Geological Histories from 4372 Ma to 26 Ma Recorded in Siliciclastic Metasedimentary Rocks from the Central Yilgarn Craton

This study presents an investigation of detrital, metamorphic, and hydrothermal minerals from siliciclastic metasedimentary rocks of the Illaara and Maynard Hills greenstone belts, central Yilgarn Craton. This research assesses how 4.3 to 3.0 Ga detrital zircon populations came to be found in dispersed metasedimentary rocks, how these rock occurrences relate to each other and what this may reveal about the early Earth and the formation of the Yilgarn Craton

Paleomagnetism indicates that primary magnetite in zircon records a strong Hadean geodynamo.

Determining the age of the geomagnetic field is of paramount importance for understanding the evolution of the planet because the field shields the atmosphere from erosion by the solar wind. The absence or presence of the geomagnetic field also provides a unique gauge of early core conditions. Evidence for a geomagnetic field 4.2 billion-year (Gy) old, just a few hundred million years after the lunar-forming giant impact, has come from paleomagnetic analyses of zircons of the Jack Hills (Western Australia). Herein, we provide new paleomagnetic and electron microscope analyses that attest to the presence of a primary magnetic remanence carried by magnetite in these zircons and new geochemical data indicating that select Hadean zircons have escaped magnetic resetting since their formation. New paleointensity and Pb-Pb radiometric age data from additional zircons meeting robust selection criteria provide further evidence for the fidelity of the magnetic record and suggest a period of high geomagnetic field strength at 4.1 to 4.0 billion years ago (Ga) that may represent efficient convection related to chemical precipitation in Earth's Hadean liquid iron core

Detrital zircon age structure within ca. 3 Ga metasedimentary rocks, Yilgarn Craton : elucidation of Hadean source terranes by principal component analysis

A multivariate approach using a similarity matrix derived from >5500 U-Pb zircon analyses was used to investigate the complex and overlapping detrital zircon age structure within ca. 3Ga metasedimentary rocks from the Yilgarn Craton, Western Australia. Detrital zircon analyses were grouped by their 207Pb/206Pb dates using a robust Chi-square grouping method which produced 74 Yilgarn-wide age groups from a pool of >3500 analyses and that were correlated between different metasedimentary rocks. Principal component analysis (PCA) was then used on a calculated similarity matrix of >65 samples which contained these age groups. PCA indicates that the main age populations of the detrital zircons in the ca. 3Ga metasedimentary rocks were derived in varying portions from the Narryer and Yarlarweelor Gneiss Complexes. Differences between the age structure of >3.9Ga zircon populations within the Mt. Alfred metasedimentary rocks with those from Mt. Narryer, Jack Hills and Maynard Hills localities is best explained by their derivation from two Hadean terranes which were joined by ca. 3.7Ga

Geochemical and ion-microprobe U–Pb zircon constraints on the Archaean evolution of Singhbhum Craton, eastern India

Geochemical and SHRIMP U–Pb zircon analyses were obtained for nine samples of the Singhbhum Cratonto investigate major regional granite intrusion, volcanism, sedimentary deposition, metamorphism and deformation episodes. Detrital zircons within a recrystallized sandstone enclave within tonalitic gneiss provided a maximum time for deposition of 3375 ± 3 Ma (all uncertainties are at 95% confidence unless otherwise specified) for the host sandstone. Igneous crystallization dates between 3380 and 3299 Mawere obtained for tonalite gneiss and foliated trondhjemite samples from widely dispersed sites, whereas unfoliated granitic samples provided igneous crystallization dates <3.3 Ga, thus constraining the timeof regional deformation and amphibolite grade metamorphism to the interval c. 3325–3300 Ma. Vol-canic, clastic sedimentary and banded iron-formation rocks assigned to the Iron Ore Group (IOG) are preserved around the margins of the Singhbhum Granite Complex. An igneous crystallization date of3285 ± 7 Ma, obtained for a biotite granodiorite that is overlain by conglomerate of the IOG, provides a maximum deposition age for components within the western (Jamda-Koira) IOG basin. A dacite tufffrom the Malayagiri IOG basin south of Palalahara yielded an igneous crystallization date of 2806 ± 6 Ma, confirming that the IOG as currently defined, includes unrelated sedimentary rocks that were deposited within different basins over an 800 Myr interval.Evolution of the Singhbhum Craton may be summarized as follows: (1) between 3530 and 3300 Ma, tonalites were emplaced, with volcanic, clastic and carbonate rocks and banded iron-formation (cycle 1) deposited onto tonalitic basement until 3375 Ma; (2) between3325 and 3300 Ma, burial, deformation and uplift transformed the central part of the Singhbhum base-ment to tonalite gneisses, with cycle 1 sedimentary rocks incorporated into the gneisses as enclaves but preserved within synforms around the basement margins; post-3.3 Ga regional metamorphism grano-diorite intrusions were emplaced until c. 3285 Ma; (3) BIF and clastic sedimentary rocks (cycle 2) were deposited around the margins of the craton onto the older (cycle 1) sedimentary rocks and adjacent gneissic basement until c. 3.1 Ga; (4) a further episode of granite intrusion at 3090 Ma was followed by uplift and erosion of the central part of the craton prior to 2806 Ma; (5) volcano-sedimentary and banded iron-formation rocks were deposited during a third sedimentary cycle at c. 2.8 Ga

Tourmaline 40Ar/39Ar geochronology and thermochronology : example from Hadean-zircon-bearing siliciclastic metasedimentary rocks from the Yilgarn Craton

We present 40Ar/39Ar ages and boron isotopes of tourmalines from a quartz-tourmaline intrusion cross-cutting the Hadean detrital-zircon-bearing metasedimentary rocks from the Mt. Alfred and Brooking Hills localities of the Illaara Greenstone Belt, Western Australia. Concordant 40Ar/39Ar plateau ages on the tourmalines at Mt. Alfred give a weighted mean age of 2935 ± 9 Ma (2σ). These results are the first reported Archean 40Ar/39Ar ages directly obtained on tourmaline, and provide a minimum depositional age for this sequence, placing constraints on the depositional timing of detrital Hadean zircons to between 2935 ± 9 Ma and c.a. 3250 Ma (youngest detrital zircon age in sequence). This is the last known occurrence of abundant detrital Hadean zircons having been directly deposited in a sedimentary sequence, and thus suggests that most Hadean zircon sources have been exhausted by this time. The 2935 ± 9 Ma tourmalines occur within a quartz-tourmaline intrusion with associated stratiform layers and veins, inferred to have grown during a hydrothermal fluid circulation event. A younger generation of tourmalines from the Brooking Hills, 30 km south of Mt. Alfred yielded concordant 40Ar/39Ar plateau ages with a weighted mean age of 2624 ± 16 Ma. These tourmalines occur within and along the margins of post-kinematic quartz veins, parallel to the foliation fabric and marks the end stage of high temperature tectono-thermal events within the Illaara Greenstone Belt. The retention of 2935 ± 9 Ma tourmaline ages within the Illaara Greenstone Belt shows that the K/Ar system of tourmaline remained closed throughout repeated upper-greenschist metamorphic events (∼450 °C) between the ages of c.a. 2930 to 2630 Ma in contrast to muscovite recording plateau ages of about 2600 Ma. This is in agreement with an approximate Ar closure temperature between ∼534 and 628 °C experimentally determined in this study

Enriched Grenvillian lithospheric mantle as a consequence of long-lived subduction beneath Laurentia

Geochemical and Nd isotopic data from mafic and newly discovered ultramafic rocks in the Adirondack Lowlands suggest widespread enrichment of the lithospheric mantle under the Grenville Province. Incompatible element abundances and previously published Hf TDM (zircon) (depleted mantle model age) and Nd TDM ages from rocks of the anorthosite-mangerite-charnockite-granite suite in the Adirondack Highlands document similar enrichment in the lower crust and its strong influence on subsequent magmatic events throughout the Ontario-Quebec-Adirondack segment of the Grenville Province. Likely the consequence of long-lived (ca. 1.4–1.2 Ga) northwest-directed subduction along the southeast edge of Laurentia (previously proposed Andean margin), this enrichment is similar to that associated with the vast (>240,000 km2) ultrapotassic province of the western Churchill Province. Enrichment of the lithospheric mantle beneath orogenic belts is a predictable and important differentiation process that has operated on Earth for at least the past 3 b.y

Rodinian Collisional and Escape Tectonics in the Hudson Highlands, New York

A new multidisciplinary research collaboration to study the western Hudson Highlands, New York, has unraveled a complex Rodinian tectonic history that will be illustrated by visiting key locations on this trip. New sensitive high-resolution ion microprobe (SHRIMP) data demonstrates a cryptic suture between a ca. 1.2-1.1 Ga island arc and sedimentary rocks from a deeply incised craton (Amazonia?). The 1.05 Ga collision between these two terranes produced westward-directed fold nappes, granulite facies metamorphism and the dominant subhorizontal gneissic foliation. Tectonic surge granite sheets were emplaced into the nappes. Bimodal (diorite and granite) plutons intruded the area prior to the onset of a steeply SE-dipping 35-kmwide dextral shear system that resulted from tectonic escape. Extensive iron remobilization and mineralization accompanied the shearing and post-kinematic pegmatite plutons mark the end of activity at ca. 980 Ma

Emplacement age and thermal footprint of the diamondiferous Ellendale E9 lamproite pipe, Western Australia

The diamondiferous Ellendale 9 (E9) pipe is a funnel-shaped maar-diatreme volcano consisting of inward-dipping tuff sequences intruded by lamproite plugs and dykes. The host rocks for the E9 pipe are Permian sandstones. The multiple lithological contacts exposed within the mined maar volcano provide a natural laboratory in which to study the effect of volcanic processes on U–Th–Pb–He systematics. Zircon from the regional sandstone and E9 lamproite display a bimodal distribution of ages on (U–Th)/He–U/Pb plots. The zircon U/Pb ages for the E9 pipe (n = 52) range from 440 to 2,725 Ma, while the cluster of (U–Th)/He ages for the lamproite dyke zircon indicate that dyke emplacement occurred at 20.6 ± 2.8 Ma, concordant with a maximum emplacement age of about ≤22 Ma from phlogopite 40Ar/39Ar. These ages indicate a xenocrystic origin for the zircon entrained in the E9 dyke. The U/Pb ages of detrital zircon from the regional sandstone host (373–3,248 Ma; n = 41) are indistinguishable from those of the lamproite zircon xenocrysts, whereas the detrital zircon in the host sandstone yield (U–Th)/He ages from 260 to 1,500 Ma. A thermochronology traverse across the E9 lamproite dyke reveals that the zircon (U–Th)/He ages in the host sandstone have not been significantly thermally reset during dyke emplacement, even at the contact. The capability of the zircon (U–Th)/He method to distinguish deep, mantle source lithologies from upper crustal source lithologies could be used in geochemical exploration for diamonds. Pre-screening of detrital samples using etching and helium assay methods will improve the efficiency and decrease the cost of greenfields exploration

A 4463 Ma apparent zircon age from the Jack Hills (Western Australia) resulting from ancient Pb mobilization

Hadean (≥4.0 Ga) zircon grains provide the only direct record of the first half-billion years of Earth’s history. Determining accurate and precise crystallization ages of these ancient zircons is a prerequisite for any interpretation of crustal evolution, surface environment, and geodynamics on the early Earth, but this may be compromised by mobilization of radiogenic Pb due to subsequent thermal overprinting. Here we report a detrital zircon from the Jack Hills (Western Australia) with 4486–4425 Ma concordant ion microprobe ages that yield a concordia age of 4463 ± 17 Ma (2σ), the oldest zircon age recorded from Earth. However, scanning ion imaging reveals that this &gt;4.4 Ga apparent age resulted from incorporation of micrometer-scale patches of unsupported radiogenic Pb with extremely high 207Pb/206Pb ratios and &gt;4.5 Ga 207Pb/206Pb ages. Isotopic modeling demonstrates that these patches likely resulted from redistribution of radiogenic Pb in a ca. 4.3 Ga zircon during a ca. 3.8 Ga or older event. This highlights that even a concordia age can be spurious and should be carefully evaluated before being interpreted as the crystallization age of ancient zircon

Paleomagnetism indicates that primary magnetite in zircon records a strong Hadean geodynamo.

Determining the age of the geomagnetic field is of paramount importance for understanding the evolution of the planet because the field shields the atmosphere from erosion by the solar wind. The absence or presence of the geomagnetic field also provides a unique gauge of early core conditions. Evidence for a geomagnetic field 4.2 billion-year (Gy) old, just a few hundred million years after the lunar-forming giant impact, has come from paleomagnetic analyses of zircons of the Jack Hills (Western Australia). Herein, we provide new paleomagnetic and electron microscope analyses that attest to the presence of a primary magnetic remanence carried by magnetite in these zircons and new geochemical data indicating that select Hadean zircons have escaped magnetic resetting since their formation. New paleointensity and Pb-Pb radiometric age data from additional zircons meeting robust selection criteria provide further evidence for the fidelity of the magnetic record and suggest a period of high geomagnetic field strength at 4.1 to 4.0 billion years ago (Ga) that may represent efficient convection related to chemical precipitation in Earth's Hadean liquid iron core