Crystallization Ages Research Articles

The role of deformation-assisted by water influx on partial melting of a granite protolith, and its role in the evolution of southwestern São Francisco Craton margin, Brazil

The Areado Granite-Migmatite Unit is part of the basement from the Southern Brasilia Orogen at southwestern São Francisco Craton margin, outcropping between Alfenas and Areado regions, Minas Gerais State, Brazil. This unit is composed of garnet meta-syenogranite, hornblende-garnet metatexite, and stromatic diatexite with different degrees of partial melting. This study characterizes the garnet meta-syenogranite, its migmatitic counterparts and investigates the different patterns of deformation and partial melting involved to produce the observed migmatites within the unit. The connection between the garnet meta-syenogranite, hornblende-garnet metatexite, and biotite diatexite is inferred from field observations and assemblage of accessory minerals. Petrography associated with modal analysis, structural data, quartz c-axis analysis, zircon U–Pb (LA-MC-ICP-MS and SHRIMP) dating, as well as trace elements chemical analysis in quartz and titanite to determine metamorphic, partial melting, and deformation evolution were used as tools in this investigation. The garnet meta-syenogranite has a crystallization age of 2069.8 ± 5 Ma. Lower intercepts from zircon indicate a metamorphic and partial melting age between 671 ± 58 and 657 ± 40 Ma. Melt crystallization ages are inferred between 622 ± 7 Ma and 604 ± 6 Ma. Different rates of partial melt are structurally connected, being higher at the base of the unit, and are related with contrasting amounts of water influx and deformation. The deformation during metamorphism/partial melting was the main cause of the differential aspect of the produced rocks. Quartz c-axis thermometer yielded deformation temperatures of 765–935 °C and other pulses occurred between 590 and 715 °C, both between 11 and 15 kbar. Those values match the Zr-in-titanite thermobarometer data, which is 670–710 °C for low-temperature titanite and 785–815 °C for high-temperature crystals. These data indicate that the protolith was generated at a Rhyacian late magmatic stage of the Mineiro Belt and passed by a Neoproterozoic deformational-metamorphic event of high temperature.

Paleoarchean evolution of the Singhbhum Craton, eastern India: New constraints from geochemistry and geochronology of granitoids of Bonai and Champua area

The Singhbhum Craton, one of five major Archean cratons in the Indian subcontinent, contains abundant well-preserved Paleoarchean supracrustals and granitoids. This study presents zircon U-Pb ages and whole rock geochemistry of tonalite-trondhjemite-granodiorites (TTGs) and granites from the Bonai Granite Complex (BGC) and Older Metamorphic Tonalite Gneiss (OMTG), which are separated from each other by the Jamda-Koira-Noamundi Iron Ore Group (IOG) supracrustals. Emplacement ages obtained in this study indicate that a major episode of TTG magmatism took place in BGC around 3368 ± 8 Ma (1σ), followed by granitic magmatism around 3331 ± 33 Ma (1σ). In contrast, a TTG from the Deo Nala area representing OMTG yielded crystallization age of 3312 ± 8 Ma (1σ). The emplacement and evolution of the BGC were coeval with granitoid magmatism from the central part of the Singhbhum Craton. Whole rock geochemical data identify both high- and low-HREE TTGs in both the BGC and OMTG to the west and east of the IOG basin, respectively. The trace element systematics of high-HREE Bonai TTG are similar to those of Icelandic dacites, suggestive of their derivation from a garnet free, plagioclase rich amphibolite. The low-HREE TTGs of the BGC and OMTG were derived from an amphibolite source with varying amounts of garnet. The potassic granites of the BGC were sourced from the older TTGs which had undergone partial melting at a shallow depth. The evolution of the BGC and OMTG can be attributed to the partial melting under a thickened mafic crust. Dome and keel structures and emplacement ages of granitoids from the west and east of the Jamda-Koira-Noamundi IOG basin, support the origin of these Paleoarchean granitoids in a stagnant lid regime. High geothermal gradients induced by heat supplied by mantle upwelling appear to have induced the melting of the thickened crust, to form the TTG. Delamination induced mafic–ultramafic underplating resulted in melting of early formed TTGs, to form the younger potassic granites of the BGC at ∼ 3.33 Ga.

Early Ordovician Age of Fluorite-Rare-Metal Deposits at the Voznesensky Ore District (Far East, Russia): Evidence from Zircon and Cassiterite U–Pb and Fluorite Sm–Nd Dating Results

This article presents new isotope-geochronological results for the granites of the Voznesensky ore district (southeastern part of the Khanka massif). The granites are associated with extensive rare-metal–fluorite, tin and tantalum mineralization. Despite the numerous published results of Rb–Sr, Sm–Nd and U–Pb dating of ore-bearing granites and associated ores, the issues of age correlation and the genetic relationship of igneous rocks and mineralization remain unclear. U–Pb zircon SHRIMP dating reveals synchronous ages of 478 ± 4 Ma and 481 ± 7 Ma for two samples of biotite leucogranites as the age of magmatic crystallization of the Voznesensky granites. The composition of the studied zircon demonstrates the typical features of magmatic zircon and has the typical features of zircon exposed to fluids at the late/post-magmatic stage. Sm–Nd ID-TIMS dating of the fluorite of the Voznesenskoe deposit yields an age of 477 ± 9 Ma, and U–Pb ID-TIMS dating of cassiterite from the Yaroslavskoe and Chapaevskoe tin deposits yields an age of 480 ± 4 Ma, which confirms the direct genetic and age relationship of ore formation with granite magmatism.

Two-billion-year-old volcanism on the Moon from Chang\u2019e-5 basalts

The Moon has a magmatic and thermal history that is distinct from that of the terrestrial planets1. Radioisotope dating of lunar samples suggests that most lunar basaltic magmatism ceased by around 2.9–2.8 billion years ago (Ga)2,3, although younger basalts between 3 Ga and 1 Ga have been suggested by crater-counting chronology, which has large uncertainties owing to the lack of returned samples for calibration4,5. Here we report a precise lead–lead age of 2,030 ± 4 million years ago for basalt clasts returned by the Chang’e-5 mission, and a 238U/204Pb ratio (µ value)6 of about 680 for a source that evolved through two stages of differentiation. This is the youngest crystallization age reported so far for lunar basalts by radiometric dating, extending the duration of lunar volcanism by approximately 800–900 million years. The µ value of the Chang’e-5 basalt mantle source is within the range of low-titanium and high-titanium basalts from Apollo sites (µ value of about 300–1,000), but notably lower than those of potassium, rare-earth elements and phosphorus (KREEP) and high-aluminium basalts7 (µ value of about 2,600–3,700), indicating that the Chang’e-5 basalts were produced by melting of a KREEP-poor source. This age provides a pivotal calibration point for crater-counting chronology in the inner Solar System and provides insight on the volcanic and thermal history of the Moon.

Petrogenesis of the Tampanchi Ultramafic–Mafic Complex (Ecuador): Geodynamic implications for the northwestern margin of South America during the late Cretaceous

The Tampanchi Ultramafic–Mafic Complex (TUMC), located in the central segment of the Cordillera Real (Ecuador), is an oval-shaped intrusive body of approximately 18 km2 emplaced within a Cretaceous metavolcano-sedimentary sequence. Field investigations, zircon geochronology, mineral and whole-rock elemental and isotopic compositions constrain the nature of the parental magma as well as the physical conditions of emplacement, age of crystallization and tectonic setting of formation of the TUMC. The Complex consist of wehrlite and olivine-hornblende clinopyroxenite crosscut by hornblende gabbros and minor dikes/veins of diorite and granite. Hornblendites formed mainly at the contact between the olivine-hornblende clinopyroxenite and the intrusive hornblende gabbros by reaction-replacement processes. Geochemical data for both the wehrlites and pyroxenites define a trend with dominantly olivine and clinopyroxene accumulation, whereas a second trend is formed by hornblende gabbros that differentiated to leucocratic rocks, with amphibole as the dominant fractionating mafic phase. The trace elements show enrichment in large-ion lithophile elements, depletion in high field strength elements (Nb, Zr, Ti) and P, and enrichment in Pb, Sr and Ba relative to primitive mantle, indicating subduction-related parental liquids. The Nd and Sr isotopic composition suggests a uniform mantle source metasomatized by subducted crustal components. Thermobarometric estimations constrain emplacement and crystallization at intermediate to shallow continental crust levels (∼12.5 km depth) of hydrated basaltic melt under oxygen fugacity conditions above the nickel-nickel oxide buffer (ΔNNO = 0 to 2). Zircon SHRIMP U-Th-Pb age data constrain crystallization at 75.1–76.0 Ma, with scarce inherited zircons that record limited recycling of the Cordillera Real basement. These results confirm magmatic arc activity at the continental margin of South America during the late Cretaceous as a result of an E-dipping subduction zone prior to the accretion of the Ecuadorian-Colombian-Caribbean oceanic plateau, and support simultaneous double subduction at the northwest margin of South America during the Cretaceous.

Source diversity of Late Neoproterozoic granitoid magmatism across an orogen-scale lineament in southern Brazil and Uruguay: Whole-rock geochemistry, zircon geochronology and Sr-Nd-Hf isotope evidence

Granitoids of similar Brasiliano age (638–540 Ma) are widespread in southern Brazil and Uruguay on both sides of an orogen-parallel lineament comprising from south to north, the Sierra Ballena, Dorsal de Canguçu and Major Gercino shear zones. We reviewed mineralogy, geochemistry and geochronology of granitoids that occur in the two domains located east and west of the mentioned lineament. Most of the granitoid magmatism is calc-alkalic to alkali-calcic and magnesian to ferroan, but variations are observed (aluminosity increases in eastern relative to western domain granitoids, and alkaline intrusions are more frequent in the western domain). Considering mineralogy and geochemistry the K-rich calc-alkaline granitoids (KCG) type of Barbarin's predominate, but amphibole-rich calc-alkaline granitoids (ACG), muscovite-bearing peraluminous granitoids (MPG) and peralkaline and alkaline granitoids (PAG) type affinities (although subordinate) do also occur. High-barium-strontium (HiBaSr) granitoids (with and without shoshonitic affinity) are frequent in the western but sporadic in the eastern domain. Granitoids intruding in the western domains of Santa Catarina, Rio Grande do Sul and Uruguay generally display crystallization ages between 638 and 540 Ma, Archaean to Neoproterozoic inherited zircon ages (3365–640 Ma) and strongly evolved isotopic signatures and model ages (ϵNd600 of −8.5 to −32; Nd TDM2 ages of 1.9–3.7 Ga; ϵHf (t) of −0.46 to −35.7 and Hf TDMCrustal ages of 1.5–3.6 Ga). Granitoids from the eastern domain, while showing a similar range of crystallization ages (630–543 Ma), generally reveal contrastingly younger inherited zircon ages (774–642 Ma) and less evolved isotope characteristics and model ages (ϵNd600 of −2.3 to – 11.8, Nd TDM2 of 1.4–2.1 Ga; ϵHf(t) of −0.21 to −12.5 and Hf TDMCrustal ages of 1.5–2.4 Ga).The assessment of the data compiled for this contribution indicate that: a) in southern Brazil and Uruguay, the Sierra Ballena-Dorsal de Canguçu-Major Gercino Lineament, act as a remarkable tectonic regional boundary between domains whose granitoids generally (but not always) show a pattern of contrasting whole-rock key chemical indicators, zircon inheritance, isotope signatures and model ages reflecting different granitoid sources; b) granitic melt production took place from the interaction of sources of different age (e.g. old Archaean to Paleoproterozoic crust and/or of Tonian age arc-materials) and composition (e.g. crustal sources: metaigneous rocks, metasediments vs. mantle-derived sources: juvenile basaltic melts and/or metasomatized mantle); c) crustal components strongly outweigh mantle ones in the source of the granitoids, suggesting that recycling was the chief mechanism; d) however, the spatial association of plutonic intermediate to basic rocks and felsic granitoids is sometimes significant in some sectors of southern Brazil and may be indicative of the adding up of new material from the mantle (i.e “crustal growth” processes) to the crust within a post-collisional environment.

GEOCRONOLOGIA U-Pb E Lu-Hf EM ORTOGNAISSES DA REGIÃO DE ESPERA FELIZ (MG): CONTRIBUIÇÃO A EVOLUÇÃO CRUSTAL DO EMBASAMENTO RIACIANO NO LIMITE DOS ORÓGENOS RIBEIRA E ARAÇUAÍ

Este artigo apresenta os resultados isotópicos de U-Pb e Lu-Hf de amostras situadas nos limites dos Orógenos Ribeira e Araçuai, em ortognaisses na região de Espera Feliz (MG). As análises petrográficas indicam paragêneses formadas Opx+Kfs+Pl+Qz+Gn formadas na fácies gramulito. Na área próxima da Serra do Caparaó são observados ortognaisses e paragnaisses além de milonitos e protomilinitos que evidenciam as zonas de cisalhamento. As rochas foram agrupadas nos seguintes litotipos: ortognaisses heterogêneos, ortogranulito e granada chanockito, além do paragnaisse, granada silimanita biotita gnaisse, e biotita leucogranito. As análises de U-Pb e Lu-Hf em grãos de zircão por LA-ICP-MS, permitiram a obtenção de idades de cristalização e de metamorfismo, além de informações sobre as fontes dos magmas. Os grãos de zirção apresentam, de forma geral, morfologia arredondada e/ou alongada, sendo possível a identificação de núcleos herdados e sobrecrescimento nos grãos através das imagens feitas no MEV. Análises geocronológicas U-Pb apresentam idades de cristalização, para os ortognaisses paleoproterozóicos, que variam de 2200 a 2075 Ma, com valores de εHf variando de -21,8 e -8,9 e TDM de 2,83 a 2,06 Ga (sugerindo fontes mantélicas com importante participação crustal na formação dos magmas destas rochas) e idades de metamorfismo de 600 a 570 Ma. Para os ortognaisses neoproterozóicos, foram encontradas idades de cristalização de 630 a 597 Ma, com valores de εHf variando de-14,6 e -6,7 e TDM de 2,22 e 1,78 Ga, sugerindo fontes crustais. Estes resultados permitem sugerir a formação do embasamento Paleoproterozoico originadas em ambiente de arco magmático durante o riaciano-orosiriano (2200-2075 Ma). Estas rochas foram intrudidas por um magmatismo edicariano (630-597 Ma). O terceiro evento identificado nas idades U-Pb das rochas estudadas é caracterizado pelo metamorfismo e anatexia (600-570 Ma) durante a colisão e formação do Gondwana.

Apatite U‐Pb Dating with Common Pb Correction Using LA‐ICP‐MS/MS

Apatite is a ubiquitous accessory mineral in various rocks (e.g., igneous, metamorphic and clastic sedimentary rocks). However, precise and accurate U‐Pb LA‐ICP‐MS dating of apatite is often compromised by high common Pb levels. Among the different common Pb correction methods, the main advantage of the 204Pb correction method is that it does not assume U/radiogenic Pb concordance. However, 204Pb is difficult to measure using ICP‐MS instruments because of the isobaric interference of 204Hg on 204Pb. We overcome this limitation by using a reaction cell sandwiched between two quadrupoles within an ICP‐MS, which can allow the online chemical separation of two different elements. Ammonia reacts efficiently (> 98%) with Hg while isotopes of Pb are not affected. The approach was tested on eight apatite reference materials (McClure Mountain, NW‐1, UWA‐1, Otter Lake, Slyudyanka, MK‐1, Durango and Fish Canyon Tuff) for which there are independent constraints on the U‐Pb crystallisation age, by comparing U‐Pb dating results employing different reaction gas mixes (NH3‐N2O and NH3 only) in two laboratories. Based on the U‐Pb data and SEM analyses on each sample, we can exclude apatite inter‐ and intra‐grain U‐Pb age heterogeneity, except for a c. 4% variability of ages in the Otter Lake sample. Our results show that accuracy and precision for U‐Pb dating are not measurably affected by different reaction gases, and we accurately reproduce ages of numerous independently characterised apatites within 4% of the reference ages, and the age reproducibility is typically better than 2%.

First evidence for the subduction initiation and boninitic magmatism from the Armutlu Peninsula (NW Turkey): geodynamic significance for the Cadomian magmatic arc system of the Gondwanan margin

ABSTRACT The Cadomian Orogeny was active along the northern margin of Gondwanaland during the Late Neoproterozoic–Early Cambrian period. Remains of this orogenic activity are traced from Western Europe to the Himalayas, including some metamorphic basement rocks of Turkey. In this study, we provide new whole-rock and mineral chemistry, and zircon U–Pb age data for the metagabbro and metagranites in the Armutlu Peninsula, north of the Intra Pontide suture zone (IPSZ), which are the equivalent of the Cadomian basement of the İstanbul Zone (İZ). Geochemically, the metagabbros show an extreme depletion in Nb, Zr, Th, and Hf; low TiO2 contents; and high Mg# values exhibiting boninitic affinity, which are commonly associated with fore-arc settings. The metagranites are calc-alkaline and high-K calc-alkaline in character. They are slightly enriched in LREE and have similar features to shear-type plagiogranites. The yielded zircon U-Pb crystallization ages are 564 ± 4.4 Ma for metagabbros and 561 ± 9.5 Ma (Late Ediacaran) for metagranites. In this study, we provide additional information for the Cadomian basement of the İZ and the geodynamic evolution of Cadomian magmatism by discussing the tectonic setting for two geochemically contrast but almost coeval magmatic products.

Zircon U–Pb geochronology and Hf isotopic compositions of Palaeoproterozoic meta‐granitoids in the Lesser Himalaya, Nepal: Tectonostratigraphic implications

The Lesser Himalayan Belt in the northern margin of Indian Shield preserves abundant sedimentary and magmatic records related to the assembly and breakup of the Columbia supercontinent. However, the Palaeoproterozoic tectonic history of the northern margin of the Indian Shield and its position in the Columbia supercontinent are still controversial. Based on detailed geological investigations in the Dailekh area of western Nepal, we conducted zircon U–Pb dating and Lu–Hf isotopic analysis for meta‐granitoids that intruded into the Lesser Himalayan Crystalline and Metasediments. The Dungeshworl granitic augen gneiss, meta‐monzogranite, and tonalitic gneiss yielded crystallization ages of 1,829 ± 12 Ma, 1,856 ± 11 Ma, and 1,852 ± 11 Ma respectively, which demonstrate that the deposition limits of both the Lesser Himalayan Crystalline and Metasediments are the Palaeoproterozoic. Zircon εHf(t) values of these meta‐granitoids range from −3.99 to +0.18, with corresponding depleted mantle two‐stage model (TDM2) ages of 2.41–2.62 Ga, indicating that they formed by reworking of Neoarchean to Palaeoproterozoic crust. Synthesizing existing geochronological, petrochemical, and zircon Lu–Hf isotopic data for metamorphic sedimentary and igneous rocks from the entire Lesser Himalayan Belt, we suggest that the Palaeoproterozoic magmatism in the northern margin of Indian Shield may be formed within subduction‐related tectonics responded to the assembly of Columbia supercontinent.

Deciphering crustal growth in the southernmost Arabian Shield through zircon U-Pb geochronology, whole rock chemistry and Nd isotopes

ABSTRACT New U-Pb zircon geochronology using high-spatial resolution secondary ion mass spectrometry fills a data gap and provides crystallization ages for granitoids from the Asir composite terrane in the southernmost Arabian Shield of Saudi Arabia. Ages of c. 810–685 Ma, c. 663–636 Ma, and 625–610 Ma reflect oceanic island arc genesis, subduction-related arc accretion (syn-collisional), and post-collisional stabilization, respectively. All samples have juvenile εNd(t) compositions with no evidence of older material being involved in their genesis, indicating that this part of the Arabian Shield grew through juvenile magmatic addition and that assimilation by syn- and post-tectonic magmatism involved an isotopically juvenile component(s). The crustal thickness derived from the (La/Yb)N proxy indicates significant thickening from 10–20 km to c. 70 km at c. 650 Ma, consistent with timing of orogenic uplift and increasing crustal thickness post-dating peak Nabitah orogeny. The age of an intrusion cross-cutting the Atura formation, when combined with other data, provides a well-constrained depositional age of c. 646–625 Ma for the Atura formation and indicates that erosion of the orogenic edifice in this part of the Arabian Shield began at latest by 625 Ma. Our new data indicate that denudation occurred 80–100 m.y. before the development of the prominent sub-Cambrian peneplain, consistent with previous assertions that major pulses of denudation occurred prior to the waning stages of Nabitah orogenesis.

THE PALEOARCHEAN (3.3 Ga) AND MESOARCHEAN (3.0 Ga) TTGs OF THE WESTERN AZOV AREA, THE UKRAINIAN SHIELD

A large anticline structure that includes the West Azov and Remivka blocks occurs in the western part of the Azov Domain of the Ukrainian Shield. These blocks are composed of rocks of the Mesoarchean (3.2-3.0 Ga) granite-greenstone association and relics of an older basement. The anticline is divided into two parts by the Bilotserkivka structure of sub-latitudinal strike; the northern part includes the Huliaipole and Remivka blocks, and the southern part is comprised of the Saltycha anticline. The Archean plagiogranitoids of the West Azov underwent intense dislocation metamorphism during the Paleoproterozoic. In many areas they were transformed into plagioclase gneisses that were attributed to the Paleoarchean “Kainkulak thickness” of the Azov Series. Detailed geological-structural and geochronological studies are required to define the age of these gneisses.We have chosen two areas for our studies: the Lantsevo anticline within the Bilotserkivka structure, and the Ivanivka area in the eastern part of the Saltycha anticline. The Bilotserkivka structure is composed of rocks of the Central Azov Series and highly deformed Archean formations. We have dated plagiogneisses of the Lantsevo anticline. These rocks contain large relics of metamorphic rocks of unknown age, including two-pyroxene and pyroxene crystalline schists, and pyroxenemagnetite quartzites (BIF). In terms of chemical composition, two-pyroxene crystalline schists correspond to tholeiitic basalts and basaltic komatiites. Ferruginous-siliceous rocks belong to the Algoma type typical for the Archean greenstone belts. Biotite gneisses are similar to the medium-pressure tonalite-trondhjemite-granodiorite rocks (TTGs). The U-Pb age of zircon crystallization from biotite gneisses is 3299 ± 11 Ma. At 30 km in the western part of the Bilotserkivka structure, we have previously identified quartz diorites having an age of 3297 ± 22 Ma. In terms of geochemical characteristics, they correspond to low-pressure TTGs. These data show that the Bilotserkivka structure is a block representing an ancient basement. In the Ivanivka area in the eastern part of the Saltycha anticline, the strike of the Archean rocks was reorientated from northwestern to latitudinal. The studied dislocated trondhjemites of the Ivanivka area correspond to TTGs in terms of the geochemical characteristics. They contain numerous relics of highly altered amphibolites. The U-Pb age of zircon crystallization from trondhjemite is 3013 ± 15 Ma. These rocks are of the same age as TTGs of the Shevchenko Complex cutting through the sedimentary-volcanogenic rocks of the greenstone structures of the Azov Domain. They share age and geochemical characteristics with biotite and amphibole-biotite gneisses of the “Kainkulak thickness” in Zrazkove village located at the Mokra Konka river (3.1-3.0 Ga) and with biotite gneisses in the lower reaches of the Kainkulak river (2.92 Ga). Thus, gneisses of the “Kainkulak thickness” in fact represent the Mesoarchean TTGs of the Shevchenko Complex, which were transformed in the Paleoproterozoic time due to the dislocation metamorphism. Late Paleoarchean (3.3 Ga) tonalites are known in the West Azov and the KMA domains; they probably also occur in the basement of the Middle Dnieper domains, where detrital zircons of this age have been reported. These data allow us to conclude the existence of a large Late Paleoarchean (3.3 Ga) protocraton, in which the Mesoarchean (3.2-3.0 Ga) greenstone belts and TTGs of the eastern part of the Ukrainian Shield and the KMA Domain were formed.

Impact-related crystallization and modification of small zircons in Apollo 15 and 16 impactites at 4.2 Ga

Because of their robustness against resetting, in situ U-Pb ages of zircons in lunar impactites have the potential to provide constraints on the lunar bombardment history that may complement the more common K-Ar ages. Most previous work has focused on relatively large zircons that show growth zoning and ages were mostly interpreted as early igneous crystallization ages. Here we combine high-resolution mineralogical imaging and in situ U-Pb dating by ion microprobe to identify, characterize and date <20 μm size zircons in thin sections of lunar impact breccias. Several tens of grains of zircons of this size range were identified in thin sections of impactites from the Apollo 15 and 16 landing sites. Small zircons are more abundant in both noritic and evolved clinopyroxene, SiO2 or K-feldspar bearing lithologies compared to anorthositic bulk compositions. Both granular zircon aggregates and overgrowth on existing zircon or baddeleyite (in breccias 15455 and 67915) are interpreted to reflect high-temperature recrystallization of zircons or its high-temperature-pressure precursor phases, following shock heating events by impact. In contrast, conchoidal or poikilitic zircons <10 μm in Fe-Ni metal bearing noritic clasts or matrix (67915, 67955) crystallized in situ from impact melt. Most U-Pb ages of the 24 analyzed grains are either concordant or reverse discordant with 207Pb-206Pb ages ranging from 4.15 to 4.25 Ga. The small age range, combined with a large textural spectrum and the frequent presence of Fe-Ni metal suggest zircon crystallization from impact melt and recrystallization of pre-existing zirconium-bearing minerals by impact heating. Such ‘impact’ zircons with 4.2 Ga ages have now been reported from most Apollo landing sites, suggesting widespread formation and modification of zircons by basin-forming impacts at this time. The contrast between U-Pb zircon (predominantly 4.2 Ga) and K-Ar feldspar ages (predominantly 3.9 Ga) likely reflects resetting of the latter chronometer by impact heating.

Re-initiation of plutonism at the Gondwana margin after a magmatic hiatus: The bimodal Permian-Triassic Longwood Suite, New Zealand

The Cambrian to Cretaceous Tuhua Intrusives, New Zealand, preserve an igneous record of Phanerozoic subduction and crustal growth at the margin of Gondwana. Within the Tuhua Intrusives, the coeval gabbroic and trondhjemitic intrusions of the c. 261–243 Ma Longwood Suite stand out as being isotopically more primitive and chemically distinct from all other New Zealand plutonic suites. We present new U-Pb crystallization ages, trace element analyses and Sr-Nd isotope compositions of the Longwood Suite. U-Pb SHRIMP zircon ages of 258.5 ± 2.5 Ma, 256.0 ± 1.8 Ma, 247.8 ± 2.7 Ma and 243.2 ± 2.4 Ma obtained from plutons on Ruapuke Island, and a dike at Bluff, affirm the restricted time range and expand the known areal extent of the Longwood Suite. Longwood Suite granitoids are I-type and sodic (K/Na < 0.4), with distinctive low Rb and Nb/Ta, flat rare earth element patterns (La/YbN < 10), unradiogenic 87Sr/86Sr(t) (0.7029 to 0.7032) and radiogenic ε143Nd(t) (+6.3 to +8.2), compared to the nearby, calc-alkaline, Late Triassic Darran Suite I-type plutons of the Tuhua Intrusives. Stable Nd isotope ratios of Longwood Suite samples are highly variable (δ146/144Nd = 233 ppm) compared to global plutonic rocks (δ146/144Nd = 44 ppm) and reflect the removal of phosphate minerals. Collectively, these geochemical characteristics are consistent with generation of the granitoids by shallow (garnet-absent) melting of an amphibolitic residue, from which we infer relatively thin lithosphere. The Longwood Suite has a maximum areal addition rate of 43 km2/Ma, substantially less than the subsequent plutonic suites when the magmatic arc was fully established. We suggest a petrotectonic model whereby Gondwana continental margin crust was tectonically underplated by Permian intra-oceanic island arc crust and mantle lithosphere, which subsequently melted to generate the isotopically primitive gabbro and trondhjemite plutons of the Longwood Suite.

Dating martian mafic crust; microstructurally constrained baddeleyite geochronology of enriched shergottites Northwest Africa (NWA) 7257, NWA 8679 and Zagami

Baddeleyite (monoclinic; m-ZrO2) is a widespread accessory phase within shergottites. However, the effects of shock loading on baddeleyite U-Pb isotopic systematics, and therefore its reliability as a geochronometer within highly shocked lithologies, are less well constrained. To investigate the effects of shock metamorphism on baddeleyite U-Pb chronology, we have conducted high-resolution microstructural analysis and in-situ U-Pb isotopic measurements for baddeleyite within enriched basaltic shergottites Northwest Africa (NWA) 7257, NWA 8679 and Zagami. Electron backscatter diffraction (EBSD) analyses of baddeleyite reveal significant microstructural heterogeneity within individual thin sections, recording widespread partial to complete reversion from high-pressure (≥3.3 GPa) orthorhombic zirconia polymorphs. We define a continuum of baddeleyite microstructures into four groupings on the basis of microstructural characteristics, including rare grains that retain magmatic twin relationships. Uncorrected U-Pb isotopic measurements form Tera-Wasserburg discordia, yielding new 238U-206Pb discordia ages of 195 ± 15 Ma (n = 17) for NWA 7257 and 220 ± 23 Ma (n = 10) for NWA 8679. Critically, there is no resolvable link between baddeleyite microstructure and U-Pb isotope systematics, indicating negligible open-system behaviour of U-Pb during zirconia phase transformations. Instead, we confirm that high post-shock temperatures exert the greatest control on Pb mobility within shocked baddeleyite; in the absence of high post-shock temperatures, baddeleyite yield robust U-Pb isotope systematics and date the age of magmatic crystallization. Low bulk post-shock temperatures recorded within Zagami (≤220 °C), and suggested within NWA 7257 and NWA 8679 by baddeleyite microstructure and other petrological constraints, confirm that the previously derived baddeleyite age of Zagami records magmatic crystallization, and provide greater age diversity to 225 Ma to 160 Ma enriched shergottites. While our data yield no resolvable link between microstructure and U-Pb isotopic composition, we strongly recommend that microstructural analyses should represent an essential step of baddeleyite U-Pb chronology within planetary (e.g., martian, lunar, asteroidal) and shocked terrestrial samples, allowing full contextualisation prior to destructive isotopic techniques. Microstructurally constrained in-situ U-Pb analyses of baddeleyite thus define new opportunities for the absolute chronology of martian meteorites and, more broadly, shocked planetary materials.

Cleaning up the record – revised U-Pb zircon ages and new Hf isotope data from southern Sweden

ABSTRACT Ten samples of felsic plutonic rocks from the Eastern Segment of the Sveconorwegian Orogen in southern Sweden, previously dated by ID-TIMS on zircon, have been dated anew using SIMS spot analysis of individual zircon grains, leading to more reliable and in most cases also more precise revised magmatic crystallization ages. A gneissic monzonite within the Protogine Zone in Småland yields a revised U-Pb age of ca. 1725 Ma, four samples of orthogneiss from Skåne all yield revised ages between 1700 and 1690 Ma, while two samples of coarse-grained granitic gneiss in the same region yield ages between 1690 and 1680 Ma. These revised ages are between 15 and 250 m.y. older than previously obtained TIMS ages. Two samples of the Gumlösa-Glimåkra granite along the Protogine Zone in northern Skåne and one sample of related syenite yield ages around 1220 Ma, similar but more precise compared to the previous ages. The U-Pb zircon data have been complemented by Hf isotope analysis by LA-ICP-MS on the same grains, and previously obtained initial Sr and Nd whole-rock isotope data have been recalculated to the revised crystallization ages. The Sr isotope data scatter, while the revised initial εNd values fall between +1 and +2 for the older rocks, and close to 0 for the younger intrusives along the Protogine Zone. Initial εHf in magmatic undisturbed zircons shows relatively little spread within each sample, between 2 and 4 Epsilon units, disregarding a few outliers, with average values for the 1725 to 1680 Ma rocks between +3 and +5.5 and for the 1220 Ma rocks at ca. +1.5. Covariation between initial εNd and initial εHf in the older rocks suggests either mixing between two isotopically distinct magma sources or one magma source which was isotopically heterogeneous. The isotopic signatures of the 1220 Ma intrusive rocks along the Protogine Zone are indicative of juvenile mantle input to their magmas, rather than pure crustal melting.

Three distinct Archean crustal growth events as recorded from 3.48 Ga migmatite, 2.70 Ga leucogranite, and 2.54 Ga alkali granite in the Bundelkhand Craton, Central India

Field relations, microstructures, phase petrology, elemental geochemistry, and zircon U-Pb geochronology of migmatites and (undeformed) granites (sensu lato) from the Central Bundelkhand Tectonic Zone (CBTZ) of the Bundelkhand Craton, central India are investigated to understand the age and origin of these rocks. Anatexis of felsic proto-crust produced in situ leucocratic melts with entrained residual assemblages, forming the metatexite and diatexite in the migmatite. The chemistry of biotite (XFe = 0.65–0.72) from the leucosome of migmatite supports its crystallization in a peraluminous felsic melt more likely in a syn-collisional tectonic setting. The residual amphibole in the leucosome from migmatites belongs to metaluminous (I-type) calc-alkaline protocrust source that originated in a subduction setting. Biotites (XFe = 0.80–0.81) and amphiboles (XFe = 0.84–0.87) from granites exposed proximal to the CBTZ, suggest alkaline (A-type) nature of the host magma, emplaced at mid-crustal depth (3.79–4.73 ± 0.5 kbar). Whole-rock geochemistry of migmatite reveals the magnesian nature of melt formed by differential degrees of melting of heterogeneous sources (i.e., TTG and mafic crust), at 12–14 kbar. The alkali granite was likely formed by the anhydrous melting of ancient felsic crustal rocks. U-Pb geochronology of inherited magmatic zircon cores (Th/U = 0.06–0.48) from migmatite yielded an upper intercept age of 3478 ± 48 Ma interpreted as zircon crystallization age in the protolith. The zircon rims of magmatic origin (Th/U = 0.06–0.72) grown over the 3.48 Ga old zircon cores provide an upper intercept age of 2700 ± 16 Ma; interpreted as zircon crystallization age in a leucocratic melt. This is synchronous with the timing of intense deformation/collision and the melting event in the CBTZ. Zircons (Th/U = 0.26–0.92) from alkali granite yielded a weighted mean crystallization age of 2539.9 ± 6.6 Ma without any noticeable zircon inheritance. The Bundelkhand Craton probably experienced at least two phases of collision-accretion orogens, the first one initiated at ca. 2.8 Ga with arc-related magmatism and ended with ca. 2.7 Ga collision-related felsic magmatic record. The second phase began with ca. 2.58 Ga subduction-related magmatism and culminated at ca. 2.54 Ga with a minor amount of post-collisional, alkali granite magmatism.

Geochronology of the Cordillera Real granitoids, the inner magmatic arc of Bolivia

The Cordillera Real granitoids are a suite of Triassic and Oligocene plutons in the core of the Eastern Cordillera of the Central Andes of Bolivia. Their tectonic setting, chemical and ore compositions make them part of the so-called “Inner Magmatic Arc”, which differs from the current “Magmatic Arc” located immediately to the west. U-Pb SHRIMP data were obtained in order to constrain crystallization ages. The Triassic group yielded the following results: 239±2 Ma for the Huato granite, 231±1 Ma for the Illampu granodiorite, 221±3 Ma for the Huayna Potosí granite and 223±2 Ma for the Taquesi granodiorite. For the Oligocene group we obtained ages of 27 Ma for two samples of the Quimsa Cruz granite. Secondary processes related to regional thermal anomalies and magmatic melt-enrichment, reset the K-Ar and U-Pb isotopic systems, producing: a) younger ages by Ar loss and b) anomalous data plot in the Concordia diagram by reorganization of U-Pb isotopic ratios. As noted in previous studies, most zircon analysed from the Zongo/Kuticucho Triassic granite exhibited extremely high U enrichment, producing reverse discordia curves that obscure the true crystallization age. Relatively abundant zircon inheritance was found in these “cold” granitoids, with ages suggesting provenance from early Paleozoic metapelites that also contained recycled older sources. This relatively abundant xenocrystic inheritance probably records the influence of the subduction process acting during the Gondwanide orogeny (336-205 Ma) as an overall subduction arc environment, punctuated at its final stage with the imprint of a continental rifting (245-220 Ma).

A new approach to SHRIMP II zircon U-Th disequilibrium dating

U-Th disequilibrium dating of zircon is used to determine the crystallisation age of zircon crystals which formed ≲350 ka. In this work we present a new analytical and data reduction workflow for zircon U-Th disequilibrium analysis using a Sensitive High Resolution Ion MicroProbe (SHRIMP), specifically the SHRIMP II instrument at Curtin University. This is the first zircon U-Th disequilibrium work carried out on the SHRIMP II, and the workflow was investigated with both 3 nA and 6 nA primary beam intensities, using an inverted mass run sequence (from high to low mass). The data reduction was affected by complex background corrections on the 230Th mass peak, extremely low counts on 230Th, and by a logarithmic high mass tail from shouldering mass peaks. A new approach to data reduction and a new computer program (Crayfish) written in Python is presented to address these complexities. Crayfish aids in visualisation of the raw count data from SHRIMP .pd files, allowing for more interaction with raw data during reduction, and formally propagates uncertainties from measurements to age.A case study testing the new approach was undertaken using trachytic samples from Jeju Island, South Korea, an active volcanic field and a UNESCO world heritage site. The newly derived SHRIMP crystallisation ages of 13 ± 27 to 212 ± 324 ka (2σ uncertainties) replicate measurements performed using a CAMECA IMS 1280.

Zircon U-Pb isotopic and geochemical study of metanorites from the chromite-mineralised Bacuri Mafic-Ultramafic Complex: Insights of a Paleoarchean crust in the Amapá Block, Guyana Shield, Brazil

The age and tectonic history of the chromite-mineralised Bacuri Complex, a layered mafic–ultramafic intrusion emplaced into Archean terranes of the Guyana Shield in the Amazonian Craton, are here investigated. The stratigraphy of the Bacuri Complex consists of an Ultramafic Zone composed of interlayered metaperidotites and chromitites and two mafic zones composed of metamafic rocks (amphibolites). In this study we present whole-rock lithogeochemical data integrated with zircon U-Pb isotopic and trace-element data from amphibolites from the Mafic Zone. The investigated amphibolites consist of hornblende + plagioclase ± diopside ± biotite with whole-rock compositions like noritic rocks of Archean layered intrusions. These samples are representative of amphibolites with variable proportions of hornblende and plagioclase, interpreted as resulting from amphibolite facies metamorphism of interlayered norite and leuconorite. We suggest that progressive fractionation of a primitive parental magma led to the formation of plagioclase and orthopyroxene cumulates in the mafic zones. We found that zircons from three metanorite samples comprise euhedral crystals with oscillatory zoning (interpreted to preserve a magmatic crystallisation age), as well as variably recrystallised metamorphic zircons. Their U-Pb 207Pb/206Pb ages span over 682 million years, from 3628 Ma to 2942 Ma. Trace element data suggest that magmatic zircons precipitated from late-stage magmatic fluids at ∼ 3.34 Ga (3343 ± 3.5 Ma weighted average age, n = 48, MSWD = 1.3), which defines the Bacuri Complex as one the oldest and best-preserved layered intrusions known. We show that fluid-mediated coupled dissolution-precipitation recrystallisation during metamorphism altered the composition and ages of magmatic zircons. We could not accurately define the number and age of metamorphic events in the 2942–3379 Ma age interval of metamorphic or ‘disturbed’ zircon domains. Our results indicate that continental crust was already established in the Amazonian craton in the Paleoarchean and possibly in the Eoarchean and that it has been reworked since at least the Mesoarchean.