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

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

Eric R Thern; PhD Thesis

 

Abstract

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 early Earth and the formation of the Yilgarn Craton.

Hadean zircons from the greenstone belts show complex age structures that have been investigated via Sensitive High Resolution Ion Microprobe (SHRIMP) spot analysis, traverses and depth profiling, Electron Probe Microanalysis (EPMA), Quantitative Evaluation of Minerals by Scanning electron microscopy (QEMSCAN), and cathodoluminescence (CL) imaging techniques. SHRIMP analyses on >4300 Ma detrital zircon cores show concordant 207 Pb/ 206 Pb dates which span >50 M.y., a significantly larger range than expected from analytical uncertainty. Later (<3800 Ma) metamorphic events have modified the 207 Pb/ 206 Pb dates obtained on many detrital grains. Evidence is presented that late-stage overprinting of thermal events has left distinct signatures in the

Hadean grains and that only the most pristine ‘cores’ of the zircons can be used when interpreting age results. The observed range of 207 Pb/ 206 Pb dates on pristine Hadean zircon cores suggests early heterogeneous (>3900 Ma) Pb-redistribution between annealed and radiation-damaged zones or channels during periodic post-crystallization thermal events.

In order to assess the provenance of the Hadean zircons, a multivariate approach using a similarity matrix derived from >5500 UPb zircon analyses was used to investigate the complex and overlapping detrital zircon age structure within ca. 3 Ga metasedimentary rocks from the Yilgarn Craton, Western Australia. Detrital zircon analyses were grouped by their 207 Pb/ 206 Pb 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. 3 Ga metasedimentary rocks were derived in varying portions from the Narryer and Yarlarweelor Gneiss Complexes. Differences between the age structure of >3.9 Ga 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 distinct Hadean terranes which were joined by ca. 3.7 Ga.

The presence of detrital Hadean zircons in ca. 3.0 Ga metasedimentary rocks of the Illaara and Maynard Hills greenstone belts implies a genetic link to the ca. 3.0 Ga metasedimentary rocks of the Narryer Terrane, which also contain abundant Hadean zircons. As granites and gneisses older than the youngest detrital zircons within these metasedimentary rocks have not been identified within the Southern Cross Terrane, the metasedimentary rocks of the Maynard Hills, Illaara and Gum Creek greenstone belts may be the oldest rocks yet identified within this terrane. Well preserved sedimentary structures are rare, as upper-greenschist to middle-amphibolite facies metamorphism and shearing has obscured primary relationships between the greenstone-hosted metasedimentary rock occurrences. More than 1000 detrital zircons from twelve samples within these belts were analyzed to determine the age and composition of their provenance sources and to better constrain their stratigraphic relationships.

Depositional ages are currently constrained by the youngest zircons from Mt. Alfred’s eastern (3318±6 Ma) and western (3264±7 Ma) horizons, and Maynard Hills ca. 3060 to 2960 Ma analyses. A minimum depositional age of ca. 2939 Ma has been calculated by the use of 40 Ar/ 39 Ar plateau ages using tourmalines from stratiform quartz-tourmaline veins within the outcrops. Increasing age complexity and maximum depositional age ‘younging’ is common across the Mt. Alfred locality from East (almost exclusively 3700-3780 Ma) to West (3300-3700 Ma and >3800 Ma). The western-most horizon at Mt. Alfred contains abundant >3800 Ma zircons, but lacks the prominent 3500 to 3300 Ma ages common to Jack Hills Hadean-zircon bearing metasedimentary rocks. This Hadean-zircon bearing horizon at Mt. Alfred is most similar in detrital zircon age characteristics to the metasedimentary rocks at Mt. Narryer but without the younger zircon ages (of Eurada and Dugel gneiss affinity; 3480-3300 Ma) found in abundance at Mt. Narryer. This makes Mt. Alfred a unique source for detrital Hadean zircons.

The detrital zircon age similarities within greenstone-hosted metasedimentary rocks throughout the Yilgarn Craton, including the Illaara, Maynard Hills and Gum Creek Greenstone Belts of the Southern Cross Terrane, the Jack Hills and Mt. Narryer of the Narryer Terrane, and the Toodyay Lake Grace Domain within the South West Terrane, strongly suggest a shared provenance of these ca. >2940 Ma metasedimentary rocks. It is likely they were deposited contemporaneously between ca. 3300 and 2940 Ma, later separated by multiple younger ca. 2950 to 2630 Ma granite-greenstone formations during rifting and collision episodes, and occur today as ‘rafts’ within younger ca. 2730-2640 Ma granite-greenstones.

A minimum depositional age, that also constrains the end-stages of deposition of the early Archean metasedimentary rocks, has been determined at ca. 2939±15 Ma (95% confidence) via multiple robust 40 Ar/ 39 Ar plateau ages on tourmalines within post-depositional stratiform layers. An initial indication of Ar closure temperature in tourmaline was experimentally calculated to be >600 ◦ C by a single step-heating experiment. The association of barite and rhythmites in parallel metasedimentary units, indications of relatively enriched δ 11 B within tourmaline, and their 2939±15 Ma 40 Ar/ 39 Ar dates suggest the host sediments were deposited prior to 2939±15 Ma within a marine-intertidal-evaporitic environment. Younger stratiform tourmaline, crystallized on the margins of late-stage quartz veins, give an age of 2622±20 Ma, defining the timing of the quartz veining, and a second stage of tourmaline growth from B-rich fluids.

The post-depositional thermal history of ca. 3.0 Ga Illaara and Maynard Hills greenstone belt siliciclastic metasedimentary rocks are characterized by a combination of SHRIMP U-Pb rutile, 40 Ar/ 39 Ar and (U-Th)/He geochronology. SHRIMP U-Pb rutile analyses from 8 siliciclastic metasedimentary rock samples reveal a complex history of events between deposition of their host sandstones (ca. 3.0 Ga) and the subsequent intrusions of mafic-ultramafics at ca. 2.8 Ga, folding, thrusting and granitic intrusions (ca. 2730 to 2630 Ma, regional D1 to D3 events). Some individual rutile grains yield multiple U-Pb dates which span from before the maximum depositional age of the metasedimentary rocks at ca. 3060 Ma to the last major metamorphic and granitic event at ca. 2630 Ma. These rutiles exhibit weakly defined core-rim younging profiles which have been interpreted to represent multiple stages of metamorphic growth and crystallization and a possible Pb-loss overprinting. The results suggest that under protracted upper greenschist to pulses of mid-amphibolite metamorphic conditions, rutile can retain signatures of multiple thermal events and even retain some of their original detrital ages and characteristics. 40 Ar/ 39 Ar plateau ages on muscovites from both greenstone belts show that late to post-deformation, planar-foliation recrystalization at 2610±15 Ma (possibly coeval with the end of D3) marks the end of high-grade stabilizing tectono-thermal events and temperatures above 350−400 ◦C. Zircon (U−Th)/He dates of 230±13 Ma define the age of exhumation, with temperatures <180 ◦C for the siliciclastic metasedimentary rocks, similar to fission track results from the northern Yilgarn. Goethite (U−Th)/He dates of 25±2 Ma are probably coeval with Fe-rich meteoric fluid influx and associated zero-age Pb-loss and Fe enrichment in metamict zones of both rutile and zircon.

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