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Econ. Environ. Geol. 2023; 56(6): 817-830

Published online December 29, 2023

https://doi.org/10.9719/EEG.2023.56.6.817

© THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY

Alice Springs Orogeny (ASO) Footprints Tracing in Fresh Rocks in Arunta Region, Central Australia, Using Uranium/Lead (U-Pb) Geochronology

Kouame Yao1, Mohammed O. Idrees2,3,*, Abdul-Lateef Balogun4, Mohamed Barakat A. Gibril5

1Macquarie University, Department of Earth and Planetary Sciences, Faculty of Science and Engineering, North Ryde, Australia
2Department of Surveying and Geoinformatics, Faculty of Environmental Sciences, University of Abuja, P.M.B. 117, Abuja, Nigeria
3Department of Surveying and Geoinformatics, Faculty of Environmental Sciences, University of Ilorin, P.M.B. 1515, Ilorin, Nigeria
4Environmental Systems Research Institute (ESRI), Melbourne, Australia
5GIS and Remote Sensing Center, Research Institute of Sciences and Engineering, University of Sharjah, Sharjah 27272, United Arab Emirates

Correspondence to : *dare.idrees@gmail.com, mohammed.idrees@uniabuja.edu.ng

Received: March 13, 2023; Revised: November 4, 2023; Accepted: November 7, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

This study investigates the age of the surficial rocks in the Arunta region using Uranium-Lead (U-Pb) geochronological dating. Rock samples were collected at four locations, Cattle-Water Pass (CP 1610), Gough Dam (GD 1622 and GD 1610), and London-Eye (LE 1601), within the Strangways Metamorphic Complex and crushed by selFragging. Subsequently, the zircon grains were imaged using Cathodoluminescence (CL) analysis and the U-Pb (uranium and lead) isotope ratios and the chrono-stratigraphy were measured. The imaged zircon revealed an anomalous heterogeneous crystal structure. Ellipses of the samples at locations GD1601, CP1610, and GD1622 fall below the intercept indicating the ages produced discordant patterns, whereas LE1601 intersects the Concordia curve at two points, implying the occurrence of an event of significant impact. For the rock sample at CP1610, the estimated mean age is 1742.2 ± 9.2 Ma with mean squared weighted deviation (MSWD) = 0.49 and probability of equivalence of 0.90; 1748 ± 15 Ma - MSWD = 1.02 and probability of equivalence of 0.40 for GD1622; and 1784.4 ± 9.1 Ma with MSWD of 1.09 and probability of equivalence of 0.37 for LE1601. But for samples at GD1601, two different age groups with different means occurred: 1) below the global mean (1792.2 ± 32 Ma) estimated at 1738.2 ± 14 Ma with MSWD of 0.109 and probability of equivalence of 0.95 and 2) above it with mean of 1838.22 ± 14 Ma, MSWD of 1.6 and probability of equivalence of 0.95. Analysis of the zircon grains has shown a discrepancy in the age range between 1700 Ma and 1800 Ma compared to the ASO dated to have occurred between 440 and 300 Ma. Moreover, apparent similarity in age of the core and rim means that the mineral crystallized relatively quickly without significant interruptions and effect on the isotopic system. This may have constraint the timing and extent of geological events that might have affected the mineral, such as metamorphism or hydrothermal alteration.

Keywords geology, rock dating, selFragging, lead loss, Cathodoluminescence imaging

  • Age and origin of zircon grains extracted from rock samples taken within the Alice Springs Orogeny were profiled through Cathodoluminescence and LA-ICP-MS analyses.

  • Core/rim intra-phase relationship suggests that the core of the mineral contains a significant number of inclusions different in composition from the host mineral.

  • The mineral may have experienced more disturbances or alterations leading to complex internal structure possibly affected by crystal-plastic deformation.

  • Discrepancies exist in the estimated age (1700 - 1755 Ma), which is much older than the ASO occurrences.

Investigating rock formation to understand its historical development is a practice that dated back to the beginning of the last century when long-lived radioactive decay systems were recognized to provide the only valid means to quantify geologic time (Davis et al., 2018). Usually, researchers utilise sediment-compacted rock specimens from various sources to discover major past geological events that have occurred (Warren, 1983). The reliability of this approach is largely dependent on the geological characteristics of the sample collected and how they are interpreted because drastic changes do occur due to rupture in the crust different from the Orogenic events (Grasemann and Huet, 2014). Violent changes in rock formations alter the order of formation and consequently, the information within the rocks sample (Kearey et al., 2009) as the orogeny leads to erosion and causes undeformed sediment to be deposited above the eroded remnants of deformed rock formation (Haines et al., 2001). This often results in information gaps that require combining several variables to understand the processes leading to the current state of the outcrop.

The Central Arunta region is a vast and remote area located in the southern part of the Northern Territory of Australia (Craven et al., 2012). The region is a fascinating geological area that contains a wide variety of rocks and geological formations that offer insight into the Earth's history and the processes that have shaped the Australian continent over billions of years (Maidment et al., 2005). The geological features of this region are diverse and complex, with a long history of geological activity. Central Arunta region is situated on the eastern margin of the Western Australian Craton, a stable block of ancient rocks that formed the core of the Australian continent (McLaren et al., 2009). The rocks of the Central Arunta are dominated by sedimentary and volcanic rocks, including sandstones, siltstones, shales, and basalts. These rocks were deposited during the Proterozoic Era, between 1.8 and 0.6 billion years ago (Page, 1988).

In Central Australia, intercontinental magnitude disturbances during the long-lived Devonian-Carboniferous epoch Alice Spring Orogeny (ASO) (~400-350 Ma), have defined a complex geology, disentombing Arunta inlier in a southdirected thrusting trend (Flottmann et al., 2004), folding sedimentary deposits to produce mountainous ranges (Haines et al., 2001), thrusting up of rocks of Proterozoic ages, and eroding sediment off mountain belt, deposing and incorporating them into remaining relics of the former sedimentary basin (Bradshaw and Evans, 1988).

ASO is characterized by misapprehension of the physical condition and the aerial extent representative of distinctive structural phenomena of the regional tectonic rocks and crust. Thus, the outcrop is interlaced with a mix of metamorphosed rocks from different geologic eras without defined aerial coverage, and consequently a distortion of the regional lithology in these zones that bear the characteristics of different epochs. The elongated lenses of pegmatite representative of Ormiston events (1076 Ma) separated from gneissic rocks affecting rocks along the central southern margin of the Arunta inlier (Marjoribanks and Black, 1974) or the Central Hart Range generated pods of Quartz and Feldspar-rich rocks migrating upward and crystallizing as pegmatites during ASO (Stewart et al., 1984) are some of the countless examples that illustrate the above assertion.

Rock dating techniques have been widely employed to understand how earth processes impacted rock formations (Wilde et al., 2001). Geoscientists have utilized geochronology dating techniques such as Rubidium–strontium (Rb-Sr) and Potassium-argon (K-Ar) to understand past events and natural phenomena leading to tectonic-induced events like earth movement and deformation, crustal shortening, and uplifts throughout central Australia (Manduca and Kastens, 2012). These methods, however, have been criticised for underestimating rock ages far younger than the actual age (Marjoribanks and Black, 1974; Page, 1978).

The Australian continent has been subjected to various tectonic events. Central Australia appears to have been affected by the late Neoproterozoic to early Cambrian Petermann Orogeny (PO) and the Devonian to Carboniferous Alice Springs Orogeny (ASO). The two intraplate orogenic events led to the creation of the Arunta and the Musgrave blocks; meanwhile, it is not clear if the surficial geology of the Arunta region belongs to the ASO (Norman, 1991). Moreover, the rock exposures and tilting of basin sediments in Central Australia resulted in unpredictable shifts, uplifts and displacements which have changed the physical and chemical characteristics of the rock and the extent of the events. This has led to complications in identifying the appropriate orogeny to which the rock formations belong (Grasemann and Huet, 2014). This study investigates the age of the surface geology of the Arunta region, Central Australia, to determine the Alice Spring Orogeny association using U-Pb (uranium-lead) geochronological dating.

2.1. Study Area

The study area lies within the Arunta region in Central Australia also known as the Alice Springs Region. Central Australia is one of the five regions in the Northern Territory that existed from 1927 to 1931 and was formed from the split of the Northern Territory in 1927 alongside the territory of North Australia (Paxton, 2016). It is a vast, dusty, arid and desert terrain (Mabbutt, 1969) composed of Precambrian metamorphosed sedimentary and igneous rocks subjected to Alice Springs Orogeny, a major intraplate tectonic responsible for the formation of a series of large mountain ranges (Wells et al., 1970). The investigation is conducted on the MacDonnell Mountain ranges, precisely on the Strangways Metamorphic Complex (SMC) in Alice Springs Sandover locality. It stretches geographically to Longitudes 133o 38' 38.38''E and 133o 38' 20.56''E and Latitudes 23o 00' 17.38''S and 23o 54' 42.97''S covering approximately 17, 936 km2. The overall map of the study area is depicted in Fig. 1.

Fig. 1. The study area. (a) Map of Australia highlighting the location of the site in the central Arunta, south of the Northern Region (b) the Strangways Metamorphic Complex (SMC) where the samples were collected, and (c) geological map of the Arunta block showing the metamorphic and igneous rock types.

2.2. Geological Setting

The Central Arunta region is characterized by a series of rugged mountain ranges, including the MacDonnell Ranges, which run east-west through the centre of the region. The MacDonnell Ranges are made up of a complex series of folded and faulted sedimentary and volcanic rocks, including sandstones, siltstones, shales, and basalts, formed in a variety of environments, including ancient river systems, shallow seas, and volcanic eruptions (Haines et al., 2001; McLaren et al., 2009) (Fig. 1c). The region is also home to a number of important geological formations, including the Uluru-Kata Tjuta National Park. Uluru, also known as Ayers Rock, is a massive sandstone monolith that rises over 340 meters above the surrounding plain. Kata Tjuta, also known as the Olgas, is a group of 36 large, domed rock formations that are composed of conglomerate and sandstone.

The Strangways Metamorphic Complex (SMC) is a geological unit located specifically in the northern part of the Arunta Region of the Northern Territory. It is an extensive region of complexly deformed rocks that have undergone intense metamorphism, resulting in significant changes in mineralogy, texture, and structure (Craven et al., 2012; Maidment et al., 2005). The complex consists of a diverse assemblage of rock types, including schists, gneisses, amphibolites, marbles, and quartzites. These rocks primarily originated as sedimentary and volcanic rocks, which were subsequently subjected to high temperatures and pressures during tectonic events. The dominant metamorphic grade within the SMC is greenschist facies, characterized by the development of minerals such as chlorite, biotite, and muscovite (Collins & Shaw, 1995; Neymark et al., 2021). However, higher-grade rocks, such as amphibolites and granulites, are also present in localized areas, indicating the occurrence of more intense metamorphic conditions. Structurally, the complex exhibits a complex pattern of folding, faulting, and shearing. The rocks have been subjected to multiple tectonic events, including compression and shearing, which have resulted in the development of numerous shear zones and faults (Aitken et al., 2009; Ding & James, 1985; Page, 1988).

2.3. Datasets

The rock samples analyzed in this study were collected from the field within the Strangways Metamorphic Complex. The samples were collected during the winter of 2016 in Central Australia, by Ass. Prof. Dr Nathan Daczko of the Department of Earth and Planetary Science at Macquarie University and his co-researchers. During the fieldwork, two sets of samples consisting of two rock units (Fig. 2) were extracted at different locations within regions A and B, taking into account the geological and physical characteristics of the outcrop, the relationship between them, and their level of deformity to understand the age of the Alice Springs Orogeny in the Arunta region. The samples collected at location A belong to coarse grain igneous granitoid setting predominantly found in all tectonic environments and orogeny thickened continental crust (Philpotts and Ague, 2022). At location A, samples were collected from the London-Eye (LE 1601) area and the Cattle Water Pass (CP 1610) close to the Cattle Water Shear Zone (CPSZ). Similarly, samples were taken from location B which originate from a retrograde schist within the Gough Dam shear zone (GDSZ) (Norman, 1991). Specifically, the rocks were extracted from the Gough Dam (GD) 1610 and 1622, respectively. The samples, presented in Fig. 2 were taken to the laboratory for examination and analysis. Also used is the 2nd Edition Northern Territory 1983 Alice Springs geological map at a scale of 1:250,000 provided by the Australia Bureau of Mineral Resources and Geoscience, Australia.

Fig. 2. Rock samples collected at Location A - (a) Igneous rocks with mainly quartz and feldspar from Cattle Water Pass (CP 1610) close to the Cattle Water Shear Zone (CPSZ), (b) Garnet granite invaded igneous dyke form London Eye (LE 1601); and at Location B – (c) Strained Felsic igneous rock from Gough Dam (GD 1622) and (d) Coarse crystalline igneous rocks from (GD 1610) within the, Shear Zone (GDSZ).

2.4. Methods

The rock samples collected were crushed to extract zircons crystals for dating using U-Pb isotope ratios. First, the zircon grains were separated magnetically using Frantz LB-1 Magnetic Barrier Laboratory Separator and the harvested zircons were imaged to assess their internal structure using Cathodoluminescence (CL) analysis. Thereafter, some quantities of zircon grains (~ 50 grains per sample) were selected and ablated using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Then, the U-Pb (uranium and lead) isotope ratios and the chronostratigraphy were measured at the sub-gr-ains scale. The samples were processed at the Mineral Processing Unit (MPU) of Macquarie University, Australia. The entire process stages involved rock sample collection, sample crushing, grain separation, zircon imaging and dating (Fig. 3).

Fig. 3. Flowchart for Zircon Geochronology dating.

2.4.1. Crushing and zircon separation

The zircon separation process starts with selFragging (selective fragmentation), a procedure where rock samples of about 3 to 4 cm blocks are pre-crushed into smaller pieces using SelFrag Lab S1.1. SelFrag Lab is a highvoltage laboratory system that discharges pulsed power to disintegrate rock samples; it has replaced the manual crushing of rocks with a hammer (Craven et al., 2012). A major advantage of the instrument is that the rocks are disaggregated along grain boundaries to recover mono-mineral fractions (Sperner et al., 2014). It also can separate morphologically intact minerals and a high yield of target specimens with minimal damage preventing dust and mechanical contamination of rock grains (SELFRAG, 2021), making it an ideal tool for zircon crystals extraction and geochronology studies. Before crushing the rock samples, the instrument was configured to reduce wear and tear in the machine and prevents the mineral grains’ destruction (Craven et al., 2012). Thus, the following settings were used: gap (10mm), pulse (200/cycle), frequency (3 pulse/second), and voltage (110 – 140 kV). Subsequently, the crushed samples were screened using 600μ and 300μ sieves. Four sample grain sizes (≥ 600μ, 600μ, 300μ, and < 300μ) were obtained, dried and panned.

The grains were separated magnetically using Frantz LB-1 Magnetic Barrier Laboratory Separator equipment (https://www.sgfrantz.com/laboratory-separators/). The objective of this is to obtain concordant grain samples (Sircombe & Stern, 2002). The Frantz LB-1 machine was calibrated to 1.7 A/ss (10°) / Fs (15°), considered the optimum condition for efficient separations with minimum damage to the mineral grains. The combined actions of the magnetic separator, electric current, and vibrator control of the Frantz LB-1 allowed the grains to fall from the feeder via a magnet and onto a split chute that separates the magnetic from the non-magnetic grains. With this, the panned concentrate (<300μ) is left with less magnetic mineral content (Silveira et al., 2019). The advantage of Frantz LB-1 is that it is more sensitive to separating materials with low ranges of paramagnetic or diamagnetic minerals (Sircombe and Stern, 2002). Based on the principle that material with gravity less than liquid floats and those with greater gravity sinks, heavy liquid separation (HLS) was done with Sodium Polytungstate liquid that enabled grains with densities lesser density (~3 to 3.1) to float while the heavy minerals sink to the bottom. Then with the aid of the microscope, the zircon grains were distinguished from other heavy minerals based on their shape. Furthermore, employing ultraviolet (UV) light allowed differentiating zircon crystals from other minerals by examining their fluoresce.

2.4.2. Zircon imaging and U-Pb dating

Cathodoluminescence (CL) analysis was applied to the internal structure of the harvested zircon which aided the selection of the area to be ablated for geochronological dating. Small and coarse zircon grains were handpicked and mounted on a double-sided adhesive tape slab moulded and polished with epoxy resin (Cluzel et al., 2011). All the grains were treated with equal importance during the polishing process. Smaller grains were carefully handled to maintain the internal structure of the grains for accurate measurement of the crystal's U and Pb isotope signatures and to prevent them from being grinded away or losing information critical to the dating process.

Before CL analysis, the zircon grains were carbon coated using the Edwards AUTO 306 carbon vacuum coating unit for Electron Microscopy. The Cathodoluminescence analysis was conducted on a Scanning Electron Microscope (SEM) at the Macquarie University Geo-Analytical (MQGA) Laboratory Centre. The CL detector attached to the SEM sends electron signals consisting of a wide range of electromagnetic radiation generated through emission processes to produce high-resolution digital CL images of luminescent materials (Götze et. al, 2012). Zircons CL images reveal information about the mineral, including growth zones, metamorphic overgrowths, alterations, fractures, areas of recrystallization, and micro-structures of magmatic zircons (Vavra, 1994). This information is critical to assessing and understanding the protoliths and geologic processes of mineral formation (Passarelli et al., 2009).

Following the CL imaging, some quantities of zircons (~ 50 grains per sample) were selected for the Laser Ablation using the Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) 7700 Series from Agilent Technologies (Karlstrom et al., 2018; Neymark et al., 2021). The LA-ICP-MS operating conditions and data acquisition parameters are: Nd:YAG laser; 30 μm beam diameter; 50– 60 μm pit depth estimate; Helium carrier gas at 0.8 l mn-1 in laser cell (with added Ar); 85% laser power output (0.08 mJ; 6J cm-2); external calibration standard (GJ-1); typical signal length for background (60s) and for age calculation (100–120 s); 2000 cps ppm sensitivity on mass 238U in laser mode. This allowed measuring the U-Pb isotope ratios at the sub-grains scale. The ICP-MS component can detect different isotopes of the same element, but it functions as a dating tool when coupled with an ablating laser (Košler and Sylvester, 2018). LA-ICP-MS is most suitable for dating late-crystallization events, particularly on highly resistant materials. Moreover, it enables highly sensitive element and isotopic analysis directly on solid samples (Latkoczy and Günther, 2002). The challenge with LAICP-MS is that it has difficulties with mass discrimination of isotopes (mass bias, (Horn et al., 2000)).

In practice, the process begins with loading zircon samples in the spectrometer, and several corrections, including matrix, signal drift and fractionation of isotopes specific to the zirconium minerals, were applied and the reference standard of the mineral to be analyzed set for calibration. This enables trace element compositions of zircon to be accurately measured by calibration against multiple reference materials (Liu et al., 2010). As samples are being ablated, particles created are transported to a secondary instrument (ICP-MS) for digestion and ionization of the sampled mass. The particles are finally transferred to a mass spectrometer for further analysis. Without the laser on, the background is measured as the gas progresses into the tube and the cell. Once the laser is turned on and the zircon grains ablated, the true signal originating from the zircon is determined by subtracting the background signal from the overall signal. LA-ICP-MS is capable of resolving UPb geochronology but well-characterized reference materials are needed to calibrate mass and determine correction factors discriminate elements (Gaynor et al., 2022).

Standards are essential to determining reliable U–Pb zircon ages provided it is homogeneous at all scales. In this study, samples were bracketed at the beginning and the end by pairs of analyses. Geochemical Evolution and Metallogeny of Continents (GEMOC) primary zircon Standard, STD GJ-1 (Elhlou et al., 2006), known for its very homogeneous age and minor error was set for a correction. It is a slightly discordant standard, that has a thermal ionization mass spectrometry (TIMS) 207Pb/206Pb age of 608.5 Ma (Jackson et al., 2004). Once the primary zircon Standard plot the signal resulting from the laser ablation, it is analyzed within the run against two other secondary standards, STD-91500, 1065 ± 1.01 Ma (Wiedenbeck et al., 1995) and the STD-Mud Tank, 732 ± 5 Ma (Black & Gulson, 1978). The secondary standards act as independent control on instrument stability and reproducibility. Thus, the latter allows control in situations where samples have inherited zircon which may be much older than the crystallization.

Field observation and laboratory analysis of the rock samples from locations A and B (Fig. 2) revealed the geology of the outcrops. The samples collected at location A belong to two distinct granitoid settings predominantly found in all tectonic environments and orogeny-thickened continental crust. The first sample, LE 1601, is a hightemperature orthopyroxene-bearing granitoid, a much larger granitoid setting but characteristically a low-strain felsic igneous rock with feldspar, quartz and matrix dynamically recrystallized biotite symplectites. The second sample, CP 1610, is a complex garnet-granitoid rock formed from meltrock interaction. It is a garnet granite invaded by igneous dyke with no strain, mostly characterized by quartz, plagioclase and Plagioclase and Quartz film between mineral grains. Both sample settings are surrounded by an interlayered high-grade metamorphic rock (Felsic and Mafic) subjected to high-temperature and moderate-pressure metamorphism (Granulite). The results agree with the previous studies in Central Arunta region (e.g., Ghatak et al., 2022; Maidment et al., 2005; Philpotts & Ague, 2022; Shaw et al., 1984).

On the other hand, the rock samples collected at location B (Gough Dam) occurred in the schist belt locality of phyllonitic biotite-rich quartzo-feldspathic gneiss and felsic, mafic granulite pods, quartz-rich rafts and garnet-bearing granulites. The samples contain quartz-rich mylonite and rocks with a very high proportion of mica (glimmerite) in contact between them. While the GD 1622 sample has a highly strained felsic igneous rock containing recrystallized quartz and K-feldspar grain with biotite and muscovite as reaction hosts, the GD 1610 sample is a coarse crystalline igneous rock of phaneritic texture containing porphyritic granite with invading K-feldspar. These structural features indicate a history of significant crustal deformation and regional-scale tectonic forces acting on the rocks, in agreement with Aitken et al. (2009). The SMC is also known for its mineralization potential. Metamorphic fluids and intrusions associated with the tectonic events have led to the formation of mineral deposits, including gold, copper, tungsten, and molybdenum. These deposits are often found in association with quartz veins and occur in various structural settings within the complex.

Fig. 4 presents the zircon core-rim proportion and internal structure of the rim. The CL imaging results in zircon internal structures with prominent detrital cores overgrown by narrow concentric uniform rims indicative of inclusionrich mineral grains. Core/rim intra-phase relationship suggests that the core of the mineral contains a significant number of inclusions or mineral grains that are different in composition from the host mineral. The inclusions represent different mineral phases incorporated from the surrounding environment during the mineralization processes under pressure and temperature (P-T), cooling and exhumation over t ime. Thus, the mineral may have experienced more disturbances or alterations leading to complex internal structure, an unusual amorphous mineral grain with banding possibly affected by crystal-plastic deformation arising from shear-zone influence resulting in convoluted history. The zircon in the SMC reveals equilibria which primarily depend on extreme temperature during the phases of rockforming silicate minerals such as garnet and rutile. Perhaps, the grains might have been affected by extensive radiation damage and hydrothermal alteration (Hoeve, 2013), the process of crystallization and transformation.

Fig. 4. core-rim structure of the imaged zircon - the inner and outer circles are the core and rim, respectively.

Additionally, the aggregation of one magma mingling into different magmas cannot be dismissed, as patchy zoning in zircons’ grains may be subjected to constantly changing parameters (Burnham and Berry, 2012), resulting in the heterogeneous crystal structure. Even though the grains appear diverse, ranging from irregular to grains with well-developed crystal faces, a considerable number of the zircons collected are symmetrical with a tetragonal crystal system. The grains harvested may have been affected by the geological structure either through intermingling of successive magmas or the crystallization process (Neymark et al., 2021) leading to heterogenous crystal structure that impacts the dating process. Thus, the core and rim were measured separately to obtain as much information as possible during the LA-ICP-MS ablation process.

The result of the LA-ICP-MS analysis consists of U-Pb isotopic and metrics for Standards that allowed plotting the ages of zircon grains per sample to determine age difference relative to the Concordia Curve (Geol, 2003). Accordingly, the plot of the single curve yielded 206Pb / 238U against 207Pb / 235U for concordant samples of various ages. The more points that agree with the Concordia curve, the higher the reliability of the analytical points and the age defined by the upper intercept (Mezger and Krogstad, 1997). Plots of the zircon samples shown in the Concordia curve produced ellipsoidal shapes which constrain the margin of error in ages defined within a specific range of the ellipse (Fig. 5). The ratio of the plot of samples from locations CP1610, GD1601, and GD1622 (Fig. 5a, 5b, and 5c, respectively) fall below the intercept. This is interpreted to mean that the ages produced a discordant pattern (Kis et al., 2023; Sircombe & Stern, 2002).

Fig. 5. Concordia Plot of zircon grain age intercept of ratios (207Pb / 206Pb) for the sample at (a) CP1610, (b) GD1601, and (c) GD1622 with mean squared weighted deviation (MSWD) of 0.61, 2.1, and 1.6, respectively and (d) LE1601. Except for LE1601 which shows the possible occurrence of an episodic Lead (Pb) loss event of significance that might have affected the zircon grains, all the plots fall below the concordant age curve indicating a discordant age pattern.

In contrast, the ellipsoidal shapes of sample LE1601 (Fig. 5d) intersect the Concordia curve at two points. The characteristic overlapping pattern of the ellipses on the Concordia Curve indicates the occurrence of an event of a significant impact which could either be the result of the date of an episodic or a continuous Lead (Pb) loss (Ashraf et al., 2017), contrary to the actual age of the rock or the time of crystallization of the minerals analyzed. When zircons are crystallized under intense heat, Lead (Pb) may be diffused out of the grain’s crystal and mixed with grains of younger age which have lower Pb content. The investigation of (Mezger and Krogstad, 1997) observed that primary zircon losing Pb by diffusion does not always produce meaningful age estimation.

Fig. 6 presents the estimated mean age of the rock samples. The sample collected at CP1610 (Fig. 6a) has estimated mean age of 1742.2 ± 9.2 Ma with MSWD = 0.49 and probability of equivalence of 0.90. Likewise, the rock sample at GD1622 (Fig. 6c) has an estimated mean age of 1748 ± 15 Ma - MSWD = 1.02 and probability of equivalence of 0.40. On the other hand, the estimated mean age of sample GD1601 (Fig. 6b) resulted in approximate age of 1792 ± 32 Ma. This appears not to represent the probable age because there are four epochs below and five above the mean age resulting in two different age groups with different means - those below the mean estimated at 1738 ± 14 Ma with MSWD of 0.109 and probability of equivalence of 0.95 (Fig. 6b1) and those above the mean approximated as 1838 ± 22 Ma with MSWD of 1.6 and probability of equivalence of 0.95 (Fig. 6b2). In the case of the sample from LE1601, the mean age is approximated at 1784.4 ± 9.1 Ma with MSWD of 1.09 and a probability of equivalence of 0.37 (Fig. 6d).

Fig. 6. Plot of the Mean age of the zircon grains at 95% confidence limit for sample at locations: (a) CP1610 - estimated at 1742.2 ± 9.2 Ma - MSWD = 0.49; probability of equivalence = 0.90; (b) Non-representative mean age of zircon grains: non-representative mean age of sample # GD1601 estimated at 1792 Ma, with 4 and 5 age groups below (b1) and above (b2) the mean, with sub-Mean age =1738 ± 14 Ma, MSWD = 0.109, probability of equivalence = 0.95 and sub- Mean and Mean age = 1838 ± 14 Ma; MSWD = 1.6; probability of equivalence = 0.95, respectively; (c) GD1622 - estimated at 1748 ± 15 Ma - MSWD = 1.02; probability of equivalence = 0.40; and (d) LE1601 with estimated mean age = 1784 ± 9.1 Ma - MSWD = 1.09; probability of equivalence = 0.37.

Table 1 summarizes the estimated age of the grain samples at the four locations while detailed information on the zircon U-Pb dating analysis is provided in the supplementary table. The ages of grains range between 1700 – 1800 Ma, four-fold ASO age is purported to have occurred between about 440 and 300 Ma (McLaren et al., 2009), which means that the zircons are detrital and probably recrystallized after a younger magmatic event. Similarity in ages of the core and rim is indicative of limited recrystallization or overgrowth events. Interpreted to mean that the overgrowth or recrystallization that occurred did not significantly affect the isotopic system. This constraint the timing and extent of geological events that might have affected the mineral, such as metamorphism or hydrothermal alteration.


Summary of the estimated age of the grain samples at the four locations


Rock SampleAGEMSWDProbability of equivalence
# CP1610Mean1742.2 ± 9.2 Ma0.490.90
# GD1601Above mean1838 ± 14 Ma1.60.95
Mean1792 ± 32 Ma--
Below mean1738 ± 14 Ma0.1090.95
# LE1601Mean1784.4 ± 9.1 Ma1.090.37
# GD1622Mean1738.2 ± 15 Ma0.1090.95


The Lead/Lead ratio (207Pb/206Pb) is published for the age of each zircon dated. The point analyzed on grains and their ages are shown in Fig. 7. Analysis of the SEM-CL imaging of the grain sample revealed that the zircons’ age might have been affected by processes that alter the internal structure leading to the observed anomalies in some of the grains falling below the Concordia curve. A comprehensive appraisal of zircon grain age discordance depends on understanding the phenomena leading to a loss in Pb and its relationship with younger age zircon. Relying solely on the amount of Pb in the grain sample to determine the age of younger zircon could result in erroneous dating. When zircons are crystallized and found in the high-grade metamorphism in an environment exposed to intense heat such as in the current study samples, it is most likely that the Pb may have diffused out of the grain’s crystal to mix with younger age zircon. Furthermore, disruptions of the U-Pb mineral component through episodic lead loss, inheritance (Corfu et al., 2018), crystal-plastic deformation (La Fontaine et al., 2017; Kovaleva and Klötzli, 2014), or thermal events may result in discordant grains inhibiting the trait of younger age zircon. It could also be the blueprints of the later magmatic event with a progressive breakdown of their structure through time-integrated radiation damage (Silver and Deutsch, 1963) amplified by contaminated trace elements.

Fig. 7. Schematic sketches of zircon U-Pb age distribution at scale 20 μm indicating the zircon core–rim proportion types and internal rim textures of grain sample from (a) CP 1610, (b) LE1601, (c) GD 1601 and (d) GD 1622. Note that P denote Point; R - Rim, and C – Core.

A considerable number of the zircons collected display well-developed crystal faces symmetrical with a tetragonal crystal system as revealed in Fig. 4. However, the most of the images reveal zircons with complex internal structure, an unusual amorphous mineral grain with banding that inhibits a convoluted history. The unusual structure may arise from the impact of crystal-plastic deformation caused by shear zone influence in the SMC which depends primarily on extreme temperature during the phases of rock-forming silicate minerals such as garnet and rutile or the processes of crystallization and transformation following successive migmatization (Ghatak et al., 2022; Marjoribanks & Black, 1974).

U-Pb loss may have occurred during the recrystallization process under temperature, pressure and chemically active fluid increase since the samples were collected within high-strain shear zones that might have been penetrated by relict of the pre-existing rock mineral formations along a network of fractures. In the Gough Dam for instance, the content of quartzite in the sample may have been modified into glimmerite (increase in biotite) during dynamic melt migration, changing the trace element and composition of Proterozoic U-Pb ages inherited from the protolith (1740–1630 Ma) to younger Palaeozoic ages in the Alice Springs Orogeny (Ghatak et al., 2022).

Moreover, the rock samples obtained from the three representative outcrops revealed that zircon grains are more abundant in the shear zone with generally equant, rounded to subrounded dimensions of up to 500 μm. The U-Pb data are mainly discordant, and the apparent 206Pb/238U dates show a large variation from the Permian to the Jurassic (Kis et al., 2023; Mezger & Krogstad, 1997; Sircombe & Stern, 2002). Isotopic data, combined with microstructural, morphological, and internal features of zircon, reveal an inherited age component that suggests partial zircon recrystallization under high-temperature conditions.

In this investigation, zircon grains extracted from rock samples taken within the ASO were progressed through various analytical means including Cathodoluminescence and LA-ICP-MS analyses to verify their age and origin. The outcome of the analysis revealed that there are discrepancies in age, estimated to be between 1700 – 1755 Ma, which i s much o lder than the ASO occurrences. The internal structure of the zircon grains as revealed in the Concordia curve plot highlights the complex anomalous contrary to the usual transparent characteristics observed in the homogeneous zone of rocks. The study further alludes to the occurrence of an event with a significant impact that may have caused the episodic Lead (Pb) loss that constrains determining the true age of the rock or the time of crystallization of the minerals. This study suggests further investigations across the Arunta region to better understand the event(s) that might have impacted the basements on a regional scale to improve dating and determine where the rocks derive their source from.

The authors would like to thank the CSIRO Veteran Dr Steve Craven, the Lab analytical instrumentation expert, Dr Timothy Murphy and the Metamorphic Petrologist, Assoc. Prof. Dr Nathan Daczko, all from Macquarie University, North Ryde, and members of the Australian Research Council Centre of Excellence for Core to Crust Fluid Systems (ARCC) and the Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC).

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Article

Research Paper

Econ. Environ. Geol. 2023; 56(6): 817-830

Published online December 29, 2023 https://doi.org/10.9719/EEG.2023.56.6.817

Copyright © THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY.

Alice Springs Orogeny (ASO) Footprints Tracing in Fresh Rocks in Arunta Region, Central Australia, Using Uranium/Lead (U-Pb) Geochronology

Kouame Yao1, Mohammed O. Idrees2,3,*, Abdul-Lateef Balogun4, Mohamed Barakat A. Gibril5

1Macquarie University, Department of Earth and Planetary Sciences, Faculty of Science and Engineering, North Ryde, Australia
2Department of Surveying and Geoinformatics, Faculty of Environmental Sciences, University of Abuja, P.M.B. 117, Abuja, Nigeria
3Department of Surveying and Geoinformatics, Faculty of Environmental Sciences, University of Ilorin, P.M.B. 1515, Ilorin, Nigeria
4Environmental Systems Research Institute (ESRI), Melbourne, Australia
5GIS and Remote Sensing Center, Research Institute of Sciences and Engineering, University of Sharjah, Sharjah 27272, United Arab Emirates

Correspondence to:*dare.idrees@gmail.com, mohammed.idrees@uniabuja.edu.ng

Received: March 13, 2023; Revised: November 4, 2023; Accepted: November 7, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

This study investigates the age of the surficial rocks in the Arunta region using Uranium-Lead (U-Pb) geochronological dating. Rock samples were collected at four locations, Cattle-Water Pass (CP 1610), Gough Dam (GD 1622 and GD 1610), and London-Eye (LE 1601), within the Strangways Metamorphic Complex and crushed by selFragging. Subsequently, the zircon grains were imaged using Cathodoluminescence (CL) analysis and the U-Pb (uranium and lead) isotope ratios and the chrono-stratigraphy were measured. The imaged zircon revealed an anomalous heterogeneous crystal structure. Ellipses of the samples at locations GD1601, CP1610, and GD1622 fall below the intercept indicating the ages produced discordant patterns, whereas LE1601 intersects the Concordia curve at two points, implying the occurrence of an event of significant impact. For the rock sample at CP1610, the estimated mean age is 1742.2 ± 9.2 Ma with mean squared weighted deviation (MSWD) = 0.49 and probability of equivalence of 0.90; 1748 ± 15 Ma - MSWD = 1.02 and probability of equivalence of 0.40 for GD1622; and 1784.4 ± 9.1 Ma with MSWD of 1.09 and probability of equivalence of 0.37 for LE1601. But for samples at GD1601, two different age groups with different means occurred: 1) below the global mean (1792.2 ± 32 Ma) estimated at 1738.2 ± 14 Ma with MSWD of 0.109 and probability of equivalence of 0.95 and 2) above it with mean of 1838.22 ± 14 Ma, MSWD of 1.6 and probability of equivalence of 0.95. Analysis of the zircon grains has shown a discrepancy in the age range between 1700 Ma and 1800 Ma compared to the ASO dated to have occurred between 440 and 300 Ma. Moreover, apparent similarity in age of the core and rim means that the mineral crystallized relatively quickly without significant interruptions and effect on the isotopic system. This may have constraint the timing and extent of geological events that might have affected the mineral, such as metamorphism or hydrothermal alteration.

Keywords geology, rock dating, selFragging, lead loss, Cathodoluminescence imaging

Research Highlights

  • Age and origin of zircon grains extracted from rock samples taken within the Alice Springs Orogeny were profiled through Cathodoluminescence and LA-ICP-MS analyses.

  • Core/rim intra-phase relationship suggests that the core of the mineral contains a significant number of inclusions different in composition from the host mineral.

  • The mineral may have experienced more disturbances or alterations leading to complex internal structure possibly affected by crystal-plastic deformation.

  • Discrepancies exist in the estimated age (1700 - 1755 Ma), which is much older than the ASO occurrences.

1. Introduction

Investigating rock formation to understand its historical development is a practice that dated back to the beginning of the last century when long-lived radioactive decay systems were recognized to provide the only valid means to quantify geologic time (Davis et al., 2018). Usually, researchers utilise sediment-compacted rock specimens from various sources to discover major past geological events that have occurred (Warren, 1983). The reliability of this approach is largely dependent on the geological characteristics of the sample collected and how they are interpreted because drastic changes do occur due to rupture in the crust different from the Orogenic events (Grasemann and Huet, 2014). Violent changes in rock formations alter the order of formation and consequently, the information within the rocks sample (Kearey et al., 2009) as the orogeny leads to erosion and causes undeformed sediment to be deposited above the eroded remnants of deformed rock formation (Haines et al., 2001). This often results in information gaps that require combining several variables to understand the processes leading to the current state of the outcrop.

The Central Arunta region is a vast and remote area located in the southern part of the Northern Territory of Australia (Craven et al., 2012). The region is a fascinating geological area that contains a wide variety of rocks and geological formations that offer insight into the Earth's history and the processes that have shaped the Australian continent over billions of years (Maidment et al., 2005). The geological features of this region are diverse and complex, with a long history of geological activity. Central Arunta region is situated on the eastern margin of the Western Australian Craton, a stable block of ancient rocks that formed the core of the Australian continent (McLaren et al., 2009). The rocks of the Central Arunta are dominated by sedimentary and volcanic rocks, including sandstones, siltstones, shales, and basalts. These rocks were deposited during the Proterozoic Era, between 1.8 and 0.6 billion years ago (Page, 1988).

In Central Australia, intercontinental magnitude disturbances during the long-lived Devonian-Carboniferous epoch Alice Spring Orogeny (ASO) (~400-350 Ma), have defined a complex geology, disentombing Arunta inlier in a southdirected thrusting trend (Flottmann et al., 2004), folding sedimentary deposits to produce mountainous ranges (Haines et al., 2001), thrusting up of rocks of Proterozoic ages, and eroding sediment off mountain belt, deposing and incorporating them into remaining relics of the former sedimentary basin (Bradshaw and Evans, 1988).

ASO is characterized by misapprehension of the physical condition and the aerial extent representative of distinctive structural phenomena of the regional tectonic rocks and crust. Thus, the outcrop is interlaced with a mix of metamorphosed rocks from different geologic eras without defined aerial coverage, and consequently a distortion of the regional lithology in these zones that bear the characteristics of different epochs. The elongated lenses of pegmatite representative of Ormiston events (1076 Ma) separated from gneissic rocks affecting rocks along the central southern margin of the Arunta inlier (Marjoribanks and Black, 1974) or the Central Hart Range generated pods of Quartz and Feldspar-rich rocks migrating upward and crystallizing as pegmatites during ASO (Stewart et al., 1984) are some of the countless examples that illustrate the above assertion.

Rock dating techniques have been widely employed to understand how earth processes impacted rock formations (Wilde et al., 2001). Geoscientists have utilized geochronology dating techniques such as Rubidium–strontium (Rb-Sr) and Potassium-argon (K-Ar) to understand past events and natural phenomena leading to tectonic-induced events like earth movement and deformation, crustal shortening, and uplifts throughout central Australia (Manduca and Kastens, 2012). These methods, however, have been criticised for underestimating rock ages far younger than the actual age (Marjoribanks and Black, 1974; Page, 1978).

The Australian continent has been subjected to various tectonic events. Central Australia appears to have been affected by the late Neoproterozoic to early Cambrian Petermann Orogeny (PO) and the Devonian to Carboniferous Alice Springs Orogeny (ASO). The two intraplate orogenic events led to the creation of the Arunta and the Musgrave blocks; meanwhile, it is not clear if the surficial geology of the Arunta region belongs to the ASO (Norman, 1991). Moreover, the rock exposures and tilting of basin sediments in Central Australia resulted in unpredictable shifts, uplifts and displacements which have changed the physical and chemical characteristics of the rock and the extent of the events. This has led to complications in identifying the appropriate orogeny to which the rock formations belong (Grasemann and Huet, 2014). This study investigates the age of the surface geology of the Arunta region, Central Australia, to determine the Alice Spring Orogeny association using U-Pb (uranium-lead) geochronological dating.

2. Materials and Methods

2.1. Study Area

The study area lies within the Arunta region in Central Australia also known as the Alice Springs Region. Central Australia is one of the five regions in the Northern Territory that existed from 1927 to 1931 and was formed from the split of the Northern Territory in 1927 alongside the territory of North Australia (Paxton, 2016). It is a vast, dusty, arid and desert terrain (Mabbutt, 1969) composed of Precambrian metamorphosed sedimentary and igneous rocks subjected to Alice Springs Orogeny, a major intraplate tectonic responsible for the formation of a series of large mountain ranges (Wells et al., 1970). The investigation is conducted on the MacDonnell Mountain ranges, precisely on the Strangways Metamorphic Complex (SMC) in Alice Springs Sandover locality. It stretches geographically to Longitudes 133o 38' 38.38''E and 133o 38' 20.56''E and Latitudes 23o 00' 17.38''S and 23o 54' 42.97''S covering approximately 17, 936 km2. The overall map of the study area is depicted in Fig. 1.

Figure 1. The study area. (a) Map of Australia highlighting the location of the site in the central Arunta, south of the Northern Region (b) the Strangways Metamorphic Complex (SMC) where the samples were collected, and (c) geological map of the Arunta block showing the metamorphic and igneous rock types.

2.2. Geological Setting

The Central Arunta region is characterized by a series of rugged mountain ranges, including the MacDonnell Ranges, which run east-west through the centre of the region. The MacDonnell Ranges are made up of a complex series of folded and faulted sedimentary and volcanic rocks, including sandstones, siltstones, shales, and basalts, formed in a variety of environments, including ancient river systems, shallow seas, and volcanic eruptions (Haines et al., 2001; McLaren et al., 2009) (Fig. 1c). The region is also home to a number of important geological formations, including the Uluru-Kata Tjuta National Park. Uluru, also known as Ayers Rock, is a massive sandstone monolith that rises over 340 meters above the surrounding plain. Kata Tjuta, also known as the Olgas, is a group of 36 large, domed rock formations that are composed of conglomerate and sandstone.

The Strangways Metamorphic Complex (SMC) is a geological unit located specifically in the northern part of the Arunta Region of the Northern Territory. It is an extensive region of complexly deformed rocks that have undergone intense metamorphism, resulting in significant changes in mineralogy, texture, and structure (Craven et al., 2012; Maidment et al., 2005). The complex consists of a diverse assemblage of rock types, including schists, gneisses, amphibolites, marbles, and quartzites. These rocks primarily originated as sedimentary and volcanic rocks, which were subsequently subjected to high temperatures and pressures during tectonic events. The dominant metamorphic grade within the SMC is greenschist facies, characterized by the development of minerals such as chlorite, biotite, and muscovite (Collins & Shaw, 1995; Neymark et al., 2021). However, higher-grade rocks, such as amphibolites and granulites, are also present in localized areas, indicating the occurrence of more intense metamorphic conditions. Structurally, the complex exhibits a complex pattern of folding, faulting, and shearing. The rocks have been subjected to multiple tectonic events, including compression and shearing, which have resulted in the development of numerous shear zones and faults (Aitken et al., 2009; Ding & James, 1985; Page, 1988).

2.3. Datasets

The rock samples analyzed in this study were collected from the field within the Strangways Metamorphic Complex. The samples were collected during the winter of 2016 in Central Australia, by Ass. Prof. Dr Nathan Daczko of the Department of Earth and Planetary Science at Macquarie University and his co-researchers. During the fieldwork, two sets of samples consisting of two rock units (Fig. 2) were extracted at different locations within regions A and B, taking into account the geological and physical characteristics of the outcrop, the relationship between them, and their level of deformity to understand the age of the Alice Springs Orogeny in the Arunta region. The samples collected at location A belong to coarse grain igneous granitoid setting predominantly found in all tectonic environments and orogeny thickened continental crust (Philpotts and Ague, 2022). At location A, samples were collected from the London-Eye (LE 1601) area and the Cattle Water Pass (CP 1610) close to the Cattle Water Shear Zone (CPSZ). Similarly, samples were taken from location B which originate from a retrograde schist within the Gough Dam shear zone (GDSZ) (Norman, 1991). Specifically, the rocks were extracted from the Gough Dam (GD) 1610 and 1622, respectively. The samples, presented in Fig. 2 were taken to the laboratory for examination and analysis. Also used is the 2nd Edition Northern Territory 1983 Alice Springs geological map at a scale of 1:250,000 provided by the Australia Bureau of Mineral Resources and Geoscience, Australia.

Figure 2. Rock samples collected at Location A - (a) Igneous rocks with mainly quartz and feldspar from Cattle Water Pass (CP 1610) close to the Cattle Water Shear Zone (CPSZ), (b) Garnet granite invaded igneous dyke form London Eye (LE 1601); and at Location B – (c) Strained Felsic igneous rock from Gough Dam (GD 1622) and (d) Coarse crystalline igneous rocks from (GD 1610) within the, Shear Zone (GDSZ).

2.4. Methods

The rock samples collected were crushed to extract zircons crystals for dating using U-Pb isotope ratios. First, the zircon grains were separated magnetically using Frantz LB-1 Magnetic Barrier Laboratory Separator and the harvested zircons were imaged to assess their internal structure using Cathodoluminescence (CL) analysis. Thereafter, some quantities of zircon grains (~ 50 grains per sample) were selected and ablated using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Then, the U-Pb (uranium and lead) isotope ratios and the chronostratigraphy were measured at the sub-gr-ains scale. The samples were processed at the Mineral Processing Unit (MPU) of Macquarie University, Australia. The entire process stages involved rock sample collection, sample crushing, grain separation, zircon imaging and dating (Fig. 3).

Figure 3. Flowchart for Zircon Geochronology dating.

2.4.1. Crushing and zircon separation

The zircon separation process starts with selFragging (selective fragmentation), a procedure where rock samples of about 3 to 4 cm blocks are pre-crushed into smaller pieces using SelFrag Lab S1.1. SelFrag Lab is a highvoltage laboratory system that discharges pulsed power to disintegrate rock samples; it has replaced the manual crushing of rocks with a hammer (Craven et al., 2012). A major advantage of the instrument is that the rocks are disaggregated along grain boundaries to recover mono-mineral fractions (Sperner et al., 2014). It also can separate morphologically intact minerals and a high yield of target specimens with minimal damage preventing dust and mechanical contamination of rock grains (SELFRAG, 2021), making it an ideal tool for zircon crystals extraction and geochronology studies. Before crushing the rock samples, the instrument was configured to reduce wear and tear in the machine and prevents the mineral grains’ destruction (Craven et al., 2012). Thus, the following settings were used: gap (10mm), pulse (200/cycle), frequency (3 pulse/second), and voltage (110 – 140 kV). Subsequently, the crushed samples were screened using 600μ and 300μ sieves. Four sample grain sizes (≥ 600μ, 600μ, 300μ, and < 300μ) were obtained, dried and panned.

The grains were separated magnetically using Frantz LB-1 Magnetic Barrier Laboratory Separator equipment (https://www.sgfrantz.com/laboratory-separators/). The objective of this is to obtain concordant grain samples (Sircombe & Stern, 2002). The Frantz LB-1 machine was calibrated to 1.7 A/ss (10°) / Fs (15°), considered the optimum condition for efficient separations with minimum damage to the mineral grains. The combined actions of the magnetic separator, electric current, and vibrator control of the Frantz LB-1 allowed the grains to fall from the feeder via a magnet and onto a split chute that separates the magnetic from the non-magnetic grains. With this, the panned concentrate (<300μ) is left with less magnetic mineral content (Silveira et al., 2019). The advantage of Frantz LB-1 is that it is more sensitive to separating materials with low ranges of paramagnetic or diamagnetic minerals (Sircombe and Stern, 2002). Based on the principle that material with gravity less than liquid floats and those with greater gravity sinks, heavy liquid separation (HLS) was done with Sodium Polytungstate liquid that enabled grains with densities lesser density (~3 to 3.1) to float while the heavy minerals sink to the bottom. Then with the aid of the microscope, the zircon grains were distinguished from other heavy minerals based on their shape. Furthermore, employing ultraviolet (UV) light allowed differentiating zircon crystals from other minerals by examining their fluoresce.

2.4.2. Zircon imaging and U-Pb dating

Cathodoluminescence (CL) analysis was applied to the internal structure of the harvested zircon which aided the selection of the area to be ablated for geochronological dating. Small and coarse zircon grains were handpicked and mounted on a double-sided adhesive tape slab moulded and polished with epoxy resin (Cluzel et al., 2011). All the grains were treated with equal importance during the polishing process. Smaller grains were carefully handled to maintain the internal structure of the grains for accurate measurement of the crystal's U and Pb isotope signatures and to prevent them from being grinded away or losing information critical to the dating process.

Before CL analysis, the zircon grains were carbon coated using the Edwards AUTO 306 carbon vacuum coating unit for Electron Microscopy. The Cathodoluminescence analysis was conducted on a Scanning Electron Microscope (SEM) at the Macquarie University Geo-Analytical (MQGA) Laboratory Centre. The CL detector attached to the SEM sends electron signals consisting of a wide range of electromagnetic radiation generated through emission processes to produce high-resolution digital CL images of luminescent materials (Götze et. al, 2012). Zircons CL images reveal information about the mineral, including growth zones, metamorphic overgrowths, alterations, fractures, areas of recrystallization, and micro-structures of magmatic zircons (Vavra, 1994). This information is critical to assessing and understanding the protoliths and geologic processes of mineral formation (Passarelli et al., 2009).

Following the CL imaging, some quantities of zircons (~ 50 grains per sample) were selected for the Laser Ablation using the Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) 7700 Series from Agilent Technologies (Karlstrom et al., 2018; Neymark et al., 2021). The LA-ICP-MS operating conditions and data acquisition parameters are: Nd:YAG laser; 30 μm beam diameter; 50– 60 μm pit depth estimate; Helium carrier gas at 0.8 l mn-1 in laser cell (with added Ar); 85% laser power output (0.08 mJ; 6J cm-2); external calibration standard (GJ-1); typical signal length for background (60s) and for age calculation (100–120 s); 2000 cps ppm sensitivity on mass 238U in laser mode. This allowed measuring the U-Pb isotope ratios at the sub-grains scale. The ICP-MS component can detect different isotopes of the same element, but it functions as a dating tool when coupled with an ablating laser (Košler and Sylvester, 2018). LA-ICP-MS is most suitable for dating late-crystallization events, particularly on highly resistant materials. Moreover, it enables highly sensitive element and isotopic analysis directly on solid samples (Latkoczy and Günther, 2002). The challenge with LAICP-MS is that it has difficulties with mass discrimination of isotopes (mass bias, (Horn et al., 2000)).

In practice, the process begins with loading zircon samples in the spectrometer, and several corrections, including matrix, signal drift and fractionation of isotopes specific to the zirconium minerals, were applied and the reference standard of the mineral to be analyzed set for calibration. This enables trace element compositions of zircon to be accurately measured by calibration against multiple reference materials (Liu et al., 2010). As samples are being ablated, particles created are transported to a secondary instrument (ICP-MS) for digestion and ionization of the sampled mass. The particles are finally transferred to a mass spectrometer for further analysis. Without the laser on, the background is measured as the gas progresses into the tube and the cell. Once the laser is turned on and the zircon grains ablated, the true signal originating from the zircon is determined by subtracting the background signal from the overall signal. LA-ICP-MS is capable of resolving UPb geochronology but well-characterized reference materials are needed to calibrate mass and determine correction factors discriminate elements (Gaynor et al., 2022).

Standards are essential to determining reliable U–Pb zircon ages provided it is homogeneous at all scales. In this study, samples were bracketed at the beginning and the end by pairs of analyses. Geochemical Evolution and Metallogeny of Continents (GEMOC) primary zircon Standard, STD GJ-1 (Elhlou et al., 2006), known for its very homogeneous age and minor error was set for a correction. It is a slightly discordant standard, that has a thermal ionization mass spectrometry (TIMS) 207Pb/206Pb age of 608.5 Ma (Jackson et al., 2004). Once the primary zircon Standard plot the signal resulting from the laser ablation, it is analyzed within the run against two other secondary standards, STD-91500, 1065 ± 1.01 Ma (Wiedenbeck et al., 1995) and the STD-Mud Tank, 732 ± 5 Ma (Black & Gulson, 1978). The secondary standards act as independent control on instrument stability and reproducibility. Thus, the latter allows control in situations where samples have inherited zircon which may be much older than the crystallization.

3. Results and Discussion

Field observation and laboratory analysis of the rock samples from locations A and B (Fig. 2) revealed the geology of the outcrops. The samples collected at location A belong to two distinct granitoid settings predominantly found in all tectonic environments and orogeny-thickened continental crust. The first sample, LE 1601, is a hightemperature orthopyroxene-bearing granitoid, a much larger granitoid setting but characteristically a low-strain felsic igneous rock with feldspar, quartz and matrix dynamically recrystallized biotite symplectites. The second sample, CP 1610, is a complex garnet-granitoid rock formed from meltrock interaction. It is a garnet granite invaded by igneous dyke with no strain, mostly characterized by quartz, plagioclase and Plagioclase and Quartz film between mineral grains. Both sample settings are surrounded by an interlayered high-grade metamorphic rock (Felsic and Mafic) subjected to high-temperature and moderate-pressure metamorphism (Granulite). The results agree with the previous studies in Central Arunta region (e.g., Ghatak et al., 2022; Maidment et al., 2005; Philpotts & Ague, 2022; Shaw et al., 1984).

On the other hand, the rock samples collected at location B (Gough Dam) occurred in the schist belt locality of phyllonitic biotite-rich quartzo-feldspathic gneiss and felsic, mafic granulite pods, quartz-rich rafts and garnet-bearing granulites. The samples contain quartz-rich mylonite and rocks with a very high proportion of mica (glimmerite) in contact between them. While the GD 1622 sample has a highly strained felsic igneous rock containing recrystallized quartz and K-feldspar grain with biotite and muscovite as reaction hosts, the GD 1610 sample is a coarse crystalline igneous rock of phaneritic texture containing porphyritic granite with invading K-feldspar. These structural features indicate a history of significant crustal deformation and regional-scale tectonic forces acting on the rocks, in agreement with Aitken et al. (2009). The SMC is also known for its mineralization potential. Metamorphic fluids and intrusions associated with the tectonic events have led to the formation of mineral deposits, including gold, copper, tungsten, and molybdenum. These deposits are often found in association with quartz veins and occur in various structural settings within the complex.

Fig. 4 presents the zircon core-rim proportion and internal structure of the rim. The CL imaging results in zircon internal structures with prominent detrital cores overgrown by narrow concentric uniform rims indicative of inclusionrich mineral grains. Core/rim intra-phase relationship suggests that the core of the mineral contains a significant number of inclusions or mineral grains that are different in composition from the host mineral. The inclusions represent different mineral phases incorporated from the surrounding environment during the mineralization processes under pressure and temperature (P-T), cooling and exhumation over t ime. Thus, the mineral may have experienced more disturbances or alterations leading to complex internal structure, an unusual amorphous mineral grain with banding possibly affected by crystal-plastic deformation arising from shear-zone influence resulting in convoluted history. The zircon in the SMC reveals equilibria which primarily depend on extreme temperature during the phases of rockforming silicate minerals such as garnet and rutile. Perhaps, the grains might have been affected by extensive radiation damage and hydrothermal alteration (Hoeve, 2013), the process of crystallization and transformation.

Figure 4. core-rim structure of the imaged zircon - the inner and outer circles are the core and rim, respectively.

Additionally, the aggregation of one magma mingling into different magmas cannot be dismissed, as patchy zoning in zircons’ grains may be subjected to constantly changing parameters (Burnham and Berry, 2012), resulting in the heterogeneous crystal structure. Even though the grains appear diverse, ranging from irregular to grains with well-developed crystal faces, a considerable number of the zircons collected are symmetrical with a tetragonal crystal system. The grains harvested may have been affected by the geological structure either through intermingling of successive magmas or the crystallization process (Neymark et al., 2021) leading to heterogenous crystal structure that impacts the dating process. Thus, the core and rim were measured separately to obtain as much information as possible during the LA-ICP-MS ablation process.

The result of the LA-ICP-MS analysis consists of U-Pb isotopic and metrics for Standards that allowed plotting the ages of zircon grains per sample to determine age difference relative to the Concordia Curve (Geol, 2003). Accordingly, the plot of the single curve yielded 206Pb / 238U against 207Pb / 235U for concordant samples of various ages. The more points that agree with the Concordia curve, the higher the reliability of the analytical points and the age defined by the upper intercept (Mezger and Krogstad, 1997). Plots of the zircon samples shown in the Concordia curve produced ellipsoidal shapes which constrain the margin of error in ages defined within a specific range of the ellipse (Fig. 5). The ratio of the plot of samples from locations CP1610, GD1601, and GD1622 (Fig. 5a, 5b, and 5c, respectively) fall below the intercept. This is interpreted to mean that the ages produced a discordant pattern (Kis et al., 2023; Sircombe & Stern, 2002).

Figure 5. Concordia Plot of zircon grain age intercept of ratios (207Pb / 206Pb) for the sample at (a) CP1610, (b) GD1601, and (c) GD1622 with mean squared weighted deviation (MSWD) of 0.61, 2.1, and 1.6, respectively and (d) LE1601. Except for LE1601 which shows the possible occurrence of an episodic Lead (Pb) loss event of significance that might have affected the zircon grains, all the plots fall below the concordant age curve indicating a discordant age pattern.

In contrast, the ellipsoidal shapes of sample LE1601 (Fig. 5d) intersect the Concordia curve at two points. The characteristic overlapping pattern of the ellipses on the Concordia Curve indicates the occurrence of an event of a significant impact which could either be the result of the date of an episodic or a continuous Lead (Pb) loss (Ashraf et al., 2017), contrary to the actual age of the rock or the time of crystallization of the minerals analyzed. When zircons are crystallized under intense heat, Lead (Pb) may be diffused out of the grain’s crystal and mixed with grains of younger age which have lower Pb content. The investigation of (Mezger and Krogstad, 1997) observed that primary zircon losing Pb by diffusion does not always produce meaningful age estimation.

Fig. 6 presents the estimated mean age of the rock samples. The sample collected at CP1610 (Fig. 6a) has estimated mean age of 1742.2 ± 9.2 Ma with MSWD = 0.49 and probability of equivalence of 0.90. Likewise, the rock sample at GD1622 (Fig. 6c) has an estimated mean age of 1748 ± 15 Ma - MSWD = 1.02 and probability of equivalence of 0.40. On the other hand, the estimated mean age of sample GD1601 (Fig. 6b) resulted in approximate age of 1792 ± 32 Ma. This appears not to represent the probable age because there are four epochs below and five above the mean age resulting in two different age groups with different means - those below the mean estimated at 1738 ± 14 Ma with MSWD of 0.109 and probability of equivalence of 0.95 (Fig. 6b1) and those above the mean approximated as 1838 ± 22 Ma with MSWD of 1.6 and probability of equivalence of 0.95 (Fig. 6b2). In the case of the sample from LE1601, the mean age is approximated at 1784.4 ± 9.1 Ma with MSWD of 1.09 and a probability of equivalence of 0.37 (Fig. 6d).

Figure 6. Plot of the Mean age of the zircon grains at 95% confidence limit for sample at locations: (a) CP1610 - estimated at 1742.2 ± 9.2 Ma - MSWD = 0.49; probability of equivalence = 0.90; (b) Non-representative mean age of zircon grains: non-representative mean age of sample # GD1601 estimated at 1792 Ma, with 4 and 5 age groups below (b1) and above (b2) the mean, with sub-Mean age =1738 ± 14 Ma, MSWD = 0.109, probability of equivalence = 0.95 and sub- Mean and Mean age = 1838 ± 14 Ma; MSWD = 1.6; probability of equivalence = 0.95, respectively; (c) GD1622 - estimated at 1748 ± 15 Ma - MSWD = 1.02; probability of equivalence = 0.40; and (d) LE1601 with estimated mean age = 1784 ± 9.1 Ma - MSWD = 1.09; probability of equivalence = 0.37.

Table 1 summarizes the estimated age of the grain samples at the four locations while detailed information on the zircon U-Pb dating analysis is provided in the supplementary table. The ages of grains range between 1700 – 1800 Ma, four-fold ASO age is purported to have occurred between about 440 and 300 Ma (McLaren et al., 2009), which means that the zircons are detrital and probably recrystallized after a younger magmatic event. Similarity in ages of the core and rim is indicative of limited recrystallization or overgrowth events. Interpreted to mean that the overgrowth or recrystallization that occurred did not significantly affect the isotopic system. This constraint the timing and extent of geological events that might have affected the mineral, such as metamorphism or hydrothermal alteration.


Summary of the estimated age of the grain samples at the four locations.


Rock SampleAGEMSWDProbability of equivalence
# CP1610Mean1742.2 ± 9.2 Ma0.490.90
# GD1601Above mean1838 ± 14 Ma1.60.95
Mean1792 ± 32 Ma--
Below mean1738 ± 14 Ma0.1090.95
# LE1601Mean1784.4 ± 9.1 Ma1.090.37
# GD1622Mean1738.2 ± 15 Ma0.1090.95


The Lead/Lead ratio (207Pb/206Pb) is published for the age of each zircon dated. The point analyzed on grains and their ages are shown in Fig. 7. Analysis of the SEM-CL imaging of the grain sample revealed that the zircons’ age might have been affected by processes that alter the internal structure leading to the observed anomalies in some of the grains falling below the Concordia curve. A comprehensive appraisal of zircon grain age discordance depends on understanding the phenomena leading to a loss in Pb and its relationship with younger age zircon. Relying solely on the amount of Pb in the grain sample to determine the age of younger zircon could result in erroneous dating. When zircons are crystallized and found in the high-grade metamorphism in an environment exposed to intense heat such as in the current study samples, it is most likely that the Pb may have diffused out of the grain’s crystal to mix with younger age zircon. Furthermore, disruptions of the U-Pb mineral component through episodic lead loss, inheritance (Corfu et al., 2018), crystal-plastic deformation (La Fontaine et al., 2017; Kovaleva and Klötzli, 2014), or thermal events may result in discordant grains inhibiting the trait of younger age zircon. It could also be the blueprints of the later magmatic event with a progressive breakdown of their structure through time-integrated radiation damage (Silver and Deutsch, 1963) amplified by contaminated trace elements.

Figure 7. Schematic sketches of zircon U-Pb age distribution at scale 20 μm indicating the zircon core–rim proportion types and internal rim textures of grain sample from (a) CP 1610, (b) LE1601, (c) GD 1601 and (d) GD 1622. Note that P denote Point; R - Rim, and C – Core.

A considerable number of the zircons collected display well-developed crystal faces symmetrical with a tetragonal crystal system as revealed in Fig. 4. However, the most of the images reveal zircons with complex internal structure, an unusual amorphous mineral grain with banding that inhibits a convoluted history. The unusual structure may arise from the impact of crystal-plastic deformation caused by shear zone influence in the SMC which depends primarily on extreme temperature during the phases of rock-forming silicate minerals such as garnet and rutile or the processes of crystallization and transformation following successive migmatization (Ghatak et al., 2022; Marjoribanks & Black, 1974).

U-Pb loss may have occurred during the recrystallization process under temperature, pressure and chemically active fluid increase since the samples were collected within high-strain shear zones that might have been penetrated by relict of the pre-existing rock mineral formations along a network of fractures. In the Gough Dam for instance, the content of quartzite in the sample may have been modified into glimmerite (increase in biotite) during dynamic melt migration, changing the trace element and composition of Proterozoic U-Pb ages inherited from the protolith (1740–1630 Ma) to younger Palaeozoic ages in the Alice Springs Orogeny (Ghatak et al., 2022).

Moreover, the rock samples obtained from the three representative outcrops revealed that zircon grains are more abundant in the shear zone with generally equant, rounded to subrounded dimensions of up to 500 μm. The U-Pb data are mainly discordant, and the apparent 206Pb/238U dates show a large variation from the Permian to the Jurassic (Kis et al., 2023; Mezger & Krogstad, 1997; Sircombe & Stern, 2002). Isotopic data, combined with microstructural, morphological, and internal features of zircon, reveal an inherited age component that suggests partial zircon recrystallization under high-temperature conditions.

4. Conclusion

In this investigation, zircon grains extracted from rock samples taken within the ASO were progressed through various analytical means including Cathodoluminescence and LA-ICP-MS analyses to verify their age and origin. The outcome of the analysis revealed that there are discrepancies in age, estimated to be between 1700 – 1755 Ma, which i s much o lder than the ASO occurrences. The internal structure of the zircon grains as revealed in the Concordia curve plot highlights the complex anomalous contrary to the usual transparent characteristics observed in the homogeneous zone of rocks. The study further alludes to the occurrence of an event with a significant impact that may have caused the episodic Lead (Pb) loss that constrains determining the true age of the rock or the time of crystallization of the minerals. This study suggests further investigations across the Arunta region to better understand the event(s) that might have impacted the basements on a regional scale to improve dating and determine where the rocks derive their source from.

Acknowledgement

The authors would like to thank the CSIRO Veteran Dr Steve Craven, the Lab analytical instrumentation expert, Dr Timothy Murphy and the Metamorphic Petrologist, Assoc. Prof. Dr Nathan Daczko, all from Macquarie University, North Ryde, and members of the Australian Research Council Centre of Excellence for Core to Crust Fluid Systems (ARCC) and the Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC).

Fig 1.

Figure 1.The study area. (a) Map of Australia highlighting the location of the site in the central Arunta, south of the Northern Region (b) the Strangways Metamorphic Complex (SMC) where the samples were collected, and (c) geological map of the Arunta block showing the metamorphic and igneous rock types.
Economic and Environmental Geology 2023; 56: 817-830https://doi.org/10.9719/EEG.2023.56.6.817

Fig 2.

Figure 2.Rock samples collected at Location A - (a) Igneous rocks with mainly quartz and feldspar from Cattle Water Pass (CP 1610) close to the Cattle Water Shear Zone (CPSZ), (b) Garnet granite invaded igneous dyke form London Eye (LE 1601); and at Location B – (c) Strained Felsic igneous rock from Gough Dam (GD 1622) and (d) Coarse crystalline igneous rocks from (GD 1610) within the, Shear Zone (GDSZ).
Economic and Environmental Geology 2023; 56: 817-830https://doi.org/10.9719/EEG.2023.56.6.817

Fig 3.

Figure 3.Flowchart for Zircon Geochronology dating.
Economic and Environmental Geology 2023; 56: 817-830https://doi.org/10.9719/EEG.2023.56.6.817

Fig 4.

Figure 4.core-rim structure of the imaged zircon - the inner and outer circles are the core and rim, respectively.
Economic and Environmental Geology 2023; 56: 817-830https://doi.org/10.9719/EEG.2023.56.6.817

Fig 5.

Figure 5.Concordia Plot of zircon grain age intercept of ratios (207Pb / 206Pb) for the sample at (a) CP1610, (b) GD1601, and (c) GD1622 with mean squared weighted deviation (MSWD) of 0.61, 2.1, and 1.6, respectively and (d) LE1601. Except for LE1601 which shows the possible occurrence of an episodic Lead (Pb) loss event of significance that might have affected the zircon grains, all the plots fall below the concordant age curve indicating a discordant age pattern.
Economic and Environmental Geology 2023; 56: 817-830https://doi.org/10.9719/EEG.2023.56.6.817

Fig 6.

Figure 6.Plot of the Mean age of the zircon grains at 95% confidence limit for sample at locations: (a) CP1610 - estimated at 1742.2 ± 9.2 Ma - MSWD = 0.49; probability of equivalence = 0.90; (b) Non-representative mean age of zircon grains: non-representative mean age of sample # GD1601 estimated at 1792 Ma, with 4 and 5 age groups below (b1) and above (b2) the mean, with sub-Mean age =1738 ± 14 Ma, MSWD = 0.109, probability of equivalence = 0.95 and sub- Mean and Mean age = 1838 ± 14 Ma; MSWD = 1.6; probability of equivalence = 0.95, respectively; (c) GD1622 - estimated at 1748 ± 15 Ma - MSWD = 1.02; probability of equivalence = 0.40; and (d) LE1601 with estimated mean age = 1784 ± 9.1 Ma - MSWD = 1.09; probability of equivalence = 0.37.
Economic and Environmental Geology 2023; 56: 817-830https://doi.org/10.9719/EEG.2023.56.6.817

Fig 7.

Figure 7.Schematic sketches of zircon U-Pb age distribution at scale 20 μm indicating the zircon core–rim proportion types and internal rim textures of grain sample from (a) CP 1610, (b) LE1601, (c) GD 1601 and (d) GD 1622. Note that P denote Point; R - Rim, and C – Core.
Economic and Environmental Geology 2023; 56: 817-830https://doi.org/10.9719/EEG.2023.56.6.817

Summary of the estimated age of the grain samples at the four locations.


Rock SampleAGEMSWDProbability of equivalence
# CP1610Mean1742.2 ± 9.2 Ma0.490.90
# GD1601Above mean1838 ± 14 Ma1.60.95
Mean1792 ± 32 Ma--
Below mean1738 ± 14 Ma0.1090.95
# LE1601Mean1784.4 ± 9.1 Ma1.090.37
# GD1622Mean1738.2 ± 15 Ma0.1090.95

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