Pb Dating Of Zircons Research Articles

Isotope-Geochemical Evidence of the Nature of the Protoliths of Diamondiferous Rocks of the Kokchetav Subduction–Collision Zone (Northern Kazakhstan)

Abstract —The isotope-geochemical features of diamondiferous metamorphic rocks of the Kokchetav subduction–collision zone (KSCZ) show that both the basement rocks and the sediments of the Kokchetav massif were their protoliths. A whole-rock Sm–Nd isochron from the diamondiferous calc-silicate, garnet–pyroxene rocks and migmatized granite-gneisses of the western block of the KSCZ yielded an age of 1116 ± 14 Ma, while an age of 1.2–1.1 Ga was obtained by U–Pb dating of zircons from the granite-gneiss basement of the Kokchetav microcontinent. Based on these data, we assume that the protoliths of the calc-silicate, garnet–pyroxene rocks and the granite-gneisses of the KSCZ were the basement rocks sharing an initially single Nd source, which was not influenced by high- to ultrahigh-pressure metamorphism (~530 Ma). Therefore, their geochemical features are probably not directly related to ultrahigh-pressure metamorphism. The corresponding rock associations lack isotope-geochemical evidence of partial melting that would occur during ultrahigh-pressure metamorphism, which suggesting that they were metamorphosed under granulite-facies conditions. At the same time, the high-alumina diamondiferous rocks of the Barchi area (garnet–kyanite–mica schists and granofelses), which were depleted to different degrees in light rare-earth elements (REE) and K, have yielded a Sm–Nd whole-rock isochron age of 507 ± 10 Ma indicating partial melting of these rocks during their exhumation stage. The close ɛNd (1100) values of the basement rocks and garnet–kyanite–mica schist with geochemical characteristics arguing against its depletion during high-pressure metamorphism indicate that the basement rocks were a crustal source for high-alumina sediments.

Petrogenesis and U–Pb zircon dating of amphibolite in the Mewengo iron deposit, Nyong series, Cameroon: fingerprints of iron depositional geotectonic setting

Petrogenesis and U–Pb zircon dating of amphibolite in the Mewengo iron deposit, Nyong series, Cameroon: fingerprints of iron depositional geotectonic setting

40Ar/39Ar dating of Paleoproterozoic shear zones in the Ellesmere–Devon crystalline terrane, Nunavut, Canadian Arctic

Paleoproterozoic gneisses of the Ellesmere–Devon crystalline terrane on southeastern Ellesmere Island are deformed by metre-scale, east-striking mylonite zones. The shear zones commonly offset pegmatitic dikes and represent the last episode of ductile deformation. Samples were dated by the 40Ar/39Ar step-heating method to put an upper limit on the time of deformation. Biotite from one tonalitic protolith and five shear zones give geologically meaningful results. Clusters of unoriented biotite grains pseudomorph granulite-facies orthopyroxene in some of the weakly deformed gneisses, whereas the shape-preferred orientation of biotite defines the mylonitic fabric. The intrusive age of the tonalitic protolith is 1958 ± 12 Ma, based on previous U–Pb dating of zircon. 40Ar/39Ar analysis of biotite from the same sample gave a plateau age of 1929 ± 23 Ma, which is interpreted as cooling from regional granulite facies metamorphism. Three nearby samples of mylonitic tonalite have 40Ar/39Ar ages in the range of ≈1870–1840 Ma. Biotite from two granitic mylonites over 80 km away return high-resolution Ar spectra in the same range, implying that widespread ductile shearing occurred at ≈1870–1840 Ma, or ≈90 million years after cooling from regional metamorphism. Although the 2.0–1.9 Ga gneisses of southeastern Ellesmere Island correlate with the Inglefield Mobile Belt in North-West Greenland and the Thelon Tectonic Zone, the late shear zones are superimposed on that juvenile arc long after the 1.97 Ga Thelon orogeny.

Structure and chronology across the Achankovil terrain boundary shear zone system (South India), and its Madagascar connection in the Gondwanaland

In South India, the WNW-striking ~ 130-km-long Achankovil Shear Zone (AKSZ) separating the Trivandrum Block and the Madurai Block is speculated to continue within southern Madagascar in the East Gondwanaland. Two aspects continue to be debated, whether the AKSZ is a terrain boundary shear zone, and if so, when the accretion occurred. We present, for the first time, a detailed field-based analysis of contrasting mesoscale structures and deformation kinematics in the granulite facies rocks across the AKSZ. The mineralogical segregation layering (S2T) in the Trivandrum Block is sub-horizontal to gently inclined and describes F3T regional-scale open asymmetric WNW-trending sub-horizontal folds. By contrast, the S2M granulite facies layering in the Madurai Block describes regional-scale steeply plunging F3M folds. Both blocks share the NNW/NNE-trending sub-parallel sinistral and dextral S3 shears of the AKSZ. Deformation microstructures and garnet-bearing melt-hosted syn-S3 extensional shears attest to the persistence of high temperature during D2–D3 deformations. Monazite chemical dates and existing Pb–Pb zircon dates are overwhelmingly Pan-African in and across the AKSZ, but the Paleoproterozoic (2.1–1.8 Ga) dates in the Trivandrum Block, equivalent to those reported in Anosyan and Androyan domains of southern Madagascar, are lacking in the Madurai Block. By contrast, the Mid Neoproterozoic (800–750 Ma) Pb–Pb zircon dates in the Madurai Block, equivalent to the Antananarivo domain (Central Madagascar), are lacking in the Trivandrum Block. The structural–chronological contrasts between the two blocks suggest the AKSZ to be a Pan-African terrain boundary shear zone system that is continuous with the Ranotsara shear zone in Madagascar. [248]

Neoproterozoic (740-680 Ma) arc-back-arc magmatism in the Sergipano Belt, southern Borborema Province, Brazil

The Novo Gosto amphibolites represent the oldest unit of the Canindé domain, a Neoproterozoic sequence located in the NW Sergipano Belt, southern Borborema Province, Brazil. The domain comprises an association of metavolcano-sedimentary rocks, gabbroic intrusions, bimodal magmatic rocks, and syn-to post-tectonic granitoids. Due to the lithostratigraphic and geochemical complexity of this domain, several controversial interpretations of the tectonic setting have been proposed over the years. The protholits of the Novo Gosto amphibolites are here classified as high Fe tholeiitic basalt type rocks. Three geochemical groups were identified: Group 1 is more depleted in incompatible elements and has a pattern similar to island arc basalts with HFSE depletion (Nb–Ta, Zr–Hf, weak Ti); Group 2 displays a pattern similar to Group 1, but with more evident anomalies (Nb–Ta, Ti) and varied enrichment in incompatible elements; Group 3 is more enriched in incompatible elements than Group 1 rocks, with the pattern being flatter with milder HFSE depletion (Nb–Ta, Zr–Hf, weak Ti, Y) and resembling a geochemical pattern slightly more enriched than E-MORB. Our results suggest an environment similar to the Ryukyu-Okinawa (Japan) and Kamchatka arc-back-arc (Russia) setting. LA-ICP-MS U–Pb zircon dating of one amphibolite sample resulted in a date of 743 ± 3 Ma and confirms the geological field relationships that indicate this unit to be the oldest in the domain. The new petrographic, geochemical, and isotopic data for the Novo Gosto amphibolites lead to the recognition of an ancient Neoproterozoic arc-back-arc system, a rare setting in the earlier phases of the Brasiliano Orogeny within the Borborema Province, which ended with the closure of the restricted Canindé sea and the associated Sergipano ocean at around 630 Ma.

In-situ hydrothermal zircon U–Pb and phlogopite 40Ar–39Ar geochronology of uranium mineralisation in Luzong ore district scientific drilling (LTZK01), Anhui Province, SE China: Constraints on the mineralisation process

The economically important Luzong ore district, located at the north-eastern margin of the South China Block, developed numerous Fe–Cu–Pb–Zn–U deposits during the Early Cretaceous. The identified uranium deposits are primarily granite-related vein-type deposits distributed in the exposed granites and adjacent Jurassic sediments along the SE edge of the Luzong volcanic basin. Recently, new uranium mineralisation has been discovered by scientific drilling in concealed syenite in the centre of the Luzong basin. In this study, we conducted detailed studies of geological characteristics and determined the geochronology of the uranium mineralisation to constrain the ore-forming process. Luzong scientific drilling (LTZK01) penetrated Quaternary loose surface deposits (0–2 m), Cretaceous volcanic rocks (2–1488.84 m), concealed syenite and monzonite (1488.84–2012.35 m), and uranium mineralisation (1506–1742 m). Uranium mineralisation predominantly occurs as uranothorite in hydrothermal veins along the intragranitic fractures and breccias, which are associated with a pyrite-quartz-albite-tourmaline-phlogopite gangue assemblage. LA-ICPMS U–Pb dating of zircons from syenite and monzonite yields crystallisation ages of 131.4 ± 1.6 (MSWD = 0.45) and 131.7 ± 1.0 Ma (MSWD = 0.87), respectively. Based on the close textural relationship with other hydrothermal minerals, morphology, and trace element features, the zircons from the uraniferous veins are of a hydrothermal origin. In-situ LA-ICPMS U–Pb dating of hydrothermal zircons yielded a weighted mean age of 131.1 ± 1.6 Ma (MSWD = 3.6), which is consistent with the hydrothermal phlogopite 40Ar–39Ar plateau age of 129.25 ± 1.51 Ma (MSWD = 7.70). These ages indicate that uranium mineralisation in LTZK01 is marginally younger than the crystallisation of the host igneous rocks and might be associated with the post-magmatic hydrothermal process. Based on the magmatic and mineralisation ages obtained in this study and previous researches, we propose that the uranium was probably fractionated from the felsic melt into F-rich magmatic fluids to form uraniferous veins in LTZK01 as the pressure and/or temperature decreased.

Petrology and geochronology of the San Martín de los Andes batholith: Insights into the Devonian magmatism of the North Patagonian Andes

Evidence is growing that a belt of Devonian plutonism is present in the Northern Patagonian segment of the present-day Andes. This plutonism formed along the paleo-Pacific margin of the Gondwana supercontinent. New whole rock chemistry and SHRIMP U–Pb zircon data for plutonic rocks of the San Martín de los Andes batholith (southwestern Argentina) provide insights into the magmatic and tectonic evolution of the southwestern Gondwana margin during the middle Palaeozoic. The San Martín de los Andes batholith was formed between 390 and 374 Ma and intruded into a Lower Paleozoic metamorphic basement that had been previously affected by Late Ordovician-middle Silurian deformation. Most of the batholith is composed of metaluminous, calc-alkaline tonalites and granodiorites, with subordinate volume of peraluminous garnet-bearing granites. Discrete and disperse gabbroic and dioritic rocks hosted in tonalites and granodiorites occur as either microgranular enclaves or syn-magmatic dykes. Tonalitic, granodioritic and granitic rocks develop transitional contacts among them revealing coetaneous building up throughout the plutonic suite, but when interpreting their chemical variability, they are not strictly comagmatic rocks. Collectively, field, petrographic, and geochemical data of the San Martín de los Andes batholith are interpreted to reflect concurrence of crystal fractionation, crustal assimilation and magma mixing in driving petrological diversification. Yet, gabbroic and dioritic rocks in the San Martín de los Andes batholith display geochemical characteristics of calc-alkaline primitive basalts that concur with what is being hypothesized to be a magmatic arc zone for the Devonian period. This study discusses how to interpret the evolution of the San Martín de Los Andes batholith together with other geodynamic models such as the Chaitenia magmatic arc.

Unveiling ductile deformation during fast exhumation of a granitic pluton in a transfer zone

Exhumation and cooling of upper crustal plutons is generally assumed to develop in the brittle domain, thus determining an abrupt passage from crystallization to faulting. To challenge this general statement, we applied an integrated approach involving meso- and micro-structural studies, thermochronology, geochronology and rheological modeling to the Miocene syn-tectonic Porto Azzurro pluton on Elba (Tuscan archipelago – Italy). This pluton is emplaced in an extensional setting, and we have realized that its fast exhumation is accompanied by localized ductile shear zones, developing along dykes and veins, later affected by brittle deformation. This is unequivocally highlighted by field studies and EBSD-aided microstructural analysis. To constrain the emplacement and exhumation rate of the Porto Azzurro pluton we performed U–Pb zircon dating and (U + Th)/He apatite thermochronology, which resulted in a magma emplacement age of 6.4 ± 0.4 Ma and an exhumation rate of 3.4–3.9 mm/yr. Thermo-rheological modeling established that localized ductile deformation occurred at two different time steps: within felsic dykes when the pluton first entered into the brittle field at 380 kyr, and along quartz-rich hydrothermal veins at c. 550 kyr after pluton emplacement. Hence, the major conclusion of our data is that ductile deformation can affect a granitic intrusion even when it is entered into the brittle domain in a fast exhuming extensional regime.

Erroneous determination of the duration of metamorphism from analysis of overlapping pre-sputtered areas during SIMS U–Pb dating of zircon

The duration of geological events has distinct implications for geological processes. Secondary ion mass spectrometry (SIMS) U–Pb dating of inherited zircon cores and metamorphic rims is the most common method used to determine the age of a protolith, and the timing and duration of metamorphism. This study investigates the age deviation induced by overlapping pre-sputtered areas during SIMS zircon core–rim U–Pb dating, using eclogite and country-rock granitic gneiss samples from the Shuanghe area in the Dabie Orogen, China. First, we analyzed only the zircon core or rim on each grain. Concordant ages of igneous zircon cores in two orthogneisses are 744 ± 10 and 768 ± 7 Ma, demonstrating that their protoliths were Neoproterozoic igneous rocks. The metamorphic zircon rims in the two granitic gneisses yielded a consistent weighted-mean 206Pb/238U age of 216 ± 2 Ma (2 SE; n = 12 and 17). Zircon grains in the studied eclogite are all metamorphic and yielded a weighted-mean 206Pb/238U age of 224 ± 2 Ma (2 SE; n = 21). The different metamorphic ages of the eclogite and orthogneisses suggest that they might be juxtaposed by later tectonic events. After being repolished, zircon grains with apparent core–rim structures from the two granitic gneisses were re-analyzed in the cores and rims of each grain. Due to the small size of the zircon grains, the pre-sputtered areas of the two spots in each zircon overlapped. Compared with the analyses in the first session, the overlapping pre-sputtered areas affacted apparent dates for zircon rims by −8.2% to +13.2%. The age deviations of the second analyses correlate with the relative position of the overlaps. The apparent 206Pb/238U dates are younger when delta X < 0 (i.e., the X coordinate of the second analysis is smaller than the first analysis), while the apparent 206Pb/238U dates are older when delta X > 0. This study proves that erroneous SIMS U–Pb dates can be produced when dating metamorphic zircon rims after analyses of the cores with overlapping pre-sputtered areas. It will lead to an apparently longer and incorrect duration of a metamorphic event.

Chronostratigraphic correlation of the volcanic 1.21 Ga Barby and Haiber Flats Formations in the Sinclair Supergroup of Namibia

This work investigates Mesoproterozoic volcanic rocks of the Haiber Flats, Barby, Welverdiend and Kairab Formations, and associated intrusive rocks in the Konkiep Terrane. Various correlations have been made in the past and recent dating work yields contrasting ages. U–Pb dating of zircons is applied in combination with Sm–Nd and Lu–Hf isotopes to determine the age, assess correlation and ascertain the much-debated tectonic setting.Ion probe U–Pb zircon dating yields ages of 1218 ± 5 Ma, 1212 ± 4 Ma and 1212 ± 5 Ma (all errors 2σ) for three samples of the Haiber Flats Formation and 1219 ± 13 Ma, 1215 ± 19 Ma for the Barby Formation. The ages obtained for the Barby Formation agree with the 1217 ± 2 Ma and 1214 ± 5 Ma ages reported from previous investigations. Rocks mapped as the supposedly older Kairab rhyolite and felsic tuff gave younger ages of 1220 ± 4 Ma and 1222 ± 10 Ma, respectively, which are coeval with the Barby and Haiber Flats Formations. Two Gorab Gabbro samples, previously mapped as Kairab Formation, yield ages of 1341 ± 8 Ma and 1336 ± 8 Ma. These older ages are within error of two Welverdiend Formation samples which gave ages of 1344 ± 6 Ma and 1337 ± 9 Ma, and the formation was previously dated in the type area at 1327 ± 10 Ma. These ages confirm the reassignment of rocks originally mapped as tholeiitic Barby Formation into the Welverdiend Formation, north of a newly identified stratigraphic hiatus corresponding to the geophysically defined Wêreldend Lineament.Sm–Nd and Lu–Hf isotopic signatures for the Barby, Haiber Flats and coeval Kairab samples indicate that they are co-magmatic and derived from partly depleted mantle contaminated by older crust. TDM model ages of these samples range from 2.5 to 1.5 Ga with a major 2.2 to 1.8 Ga group. This suggests that the older crustal component was Paleoproterozoic but included some Archean material. The Welverdiend Formation and Gorab Gabbros show similarity in age, Sm–Nd and Lu–Hf characteristics and were probably derived from partially depleted mantle, or from a well-homogenised mixture of depleted mantle and older crust.REE diagrams indicate enrichment in LREE (La–Gd) and a flat HREE pattern (Tb to Lu). On spidergrams, most samples are characterised by negative Nb–Ta anomalies and positive K and Pb anomalies which indicate a subduction-related setting (with positive Sr anomaly) or a reworked crustal origin. The similarity in ages, isotopic signatures and geochemistry confirms the correlation of the Barby and Haiber Flats Formation.

Timing and provenance of Paleoproterozoic supracrustal rocks in the central Thelon tectonic zone, Canada: implications for the tectonic evolution of western Laurentia from ca. 2.1 to 1.9 Ga

Depositional ages and provenance of metasedimentary rocks provide constraints on the architecture of the interface between the Slave and Rae cratons and processes related to the Thelon Orogen. Clastic rocks analysed from the central Thelon tectonic zone are Paleoproterozoic in age and not remnants of the Archean Yellowknife Supergroup (Slave Province), as originally considered. Two assemblages are recognized. An older clastic assemblage deposited after 2.09 Ga contains detrital zircon age modes of 2.3 and 2.17 Ga, with subordinate Neoarchean and Paleoarchean detritus. Its deposition is interpreted to predate Thelon magmatic activity given that (1) it lacks ca. 2.01–1.97 Ga detritus of Thelon magmatic origin, and (2) correlative clastic rocks occur as inclusions in Thelon plutons and contain ca. 2.0 Ga metamorphic monazite. This assemblage is correlative with both the Mary Frances and Rutledge River groups, establishing a &gt;800 km long basin at ca. 2.1 Ga that received detritus from the western Rae and (or) Buffalo Head terrane(s). Separation from the Slave craton at this time is consistent with the absence of any Slave-affinity detritus. A younger assemblage deposited after 1.95 Ga and prior to 1.91 Ga contains mainly 2.02–1.95 Ga detrital zircon, age modes comparable with adjacent Thelon convergent-margin plutonic rocks. The younger assemblage records deposition of the uplifted and eroded Thelon magmatic arc in an intermontane or foreland basin setting during the later stages of post-collisional convergence. These U–Pb zircon data support a tectonic model for western Laurentia that reconciles differences between the Thelon and Taltson magmatic zones involving ca. 2.1 Ga rifting, ca. 2.01–1.97 Ga convergence, followed by &lt;1.95 Ga thrust-driven exhumation.

A geomagnetic polarity timescale for the Carboniferous

Abstract The geomagnetic polarity pattern for the Carboniferous is incompletely known with the best-resolved parts in the Serpukhovian and Bashkirian. Hence, data from both igneous and sedimentary units are also used in an additional polarity bias evaluation. In the Tournaisian to mid Visean interval polarity is mainly derived from palaeopole-type palaeomagnetic studies, allowing identification of polarity bias chrons. Seven polarity bias chrons exist in the Mississippian (MI1n B to MI4n B ) with an additional 33 conventional magnetochrons and submagnetochrons (MI4r to MI9r). The Moscovian and Gzhelian polarity is best resolved in magnetostratigraphic studies from the Donets Basin and the southern Urals. Dispute about the reliability of these data is ill-founded, since an assessment of supporting data from palaeopole-type studies suggests that these datasets currently provide the best magnetic polarity data through the Pennsylvanian. Polarity bias assessment indicates a normal polarity bias zone in the Kasimovian. In the Pennsylvanian there are 27 conventional magnetochrons and submagnetochrons (PE1n to CI1r) and one normal polarity bias chron (PE8n B ). The Kiaman Superchron begins in the mid Bashkirian, with clear data indicating brief normal polarity submagnetochrons within the Superchron. The magnetochron timescale is calibrated using 31 U–Pb zircon dates and a quantitative Bayesian-based age-scaling procedure.

Geochronology, geochemistry and mineral chemistry of Malayer–Boroujerd–Shazand pegmatite dikes, Sanandaj–Sirjan zone, NW Iran

Pegmatite dikes in the Malayer–Boroujerd–Shazand magmatic complex, located in the Sanandaj-Sirjan zone of the Zagros orogen, are hosted in both granitoid rocks and their contact and regional metamorphic aureoles. In this study, we report the geochronology, geochemistry and mineral chemistry of these pegmatite dikes. The granitoid rocks in the Malayer–Boroujerd–Shazand magmatic suites are mainly monzogranite, quartz diorite, granodiorite and quartz monzonite characterized by metaluminous to peraluminous I-type compositions of the calc-alkaline series and display the geochemical characteristics of volcanic arc granites related to an active continental margin. The pegmatite dikes mainly consist of quartz, feldspar (albite and orthoclase), tourmaline, muscovite and less common biotite and garnet. The tourmaline content in the igneous-hosted pegmatite dikes is higher than those of metamorphic rocks, whereas the muscovite content is lower in the igneous-hosted pegmatite dikes. The K/Rb ratio and the Rb content of pegmatite dikes and their minerals (K-feldspar and muscovite) indicate that they are less evolved. Pegmatite dikes display a typical calc-alkaline affinity of I-type granites and are classified as peraluminous Lithium-Caesium-Tantalum (LCT) family (barren sub-type) to muscovite class. The tourmaline is a schorl-foitite type, which is common in the early crystallization stage of less evolved pegmatites. U–Pb zircon dating of the Boroujerd and Shazand pegmatite dikes yield weighted mean 206Pb/238U ages of 160.2 ± 1.8 and 121.8 ± 1.5 Ma, respectively. In contrast, U–Pb zircon dating of the Malayer pegmatite dike gives a mean 206Pb/238U age of 43.2 ± 1.5 Ma. Our new geochronological data indicate that the first group of pegmatite dikes (older pegmatite dikes) in the Boroujerd district (160.2 ± 1.8 Ma) shows an overlap with the main and first magmatic episode in the Sanandaj–Sirjan zone during Jurassic times, simultaneous with the Middle Cimmerian Orogenic phase, and formed during the subduction initiation. The second group of pegmatite dikes in the Shazand district (121.8 ± 1.5 Ma) formed in the Early Cretaceous and shows a temporal overlap with previously reported ages (ca. 130–114 Ma) during the early Alpine tectonic movements (Late Cimmerian Orogenic phase). Finally, the younger group of pegmatite dikes in the Malayer district (43.2 ± 1.5 Ma) formed in the Eocene overlapping with previously reported ages (ca. 34–52 Ma) which is equivalent to the Laramian phase. These data reveal that the magmatism in the Malayer–Boroujerd–Shazand magmatic complex started in the Early-Middle Jurassic resembling adjacent areas in the Sanandaj–Sirjan zone (e.g., Hamedan region) by melting of crustal protoliths in a subduction tectonic setting. The second and third groups of pegmatite dikes formed during the late subduction to syn-collisional process, respectively. These pegmatite generation marked the subduction initiation to syn-collisional span in the region. The zircon geochemistry (negative Ce anomalies and positive Eu anomalies) is indicative of the relatively oxidized conditions of the parent magma and shows characteristics of continental crust zircon.

Polymetamorphic events in the Jiao-Liao-Ji Belt, North China Craton：Evidence from integrated zircon, xenotime, and monazite LA–ICP–MS U–Pb dating

ABSTRACT Poly-metamorphism has been observed in many Precambrian metamorphic terranes. The metamorphic overprinting in the poly-metamorphic rocks makes it difficult to determine the timing and P − T path of each metamorphic event. In this study, in situ U–Pb dating of zircon, xenotime, and monazite, with simultaneously determination of trace element compositions, is utilized to provide new constraints on the timing of discrete metamorphic events involving reactivation of the Jiao-Liao-Ji Belt, the eastern edge of the North China Craton. Detrital zircons from metasandstones in the Gaixian Formation record a mixed provenance that includes late Neoarchean (ca. 2532 Ma) and middle Paleoproterozoic (ca. 2181 Ma). Xenotimes from the Gaixian pelitic schists yield an upper intercept age of 1906 ± 25 Ma, and this age is interpreted as the timing of greenschist-facies regional metamorphism. However, monazites from the same pelitic schists yield a distinctly younger age of 242 Ma, with late Paleoproterozoic inherited ages of 1894 and 1809 Ma, which are broadly consistent with the xenotime ages. In addition, magmatic zircons from a granitic mylonite vein yield a similar age of 245 Ma, making it coeval with the monazite. The interpretation of these ages is also reinforced by the mineral assemblage of inclusions in the dated accessory mineral grains. These new geochronological data, combined with published data from the Jiao-Liao-Ji Belt and the related regions, suggest that the Gaixian Formation in the Yanghe area was likely some time deposited at 2181–1900 Ma, and experienced both late Paleoproterozoic and Triassic low-grade metamorphic events related to two distinct orogenic phases in the eastern North China Craton.

Zircon U–Pb geochronology of Paleoproterozoic Statherian intraplate A-Type magmatic associations of the Lagoa Real Uranium Province, São Francisco Craton (Bahia, Brazil)

The Lagoa Real Complex, in the Lagoa Real Uranium Province, is a manifestation of extensive intraplate A-type magmatism that occurred in the São Francisco Craton during the Paleoproterozoic. U–Pb dating of zircon by LA-MC-ICP-MS shows that granites were emplaced during a Statherian, rift-related, 1.76–1.72 Ga magmatic episode, coeval with emplacement of the A-type Borrachudos suite and felsic metavolcanic rocks from the southeastern, central and northern Espinhaço Igneous Province of the São Francisco Craton. The Lagoa Real Complex involves at least three co-genetic pulses, starting with the initial emplacement of the alkali-feldspar-bearing Lagoa do Barro granite, which crystallized at 1762 ± 9 Ma. This was followed by emplacement of the Juazeirinho granite at 1755 ± 6 Ma, and finally the intrusion of the São Timóteo granite, which represents the main magmatic peak of the Lagoa Real Uranium Province, at 1741 ± 4 Ma. Although the São Timóteo granite has some characteristics that suggest it may be a rapakivi granite, there is no classical cogenetic association with basic rocks. This study confirms that the charnockites of the Lagoa Real Complex have an age of 2047 ± 5 Ma. They are therefore not part of the evolution of the 1.76–1.74 Ga magmatic episode. Also, rocks within the Lagoa Real Uranium Province that were previously mapped as leucodiorites are, in fact, albite-gneiss with ages of 1743–1740 Ma, which originated through the gneissification processes that affected the São Timóteo granite and can also be classified as albitites. The highly negative values of ϵNd(0) from −27 to −33 and high initial Sr ratio are compatible with the Lu–Hf isotopic data of previous studies. These results suggest the Lagoa Real Complex has a crustal derivation with minimum mantle involvement. The formation of the Lagoa Real Complex granites is inferred to be a result of fractional crystallization. All studied granites have a similar geochemical signature in terms of trace elements including REEs, which indicates that the granites come from the same magmatic source and are related to rifting and the initial stages of the Espinhaço Supergroup evolution. The source of the Lagoa Real Complex would be reworked Archean crustal material, probably in the mid to deep crust. Finally, the studied granites, although predominantly metaluminous, have U contents (maximum 12 ppm) below the world average for A-type granites (20 ppm). The fractionation values of Th in relation to U, and the absence of metamictization of the minerals (mostly allanite) that potentially host uranium confirms this observation. Therefore, the granites of the Lagoa Real Complex, despite being the protolith of bodies that host U mineralization, should not be considered as the principal source of U in the U mineralization found in the Lagoa Real albitites. However, the provenance of an important part of the uranium could be associated with Statherian meta-rhyolites and meta-rhyodacites, contemporaneous with the São Timóteo granite, and occurring to the west of the Lagoa Real Complex.

A middle Triassic seamount within the western Yarlung Zangbo suture zone, Tibet: The earliest seafloor spreading record of Neotethys to the North of East Gondwana

Initial opening time of Neotethyan ocean in the eastern Gondwana realm is still controversial. Ocean plate stratigraphy (OPS) forming in or near middle ocean ridges is of significance to trace the early stage of seafloor spreading. An over 10-km-long and ~ 5-km-wide Gongzhu seamount OPS is well preserved in the western Yarlung Zangbo suture zone (Southern Tibet), and consists mainly of neritic limestone, siliceous limestone, mudstone, mud-riched basaltic tuff, globular basalt and basaltic breccia interbedded with basaltic lava flows, and basaltic tuffaceous breccia. Geochemical and isotopic compositions of basaltic tuffceous breccias and basaltic lavas exhibit OIB-type affinities, basaltic lavas display slightly high (87Sr/86Sr)t ratios of 0.70430–0.70523, low (143Nd/144Nd)t ratios of 0.51244–0.51247 and εNd(t) values of +2.21 to +2.84, initial ratios of 206Pb/204Pb = 17.571–18.176, 207Pb/204Pb = 15.612–15.652, and 208Pb/204Pb = 37.290–38.706. These features are similar to those of alkaline basalts from the Vening-Meinesz Seamount Province in the northeast Indian ocean. The Gongzhu basaltic rocks are geochemically comparable with the mixing products of melts derived from the Pacific/Atlantic MORB and a metasomatized subcontinental lithosphere mantle. In-situ U--Pb zircon dating of two basaltic tuffaceous braccias has yielded mean ages of 245.1 ± 2.5 Ma and 241.9 ± 4.1 Ma. We propose that 272–290 Ma Panjal tholeiitic lavas exposed in the Kashmir region of NW India in the Himalayan orogenic belt developed during rifting of the Gondwana continental margin, whereas the slightly younger, 242–245 Ma Gongzhu seamount OPS formed during the initial seafloor spreading stage of the opening of the Neotethyan ocean.

Paleogene magmatic flex and flux in the Ladakh Arc, NW Himalaya: Chronostratigraphy of the Khardung Formation

The Ladakh Arc was formed by subduction of Neotethyan oceanic lithosphere along the southern Eurasian margin before magmatic activity ceased upon the continental collision between India and Eurasia. The onset of arc magmatism is critical in terms of understanding when this subduction began. Termination of arc activity is associated with continental collision; thus, the youngest arc magmatism provides a maximum age for this collision. Intra-arc volcaniclastic sequences provide a stratigraphic record of arc magmatism, but also information on input from older basement rocks. We provide detailed U–Pb detrital zircon chronostratigraphy for the Khardung Formation, which represents the volcanic equivalent of the Ladakh Batholith. The lower section of the Khardung Formation is mafic to andesitic and has a maximum depositional age of c. 60 Ma and the oldest inherited zircon age population from this lowermost section is c. 69 Ma. A switch to felsic volcanism occurred soon after and continued until at least c. 52 Ma. The youngest portion of the sequence has been removed by erosion or faulting along the Miocene dextral Karakoram Fault. However, detrital zircon data from the post-collisional Indus Group indicate that magmatism within the Ladakh Arc continued until c. 41 Ma. A compilation of U–Pb zircon data for the Ladakh Batholith paired with our new data from the volcaniclastic sequence, reveal an inverse relationship between alternating pulses of volcanic and plutonic activity within the arc. We suggest, this magmatic pattern may relate to alternating episodes of extension (volcanism) and compression (plutonism) imposed on the arc.

Discovery of Precambrian deep-water turbidites and submarine volcanism in the Brasília Belt, Central Brazil: The Quilombo Formation

A deep-water volcano-sedimentary sequence exposed along the Brasília Orogenic Belt was discovered and is herein presented and ascribed to a new stratigraphic unit. This sequence was previously unknown to contain volcanic rocks and to represent a turbidite record. It has been earlier loosely assigned to the Vazante Group based only on map continuity and interpreted as part of the Neoproterozoic passive margin of the western São Francisco Craton, but comprehensive studies have not been conducted for this specific sequence. Based on recent detailed fieldwork integrated with petrology, stratigraphy, geochronology and geochemistry, we describe a previously ignored succession of volcanic rocks interleaved in deep-water turbidites in the Brasilia Belt, and named as the Quilombo Formation. This formation comprises two sub-units, the Ribeirão da Porteira Member (lithoarenite, greywacke, siltstone, shale, and minor conglomerate) and the Ribeirão do Franco Member (fine-grained greywacke, siltstone, shale, and minor intermediate volcanic layers interpreted as subaqueous lavas). The lavas are strongly altered by SiO2 enrichment and depletion of Ca, K and Na. Based on immobile and rare earth elements analysis results, the volcanic rocks are classified as andesite to basaltic-andesite with a calk-alkaline to shoshonitic signature. U–Pb zircon dating of crystals extracted from lava samples yielded an age of 1394.46 ± 19.4 Ma, considered as the maximum depositional age of the sequence. Nonetheless, given the scarcity of zircon grains in the volcanic sample and the possible presence of xenocrysts, the exact age of the volcanic rocks still demands confirmation. The Quilombo Formation is for sure stratigraphically positioned between the Traíras Group (maximum depositional age of ca. 1543 ± 31 Ma) and the Bambuí Group (Ediacaran age). However, it is not yet possible to establish a confident correlation with either the Vazante, Paranoá, Canastra or (Upper) Traíras groups. Nonetheless, the volcanic rocks found represent the first tangible chance to obtain an absolute age for one of these groups, up to now only dated via detrital zircon ages.

Petrogenesis and tectonic implications of Early Paleozoic granitoids in the Qiaerlong district of the West Kunlun orogenic belt: constraints from petrology, geochronology, and Sr-Nd-Hf isotope geochemistry

ABSTRACT The West Kunlun orogenic belt (WKOB) is a complex orogenic belt that preserves the evidence of Early Palaeozoic and Late Palaeozoic orogenic events, with the former resulting from the closure of the Proto-Tethys Ocean. To better reveal the relationships between Early Palaeozoic granitoids in the WKOB and the tectonic evolution of the Proto-Tethys Ocean, we analysed the rocks of Akedala pluton, which is exposed within the Qiaerlong basin, and compared the results with contemporaneous magmatic rocks elsewhere in the WKOB. Integrated petrology, geochronology, and Sr–Nd–Hf isotope analyses of the rocks revealed the following: (1) The Akedala pluton is mainly composed of metaluminous–peraluminous I-type quartz monzonite and monzogranite; U–Pb dating of zircon indicates that quartz monzonite and monzogranite components is emplaced at 473–479 Ma and 462 Ma, respectively. (2) Both of them were enriched in large-ion lithophile elements and light rare earth elements and depleted in high field strength elements with negative Nb, Ta, Zr, P and Ti anomalies, indicating that they formed in a subduction zone environment. (3) Quartz monzonites were enriched in MgO concentrations (>3.10 wt.%), and show variable whole rock Sr-Nd isotopic ratios ((87Sr/86Sr)i = 0.7042–0.7058, εNd(t) = – 0.4 to +1.1), and had positive zircon εHf(t) (+0.0 to +6.8) values, which suggests they are derived from a mixed source of juvenile crust and older lower crust. (4) Monzogranites were characterized by high Sr (461–804 ppm) and low Y (2.00–3.68 ppm) and Yb (0.19–0.31 ppm) contents with high Sr/Y (164–325) and (La/Yb)N (12.6–46.7) ratios, indicating an adakitic affinity; they had relatively homogeneous whole rock (87Sr/86Sr)i (0.7044–0.7045) and positive εNd(t) (+2.0 to +2.9) and positive zircon εHf(t) (+1.1 to +4.3) values with low MgO, Cr, Ni and Nb contexts, suggesting that they were mainly generated from partial melting of thickened lower crust. Comparative analysis of these results with other Early Palaeozoic arc magmas in the WKOB shows that the Akedala pluton was formed due to the southward subduction of the Proto-Tethys Ocean. Divergent bidirectional subduction of the Proto-Tethys Ocean was initiated in the Early Cambrian, continued through the Middle Ordovician, and ended in the Early Silurian, due to terminal collision between the Tarim block and the West Kunlun terrane, which created the Early Palaeozoic orogenic belt.

Early Miocene calc-alkaline felsic tuffs within deep-marine turbidites in the Kyrenia Range, north Cyprus, with a possible post-collisional eruptive centre in western Anatolia

AbstractFelsic tuff as a direct fallout deposit is known from one small area in the Kyrenia Range, north Cyprus, within deep-sea terrigenous turbidites. Nearby tuffaceous siltstones contain compositionally similar felsic volcanic rocks (c. 5–10%), mixed with terrigenous material. Sedimentary evidence indicates that the fallout tuff was variable reworked locally, whereas the tuffaceous siltstones are interpreted as turbidites mixed with terrigenous material derived from Anatolia. U–Pb dating of zircons that were extracted from a sample of relatively homogeneous tuff yielded a dominant age of 16.64 ± 0.12 Ma (Burdigalian). Zircon trace-element analysis indicates predominant derivation from within-plate-type felsic magma. Whole-rock chemical analysis of the tuffaceous sediments as a whole is compatible with a felsic arc source, similar to the post-collisional magmatism within Anatolia. Regional comparisons suggest that the nearest volcanism of similar age and composition is locatedc. 500 km away, within the Kırka area (Eskişehir region) of the Western Anatolia Volcanic Province. Evidence of tephra dispersal in the western Mediterranean region and climatic modelling suggests E-wards prevailing winds and therefore tephra transport over southern Anatolia and adjacent areas during early Miocene time. The north Cyprus tuffs could represent powerful Minoan (Plinian)-type eruptions in western Anatolia, coupled with SE-wards tephra transport during and soon after the onset of post-collisional magmatism.