal architecture consisting of a pile (or stack) of several coherent tectonic thrust sheets or units. Their identification is fundamental for understanding the scale and mechanisms active during subduction and exhumation of these crustal slices. This study investigates the geometry of the northern Dora-Maira Massif and the kinematics of the major tectonic boundaries, combining field and geochronological data. The tectonic stack of the northern Dora-Maira Massif comprises the following units. The lowermost unit (the Pinerolo Unit) is mainly characterized by Upper Carboniferous fluvio-lacustrine (meta-)sediments. The Pinerolo unit is overthrust by a pre-Carboniferous basement. The latter is subdivided in two tectonic units (the Chasteiran and Muret Units) with different Alpine metamorphism (ultra-high-pressure and high-pressure, respectively). The pre-Carboniferous basement of the Muret Unit is thicker than previously thought for two main reasons. Firstly, some paragneisses, traditionally assumed to be Carboniferous and/or Permian in age, display detrital zircon ages indicating a main source at about 600 Ma. Secondly, three samples of the Granero Orthogneiss, previously assumed to be a Permian intrusive body, have provided zircon U–Pb ages of 447 ± 1 Ma, 456 ± 2 Ma and 440 ± 2 Ma, indicating a late Ordovician or early Silurian age for the protoliths. The uppermost unit (the Serre Unit) comprises porphyritic (meta-) volcanic and volcaniclastic rocks dated to the Permian (271 ± 2 Ma), on top of which remnants of the Mesozoic cover is preserved. Detailed mapping of an area about 140 km2 shows that (i) the ultra-high pressure Chasteiran Unit is localized at the boundary between the Pinerolo and Muret Units, (ii) the Granero Orthogneiss may be considered as the mylonitic sole of the Muret Unit, characterized by a top-to-W sense of shear, and (iii) the contact between the Muret and Serre Units displays ductile-to brittle structures (La Fracho Shear Zone), indicating a top-to-the-NW displacement of the hangingwall with respect to the footwall. A final episode of brittle faulting, cutting across the nappe stack (the Trossieri Fault), indicates an extensional stage in the core of the Alpine belt, as previously documented in more external zones. This work provides a necessary and robust basis before an accurate discussion of processes acting during continental subduction of the Dora-Maira Massif may be understood. [ABSTRACT FROM AUTHOR]

The Oaxacan Complex Carboniferous-Permian sedimentary cover in southern México records provenance shifts through time, reflecting the collision between Gondwana and Laurentia to amalgamate Pangea. The integration of petrological analysis and LA-ICP-MS U-Pb zircon geochronology from Santiago, Ixtaltepec and Yododeñe formations compared with adjacent terranes suggests that: (1) during the Early Mississippian, the Santiago Formation received sediments mainly from local sources such as the Oaxacan Complex and Tiñu Formation, with minor contributions from adjacent peri-Gondwana sources. The magmatic activity may have started during this time (~359–346 Ma) (2) during the Late Mississippian (Ixtaltepec Formation), detrital zircon grains of Ediacaran-Cambrian age are dominant, derived from sediments either related to the Pan-African/Brasiliano orogeny or the opening of the Iapetus Ocean; (3) during the Late Mississippian-Middle Pennsylvanian, intercalated marine volcaniclastic sandstone (Ixtaltepec Formation) provides the first record of Carboniferous arc-related volcanism reported in southern Mexico, dated between 330 and 308 Ma; (4) the early Permian Yododeñe Formation records the exhumation and erosion of the sedimentary cover during the final stage of Pangea assembly. Rhyodacitic subvolcanic sills and lavas dated at ~282–270 Ma are present throughout the succession. Volcanism and ca. 360–308 Ma detrital zircon grains could be associated with a Carboniferous magmatic arc formed by subduction of the Rheic oceanic plate beneath Gondwana. Slightly younger detrital zircon and subvolcanic rocks dated at ~300–270 Ma are linked to a western Pangea arc developed in response to the subduction of the Paleo-Pacific Ocean following Pangea assembly. Our results suggest that the Carboniferous-Permian units were deposited in a peri-arc basin, sharing sediment provenance with the Maya and Coahuila blocks, the Sierra de Juárez Complex, and northwestern South America. [ABSTRACT FROM AUTHOR]

The Pseudoreticulatispora confluens–P. pseudoreticulata spore-pollen zonal datum typically coincides with the end of widespread Permian glacial deposits in Western Australia. Although previously attributed to the mid-Sakmarian, chemical abrasion isotope dilution thermal ionisation mass spectrometry (TIMS) dating of zircons from volcanic tuffs in the Ditji Formation of the Bonaparte Basin and the Grant Group in the Canning Basin point to an Asselian age of about 295.25 Ma for this datum. All dated zircons from the Ditji Formation came from petroleum well cuttings but the accompanying palynology was mostly from sidewall cores; however, all Grant Group samples were from conventional core. TIMS dates from the Ditji Formation range in age from 295.2 to 292.7 Ma whereas the only productive tuff from the Grant Group yielded a 296.26 Ma date. By comparison, there are no zircon dates to constrain the onset of glacial deposition in Australia. The Bonaparte Basin ages overlap with those for the Edie Tuff (296.1–294.5 Ma) in Queensland's Galilee Basin, approximately 2000 km to the southeast, which also lies close to the base of the P. pseudoreticulata Zone. To date the only fossil group within the P. confluens Zone in Western Australia to provide independent age control, albeit loosely, are goniatites from the northern Perth Basin (Uraloceras irwinense and Juresanites jacksoni) that have consistently been attributed to the Sakmarian; these require a reassessment of their affinity with Russian faunas and therefore to global stratotypes. The position of the Carboniferous–Permian boundary is elusive in Australia and will remain so until additional volcanic tuffs containing young datable zircons are found; however, spore-pollen and zircon dates from Namibia place this boundary within the P. confluens Zone. [ABSTRACT FROM AUTHOR]

The Koras Group is a bimodal volcanosedimentary group located in post-tectonic grabens in a foreland thrust complex in the Kaaien Terrane of the Mesoproterozoic Namaqua-Natal Province of southern Africa. It contains two sequences of mafic and felsic volcanic rocks with an unconformity between them, only the lower sequence being slightly folded. The Koras Group was long regarded as having formed at the end of the 1 210 to 1 000 Ma Namaqua Orogeny, because it lacks the severe deformation and metamorphism of the underlying rocks, with igneous minerals preserved in many samples. Following years of unsuccessful attempts to precisely date the volcanic rocks, the first two ion probe U-Pb zircon studies both reported ages of ~1 172 Ma for the Swartkopsleegte Formation felsic lava in the slightly folded lower sequence (based on relatively few dated zircons) and ~1 100 Ma for the Leeuwdraai Formation rhyolite in the undeformed upper sequence. Thus a major 70 m.y. hiatus seemed apparent between the lower and upper sequences despite their similar geochemistry and rift-related setting. This gave rise to models which envisaged the Kaaien Terrane being unaffected by the syn- to late-tectonic deformation, migmatisation and granite intrusions, documented between 1 200 and 1 150 Ma in the adjoining Namaqua-Natal terranes to the west. A high-pressure (10 kbar) metamorphic event, recognised in the Kaaien Terrane basement just south of hardlydeformed Koras Group exposures and dated at 1 150 Ma, is inconsistent with such models. A re-investigation and microbeam dating campaign on the Koras Group confirms the 1 101 ± 2 Ma (n = 6) age for felsic volcanic rocks of the upper sequence, but establishes a new reliable age of 1 114 ± 4 Ma for the lower one (n = 2). The 1 170 ages obtained in the earlier two studies were revisited and are now considered to reflect the age of zircon xenocrysts from the source rocks, which dominate the zircon population of some Swartkopsleegte Formation samples. Several criteria to distinguish autocrystic (magmatic) from antecrystic (age-overlappping xenocrystic) data points were investigated. One sample had high Th levels in only the younger zircons, but histograms of sufficiently precise 207Pb/206Pb ages provided the main criterion. Calculations of zircon crystallisation temperature intervals were not useful in predicting the abundance or proportions of magmatic and antecryst zircons. A multi-episode model of magmatic generation and crystallisation events is probably appropriate. In cases when felsic volcanic samples yield few zircons, care must be taken to avoid the problem exposed in this study. The Koras Group sediments have similar detrital zircon U/Pb age distributions to those of the Rehoboth Basement Inlier. This supports the concept that the Kaaien Terrane originated as the southern part of the Rehoboth Province. [ABSTRACT FROM AUTHOR]

In central Victoria, inherited zircon in Devonian igneous rocks and detrital zircon in metasedimentary country rocks and an amphibolite-facies xenolith show that Mesoproterozoic parts of the underlying Selwyn Block cannot be the source for all the silicic magmas. Zircon inheritance in S-type samples reveals significant thermal events at 525–425 Ma and 1200–1100 Ma. Both S- and I-type samples have prominent zircon age peaks at 420–410 Ma, which record high-grade metamorphism of the deep crust during the terminal phases of the Benambran and Bindian orogenies. All I-type rocks have 650–500 Ma peaks, suggesting derivation from an arc-related metavolcanic source in the upper Selwyn Block. Protoliths of the greenschist-facies Ordovician metasediments and the amphibolite-facies Cambrian metasedimentary xenolith were deposited in distal backarc settings. Most inherited zircon cores are metamorphic, and the strongest zircon inheritance occurs in hornblende-bearing I-type rocks, highlighting their largely crustal origin. Zircon populations at ca 1400 Ma, thought to signal sediment derivation from East Antarctica and Rodinia-Nuna, are mostly absent in I-type samples and some S-types. The ca 1400 Ma signal probably applies to the upper, metasedimentary Selwyn Block, so Devonian S-type magmas were sourced mainly in the deeper sections. Zircon inheritance in the Devonian igneous rocks was not influenced by the exposed metasedimentary country rocks. Two samples from one of the smaller plutons have contrasting patterns of zircon inheritance, suggesting relatively small-scale source heterogeneity. Many rounded and corroded cores in zircon crystals yield the same ages as the crystallisation dates for the rocks, and thus are antecrysts. Higher whole-rock Zr contents generally correlate with higher proportions of inherited zircon, and differentiation does not affect this relationship. The degree of partial melting of a magma source and the efficiency of crystal entrainment are critical in governing zircon inheritance. Mesoproterozoic sections of the Selwyn Block cannot be the sources for all the Devonian silicic magmas in central Victoria. I-type rocks have 650–500 Ma zircon age peaks, suggesting derivation from arc-related metavolcanic rocks in the upper Selwyn Block. Hornblende-bearing I-type rocks have the strongest zircon inheritance patterns, indicating the largely crustal origins of I-type magmas. Exposed metasedimentary country rocks were not involved in magma genesis. [ABSTRACT FROM AUTHOR]

Metamorphic rocks from the Diancang Shan (DCS) complex along the Ailao Shan-Red River belt between the South China plate and Simao-Indochina block are studied in this contribution to constrain the Proto- to Palaeo-Tethys tectonic evolution of the eastern Tethys domain. LA-ICP-MS zircon U-Pb geochronological dating is conducted in this study on the metasedimentary rocks (garnet-biotite-plagioclase gneisses, biotite-quartz schists and phyllonites) from different stratigraphical horizons (Proterozoic and Triassic) of the DCS complex. The analyses yielded three major age peaks at ca. 248 Ma, ca. 441 Ma, ca. 1000–900 Ma and one subordinate age peak at ca. 828 Ma. The youngest ages for the samples help to constrain the maximum ages of deposition of the complex at Early Carboniferous and Late Triassic, respectively. The existence of the diagnostic age peak at ca. 440 Ma and the euhedral-subhedral morphology of the detrital zircons in the Palaeozoic and Mesozoic strata may indicate that there were early Palaeozoic magmatic activities along the eastern margin of the Simao-Indochina block. At the meantime, the age peak at ca. 440 Ma was never reported in the early Palaeozoic strata from the western margin of the South China plate, which supports the existence of an early Palaeozoic rift zone, i.e., the Proto-Tethys Jinsha Jiang-Ailao Shan-Song Ma rift zone, being consistent with geochemical discriminations. Furthermore, we conclude, from the significant difference between detrital zircon age data of Silurian to Devonian sedimentary rocks from the Simao-Indochina block and South China plate, the opening of the Palaeo-Tethys ocean since Early Carboniferous, the absence of evidence for the Palaeozoic orogeny, and the successive sedimentation from the Cambrian to Triassic along the western margin of the South China plate, that the Proto-Tethys rifting and subsequent Palaeo-Tethys ocean evolution along the Jinsha Jiang-Ailao Shan-Song Ma zone formed actually a complete 'Wilson Cycle' from early Palaeozoic to the Late Triassic. [ABSTRACT FROM AUTHOR]

The Urals is one of the reference mobile belts of the mafic type characterized by a wide development of ophiolites which are associated with numerous deposits of chromites of significant industrial importance. In this regard, the estimation of the age of the rocks of the ophiolite association will be useful for analyzing the regularities of the formation of chromite deposits. This work presented the results of age dating of the rocks of the chromite-bearing Kluchevskoy mafic–ultramafic massif, one of the most representative of all the ophiolite-type massifs in the Urals, by two isotopic methods. The U–Pb (SHRIMP-II, VSEGEI) dating of the zircon dominated assemblage from rocks of different composition of both crustal and mantle sections of the Kluchevskoy ophiolite massif yielded similar dates ranging from 456 to 441 Ma. The study of the composition of silicate inclusions in zircon grains of this assemblage showed that they are represented by typical metamorphic minerals: low-T amphibole, albite, and an epidote-group mineral. The P–T conditions of zircon crystallization established via the examination of the composition of minerals in these inclusions showed that the crystallization of the predominant fraction of zircons coincides in time with the lower epidote–amphibolite and upper green-schist facies metamorphism of rocks happening under the decompression conditions, i.e., during their exhumation from the deep crustal level (8–13 km). A small amount of zircons of late generation showed a wide spread in age (277.4–318.1 Ma). The time of their crystallization corresponds to the stage of metamorphism associated with the collision orogeny in the Ural mobile belt. The more ancient 147Sm–143Nd age of 514 Ma should be assumed as the formation time of the rocks (or its upper age boundary). [ABSTRACT FROM AUTHOR]

(1) Background: The widely developed thrust structures in the Jingzhushan Formation of late Cretaceous in Nyima Basin are related to the collision which leads to orogeny between the Lhasa and Qiangtang blocks. Clarifying the tectonic properties of the Nyima Basin is of great significance to characterize the evolution of the collision between the Lhasa block and the Qiangtang block. (2) Methods: Combined with mineralogy and petrology and U-Pb zircon dating of extrusive rocks, a detailed magnetic fabric test and rock magnetic experiment were carried out on the red sandstones of the Late Cretaceous Jingzhushan Formation in Nyima Basin. (3) Results: The sedimentary magnetic fabrics developed at the three sampling locations in the southern part of the basin are related to paleocurrents. The strained magnetic fabrics developed at one sampling site in the north, including pencil-like fabrics and tensile linear fabrics, are related to SE-NW tectonic stress. The average magnetic susceptibility value of the strained fabric with sandstone is lower than that of the sedimentary fabric, and its size is controlled by mica paramagnetic minerals. Ferromagnetic minerals are magnetite or hematite. In addition, the age of sandstone in the Jingzhushan Formation from the Nyima Basin matches the age of the eruptive rock in the Jingzhushan Formation from Coqen Basin, which is 92 Ma. (4) Conclusions: The strained fabric is developed near the thrust fault, which can explain the widely developed thrust structures in the basin. The Nyima Basin of the Jingzhushan Formation in the Late Cretaceous was an ocean-continent transitional foreland basin developed under the background of arc-arc "soft collision". [ABSTRACT FROM AUTHOR]

Kimberlites are deep and relatively rare mantle‐derived igneous rocks, which provide key insights into the lithospheric mantle composition beneath continents (i.e., from the entrained mantle materials) at the time of magma emplacement. In addition, the temporal distribution of kimberlites throughout Earth's history may reveal secular changes in global geodynamics. However, alteration and abundant exotic material (i.e., xenoliths and xenocrysts) of different ages, both typical of kimberlites, often hinder reliable dating of these rocks. In this study, we investigated zircon xenocrysts and their replacement by desilicification reaction rims (DSRs), which consist mainly of baddeleyite, in the metamorphosed Kimozero kimberlites from the Karelian Craton, Russia. Similar DSRs have been identified previously on mantle‐derived zircons in numerous kimberlites, but the precise timing and location of formation of these rims remain unclear. U–Pb isotope dating of mantle zircon xenocrysts in the Kimozero kimberlites indicates that these rocks were emplaced at ca. 1.97 Ga. DSRs with similar textures and baddeleyite chemistry occur on both the 1.97 Ga mantle and 2.70–2.38 Ga crustal zircon xenocrysts in these kimberlites. Pb–Pb isotope dating of the DSRs yields the same age (i.e., ca. 1.97 Ga) as the mantle zircons. The data constrain the formation of DSRs to crustal levels and, more importantly, indicate that these rims record the timing of kimberlite emplacement. Therefore, we suggest that such baddeleyite reaction rims are a new and promising material for kimberlite geochronology. Plain Language Summary: Kimberlites are magmatic rocks produced by magma that ascended from mantle depths. It is important to obtain accurate kimberlite ages mainly because: (a) mantle rocks and minerals, carried by these magmas, allow assessment of the composition of the lithospheric mantle beneath continents at the time of kimberlite emplacement; and (b) the worldwide distribution of kimberlite ages may record secular changes in global geodynamics. However, kimberlites are readily altered and commonly contain exotic material of different ages and origins, which both hinder reliable kimberlite dating. Here, we investigated xenocrystic zircons and their reaction rims, which consist mainly of baddeleyite, in the strongly altered Kimozero kimberlites. Similar rims have been identified on mantle‐derived zircons in numerous kimberlites, but the timing and location of rim formation is poorly constrained. We found that the rims have replaced zircon xenocrysts of different ages and with both mantle and crustal origins, implying the rims formed at crustal levels. The results of Pb–Pb dating for these rims compared with those obtained from zircon U–Pb dating indicate that the rims record the timing of Kimozero kimberlite emplacement at ca. 1.97 Ga. Therefore, such baddeleyite rims are suggested as a promising material for obtaining reliable kimberlite ages. Key Points: Desilicification (i.e., baddeleyite) rims replace kimberlite‐hosted zircon xenocrysts of mantle and crustal originsThese rims represent a promising material for kimberlite geochronology, as they form at the time of kimberlite emplacementThe Kimozero kimberlites intruded the Karelian Craton at ca. 1.97 Ga [ABSTRACT FROM AUTHOR]

The Guide Basin, located in the West Qinling orogen of central China, is a typical geothermal field. However, the granodiorites occurring in the Zhacanggou area have been poorly restricted. Herein, we report new isotopic compositions of whole‐rock major‐trace elements, zircon U–Pb, and Lu–Hf, on the host granodiorites and their microgranular enclaves (MEs) in the Zhacanggou area. Zircon U–Pb dating for the host rocks (porphyritic granodiorite and granodiorite) revealed ages of 230.5 ± 1.4 Ma and 229.0 ± 1.6 Ma, similar to those (230.5 ± 1.5 Ma and 231.4 ± 1.4 Ma) of the MEs. Zircon Hf isotope analyses for the granodiorite and MEs yielded εHf(t) values from −11.0 to −1.5 and from −7.8 to −0.7, with corresponding TDM2 ages of 1,355–1,959 and 1,304–1,755, respectively. The MEs possess plagioclase phenocrysts, acicular apatites, and chilled borders. They were characterized by relatively low SiO2 (58.39–65.24 wt%) and high Cr, Ni, and MgO contents, leading to relatively high Mg# values (44–59) and low Sr/Y (14.23–19.84) ratios. The host granodioritic rocks show similar adakitic geochemical characteristics with SiO2 = 64.23–68.57 wt%, MgO = 1.29–2.22 wt%, Mg# = 48–53, Sr = 340–596 ppm, Y = 11.3–13.7 ppm, Sr/Y = 28.39–47.89, and no obvious Eu anomalies. These geochemical and isotopic data, combined with the mineralogical features, indicate that the granodiorites in the Zhacanggou area were produced by the mixing of high‐Mg# melts originating from an enriched lithospheric mantle source, and felsic magmas deriving by the partial melting of the normal lower continental crust with the tectonic regime transition from a syn‐collision to a post‐collision setting. [ABSTRACT FROM AUTHOR]

The eastern flank of the Talesh range of western Alborz, northwestern Iran, exposes volumetrically significant Late Cretaceous volcanic and volcaniclastic associations as well as cross-cutting dykes and minor subvolcanic equivalents. The volcanic units appear as massive to pillowed flows interlayered with volcaniclastic beds and subordinate limestones. We present new field data, zircon U Pb ages, bulk-rock major and trace element geochemistry, and Sr Nd isotopes on the igneous rock suite. Geochemical data show that the lava units and dykes are mainly basaltic in composition with minor basaltic andesites and trachytes. They form two contrasting series of arc-related calc-alkaline (Group 1) and OIB-like (Group 2) rocks. The zircon U Pb age of Group 1 rocks is 95.6 ± 1.8 Ma (2σ), whereas for Group 2 rocks, cross-cutting relationships and stratigraphic constraints indicate a somewhat younger age. Group 1 rocks have 87Sr/86Sr (i) varying from 0.7038 to 0.7070 and ε Nd (t) values ranging from +0.5 to +5.0. Group 2 rocks have 87Sr/86Sr (i) = 0.7040–0.7065 and ε Nd (t) from −3.0 to +3.7. Trace element and isotopic modelling shows that the primitive Group 1 magmas were derived from a metasomatized mantle source enriched by the addition of ∼6–10 % sediment melt component, while Group 2 rocks are consistent with melts of an asthenospheric mantle source enriched by ∼1–5 % EMII component. Trace element modelling indicates that Group 1 rocks formed from ∼6–13 % partial melts of a spinel-garnet lherzolite with garnet:spinel ratios of 45:55 to 10:90, whereas primitive Group 2 melts were generated through ∼1–3 % partial melting of a garnet-spinel-lherzolite with garnet:spinel ratios of 90:10 to 80:20. We propose a geodynamic model in which a north-dipping Neotethyan slab beneath the western Alborz (Talesh area) continental margin produced arc assemblages of Group 1 rocks and subsequent slab rollback led to the upwelling of an asthenospheric mantle to generate OIB-like magmatism of Group 2 rocks in an extensional intra-arc to back-arc setting. [Display omitted] • The Talesh magmatic rocks are dominated by basaltic lavas. • They were emplaced during the Late Cretaceous. • The magmatic rocks include both arc-type calc-alkaline and OIB-like alkaline rocks. • The mantle source of arc-type rocks was enriched by slab-derived sediment melts. • Slab rollback triggered asthenospheric melting and formation of OIB-like magmas. [ABSTRACT FROM AUTHOR]

The Tianshui-Sunjiaxia area is located in the connection zone of West Qinling Orogen and North Qilian Orogen, which could provide great insights into the amalgamation processes between the northern and southern blocks of China. Three subduction- and rift-related rocks gneissic granite from North Qilian arc-interarc belt (NQAI) granite and metabasalt from North Qinling back-arc basin (NQBA) are distinguished across the connection zone. The gneissic granite was generated by melts from older crustal materials of Longshan Group with the addition of a relatively juvenile basaltic source from the lower crust during the collision process. The Liwanxincun metabasalt reflects the mixing of the partial melting of the shallow asthenospheric mantle and the metasomatized mantle in a back-arc extension setting. The LA-ICP-MS zircon U-Pb dating of gneissic granite (068, 069) yields crystallization ages of 457.0 ± 1.6 Ma and 445.9 ± 2.1 Ma. The study area is divided into six tectonic units in Early Paleozoic time involving NQAI (Yanjiadian-Xinjie) continental arc, interarc rift basin (Maojiamo-Xiwali), continental arc (Chenjiahe-Wangjiacha); NQBA back-arc rift basin (Huluhe-Hongtubao), island arc and ophiolitic melange belt (North Qinling-Shangdan). A tectonic model is proposed in which the NQAI continental arc (Yanjiadian-Xinjie) might represent the early period of subduction of North Qilian Ocean (NQO) and the interarc rift is the product of the extension triggered by southward subduction of NQO. The ongoing subduction of NQO then leads to the formation of Chenjiahe-Wangjiacha continental arc, as well as the Hongtubao back-arc spreading ridge in NQBA back-arc basin (Huluhe). The tectonic evolution of the connection zone is closely associated with the closure of the North Qilian Ocean and North Qinling-Shangdan Ocean in the context of the convergence process of micro-continental blocks, including North China block, Longshan group and North Qinling Terrane. [ABSTRACT FROM AUTHOR]

High-grade epithermal Au–Ag veins of the Vodorazdelnaya district in far eastern Russia are hosted by Early Cretaceous volcanic rocks of the Tytylveem belt. The largest deposit is Zone 37, a 1-km long, northeast-striking vein that is up to 35-m wide, containing at least 32 t Au. The main shoot consists of crustiform—colloform banded quartz-chalcedony-adularia-sulfide veins, of low-sulfidation style. The vein is cut by late-mineral rhyolite sills and dikes. Zone 37 vein is dated at 117 ± 2 Ma, overlapping within error with the cross-cutting rhyolites (121–119 Ma) and with intrusions of the nearby Ilirney granitic pluton (119–117 Ma). The volcanic host rocks are dated at 121–115 Ma. At September Northeast, 15 km west-northwest of Zone 37, mineralisation consists of veins up to 4 m wide, with crustiform banded quartz ± chalcedony ± adularia ± chlorite, and Zn-Pb-Cu sulfides, of intermediate sulfidation style, in a 1-km-long, north-northeast striking trend. High grades are associated with gold dendrites in fine-grained quartz bands and with minor late-stage tellurides. The veins were disrupted by intrusion of rhyolite dikes and associated phreatic breccias. Some breccias host mineralisation as vein clasts and minor matrix sulfides. The Tytylveem belt is preserved below a regional unconformity that forms the base to the overlying Late Cretaceous (106–77 Ma) Okhotsk-Chukotka Volcanic Belt (OCVB). Intrusive rocks attributed to OCVB magmatism were emplaced at about 96 Ma and are associated with overprinting hydrothermal alteration, sub-economic veins and thermal resetting of some vein adularia ages at Zone 37, to 91–97 Ma. [ABSTRACT FROM AUTHOR]

The Bengge biotite aegirine‐augite syenite (BAS) is located in the southern part of Yidun Arc, eastern margin of the Tibetan Plateau area. It is an integral part of the Bengge ultrapotassic rocks (UPR), an alkalic complex mainly consisting of BAS, biotite pyroxene syenite (BPS), and biotite syenite (BS). Geochronological studies including zircon LA‐ICPMS U–Pb dating and biotite 40Ar/39Ar dating have been conducted on the Bengge BAS to constrain its emplacement age. The zircon U–Pb dating yields a wide range of ages from 209 ± 2 Ma to 239 ± 2 Ma, with the concordia 206Pb/238U age being 228 ± 8.9 Ma (MSWD = 0.13). Meanwhile, the biotite 40Ar/39Ar dating yields a plateau age of 205.7 ± 1.5 Ma. The whole‐rock Zr‐saturation temperature (Tzir‐sat) of the Bengge BAS has shown to be very low (86 ± 13°C to 634 ± 13°C), which is much lower than Ti‐in‐zircon temperature (>758°C) of the zircons used for U–Pb dating. This indicates that the zircons in the Bengge BAS are inherited, and they were already present in the BAS melts before the melts became Zr‐saturated. Similarly, the Tzir‐sat of the Bengge BPS and BS are 346 ± 14°C ~ 549 ± 13°C and 281 ± 14°C ~ 525 ± 13°C, respectively, which is also too low for magmatic zircon crystallization in the melts. The inherited zircons of Bengge BAS have typical features of magmatic zircon, with high (Sm/La)N ratios (2.54–819), Ce/Ce* ratios (4.77–304) and low La concentration. Low εHf (T) values (−5.0 to 0.42) and high U/Yb (0.67–12.6) values imply that the inherited zircons (209–239 Ma) in the Benge BAS are derived from an early arc‐magma system of the Yidun Arc. The biotite 40Ar/39Ar dating constrains the Bengge BAS emplacement age at 205.7 ± 1.5 Ma, the time when the arc‐magma activity had ceased, and a regional geodynamic transition in the Yidun Arc from convergence to back‐arc spreading had started. [ABSTRACT FROM AUTHOR]

The northern Alxa orogenic belt, located in the middle segment of the Central Asian Orogenic Belt, is a junction between the North China Craton, Tarim Craton and Mongolian microcontinents. However, its Permo-Carboniferous tectono-palaeogeographic evolution has not been well established. In this study, new zircon U–Pb–Hf isotopic data for the Permo-Carboniferous clastic rocks were used to address the above issue. Based on our dating work and fossil assemblages, we confirmed that the Amushan, Maihanhada, Aqide and Haersuhai formations were formed in the late Carboniferous to early Permian, early to middle Permian, middle Permian and late Permian periods, respectively. The Amushan Formation sandstone in the Zhusileng area contains abundant Palaeozoic zircons (with age peaks at 440 Ma, 475 Ma and 539 Ma) and some Precambrian zircons. These zircon ages and the southeast transgressive direction suggest the provenance to be the local Zhusileng–Hangwula block and the nearby Yagan continental margin. The zircon age pattern with a high proportion of detrital zircons much older than the depositional time and no detrital zircons close to the depositional time suggest an extensional basin depositional setting. The clastic rocks of the Maihanhada, Aqide and Haersuhai formations inherited the source for the Amushan Formation, with a greatly increased input of Permo-Carboniferous volcanic rocks with post-collision or intraplate chemical affinity. Accordingly, a Permo-Carboniferous extensional stage was suggested. Finally, a tectono-palaeogeographic model was reconstructed for the northern Alxa orogenic belt, evolving from a late Carboniferous transgression with crustal extension to early to middle Permian rapid basin filling and a late Permian marine regression. [ABSTRACT FROM AUTHOR]

We present results of geochemical and Sr-Nd isotope studies of rocks and of local dating and determination of the Lu-Hf isotope composition of zircons from late Vendian-early Cambrian and Cambrian-Ordovician intrusive associations (granitoids and gabbroids) of the Kaa-Khem and East Tannu-Ola batholiths in Eastern Tuva. The wide ranges of the eNd values (6.9 to 0.5) of rocks and the eHf values of magmatic and inherited zircons reflect the diversity of the magma sources of late Vendian-early Paleozoic intrusive associations formed at the island arc and accretion-collision stages. Late Vendian (572-562 Ma, Kopto and Buren massifs) and early Cambrian (522- 518 Ma, East Tannu-Ola batholith) island arc tholeiitic and calc-alkalic plagiogranitoids resulted from the melting of the Vendian-early Cambrian island arc crust without the contribution of a more ancient crustal material. The subalkalic gabbro-monzodiorite-granosyenite association of the Zubovka massif (510 Ma) formed from a mantle source depleted isotopically but enriched in incompatible elements, with the participation of an island arc crust material; this process took place in the early phase of plume activity at the accretion-collision stage. Island arc complexes were the main source of Cambrian-Ordovician accretion-collision calc-alkalic plagiogranitoids (500-450 Ma, Terektyg-Cheder, Karaos, Tapsa, Baisyut, and other massifs). Variations in their composition were due to the melting of thick crust, whose isotopic heterogeneity was caused by the different contributions of a more ancient crustal source. The crust of the Tuva-Mongolian terrane made the main contribution to the formation of the potassic granitoids of the Bren' massif (450 Ma), marking the completion of accretion- collision processes in this region. The isotope parameters of the Vendian-early Paleozoic granitoids are indicators of the crust formation and evolution in the course of subduction and accretion-collision processes. [ABSTRACT FROM AUTHOR]

The Lengshuikeng Ag‐Pb‐Zn ore field is located in the North Wuyi Mesozoic volcanic belt south of the Qinzhou–Hangzhou suture zone between the Yangtze and Cathaysia paleo‐plates. Previous zircon U–Pb geochronological studies on ignimbrites and tuffs from this area have yielded conflicting ages of 157–161 Ma (Early Upper Jurassic) and 137–144 Ma (Early Lower Cretaceous). Volcanic rocks in the ore field have even been proposed to include both ages. Our SHRIMP zircon U–Pb dating of the ignimbrite and tuff samples from the ore field, along with field observations and results from geochronological work on other volcanic and sub‐volcanic rocks in the region, shows that two populations of magmatic zircons, one autocrystic and the other xenocrystic, are present in the pyroclastic rocks. The autocrystic zircons have ages suggesting formation/eruption at approximately 140 Ma, whereas the xenocrystic zircons give ages of 155–159 Ma, indicating intrusion of granitic porphyries in the Early Upper Jurassic. Therefore, the pyroclastic rocks in the Lengshuikeng Ag–Pb–Zn ore field formed in the Early Lower Cretaceous. The youngest zircon U–Pb ages from pyroclastic rocks may not represent the formation/eruption ages of the host rock, depending most likely on the existence and/or abundance of juvenile or vitric pyroclasts in the rocks. [ABSTRACT FROM AUTHOR]

The provenance of Triassic Vieux Emosson Formation, autochthonous sediments of the Aiguilles Rouges massif (External Alps), was determined from U–Pb ages of detrital zircons. In addition, two samples of Late Carboniferous sandstones from the Salvan‐Dorénaz basin were included to extend the database of potential source rocks. Overall, age data from the four samples are largely comparable, each with a wide range of ages, indicating similar source rocks. The compositionally and texturally immature strata of the Vieux Emosson Formation suggest a local sediment source. However, not all detrital zircons can be accounted for by the ages of the local polymetamorphic pre-Mesozoic basement, which consists mainly of paragneiss, Ordovician orthogneiss and Late Carboniferous, Variscan-related, magmatic and sedimentary rocks. Ordovician zircons were common in the samples, but Late Carboniferous zircons were only abundant in one sample. Early Cambrian and older zircons were likely recycled from the paragneiss. Recycled zircons primarily represent Cadomian orogenic events (~ 550–650 Ma); older zircons are from eastern Gondwana and West African craton sources. Abundant late Cambrian (~ 500 Ma) and Silurian (~ 425 Ma) zircons likely represent Cadomian rifting and the magmatic events that produced the Ordovician orthogneiss, respectively. Permian zircons were only found in one Triassic sample, and are possibly related to magmatic activity associated with post-Variscan extension. The age data of the detrital zircons in the Triassic Vieux Emosson Formation and in the sandstones from the Late Carboniferous Salvan-Dorénaz basin indirectly dates the source rocks and for the first time confirms Cadomian basement in the Aiguilles Rouges massif. Source rocks for the Silurian, Permian, and to a lesser degree, late Cambrian zircons are not documented in the local basement, and either have been eroded away or are now located to the southeast beneath Penninic nappes. [ABSTRACT FROM AUTHOR]

SIMS U‐Pb zircon dating of metaigneous rocks of the Nordbreen Nappe from the West Ny‐Friesland terrane (Eastern Basement Province of Svalbard) yields crystallization ages of 1,761 ± 4 Ma for a felsic metatuff and 1,373 ± 4 Ma for a metagabbro dyke. The Palaeoproterozoic age of the metatuff is similar to previously obtained ages for various felsic rocks from the study area, whereas the Mesoproterozoic age of the metagabbro has not been thus far documented on Svalbard. However, a similar age pattern has been reported from Northeast Greenland. Therefore, we conclude that the West Ny‐Friesland terrane can be correlated with age equivalent units of Northeast Greenland. We also explore similarities and dissimilarities between the study area and other terranes of Svalbard and speculate on regional‐scale correlations. Together, the presented new ages provide critical piercing points in palaeogeographic reconstructions of the relatively poorly understood circum‐Arctic region. [ABSTRACT FROM AUTHOR]

The zircon U-Pb system is one of the most robust geochronometers, but during an impact event individual crystals can be affected differently by the passage of the shock wave and impact generated heat. Unraveling the potentially complex thermal history recorded by zircon crystals that experienced variable levels of shock and heating, as well as additioanl pre- and post-impact thermal events, has been difficult using classical geochronological methods. The existing high-precision 40Ar/39Ar age constraints for the K-Pg Chicxulub event, and the previous U-Pb dating of the basement rocks from the impact site, make Chicxulub an ideal location to study impact-induced effects on the zircon U-Pb systematics and to evaluate potential 'memory effects' of pre-impact U-Pb signatures preserved within those individual zircon crystals. Recent IODP-ICDP drilling of the Chicxulub impact structure recovered 580 m of uplifted shocked granitoid and 130 m of melt and suevite, providing an unprecedented opportunity to study zircon crystals subjected to a range of shock pressures, thermal, and deformational histories. Zircon morphologies were classified using scanning electron microscopy (SEM) imaging and then samples were depth profiled using laser ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS) to document the range of preserved age domains from rim-to-center within individual crystals. The results show U-Pb ages range from 66 to 472 Ma, which are consistent with both inherited Carboniferous and Late Paleozoic basement ages as well as Pb loss ages in response to the K-Pg impact event. While the bulk of the zircon grains preserve Paleozoic ages, high U (metamict) zones within fractured zircon crystals exhibited an age within uncertainty (66 ± 6.2 Ma) of the impact age (66.038 ± 0.049 Ma), indicating that inherited intragrain U-Pb kinetics and/or hydrothermal fluid flow may have controlled age resetting those zircon crystals rather than impact-induced shock and heating alone. Moreover, the calculated α-decay doses suggest that the zircon crystals experienced Stage 1 or early Stage 2 radiation damage accumulation. Therefore, we suggest that the lowered crystal annealing temperature in crystals that previoulsy experienced radiation damage make the zircon U-Pb clock either more susceptible to the relatively short heat pulse of the impact event, the moderate pressure and temperature conditions in the peak ring, and/or to hot-fluid flow in the long-lasting post impact hydrothermal system. • Shocked zircon crystals extracted from Chicxulub's peak ring rocks are analyzed via U-Pb LA-ICP-MS depth profiling. • U-Pb LA-ICP-MS depth profiling and the subsequent data export in 1 s increments allows to spatially resolve age trends. • Zircon U-Pb clock partially and possibly fully reset due to the impact event. • Younger U-Pb ages are preserved in fractured zircon with a high U [ppm] concent. or in zircon displaying granular textures. [ABSTRACT FROM AUTHOR]

Detrital zircons in modern stream sands and terraces of the Snake River drainage upstream of and within Hells Canyon, western Idaho, U.S.A, effectively reproduce the geochronometry of Idaho magmatic rocks, both from Laurentia in eastern Idaho, and from the accreted terranes west of the Salmon River Suture. We used large-n (300 grain) analysis of detrital zircon ages and Monte Carlo-based mixture modeling to estimate the percent contributions from the upper Snake River and several tributaries. The large number of grains analyzed allows more precise statistical modeling than previous studies, which used 60 or 100 grains. Immediately upstream of the Hells Canyon Complex of dams modeling suggests that the Snake River at Farewell Bend contains detrital zircon contributions of Payette River 50 % and Boise River 10 % (mainly from the Atlanta lobe, Idaho batholith). The upper Snake River has 30 % contribution and contains a complex detrital zircon signature including both primary zircons derived from magmatic sources and recycled zircons derived from sedimentary rocks with an ultimate magmatic source. Thus, 60 % of the detrital zircons come from the 98–73 Ma southern Idaho batholith, with less than a third coming from the upper Snake River drainage, despite it representing 60 % of the drainage area. The Owyhee, Malheur, and Weiser rivers contribute <5 % each. Within Hells Canyon, modeling suggests that the main Snake River contributes by far the largest component (85 %); side canyons contribute <5 % of the zircon load. Mixing models for Snake River stream sediment downstream of Lewiston, below Lower Granite Dam (sampled from sands deposited before installation of the dam), suggest 53 % contribution from the main Snake River above the Salmon River, 34 % from the Clearwater River, 7 % from the Salmon River, and 6 % from the Grande Ronde River. Comparing relative zircon inputs from the modern Snake and Salmon rivers to a pre-dam river terrace just downstream from their confluence suggests that the Snake River in Hells Canyon today contributes only 4 % of the detrital zircons whereas before installation of the Hells Canyon Dams it contributed 29 %. We interpret this reduction in detrital zircon supply to be caused by sediment trapping above the dams. One influence on the detrital zircon contribution of various streams is peak flow volume. Zircon fertility of specific source rocks is the main influence on detrital zircon percentages within individual drainages. Mesoproterozoic, Cretaceous and Eocene granitic rocks produce euhedral magmatic detrital zircon grains that dominate several drainages. Mesozoic metavolcanic and Eocene Challis Volcanic Group rocks yield fewer and often broken detrital zircon grains. Large outcrop areas of Neogene Columbia River and Snake River Plain basalts provide almost none. Recycling of older magmatic detrital zircon through Proterozoic and Paleozoic sandstones is a signature of the eastern and central Idaho thrust belt. [ABSTRACT FROM AUTHOR]

The Early Caledonian folded region of the southern framing of the Siberian platform, between the Zavkhan and Tuva-Mongolian terranes, contains blocks of the Songino ledge crystalline rocks. In the Bayan Nuur block (southern part of the Songino ledge), the Neoproterozoic (890–780 Ma) Bayan Nuur gneiss-migmatitic and Kholbo Nuur metaterrigenous-volcanogenic complexes are distinguished. The zone of their junction is traced by thrust structures, which are reflected in all structural-material complexes of the Bayan Nuur block. There are tectonic slabs in the junction zone. Their peculiarity is the presence of rocks formed before the formation of thrust gabbro-ultrabasite complex and post-thrust gabbroids and gabbro-diorites. The upper limit of the thrust formation interval is defined by Bayan Nuur pluton granitoids having the age of 790 ± 3 Ma and by gabbroids and anorthosites of Onzula tectonic plate giving the ages of 785 ± 3 and 784 ± 3 Ma. The lower limit of the thrust formation interval is determined by the age of the Bayan Nuur ultrametamorphic granitoid complex (802 ± 6 Ma). Bodies of massive (post-thrust) pegmatoid gabbros and rocks of gabbro-ultrabasite complex possessing clearly manifested thrust-associated structures are recognized in the tectonic slab of the ridge reaching the area of Mount Sant Uul. These rocks are dated at 782 ± 2 and 806 ± 10 Ma, respectively (ID-TIMS). The data indicate that the post-thrust and pre-thrust units cannot be parts of a united layered complex. In turn, pre-thrust units can be considered as paleooceanic fragments in the accretionary structure of the Bayan Nuur block, Songino ledge. [ABSTRACT FROM AUTHOR]

U-Pb geochronology requires knowledge of a precise and accurate value for the 238 U/ 235 U ratio of any mineral or material being dated by this method. It is now well established that the long-accepted value of 137.88 is not representative of most terrestrial materials and that the ratio is variant both between and within samples. As most samples used for chronology have insufficient amounts of U to determine a precise isotopic composition, the U-Pb geochronology community has informally adopted the recommended value of 137.818 ± 0.045 based on a single data set of 44 zircons by Hiess et al. (2012). To evaluate the robustness of this value and the true range of U isotopic compositions in U-bearing minerals, we have measured the U isotopic composition of 28 bulk zircon samples as well as several co-existing accessory minerals from rocks with a wide range of ages and geographical distributions. The 238 U/ 235 U ratio of 137.817 ± 0.031 represents the mid-point value of our data with an uncertainty that includes all 28 zircon populations analyzed and their respective uncertainties. This value is indistinguishable from the previously recommended value but with a smaller uncertainty that may reflect the refined analytical methods employed in our study. This result provides confidence to the U-Pb geochronology community that the value recommended by Hiess et al. (2012) is appropriate and that their stated uncertainty appears to be conservative. We find variability in the 238 U/ 235 U ratio of coexisting phases within single samples of up to 2.9 ε-units (parts per 10,000). In addition, our range of U isotopic compositions for zircon is heavier than that reported for bulk continental crust in general and most granitoid intrusive rocks specifically. We attribute these observations to differences in the coordination environment of U in zircon and other U-bearing phases. With the amount of zircon formed in magmatic systems limited by the abundance of Zr, we estimate that the majority of U in granitoid rocks must reside in other phases with a bulk isotopic composition that represents a lighter complement to that of zircon. [ABSTRACT FROM AUTHOR]

Zircon has been widely used as a geochronometer with the U–Pb decay system but rarely with the Th–Pb system. However, zircon from carbonatite contains very little U and has a high Th/U ratio, making Th–Pb dating preferable. As a one-dimensional system, a series of consistent Th–Pb ages can be used to date a geological event. In contrast, a wide variation in Th–Pb ages could result from Pb loss or multiple growth events, making it difficult to link to specific geological events. In this study, we described Th–Pb dating protocols on zircon and carried out Th–Pb measurements on seven zircon reference materials using a Cameca IMS 1280HR ion microprobe. The results demonstrated that these seven U–Pb zircon standards have similar absolute concentrations of common lead. The radiogenic 208Pb concentrations (depending on the Th content and age) determine the proportion of common lead and define the extent of variation in the Th–Pb system under certain analytical conditions. This relationship could be used as a criterion to evaluate whether it is a single population or not based on Th–Pb dating results of unknown zircons. By comparison, M257 and Qinghu zircons are suggested as the most suitable Th–Pb dating standards, with ID-TIMS U–Pb ages representing the best estimate of the Th–Pb reference ages. A zircon sample from the Wu dyke, an extremely rare earth element (REE)-rich carbonatite dyke in the Bayan Obo area, was dated with the established Th–Pb dating procedure and yielded an age of 1327 ± 20 Ma as the emplacement time. A pronounced relationship between apparent Th–Pb ages and corresponding Th contents (＞1500 ppm) indicates that zircons with accumulated radiogenic doses of ＞1.7 × 1018α per g may tend to lose radiogenic 208Pb. [ABSTRACT FROM AUTHOR]

The southern Midyan terrane is a composite Tonian to Ediacaran tectonostratigraphic crustal block in the northern Arabian Shield that prior to Red Sea opening was contiguous with coeval rocks in the Eastern Desert of Egypt and Sinai. Ion microprobe (sensitive high-resolution ion microprobe [SHRIMP]) dating of 12 rock samples described here and the results of other dating programmes establish a clear timeframe for depositional, intrusive, and structural events in the region and provide a chronology of tectonism in this part of the Arabian-Nubian Shield. Deposition of Zaam and Bayda group volcanosedimentary rocks and emplacement of mafic-ultramafic complexes and TTG-type diorite, tonalite, and granodiorite denote formation of the Tonian (780-715 Ma) Zaam arc and fore-arc ophiolite above a possible west-dipping subduction system in the southern part of the Midyan terrane. Convergence with the Hijaz terrane farther south and obduction of ophiolite nappes resulted by ~700 Ma in development of the Yanbu suture. Ongoing or a new subduction system led to a ~705-660 Ma Cryogenian pulse of magmatism represented by I-type calc-alkaline diorite, granodiorite, and granite that have volcanic-arc and syn-collisional granite affinities. This was followed, after a brief end-Cryogenian hiatus, by a 635-~570 Ma period of Ediacaran magmatism marked by monzogranite, syenogranite, and minor gabbro and diorite. These rocks are reported to have within-plate to volcanic-arc and syncollision chemical characteristics but their precise tectonic setting is uncertain. Structurally, the intrusions are diapiric and were evidently emplaced in an extensional regime consistent with an overlap between intrusion and Najd faulting associated, at this time, with transpressional collision and northward extension through much of the ANS. Terminal magmatism in the southern Midyan terrane postdated cessation of Najd faulting at ~575 Ma and resulted in the emplacement of undeformed within-plate A-type alkali-feldspar granites and mafic (lamprophyre) and felsic dikes. [ABSTRACT FROM AUTHOR]

The results of geochronological (U-Pb), isotope-geochemical (Lu-Hf), and geochemical (REEs) studies of young (MZ, KZ) and xenogenic (AR, PR) zircons from magmatic rocks of the Central Arctic rises of the Arctic Ocean (AO) and the crest zone of the Mid-Atlantic Ridge (MAR) are presented. The data obtained show that the depleted mantle could be a source of young (KZ) zircons of the MAR, whereas young (MZ) zircons of the MAR and all xenogenic (AR, PR) zircons of the AO and MAR are from crustal rocks of the continental lithosphere. [ABSTRACT FROM AUTHOR]

Abstract: The Chinese Western Tianshan terrane is a well‐known ultrahigh‐pressure (UHP) metamorphic belt that mainly consists of subducted oceanic crustal materials. While two uplift stages have been recognized, the early exhumation history has not been fully constrained. We conducted U–Th–Pb and trace element microanalysis of titanite as well as zircon from UHP eclogites. Apart from several inclusions in garnet, three types of retrograde titanite have been identified based on their petrographic occurrences: (a) in matrix enclosing relict rutile; (b) as coarse‐grained coronas around garnet; and (c) in veins coexisting with epidote and/or apatite. The three types of titanite are homogeneous in chemical composition, low in Al, enriched in middle rare earth elements (REEs), and depleted in light REEs and heavy REEs, suggesting their growth during retrograde decompression. Sensitive high‐resolution ion microprobe (SHRIMP) and LA‐ICP‐MS U–Pb dating of titanite from four samples, regardless of their textural occurrences, gave consistent ages of c. 306 Ma. Phase equilibria modelling and Zr‐in‐titanite thermometry indicate that titanite replaced rutile at ~14 kbar and ~570°C, following a nearly isothermal decompression from the UHP peak conditions of 30 kbar and 550–570°C. SHRIMP zircon U–Pb dating yields two age groups: c. 320 Ma and c. 305 Ma. The former is generally interpreted to be the age of UHP stage. The latter, overlapping the titanite ages, represents the timing of titanite growth during exhumation. Thus, we estimate that the near‐isothermal exhumation from the UHP peak conditions to the epidote–amphibolite facies retrogression (14 kbar) lasted c. 15 Ma, corresponding to an average exhumation rate of 3.4 ± 2.5 mm/yr (2σ). This exhumation rate lies within the range of oceanic LT‐HP belts in the absence of continental subduction, suggesting a similar slab exhumation mechanism that operates in the UHP regime. The consistent retrograde metamorphic ages recorded by titanite from different localities, and the general agreement among P–T paths in the adjacent area, suggest that the UHP unit of Western Tianshan metamorphic belt was likely exhumed as a coherent unit. [ABSTRACT FROM AUTHOR]

The Yukon-Koyukuk Basin is a large depression that covers ~118,000 km2 in western interior Alaska and is divided into two subbasins by a volcanic arc assemblage. Interpretations of the depositional setting of the northern Kobuk Koyukuk subbasin vary from a syncollisional forearc basin to a postorogenic successor basin formed by lithospheric extension. New results from sandstones and conglomerates collected from the Kobuk Koyukuk subbasin provide evidence for the timing of basin development, insight into the provenance of coarse siliciclastic sediments, and understanding of the nature of Cretaceous paleogeography and paleodrainage of Arctic Alaska. Early sedimentary rocks of the Kobuk Koyukuk subbasin contain abundant mafic to ultramafic volcanic and plutonic lithic fragments and mafic heavy minerals (e.g., spinel, clinopyroxene, and amphibole). They also contain abundant Middle Triassic to early Late Jurassic zircons (240-160 Ma; peak maximum ca. 200 Ma) that yield highly juvenile Hf isotopic compositions. Geochemistry of chromium spinels (Cr# = 0.17-0.86) suggests crystallization in an immature arc setting that likely developed over mid-ocean-ridge basalt-type crust. These early sediments originated from the mafic and ultramafic rocks of the Angayucham terrane, which was once much more extensive. These results suggest that the Angayucham terrane consists of an obducted Middle Triassic to early Late Jurassic oceanic arc complex that was coeval with oceanic- to continental-margin mafic arc magmatism in the Canadian Cordillera. Our generalized stratigraphy, along with U-Pb ages and Lu-Hf isotope analyses of zircons from sedimentary rocks of the Kobuk Koyukuk subbasin, reflects the tectonic and/or erosional unroofing of the adjacent southern Brooks Range and Ruby terrane. U-Pb ages of detrital zircons collected from the stratigraphically lowest mafic- to ultramaficrich strata yield maximum depositional ages (107 Ma) that reflect initial erosion of the structurally highest Angayucham terrane and initiation of basin formation and deposition in the late Early Cretaceous. Continued uplift and erosion exposed structurally deeper metamorphic rocks, as revealed by incorporation of low-grade phyllites and eventually higher-grade metamorphic schistose lithic detritus and intermediate-composition (e.g., biotite) to metamorphic (e.g., chloritoid and xenotime) heavy mineral suites into the basin sediments. Differences in detrital zircon signatures between similar-age strata in the Colville foreland basin to the north of the Brooks Range and the Kobuk Koyukuk subbasin indicate that the sediments within the two basins were derived from two different sources, and the Brooks Range orogen acted as a drainage divide during late Early Cretaceous deposition. [ABSTRACT FROM AUTHOR]

We present our new investigation into the depositional environment and provenance of the Yingshuiquan Formation in the central Qiangtang region of northern Tibet, in order to further our understanding of the environment of the Longmu Co-Shuanghu Palaeo-Tethys during the Early Triassic. The Yingshuiquan Formation is composed of oolitic limestone, calcareous sandstone, calcarenite, thin-bedded ribbon limestone, bioclastic limestone, and coarse oolite limestone that were deposited in a shallow-marine basin and contain abundant Lower Triassic conodont fossils (e.g. Hadrodontina anceps, Pachycladina sp., gen. et sp, Pachyclaina oblique, Hibbardelloides sp). We selected detrital zircons from four calcareous sandstone samples for U-Pb dating, yielding minimum age peaks of 263, 269, 275, and 280 Ma, respectively, and a minimum age of 249 Ma, based on several zircons around the same age. Analysis of the conodont biofacies and zircon LA-ICP-MS dating of calcareous sandstone indicates that the data is consistent with deposition in the Early Triassic. The Yingshuiquan Formation records Early Triassic shallow-water sediment in the Longmu Co-Shuanghu Palaeo-Tethys, and has a Southern Qiangtang and Northern Qiangtang terranes provenance. During the Early Triassic, the carbonate sediments of the Yingshuiquan Formation were deposited in an active environment around the Longmu Co-Shuanghu Palaeo-Tethys, which has became a residual sea basin. [ABSTRACT FROM AUTHOR]

Continent–continent collision between the North China Block (NCB) and South China Block (SCB) took place along the Qinling–Tongbai–Hong'an–Dabie orogens during the Triassic. A micro-continent with Paleozoic arc magmatism has been recognized in the northern Qinling–Tongbai orogens; however, it remains unclear whether the micro-continent extended to the Dabie Orogen to form a ribbon-shaped micro-continent, due to later burial by the Hefei Basin in the north. To solve this problem, we conducted LA–ICP–MS U Pb dating of zircons from Silurian to Cretaceous sandstones and volcanic rocks from the southern margin of the basin. The age spectra of detrital zircons suggest that the Dabie Orogen and later basin cover were the sources of the analyzed sandstones. The detrital and inherited zircons indicate Neoproterozoic, early and late Paleozoic magmatism in the Beihuaiyang unit in the north of the Dabie Orogen. The zircon and previous geophysical data show that a micro-continent bounded by the Feizhong Suture in the north and the Xiaotian–Mozitang Suture in the south existed between the NCB and the Triassic Dabie Orogen, and its northern half is buried by the Jurassic–Paleogene Hefei Basin. The Beihuaiyang micro-continent experienced early Paleozoic arc magmatism caused by southward subduction of the Erlangping oceanic crust and late Paleozoic magmatism related to northward subduction of the Paleotethyan oceanic crust. The micro-continent was accreted to the southern edge of the NCB at the end of the Early Devonian (ca. 400 Ma) via arc–continent collision. Similarly to the Qinling–Tongbai orogens, the Dabie Orogen contains a Paleozoic accretionary system in the north and a Triassic collisional system in the south; thus, it is suggested that a ribbon-shaped micro-continent, > 900 km long and 50–100 km wide, was present along the entire Qinling–Tongbai–Hong'an–Dabie orogens prior to the middle Paleozoic. This micro-continent might have originated as a result of middle Neoproterozoic rifting along the margin of the SCB. [ABSTRACT FROM AUTHOR]

The Tiegelongnan Cu (Au) deposit is the largest copper deposit newly discovered in the Bangong-Nujiang metallogenic belt. The deposit has a clear alteration zoning consisting of, from core to margin, potassic to propylitic, superimposed by phyllic and advanced argillic alteration. The shallow part of the deposit consists of a high sulphidation-state overprint, mainly comprising disseminated pyrite and Cu-S minerals such as bornite, covellite, digenite, and enargite. At depth porphyry-type mineralization mainly comprises disseminated chalcopyrite, bornite, pyrite, and a minor vein molybdenite. Mineralization is disseminated and associated with veins contained within the porphyry intrusions and their surrounding rocks. The zircon U-Pb ages of the mineralized diorite porphyry and granodiorite porphyry are 123.1 ± 1.7 Ma (2σ) and 121.5 ± 1.5 Ma (2σ), respectively. The molybdenite Re-Os age is 121.2 ± 1.2 Ma, suggesting that mineralization was closely associated with magmatism. Andesite lava (zircon U-Pb age of 111.7 ± 1.6 Ma, 2σ) overlies the ore-bodies and is the product of post-mineralization volcanic activity that played a critical role in preserving the ore-bodies. Values of −4.6 ‰ to + 0.8 ‰ δ34S for the metal sulfides (mean − 1.55 ‰) suggest that S mainly has a deep magmatic source. The H and O isotopic composition is (δD = −87 ‰ to −64 ‰; δ18OH2O = 5.5 ‰ to 9.0 ‰), indicating that the ore-forming fluids are mostly magmatic-hydrothermal, possibly mixed with a small amount of meteoric water. The zircon εHf(t) of the diorite porphyry is 3.7 to 8.3, and the granodiorite porphyry is 1.8 to 7.5. Molybdenite has a high Re from 382.2 × 10−6 to 1600 × 10−6. Re and Hf isotope composition show that Tiegelongnan has some mantle source, maybe the juvenile lower crust from crust-mantle mixed source. Metallogenesis of the Tiegelongnan giant porphyry system was associated with intermediate to acidic magma in the Early Cretaceous (~120 Ma). The magma provenance of the Tiegelongnan deposit has some mantle-derived composition, possibly mixed with the crust-derived materials. [ABSTRACT FROM AUTHOR]

Using techniques of microprobe analysis and Raman microspectroscopy, a study of the composition, structure, and age of high-uranium zircon from the Sharatash intrusion pegmatite was performed. The presence of a number of zones in crystals having different physicochemical characteristics has been revealed. The Id-e zones affected by a radiation dose of (30-130) × 10 α-dec/g were completely amorphized, and their chemical isochron age was estimated at 311.4 ± 7.1 Ma: this is interpreted as the time of zircon formation in the pegmatite. The age of uraninite coupled with zircon is 317.1 ± 2.3 Ma. These dating results clarify the formation time of the final phase (pegmatite) of the Shartash intrusion and determine the upper age limit of hydrothermal gold mineralization (Berezovsk deposit) associated with the granite massif. [ABSTRACT FROM AUTHOR]

The Nan-Uttaradit suture is marked by a narrow N-S trending and discontinuous ophiolite belt in northern Thailand. This suture zone is a mélange composed of gabbro, tholeiitic metabasalt, andesite and radiolarian chert. Samples of gabbro and meta-basalt in the Nan-Uttaradit suture yield zircon U-Pb ages of 311±10 and 316±3 Ma, respectively, interpreted as the crystallization ages of the rocks, suggesting the Nan-Uttaradit Ocean existed in the Late Carboniferous. Our results indicate that the Nan-Uttaradit Ocean co-existed with the Ailaoshan-Jinshajiang Ocean to the north and was probably an along-strike extension of the latter. [ABSTRACT FROM AUTHOR]

This paper reports geochronological data of detrital zircons from the country rock and sedimentary xenoliths of the Cilincuo pluton (79±0.7 Ma) in the southern Yidun arc belt and the inherited zircons from the Late Triassic granites in the eastern Yidun arc belt, eastern Tibet Plateau. Detrital zircons ages from the sedimentary xenoliths have four prominent peaks at 2.5-2.4 Ga, 1.9-1.8 Ga, 480-400 Ma, and 350-300 Ma, whereas those from the country rock exhibit another four prominent peaks at 1.9-1.8 Ga, 850-700 Ma, 480-400 Ma, and 300-250 Ma. Based on comparison with age data from previous studies, we suggest that the sedimentary xenoliths are from the Lanashan Formation and the major provenance of them is Qiangtang Block, Zhongza massif and South China Block, whereas the country rock belongs to the Lamaya Formation and the major provenance of them is similar to those of the neighbouring Songpan-Garzê terrane. In addition, the inherited zircons from the Late Triassic granites in the eastern Yidun arc belts have a prominent Neoproterozoic age population (900-700 Ma), which suggests that there is an old basement with west Yangtze Craton affinity beneath the Triassic sediments. Combining with previous studies, we propose that the provenances of the formations vary from the Lanashan Formation to the Lamaya Formation which may indicate a record of the final closure of the Garzê-Litang Ocean. [ABSTRACT FROM AUTHOR]

We determined detailed U–Pb ages for zircons in migmatitic granites densely exposed in the Namche Barwa peak (NBP) area, the core area of the eastern Himalayan syntaxis (EHS). The cores of these zircons show ages ranging from 863 to 1,256 Ma, with high Th/U ratios. These ages correspond to the crystallization age of the Proterozoic Indian continental basement. The majority of zircon rims from these migmatitic granites yield a significant age peak at about 8 Ma with an increase in Th/U ratios, suggesting that a large-scale partial crustal melting event occurred at 8 Ma in the NBP area. Because thermochronologic results have demonstrated that a rapid denudation started around 8 Ma, we interpret this large-scale partial melting of the crust as a decompression melting caused by rapid denudation in the NBP area. The rapid denudation at 8 Ma probably resulted from the combination of tectonic activities and surface erosion. [ABSTRACT FROM PUBLISHER]

The Mesoarchean chromitites in the Singhbhum craton provide the main source of chromium in India. We report here results of dating zircon grains that were found in the chromitite of ultramafic rocks belonging to the southern part of the Archean Baula Complex. The chromite-bearing ultramafic rocks are cut by a gabbronorite intrusion, which had earlier been dated by SHRIMP at 3.12Ga. We have performed isotopic analysis of the zircon grains by Laser Ablation Multicollector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS) and obtained an age of 2.84Ga, which is inconsistent with the previous age reported for the gabbronorite. We have also determined the Pb isotopic composition of galena found in the chromitite and found out it was highly radiogenic given the Archean age of its host rock. We discuss the potential reasons for these results, such as later Neoarchean or even Proterozoic tectono-magmatic/hydrothermal events, and stress the importance of future detailed petrographic, mineralogical and isotopic studies to solve the dilemmas. [ABSTRACT FROM AUTHOR]

The U-Pb geochronological data on detrital zircons from placers confined to Neoproterozoic quartzite-schist sequences, which are widespread in the Kokchetav massif of northern Kazakhstan, are discussed. Detrital zircons (332 grains in total) originate from the ore occurrences in the central, northern, and western parts of the massif. The concordant ages of detrital zircons from all the examined occurrences largely correspond to intervals of 1017-1528, 1628-1946, and 2653-2739 Ma. The obtained data imply that material of quartzite-schist sequences of the Kokchetav massif was provided by Mesoproterozoic, Paleoproterozoic, and Neoarchean rock complexes. The lower age limit determined for these sequences is approximately 1.06 Ga. The dates obtained for detrital zircons are most consistent with events that took place in Laurentia. They correspond to the formation and breakup of the Columbia/Nuna supercontinent (approximately 1650-1580 and 1450-1380 Ma ago, respectively) and formation of the Rodinia supercontinent in the period of 1300-900 Ma ago. [ABSTRACT FROM AUTHOR]

The first data on the Middle Riphean (Stenian) age of the Isherim Formation developed in the synonymous anticlinorium of the North Urals and U-Pb age of detrital zircons from its arkosic sandstones are discussed. [ABSTRACT FROM AUTHOR]

The Paleo-Tethyan arc belt is host to a small number of porphyry Cu deposits the key controls on which remains poorly understood. It has been hypothesized that the Paleo-Tethyan arc magmas were reduced and therefore less fertile for porphyry Cu formation but few studies have been conducted to directly assess the magmatic oxygen fugacity. We investigate the magma fertility of eastern Paleo-Tethyan arc igneous rocks in the Sanjiang region, SW China, represented by the Maoding (268.3 ± 2.3 Ma), Suwalong (275.6 ± 1.4 Ma to 264.7 ± 1.4 Ma) and Baimaxueshan (255.7 ± 1.5 Ma to 249.3 ± 1.1 Ma) granitoids. They have geochemical features consistent with typical continental arc rocks, formed from partial melting of the mantle wedge, with increasing contamination of crust-derived melts over time. The presence of amphibole, high Sr/Y ratios (35 ± 20, n = 17), and lack of negative Eu anomalies (Eu N /Eu N ⁎ = 1.04 ± 0.3, n = 17) are consistent with derivation from hydrous magmas. Zircons from the intrusive rocks have relatively low Eu N /Eu N ⁎ (mostly <0.4) ratios and calculated ΔFMQ values (−2.1 ± 1.2, n = 86). Combined with apatite microphenocrysts with extremely low sulfur contents (< 0.12 wt% SO 3), it suggests that the intrusions originated from water-rich but reduced arc magmas, that were less fertile for porphyry Cu formation. Our study highlights that magmatic f O 2 is probably a critical control on the prospectivity of arc systems and confirms that zircon and apatite compositions can be used to evaluate porphyry Cu fertility. • The Paleo-Tethyan arc igneous rocks were dated from ~275 to 249 Ma. • They were derived from partial melting of mantle wedge during MASH processes. • These arc magmas were reduced and less fertile for porphyry Cu formation. • Zircon and apatite compositions can be used to evaluate porphyry Cu fertility. [ABSTRACT FROM AUTHOR]

Zircon (ZrSiO4) is the most commonly used geochronometer, preserving age and geochemical information through a wide range of geological processes. However, zircon U-Pb geochronology can be affected by redistribution of radiogenic Pb, which is incompatible in the crystal structure. This phenomenon is particularly common in zircon that has experienced ultra-high temperature metamorphism, where ion imaging has revealed submicrometer domains that are sufficiently heterogeneously distributed to severely perturb ages, in some cases yielding apparent Hadean (>4 Ga) ages from younger zircons. Documenting the composition and mineralogy of these Pb-enriched domains is essential for understanding the processes of Pb redistribution in zircon and its effects on geochronology. Using high-resolution scanning transmission electron microscopy, we show that Pb-rich domains previously identified in zircons from East Antarctic granulites are 5-30 nm nanospheres of metallic Pb. They are randomly distributed with respect to zircon crystallinity, and their association with a Ti- and Al-rich silica melt suggests that they represent melt inclusions generated during ultra-high temperature metamorphism. Metallic Pb is exceedingly rare in nature and previously has not been reported in association with high-grade metamorphism. Formation of these metallic nanospheres within annealed zircon effectively halts the loss of radiogenic Pb from zircon. Both the redistribution and phase separation of radiogenic Pb in this manner can compromise the precision and accuracy of U-Pb ages obtained by high spatial resolution methods. [ABSTRACT FROM AUTHOR]

The U–Pb dating system has been widely used in geochronology because the system contains two independent parent/daughter pairs yielding three ages (i.e., 238U/206Pb, 235U/207Pb and 207Pb/206Pb ages) to internally check self-consistency. Among numerous U-bearing minerals, zircon has been recognized as the best mineral for U–Pb geochronology owing to its moderate U content, negligible initial unradiogenic Pb (or common Pb) and occurrence in a wide range of rock types. With the development of Secondary Ion Mass Spectrometry (SIMS) and in situ dating methods, the 238U/206Pb zircon age uncertainty could be achieved at an ∼1% level. However, the 207Pb/206Pb age uncertainty of Phanerozoic zircon is always very poor, when single-collector SIMS is used. The low level precision often hampers effective examination of concordance of young zircon between U–Pb and Pb/Pb ages, which is crucial to the data quality evaluation and chronological interpretations. In this study, we developed a hybrid “dynamic multi-collector U–Pb dating technique”. It takes advantage of both the static multi-collector mode and peak-hopping mono-collector mode. The technique is able to simultaneously measure with high-precision the 207Pb/206Pb ratio as in the static multi-collector mode without trade off in the analytical precision of the 238U/206Pb ratio of the conventional peak-hopping mono-collector mode. Four zircon reference materials (91500, M257, Temora and Plešovice) were measured to demonstrate that this new analytical protocol is able to achieve a higher precision for the 207Pb/206Pb age by a factor of two than the conventional mono-collector mode within the same working time. It is possible to simultaneously obtain the 207Pb/206Pb age and 238U/206Pb age with comparable quality to effectively evaluate the concordance of the U–Pb system for Phanerozoic samples. [ABSTRACT FROM AUTHOR]

In northwestern California, the Franciscan subduction complex has been subdivided into seven major tectonostratigraphic units. We report U-Pb ages of ≈2400 detrital zircon grains from 26 sandstone samples from 5 of these units. Here, we tabulate each unit’s interpreted predominant sediment source areas and depositional age range, ordered from the oldest to the youngest unit. (1) Yolla Bolly terrane: nearby Sierra Nevada batholith (SNB); ca. 118 to 98 Ma. Rare fossils had indicated that this unit was mostly 151–137 Ma, but it is mostly much younger. (2) Central Belt: SNB; ca. 103 to 53 Ma (but poorly constrained), again mostly younger than previously thought. (3) Yager terrane: distant Idaho batholith (IB); ca. 52 to 50 Ma. Much of the Yager’s detritus was shed during major core complex extension and erosion in Idaho that started 53 Ma. An Eocene Princeton River–Princeton submarine canyon system transported this detritus to the Great Valley forearc basin and thence to the Franciscan trench. (4) Coastal terrane: mostly IB, ±SNB, ±nearby Cascade arc, ±Nevada Cenozoic ignimbrite belt; 52 to <32 Ma. (5) King Range terrane: dominated by IB and SNB zircons; parts 16–14 Ma based on microfossils. Overall, some Franciscan units are younger than previously thought, making them more compatible with models for the growth of subduction complexes by progressive accretion. From ca. 118 to 70 Ma, Franciscan sediments were sourced mainly from the nearby Sierra Nevada region and were isolated from southwestern US and Mexican sources. From 53 to 49 Ma, the Franciscan was sourced from both Idaho and the Sierra Nevada. By 37–32 Ma, input from Idaho had ceased. The influx from Idaho probably reflects major tectonism in Idaho, Oregon, and Washington, plus development of a through-going Princeton River to California, rather than radical changes in the subduction system at the Franciscan trench itself. [ABSTRACT FROM AUTHOR]

U–Pb zircon ages of igneous and metaigneous rocks in orogenic belts provide important information concerning the occurrence and duration of magmatic and metamorphic events whereas detrital zircon ages of metasedimentary sequences place constrains on the sources and deposition ages of supracrustal rocks. With these data, it is possible to compare and establish correlations between different provinces or between domains of the same province. A sufficiently large dataset of U–Pb ages is now available from the Borborema Province, northeastern Brazil, allowing the use of statistical methods to identify age peaks. 210 U–Pb crystallization ages from magmatic rocks and ca. 2300 U–Pb zircon ages of detrital grains from metasedimentary rocks were compiled. The igneous age spectrum shows two large age clusters. The first spans about 300 Ma, from 2.25 Ga to 1.97 Ga. Orthogneissic complexes in the Rio Grande do Norte Domain have ages mainly between 2.22 and 2.15 Ga whereas in the remainder of the province they more commonly range from 2.18 to 2.0 Ga. The long duration of this magmatism is comparable to that of Neoproterozoic and Phanerozoic accretionary/collisional orogens and is inferred to have resulted from amalgamation of preexisting crustal blocks and juvenile material to the active margin of the Rio Grande do Norte Domain. The other large cluster of ages is much narrower, from 650 Ma to 550 Ma, which is more consistent with the duration of orogenesis in collisional belts. Orogenic granitoids older than 600 Ma were not yet identified in the Rio Grande do Norte Domain, perhaps reflecting a thicker lithosphere that delayed initiation of the Brasiliano Orogeny. In contrast with the igneous rock-zircon record, the detrital zircon record requires sources with ages that span almost the whole Proterozoic eon. The presence of zircons ages in the 1.5–1.0 Ga and 0.85–0.65 Ga intervals, which contrasts with their absence, in the first case, or rarity, in the second case, amongst the igneous rocks, may be explained by derivation from igneous sources as yet undiscovered or removed by erosion, and/or by provenance from distal sources. The occurrence of 2.15–2.05 Ga-old detrital zircons in the Rio Grande do Norte Domain and of Tonian zircons in this domain and in the Ceará Central and Médio Coreaú domains imply the inexistence of physical barriers separating the northern and southern domains of the Borborema Province during the Neoproterozoic. The results are consistent with deposition in an intracratonic setting that became the site of lithospheric extension in response to far-field stresses. The ages of the youngest detrital zircons (650–630 Ma) indicate that deposition ended just before or at the beginning of the Brasiliano Orogeny. [ABSTRACT FROM AUTHOR]

The new data obtained during thorough geological-structural, geochronological, petrographic, and petrochemical investigations of Late Cretaceous granitoids from the southern Sredinnyi Range in Kamchatka and zircon dating by the U-Pb SIMS and LA-ICPMS methods indicate that this region was characterized by intense granitoid magmatism in the Campanian (83.1 ± 2.0 to 76.2 ± 1.5 Ma). The Campanian granitoids mark initiation of the 'newly formed' continental crust in Kamchatka. The composition of these Late Cretaceous rocks demonstrates similarity to peraluminous I-type granites. The data allow the Campanian stage of magmatic activity to be defined in southern Kamchatka. [ABSTRACT FROM AUTHOR]

The calc-alkaline Bergell intrusion is classically mapped as a main tonalite and main granodiorite unit, whose ages have been determined at 32 and 30 Ma, respectively. These units are separated by a mostly thin band interpreted as magmatic 'transition zone'. High precision U-Pb dating of zircons and whole rock geochemistry of transition zone rocks are combined with a compilation of available geochemical analyses and ages to characterize the emplacement and magmatic evolution of the Bergell intrusion. Detailed field work shows that the transition zone is heterogeneous, characterized by multiple intrusions of tonalites to granodiorites to quartz-monzonites containing mafic enclaves and leucocratic dykes. Commonly, magmatic flow structures such as compositional banding, schlieren and accumulation of K-feldspar megacrystals are observed. The compositions of the transition zone intrusives as well as of the geochemically fairly heterogeneous tonalite and granodiorite units span a continuous range from 49-76 wt% SiO for mafic enclaves to leucocratic dykes. Major and trace element trends are consistent with a predominant fractional crystallization. High precision U-Pb dating of single zircon grains separated from tonalites and granodiorites of the transition zone yields crystallization ages of 31.22 ± 0.04 and 31.13 ± 0.10 Ma, respectively. Hence, the transition zone represents an own phase of intrusion during which a range of magma types were coeval. The available ages and compositional data suggest magmatic activity over the entire range from 28 to 33 Ma across the Bergell intrusion, which should not be described by two main 'tonalite' and 'granodiorite' stages but by a model with several magmatic main phases during which most magma types developed multiply. Geobarometry of tonalite boulders in the Como molasse (yielding a zircon crystallization age of 32.06 ± 0.13 Ma) indicate average erosion rates for the Bergell intrusion of 1.9 mm/a between 32-25 Ma and of 0.5 mm/a since then. [ABSTRACT FROM AUTHOR]

The carbonaceous-siliceous-argillitic rock type uranium deposit in the Zoige area is located in the northeastern margin of the Tibetan Plateau, and has gained much attention of many geologists and ore deposit experts due to its scale, high grade and abundant associated ores. Because of the insufficient reliable dating of intrusive rocks, the relationship between mineralization and the magmatic activities is still unknown. In order to study this key scientific issue and the ore-forming processes of the Zoige uranium ore field, the LA-ICP-MS zircon U-Pb dating of magmatic rocks was obtained: 64.08±0.59 Ma for the granite-prophyry and ∼200 Ma for the dolerite. U-Pb dating results of uraninite from the Zoige uranium ore field are mainly concentrated on ∼90 Ma and ∼60 Ma. According to LA-ICP-MS U-Pb zircon dating, the ages for the dolerite, porphyry granite and granodiorite are 200 Ma, 64.08 Ma approximately and 226.5-200.88 Ma, respectively. This indicates that the mineralization has close relationship with activities of the intermediate-acidic magma. The ages of the granite porphyry are consistent with those uraninite U-Pb dating results achieved by previous studies, which reflects the magmatic and ore-forming event during the later Yanshanian. Based on the data from previous researches, the ore bodies in the Zoige uranium ore field can be divided into two categories: the single uranium type and the uranium with polymetal mineralization type. The former formed at late Cretaceous (about 90 Ma), while the latter, closely related to the granite porphyry, formed at early Paleogene (about 60 Ma). And apart from ore forming elemental uranium, the latter is often associated with polymetallic elements, such as molybdenum, nickel, zinc, etc. [ABSTRACT FROM AUTHOR]

A U-Pb-He double-dating method is applied to detrital zircons with core-rim structure from the Ganges River in order to determine average short- and long-term exhumation rates for the Himalayas. Long-term rates are calculated from the U/Pb ages of metamorphic rims of the grains that formed during the Himalayan orogeny and their crystallization temperatures, which are calculated from the Ti-in-zircon thermometer. Short-term rates are calculated from (U-Th)/He ages of the grains with appropriate closure temperatures. The results show that short-term rates for the Himalayas, which range from 0.70 ± 0.09 to 2.67 ± 0.40 km/Myr and average 1.75 ± 0.59 (1 σ) km/Myr, are higher and more varied than the long-term rates, which range from 0.84 ± 0.16 to 1.85 ± 0.35 km/Myr and average 1.26 ± 0.25 (1 σ) km/Myr. The differences between the long-term and short-term rates can be attributed to continuous exhumation of the host rocks in different mechanisms in continental collision orogen. The U/Pb ages of 44.0 ± 3.7 to 18.3 ± 0.5 Ma for the zircon rims indicate a protracted episode of ∼25 Myr for regional metamorphism of the host rocks at deeper crust, whereas the (U-Th)/He ages of 42.2 ± 1.8 to 1.3 ± 0.2 Ma for the zircon grains represent a protracted period of ∼40 Myr for exposure of the host rocks to shallower crustal level. In particular, the oldest (U-Th)/He ages of the zircon grains are close to the oldest U/Pb ages for the rims, indicating that some parcels of the rocks that contain zircons were rapidly exhumed from deep to shallow levels in the stage of collisional orogeny. On the other hand, some parcels of the rocks may have been carried upwards by thrust faults in the post-collisional stage. The parcels could be carried upwards by the thrust faults that steepen as they near the surface, or by transient movement faults so that areas of rapid exhumation became areas of slow exhumation and visa versa on a time scale of a few Myr in order to maintain the continuous exhumation. In this regard, the Ganges River must be preferentially sampling areas that are currently undergoing above average rates of uplift. [ABSTRACT FROM AUTHOR]