Mount Narryer Research Articles

How old are the Jack Hills metasediments really?: The case for contamination of bedrock by zircon grains in transported regolith

Abstract The enigmatic occurrence of Neoarchean- and Proterozoic-aged zircon grains in some samples of metasedimentary rocks from the Jack Hills, Western Australia, that otherwise appear to have been deposited before 3.0 Ga is explained by contamination with embedded zircon grains sourced from transported regolith. Zircon U-Pb age spectra obtained by laser ablation–inductively coupled plasma mass spectrometry (LA-ICPMS) analysis of modern alluvium in creek beds draining the metasedimentary belts of Mount Narryer and Jack Hills are dominated by these exotically sourced younger components, as is the red soil that partly covers and infiltrates fractures in the exposed Hadean zircon-bearing bedrock at Jack Hills. The consistency of the age patterns found in superficial deposits sampled over some 100 km implies a dominance of colluvium transported from the wider Yilgarn craton, adjacent Capricorn orogen, and more distal sources rather than material produced from the local (Mesoarchean and older) bedrocks. The potential for similar cases of “environmental contamination” of surface rocks by exogenous zircons needs to be considered more widely, particularly in situations where interpreted maximum deposition ages are based on only a small number of outlying youngest analyzed grains.

Heterogeneous Hadean crust with ambient mantle affinity recorded in detrital zircons of the Green Sandstone Bed, South Africa

The nature of Earth's earliest crust and the processes by which it formed remain major issues in Precambrian geology. Due to the absence of a rock record older than ∼4.02 Ga, the only direct record of the Hadean is from rare detrital zircon and that largely from a single area: the Jack Hills and Mount Narryer region of Western Australia. Here, we report on the geochemistry of Hadean detrital zircons as old as 4.15 Ga from the newly discovered Green Sandstone Bed in the Barberton greenstone belt, South Africa. We demonstrate that the U-Nb-Sc-Yb systematics of the majority of these Hadean zircons show a mantle affinity as seen in zircon from modern plume-type mantle environments and do not resemble zircon from modern continental or oceanic arcs. The zircon trace element compositions furthermore suggest magma compositions ranging from higher temperature, primitive to lower temperature, and more evolved tonalite-trondhjemite-granodiorite (TTG)-like magmas that experienced some reworking of hydrated crust. We propose that the Hadean parental magmas of the Green Sandstone Bed zircons formed from remelting of mafic, mantle-derived crust that experienced some hydrous input during melting but not from the processes seen in modern arc magmatism.

Paleoseismology of the Mount Narryer fault zone, Western Australia: A multistrand intraplate fault system

Our paleoseismological study of faults and fault-related folds comprising the Mount Narryer fault zone reveals a mid- to late Quaternary history of repeated morphogenic earthquakes that have influenced the planform and course of the Murchison, Roderick, and Sanford Rivers, Western Australia. The dominant style of deformation involves folding of near-surface sediments overlying discrete basement faults. Carbon-14, optically stimulated luminescence, and in situ–produced 10 Be constrain the timing of the events and late Quaternary slip rates associated with fault propagation folds in tectonically uplifted and deformed alluvial channel deposits. A flight of five inset fluvial terraces is preserved where the Murchison River flows across the Roderick River fault. These terraces, which we infer to be coseismic, are consistent with at least four late Quaternary seismic events on the order of moment magnitude (M w ) 7.1 within the last ~240 k.y. Secondary shears expressed on the folds indicate a component of dextral strike-slip displacement. Quaternary slip rates on the underlying faults range from 0.01 to 0.07 mm yr –1 , with a total slip rate for the zone between 0.04 and 0.11 mm yr –1 . These rates are intermediate to those in the adjacent Mesozoic basin (>0.1 mm yr –1 ) and Precambrian craton ( –1 ) and so provide insight into how tectonic strain is partitioned and transferred across a craton margin.

Resolving poly-metamorphic Paleoarchean ages by chemical dating of monazites using multi-spectrometer U, Th and Pb analyses and sub-counting methodology

Chemical U–Th–total Pb dating method is a low-cost and reliable technique to estimate the age of monazites of detrital and diagenetic origin or those found in low to high-T metamorphic or magmatic rocks using electron microprobe (EMP) analyses. The method has been commonly applied for age estimation in Phanerozoic and Proterozoic rocks and less commonly in Neoarchean rocks. Monazite dates from the Paleoarchean era are rare and the associated error % of spot age (=[2σ error/absolute age]×100) is high, i.e., between 2 and 26%. The errors associated with chemical dating of monazite are significantly larger compared to isotope methods, which impede precise age estimation and age resolution in polygenetic monazites of Neoarchean and older rocks.In this study, simultaneous element analyses in multiple spectrometers and the sub-counting method of analyses are employed to obtain precise estimates of U, Th and Pb concentrations in monazite. Using vanadinite [Pb5(VO4)3Cl] as the Pb standard (instead of PbS), the method yields an age of 3118+/−28Ma with Mount Narryer Mesoarchean monazite (Thermal Ionization Mass Spectrometry [TIMS] age: 3132+/−1Ma) and 516+/−12Ma with Moacyr monazite (TIMS age: 509+/−1Ma; EMP monazite age: 497+/−10Ma) as the reference samples. The modifications led to ~40% reduction in spot age errors in the Mount Narryer sample relative to those obtained by conventional monazite dating method. The reduced errors in spot ages helped in resolving statistically un-mixed age populations (3152+/−16Ma, 3322+/−15Ma and 3420+/−140Ma) in polygenetic monazites within muscovite-quartz schists from the Singhbhum Craton, Eastern India. The significance of these dates is briefly discussed.

Tracing the provenance and recrystallization processes of the Earth's oldest detritus at Mt. Narryer and Jack Hills, Western Australia: An in situ Sm–Nd isotopic study of monazite

Mount Narryer and Jack Hills metasedimentary rocks in the Narryer Gneiss Complex of the Yilgarn Craton, Western Australia, contain zircons with ages up to 4.4 Ga, the oldest known crustal materials on Earth, and monazites up to 3.6 Ga. In this study, we have investigated 147Sm– 143Nd systematics of detrital and metamorphic monazites from these metasedimentary rocks using laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS). All detrital monazites have negative initial εNd(t) values, indicating that their parental magmas formed by remelting of older crustal materials. A comparison between the initial εNd(t) values of the detrital monazites and granitoids in the Narryer Gneiss Complex indicates that the Mt. Narryer and Jack Hills sediments were partly derived from the most isotopically enriched surrounding granitoids with ages of ca. 3.6 and 3.3 Ga. The metamorphic monazites generally have lower initial εNd(t) values when compared to the detrital monazites. However, the detrital and metamorphic monazites show similar distributions of εNd(t) at the ages of sediment deposition (3.28 Ga for Mt. Narryer and 3.05 Ga for Jack Hills). In addition, multiple analyses on single monazite grains having core-rim structures reveal that the cores and the recrystallized rims had identical Nd isotopic compositions at the time of recrystallization. These findings indicate that older monazites are source of light rare earth elements for younger metamorphic monazite formation and, therefore, that monazite can inherit its primary Sm–Nd isotopic signature during the recrystallization processes. We calculated the Nd model ages for all analyzed monazites to estimate crustal residence time of their source materials. We find that no igneous monazites older than 4.0 Ga were recrystallized to form the monazites. This implies that the lack of Hadean monazites is not due to recrystallization of ancient monazites during later metamorphism, but due to high-Ca compositions of the parental magmas of Hadean detritus, which prevent growth of magmatic monazite.

Thermal events documented in Hadean zircons by ion microprobe depth profiles

We report the first U–Th–Pb ion microprobe depth profiles of four Hadean zircons from the Jack Hills and Mount Narryer supracrustal belts of the Narryer Gneiss Complex (NGC), Western Australia. This ultra-high spatial resolution technique probes the age and origin of sub-micron features in individual crystals that can record episodes of zircon growth. Near-surface grain dates of 2700 Ma or older are coincident with post-depositional growth/modification. Some ages may coincide with documented pre-deposition metamorphic events for the NGC and igneous emplacement at ca. 3700 Ma. Separate events that do not correlate in time with known geologic episodes prior to the preserved rock record are also present on pre-4000 Ma zircons. We find evidence for a ∼3.9 Ga event, which is coterminous within age uncertainty with one or several large basin-forming impacts (e.g. Nectaris) on the Moon attributed to the late heavy bombardment of the inner solar system.

Detrital Zircon from the Jack Hills and Mount Narryer, Western Australia: Evidence for Diverse >4.0 Ga Source Rocks

Abstract Geochronological, geochemical, and morphological analysis of detrital zircon in the Jack Hills and Mount Narryer metasedimentary belts, Western Australia, indicates the grains were derived from diverse rocks, including >4000 Ma sources that predate the oldest known terrestrial rock. In three metaconglomerate layers in the western part of the Jack Hills, 4200–3800 Ma zircon makes up 14% of the population, 3800–3600 Ma grains form only 2%, and 3550–3250 Ma zircon is dominant with a significant peak at 3380 Ma (U‐Pb ages and trace element concentrations were obtained by laser‐ablation microprobe inductively coupled plasma mass spectrometry). These grains are interpreted as being derived from similar rock types because they are indistinguishable in U concentration (50–200 ppm), internal zoning (both oscillatory and sector zoning within the same grain), and morphology (subequant fragments of grains). We conclude that a previously proposed evolved granitic source is unlikely because the zircon differs ...

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.

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 176Hf 177Hf ratio measured at several spots on a single grain. Unfractionated (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 ϵ Hf (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 ϵ Hf of −23, implying a metamorphic origin as a reworked product of ancient crust. SHRIMP 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 176Hf 177Hf 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.

Oldest known terrestrial anorthosite at Mount Narryer, Western Australia

Fragments of a deformed and metamorphosed, layered anorthosite-gabbro-ultramafic intrusion called the Manfred Complex occur as inclusions in ∼ 3650 and ∼ 3400 Ma old banded granite gneisses at Mount Narryer in the Western Gneiss Terrain of the Yilgarn Block. The complex is typical of a world-wide group of Archaean anorthosites, characterized by very large primary grain sizes of equidimensional calcic plagioclase. This is the first recorded occurrence of these anorthosites in Australia. Pb-Pb and Sm-Nd whole-rock, and U-Pb zircon geochronology given in the accompanying two papers (Fletcher, I.R., Rosman, K.J.R. and Libby, W.G., 1988. Precambrian Res., 38: 343–354; Kinny, P.D., Williams, I.S., Froude, D.O., Ireland, T.R. and Compston, W., 1988. Precambrian Res., 38: 325–341) shows that the Manfred Complex is about 3730 Ma old. It is therefore the oldest known terrestrial anorthosite complex and one of the oldest known rock units on Earth.

Early archaean zircon ages from orthogneisses and anorthosites at Mount Narryer, Western Australia

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.

Sm-Nd, Pb-Pb and Rb-Sr geochronology of the manfred complex, Mount Narryer, Western Australia

The Sm-Nd, Pb-Pb and Rb-Sr systems in Manfred Complex samples ranging from metaleucogabbro to metapyroxenite indicate formation of the complex ∼ 3700 Ma ago and multiple metamorphic disturbances over the succeeding ∼ 2000 Ma. The isotopic responses to metamorphism were largely determined by the accessory mineral contents of the rocks and by country rock influences. Whole-rock data give a five-point Sm-Nd isochron date of 3680±70 Ma, and the six samples with least radiogenic Pb give a Pb-Pb isochron date of 3689±146 Ma. Whole-rock Rb-Sr data reflect the widespread Yilgarn ‘event’ at ∼ 2700 Ma, but some samples show additional Rb-Sr disturbance < 2000 Ma ago. Data for various mineral fractions show substantial internal re-equilibration of Sm-Nd during granulite facies metamorphism at ∼ 3320 Ma, and further disturbance at least as recently as ∼ 3100 Ma. About half of the samples examined show evidence of partial Pb isotopic re-equilibration or U gain at ∼ 2900 Ma, possibly a time of retrograde amphibolite facies metamorphism. A high apparent source μ = 10.2 ± 0.4 from the Pb-Pb whole-rock isochron could indicate assimilation of significant quantities of older (? > 4000 Ma) crustal material. The measured initial ϵ Nd = −0.2 ± 0.7 is consistent with this, but is not definitive. The various dates correspond well to those determined by Kinny et al. (Kinny, P.D., Williams, I.S., Froude, D.O., Ireland, T.R. and Compston, W., 1988. Precambrian Res., 38: 325–341) and Wijbrans (Wijbrans, J.R., 1985, Ph.D. thesis. Australian National University), and complement the geological framework presented by Myers (Myers, J.S., 1988. Precambrian Res., 38: 309–323).

Early Precambrian crustal evolution at Mount Narryer, Western Australia

The oldest known rocks in Australia occur at Mount Narryer and are associated with quartzite that contains detrital zircons far older than any known terrestrial rocks.The region forms part of the Western Gneiss Terrain of the Archaean Yilgarn Block. The main rock unit is a banded biotite adamellite gneiss which gives a Sm-Nd model age of 3630 Ma. It is intruded by a leucocratic granite gneiss which gives a Sm-Nd model age of 3510 Ma and contains abundant inclusions of a meta-anorthosite—gabbro—ultramafic suite. Quartzites and banded quartz—pyroxene—magnetite rocks form concordant layers within both gneisses.Most rocks are strongly deformed, are folded together into large-scale fold interference patterns, and show mineral assemblages in granulite or retrograded granulite facies. One large body of quartzite has internally escaped intense deformation and contains cross-bedding and graded-bedding structures as well as horizons of both polymict and quartz-pebble conglomerates. The quartzites contain detrital zircons which mostly give U-Pb ages of ∼3750–3500 Ma, but a few zircon grains give ages of 4100–4200 Ma. Samples of the adamellite gneiss give a Rb-Sr whole-rock isochron age of 3348±43 Ma which might date a peak of metamorphism.

Early Archaean gneisses from the Yilgarn Block, Western Australia

The Yilgarn Block is an ancient crustal block which extends over ∼650,000km2 in the south-west of Western Australia and consists of high-grade gneiss and granite–greenstone terrains1. Being one of the largest segments of Archaean crust in the world, it is important to studies of early crustal evolution. We report here a geochronological study of banded gneisses near Mount Narryer, in the north-west of the Yilgarn Block, which has given a Rb–Sr whole-rock isochron age of 3,348±43 Myr with an initial 87Sr/86Sr ratio of 0.7037±0.0005. The data indicate that the rocks had a prior crustal residence time of ∼200 Myr. The validity of this early Archaean age is supported by model Sm–Nd ages of 3,510 Myr and 3,630 Myr for two of the samples. Taken together, these ages represent the oldest evolutionary sequence so far identified in the Yilgarn Block, and are comparable with old ages from other Archaean cratons elsewhere in the world.