Ion Microprobe Dating Of Zircon Research Articles

High-temperature S-type granitoids (charnockites) in the Jining complex, North China Craton: Restite entrainment and hybridization with mafic magma

The Liangcheng garnet granitoids are parautochthonous S-type granites that occur within granulite-facies metasediments in the ultrahigh-temperature (UHT) Jining Complex, eastern part of the Khondalite Belt, North China Craton. These garnet granitoids contain mafic microgranular enclaves and are intimately associated with numerous small intrusions of the Xuwujia gabbronorites, showing magma mingling and mixing textures. Typical mineralogy in the garnet granitoids is plagioclase + K-feldspar + quartz + garnet + biotite ± orthopyroxene. UPb ion microprobe dating of zircon suggests that garnet granitoids and associated gabbronorites were coeval with UHT metamorphism at ~1.93–1.92 Ga, and also recorded ~1.92 Ga metamorphism during slow cooling. Garnet granitoids are characterized by the presence of orthopyroxene, high modal proportion of garnet (up to 25%) and strongly peraluminous (ASI = 1.11–1.55) along with lower SiO2 (57.8–70.1 wt%) and higher FeOt + MgO (5.6–14.4 wt%) contents than typical S-type granites. Mineral chemistry (high Ti biotite and high Al orthopyroxene), high Al-in-orthopyroxene temperature (910–989 °C) and Zr saturation temperature (825–901 °C), and low whole-rock Al2O3/TiO2 ratios (11–28), indicate that garnet granitoids were derived from biotite dehydration melting of metasedimentary rocks under UHT granulite-facies conditions. Textural and compositional evidence suggests that a significant proportion of garnets in the Liangcheng garnet granitoids are potentially a peritectic phase entrained from the source. Garnet aggregates may also represent entrained garnet-rich restites. The large HREE variations of garnet granitoids also support the entrainment of garnet-rich restites. Garnet granitoids display nearly linear trends with the mafic enclaves and gabbronorite for most major elements that demonstrates the involvement of mafic magma in the granitoid petrogenesis. The εHf(t) values (−1.0 − +5.6) and δ18O values (9.2–11.6‰) of zircons in garnet granitoids partly overlap with those in gabbronorites (εHf(t) = −2.9 − +3.5, δ18O = 7.7–10.9‰), suggesting the strong interaction and mixing process between them. Major element modelling suggests that the addition of 20–40% restites (garnetites) and gabbronorites to the anatectic melts was responsible for the large chemical variations. Therefore, the Liangcheng garnet granitoids represent high-temperature (> 900 °C), H2O-undersaturated, strongly peraluminous granitoids, and are produced by large-scale melting of metasedimentary rocks and subsequent mixing between restite-rich anatectic melt and mafic magma. The voluminous high-temperature granitoids, coupled with UHT granulites in the Jining Complex, manifest the thermal response of a granulitic lower crust to the mafic underplating.

Ion microprobe dating of zircons from active Dayingshan volcano, Tengchong, SE Tibetan Plateau: Time scales and nature of magma chamber storage

The rare occurrences of active volcanoes on the Tibetan Plateau provide a unique opportunity to investigate time scales of magma chamber processes for post-collisional potassic volcanism. This research utilizes high-spatial resolution U–Th disequilibrium dating methods to date zircons from the active Dayingshan volcano, Tengchong, SE Tibetan Plateau, which erupted last during the Holocene. Zircon crystal ages can be used to provide constraints on the magma chamber storage time. Measured zircon interiors yielded an isochron age of 87.5±6.5ka, while zircon outermost rims yielded an isochron age of 58±13ka. The calculated zircon crystal storage time for the most recent eruption is 48ka. At least two separate late Pleistocene zircon crystallization events have occurred where the older age population represents antecrysts remobilized from an earlier magmatic episode, but protracted crystallization cannot be ruled out. Regardless, the similarity in zircon age populations between Dayingshan volcano and Maanshan volcano 13km apart in the Tengchong volcanic field suggests a synchronous thermal history of the magma reservoir, possibly in an extensive interconnected magma chamber beneath the Tengchong volcanic field.

U–Pb SHRIMP zircon geochronology and T– t– d history of the Kampa Dome, southern Tibet

Structural, petrographic and geochronologic studies of the Kampa Dome provide insights into the tectonothermal evolution of orogenic crust exposed in the North Himalayan gneiss domes of southern Tibet. U–Pb ion microprobe dating of zircons from granite gneiss exposed at the deepest levels within the dome yields concordia 206Pb/ 238U age populations of 506 ± 3 Ma and 527 ± 6 Ma, with no evidence of new zircon growth during Himalayan orogenesis. However, the granite contains penetrative deformation fabrics that are also preserved in the overlying Paleozoic strata, implying that the Kampa granite is a Cambrian pluton that was strongly deformed and metamorphosed during Himalayan orogenesis. Zircons from deformed leucogranite sills that cross-cut Paleozoic metasedimentary rocks yield concordant Cambrian ages from oscillatory zoned cores and discordant ages ranging from ca. 491–32 Ma in metamict grains. Since these leucogranites clearly post-date the metasedimentary rocks they intrude, the zircons are interpreted as xenocrysts that are probably derived from the Kampa granite. The Kampa Dome formed via a series of progressive orogenic events including regional ~ N–S contraction and related crustal thickening (D 1), predominately top-to-N ductile shearing and crustal extension (D 2), top-to-N brittle–ductile faulting and related folding on the north limb of the dome, localized top-to-S faulting on the southern limb of the dome, and crustal doming (D 3), and continued N–S contraction, E–W extension and doming (D 4). Structural and geochronologic variability amongst adjacent North Himalayan gneiss domes may reflect changes in the magnitude of crustal exhumation along the North Himalayan antiform, possibly relating to differences in the mid-crustal geometry of the exhuming fault systems.

U-Pb dating of zircon in the Bishop Tuff at the millennial scale

Zircon from the Bishop Tuff (eastern California) was dated by the U-Pb isotope dilution thermal ionization mass spectrometry (ID-TIMS) method to evaluate time scales of magmatic evolution and applicability of the method to rocks younger than 1 Ma. The 206 Pb/ 238 U dates from 17 of 19 grains are equivalent with a weighted mean of 767.1 ± 0.9 ka that overlaps with the published 40 Ar/ 39 Ar sanidine age, requiring that most zircon crystallized immedi- ately before eruption. Dates from two grains that are 6 and 12 k.y. older indicate minimum times that a small portion of the grains resided in the magma. These fi ndings are consistent with chemical zoning patterns in zircon that suggest a relatively simple crystallization history, yet contrast with previous results of ion microprobe dating of zircon that show 50-80 k.y. of magma residence. The weighted mean 207 Pb/ 235 U date is ~52 k.y. older than the 206 Pb/ 238 U date, a difference that is attributed to excess 207 Pb from initial Pa/U disequilibrium. The results dem- onstrate that high-precision ID-TIMS geochronology can resolve magma chamber dynamics of <1 Ma silicic eruptions at the millennial scale.

Diachronous evolution of volcano-sedimentary basins north of the Congo craton: Insights from U–Pb ion microprobe dating of zircons from the Poli, Lom and Yaoundé Groups (Cameroon)

Ion microprobe U–Pb dating of zircons from Neoproterozoic volcano-sedimentary sequences in Cameroon north of the Congo craton is presented. For the Poli basin, the depositional age is constrained between 700–665 Ma; detrital sources comprise ca. 920, 830, 780 and 736 Ma magmatic zircons. In the Lom basin, the depositional age is constrained between 613 and 600 Ma, and detrital sources include Archaean to Palaeoproterozoic, late Mesoproterozoic to early Neoproterozoic (1100–950 Ma), and Neoproterozoic (735, 644 and 613 Ma) zircons. The Yaoundé Group is probably younger than 625 Ma, and detrital sources include Palaeoproterozoic and Neoproterozoic zircons. The depositional age of the Mahan metavolcano-sedimentary sequence is post-820 Ma, and detrital sources include late Mesoproterozoic (1070 Ma) and early Neoproterozoic volcanic rocks (824 Ma). The following conclusions can be made from these data. (1) The three basins evolved during the Pan-African event but are significantly different in age and tectonic setting; the Poli is a pre- to syn-collisional basin developed upon, or in the vicinity of young magmatic arcs; the Lom basin is post-collisional and intracontinental and developed on old crust; the tectono-metamorphic evolution of the Yaoundé Group resulted from rapid tectonic burial and subsequent collision between the Congo craton and the Adamawa–Yade block. (2) Late Mesoproterozoic to early Neoproterozoic inheritance reflects the presence of magmatic event(s) of this age in west–central Africa.

Metamorphic Evolution of Iron-rich Mafic Cumulates from the Otztal-Stubai Crystalline Complex, Eastern Alps, Austria

Olivine-rich rocks containing olivine þ orthopyroxene þ spinel þ Ca-amphibole clinopyroxene garnet are present in the central Otztal–Stubai crystalline basement associated with eclogites of tholeiitic affinity. These rocks contain centimetre-sized garnet layers and lenses with garnet þ clinopyroxene corundum. Protoliths of the olivine-rich rocks are thought to be olivine þ orthopyroxene þ spinel dominated cumulates generated from an already differentiated Fe-rich (X Mg 1⁄4 0 57– 0 65) tholeiitic magma that was emplaced into shallow continental crust. Protoliths of the garnetrich rocks are interpreted as layers enriched in plagioclase and spinel intercalated in a cumulate rock sequence that is devoid of, or poor in, plagioclase. U–Pb sensitive high-resolution ion microprobe dating of zircons from a garnet layer indicates that emplacement of the cumulates took place no later than 517 7Myr ago. After their emplacement, the cumulates were subjected to progressive metamorphism, reaching eclogite-facies conditions around 800 C and >2 GPa during a Variscan metamorphic event between 350 and 360Ma. Progressive high-P metamorphism induced breakdown of spinel to form garnet in the olivine-rich rocks and of plagioclase þ spinel to form garnet þ clinopyroxene corundum in the garnet layers. Retrogressive metamorphism at T 650–680 C led to the formation of Ca-amphibole, chlorite and talc in the olivine-rich rocks. In the garnet layers, hogbomite formed from corundum þ spinel along with Al-rich spinel, Ca-amphibole, chlorite, aspidolite– preiswerkite, magnetite, ilmenite and apatite at the interface between olivine-rich rocks and garnet layers at P <0 8GPa. Progressive desiccation of retrogade fluids through crystallization of hydrous phases led to a local formation of saline brines in the garnet layers. The presence of these brines resulted in a late-stage formation of Feand K-rich Ca-amphibole and Sr-rich apatite, both characterized by extremely high Cl contents of up to 3 5 and 6 5 wt % Cl, respectively.

Exhumation of the Ronda peridotite and its crustal envelope: constraints from thermal modelling of a P – T –time array

The Ronda peridotite in the Betic Cordillera of southern Spain is a relic of the sub-orogenic lithospheric mantle that was exhumed during earliest Miocene time from about 66 km depth. Overlying crustal rocks show an apparently coherent metamorphic zonation from high-pressure granulite-facies rocks at the contact to unmetamorphosed rocks 5 km higher in the structural sequence, indicating drastic tectonic thinning of the whole orogenic crust during exhumation. P – T paths from the peridotite and its crustal envelope indicate decompression with rising temperature to shallow depths. U–Pb ion microprobe dating of zircon, Ar/Ar dating of hornblende, Ar/Ar laserprobe dating of muscovite and biotite, and fission-track analysis of zircon and apatite reveal that cooling was extremely rapid in the interval 21.2–20.4 Ma. One-dimensional thermal modelling of the array of P – T –time paths indicates that an asthenospheric heat source at an initial depth of about 67.5 km is required to explain heating during exhumation, and that the main period of exhumation lasted 5 Ma, starting at around 25 Ma. Exhumation must therefore have directly followed removal of most, but not all, of the lithospheric mantle beneath the Betic orogen, and was coeval with a period of late orogenic extension that profoundly modified the crustal structure and created the present-day Alboran Sea in the western Mediterranean.

Time scales of magma storage and differentiation of voluminous high-silica rhyolites at Yellowstone caldera, Wyoming

Ion microprobe dating of zircons from post-collapse rhyolites at Yellowstone caldera reveals the time scales of crystallization and storage of silicic magma in a differentiating magma reservoir, the role of recycling of crystals from the caldera-forming magmatism, and the timing and efficacy of crystal-melt separation. Zircons in the voluminous (~900 km3) Central Plateau Member lavas, which progressively erupted between 70 to 160 ka, yield 238U-230Th disequilibrium ages dominantly spanning the range from those of their respective eruptions to ~200 ka; mean zircon ages range to ca. 60,000 years before eruption. When considered together with the trace element and Sr- and Nd-isotope compositions of their host melts, the age distributions of the CPM zircons show that the rhyolites are cogenetic and differentiated tens of thousands of years prior to eruption from an evolving magma reservoir. Thus, the post-caldera CPM rhyolites were not erupted from a long-standing body of rhyolitic magma left over from the caldera-forming eruption, nor do they represent significant remobilization of the plutonic roots of the caldera. Rather, the CPM magma was generated and differentiated by episodes of effective crystal-melt separation at ~200 and ~125 ka and, sustained by thermal inputs, stored for timescales on par with estimates for other voluminous caldera-related rhyolites.

Mesozoic formation of pyroxenites and gabbros in the Ronda area (southern Spain), followed by Early Miocene subduction metamorphism and emplacement into the middle crust: U–Pb sensitive high-resolution ion microprobe dating of zircon

A combination of cathodoluminescence studies and U–Pb sensitive high-resolution ion microprobe dating of distinct magmatic and metamorphic zircon domains constitutes a powerful tool to decipher the geological history of complex metamorphic terranes. This combination of techniques has been applied to the internal zone of the Betic chain (southern Spain), where several orogenic lherzolite massifs — the Ronda peridotites — occur as a slice of mantle rocks, containing ∼5% of pyroxenite layers. The Ronda peridotites are emplaced within metamorphic country rocks that consist mainly of granulites and eclogite-bearing migmatites. In order to determine both the protolith ages of the pyroxenites and the age of the high-pressure (HP) metamorphism in the country rocks, mafic layers within the peridotites and eclogite relics within the country rocks have been studied. Anatectic granites (formed by partial melting of the metasedimentary country rocks during hot emplacement of the peridotites) were also studied, in order to determine the age of tectonic emplacement of the ultramafic rocks into the continental crust. Early Jurassic and Late Jurassic–Early Cretaceous protolith ages have been obtained from magmatic oscillatory-zoned domains of the zircons separated from three HP/ultra-HP garnet pyroxenites (178±6 Ma, 143±16 Ma and 131±3 Ma). Magmatic zircons of an eclogite yielded an Early Jurassic age (183±3 Ma). These ages are interpreted to reflect extensional/transtensional stages in the area of the Alboran Sea, related to Pangea's break-up and the opening of the Atlantic Ocean. HP/high-temperature (HT) metamorphism has been recorded in unzoned rims of the zircons of the eclogite at 19.9±1.7 Ma, which is interpreted to represent the T-peak of the metamorphism (790±15°C), immediately following the P-peak (17±1 kbar). Thus, Miocene subduction-zone metamorphism could be deduced geochronologically for the first time in the Betic chain. Crystallisation of anatectic granites crosscutting both peridotites and country rocks has been dated at 18.8±4.9 Ma. Our data suggest very high exhumation (>31 mm/y) and cooling rates (200–340°C/Ma) for the area of Ronda.

SHRIMP monazite and zircon geochronology of high‐grade metamorphism in New Zealand

Ion microprobe dating of zircon and monazite from high‐grade gneisses has been used to (1) determine the timing of metamorphism in the Western Province of New Zealand, and (2) constrain the age of the protoliths from which the metamorphic rocks were derived. The Western Province comprises Westland, where mainly upper crustal rocks are exposed, and Fiordland, where middle to lower crustal levels crop out. In Westland, the oldest recognisable metamorphic event occurred at 360–370 Ma, penecontemporaneously with intrusion of the mid‐Palaeozoic Karamea Batholith (c. 375 Ma). Metamorphism took place under low‐pressure/high‐temperature conditions, resulting in upper‐amphibolite sillimanite‐grade metamorphism of Lower Palaeozoic pelites (Greenland Group). Orthogneisses of younger (Cretaceous) age formed during emplacement of the Rahu Suite granite intrusives (c. 110 Ma) and were derived from protoliths including Cretaceous Separation Point suite and Devonian Karamea suite granites. In Fiordland, high‐grade paragneisses with Greenland Group zircon age patterns were metamorphosed (M1) to sillimanite grade at 360 Ma. Concomitant with crustal thickening and further granite emplacement, M1 mineral assemblages were overprinted by higher‐pressure kyanite‐grade metamorphism (M2) at 330 Ma. It remains unclear whether the M2 event in Fiordland was primarily due to tectonic burial, as suggested by regional recumbent isoclinal folding, or whether it was due to magmatic loading, in keeping with the significant volumes of granite magma intruded at higher structural levels in the formerly contiguous Westland region. Metamorphism in Fiordland accompanied and outlasted emplacement of the Western Fiordland Orthogneiss (WFO) at 110–125 Ma. The WFO equilibrated under granulite facies conditions, whereas cover rocks underwent more limited recrystallization except for high‐strain shear zones where conditions of lower to middle amphibolite facies were met. The juxtaposition of Palaeozoic kyanite‐grade rocks against Cretaceous WFO granulites resulted from late Mesozoic extensional deformation and development of metamorphic core complexes in the Western Province.

An example of reverse discordance during ion microprobe zircon dating: An artifact of enhanced ion yields from a radiogenic labile Pb

Sample #74459 contains 204Pb-rich zircons which, when analyzed by ion microprobe, yield U/Pb ages that are exceptionally older than their 207Pb/ 206Pb ages. The 64 ion microprobe measurements define an age of 2681 ± 6 Ma (1 σ) for the zircons, with individual analyses giving dilution analyses were conducted on 12 single grains previously analyzed by SHRIMP: the IDTIMS measurements yielded radiogenic 238U/ 206Pb ratios between 2.0 and 8.5 whereas the 34 ion microprobe analyses on the same 12 grains gave systematically lower values between 0.6 and 2.5. The discrepancy between the SHRIMP and IDTIMS data has been traced to the labile Pb. By plotting the 204Pb vs. 206Pb concentrations observed by the ion microprobe, an unusually low 204Pb/ 206Pb ratio of ∼0.02 has been defined for the labile component. Apparently the secondary ion emission yield of the labile Pb was significantly greater than that of in situ radiogenic Pb, thereby invalidating SHRIMP's interelement calibration and leading to inaccurate U/Pb ages. These data provide important insights into the geochemistry of labile Pb; in #74459 the labile Pb was locally derived and was well mixed at the kilogram scale prior to becoming housed within the zircon grains. High-resolution TEM data suggest that the labile Pb is contained within amorphous microdomains which are intimately associated with crystalline zircon. Another feature of the labile Pb is its ubiquitous presence in the zircons whereas it was not taken by the coexisting feldspar. This labile component has an age of ∼0 Ma.

The age of the Permian-Triassic boundary

The 5 cm boundary clay bed in the Chinese stratotype section through the Permian-Triassic boundary has been recognised as a bentonite. SHRIMP ion microprobe dating of zircons in the bentonite indicates a magmatic age of 251.2 ± 3.4 Ma (2σ); this is the first direct constraint on the numerical age of the Permian-Triassic boundary. Future refinements of ages at this important, but poorly constrained, level of the Phanerozoic timescale may depend on re-analysis of this uniquely placed volcanic horizon, and other bentonites in the fossiliferous Chinese Upper Permian and Lower Triassic. The utility of defining the Permian-Triassic boundary in the Chinese stratotype section, in the vicinity of known dateable horizons, should be considered.

Ion-microprobe dating of zircon from quartz-graphite veins at the Bristol, New Hampshire, metamorphic hot spot

Detrital zircons entrained in hydrothermal quartz-graphite-rutile veins found near the Bristol, New Hampshire, metamorphic hot spot are overgrown by thin rims. Ion-microprobe analyses of these rims date their growth at 408 {plus minus} 6 Ma. These measurements quantitatively confirm textural evidence that the graphite veins were emplaced during peak metamorphism associated with the Acadian orogeny, and they provide a direct positive test of the hypothesis, based on petrological and stable-isotope evidence, that the hydrothermal systems responsible for the quartz-graphite veins were also responsible for the hot-spot metamorphism.

The development of the Central European continental crust since the Early Archaean based on conventional and ion-microprobe dating of up to 3.84 b.y. old detrital zircons

Archaean components up to 3.84 b.y. old have been detected by ion-microprobe dating of zircon from various sediments and metasediments of the European Hercynides (Moldanubian of northeastern Bavaria and the Massif Central). The Early Archaean age of 3.84 b.y. was obtained within the euhedral core of a detrital grain from a Moldanubian paragneiss. The same detrital zircon grain reveals at least three metamorphic overprints at 2.59 b.y. B.P., 1.94 b.y. B.P. and 460 m.y. B.P., and at least two cycles of mechanical abrasion during surface recycling in the Early Proterozoic and after the Pan-African cycle. It is, thus, the crystal with the most complex geological history ever detected. Further Archaean ages for the crystallization of primary magmatic zircons, probably in the course of continental crust formation in the first cycle provenances of the two studied areas, were found at 3.15, 2.9, 2.76 and 2.65 b.y. B.P., and in a few cases between 2.6 and 2.5 b.y. B.P. Early Proterozoic crust-forming events between about 1.75 and 2.15 b.y. B.P. play an important role, especially for the sediments and metasediments of the Montagne Noire at the southern rim of the Massif Central. Middle and Late Proterozoic ages cluster around 1.0 b.y. B.P. and ≈ 600 m.y. B.P. for all analysed samples. Therefore, the deposition of the metasedimentary precursors took place after the Pan-African cycle, which is in line with the Rb-Sr whole-rock systematics of Central European metasediments. The ages of metamorphism in the different source areas could be inferred in some cases by lower intercept ages obtained on single detrital grains. The following sequence coinciding with times of magmatic events has been so far established: 2.59,1.94 and 1.0 b.y. B.P., and 600 m.y. B.P. Thus, ion-probe dating is also capable of defining secondary effects in the respective provenances which are typical for orogenic cycles. The ubiquitous presence of Pan-African detrital zircons in European metasediments is a strong indication that the detrital material was derived from Gondwana. However, from the presence of metamorphic units including eclogitefacies rocks which probably formed at different continental margins and at various times between about 500 m.y. and 330 m.y. ago it is probable that various Gondwana-derived microcontinental plates contributed to the different basin fillings. The Gondwana-derived detritus is also supported by the observation of two periods of relative quiescence or non-activity around 2.3 b.y. ago, and especially around 1.5 b.y. ago, which is a typical feature observed in Gondwana (Cahen et al., 1984) but not in Laurasia, the other possible source of the detritus. The lack of zircons crystallized between 3.84 and 3.15 b.y. ago might have different causes. Thus, the continental crust of the European Hercynides probably developed since the Early Archaean and grew via many crustal accretion events to its present state.