Complex Zircon Research Articles

The interpretation of complex zircon U–Pb systems in Archaean granitoids and gneisses from the Jack Hills, Narryer Gneiss Terrane, Western Australia

SHRIMP U–Pb ages of zircons from Archaean TTG granitoids and gneisses from the Jack Hills region of Western Australia show a range of complexity which reflects the origin and geological history of the rocks and the stability of the zircon U–Pb systems under metamorphic and magmatic conditions. Strong light rare earth element (LREE) enrichment and similarities in the major and trace element chemistry suggest that TTG gneisses formed by fractional melting of a garnet-bearing amphibolitic source. Maximum ages, of 3.75 to 3.6 Ga, obtained from low U zircon cores are interpreted as the period of formation of the TTG parent rocks. However, most zircons in these populations are complex, consisting of zoned and unzoned parts, and have disturbed U–Pb systems which form a concordant to discordant array between 3.7 and 3.3 Ga on a concordia plot, suggesting these grains experienced one or more disturbance events within this time span. The range of complexity of U–Pb systems observed in a number of the zircon populations is also present within individual zircon crystals indicating that secondary movement and loss of Pb and possibly U has occurred within and from zircons, causing variable isotopic discordance and yielding a range of apparent ages within a single grain. Disturbance of the zircon isotopic systems appears to be due to a combination of early Archaean events together with recent Pb loss. The presence of 3.3 Ga zircon rims around older cores in samples of tonalite and porphyritic granodiorite, together with the exclusive presence of 3.3 Ga zircons in one sample of porphyritic granite, is interpreted as evidence of a major magmatic and possibly metamorphic event at this time. The presence of ca 3.5 Ga ages in a number of unzoned zircons in the tonalite and two samples of granodiorite suggests that this could also have been a time of magmatism and zircon crystallization. Evidence for 3.3 Ga magmatism appears to be restricted to zircons from TTG rocks south of the Jack Hills Metasedimentary Belt, raising the possibility that the present position of the belt defines a major suture within the Narryer Gneiss Terrain. The post-tectonic monzogranites have LREE enriched patterns with strong negative Eu anomalies, similar to the older gneisses, but in contrast to these rocks, zircons from the monzogranites have relatively simple U–Pb systems, which define an age of 2654±7 Ma, and only rarely do zircons have inherited cores with nearly concordant ages of ca 3.6 Ga. The very low content of inherited zircon raises doubts about the formation of the granites by reworking of older TTG gneisses and an alternative explanation that the monzogranites formed from new 2.65 Ga crust, with minor 3.6 Ga crustal contamination, is seen as more likely.

The evolution of a layered metaigneous complex in the Rauer Group, East Antarctica: evidence for a distinct Archaean terrane

The Rauer Group in Prydz Bay, East Antarctica, contains both Archaean and Proterozoic crustal components. Archaean geological relationships have been defined from the Scherbinina layered complex (SLC), a polydeformed and metamorphosed Fe-tholeiite intrusive cut by several generations of deformed metabasite dykes. The SLC and three of these dyke generations are intruded by tonalitic and trondhjemitic sheets representative of Archaean felsic orthogneisses present outside the complex. UPb shrimp dating of zircons precisely defines the magmatic age of the SLC as 2844 ± 6 Ma. A tonalite sheet that cuts the SLC preserves a complex zircon isotopic array dominated by inherited grains derived from source rocks up to 3470 ± 30 Ma old, but from field evidence was emplaced at ca 2800-2550 Ma. The presence of 3470 Ma source rocks is unique in the East Antarctic Shield, and this feature as well as the prominence of 2840-2800 Ma orthogneisses establishes the Rauer Group as a distinct Archaean terrane. The Archaean of the Rauer Group is not comparable with the nearby Vestfold Hills, where ca 2520-2480 Ma orthogneisses form the dominant rock units. It is likely that the Vestfold Hills and Rauer Group terranes were amalgamated at either 1000 or 500 Ma, rather than at the end of the Archaean. A mid-Proterozoic tectonothermal event documented in the Rauer Group from syn-metamorphic 1030-1000 Ma intrusives (Kinny, P.D., Black, L.P. and Sheraton, J.W., 1993. Zircon ages and the distribution of Archaean and Proterozoic rocks in the Rauer Islands. Antarctic Sci., 5, 193–206) has not been detected in the zircon UPb data obtained on the Archaean gneisses examined here. Instead, the SLC and intrusive tonalite show resetting of zircon UPb systematics at ca 520 Ma. This resetting reflects Pb-loss within irregular compositional zones and healed cracks in older zircon, and is attributed to the effects of fluid infiltration under greenschist to amphibolite facies conditions. These effects correlate in time with ca 530-510 Ma higher-grade deformation and metamorphism evident in Proterozoic paragneisses exposed in areas to the south-west of the Rauer Group, and may record amalgamation of the Rauer Group with these areas during the East Gondwanian Pan-African orogeny.

The Microtome: An Innovative Tool Prerequisite for Applying Multiple High-Precision/Resolution Analytical Techniques to Wafers of Same Sub-Grain Scale Sample

The modern high-precision/resolution analytical techniques call for single-grain samples, like individual zircon crystals. The problem is that even single zircon grains frequently demonstrate a complex, inhomogeneous nature due to the presence of chemically different phases, of structurally distinct crystal domains or of growth zones which may reflect diverse and sometimes independent stages of the history of the momentary host or precursor rocks. The most reliable interpretation for the complex mineral grains can be achieved by applying all relevant analytical methods to the same mineral domain within a single mineral grain. However, up to now the conventional preparation of small samples for ion microprobe, electron microprobe/microscope, cathodoluminescence, radiography and other microanalytical studies required mechanical polishing which usually results in irretrievable loss of at least half of the original sample. In addition, this way of sample preparation does not allow to retain the original material either for future cross-checking or for correlative investigations, such as studying the effects of chemical or mechanical treatment on the initial substance. Thus, the necessity of having closely equivalent portions of a specimen at hand is obvious. Crystal portions of such properties are most easily produced in the shape of thin wafers. Our novel sub-grain scale technique based on a recently developed miniature wire-saw (microtome) expressly permits precise cutting of hard crystalline objects, in particular of submillimeter-sized crystals. The instrument drives a tungsten cutting wire, 4 to 13 micrometer in diameter, passing through a slurry of 1 to 0.5 micrometer diamond particles and can provide individual or sets of thin slices from single crystals, in any direction desired. The microtome has proved its worth in slicing mineral grains without damage to the original sample and without significant loss of material. The resulting cuts were found to be no wider than the diameter of the cutting wire. The exact and firm adjustment of the sample as well as the precise and continuously controlled guidance of the wire results in planar and strictly vertical cuts. Under the microscope, all the obtained mineral slices exhibit very light sawing tracks. These prove useful for the documentation of the microsampling process, such as orientation and identification of the slices and proper restoration of the grain. The sawing tracks have a negligible depth (fractions of micrometer) and if desired can be easily erased with a few passes in 1-micrometer diamond abrasive. This approach to producing equivalent wafers attains particular significance in creating standards for microanalytical work from existing rare and highly valuable reference materials. An automated version of the prototype microtome described by Sergeev et al. [1997] has since been developed. This advanced instrument is now equipped with a slow-running electrical motor and an upgraded system for wire-tension control. This permits unidirectional and steadier motion of an extended portion of the cutting wire that further improves the quality of the cut surface. The practicability of the microtome technique in zircon U/Pb analysis has been tested in two ways: Firstly, we sliced a chip of the zircon standard, routinely used for calibration of the ion microprobe into three pieces. The two lateral parts were first checked by cathodoluminescence imaging for internal homogeneity and then placed onto the sticky tape with the cut surfaces down like the other unknowns before filling up the mount with epoxy resin. The remaining central part of the standard zircon was analysed by isotope dilution techniques in order to get precise calibration results for the standard material. Secondly, with the aim of extracting a maximum of age and petrogenetic information from complex zircon grains we cut selected crystals along the c-axis to obtain two mirror-image sections using the 5micrometer wire. One half-zircon (part A) was investigated by cathodoluminescence imaging and

Determination of Radiation Damage Ages on Parts of Zircon Grains by Raman Microprobe: Implications for Annealing History and U-Pb Stability

The advent of ion microprobes has made it possible to determine U-Pb ages on micro-sized (10-30 micron diameter) areas on the polished surface of a U-bearing mineral such as zircon. Previously observed cores and rims can now be dated directly, whereas using conventional techniques, even by measuring single abraded grains, analyses of mixed core-rim phases can rarely be avoided. In making micro-scale analyses geochronologists have become aware that zircons (and other U-bearing minerals) are more complex than previously thought and have a variety of internal structures that reflect geological processes. These structures are studied extensively using back scattered electron and cathodoluminescence imaging. However these techniques do not provide information on the crystallinity of various parts of a complex zircon crystal. It is well known that zircon breaks down to the metamict state through time integrated radiation damage and a measure of the degree of this breakdown on a micro-scale will provide basic information on questions such as the geological significance of recent lead loss observed in many zircon U-Pb analytical results, and together with Th-U concentrations determined by ion microprobe or other techniques, will provide a means for determining radiation damage ages of selected areas of a zircon crystal. The determination of radiation damage ages of zircon on a micro-scale is therefore a new innovation applied to an old idea with scope to provide previously unavailable information for the interpretation of zircon U-Pb ages and the thermal evolution of the host rock.

Proterozoic zircon growth in Archean lower crustal xenoliths, southern Superior craton - a consequence of Matachewan ocean opening

Granulite-grade, anorthositic and mafic xenoliths recovered from a Jurassic kimberlite pipe near Kirkland Lake, Ontario are fragments of the lower crust that underlies the ca. 2.7 Ga Abitibi greenstone belt of the Superior craton. Cathodoluminescence imaging and/or backscatter electron microscopy of zircon from four individual xenoliths reveals a complex crystallization history, characterized by two main stages of zircon growth. The age of the two stages has been constrained by combining imaging results with isotope dilution U-Pb dating of grain fragments and single grains. Minimum ages for the first crystallization stage in individual xeno liths are 2584 ± 7 Ma, 2629 ± 8 Ma, 2633 ± 3 Ma, whereas an approximate crystallization age for a fourth sample is 2788 ± 57 Ma. The second main stage of growth consists of chemically and isotopically distinct metamorphic zircon overgrowths. Times of solid-state zircon growth are most broadly constrained in three samples to the interval between 2.52 Ga to 2.40 Ga, and most precisely dated in a meta-anorthosite at 2416 ± 30 Ma. These complex zircons are intergrown with garnet and clinopyroxene of the host granulite-facies assemblage, and thus the Paleoproterozoic ages of the metamorphic overgrowths are interpreted to reflect an interval of isobaric, granulite-grade metamorphism of the lower crust beneath the greenstone belt approximately 150 million years after craton formation. This interval of metamorphism is broadly coeval with the intrusion of the Matachewan dyke swarm across the southern Superior craton, and with mafic magmatism and deposition of Huronian rift-margin sediments 200 km to the south during the opening of the Matachewan ocean. It is proposed that a significant volume of magma intruded the crust-mantle interface during rifting, promoting isobaric metamorphism and zircon growth in the deep levels of the Superior craton. Subsequent major rifting events along this margin apparently failed to produce a similar lower crustal response. The results have important implications for the structure of lithosphere beneath Archean continental crust.

Resolving high precision U-Pb ages from Tertiary plutons with complex zircon systematics

Success in establishing high precision crystallization ages with the U/Pb zircon method is highly dependent on the ability to isolate or remove the effects of inherited xenocrystic components or secondary lead-loss, or the combined effects of both. Current “conventional” single crystal and ion probe (SHRIMP) methods are successfully applied to Paleozoic and older samples but typically suffer from imprecision when applied to younger (Mesozoic-Cenozoic) samples due to generally reduced radiogenic lead concentrations. Here we apply a series of intense partial dissolution steps to multigrain zircon fractions from a series of Paleocene tonalitic to granodioritic intrusive rocks from southeastern Alaska. The zircon systematics are complicated by both minor inheritance and postcrystallization lead-loss. Physical and isotopic evidence demonstrate that the partial dissolution steps preferentially remove outer layers susceptible to lead loss, as well as core regions containing inherited components. The final residues are often hollow shells of low-U primary igneous zircon that yield highly precise, reproducible, and concordant ages. The resulting age determinations commonly statistically differ at the 95% confidence level from ages based on apparently concordant, but less precise conventional isotope dilution analyses, the uncertainties of which masked minor, subtle isotopic complexities. This observation strongly cautions against basing age assignments of samples yielding slightly discordant data on (1) single “concordant” analyses accompanied by an array of discordant data or (2) the mean of several 206Pb/ 238U or 207Pb/ 206Pb ages. The step-wise dissolution technique allows age determinations on young, relatively low U and Pb zircons at a resolution not currently possible with techniques such as single-grain conventional or spot ion probe analyses. Widespread application of the technique will likely prove instrumental in resolving detailed magmatic histories of igneous complexes where zircon systematics are typically complex.

Southernmost Andes and South Georgia Island, North Scotia Ridge: Zircon U-Pb and muscovite [formula omitted] age constraints on tectonic evolution of Southwestern Gondwanaland

Zircon U-Pb and muscovite 40Ar 39Ar isotopic ages have been determined on rocks from the southernmost Andes and South Georgia Island, North Scotia Ridge, to provide absolute time constraints on the kinematic evolution of southwestern Gondwanaland, until now known mainly from stratigraphic relations. The U-Pb systematics of four zircon fractions from one sample show that proto-marginal basin magmatism in the northern Scotia arc, creating the peraluminous Darwin granite suite and submarine rhyolite sequences of the Tobifera Formation, had begun by the Middle Jurassic (164.1 ± 1.7 Ma). Seven zircon fractions from two other Darwin granites are discordant with non-linear patterns, suggesting a complex history of inheritances and Pb loss. Reference lines drawn through these points on concordia diagrams give upper intercept ages of ca. 1500 Ma, interpreted as a minimum age for the inherited zircon component. This component is believed to have been derived from sedimentary rocks in the Gondwanaland margin accretionary wedge that forms the basement of the region, or else directly from the cratonic “back stop” of that wedge. Ophiolitic remnants of the Rocas Verdes marginal basin preserved in the Larsen Harbour complex on South Georgia yield the first clear evidence that Gondwanaland fragmentation had resulted in the formation of oceanic crust in the Weddell Sea region by the Late Jurassic (150 ± 1 Ma). The geographic pattern in the observed age range of 8 to 13 million years in these ophiolitic materials, while not definitive, is in keeping with propagation of the marginal basin floor northwestward from South Georgia Island to the Sarmiento Complex in southern Chile. Rocks of the Beagle granite suite, emplaced post-tectonically within the uplifted marginal basin floor, have complex zircon U-Pb systematics with gross discordances dominated by inheritances in some samples and Pb loss in others. Of eleven samples processed, only two had sufficient amounts of zircon for multiple fractions, and only one yielded colinear points. These points lie close to the lower concordia intercept for which the age is 68.9 ± 1.0 Ma, but their upper intercept is not well known. Inasmuch as this age is similar to the 40Ar 39Ar age of secondary muscovite growing in extensional fractures of pulled-apart feldspar phenocrysts in a Beagle suite granitic pluton (plateau age is 68.1 ± 0.4 Ma), we interpret the two dates as good time constraints for cooling following a period of extensional deformation probably related to the tectonic denudation of the highgrade metamorphic complex of Cordillera Darwin in Tierra del Fuego.

Conventional U-Pb dating versus single-grain Pb evaporation dating of complex zircons from a pegmatite in the high-grade gneisses of southwestern Sweden

U-Pb multi-grain dating of zircons from a pegmatite dyke in southwestern Sweden yield analyses which define a discordia with upper and lower intercept ages of 1510 ± 32 Ma and 631 ± 64 Ma respectively. Dating by the single-zircon evaporation method on prismatic zircons from the same dyke indicates a primary crystallization age of 1409 ± 20 Ma. In addition, four oval zircon grains yield a 207Pb 206Pb age of 969 ± 27 Ma. The older single-zircon evaporation age is interpreted as representative of pegmatite emplacement, whereas the younger age is suggested to correspond to isotopic disturbance and new zircon growth during a late Sveconorwegian metamorphic event. To explain the false intercept ages in the U-Pb concordia diagram, this study suggests a three-stage model including (1) crystallization of zircons during pegmatite intrusion, (2) episodic lead-loss and new zircon growth during high-grade metamorphism and (3) a recent lead-loss event. As the dykes are strongly deformed, the protolith age of the pegmatite provides an upper age limit for the gneiss-forming event. The single-zircon age of 969 ± 27 Ma is interpreted to be associated with this event, which took place during high-grade metamorphic conditions.

U-Pb zircon geochronology of the Selbaie Cu-Zn-Ag-Au mine, Abitibi Subprovince, Canada

The Selbaie mine is a large tonnage, low-grade Cu-Zn-Ag-Au deposit with both stratiform and epithermal characteristics. Mineralization comprises lower grade, disseminated Zn-Ag ore and pyritic siliceous sinter of the A1 zone at the surface; and higher grade Cu-Zn-Ag-Au quartz sulfide lenses of the A2 zone below and transitional with the A1 zone, and of the B zone  1 km to the west. The copper-rich quartz sulfide lenses comprise networks of subvertical quartz sulfide veins, vein arrays, and breccia. They have irregular, locally gradational boundaries and variable attitudes, with dips from 25 o to 90os The mineralization is within a volcanic stratigraphic succession of heterolithic breccias and felsic and intermediate tuffs and flows, which are cut by mafic and felsic intrusions and felsite dikes. The felsic volcanic rocks and a tonalitic marginal phase from the adjacent Brouillan pluton are calc-alkalic and have geochemical signatures that support a cogenetic relationship. The mineralization and mine stratigraphic succession are truncated by the Brouillan faults, a series of subparallel, west-directed, predominantly brittle thrusts with the Brouillan pluton on the upper plate above the mine stratigraphic succession. Galenas from stratiform and vein ore have uniform lead isotope compositions of 2o6pb/2o4Pb = 13.184 to 13.211, '2o7pb/2o4pb = 14s351 to 14.369, and '2ospb/2o4pb = 33s039 to 33.100 , with model/ values of 8.1-8.2, and model K values of 4.1 to 4.2 (n = 5); among the most primitive of Abitibi volcanic-associated massive sulfide depositss They have model ages that range from 2.71 to 2.68 Ga. A U-Pb zircon age of 2729_+ Ma for a sericite-altered, calc-alkalic quartz porphyry flow 150 m up section from stratiform mineralization closely approximates the age for this mineralization type. A U-Pb zircon age for a felsite dike that cuts postore east-west foliation is 2726 _+ 3 Ma, slightly younger but overlapping at the 20- level with the age for the quartz feldspar porphyry flow, indicating that the copper-rich quartz sulfide lenses are essentially coeval with the lower grade stratiform mineralization. A sample of sheared, chloritic granodiorite taken from one of the Brouillan faults near the margin of the Brouillan pluton has a complex zircon opulation Six concordant anal ses of abraded, multigrain fractions, individual grains, or grain tips 20, P, 206 '. s

Heterogeneous metamictization of zircon on a microscale

The ability of modern Raman microprobes to determine the crystallinity of zircon with a lateral resolution of about 1 μm and a volume resolution of less than 5 μm 3 opens up new opportunities for correlating structural, chemical, and isotopic data on the same micro-area within a zircon grain. Combining this information we confirm by direct measurement the inhomogeneity in the degree of crystallinity of complex zircon grains and the correlation between U-Th concentration, isotopic discordance, and degree of metamictization in micro-areas of these grains. Micro-areas with a higher degree of metamictization are more chemically heterogeneous than neighbouring more crystalline areas and contain higher concentrations of Ca, Fe, U, Th, and Y but not always Hf, which in some cases shows higher concentration in the less metamict areas. Raman observations that there are no “radiation-damage halos” extending over distances greater than 1–1.5 μm preclude alpha and beta radiation as well as fission recoil as viable causes for amorphization, and confirm that alpha recoil is the most important mechanism causing zircons to disorder into the metamict state.

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. ɛNd(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 ɛNd(t) values from −7.7 to −2.8 (TDM 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.

Palaeoproterozoic U-Pb zircon ages from Belorussia: new geodynamic implications for the East European Craton

Abstract The crust in the northwest of the East European Craton has traditionally been regarded as part of an Archaean nucleus in eastern Europe. Recent correlation with the Baltic Shield in Scandinavia, however, suggests the need of reappraisal. To study this problem, zircons from three boreholes in the basement of the Russian Platform in Belorussia (Belarus) have been dated by the U-Pb and Pb-Pb methods. The ages indicated by two U-Pb discordias are 1982±26 Ma for a metadacite from the Central Belorussian Belt of volcanic-arc igneous rocks, and 1800±7 Ma for mafic granulite from the Baltic-Belorussian Granulite Belt largely built up of mafic igneous rocks. For another Central Belorussian metadacite, a Pb-Pb age of 1975±10 Ma was obtained. In previous “stratigraphies” of Belorussia, all three dated rocks have been considered to be important components of an Archaean, even Early Archaean crust. Analyses of abraded crystal cores of complex zircons in one sample and separated core and rim zircons in another do not suggest substantially older ages of the core materials. Together with previous ages of granitoids and charnockites, which had, however, been explained as a result of late magmatism and metamorphism, and therefore not characterizing the age of the crust, our data indicate the dominance of Palaeoproterozoic rocks in the basement of the northwestern part of the East European Craton that adjoins the Tornquist-Line boundary between central and northeastern Europe. In conjunction with the results of a Finnish-Estonian study of similarly allegedly Archaean, but in actual fact Proterozoic rocks in southern Estonia, our results suggest the existence of large Palaeoproterozoic crustal belts extending all the way from the Finnish Gulf to eastern Poland. This implies a substantial revision of established concepts on the Precambrian geology in the northeastern half of the European continent.

UPb zircon ages from the Hook granite massif and Mwembeshi dislocation: constraints on Pan-African deformation, plutonism, and transcurrent shearing in Central Zambia

The Hook granite massif in the inner part of the Lufilian arc in central Zambia generally has been interpreted as granitized Archean or Paleoproterozoic sialic basement partly remobilized during Pan-African orogenesis. Reconnaissance field studies and UPb zircon geochronology reveal no evidence for exposed basement within the massif, which instead is shown to be a large, syn- to post-tectonic, composite batholith intrusive into upper Katangan (Kundelungu) strata. Syntectonic parts of the massif contain a single, moderate- to high-temperature, solid-state foliation that is continuous with the regionally developed S1 fabric in the adjacent lower-grade Kundelungu metasedimentary rocks. Two separate phases of syntectonic granite yield UPb zircon upper intercept ages of 559±18 and 566±5 Ma. Both samples show normal, linear discordance patterns, and zircon residues from acid leaching plot nearer concordia. An undeformed rhyolite dike intruding a Katangan pendant in the central part of the massif shows more complex zircon isotopic systematics, but a nearly concordant fraction has an age of 538±1.5 Ma. Post-tectonic granite yields an upper intercept age of 533±3 Ma. These ages constrain Kundelungu deposition in central Zambia as pre-570 Ma and show that regional deformation and voluminous syntectonic granite plutonism occurred in the inner part of the Lufilian arc at 560–570 Ma, with the major tectonic activity ending by 530–540 Ma. The Mwembeshi dislocation, a regionally significant Pan-African transcurrent shear zone, runs along the southern margin of the Hook massif. Syntectonic rhyolite intruded in the dislocation yields an upper intercept age of 551±19 Ma, showing that transcurrent shearing occurred in the same time frame as batholith emplacement, probably within an overall transpressive regime. Deformation and batholith emplacement in the inner part of the Lufilian arc were synchronous with a major pulse of syn- to post-tectonic granite plutonism in the Damara belt of Namibia, supporting a direct link between these regions during Pan-African orogenesis. In contrast, tectonothermal activity in central Zambia is unrelated to main-phase orogenesis in the Zambezi belt to the east, dated at ∼820 Ma. Models that propose a direct kinematic link between deformation in the Lufilian arc, shearing along the Mwembeshi dislocation, and main-phase orogenesis in the Zambezi belt are not supported by the present age data.

Lead geochronology of zircon by LaserProbe-inductively coupled plasma mass spectrometry (LP-ICPMS)

LaserProbe-inductively coupled plasma mass spectrometry (LP-ICPMS) provides a sensitive, fast, and simple means to determine 207Pb 206Pb ages in single zircon grains. A Nd:YAG laser is used to irradiate the zircon surface and leaves a cylindrical pit of 30–60 μm, from which the vaporised materials are transported by argon gas to a Fisons-VG PQII+ ICPMS for analysis. No zircon abrasion, cleaning nor chemical separation procedures are required. The accuracy and the limitation of the method were evaluated by analyzing twenty-one zircon samples ranging in age from 2.7 Ga to 1.0 Ga, which have also been dated by the conventional U-Pb thermal ionization mass spectrometry technique (TIMS). The LP-ICPMS 207Pb 206Pb ages for zircons with grain size > 60 μm and 207Pb concentration > 3 ppm are within 1% of the TIMS ages. Smaller zircons (≤60 μm) and those with 207Pb concentration < 2 ppm yield inaccurate ages. Operating the LP-ICPMS at conditions that give a compromise between the ideal spatial resolution and instrument sensitivity, the limits of detection were found to be 0.2 ppm for 206Pb, 207Pb, and 208Pb. The precision of the 207Pb 206Pb ratio is generally 0.5–6% (1σ) from each sampling pit and is strongly dependent on the lead concentration. However, the precision for the average of the mean ratios from different pits in one grain or several grains of the same population are generally <1.5% (1σ). The results presented here demonstrate that the LP-ICPMS can be used to determine 207Pb 206Pb ages of zircons and that reliable ages can be obtained from high quality, limpid zircons with a simple Pb-loss history. Uses of the technique include screening of zircon populations from different rocks in areas of poorly known age relationships and provenance studies of detrital zircons from ancient and modern sedimentary sequences. Other applications include the study of growth zones and of inherited components in complex zircon populations. In comparison with the SHRIMP (Sensitive High Resolution Ion MicroProbe) technique, the LP-ICPMS has the disadvantage of partially destroying the sample and inability to obtain accurate and precise U Pb ratio at present. However, LP-ICPMS has better sensitivity, similar precision, but slightly worse spatial resolution for the measurement of 207Pb 206Pb relative to the SHRIMP. Low purchasing and operating cost for the LP-ICPMS provide a simple and economic alternative to the SHRIMP method.

The numerical age of the Devonian-Carboniferous boundary

AbstractThe SHRIMP ion microprobe has been used to date zircons in a 1 cm thick bentonite located in the Hasselbachtal auxiliary global stratotype section through the Devonian-Carboniferous boundary in Germany, and a tuff located at a similar biostratigraphic level in Australia. Multiple replicate analyses have yielded indistinguishable ages, and indicate 353.2±4.0 Ma (2σ) as the age of the boundary. Obtaining an age for the boundary in the German reference section depended on the ability of SHRIMP to date a small sample and to target small areas of structurally complex zircons. Future refinements of the age of this boundary may depend on re-analysis of this uniquely placed bentonite, and this highlights the value of defining biostratigraphic stratotypes in the vicinity of known dateable horizons.

Chronology of early Archaean granite-greenstone evolution in the Barberton Mountain Land, South Africa, based on precise dating by single zircon evaporation

We report precise 207Pb 206Pb single zircon evaporation ages for low-grade felsic metavolcanic rocks within the Onverwacht and Fig Tree Groups of the Barberton Greenstone Belt (BGB), South Africa, and from granitoid plutons bordering the belt. Dacitic tuffs of the Hooggenoeg Formation in the upper part of the Onverwacht Group yield ages between 3445 ± 3 and 3416 ± 5 Ma and contain older crustal components represented by a 3504 ± 4 Ma old zircon xenocryst. Fig Tree dacitic tuffs and agglomerates have euhedral zircons between 3259 ± 5 and 3225 ± 3 Ma in age which we interpret to reflect the time of crystallization. A surprisingly complex xenocryst population in one sample documents ages from 3323 ± 4 to 3522 ± 4 Ma. We suspect that these xenocrysts were inherited, during the passage of the felsic melts to the surface, from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite-trondhjemite plutons along the southern and western margins of the BGB, and units predating any of the exposed greenstone or intrusive rocks. Several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 Ma, coeval with or slightly older than Onverwacht felsic volcanism, while the Kaap Valley pluton along the northwestern margin of the belt is coeval with Fig Tree dacitic volcanism. These results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites. Some of the volcanic and plutonic units contain zircon xenocrysts older than any exposed rocks. These indicate the existence of still older units, possibly stratigraphically lower and older portions of the greenstone sequence itself, older granitoid intrusive rocks, or bodies of older, unrelated crustal material. Our data show that the Onverwacht and Fig Tree felsic units have distinctly different ages and therefore do not represent a single, tectonically repeated unit as proposed by others. Unlike the late Archaean Abitibi greenstone belt in Canada, which formed over about 30 Ma, exposed rocks in the BGB formed over a period of at least 220 Ma. The complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones. A surprising similarity in rock types, tectonic evolution, and ages of the BGB in the Kaapvaal craton of southern Africa and greenstones in the Pilbara Block of Western Australia suggests that these two terrains may have been part of a larger crustal unit in early Archaean times.

Single-zircon dating by stepwise Pb-evaporation constrains the Archean history of detrital zircons from the Jack Hills, Western Australia

Pb isotope analyses have been carried out on 42 zircon grains from a Western Australian metaconglomerate using stepwise Pb-evaporation directly in the ion source of a thermal ionization mass spectrometer. The metaconglomerate is from the Archean Jack Hills Metasedimentary Belt, and is known from ion microprobe (“SHRIMP”) analyses to contain a complex zircon population with ages between 4.2 Ga and 3.1 Ga. The same complex pattern of ages is found by the Pb evaporation studies. Five grains yielded minimum crystallization ages from 4.17 Ga to 4.07 Ga. The main population appears significantly younger, having been generated at about 3.55-3.3 Ga. The agreement between the two analytical approaches confirms the SHRIMP results and demonstrates the value of the stepwise-evaporation technique in determining the age patterns of mixed zircon populations. In many of the evaporative Pb isotope records the 207/206 ratios remained constant for all evaporation steps, which we interpret as evaporation from concordant zircon phases. However, for the majority of zircons 207/206 ratios increased with increasing evaporation temperature, and usually approached constant values during evaporation at the highest temperatures. This can be attributed to mixing of different radiogenic Pb components from either crystalline zircon phases of different ages or from domains of isotopically disturbed metamict zircon. Present results confirm > 4 Ga zircon ages in the metaconglomerate from the Jack Hills and substantiate formation of crust at a very early stage in the evolution of the earth. Results also confirm a major crust-forming event 3.55-3.3 Ga ago.

Age of the barramundi orogeny in northern Australia by means of ion microprobe and conventional U-Pb zircon studies

U-Pb zircon geochronological studies are used to quantify the tectonostratigraphic history of a major crust-forming event in early Proterozoic terranes of northern Australia. Magmatism and tectonic processes associated with this event, the Barramundi Orogeny, appear similar over very wide areas, about one third of the Australian continent. The Barramundi event marks the deformation, metamorphism and cratonization of the earliest Proterozoic ∼2.0−1.9 Ga basinal successions, and it was followed by and closely linked to extensive orogenic felsic magmatism. The Yaringa Metamorphics are an inlier of paragneiss and schist that pre-dates 1865 ± 3 Ma Leichhardt felsic magmatism and younger basin formation in the Mount Isa Inlier. From an ion microprobe U-Pb zircon study of massive and migmatitic gneiss, the time of Yaringa metamorphism is established at 1890±8 Ma. Recognized younger Proterozoic deformations in the interval 1610−1550 Ma are generally not evident in zircon U-Pb systems, but have totally reset local Rb-Sr whole-rock values. Conventional U-Pb work on multi-grain zircon samples of Yaringa gneiss gives a geologically meaningless age (2090±40 Ma) that is an artifact of complex zircon growth, part of which was initially inherited from a 2.55 to 2.2 Ga provenance, and the other major part of which was formed in situ during the 1890 Ma high-grade metamorphism. Predictably, for simple zircon systems in low-grade metavolcanic térranes, conventional and ion microprobe zircon ages are in close agreement. Dating of volcaniclastic sequences above and below the Barramundi tectonic break in the Pine Creek Inlier brackets the orogeny in this terrane to a similar time frame, 1885–1870 Ma. Some zircons which crystallized at this time form selvedges around older cores whose age of 2100-2000 Ma may be relics of an earlier deep crustal underplating. The uniform chemistry of post-Barramundi felsic magmatism, and the similar tectonic setting of the areally extensive Barramundi event, are mirrored by their temporal coherency. Cycles of geosynclinal deposition, burial, metamorphism and uplift took place during intervals of several million years or less, implying comparable rates for early Proterozoic orogenesis to those established for Tertiary and Quaternary settings.

Zircon U-Pb dating in the vicinity of the Olympic Dam Cu-U-Au deposit, Roxby Downs, South Australia

Isotopic data are presented for granitoids and felsic volcanics from the Stuart shelf of South Australia, some of which are spatially associated with the Olympic Dam Cu-U-Au deposit. Relatively unaltered quartz monzodiorite, more extensively altered quartz syenite, and felsic volcanics yield zircon U-Pb ages of 1,590 (super +5) (sub -4) , 1,613 (super +20) (sub -19) , and 1,576 (super +22) (sub -19) m.y., respectively. The zircons from this group of rocks, the Olympic Dam suite, have similar morphologies and similar 206 Pb/ 208 Pb ratios, suggesting an intimate genetic relationship. Their ages are comparable to the Gawler Range Volcanics cropping out approximately 130 km to the southwest in the Gawler orogenic domain.Close to the Olympic Dam deposit, the zircon U-Pb system of Olympic Dam suite quartz syenite was contaminated by barite precipitated from hydrothermal fluids associated with the mineralization. The barite is readily removed by air abrasion, which restores the reversely discordant and spuriously young zircons to more normally discordant positions on the concordia diagram. The abraded zircons then have U-Pb ages similar to uncontaminated Olympic Dam suite zircons. The barite lead must be enriched in 206 Pb which is concluded to be derived from radium extracted from U-bearing rocks and admitted to the Ba crystal lattice site.Deformed and altered basement granitoids on the Stuart shelf have complex zircons with discrete cores and overgrowths which yield complicated U-Pb isotope systematics. The cores are estimated to be about 1,800 m.y. old, and the overgrowths probably less than 1,730 m.y. old. These granitoids are temporally correlated with basement rocks of the Gawler orogenic domain.Comparable apatite U-Pb and biotite Rb-Sr isotope dates from all the granitoids and from thin veins in the Acropolis prospect suggest regional cooling through the respective blocking temperatures (about 350 degrees -400 degrees and 300 degrees C) between 1,602 + or - 7 and about 1,542 m.y. The Acropolis apatite date suggests at least some Stuart shelf mineralization was emplaced at, or before, about 1,602 m.y. A maximum age for the Olympic Dam mineralization is set by the Olympic Dam suite quartz syenite zircon age, 1,613 (super +20) (sub -19) m.y. Our data do not place minimum age limits on the mineralization.

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.