P. D. Kinny

3820 Ma zircons from a tonalitic Armîsoq gneiss in the Godthåb district of Southern West Greenland

Abstract Zircons from a sample of the tonalitic Amitsoq gneisses near Godthhb, West Greenland, exhibit several stages of crystal growth. Measurements of the U-Th-Pb isotopic composition of 30 μm areas within individual grains using the ion microprobe SHRIMP reveal that components of magmatic zircon from the original igneous protolith are at least 3822 ± 5 Ma (lσ) old, slightly but significantly older than the age of zircons from a fclsic volcanic unit in the Isua supracrustal belt as measured at high precision by the same technique, and older than all ages previously determined by mineral and whole-rock isotopic techniques for Amitsoq gneisses at Godthab and at Isukasia. Overgrowths of younger zircon formed at ca. 3630 Ma, coinciding with an episode of major Pb loss from the older grains. These results indicate that previous whole-rock isochron ages refer to later isotopic redistribution and/or that other phases of the Amitsoq gneisses intruded during this later period and dominate the isotopic systematics. Comparison of the ion-probe results with previous conventional U-Th-Pb data for the same zircon sample demonstrates the strong influence of discordant high-uranium parts of zircon on the mean isotopic composition of multi-grain samples.

Svecofennian detrital zircon ages: implications for the Precambrian evolution of the Baltic Shield

Metasediments intruded by 1.90-1.87 Ga old plutonic rocks form the oldest major Proterozoic crustal component in the Svecofennian Domain of the Baltic (Fennoscandian) Shield. Their NdTDM model ages and conventional multigrain zircon Uue5f8Pb ages between 2.4 and 2.1 Ga have previously been interpreted either as mixing ages between ∼ 1.9 Ga old juvenile materials and a minor Archaean component, or as actual rock and protolith ages. To resolve the ensuing controversy, 120 individual detrital zircons from Svecofennian metasediments in Sweden and Finland were analysed using the SHRIMP ion microprobe. n nThe oldest materials in this array are a 3.44 Ga old zircon from the Tampere Schist Belt in Finland and a 3.32 Ga old crystal from southeastern Sweden. About 30% of the analysed crystals are 2.97-2.60 Ga old, while ∼ 65% have ages between 2.12 and 1.88 Ga. Thus there is no evidence of 2.6-2.1 Ga old protoliths, but the age range of the Proterozoic zircons indicates that a major area of 2.1-1.9 Ga old crust was in erosional position 1.9 Ga ago. This implies that the formation of Palaeoproterozoic crust in the Baltic Shield or its one-time close neighbourhood must have commenced 100–200 Ma earlier than hitherto assumed. n nIn conjunction with previously obtained isotopic data, the youngest detritus ages of the present study constrain the age of Svecofennian sedimentation. It can also be concluded that the Archaean zircons found in quartzites from southern Sweden may have been derived from source areas to the southwest of the central-Svecofennian marine depositional basin, the so-called Bothnian Basin, separating southern Sweden from the Archaean craton in the northeastern part of the Shield.

The age and Pb loss behaviour of zircons from the Isua supracrustal belt as determined by ion microprobe

Ion microprobe UThPb analyses of zircons from a conglomeratic metavolcanic unit in the Isua supracrustal belt of West Greenland show that their magmatic age is3807 ± 1Maσ and failed to detect any older zircon cores or xenocrysts. Low U areas within zircons, at a 30 μm scale, lost only small amounts of radiogenic Pb and only in the recent past, whereas high U areas within grains lost much greater proportions of their Pb, both recently and as early as the Archaean. Some areas in the zircons have concordant UPb and ThPb ages. The ion-probe results agree with earlier determinations for the Isua supracrustals and clarify minor discrepancies between previous multigrain and single crystal analyses.

Age constraints on the geological evolution of the Narryer Gneiss Complex, Western Australia

Zircon U‐Th‐Pb and mineral K‐Ar and 40Ar/39Ar isotopic studies indicate that the maximum deposition age of the Mt Narryer quartzite (which contains detrital zircons up to 4200 Ma old) is 3280 Ma, or by association with other sequences possibly 3100 Ma. This postdates a major episode of high‐grade metamorphism, granite emplacement and deformation at 3300 Ma, which affected adjacent gneiss terranes and which previously had been considered to have affected metasediments and basement gneisses alike. Prograde metamorphism of the Narryer metasediments to amphibolite facies evidently took place during a younger event culminating ca 2700 Ma, prior to injection of granite sheets ca 2650 Ma in age, by which time the present tectonic framework had been assembled.

EARLY ARCHAEAN ZIRCON AGES FROM ORTHOGNEISSES AND ANORTHOSITES AT MOUNT NARRYER, WESTERN AUSTRALIA

Abstract U-Th-Pb isotopic analyses have been made with an ion microprobe of 20–30 μm areas within zircons from gneisses adjacent to the Mount Narryer quartzite, Western Australia, in which detrital zircons up to 4200 Ma old have been found. The cores of the zircons yield protolith crystallization ages of 3678±6 Ma (95% confidence limits) for the Meeberrie banded monzogranite gneiss and 3381 ± 22 Ma for the Dugel syenogranite leucogneiss, in agreement with observed intrusive relationships. Zircons from inclusions of deformed leucogabbro and meta-anorthosite within the Dugel gneiss (formerly part of the Manfred layered igneous complex) are 3730±6 Ma old. The remnants of the Manfred Complex are therefore the oldest rocks currently known on the Australian continent. Analyses of discrete post-magmatic grains and of overgrowths on older grains indicate an episode of subsolidus zircon growth in the region at 3296±4 Ma, which is interpreted as a time of local metamorphism of the gneisses. Subsequently, the zircons have been rounded and embayed and have lost radiogenic Pb in both ancient and recent times. All the zircon ages determined in this study are within the age range for detrital zircons in the Mount Narryer quartzite; it is therefore possible that these felsic gneisses and anorthositic rocks were a local source for some of the sedimentary material. However, the lateral extent of this early Archaean terrain and the source rocks for the rare 4100–4200 Ma old zircons in the metasediments are yet to be determined.

A reconnaissance ion-probe study of hafnium isotopes in zircons

A SIMS technique for the isotopic analysis of hafnium in zircons using the SHRIMP ion microprobe has been developed, and a precision of typically 0.5%. (2σ) achieved in the mean reduced 176Hf177Hf ratio measured at several spots on a single grain. n nUnfractionated (chondritic) initial Hf isotopic compositions have been measured on a number of Archaean zircon populations. These include the oldest-known terrestrial minerals, the 4.2 Ga-old Mount Narryer detrital zircons, thereby confirming their antiquity. In contrast, positive initial ϵHf (relative to the chondritic model composition) has been found in several post-Archaean zircon populations, reflecting the increasing involvement of isotopically evolved depleted mantle sources in the formation of younger crust. The 570 Ma-old Sri Lankan zircon standard SL7 yielded an exceptionally low initial ϵHfof −23, implying a metamorphic origin as a reworked product of ancient crust. n nSHRIMP U-Pb analyses of zircons from Archaean tonalitic gneiss at Watersmeet, Michigan, yield a precise crystallization age of 3636 ± 6 Ma (2σ), and show that a previously reported correlation between 176Hf177Hf and U-Pb isotopic discordance in bulk zircon samples (Patchett, 1983) was caused by the addition of radiogenic Hf in discrete overgrowths of new zircon ca. 2.7 Ga ago. The original 3.64 Ga grains show no evidence of disturbance to their original (chondritic) Hf isotopic composition. There is presently no evidence for significant isotopic exchange of Hf between zircon and other minerals in crustal rocks.

Zircon ages and the distribution of Archaean and Proterozoic rocks in the Rauer Islands

The application of zircon U-Pb geochronology using the SHRIMP ion microprobe to the Precambrian high-grade metamorphic rocks of the Rauer Islands on the Prydz Bay coast of East Antarctica, has resulted in major revisions to the interpreted geological history. Large tracts of granitic orthogneisses, previously considered to be mostly Proterozoic in age, are shown here to be Archaean, with crystallization ages of 3270 Ma and 2800 Ma. These rocks and associated granulite-facies mafic rocks and paragneisses account for up to 50% of exposures in the Rauer Islands. Unlike the 2500 Ma rocks in the nearby Vestfold Hills which were cratonized soon after formation, the Rauer Islands rocks were reworked at about 1000 Ma under granulite to amphibolite facies conditions, and mixed with newly generated felsic crust. Dating of components of this felsic intrusive suite indicates that this Proterozoic reworking was accomplished in about 30–40 million years. Low-grade retrogression at 500 Ma was accompanied by brittle shearing, pegmatite injection, partial resetting of U-Pb geochronometers and growth of new zircons. Minor underformed lamprophyre dykes intruded Hop and nearby islands later in the Phanerozoic. Thus, the geology of the Rauer Islands reflects reworking and juxtaposition of unrelated rocks in a Proterozoic orogenic belt, and illustrates the important influence of relatively low-grade fluid-rock interaction on zircon U-Pb systematics in high-grade terranes.

SHRIMP U-Pb zircon geochronology of the Narryer Gneiss Complex, Western Australia

Abstract SHRIMP U-Pb zircon geochronology of the Narryer Gneiss Complex of the Yilgarn Craton, Australia, has confirmed that its eastern half contains abundant early Archaean gneisses (3300–3730 Ma), but has found gneisses with ages of only 3000 Ma or less in its western half. In the more intensively-studied southern part of the Narryer Gneiss Complex, the early Archaean gneisses are divided into the Eurada Gneiss Association (oldest rocks 3490 Ma) and the Nookawarra Gneiss Association (oldest rocks 3730 Ma). In both associations, only the oldest components are tonalitic, and all younger components are granodioritic or granitic in composition. These associations show different histories until emplacement of granites and pegmatites at 3280–3300 Ma, when they might have been tectonically juxtaposed. These two gneiss associations are found intercalated with the Narryer Supracrustal Association, which contains abundant detrital, quartz-rich sediments. The largest exposures of this association form Mount Narryer and the Jack Hills, where a small proportion of the detrital zircons within sediments are >4000 Ma old. Detrital zircon age populations vary from sample to sample in the Narryer Supracrustal Association. Analysis of these age populations shows that they could have been derived from Nookawarra and Eurada Gneiss Association rocks in varying proportions, apart from 3800 to 4280 Ma zircons, whose source remains unknown. The original relationships between the early Archaean gneiss associations have been obliterated by intrusion of several generations of granite sheets, intercalation with the Narryer Supracrustal Association and folding between 2750 Ma and 2620 Ma, followed by movement on subvertical shear zones either at the end of the Archaean or in the Proterozoic. The late Archaean events coincide with similar ones throughout the Yilgarn Craton, and are thought to be concomitant with growth of the craton by terrane assembly, probably in several distinct episodes.

The difficulties of dating mafic dykes: an Antarctic example

Archaen gneisses of the Vestfold Hills of East Antarctica are transected by several compositionally discrete suites of tholeiitic dykes. A representative of one of those suites, which has been dated in the present study, shows that not only Rb−Sr whole-rock isochrons, but also U−Pb zircon techniques (if not properly applied) can produce erroneous crystallisation ages. Two zircon populations were recovered from the mafic dyke itself, one of which is 2,483±9 Ma in age and clearly of xenocrystic origin. The other yields an age of 1,025±56 Ma, which is not ascribed to the magmatic crystallisation of the dyke, but rather to the time that it underwent metamorphic/metasomatic alteration as a response to high-grade tectonism in the adjacent mobile belt. It is presumed that the zircon in question formed by the breakdown of another mineral or minerals (possibly magmatic baddeleyite), due either to ingress of a siliceous fluid, or more probably by the release of silica from the breakdown of pyroxene to amphibole. A cogenetic 1–2 cm wide felsic vein, of late magmatic/early hydrothermal origin, also contains two zircon populations. Again, most of the grains therein, which are interpreted as of xenocrystic origin, grew at 2,483±9 Ma. However, a few euhedral zircons with very high U and Th contents grew at 1,248±4 Ma, which is taken to be the formation age of both the felsic vein and the enclosing mafic dyke.

Zirconology of the Meeberrie gneiss, Yilgarn Craton, Western Australia: an early Archaean migmatite

Abstract The Meeberrie gneiss, a major component of the early Archaean Narryer Gneiss Complex of Western Australia, is a polyphase migmatite, comprising monzogranites together with minor tonalitic and trondhjemitic components, ranging in age from 3730 to 3300 Ma, with important resolvable age components at ∼ 3670, 3620 and 3600 Ma. Ion-probe U-Pb studies of zircons from numerous localities show that almost all hand specimens of the gneiss consist of multiple generations of magmatically crystallized zircons, even down to the scale of individual centimetre-sized bands. The earliest age components generally are not found in inherited cores but rather in separate zircon grains, usually indistinguishable from the younger grains in terms of morphology, internal structure and trace-element composition. Only in rare low-strain zones are original cross-cutting relationships between individual magmatic components and pegmatite segregations preserved. For the remainder, late Archaean deformation has blended units and erased pre-existing structures. However, deformation state appears to have little bearing on the zircon isotopic systematics. Geochemical data for such rocks are thus composite, as are Nd isotopic compositions which nonetheless show that the younger generations of material in the gneisses are produced predominantly by reworking of older crust.