Major Magmatic Events Research Articles

High-precision U-Pb zircon dating identifies a major magmatic event on the Moon at 4.338 Ga.

The Moon has had a complex history, with evidence of its primary crust formation obscured by later impacts. Existing U-Pb dates of >500 zircons from several locations on the lunar nearside reveal a pronounced age peak at 4.33 billion years (Ga), suggesting a major, potentially global magmatic event. However, the precision of existing geochronology is insufficient to determine whether this peak represents a brief event or a more protracted period of magmatism occurring over tens of millions of years. To improve the temporal resolution, we have analyzed Apollo 14, 15, and 17 zircons that were previously dated by ion microprobe at ~4.33 Ga using isotope dilution thermal ionization mass spectrometry. Concordant dates with sub-million-year uncertainty span ~4 million years from 4.338 to 4.334 Ga. Combined with Hf isotopic ratios and trace element concentrations, the data suggest zircon formation in a large impact melt sheet, possibly linked to the South Pole-Aitken basin.

Archaean multi-stage magmatic underplating drove formation of continental nuclei in the North China Craton

The geodynamic processes that formed Earth’s earliest continents are intensely debated. Particularly, the transformation from ancient crustal nuclei into mature Archaean cratons is unclear, primarily owing to the paucity of well-preserved Eoarchaean–Palaeoarchaean ‘protocrust’. Here, we report a newly identified Palaeoarchaean continental fragment—the Baishanhu nucleus—in northeastern North China Craton. U–Pb geochronology shows that this nucleus preserves five major magmatic events during 3.6–2.5 Ga. Geochemistry and zircon Lu–Hf isotopes reveal ancient 4.2–3.8 Ga mantle extraction ages, as well as later intraplate crustal reworking. Crustal architecture and zircon Hf–O isotopes indicate that proto-North China first formed in a stagnant/squishy lid geodynamic regime characterised by plume-related magmatic underplating. Such cratonic growth and maturation were prerequisites for the emergence of plate tectonics. Finally, these data suggest that North China was part of the Sclavia supercraton and that the Archaean onset of subduction occurred asynchronously worldwide.

Geochemistry and geochronology of the TTG-sanukitoid suite in the Zhulagou area: Constraints on the Neoarchean crustal evolution of the western North China Craton

The Archean basement in the Yinshan Block of the western North China Craton consists of a variety rock of assemblage, including TTG (tonalite-trondhjemite-granodiorite) rocks and some dioritic gneiss (sanukitoid). Understanding the petrogenesis of these rocks is crucial for comprehending the late Neoarchean crustal evolution of the western North China Craton. This study provides a new geochronological, geochemical, and Sr-Nd-Pb-Hf isotopic data for the TTG-sanukitoid suit that was newly identified in the Zhulagou area. The late Neoarchean TTG gneisses were emplaced around 2.54–2.50 Ga, while dioritic gneiss was emplaced between 2.53 and 2.50 Ga. The TTG gneisses exhibit high SiO2 (64.46–72.24 wt%) and Al2O3 (13.37–16.48 wt%) contents, relatively elevated (La/Yb)N and Sr/Y ratios, as well as low Y and Yb contents. They display variable εHf(t) (+0.94 to +3.57) and εNd(t) (−1.02 to +0.82) values, with relatively low Pb isotopic compositions, suggesting that they were derived from the partial melting of a thickened mafic lower crust. On the other hand, dioritic gneiss shows moderate SiO2 (58.05–61.87 wt%) content, high MgO (2.62–4.06 wt%), Al2O3 (14.42–18.44 wt%) and Cr contents, but relatively low (La/Yb)N values, showing geochemical affinity to the Archean sanukitoids. The dioritic gneiss exhibits variable εHf(t) values (−2.29 to +5.56) and εNd(t) (−2.05 to +2.02), with relatively low Pb isotopic compositions. Furthermore, the presence of amphibolite xenoliths, along with the high Mg# contents (36.84–53.49), indicating the addition of mantle-derived components to its source region. Therefore, the formation of the dioritic gneiss is primarily attributed to partial melting of the thickened (mafic) lower crust and the addition of mantle material. By combining previous studies with the new data presented in this study, we propose that a late Neoarchean tectonic regime involving a mantle plume is the most favorable explanation for the two major magmatic events in the Yinshan Block. Firstly, the earlier pulse/stage of mantle plume resulted in the ∼2.7 Ga thickened (mafic) lower crust and a limited amount of TTG rocks. Subsequent pulse/stage of mantle plume triggered partial melting of pre-existing thickened (mafic) lower crust and generated 2.54–2.50 Ga TTG rocks and dioritic gneiss.

Melting of Subducted Continental Crust in the Cimmerian Collisional Orogenic Belt, NE Iran

Abstract The subduction of continental crust in orogenic belts that are not associated with high-pressure metamorphism is still poorly understood. The Late Triassic postcollisional granitic rocks of NE Iran are related to the convergence between the Central Iran terrane and the Turan terrane on the southern margin of the Eurasian continent. They intruded into the Paleo-Tethyan suture between the Central Iran and Turan terranes. Granitoids of the Torbat-e-Jam pluton were emplaced at ca. 217 Ma, and their moderately high SiO2 (>64 wt. %), low MgO contents (mostly <2 wt. %), slightly depleted Sr-Nd, and zircon Hf isotopes suggest partial melting of a juvenile crust. Granodiorites from NW and SE Mashhad were emplaced at ca. 217 and 200 Ma, and their geochemical features and enriched Sr-Nd and zircon Hf isotopes suggest melting of continental crust. Inherited zircon cores in both the NW and SE Mashhad intrusions have dominant age peaks of ca. 540 and 770 Ma, similar to the age spectrum of adjacent Paleozoic sediments derived from the Central Iran terrane, but distinct from Triassic sediments sourced from the Turan terrane. The inherited zircon cores cannot be explained by crustal contamination during magma ascent, rather the age pattern of inherited zircon cores coincides with major magmatic events in the Central Iran terrane and fingerprint their source, suggesting partial melting of the subducted Central Iran continental crust. This study suggests that the melting of subducted continental crust can occur in simple collisional belts, rather than being confined to ultrahigh-pressure metamorphic orogenic belts.

Organic walled microfossils in wet peperites from the early Cretaceous Paraná-Etendeka volcanism of Brazil

Large igneous provinces (LIPs) are major magmatic events that have a significant impact on the global environment and the biosphere, for example as triggers of mass extinctions. LIPs provide an excellent sedimentological and geochemical record of short but intense periods of geological activity in the past, but their contribution towards understanding ancient life is much more restricted due to the destructive nature of their igneous origin. Here, we provide the first paleontological evidence for organic walled microfossils extracted from wet peperites from the Early Cretaceous Paraná-Etendeka intertrappean deposits of the Paraná basin in Brazil. Wet peperites are a volcaniclastic rock formed by the interaction of lava and subaqueous sediments.The Paraná-Etendeka was formed during the Valanginian (ca. 132 Ma) as a continental flood basalt in present day South America and Namibia, and released enormous amounts of carbon dioxide, sulfur dioxide, methane and hydrogen fluoride into the atmosphere. The organic walled microfossils recovered from the Paraná-Etendeka peperites include pollen grains, spores, acritarchs, and other remains of unidentifiable organic matter. In addition to the peperites, organic walled microfossils were also found in heterolithic sandstones and interpillow sandstones. Our findings represent the first insight into the biodiversity of the Paraná Basin during the Early Cretaceous during a period of intense magmatism, and the microfossil assemblages corroborate a regional paleoclimatic transition from arid to more humid conditions that were likely induced by the volcanic activity. We corroborate the potential of wet peperite rocks as a valuable source of paleobiological data and emphasize the importance of sampling volcaniclastic units that have been traditionally considered with lower fossiliferous potential due to their igneous origin.

Petrogenesis and tectonic implications of the Late Cretaceous to Paleogene calc-alkaline volcanic rocks, Ladakh Himalaya

One of the most widespread volcanic suites exposed in the northwest Himalaya is the Khardung volcanic suite sandwiched between the Ladakh Batholith in the south and the Karakoram Batholith in the north. These volcanic rocks show a transition from andesite, dacite to more evolved rhyolite and have a calc-alkaline affinity. They display large ion lithophile elements (Rb, Sr, Ba, U, Pb) enrichment and depletion of high field strength elements (Nb, Ta, Hf, Ti) with increasing Eu negative anomaly from andesite to rhyolite. Bulk-rock Liquid Line of Descent (LLD) modelling shows that fractional crystallization played a dominant role in their petrogenesis. The calculated zircon saturation temperature (TZr) in andesitic magma (745 °C) is higher than the dacitic and rhyolitic magmas (682 °C). Andesites have zircon ages of 69–68 Ma, whereas more evolved dacite and rhyolite yield younger ages (59–64 Ma) that are consistent with their generation in the last stage of magmatism in an active continental marginal setting. Geochemical and geochronological findings of this study demonstrate that Khardung volcanic rocks were derived through two major magmatic events. These events occurred during the Late Cretaceous to the Paleocene Epoch due to the subduction of the Neo-Tethyan oceanic slab underneath the Eurasian plate boundary before the main collision between India and Eurasia.

U–Pb ages via LA-ICP-MS of carbonate from brittle structures of Jandaíra formation, Potiguar basin, NE-Brazil

U–Pb carbonate dating by LA-ICP-MS is becoming a popular tool to investigate the timing of brittle deformation. In northeast Brazil, the absolute dating of brittle deformation in a series of rift-related intracontinental basins associated with the South Atlantic Rift has been little explored. Here we add on that, by investigating carbonate infilling and dating of fractures/veins from the Jandaíra limestone, part of the post-rift sequence of the Potiguar Basin. Until now, these features' ages (Lower Cretaceous to Middle Miocene) were inferred by field relationship and stratigraphic studies. Carbonates that were previously considered impossible to date, have been dated using a recent and challenging technique of U–Pb dating via LA-ICP-MS. Our data, the first ones with this method on this basin, indicate that post-rift brittle deformation can be constrained between 45 and 30 Ma. This interval is coincident with a major magmatic event (Macau-Messejana) suggesting a causal relationship between magmatism and brittle deformation, but the origin of the stress associated with this tectono-magmatic remains debatable. We validate here the use of U–Pb carbonate dating and its application for dating brittle deformation in northeast Brazil rift-related basins.

Zircon U–Pb geochronology and Sm–Nd and Rb–Sr isotope systematics of Neoproterozoic granitoïds from Bou Azzer (Anti-Atlas - Morocco): The obduction trigger of the central Anti-Atlas terrane

This study presents new U–Pb zircon ages for the Neoproterozoic granitoids of the Bou Azzer inlier at the central part of the Anti-Atlas chain (Morocco), in the northwestern edge of the West African Craton (WAC). Analyzes of zircon grains yield the following ages: 658 ± 3 Ma for Ait Ahmane pluton, 647 ± 5 Ma for Bou-Izbane intrusion, 656 ± 3 Ma for Bou Azzer quartz-diorites, 583 ± 12 Ma in the Oustrate intrusion and 644 ± 4 Ma for the Ait Ahmane leucogranite. These ages confirm a major magmatic event between 640 and 660 Ma contemporaneous with the obduction of oceanic crust marking the formation of the ophiolites of Bou Azzer and Sirwa. The emplacement of granitoids would have led to a decrease in the density of the oceanic crust and triggered the process of obduction. The granitoids display the geochemical features of volcanic arc magmas and have positive initial ϵNd (+4.67 to +6.57) and low initial 87Sr/86Sr (0.7002–0.7055). The quartz-diorite group displays young Depleted Mantle Model (TDM) ages that range from 904 to 1065 Ma. The leucogranites have younger TDM of 869–884 Ma. These new data attest to their juvenile nature and the absence of any crustal contribution. The NNW-SSE extended inlier band of Bou Azzer - Sirwa constitutes an exotic terrane of Neoproterozoic age and of a juvenile nature wedged between the SW and NE domains, respectively, formed by or surmounting a Meso-Paleoproterozoic or even Archean basement. The Bleida granodiorite, dated at 559 ± 4, displays older U–Pb ages from inherited zircon cores at 1234, 1748 and 2085 Ma. The Bleida granodiorite was emplaced during a later magmatic episode that belongs to the Ouarzazate Group, generated during the post-collisional metacratonic evolution of the northern edge of the West African craton.

Evolution of the preserved European continental crust, constrained by U-Pb, O and Hf isotopic analyses of river detrital zircons

Integrated U-Pb, O and Lu-Hf isotopic analyses of detrital zircons, collected from fourteen European river sands and one beach sand from northern Germany, were carried out to study the evolution of the preserved European continental crust. The drainage area of these sand samples covers large areas in eastern and western Europe, which evolved independently until their amalgamation during the Paleozoic. U-Pb ages of detrital zircons from eastern and western Europe generally match the ages of the major magmatic events in the two tectonic regions. The combined histogram of 2403 zircon U-Pb ages, obtained for this study, shows five major periods of zircon crystallisation: 2.80–2.65, 1.95–1.35, 1.25–0.85, 0.65–0.40, and 0.40–0.25 Ga, which correspond broadly with the timing of supercontinent assembly. The correlation could be due to enhanced granitoid magmatism and zircon growth during the supercontinent amalgamation, or the high preservation potential of igneous rocks generated during continental collisions. The O and Hf isotopic data of the analysed European river zircons show wide spreads in δ18O and εHf(t) values at ca. 1.9 Ga and 650–250 Ma, contrasting with the limited range for the Mesoproterozoic zircons. This indicates that extensive reworking of pre-existing crustal rocks occurred during the late Paleoproterozoic and Paleozoic in the evolution of the European continental crust, whereas the formation of new crust played an important role during the Mesoproterozoic.Arc mantle Hf model ages for the detrital zircons were calculated to estimate when European juvenile crust was extracted from the mantle. Model age uncertainties were estimated using the bootstrap method, considering three sources of uncertainty: (i) uncertainty in the Hf isotopic composition of the mantle reservoir, (ii) uncertainty in the 176Lu/177Hf for the crustal source region, and (iii) analytical uncertainties. Two types of model ages are calculated based on constant crustal 176Lu/177Hf (TAMC), and on variable176Lu/177Hf (TAMV), the latter being constrained by zircon δ18O values. Histograms of the calculated TAMC and TAMV model ages both show major juvenile extraction events between the late Paleo- and Mesoproterozoic. Although the principal periods in the European crustal growth contrast with those identified in the African, Russian and Mississippi river basins, the overall growth curves for the four continents show comparable results: (i) the formation of little or no continental crust before 4.0 Ga, and (ii) the formation of 50 % of the present crustal volume by ca. 2.0–1.6 Ga.

Tectono-magmatic evolution during the extensional phase of a Wilson Cycle: a review of the Alpine Tethys case and implications for Atlantic- type margins

Despite the recent progress in seismic imaging and dynamic modelling, the tectono-magmatic evolution during late orogenic extension and rifting is at present little understood. Answering questions such as when and where magma forms, how it percolates and how it is emplaced requires access to magmatic rocks from different crustal and mantle levels, which is impossible in modern rift systems. An alternative, presented in this study, is to use remnants of fossil extensional systems exposed in collisional orogens. Here we review the Permian to Early Cretaceous tectono-magmatic evolution recorded in the Alpine realm of Western Europe. While most of the previous studies reviewed the geochemical and petrological characteristics of the magmatic rocks, here we rely on a holistic approach to link the magmatic evolution to extensional processes on a large scale. We synthesize our observations on the timing and location of magma production in a Wheeler diagram. Our study shows that two major magmatic events can be defined: the first one took place during the Permian and was linked to post- orogenic extension of the Variscan belt. It affected a large portion of Western Europe, where it was responsible for significant sub-aerial volcanic activity, for the emplacement of massive magmatic additions at the base and within the crust, and for the depletion of the lower subcontinental lithospheric mantle. The second included several pulses of magmatic activity related to the multiphase rifting event that led to the successive opening of the Meliata-Vardar, Alpine Tethys and southern North Atlantic (proto-)oceanic domains. In the Alpine Tethys, the first rift-related magmatic activity was linked to melt infiltration within the subcontinental lithospheric mantle during the so-called necking stage. Basalts were emplaced only later, during and after mantle exhumation. Their chemical signature reflects the interaction between melts derived from Depleted Mid-oceanic ridge basalt Mantle (DMM) and the previously refertilized/infiltrated subcontinental mantle. Late alkaline magmatic activity post- dated breakup. A main result of this study is that the tectono-magmatic evolution of extensional systems cannot be explained by simple models using ß-factors or mantle temperatures alone but also needs to integrate the inheritance acquired during previous tectonic phases.

Supercontinent-paced magmatic destabilisation and recratonisation of the Yilgarn Craton

Knowledge of the evolution of ancient cratonic lithospheres underpins our understanding of Precambrian Earth. The Yilgarn Craton has exceptionally well-preserved Archean geology, with juvenile crust formation and major orogenesis concluding in the Neoarchean, and a stabilised upper-crustal architecture developing before 2.42 Ga. However, in an apparent dichotomy, geophysical models resolve lithospheric mantle composition outside the range of xenolith data from Archean regions, indicating the lithospheric mantle has since been extensively refertilised. Post-Archean igneous and sedimentary rocks record a prolonged lithospheric evolution that is not well resolved in datasets recording bulk crustal isotopic evolution. Reconciling these, we combine interpretation of geological and geophysical data to resolve two phases of lithosphere destabilisation driven by major magmatic events at ∼2.06 Ga and at ∼1.08 Ga. During destabilisation, sub-lithospheric and sub-crustal mantle fluxes caused extensive mantle refertilisation. For 200–400 Ma post-refertilisation, distributed sedimentary basins formed during recratonisation of the now denser lithosphere. The timing of these events suggests a relationship with the early stages of supercontinent assembly: Dominant downwelling beneath the assembling supercontinent sustains a sufficiently non-tensile tectonic setting to inhibit lithospheric thinning and breakup and enhances lateral flow of any upwelling mantle. This setting allows widespread intraplate refertilisation to occur while later the assembled supercontinent provides a stable setting allowing thermal re-equilibration and recratonisation to occur. In contrast, lithospheric refertilisation during supercontinent breakup will be more susceptible to density instabilities and recycling in later collisions. Consequently, we suggest that refertilisation of extant cratonic lithosphere may dominantly have occurred during the assembly of supercontinents.

Climate changes as recorded in stable carbon isotopic compositions of the Late Jurassic marine sedimentary succession in the Qiangtang Basin, Northern Tibet

The Jurassic oceanographic and climatic evolution is considered to be related to the breakup of Pangaea. As a crucial component of the carbon cycle, atmospheric CO2 concentration (pCO2) has been postulated as a main driver for climate change during the Jurassic, and concomitant changes in paleo-oceanographic conditions occurred as a result. In this study, we present a high-resolution organic matter (kerogen) carbon isotope dataset (δ13Ckerogen) from Upper Jurassic marine sedimentary rocks in the Qiangtang Basin, Tibet. The δ13Ckerogen result contains a genuine record concerning the response of the eastern Tethys to exogenic carbon cycle perturbations in both marine and atmospheric reservoirs and is also consistent with the high-resolution bulk carbonate and organic matter carbon-isotope records from the Atlantic and western Tethys. The relative fractionation of carbon isotopes in organic matter vs. carbonate species, defined as Δ13C (δ13Ccarb-δ13Ckerogen), and the secular trend of atmospheric pCO2 over the Late Jurassic that is calculated from the high-resolution δ13Ckerogen values indicate a cold Callovian-Oxfordian transition, a long-term increasing but fluctuating Kimmeridgian and a prominent early Tithonian cooling event (ETCE). The pronounced temperature plateau during the late Kimmeridgian-early Tithonian was contemporaneous with the occurrence of major magmatic events during the Late Jurassic, while the ETCE has been possibly attributed to major changes in oceanic circulation patterns. Additionally, reconstructed atmospheric pCO2 values show very small differences to values using phytane- and stomatal density-based pCO2 calculations, providing an alternative estimate for accurate identification of the paleoclimatic framework of this enigmatic interval in the Mesozoic.

Crustal growth/reworking and stabilization of the western Superior Province: Insights from a Neoarchean gneiss complex of the Winnipeg River terrane

AbstractLong-term stability of the continental lithosphere is attained through a cumulative increase in net buoyancy and rigidity due to progressive compositional differentiation (i.e., cratonization). As stable cratons provided the nucleus for the subsequent accretionary growth and tectono-magmatic reworking that produced modern continental crust, the geodynamic processes that facilitated the stabilization of cratons are critical for understanding the evolution of Earth’s lithosphere. This study uses a portion of the Winnipeg River terrane, one of the oldest terranes of the western Superior Province, as a natural laboratory to investigate Archean crustal growth (partial melting of mantle) and reworking (partial melting of crust) and provides insights into the geodynamic processes driving mantle depletion and crustal remelting. Zircon U-Pb data obtained by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) from an extensive Winnipeg River terrane gneiss complex reveal six major magmatic events at ca. 3060 Ma, 2930–2920 Ma, ca. 2910 Ma, 2830–2800 Ma, 2735–2730 Ma, and ca. 2700 Ma and regional metamorphism at ca. 2900 Ma. Whole-rock geochemistry and zircon Lu-Hf and trace element data indicate that (1) the magmatism at ca. 3060 Ma and ca. 2930–2920 Ma represents reworking of the isotopically evolved components of the incipient Winnipeg River terrane at shallow depths, (2) the ca. 2910 Ma magmatism features a step-change of Hf isotopic compositions from subchondritic to suprachondritic and records the formation of new juvenile magmas and the first reworking of existing juvenile crust, and (3) the magmatism after ca. 2830 Ma largely reflects reworking of the juvenile components of the incipient Winnipeg River terrane at medium to shallow depths prior to the ca. 2700 Ma trans-crustal magmatism associated with the convergence of the Winnipeg River terrane and western Wabigoon terrane. Juvenile magmatism and crustal growth in the Winnipeg River terrane at ca. 2910 Ma are inferred to correspond with significant mantle depletion below the Winnipeg River terrane, which led to a more stable lithospheric mantle in this part of the western Superior Province. Zircon trace element data support a mantle upwelling model rather than lithosphere recycling models for the depletion of mantle at ca. 2910 Ma. This study suggests that crustal growth and mantle depletion bracketed by prolonged, episodic crustal reworking may be a fundamental characteristic of the cratonization process.

Cambro–Ordovician ferrosilicic magmatism along the northern Gondwana margin: constraints from the Cézarenque–Joyeuse gneiss complex (French Massif Central)

It is well-acknowledged that the northern margin of the Gondwana supercontinent was affected by a major magmatic event at late Cambrian (Furongian) to early Ordovician (Tremadocian–Floian) times. However, an accurate assessment of its extent, origin, and significance is partly hampered by the incomplete characterization of the numerous gneiss massifs exposed in the inner part of the Variscan belt, as some of them possibly represent dismembered and deformed Furongian–Lower Ordovician igneous bodies. In this study, we document the case of the “Cézarenque–Joyeuse” gneiss complex in the Cévennes parautochthon domain of the French Massif Central. The gneisses form decametre- to kilometre-thick concordant massifs interlayered within a pluri-kilometric sequence of mica- and quartz schists. They encompass two main petrological types: augen gneisses and albite gneisses, both typified by their blue and engulfed quartz grains with the augen facies differing by the presence of centimetre-sized pseudomorphs after K-feldspar and the local preservation of igneous textures. Whole-rock geochemistry highlights that many gneisses have magmatic ferrosilicic (acidic with anomalously high FeOt and low CaO) compositions while others are akin to greywackes. Collectively, it is inferred that the bulk of the Cézarenque–Joyeuse gneisses represents former rhyodacite lava flows or ignimbrites and associated epiclastic tuffs. Volumetrically subordinate, finer grained, and strongly silicic leucogneisses are interpreted as microgranite dykes originally intrusive within the volcanic edifices. LA–ICP–MS U–Pb dating of magmatic zircon grains extracted from an augen gneiss and a leucogneiss brackets the crystallization age of the silicic magmas between 486.1 ± 5.5 Ma and 483.0 ± 5.5 Ma which unambiguously ties the Cézarenque–Joyeuse gneisses to the Furongian–Lower Ordovician volcanic belt of SW Europe. Inherited zircon date distributions, Ti-in-zircon and zircon saturation thermometry demonstrate that they formed by melting at 750–820 °C of Ediacaran sediments. Zircon Eu/Eu* and Ce/Ce* systematics indicate that the melts were strongly reduced (fO2 probably close to the values expected for the iron–wüstite buffer), possibly because they interacted during ascent with Lower Cambrian black shales. This would have enhanced Fe solubility in the melt phase and may explain the peculiar ferrosilicic signature displayed by many Furongian–Lower Ordovician igneous rocks in the northern Gondwana realm. We infer that crustal melting resulted from a combination of mantle-derived magma underplating in an intracontinental rift setting and anomalously elevated radiogenic heat production within the Ediacaran sedimentary sequences.

Magmatism in the Siang window of the Eastern Himalayan Syntaxis, NE India: a vestige of Kerguelen mantle plume activity

Abstract We report new U–Pb zircon ages for mafic plutonic (gabbro) and volcanic (andesite) rocks, along with the whole-rock chemistry of a mafic–felsic suite of volcanic rocks from the Siang window of the Eastern Himalayan Syntaxis, NE India. Field relationships, and mineralogical and geochemical characteristics, of the studied mafic–intermediate–felsic rocks suggest their co-magmatic linkage that was generated in an extensional tectonic environment. Incompatible trace elements and low concentrations of large ion lithophile elements (LILEs) and REE behaviour reflect both the enriched nature of the mafic rocks and the limited influence of crustal contamination in their genesis. Partial melting and fractional crystallization processes have played a major role during the genesis of these felsic volcanics from the parental mafic magma. The laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb ages suggest that the mafic plutonic rock was emplaced at c. 121.18 ± 1 Ma and intermediate volcanic rock was emplaced at c. 135.48 ± 0.50 Ma during the Early Cretaceous period. The new ages are consistent with earlier reported zircon U–Pb ages (133.0 ± 1.9–130.7 ± 1.8 Ma) of felsic volcanic rocks from the present study area. Our new field observations, and mineralogical and geochemical characteristics, in conjunction with the U–Pb isotopic database suggest that the major magmatic event in the core of the Siang window of the Eastern Himalaya is coeval with the Rajmahal–Sylhet–Mikir–Shillong flood basalts of eastern and northeastern India, and the Comei–Bunbury Large Igneous Province of southeastern Tibet and SW Australia. These events are related to the break-up of eastern Gondwana and outbreak of the Kerguelen plume.

Provenance of northern Gondwana Lower Ordovician sandstones (Khabour Formation, northern Iraq) revealed by detrital zircon using LA‐ICP‐MS dating

We present new zircon U/Pb provenance data using laser ablation induced coupled plasma mass spectroscopy (LA‐ICP‐MS) for eight samples of mature quartz arenites and subarkosic sandstones of the Lower Ordovician Khabour Formation, northern Iraq, to establish their depositional age and provenance. The samples were collected from thin‐ to medium‐bedded sandstones along profiles of the Chalky Nasara and Ora sections. Most analysed detrital zircon analyses are concordant within 100 ± 10% and include a prevalence of Proterozoic over Archean ages. Collectively, the data can be divided into four main age groups: (a) Neoproterozoic ages, 570–1,000 Ma (58%); (b) Mesoproterozoic ages, 1,000–1,500 Ma (10%); (c) Palaeoproterozoic ages, 1,620–2,480 Ma (21%); and (d) Archean ages, 2,500–3,300 Ma (11%). The predominance of Cryogenian–Ediacaran ages (850–550 Ma) reflects major magmatic events associated with the assembly of northern Gondwana. The major age group at ~600 Ma likely represents the unroofing of late‐stage granitic rocks from the Arabian‐Nubian Shield. The lack of zircons younger than 490 Ma suggests that Lower Ordovician sandstones in northern Iraq were probably deposited on the distal shelf of the northern Gondwana passive margin and were not influenced by rifting and magmatism in northern Gondwana caused by the opening of Palaeo‐Tethys. Diagenetic xenotime overgrowths are common on detrital zircon grains, but radiometric dating of these was not possible.

Timing of Proterozoic magmatism in the Sunsas belt, Bolivian Precambrian Shield, SW Amazonian Craton

We present new U–Pb zircon and monazite ages from the Sunsas belt granitic magmatism in Bolivia, SW Amazonian Craton. The geochronological results revealed four major magmatic events recorded along the Sunsas belt domains. The older igneous event formed a granitic basement coeval to the Rio Apa Terrane (1.95 – 1.85 Ga) in the southern domain. The second magmatic episode is represented by 1.68 Ga granites associated to the Paraguá Terrane (1.69–1.66 Ga) in the northern domain. The 1.37–1.34 Ga granites related to San Ignacio orogeny represent the third and more pervasive magmatic event, recorded throughout the Sunsas belt. Moreover, magmatic ages of ~1.42 Ga revealed that the granitogenesis associated to the Santa Helena orogeny also affected the Sunsas belt, indicating that it was not restricted to the Jauru Terrane. Lastly, the 1.10–1.04 Ga youngest magmatism was developed during the Sunsas orogeny and represents the final magmatic evolution related to Rodinia assembly. Likewise, the 1.95–1.85 and 1.68 Ga inherited zircon cores obtained in the ~1.3 Ga and 1.0 Ga granite samples suggest strong partial melting of the Paleoproterozoic sources. The 1079 ± 14 Ma and 1018 ± 6 Ma monazite crystallization ages can be correlated to the collisional tectono-thermal event of the Sunsas orogeny, associated to reactions of medium- to high-grade metamorphism. Thus, the Sunsas belt was built by heterogeneous 1.95–1.85 Ga and 1.68 Ga crustal fragments that were reworked at 1.37–1.34 Ga and 1.10–1.04 Ga related to orogenic collages. Furthermore, the 1.01 Ga monazite age suggests that granites previously dated by zircon can bear evidence of a younger thermal history. Therefore, the geochronological evolution of the Sunsas belt may have been more complex than previously thought.

PLATINUM-BEARING PLACERS: MINERAL ASSOCIATIONS AND THEIR 190Pt-4He AND Re-Os AGES, AND POTENTIAL LINKS WITH LARGE IGNEOUS PROVINCES IN THE SIBERIAN CRATON

AbstractPlacer deposits in the Siberian platform are closely associated with well-known platiniferous intrusions (Kondyor, Inagli, Noril’sk, Chiney, and Guli) but there are also abundant platinum-group mineral (PGM) placers of large areal extent in the river basins of Siberia (Vilyui, Lena, Aldan, Anabar, and Olenek) for which the primary sources are unknown. The 190Pt-4He dating method is introduced, compared with existing Re-Os isotope dating of PGMs, and applied to Pt-bearing placers of Siberia. Correlations between the 190Pt-4He and Re-Os isotope ages of the PGMs and with ages of major magmatic events in the Siberian craton show that the potential sources for some of the placers are likely buried Precambrian mafic-ultramafic intrusions of Large Igneous Provinces (LIP). The approach of this paper can be applied to other cratonic areas with alluvial PGMs of unknown origin.

A clearer view of crustal evolution: U-Pb, Sm-Nd, and Lu-Hf isotope systematics in five detrital minerals unravel the tectonothermal history of northern China

AbstractMuch of the global picture of crustal evolution has been constructed using zircon. While this has revealed a rich and complex history, this view is necessarily incomplete because of the lithology-specific affinity of zircon and the high temperatures needed to reset the U-Pb and Lu-Hf systems inherent within it. Here we use a five mineral, multi-isotope system approach to compare the record of crustal evolution recorded by zircon versus the picture provided by monazite, titanite, apatite, and rutile from the Yong-Ding and Luan rivers, northern China. These other minerals sample more diverse lithologies and temperature-pressure conditions that reflect additional tectonothermal events to those recorded solely by zircon. Zircon from both studied rivers predominantly reflects magmatic features, yielding age peaks at 2.6–2.3, 2.0–1.8, and 0.38–0.13 Ga, corresponding to the major magmatic events in their catchments. However, the detrital zircon record from both catchments fails to record and detail several important tectonothermal events. Specifically, the detrital monazite U-Pb ages cluster into two Paleoproterozoic peaks of ca. 1.95 and 1.85 Ga, while detrital apatite and rutile ages document unimodal and protracted U-Pb age peaks at 1.9–1.6 Ga. The different U-Pb closure temperatures of monazite, apatite, and rutile likely record two metamorphic events and the subsequent cooling history—key details that are absent from or obscured in the zircon record. The Phanerozoic mineral U-Th-Pb ages correspond to multiple magmatic events between 0.40 and 0.24 Ga and subsequent 0.24–0.20 Ga metamorphism. The 0.60–0.25 Ga rutile U-Pb ages along with 0.33–0.24 Ga U-Pb ages in some zircon grains with radiogenic Hf isotope compositions from the Luan River do not match the geological records in the North China Craton, but instead reflect the protracted subduction-accretionary history of the Central Asian Orogenic Belt. In addition to their U-Th-Pb ages, Nd model ages of monazite, titanite, and apatite, plus zircon Hf model ages provide additional constraints on regional crustal evolution. The Nd model age information is blurred by the fact that the relationship between the Sm/Nd of these minerals and their former host rocks is not precisely known. Taken at face value, the monazite Nd model ages have two Neoarchean peaks at 2.9–2.7 and ca. 2.5 Ga, that may correspond to two crustal growth episodes, while the titanite Nd model ages with predominant peaks at 2.2–1.8 and 1.5–1.3 Ga broadly correspond with those derived from the whole-rock analyses of the wide spread Phanerozoic granitoids, and hence record extensive crustal reworking. In contrast, the zircon Hf model ages are strongly skewed to a 2.9–2.7 Ga period and fail to record the post-Archean evolution of this region. These data highlight the power of integrating the U-Th-Pb age and Lu-Hf/Sm-Nd isotope compositions of multiple detrital minerals, with a broad range in geochemical behavior and closure temperatures, to gain a more complete understanding of tectonothermal history and crustal evolution than zircon alone.

Age and isotope geochemistry of magmatic rocks of the Lohit Plutonic Complex, eastern Himalaya: implications for the evolution of Transhimalayan arc magmatism

We present integrated in situ zircon U–Pb and Hf isotope data, along with whole-rock Sr–Nd isotope and geochemical data for plutonic rocks of the Lohit Plutonic Complex of eastern Himalaya to understand the age distribution and petrogenetic–tectonic evolution of the Transhimalayan batholiths in the eastern Himalaya. The plutonic rocks are gabbro, diorite and quartz-diorite in composition, have calc-alkaline signatures with metaluminous character and have an affinity with Gangdese I-type granitoids of southern Tibet. The enrichment of large ion lithophile elements with highly depleted negative Nb anomalies is consistent with their origin in a subduction-related environment. Our laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb ages suggest that two major stages of magmatic emplacement took place at c. 145 Ma (Early Cretaceous) for gabbro and c . 100–90 Ma (early Late Cretaceous) for diorite and quartz-diorite. The zircon Hf isotopes in both the gabbro and diorite samples show high positive ε Hf ( t ) values between +11 and +20, suggesting crystallization from juvenile crust or depleted mantle derived melts, as also constrained by the low initial 87 Sr/ 86 Sr ratios (0.7038–0.7045) and high positive ε Nd ( t ) values (+2.9 to +5.9). Results of REE modelling indicate that the source of these rocks is garnet lherzolite. Field observations and mineralogical and geochemical characteristics in conjunction with the isotopic database suggest that the Transhimalayan magmatic rocks in eastern Himalaya were generated by partial melting of the lithospheric mantle wedge and emplaced into the lower crust ( c . 25–30 km) during at least two major magmatic events that took place in the Cretaceous period. Supplementary material: LA-ICP-MS zircon U–Pb and Hf isotope data are available at https://doi.org/10.6084/m9.figshare.c.4665305