U-Pb age and genetic significance of heterogeneous zircon populations in rocks from the Favourable Lake area,Northwestern Ontario

The study evaluates the relationships between measured U-Pb ages and zircon characteristics of five morphologically, texturally, and isotopically complex zircon populations and compares the zircon development stages to the orogenic evolution of the Favourable Lake area. Two distinct zircon types from a hornblendite xenolith in a granitoid batholith of the Sachigo subprovince of the Superior Province yield U-Pb ages of 2729.0±6.8 Ma and 2714.8 _–6.4 ^+7.4 Ma, which date specific metamorphic phases coinciding with major plutonic pulses in the batholith. Zircons from a metamorphosed felsic dike, crosscutting the hornblendite, consist of an old zircon component with a minimum age of 2788 Ma possibly reflecting igneous crystallization 2950 Ma ago, and a younger component with an inferred age of 2725±15 Ma, probably reflecting metamorphism during batholith emplacement.In the Berens River subprovince to the south, granodiorite forms both a late tectonic phase in a large batholith and a post-tectonic pluton intruded into the batholith, yet zircons from these granodiorites have identical ages of 2697.3±1.7 Ma and 2696.2±1.2 Ma, respectively. The late tectonic granodiorite also contains inherited zircons with a minimum age of 2767 Ma which are indirect evidence for the presence of old sialic crust in this subprovince. Zircons from a sheared monzonite near the boundary fault between the two subprovinces yield an upper intercept age of 2769 _–26 ⁺⁶³ Ma, which we interpret as the intrusion age of the monzonite. This rock is older than most dated units in the surrounding batholiths and suggests that the boundary is a long-lived Archean structure.A lower intercept age of about 1750 Ma for zircons of the hornblendite is the result of chemical alteration of the zircons. This, and a similar lower intercept age shown by the sheared monzonite zircons, are thought to reflect increased fluid activity and possibly shearing during the Early Proterozoic Hudsonian orogeny which occurred in the Churchill Province to the northwest. A later Pb-loss mainly from near-surface domains of the zircons is indicated by lower intercept ages of about 500–100 Ma.Publication approved by the Director, Ontario Geological Survey     

U–Pb and Hf-isotope analysis of zircons in mafic xenoliths from Fuxian kimberlites: evolution of the lower crust beneath the North China craton

Mafic xenoliths from the Paleozoic Fuxian kimberlites in the North China craton include garnet granulite, and minor pyroxene amphibolite, metagabbro, anorthosite and pyroxenite. The formation conditions of the amphibolites are estimated at 745–820 °C and 7.6–8.8 Kb (25–30 km); the granulites probably are derived from greater depths in the lower crust. LAM-ICPMS U–Pb dating of zircons from four granulites reveals multiple age populations, recording episodes of magmatic intrusion and metamorphic recrystallisation. Concordant ages and upper intercept ages, interpreted as minimum estimates for the time of magmatic crystallisation, range from 2,620 to 2,430 Ma in three granulites, two amphibolites and two metagabbros. Lower intercept ages, represented by near-concordant zircons, are interpreted as reflecting metamorphic recrystallisation, and range from 1,927 to 1,852 Ma. One granulite contains two metamorphic zircon populations, dated at 1,927±55 Ma and 600–700 Ma. Separated minerals from one granulite and one amphibolite yield Sm–Nd isochron ages of 1,619±48 Ma (¹⁴³Nd/¹⁴⁴Nd)i=0.51078), and 1,716±120 Ma (¹⁴³Nd/¹⁴⁴Nd)i=0.51006), respectively. These ages are interpreted as recording cooling following metamorphic resetting; model ages for both samples are in the range 2.40–2.66 Ga. LAM-MC-ICPMS analyses of zircon show a range in ¹⁷⁶Hf/¹⁷⁷Hf from 0.28116 to 0.28214, corresponding to a range of _Hf from –34 to +12. The relationships between ²⁰⁷Pb/²⁰⁶Pb age and _Hf show that: (1) the granulites, amphibolites and metagabbro were derived from a depleted mantle source at 2.6–2.75 Ga; (2) zircons in most samples underwent recrystallisation and Pb loss for 100–200 Ma after magmatic crystallisation, consistent with a residence in the lower crust; (3) metamorphic zircons in several samples represent new zircon growth, incorporating Hf liberated from breakdown of silicates with high Lu/Hf; (4) in other samples metamorphic and magmatic zircons have identical ¹⁷⁶Hf/¹⁷⁷Hf, and the younger ages reflect complete resetting of U–Pb systems in older zircons. The Fuxian mafic xenoliths are interpreted as the products of basaltic underplating, derived from a depleted mantle source in Neoarchean time, an important period of continental growth in the North China craton. Paleoproterozoic metamorphic ages indicate an important tectonic thermal event in the lower crust at 1.8–1.9 Ga, corresponding to the timing of collision between the Eastern and Western Blocks that led to the final assembly of the North China craton. The growth of metamorphic zircon at 600–700 Ma may record an asthenospheric upwelling in Neoproterozoic time, related to uplift and a regional disconformity in the North China craton.     

Evolution of the Yunkai Terrane, South China: Evidence from SHRIMP zircon U–Pb dating, geochemistry and Nd isotope

The Yunkai Terrane is one of the most important pre-Devonian areas of metamorphosed supracrustal and granitic basement rocks in the Cathaysia Block of South China. The supracrustal rocks are mainly schist, slate and phyllite, with local paragneiss, granulite, amphibolite and marble, with metamorphic grades ranging from greenschist to granulite facies. Largely on the basis of metamorphic grade, they were previously divided into the Palaeo- to Mesoproterozoic Gaozhou Complex, the early Neoproterozoic Yunkai ‘Group’ and early Palaeozoic sediments. Granitic rocks were considered to be Meso- and Neoproterozoic, or early Palaeozoic in age. In this study, four meta-sedimentary rock samples, two each from the Yunkai ‘Group’ and Gaozhou Complex, together with three granite samples, record metamorphic and magmatic zircon ages of 443–430 Ma (Silurian), with many inherited and detrital zircons with the ages mainly ranging from 1.1 to 0.8 Ga, although zircons with Archaean and Palaeoproterozoic ages have also been identified in several of the samples. A high-grade sillimanite–garnet–cordierite gneiss contains 242 Ma metamorphic zircons, as well as 440 Ma ones. Three of the meta-sedimentary rocks show large variations in major element compositions, but have similar REE patterns, and have t_DM model ages of 2.17–1.91 Ga and ε_Nd (440 Ma) values of −13.4 to −10.0. Granites range in composition from monzogranite to syenogranite and record t_DM model ages of 2.13–1.42 Ga and ε_Nd (440 Ma) values of −8.4 to −1.2. It is concluded that the Yunkai ‘Group’ and Gaozhou Complex formed coevally in the late Neoproterozoic to early Palaeozoic, probably at the same time as weakly to un-metamorphosed early Palaeozoic sediments in the area. Based on the detrital zircon population, the source area contained Meso- to Neoproterozoic rocks, with some Archaean material. Palaeozoic tectonothermal events and zircon growth in the Yunkai Terrane can be correlated with events of similar age and character known throughout the Cathaysia Block. The lack of evidence for Palaeo- and Mesoproterozoic rocks at Yunkai, as stated in earlier publications, means that revision of the basement geology of Cathaysia is necessary.     

Differential response of zircon U−Pb isotopic systematics to metamorphism across a lithologic boundary: an example from the Hope Valley Shear Zone,southeastern Massachusetts,USA

A detailed morphological, chemical and isotopic study of zircons from a single outcrop of two mineralogically and chemically distinct units of the late Precambrian Ponaganset gneiss was undertaken to investigate the effects of mylonitization and metamorphism on U-Pb isotopic systematics. Late Paleozoic, amphibolite-grade (approx. 600°C) mylonitization of the Ponaganset gneiss at this locality is associated with movement along the Hope Valley Shear Zone. The response of zircon to metamorphism in each gneiss unit is distinct: zircons in gray augen gneiss are uncorroded and not overgrown, whereas zircons from fluorite-bearing pink granitic gneiss are variably corroded and over 50% bear opaque overgrowths. The zircon overgrowths are chemically distinct from the primary cores, and contain high conentrations of Hf, U, HREE, and Th. Mylonite derived from the gray gneiss contains only a small population of Hf-U-rich metamorphic zircon, but zircons in the pink gneiss-derived mylonite are dominated by the Hf-U-rich metamorphic component. In terms of their U-Pb isotopic systematics, overgrowth-free zircons from both units are markedly discordant (gray, 10–20%, pink, 35%), but overgrown zircons from the pink gneiss are up to 70% discordant. Zircons from the mylonites yield younger Pb–Pb and U–Pb ages than those of the protolith gneisses, and isotopic data from each gneiss + mylonite pair define a linear array on concordia plots. Upper intercept ages of the gray gneiss (621+/–27 Ma) and the pink gneiss (635+/–50 Ma) indicate that the crystallization of both units was coeval, and the lower intercept ages (gray, 270+/–92 Ma; pink, 285+/–26 Ma) fall within the range of other published age estimates for Alleghanian metamorphism in southeastern New England (e.g., Zartman et al. 1988). New growth of zircon suggests that Zr was mobile during metamorphism. The presence of fluorite in the pink gneiss, and a discontinuity in log values obtained from biotite across the pink gneiss-gray gneiss contact indicates that dissolution and reprecipitation of zircon may be related to local variations in HF fugacity. Zircon dissolution/reprecipitation in the pink gneiss, and the lack of similar features in the contiguous gray gneiss, suggests that the degree of isotopic perturbation of zircon during metamorphism is related to bulk chemistry, fluid chemistry and/or the degree of fluid-rock interaction.     

K-Ar ages of the metamorphic sole of the Semail Ophiolite: implications for ophiolite cooling history

K–Ar ages have been determined on micas and hornblendes in the basal metamorphic sequence and in metamorphic rocks squeezed into the mantle sequence of the Semail Ophiolite. The hornblende ages of 99±0.5 and 102±0.8 Ma and the 90 Ma ages of coexisting micas from the high-grade metamorphic portion of the sequence are interpreted as cooling stages following the peak of metamorphism (T 800–850° C, P 6.5–9 kbar). The new pressure estimates are based on findings of kyanite in garnet-amphibolite and cordierite in quartzitic rocks. These data indicate a cooling rate of 10–30° C/Ma. The oldest mica ages of 95±1 Ma are observed in the lowest-grade greenschists. These also largely represent cooling ages, but might in part also include formation ages. The pattern of the muscovite ages across the metamorphic sole indicates that the cooling front moved from the low-grade metamorphic zone, through the high-grade rocks and into the base of the overlying ophiolite. Radiometric ages of hornblendes (92.3±0.5 and 94.8±0.6 Ma) indicate that the crustal gabbro sequence cooled below 500° C later than the base of the ophiolite sequence. Metamorphism of the sole rocks occurred during subduction of oceanic sediments and volcanic or gabbroic rocks as they progressively came into contact with hotter zones at the base of the overriding plate. The peak of metamorphism must have been contemporaneous with the main magmatism in the Semail Ophiolite. One of the dated muscovites yields an age of 81.3±0.8 Ma, but this is related to discrete deformation zones that were active during late-stage emplacement of the ophiolite.     

Jurassic ophiolites within the Valais domain of the Western and Central Alps: geochronological evidence for re-rifting of oceanic crust

Metabasic rocks from different parts of the Antrona ophiolites, Western Alps, as well as from the Misox zone, Central Alps, were dated using ion microprobe (SHRIMP) U-Pb analyses of zircon, in association with cathodoluminescence (CL) imaging. HP metamorphism must have affected at least the major part of the Antrona ophiolites, although HP relics are rarely preserved, probably due to the Lepontine metamorphic overprint. HP metamorphism has affected also the area of the Misox zone. The origin of the Antrona ophiolites is arguable. They were interpreted as part of both the Piemont–Ligurian (PL) and the Valais ocean, the two main oceans in the area of the Alps before Alpine convergence. SHRIMP-analyses of co-magmatic zircon domains from the Antrona ophiolites (Guggilihorn, Passo del Mottone and Quarata areas) yielded identical (within uncertainty) weighted mean ²⁰⁶ Pb/²³⁸U ages of 155.2±1.6 Ma, 158±17 Ma (or 163.1±2.4 Ma: one analysis; 1 error) and 155.6±2.1 Ma, respectively, interpreted as the time of crystallization of the magmatic protoliths. These Late Jurassic ages fit well to the time span considered for the formation of Piemont–Ligurian oceanic crust. The metagabbro of the Misox zone (Hinterrhein area), for which a Valaisan origin is generally accepted, gave also a Late Jurassic, PL protolith age of 161.0±3.9 Ma. The metamorphic zircon domains from the amphibolitized eclogite of Mottone yielded an age of 38.5±0.7 Ma, interpreted as the time of HP metamorphism. This age is in good agreement with the time of metamorphism reported from previous zircon SHRIMP-data for eclogites and amphibolites of other parts in the Valais domain. In order to bring in line the PL protolith ages with the Valaisan metamorphic ages, we suggest a scenario involving emplacement of part of the PL oceanic crust to the north of the newly formed Briançonnais peninsula, inside the Valais geotectonic domain. This paleotectonic configuration was probably established when younger Valaisan oceanic crust formed by spreading and re-rifting, partly within PL oceanic crust.     

Latest precambrian (Cadomian) zircon ages,Nd isotopic systematics and P-T evolution of granitoid orthogneisses of the Erzgebirge,Saxony and Czech Republic

Single zircons from two orthogneiss complexes, the Grey Gneiss and Red Gneiss, the lowermost tectonic units in the Erzgebirge, were dated. The grey Freiberg Gneiss is of igneous origin and has a ²⁰⁷Pb/²⁰⁶Pb emplacement age of 550±7 Ma. A quartz monzonite from Lauenstein contains idiomorphic zircons with a mean ²⁰⁷Pb/²⁰⁶Pb age of 555±7 Ma as well as xenocrysts ranging in age between 850 and 1910 Ma. Red gneisses from the central Erzgebirge contain complex zircon populations, including numerous xenocrysts up to 2464 Ma in age. The youngest, idiomorphic, zircons in all samples yielded uniform ²⁰⁷Pb/²⁰⁶Pb ages between 550±9 and 554±10 Ma. Nd isotopic data support the interpretation of crustal anatexis for the origin of both units. _Nd(t) values for the grey gneisses are –7.5 and –6.0 respectively, (mean crustal residence ages of 1.7–1.8 Ga). The red gneisses have a wider range in _Nd(t) values from –7.7 to –2.8 (T _DM ages of 1.4–1.8 Ga). The zircon ages document a distinct late Proterozoic phase of granitoid magmatism, similar in age to granitoids in the Lusatian block farther north-east. However, Palaeozoic deformation as well as medium pressure metamorphism ( 8 kbar/600–650° C) are identical in both gneiss units and distinguish these rocks from the Lusatian granitoids. The grey and red gneisses were overthrust by units with abundant high-pressure relicts and a contrasting P-T evolution. Zircon xenocryst and Nd model ages in the range 1000–1700 Ma are similar to those in granitoid rocks of Lusatia and the West-Sudetes, and document a pre-Cadomian basement in parts of east-central Europe that, chronologically, has similarities with the Sveconorwegian domain in the Baltic Shield.     

U-Pb zircon,Rb-Sr and Sm-Nd geochronology of high- to very-high-pressure meta-acidic rocks from the western Alps

Three meta-acidic rocks from the western Italian Alps, a magnesiochloritoid-bearing metapelite from the Monte Rosa massif, a coesite-pyrope-quartzite from the Dora Maira massif and the Monte Mucrone granite in the Sesia Zone, have been studied by U-Pb zircon, Rb-Sr on whole-rock, apatite and phengite and Sm-Nd wholerock methods. The mineral parageneses of the investigated rocks indicate high- to very-high-pressure and medium-to-high-temperature metamorphism. This combined isotopic study has enabled us to constrain the ages of magmatic and metamorphic events and also to compare the behaviour of U-Pb zircon systems in three intensely metamorphosed areas of the Pennine domain. The U-Pb zircon data have yielded a magmatic age for the Monte Mucrone granite at 286±2 Ma. This result confirms the occurence of late-Hercynian magmatism in the Sesia Zone, as in other Austro-Alpine units and in other areas of the European crystalline basement. In the Monte Rosa massif, a geologically meaningless lower intercept age of 192±2 Ma has been interpreted as an artefact due to a complex evolution of the U-Pb zircon system. The magmatic shape of the zircons implies a magmatic or volcano-sedimentary protolith for this rock, originally considered as a metasediment. The very-high-pressure metamorphism in the Dora Maira quartzite has produced an opening of the U-Pb zircon system at 121+12–29 Ma. The Rb-Sr data support the occurence of high-grade metamorphism during Cretaceous times. Phengites model ages are slightly younger than the U-Pb zircon lower intercept ages due to cooling phenomena or possible response of the phengites to a later deformation. The Nd model ages from the whole-rock samples, as well as the U-Pb upper intercept ages from zircons of all three investigated rocks, indicate the presence of Proterozoic crustal components inherited from the precursors of these meta-acidic rocks. The studied zircon populations and their U-Pb systems apparently showed open-system behaviour only when affected by extreme metamorphic conditions (700–750° C, > 28 kbar), whereas eclogite-facies conditions of 500–550° C and 14–16 kbar were not enough to disturb significantly the U-Pb zircon evolution. It is also probable that the sedimentary or magmatic origin of the protoliths of these meta-acidic rocks, which involved different characteristics such as grain-size and fluid phase concentration and composition, could be another important factor controlling the U-Pb zircon system behaviour during metamorphic events.     

The youngest basic oceanic magmatism in the Alps (Late Cretaceous<Emphasis Type="Italic">;</Emphasis> Chiavenna unit,Central Alps): geochronological constraints and geodynamic significance

Cathodoluminescence-controlled radiometric dating (U–Pb SHRIMP) was carried out on zircon domains from metabasic rocks of the Chiavenna unit, a major mafic/ultramafic-bearing unit in the Central Alps. Co-magmatic zircon domains from amphibolites near Chiavenna and Prata areas yielded weighted mean ²⁰⁶Pb/²³⁸U ages at 93.0±2.0 and 93.9±1.8 Ma, respectively, interpreted as the age of crystallization of the magmatic protoliths. These ages fit well with the time of late spreading in the Valais Ocean, as suggested by previous paleogeographic reconstructions. Inherited zircon grains and/or core domains (Permo-Triassic, Carboniferous, Proterozoic) are abundant, indicating proximity of the Chiavenna unit to thinned continental crust. This is in line with the origin of this unit from subcontinental mantle sources, as suggested previously on petrological and structural grounds. Metamorphic zircon domains from one amphibolite near Chiavenna yielded a weighted mean ²⁰⁶Pb/²³⁸U age at 37.1±0.9 Ma, identical to the 38.5±0.9 Ma SHRIMP age of an amphibolitized eclogite of the Antrona ophiolites (Valais domain, Western Alps). Precise metamorphic ages were difficult to obtain from the composite (poly)metamorphic rim domains of the Prata amphibolite. This is attributed to the location of the Prata area close to the granulite-facies Gruf unit (metamorphosed at ca. 33 Ma) and to the 24–25 Ma old Novate granite, where metamorphic/fluid events probably caused multiple resetting to various degrees. The ca. 93 Ma old magmatism, identified for the first time in the Chiavenna unit, is the youngest basic oceanic magmatism reported in the Alps. The 37.1±0.9 Ma old metamorphism in the Chiavenna unit, attributed to the Valais domain, confirms the model suggesting stepwise younging of metamorphic ages from the south (Adriatic plate) to the north (European plate). It is older than metamorphism in the European margin (ca. 35–31 Ma) lying to the north of the Valais domain and younger than that in the Piemont–Ligurian Ocean (ca. 44–45 Ma) lying to the south of the Valais domain.Editorial responsibility: W. Schreyer     

Single zircon ages of migmatitic gneisses and granulites in the Obudu Plateau: Timing of granulite-facies metamorphism in southeastern Nigeria

A single zircon geochronological study of gneisses from the Obudu Plateau of southeastern Nigeria, using the evaporation technique, indicates that zircons recorded several Precambrian high-grade metamorphic events (Eburnean and Pan-African). Igneous and multifaceted metamorphic zircons yielded ²⁰⁷Pb/²⁰⁶Pb ages of 2062.4 ± 0.4 Ma, 1803.8 ± 0.4 Ma and 574 ± 10 Ma, respectively and confirm for the first time that granulite-facies metamorphism affected the basement of southeastern Nigeria, resulting in the formation of charnockites and granulitic gneisses. The Pan-African high-grade event was coeval with the formation of granulites in Cameroon, Togo and Ghana and resulted from collisional processes during continental amalgamation to form the Gondwana supercontinent. The sources of the sediments, which were deposited at ≈605 Ma and metamorphosed at 574 Ma, comprise older igneous and metamorphic protoliths (including inherited xenocrystic zircons up to 2.5 Ga in age). The Palaeoproterozoic zircons seem to have survived Pan-African melting.     

SHRIMP and electron microprobe chronology of UHT metamorphism in the Napier Complex,East Antarctica: implications for zircon growth at >1,000 °C

Zircons in ultra-high-temperature (UHT) metamorphosed paragneisses from Mt. Riiser-Larsen in the Napier Complex, East Antarctica, were dated by using ion microprobe (SHRIMP) and electron microprobe (EMP). Both SHRIMP and EMP analyses yield consistent 2520–2460 Ma age populations for garnet–orthopyroxene-bearing paragneiss and leucosomes enclosed within. The peak UHT event was dated at 2480 Ma by SHRIMP analyses on metamorphic zircons from the garnet–orthopyroxene paragneiss and those on magmatic zircons from the leucosomes which are interpreted to be formed at syn-UHT. As obtained by SHRIMP, the UHT metamorphic event was terminated no later than 2460 Ma. Minor 2520-Ma SHRIMP age suggests either the onset of prograde metamorphism or another high-grade metamorphic event unrelated to the UHT. EMP analyses on metamorphic zircons from sapphirine–quartz and osumilite-bearing magnesian paragneisses give c. 2500–2450 Ma ages. Inherited igneous zircon cores of the magnesian paragneisses yield relatively scattered EMP ages ranging over c. 3000–2650 Ma, suggesting that igneous materials of these ages sourced the protoliths of the paragneisses and that they were deposited during the interval c. 2650–2520 Ma.     

A Paleoproterozoic orogeny recorded in a long-lived cratonic remnant (Wuyishan terrane), eastern Cathaysia Block, China

The Precambrian basement of northern Wuyishan (southern Zhejiang Province, eastern Cathaysia Block, South China), consists mainly of Paleoproterozoic granites and metamorphic rocks of the Badu Complex, which are the oldest rocks found in the Cathaysia Block. LA-ICPMS zircon U–Pb ages for a gneiss and five gneissic granites from the Tianhou, Danzhu, Xiaji and Lizhuang plutons indicate that magmatism and metamorphism took place between 1888 and 1855 Ma. The Xiaji (1888 ± 7 Ma) and Lizhuang (1875 ± 9 Ma) granites have high SiO₂, K₂O and Rb contents, high A/CNK (1.09–1.40) and Rb/Sr, and low contents of Sr, REE and mafic components (Mg, Fe, Ti, Mn and other transition metals). They have the geochemical signature of S-type granites, and a sedimentary protolith is confirmed by the presence of abundant inherited zircons with a range of ages and Hf-isotope compositions. The Tianhou and Danzhu granites are metaluminous to weakly peraluminous (A/CNK = 0.80–1.07), and have low SiO₂ contents, high Ga/Al and FeO/(FeO + MgO) ratios, and Zn and HFSE concentrations typical of A-type granites. They also record high crystallization temperatures (885–920 °C), consistent with A-type granites. High Y/Nb ratios (>1.4) indicate that they belong to the A₂ subgroup, suggesting that they probably formed in a post-orogenic tectonic setting. Their ages range from 1867 to 1855 Ma, slightly later than the syn-collisional Lizhuang and Xiaji S-type granites. These granitic rocks and the metamorphic rocks of the Badu Complex define a late Paleoproterozoic orogenic cycle in the area. All the 1.86–1.90 Ga zircons, whether derived from S- or A-type granites, show similar Hf-isotopic compositions, with Hf model ages clustering at 2.8 Ga. These model ages, and inherited zircons (ca. 2.5–2.7 Ga) found in some rocks, indicate that the late Paleoproterozoic magmatism and tectonism of the eastern Cathaysia Block represent an overprint on an Archaean basement. This Paleoproterozoic orogeny in the Wuyishan terrane coincides with the assembly of the supercontinent Columbia, suggesting that the Wuyishan terrane was the part of this supercontinent.Zircon ages also record an early Mesozoic (Triassic) tectonothermal overprint that was very intensive in the northern Wuyishan area, leading to high-grade metamorphism of Paleoproterozoic basement, Pb loss from Paleoproterozoic zircons and overgrowth of new zircon. The central and southern parts of Wuyishan and the Chencai area (northern Zhejiang Province) also experienced strong reworking in Neoproterozoic and early Paleozoic times. The Wuyishan terrane (especially in the north) represents a long-lived remnant of the old craton, which has survived for at least one billion years. The compositions of the basement rocks, the Paleoproterozoic orogeny and the Triassic tectonothermal imprint in the Wuyishan terrane are similar to those recognized in the Yeongnam massif of South Korea, suggesting that the two terranes may have been connected from Paleoproterozoic to Triassic time.     

The thermal evolution of Corsica as recorded by zircon fission-tracks

New zircon fission-track (ZFT) ages from Corsica record multiple thermal events that can be tied to the structural evolution of the western Mediterranean region. The Corsican zircons have a wide scatter of ZFT grain ages (243–14 Ma), which together define several age domains. Western Corsica consists largely of stable Hercynian basement characterized by ZFT ages in the range 161–114 Ma. We interpret these ages (Late Jurassic–Early Cretaceous) as the product of a long-lived Tethyan thermal event related to continental rifting and subsequent drifting during the separation of the European and African plates and the formation of the Liguro–Piemontese ocean basin. In contrast to Hercynian Corsica, Alpine Corsica (northeast Corsica) experienced widespread deformation and metamorphism in Late Cretaceous(?)–Tertiary time. Dated samples from Alpine Corsica range in age from 112 to 19 Ma and all are reset or partially reset by one or more Alpine thermal events. The youngest ZFT grain ages are from the northernmost Alpine Corsica and define an age population at  24 Ma that indicates cooling after Tertiary thermal events associated with the Alpine metamorphism and the opening of the Liguro–Provençal basin. A less well-defined ZFT age population at  72 Ma is present in both Alpine Corsica and Hercynian basement rocks. The thermal history of these rocks is not clear. One interpretation is that the ZFT population at  72 Ma reflects resetting during a Late Cretaceous event broadly synchronous with the early Alpine metamorphism. Another interpretation is that this peak is related to variable fission-track annealing and partial resetting during the Tertiary Alpine metamorphic event across central to north-eastern Corsica. This partial age resetting supports the presence of a fossil ZFT partial annealing zone and limits the peak temperature in this area below 300 °C, for both the affected pre-Alpine and Alpine units.     

U-Pb data on zircons for the thermal peak of metamorphism in the Variscan Odenwald,Germany

U-Pb data are presented for zircons from metamorphic rocks of the crystalline Odenwald that comprises part of the Mid-German Crystalline Rise. The Odenwald can be divided into four principal domains with different magmatic and metamorphic histories on the basis of published structural and petrological data and the presented ages. In the eastern (Böll-stein) Odenwald the metasedimentary rocks contain zircons derived from basement rocks that exceed 2.2 Ga in age. The intrusion of granitic magmas into these metasedimentary rocks took place before or close to the end of the thermal peak of metamorphism at 375 ± 5 Ma ago. Rapid uplift of the Böllstein Odenwald occurred at about 320 Ma bp, later than in the Bergsträsser Odenwald. The western (Bergsträsser) Odenwald can be subdivided, by geochronological, petrological and especially structural data, into a northern, a central and a southern part. In the northern part basic magmatism has an age of 362 ± 7 Ma. The peak of thermal metamorphism in the southernmost Odenwald was accompanied by partial melting of metasedimentary rocks and the formation of new zircons with ages between 332 and 342 Ma. Probable peak metamorphic conditions, revealed by two samples from the heterogeneous central part, occurred at about 336 Ma, i.e. in the same time span as in the southernmost part, but later than in the Böllstein area and the northern Odenwald. Rapid uplift started earlier in the central and southern part and 10–20 Ma earlier than in the Böllstein Odenwald. Cooling from 500 to 300°C occurred in the northern part in the time interval between the elevation of the other parts of the Bergstraesser Odenwald and the Böllstein Odenwald. There is no indication for any thermal equilibrium between 2.25 Ga and 375 Ma recorded in the zircons of both areas.     

U-Pb zircon dating of mafic dykes and its application to the Proterozoic geological history of the Vestfold Hills,East Antarctica

The Vestfold Hills, one of several Archaean cratonic blocks within the East Antarctic Shield, comprises a high-grade metamorphic basement complex intruded by at least nine generations of Early to Middle Proterozoic mafic dykes. Extensive U-Pb ion microprobe (SHRIMP) analyses of zircons, derived predominantly from late-stage felsic differentiates of the mafic dykes, provide precise crystallisation ages for several dyke generations. These new ages enable constraints to be placed on both the history of mafic magmatism in the Vestfold Hills and the timing of the various interspersed Proterozoic deformation events. In addition to demonstrating the utility of zircons derived from felsic late-stage differentiates for the dating of co-genetic mafic dykes, this study also places doubt on previous wholerock Rb-Sr dating of mafic dyke suites in this and other areas of East Antarctica. The ²⁰⁷Pb/²⁰⁶Pb zircon ages of 2241±4 Ma and 2238±7 Ma for the Homogeneous and Mottled Norites, respectively, provide a younger emplacement age for associated group 2 High-Mg tholeiite dykes than the whole-rock Rb-Sr date (2424±72 Ma) originally interpreted as the age of all high-Mg intrusives in the Vestfold Hills. Zircon ages of 1754±16 Ma and 1832±72 Ma confirm the previously defined Rb-Sr age of the group 2 Fe-rich tholeiites. Two later dyke generations, the group 3 and 4 Fe-rich tholeiites, are distinguished on the basis of field orientations and cross-cutting relationships, and yield zircon emplacement ages of 1380±7 Ma and 1241±5 Ma which also define minimum ages for two suites of lamprophyre dykes. Xenocrystic zircons within both felsic segregations and mafic dykes yield zircon ages of 2478±5 Ma to 2740 Ma, indicating the presence of Archaean crustal source rocks of this antiquity beneath the Vestfold Hills.     

Proterozoic–Paleozoic development of the basement of the Central Andes (18–26°S) — a mobile belt of the South American craton

The Late Precambrian–Early Paleozoic metamorphic basement forms a volumetrically important part of the Andean crust. We investigated its evolution in order to subdivide the area between 18 and 26°S into crustal domains by means of petrological and age data (Sm–Nd isochrons, K–Ar). The metamorphic crystallization ages and t_DM ages are not consistent with growth of the Pacific margin north of the Argentine Precordillera by accretion of exotic terranes, but favor a model of a mobile belt of the Pampean Cycle. Peak metamorphic conditions in all scattered outcrop areas between 18 and 26°S are similar and reached the upper amphibolite facies conditions indicated by mineral paragensis and the occurrence of migmatite. Sm–Nd mineral isochrons yielded 525±10, 505±6 and 509±1 Ma for the Chilean Coast Range, the Chilean Precordillera and the Argentine Puna, and 442±9 and 412±18 Ma for the Sierras Pampeanas. Conventional K–Ar cooling age data of amphibole and mica cluster around 400 Ma, but are frequently reset by Late Paleozoic and Jurassic magmatism. Final exhumation of the Early Paleozoic orogen is confirmed by Devonian erosional unconformities. Sm–Nd depleted mantle model ages of felsic rocks from the metamorphic basement range from 1.4 to 2.2 Ga, in northern Chile the average is 1.65±0.16 Ga (1σ; n=12), average t_DM of both gneiss and metabasite in NW Argentina is 1.76±0.4 Ga (1σ; n=22), and the isotopic composition excludes major addition of juvenile mantle derived material during the Early Paleozoic metamorphic and magmatic cycle. These new data indicate a largely similar development of the metamorphic basement south of the Arequipa Massif at 18°S and north of the Argentine Precordillera at 28°S. Variations of metamorphic grade and of ages of peak metamorphism are of local importance. The protolith was derived from Early to Middle Proterozoic cratonic areas, similar to the Proterozoic rocks from the Arequipa Massif, which had undergone Grenvillian metamorphism at ca. 1.0 Ga.     

东昆仑西段阿确墩地区白沙河岩组锆石U-Pb年龄 ——对前寒武纪基底演化的约束

对东昆仑西段阿确墩地区金水口岩群白沙河岩组片麻岩进行了LA-ICP-MS锆石U-Pb年代学研究.结果显示,片麻岩中锆石的年龄范围为2400～410 Ma,峰值范围为1650～1400 Ma,其中存在少量新元古代(1.0～0.9 Ga)和早古生代(410 Ma)的变质锆石,从而限定了原岩的最早沉积时代,为中元古代末期,指...     

Zircon dating of North Bohemian granulites, Czech Republic: further evidence for the Lower Carboniferous high-pressure event in the Bohemian Massif

U-Pb zircon and rutile multigrain ages and ²⁰⁷Pb/²⁰⁶Pb zircon evaporation ages are reported from high-pressure felsic and metapelitic granulites from northern Bohemia, Czech Republic. The granulites, in contrast to those from other occurrences in the Bohemian Massif, do not show evidence of successive HT/MPLP overprints. Multigrain size fractions of nearly spherical, multifaceted, metamorphic zircons from three samples are slightly discordant and yield a U-Pb Concordia intercept age of 348 ± 10 Ma, whereas single zircon evaporation of two samples resulted in ²⁰⁷Pb/²⁰⁶Pb ages of 339 ± 1.5 and 339 ± 1.4 Ma, respectively. A rutile fraction from one sample has a U-Pb Concordia intercept age of 346 ± 14 Ma. All ages are identical, within error, and a mean age of 342 ± 5 Ma was adopted to reflect the peak of HP metamorphism. Because rutile has a lower closing temperature for the U-Pb isotopic system than zircon, the results and the P-T data imply rapid uplift and cooling after peak metamorphism. The above age is identical to ages for high-grade metamorphism reported from the southern Bohemian Massif and the Granulite Massif in Saxony. It can be speculated that all these granulites were part of the same lower crustal unit in early Carboniferous, being separated later due to crustal stacking and subsequent late Variscan orogenic collapse.     

Geochronology and geochemistry of a dyke–host rock association and implications for the formation of the Bavarian Pfahl shear zone, Bohemian Massif

To place constraints on the formation and deformation history of the major Variscan shear zone in the Bavarian Forest, Bavarian Pfahl zone, SW Bohemian Massif, granitic dykes and their feldspar-phyric massive host rock (so-called palite), zircons were dated by the U–Pb isotope dilution and Pb-evaporation methods. The dated samples comprise two host rocks and four dykes from a K-rich calc-alkaline complex adjoining the SW part of the Bavarian Pfahl shear zone. The palites, which appear to be the oldest magmatic rocks emplaced in the shear zone, yield ages of 334±3, 334.5±1.1 Ma (average ²⁰⁷Pb/²⁰⁶Pb-evaporation zircon ages) and 327–342 Ma (range of U/Pb zircon ages) suggesting a Lower Carboniferous age for the initiation of the Pfahl zone. Absence of inherited older cores in all investigated zircons indicates that incorporation of crustal zircon material has played virtually no role or that the melting temperature was very high. Determination of the dyke emplacement age is complicated by partial Pb-loss in most of the fractions analysed. This Pb-loss can be ascribed to higher U content of the dyke zircons compared to those from host rock. Upper discordia intercept ages of the different dykes range from 322±5 to 331±9 Ma. The dykes are pre- to synkinematic with respect to penetrative regional mylonitisation along the Pfahl zone, and the upper intercept ages provide a maximum age for this tectonic event.     

U-Pb geochronology of the Grenville Orogen of Ontario and New York: constraints on ancient crustal tectonics

Based on lithological, structural and geophysical characteristics, the Proterozoic Grenville Orogen of southern Ontario and New York has been divided into domains that are separated from each other by ductile shear zones. In order to constrain the timing of metamorphism, U-Pb ages were determined on metamorphic and igneous sphenes from marbles, calc-silicate gneisses, amphibolites, granitoids, skarns and pegmatites. In addition, U-Pb ages were obtained for monazites from metapelites and for a rutile from an amphibolite. These mineral ages constrain the timing of mineral growth, the duration of metamorphism and the cooling history of the different domains that make up the southern part of the exposed Grenville Orogen. Based on the ages from metamorphic minerals, regional and contact metamorphism occurred in the following intervals:Central Granulite Terrane:Adirondack Highlands: 1150 Ma; 1070–1050 Ma; 1030–1000 MaCentral Metasedimentary Belt:Adirondack Lowlands 1170–1130 MaFrontenac domain 1175–1150 MaSharbot Lake domain ca. 1152 MaFlzevir domain: 1240 Ma; 1060–1020 MaBancroft domain: ca. 1150 Ma; 1045–1030 MaCentral Gneiss Belt: ca. 1450 Ma; ca. 1150 Ma; 1100–1050 MaGrenville FrontTectonic Zone ca. 1000 Ma.Combination of mineral ages with results from thermobarometry indicates that metamorphic pressures and temperatures recorded by thermobarometers were reached polychronously in the different domains that are separated by major shear zones. Some of these shear zones such as the Robertson Lake shear zone and the Carthage-Colton shear zone represent major tectonic boundaries. The Grenville Orogen is made up of a collage of crustal terranes that have distinct thermal and tectonic histories and that were accreted laterally by tectonic processes analogous to those observed along modern active continental margins. The subsequent history of the orogen is characterized by slow time-integrated cooling rates of 3±1°C/Ma and denudation rates of 120±40m/Ma.