Igneous Zircon Cores Research Articles

On the occurrence of a Variscan eclogite in the Argentera-Mercantour Massif, Western Alps: Implications for the evolution of the southern Variscan belt

The newfound Bois de Sélasse eclogite in the eastern Argentera-Mercantour Massif (External Crystalline Massifs, Western Alps) is crucial for better constraining the tectonic evolution of the southern part of the European Variscan belt. The whole-rock composition of this eclogite aligns with that of a basaltic protolith with a normal mid-oceanic-ridge affinity, and U-Pb dating on igneous zircon cores reveals an emplacement age of 524 ± 5 Ma. The emplacement may have occurred either in the oceanic lithosphere to the north of the active Gondwana margin or within a back-arc basin during the subduction beneath Gondwana. Exceptionally preserved prehnite−pumpellyite to eclogite facies minerals provide evidence of prograde metamorphism along a standard oceanic subduction geotherm (≤10 °C/km). Peak eclogite facies conditions are constrained at 610−660 °C and 1.9−2.3 GPa by thermodynamic modeling combined with Ti-in-zircon and Zr-in-rutile thermometry. A minimum age for eclogite facies metamorphism is established at 339 ± 6 Ma by U-Pb dating on metamorphic zircon rims. The protolith of the Bois de Sélasse eclogite is indeed older than the Variscan oceans, but it was similarly affected by Variscan subduction. We discuss the implications of this new finding in the context of the European Variscan belt.

An Iapetus origin for a layered eclogite complex in the northern Western Gneiss Region, Scandinavian Caledonides

AbstractThe Western Gneiss Region (WGR) is a Precambrian basement domain in the Scandinavian Caledonides and one of the world's largest high‐ and ultrahigh‐pressure terranes. The south–central WGR underwent regional eclogite facies metamorphism 415–400 Ma ago when Baltica subducted beneath Laurentia, during the Scandian orogeny. Eclogites in the WGR group into two traditional types: (1) Precambrian mafic intrusions metamorphosed in situ during Scandian continental subduction and (2) eclogites, garnet peridotites and garnet pyroxenites within ultramafic complexes derived from the subcontinental mantle beneath Laurentia. We document, using field relations, petrography, whole‐rock geochemistry and secondary ion mass spectrometry (SIMS) zircon geochronology, a hitherto unrecognized third type of eclogite in the WGR that places new constraints on its tectonic architecture: an eclogitized fragment of oceanic crust from the Iapetus Ocean. The Kråkfjord eclogite complex is a km2‐sized body with an interior consisting of kyanite eclogite (meta‐troctolite) and subordinate layers and lenses of garnet peridotite, garnet websterite and kyanite–garnet leucotonalite. This interior is capped by Fe–Ti‐rich eclogite, which locally contains subordinate pockets of migmatitic aluminous gneiss. The elemental abundances and isotopic compositions of the Fe–Ti‐rich eclogites resemble those of mid‐ocean ridge basalt (MORB). In contrast, the interior kyanite eclogites, peridotites and pyroxenites have compositions similar to the gabbroic cumulates in the lower oceanic crust of slow‐spreading ridges. U–Pb SIMS dating of igneous zircon cores from a leucotonalite pod in the interior of the Kråkfjord complex yields Cambro‐Ordovician igneous ages of 500–440 Ma, with the ~500 Ma age interpreted as the isotopically undisturbed age. This age matches those of Iapetan oceanic rocks exposed elsewhere in the mountain belt. Metamorphic zircon from an Fe–Ti‐rich eclogite in the carapace of the Kråkfjord complex dates the eclogite facies metamorphism at 421.9 ± 2.2 Ma, synchronous with the continental collision. Zircon from a leucosome in Fe–Ti‐rich retro‐eclogite indicates an age of 408.5 ± 2 Ma for the crystallization of partial melt following the decompression. Detrital zircon core ages from a pocket of aluminous migmatitic gneiss in the carapace indicate derivation of sediment from the Baltic crust. Collectively, the data show that the eclogite complex (1) originated at an Iapetus spreading centre near the continent Baltica, (2) subducted to eclogite conditions during Scandian continental collision and (3) was tectonically intercalated with the Precambrian Baltica basement of the WGR.

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.

Mesoarchaean (2820 Ma) high-pressure mafic granulite at Uauá, São Francisco Craton, Brazil, and its potential significance for the assembly of Archaean supercratons

High pressure (HP) granulites of regional scale form as a result of tectonic events that lead to crustal thickening or subduction of the crust into the mantle. Most HP granulites are Phanerozoic, a few are Proterozoic, and Archaean HP granulites are even scarcer. Here we present field relationships, mineral chemistry and zircon U-Pb ages, Hf isotope data and trace elements data for the mafic granulite and associated rocks of the Uauá terrane, São Francisco Craton, Brazil, as evidence for the likely existence of a thick continental crust in the Mesoarchaean/Neoarchaean transition. The HP mafic granulite occurs as lensoid bodies within shallow dipping diorite to leucodiorite gneisses. Small scale layering between mafic granulite and diorite gneiss indicate these rocks are cogenetic. Garnet-clinopyroxene pairs with quartz, zircon, ilmenite, plagioclase, and clinopyroxene inclusions in garnet characterize the HP assemblage. Garnet porphyroblasts also show opx-cpx-plag symplectite coronas, which coupled with hornblende and plagioclase define PT conditions to lower grade granulite and amphibolite facies. Microprobe data combined with phase equilibria modelling (pseudosections) indicate 16–18 kbar and 930–960 °C for the peak HP assemblage, and 6.2–7.0 kbar and 660–760 °C for lower pressure granulite to amphibolite facies symplectite coronas. Metamorphic zircon rims in equilibrium with garnet have SHRIMP ages of 2819 ± 14, and igneous zircon cores of 3127 ± 14 Ma. The cores of zircon in associated gneiss samples have U-Pb ages between 3090 ± 13 Ma (LA-ICP-MS) and 3125 ± 15 Ma (SHRIMP) with age cluster at 3120 ± 6 Ma. εHf(t) values on igneous zircon of the HP mafic granulite are slightly positive whereas metamorphic zircon rims are negative. The associated diorite gneiss invariably yielded negative zircon εHf(t) values. Trondhjemite sheets intrusive in the mafic granulite are ca. 20 m.y. younger (2794 ± 13 Ma) than the host granulite. We interpret the igneous protoliths of the mafic granulite and leucodiorite gneiss as a single igneous complex emplaced in the continental crust, later deformed and metamorphosed by contraction and crustal thickening during collision of blocks/cratons to form Earth's first supercratons by the end of the Mesoarchaean.

High-spatial resolution dating of monazite and zircon reveals\xa0the timing of subduction\u2013exhumation of the Vaimok Lens in the Seve Nappe Complex (Scandinavian Caledonides)

In-situ monazite Th–U–total Pb dating and zircon LA–ICP–MS depth-profiling was applied to metasedimentary rocks from the Vaimok Lens in the Seve Nappe Complex (SNC), Scandinavian Caledonides. Results of monazite Th–U–total Pb dating, coupled with major and trace element mapping of monazite, revealed 603 ± 16 Ma Neoproterozoic cores surrounded by rims that formed at 498 ± 10 Ma. Monazite rim formation was facilitated via dissolution–reprecipitation of Neoproterozoic monazite. The monazite rims record garnet growth as they are depleted in Y2O3 with respect to the Neoproterozoic cores. Rims are also characterized by relatively high SrO with respect to the cores. Results of the zircon depth-profiling revealed igneous zircon cores with crystallization ages typical for SNC metasediments. Multiple zircon grains also exhibit rims formed by dissolution–reprecipitation that are defined by enrichment of light rare earth elements, U, Th, P, ± Y, and ± Sr. Rims also have subdued Eu anomalies (Eu/Eu* ≈ 0.6–1.2) with respect to the cores. The age of zircon rim formation was calculated from three metasedimentary rocks: 480 ± 22 Ma; 475 ± 26 Ma; and 479 ± 38 Ma. These results show that both monazite and zircon experienced dissolution–reprecipitation under high-pressure conditions. Caledonian monazite formed coeval with garnet growth during subduction of the Vaimok Lens, whereas zircon rim formation coincided with monazite breakdown to apatite, allanite and clinozoisite during initial exhumation.

In situ zircon U-Pb dating and whole-rock geochemistry of metasedimentary rocks from South Liaohe Group, Jiao-Liao-Ji orogenic belt: Constraints on the depositional and metamorphic ages, and implications for tectonic setting

The Liaohe Group is an important Paleoproterozoic lithostratigraphic unit of the Jiao-Liao-Ji orogenic belt and traditionally subdivided into the North and South Liaohe Groups. However, their tectonic setting and evolution are still a matter of debate. The precise depositional and metamorphic data of the studied metasedimentary rocks from the South Liaohe Group provide important information on the controversial discussion of the tectonic setting and evolution. Based on cathodoluminescence (CL) imaging and connecting LA-ICP-MS U-Pb geochronology, zircon grains separated from metasedimentary rocks of different formations of the South Liaohe Group were subdivided into detrital and metamorphic zircons or zircon domains. Igneous zircon cores of detrital zircons of the Li’eryu, Gaojiayu, Dashiqiao and Gaixian Formations yield U-Pb age peaks at ∼2130 Ma, ∼2520 Ma, ∼2160 Ma/∼2500 Ma, and ∼2040 Ma, indicating that their sediments were transported from the Liaoji Granitoids, the Archean basement rocks of the adjacent Complex. In addition, deposition age of the sedimentary precursors was investigated which must have taken place at some time after ∼2.05 Ga, ∼2.07 Ga, ∼2.04 Ga and ∼1.92 Ga, respectively. These new geochronological data can be well explained by (1) a continent-arc collision model which suggests that the South Liaohe Group formed an intervening island arc and foreland basin sandwiched between the Longgang and Nangrim Complexes, and (2) a northward subduction of the Nangrim Complex source rocks of the South Liaohe Group; these source rocks were successively deposited along the margin of the two Complexes.

Trace element features of hydrothermal and inherited igneous zircon grains in mantle wedge environment: A case study from the Myanmar jadeitite

Jadeitites are considered to crystallise in ultramafic rocks in the subduction channel presumably from the overlying mantle wedge, and therefore zircons from these rocks provide important insights into mantle wedge processes. Here we investigate hydrothermal zircon (Group II) formed within a subduction zone and compare these with the igneous zircon cores (Group I) from the Myanmar jadeitite. Previous U–Pb studies reported ages of Groups I and II zircons as ~163Ma, and ~147Ma respectively, and both show isotope signature of the depleted mantle. Group I zircons have much higher total concentrations of rare earth elements (REEs) (500–1945ppm) than those of Group II zircon (112–307ppm), and contains relatively higher abundance of Y, Nb, Ta, Ti, Th and U with higher (Sm/La)N ratios (25.3–501) and Ce-anomalies (8.04–140) but lower (Yb/Gd)N ratios (9.76–57.0) than those of the Group II ( (Sm/La)N ratios=2.12–32.2, Ce-anomalies=1.63–19.6, (Yb/Gd)N ratios=44.8–142). Hf concentrations are broadly similar in both Groups. The Group I zircons are considered to be magmatic and crystallised from H2O–rich basaltic melt at relatively high pressure in the mantle wedge, whereas the Group II zircon overgrowth took place through recrystallisation and precipitation with distinct dissolution of the Group I zircons. Variation in the concentration of trace elements in zircons from Groups I to II in the mantle wedge is related to an intra-oceanic subduction system in the presence of Na-rich hydrothermal fluids under high-pressure and low-temperature. The Ti-in-zircon thermometer yield a mean crystallisation temperature of 742±141°C for Group I zircons, whereas the Group II zircons yield 339±33°C. The two groups of zircons also provide insights into the probable protolith involved in formation of the Myanmar jadeitite.

Hallandian 1.45 Ga high-temperature metamorphism in Baltica: P–T evolution and SIMS U–Pb zircon ages of aluminous gneisses, SW Sweden

The southernmost Baltic Shield exposes polymetamorphic continental crust that was largely formed and accreted during a series of 1.92–1.66Ga Paleoproterozoic orogenic events and later reworked during the 1.14–0.90Ga Sveconorwegian orogeny. An intermediate period of metamorphism, deformation and magmatism at 1.47–1.38Ga has been attributed to the Hallandian orogeny, but due to overprinting by Sveconorwegian high-grade metamorphism and deformation, the P–T-t evolution and deformation of the Hallandian event have remained obscure. This study presents the first quantitative P–T model of the Hallandian event using high-temperature aluminous gneisses in the south-easternmost marginal part of the Sveconorwegian orogen. The high-grade metamorphism and spatially associated granite magmatism are dated using U–Pb SIMS analysis of zircon. Petrography, bulk and mineral geochemistry, and pseudosection models demonstrate prograde staurolite-sillimanite-grade metamorphism reaching granulite-facies temperatures (700–750°C) at low pressures (4–5kbar), with the formation of Crd+Sil+Grt+K-fsp+Ilm+Melt±Bt. The rocks followed a clockwise P–T path. Later stages involved the formation of sillimanite+biotite at the expense of garnet and cordierite. Local low-temperature and fluid-assisted retrogression also caused the formation of chlorite and muscovite at the expense of cordierite. Both granite and aluminous gneisses contain complex zircon with inherited 1.70Ga igneous cores and high-U, secondary zircon, mainly formed by reworking of protolith cores. The latter date the Hallandian high-grade metamorphism at 1451±6Ma and the granite magmatism at 1445±8Ma. The presence of 1.70Ga igneous zircon cores in both metamorphic and magmatic rocks suggests that they formed from similar protoliths. The protolith ages correlate with the youngest generation of magmatic rocks of the Transscandinavian Igneous Belt. The aluminous gneisses are of supracrustal origin, and may have formed by chemical alteration of magmatic rocks. Hallandian regional metamorphism took place under a strongly elevated geotherm and was associated with granitic magmatism, suggesting an accretionary orogenic setting. The Hallandian event may demonstrate an 1.47–1.38Ga Andean-type continental margin at the SW margin of Baltica.

Temperature–time evolution of the Assynt Terrane of the Lewisian Gneiss Complex of Northwest Scotland from zircon U-Pb dating and Ti thermometry

The Lewisian Gneiss Complex of Northwest Scotland is a classic Precambrian basement gneiss complex. The Lewisian is divided into a number of terranes on the basis of structural, metamorphic and geochronological evidence. The most well-studied of these is the Assynt Terrane, which forms the central part of the Lewisian outcrop on the Scottish mainland. Field evidence shows that it has a complex tectonothermal history, the early stages of which remain poorly constrained. This paper sets out to better understand the chronology and thermal evolution of the Assynt Terrane through zircon U-Pb dating and Ti-in-zircon thermometry, the latter applied to the Lewisian for the first time. This is placed in context by integration with detailed field mapping, sample petrography, zircon cathodoluminescence (CL) imaging and rare earth element (REE) analysis.Zircons from six tonalite-trondhjemite-granodiorite (TTG) gneiss samples and two metasedimentary gneiss samples were analysed. The TTG gneisses were predominantly retrogressed to amphibolite-facies; zircons showed a range of CL zoning patterns and REE profiles were similar to those expected for magmatic zircon grains. Zircons from the metasedimentary gneisses also displayed a range of CL zoning patterns and are depleted relative to chondrite in heavy REEs due to the presence of garnet.Zircon analysis records a spread of concordant U-Pb ages from ∼2500 to 3000Ma. There is no evident correlation of ages with location in the crystal or with CL zoning pattern. A weighted average of 207Pb/206Pb ages from the oldest igneous zircon cores from the TTG gneiss samples gives an age of 2958±7Ma, interpreted to be a magmatic protolith crystallisation age. A weighted average of 207Pb/206Pb ages of the youngest metamorphic rims yields an age of 2482±6Ma, interpreted to represent the last high-grade metamorphism to affect these rocks. Ti-in-zircon thermometry records minimum temperatures of 710–834°C, interpreted to reflect magmatic crystallisation.REE profiling enabled the zircons in the metasedimentary rocks to be linked to the presence of metamorphic garnet, but resetting of U-Pb systematics precluded the determination of either protolith or metamorphic ages. Zircons from the metasedimentary gneisses generally record higher minimum temperatures (803–847°C) than the TTG gneisses, interpreted to record zircon crystallisation in an unknown protolith.

1.8 billion years of fluid–crust interaction: A zircon oxygen isotope record for the lower crust, western Churchill Province, Canadian Shield

The western Churchill Province of the Canadian Shield experienced a prolonged and complex formation history (ca. 4.04 to 1.70Ga), with evidence for multiple episodes of orogenesis and regional magmatic activity. Here we report on the oxygen isotopic compositions of garnet and zircon recovered from lower crustal xenoliths, which have U–Pb ages between ca. 3.5 and 1.7Ga. Overall, zircon from four metabasite xenoliths from the Rankin Inlet sample suite have δ18O values ranging from +5.5 to +8.6‰. Zircon from three metatonalite/anorthosite xenoliths and five metabasite xenoliths from the Repulse Bay sample suite have δ18O values of +5.6 to +8.3‰. High δ18O values (>+6.0‰) for the oldest igneous zircon cores (ca. 3.5Ga and 3.0–2.6Ga) indicate that their metatonalite/anorthosite protolith magmas were generated from, or had assimilated, supracrustal rocks that interacted previously with surface-derived fluids. Igneous zircon cores (ca. 2.9–2.6Ga) from one metabasite xenolith have δ18O values of +5.6 to +6.4‰, which suggests a formation from a mantle-derived basaltic/gabbroic magma. Metamorphic zircon cores (ca. 2.0–1.9Ga) from one metabasite xenolith commonly have δ18O values between +6.0 and +6.3‰, which is indicative of a basalt/gabbro protolith and localized reworking of the lower crust caused by regional-scale plate convergence. The wide range of δ18O values (+5.5 to +8.3‰) for ca. 1.75–1.70Ga metamorphic zircon rims (identified in all xenoliths) indicates regional transient heating and reworking of mantle- and supracrustal-derived crust, induced by magmatic underplating along the crust–mantle boundary.

Geology, age and field relations of Hadean zircon-bearing supracrustal rocks from Quad Creek, eastern Beartooth Mountains (Montana and Wyoming, USA)

Quad Creek Paleoarchean (≤3250Ma) quartzites in southern Montana host Hadean (pre-3850Ma) detrital zircons. Although an accessible resource for investigating early Earth processes distinct from other better known ancient zircon localities, the outcrop-scale geological and geochemical context of these rocks has not previously been well documented. New (1:250) mapping reveals a varied suite of isoclinally folded, sheared and variably deformed chromite-bearing banded and massive quartzites, garnetiferous siliceous (migmatitic) paragneisses, amphibolite, quartz–biotite schists and quartz+magnetite rocks (banded iron-formation; BIF). Conventional ion microprobe U–Pb zircon ages of populations from different quartzites and a paragneiss show outgrowth rim ages on older inherited detrital igneous zircon cores that match documented regional metamorphic events evidenced elsewhere in the Wyoming Craton. Weighted mean 207Pb/206Pb ages for the youngest concordant zircon cores of igneous derivation indicate the Quad Creek sediments were deposited by about 3250Ma. Coupled with a large zircon 207Pb/206Pb age survey (n=1274), and an extended U–Th–Pb depth-profile of the oldest grain in our sample set, these data support the notion that the oldest crust tapped by these sediments was comparable in age to the ca. 4000Ma Acasta Gneiss Complex. This similarity is suggestive of both of a linkage between the Wyoming Craton and the Western Slave Province, and the lingering influence of Hadean crust well into the Archean.

Recrystallization rims in zircon (Valle d'Arbedo, Switzerland): An integrated cathodoluminescence, LA-ICP-MS, SHRIMP, and TEM study

Recrystallization rims are a common feature of zircon crystals that underwent metamorphism. We present a microstructural and microchemical study of partially recrystallized zircon grains collected in polymetamorphic migmatites (Valle d’Arbedo, Ticino, Switzerland). The rims are bright in cathodoluminescence (CL), with sharp and convex contacts characterized by inward-penetrating embayments transgressing igneous zircon cores. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data and transmission electron microscopy (TEM) imaging indicate that the rims are chemically and microstructurally different from the cores. The rims are strongly depleted in REE, with concentrations up to two orders of magnitude lower than in the cores, indicating a significant loss of REE during zircon recrystallization. Enrichment in non-formula elements, such as Ca, has not been observed in the rims. The microstructure of zircon cores shows a dappled intensity at and below the 100 nm scale, possibly due to radiation damage. Other defects such as pores and dislocations are absent in the core except at healed cracks. Zircon rims are mostly dapple-free, but contain nanoscale pores and strain centers, interpreted as fluid inclusions and chemical residues, respectively. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages show that the recrystallization of the rims took place &gt;200 Ma ago when the parent igneous zircon was not metamict. The chemical composition and the low-Ti content of the rims indicate that they form at sub-solidus temperatures (550-650 °C). Recrystallization rims in Valle d’Arbedo zircon are interpreted as the result of the migration of chemical reaction fronts in which fluid triggered in situ and contemporaneous interface-coupled dissolution-reprecipitation mechanisms. This study indicates that strong lattice strain resulting from the incorporation of a large amount of impurities and structural defects is not a necessary condition for zircon to recrystallize. Our observations suggest that the early formation of recrystallization rims played a major role in preserving zircon from the more recent Alpine metamorphic overprint.

Zircon U-Pb isotope, 18O and trace element response to 80 m.y. of high temperature metamorphism in the lower crust: Sluggish diffusion and new records of Archean craton formation

Coordinated cathodoluminescence (CL) imaging and ion microprobe (SHRIMP and CAMECA 1280) analysis document micron-scale U-Pb-O isotope and trace element zoning in zircons from deep crust exposed to 80 m.y. of high temperature and pressure metamorphism. Three, along-strike paragneiss samples across the amphibolite to granulite facies transition in the Kapuskasing Uplift crustal cross-section in the Archean Superior province yield detrital, originally igneous zircon cores overgrown by progressively larger volumes of metamorphic zircon with increasing grade. The cores generally retain primary age (2.85±0.03 to 2.67±0.02 Ga), oxygen isotope (5.1 to 7.0‰) and trace element compositions similar to those reported for magmatic arc sources. Dark CL, metamorphic zircon rims record nearly continuous overgrowth events for ∼80 m.y. from 2.66±0.01 to 2.58±0.01 Ga during uppermost amphibolite to granulite facies regional metamorphism. These rims have significantly higher δ18O values (8.4 to 10.4‰) and trace element compositions quite distinct from those of the cores; these differences indicate that their δ18O and trace element compositions were not inherited from the igneous cores, consistent with extensive textural evidence for rim formation as metamorphic overgrowths. Multi-spot traverses record steep oxygen isotope discontinuities (4‰ over <10 μm) at core-rim boundaries, confirming the extremely sluggish rates of volume diffusion of O in non-metamict zircon during extended (∼80 m.y.) granulite-grade metamorphism (peak T=750-800 °C) at substantial f(H2O) but water-undersaturated (fluid-absent) conditions. Likewise no evidence of significant diffusive exchange of δ18O could be detected along deformation microstructures such as annealed fractures in cores infilled with high δ18O zircon. Application of simple diffusion models to detailed δ18O profiles in a large number of zircon grains constrain maximum values of the diffusivity of oxygen in zircon (logDZrcox) to the range −27.5 to −26.4 m2/s. For the estimated 80 m.y. and 700 to 800 °C time-T window of rim formation, these maximum values are similar to or slower than values reported by [Page and others (2007][1], [2010)][2] and the experimentally-determined “dry” diffusivity of oxygen in zircon ([Watson and Cherniak, 1997][3]), but are markedly slower than the experimentally-determined “wet” diffusivity of oxygen in zircon ([Watson and Cherniak, 1997][3]). Fast diffusion of oxygen in zircon predicted by hydrothermal experiments may, in nature, require the presence of a hydrous fluid rather than a threshold value of f(H2O). Our test demonstrates that unrecrystallized metamorphosed igneous zircons and metamorphic zircons will retain the geochemical (U-Pb age, trace element and δ18O) record of their origin and evolution despite prolonged, high-grade metamorphism at significant f(H2O) but water under-saturated (fluid-absent) conditions. Such zircons, particularly those that exhibit δ18O zoning, are micron-scale records for the T-time-fluid interaction history of deep crustal rocks. Such records will not be preserved in less refractory phases and promise new insights into the processes of continent formation and evolution. [1]: #ref-64 [2]: #ref-65 [3]: #ref-96

A comparison of U–Pb and Hf isotopic compositions of detrital zircons from the North and South Liaohe Groups: Constraints on the evolution of the Jiao-Liao-Ji Belt, North China Craton

The Jiao-Liao-Ji orogenic belt is one of the important Paleoproterozoic mobile belts in the North China Craton (NCC) and its central segment consists of the North and South Liaohe Groups and the Liaoji Granitoids. The North and South Liaohe Groups consist of sedimentary and volcanic successions metamorphosed from greenschist to lower amphibolite facies, and the Liaoji granitoids are divisible into 2.2–2.1 Ga (pre-tectonic) monzogranitic gneisses and 1.88–1.85 Ga (post-tectonic) porphyritic monzogranites and alkaline syenites. Combined with cathodoluminescence (CL) imaging, we report LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry) U–Pb and Hf isotopic data for detrital zircons from the North and South Liaohe Groups. Zircons grains from both groups have igneous zircon cores that yielded two U–Pb age populations at 2.0–2.2 Ga (major) and ∼2.5 Ga (minor), indicating that their provenances were similarly dominated by Paleoproterozoic Liaoji Granitoids and late Archean basement rocks. Some structureless zircon rims in both groups yield concordant or upper intercept ages ∼1.9 Ga, interpreted as the peak metamorphic age of the two groups. These results suggest that both groups were deposited in the period 2.0–1.9 Ga, shortly after the emplacement of the Liaoji Granitoids. Moreover, zircons from both groups have similar Hf model age peaks ( T DM C peaks at 2.66–2.75 Ga and 2.80–3.00 Ga) and ɛHf values (−5 to +9 and −7 to +5). Based on these striking similarities, we propose that the protoliths of the North and South Groups may have formed simultaneously, and that the basement rocks underneath the two groups belong to the same Archean continental block rather than two different blocks as previously considered. The Archean basement underwent Paleoproterozoic rifting, accompanied by the emplacement of the Liaoji granitoids at 2.2–2.1 Ga and the deposition of the North and South Liaohe Groups at 2.1–1.9 Ga, and the rift was subsequently closed by ∼1.9 Ga.

Creation of a continent recorded in zircon zoning

We have discovered a robust microcrystalline record of the early genesis of North American lithosphere preserved in the U-Pb age and oxygen isotope zoning of zircons from a lower crustal paragneiss in the Neoarchean Superior province. Detrital igneous zircon cores with δ 18 O values of 5.1‰–7.1‰ record creation of primitive to increasingly evolved crust from 2.85 ± 0.02 Ga to 2.67 ± 0.02 Ga. Sharp chemical unconformity between cores and higher δ 18 O (8.4‰–10.4‰) metamorphic overgrowths as old as 2.66 ± 0.01 Ga dictates a rapid sequence of arc unroofing, burial of detrital zircons in hydrosphere-altered sediment, and transport to lower crust late in upper plate assembly. The period to 2.58 ± 0.01 Ga included ∼80 m.y. of high-temperature (∼700–650 °C), nearly continuous overgrowth events reflecting stages in maturation of the subjacent mantle root. Huronian continental rifting is recorded by the youngest zircon tip growth at 2512 ± 8 Ma (∼ 600 °C) signaling magma intraplating and the onset of rigid plate behavior. This >150 m.y. microscopic isotope record in single crystals demonstrates the sluggish volume diffusion of U, Pb, and O in zircon throughout protracted regional metamorphism, and the consequent advances now possible in reconstructing planetary dynamics with zircon zoning.

U‐Pb Age and Hf Isotope Study of Detrital Zircons from the Wanzi Supracrustals: Constraints on the Tectonic Setting and Evolution of the Fuping Complex, Trans‐North China Orogen

Abstract: Located in the middle segment of the Trans‐North China Orogen, the Fuping Complex is considered as a critical area in understanding the evolution history of the North China Craton (NCC). The complex is composed of various high‐grade and multiply deformed rocks, including gray gneiss, basic granulite, amphibolite, fine‐grained gneiss and marble, metamorphosed to upper amphibolite or granulite facies. It can be divided into four rock units: the Fuping TTG gneisses, Longquanguan augen gneisses, Wanzi supracrustals, and Nanying granitic gneisses. U‐Pb age and Hf isotope compositions of about 200 detrital zircons from the Wanzi supracrustals of the Fuping Complex have been analyzed. The data on metamorphic zircon rims give ages of 1.82‐1.84 Ga, corresponding to the final amalgamation event of the NCC, whereas the data for igneous zircon cores yield two age populations at ∼2.10 and ∼2.51 Ga, with some inherited ages scattering between 2.5 and 2.9 Ga. These results suggest that the Wanzi supracrustals were derived from the Fuping TTG gneisses (∼2.5 Ga) and the Nanying granitic gneisses (2.0–2.1 Ga) and deposited between 2.10 and 1.84 Ga. All zircons with ∼2.51 Ga age have positive initial ∍Hf values from +1.4 to +10.9, suggesting an important crustal growth event at ∼2.5 Ga through the addition of juvenile materials from the mantle. The Hf isotope data for the detrital zircons further imply that the 2.8 Ga rocks are important components in the lower crust, which is consistent with a suggestion from Nd isotope data for the Eastern Block. The zircons of 2.10 Ga population have initial ∍Hf values of −4.9 to +6.1, interpreted as mixing of crustal re‐melt with minor juvenile material contribution at 2.1 Ga. These results are distinct from that for the Western Block, supporting that the Fuping Complex was emplaced in a tectonic active environment at the western margin of the Eastern Block.

Anorthosites in the Eastern Granulites of Tanzania—New SIMS zircon U–Pb age data, petrography and geochemistry

Several occurrences of anorthosites are known in the Neoproterozoic Mozambique Belt in Tanzania. These are tectonically incorporated into a suite of enderbitic rocks and migmatitic orthogneisses of the Eastern Granulites. Two larger anorthosite bodies and associated rocks from the Pare Mountains and the Uluguru Mountains have been chosen for a comparative study regarding their formation age, age of subsequent metamorphic processes, mineral chemistry and rock chemistry. All the investigated magmatic bodies were overprinted by Neoproterozoic high-pressure granulite facies metamorphism and deformation, documented by similar petrography and mineralogy, metamorphic textures and deformational characteristics. However, mineral chemistry, geochemistry and geochronology reveal major differences between the two occurrences. The Pare anorthosite contains igneous zircons that yield Archean formation ages of ca. 2.64 Ga and a more calcic mineral and rock chemistry that is typical for Archean-type anorthosites. Extremely narrow metamorphic rims around zircons could not be dated but may have grown during the Neoproterozoic metamorphic overprint. In contrast, the Uluguru anorthosite contains igneous zircon cores with U/Pb ages of 880–820 Ma. A broad metamorphic rim was dated at ca. 640–650 Ma. The adjacent migmatitic basement gneisses yield formation ages of ca. 986 Ma followed by Pb-loss. Early Neoproterozoic events can also be inferred from both anorthosite suite and basement samples of the Mahenge Mountains. There U/Pb ages from zircon cores cluster around 730–800 Ma. Well-developed metamorphic rims yield Neoproterozoic ages of 650 Ma. The basement gneisses are related to a destructive plate margin setting. The anorthosites may have formed when parts of Central Madagascar rifted off East Africa at the time considered.

Sm-Nd and zircon SHRIMP U-Pb dating of Huilanshan mafic granulite in the Dabie Mountains and its zircon trace element geochemistry

The mafic granulites from Huilanshan are outcropped on the center of the Luotian dome in the northern Dabie Mountains. The Sm-Nd isochron defined by granulite-facies metamorphic minerals (garnet+clinopyroxene+hypersthene) yields an age of 136 ± 18 Ma indicating the early Cretaceous granulite-facies metamorphism. The cathodoluminescence (CL) images of zircons from the granulite show clearly core-mantle-rim structures. The zircon cores are characterized by typical oscillatory zoning and highly HREE enriched patterns, which suggests their magma origin. Some zircon cores among them with little Pb loss give SHRIMP U-Pb ages ranging from 753 to 780 Ma, which suggests that the protolith of Huilanshan granulite is Neoproterozoic mafic rocks. The zircon mantles usually cut across the oscillatory zone of the zircon cores have 3-10 times lower REE, Th, U, Y, Nb and Ta contents than the igneous zircon cores but have high common Pb contents. These characteristics suggest that they were formed by hydrothermal alteration of the igneous zircons. The part of zircon mantles with little Pb loss give a similar SHRIMP U-Pb age (716-780 Ma) to the igneous zircon cores, which implies that the hydrothermal events occurred closely to the magmatic emplacement. In view of the strong early Cretaceous magmatism in the Luotian dome, consequently, the Huilanshan mafic granulite was formed by heating of the Neoproterozoic mafic rocks in mid-low crust, which caused the granulite-facies metamorphism underneath the Dabie Mountains. The similarity between the granulite metamorphic age (136±18 Ma) defined by Sm-Nd isochron and K-Ar age of 123-127 Ma given by amphible from the gneiss in Luotian dome suggests a rapid uplifting of the Luotian dome, which may result in further exhumation of the ultrahigh pressure metamorphic rocks in the Dabie Mountains.

A SHRIMP U–Pb and LA-ICP-MS trace element study of the petrogenesis of garnet–cordierite–orthoamphibole gneisses from the Central Zone of the Limpopo Belt, South Africa

The Central Zone of the Limpopo Belt (South Africa) underwent high-grade metamorphism at ∼2.7–2.5 and ∼2.03 Ga. Quartz-rich, garnet-, cordierite-, biotite- and orthoamphibole-bearing, feldspar-free gneisses from the western Central Zone reached granulite-facies conditions (∼800 °C at ∼8–10 kbar) followed by decompression. Garnet from one such sample shows significant zonation in trace elements but little zonation in major elements. Zoning patterns suggest that the early prograde breakdown of REE-rich accessory phases contributed to the garnet trace element budget. Monazite from the sample yields a SHRIMP weighted mean 207Pb– 206Pb age of 2028 ± 3 Ma, indistinguishable from a SHRIMP zircon age of 2022 ± 11 Ma previously measured on metamorphic overgrowths on ∼2.69 Ga igneous zircon cores. New zircon and monazite formed before, or at, the metamorphic peak, and occur as inclusions in garnet. Monazite appears to have formed through the breakdown of early allanite ± xenotime ± apatite. Trace element zoning patterns in garnet and the age of accessory phases are most consistent with a single tectonometamorphic event at ∼2.03 Ga. The plagioclase and K-feldspar-free composition of the garnet–cordierite–orthoamphibole gneisses requires open system processes such as intense hydrothermal alteration of protoliths or advanced chemical weathering. In the studied sample, the ∼2.69 Ga igneous zircons show a prominent negative Eu anomaly, suggesting equilibrium with plagioclase, or plagioclase fractionation in the precursor magma. In contrast, the other minerals either show small negative (∼2.03 Ga monazite), no (∼2.02 Ga zircon and garnet) or positive Eu anomalies (orthoamphibole). This suggests that the unusual bulk compositions of these rocks were set in after ∼2.69 Ga but before the peak of the ∼2.03 Ga event, most probably while the protoliths resided at shallow or surficial crustal levels.

Exsolution of thortveitite, yttrialite, and xenotime during low-temperature recrystallization of zircon from New Caledonia, and their significance for trace element incorporation in zircon

Recrystallized zircon grains in a phengite-epidote-chlorite schist from north-eastern New Caledonia contain as inclusions a mineral assemblage consisting of celadonite, kaolinite, quartz, Fe-oxy-hydroxide, smectite, chlorite, xenotime-(Y), thortveitite, yttrialite, and allanite-(Ce). This assemblage formed during low-temperature (&lt;100 °C) seafloor alteration of a plagioclase-rich mafic rock and represents the first documented evidence of this alteration event in the high-P belt of northeastern New Caledonia. The survival of this low-temperature mineral paragenesis in zircon of a rock that has undergone subsequent eclogite-facies metamorphism testifies to the strength and value of zircon as a container for mineral inclusions. Thortveitite (Sc 2 Si 2 O 7 ) and yttrialite (Y 2 Si 2 O 7 ) inclusions in the altered zircon cores represent a new occurrence for these minerals and suggest that they may be more common than is currently recognized. The altered zircon cores generally have lower trace-element contents than the pristine igneous zircon cores and we suggest that thortveitite, yttrialite, and xenotime-(Y) formed as a result of trace-element expulsion from zircon during low-temperature recrystallization. Trace-element concentrations in the zircon cores indicate that trivalent cations (REE, Y, Sc) in zircon cannot be charge-balanced by xenotime substitution alone. Pentavalent cation concentrations (Nb, As) are also insufficient for charge balance. The presence of thortveitite (Sc 2 Si 2 O 7 ) and yttrialite (Y 2 Si 2 O 7 ) suggests that trivalent cations in zircon might be charge-balanced either by monovalent anions substituting for oxygen or by small monovalent cations such as H occurring interstitially in the zircon lattice.