School of Earth Sciences - Theses
Permanent URI for this collection
Search Results
1 - 3 of 3
-
ItemAge, origin and extent of the Newer Volcanic Province in the Melbourne region, Australia( 2019)Abstract Intraplate basaltic volcanism is present on every continent, untethered to any specific tectonic setting. The ca. 4.6 Ma – 5 ka Newer Volcanic Province (NVP) in south-eastern Australia is a chemically and morphologically diverse intraplate basaltic province. Its diversity, preservation and accessibility make it an ideal natural laboratory for investigating the causes and evolution of intraplate magmatism worldwide. Previous studies in the NVP have put forward competing magmatic models, but their validity is dependent on a limited geochronological dataset. Furthermore, previous geochronology has seldom been supported by geochemical or geomorphological studies on the same volcanic products, such that there is often a disconnect between the absolute ages of NVP rocks and the extent and composition of their associated lava flows or eruption points. This study utilised a holistic approach to lava flow mapping, with a focus on the diagnostic petrographic and geochemical features of individual basaltic lava flows selected for geochronology. The current study area of Melbourne, which is located at the eastern margin of the NVP, was selected for its age range and geochemical complexity, rivalling those of the entire NVP. Drill core was utilised to trace lava flows at depths of up to 80 m, thereby unravelling complex flow networks and facilitating the construction of a detailed lava flow map for the Melbourne area. Trace element geochemistry was utilised to distinguish between petrographically similar flows, and to investigate the eruption of three chemically distinct magma batches erupted consecutively from Mount Fraser. This mapping and geochemical work is complemented by new, high-precision 40Ar/39Ar age constraints, spanning ~7.9 Ma – 0.8 Ma. The new geochronological constraints on Melbourne lava flows reveal that the earliest activity (~7.9 – 3.8 Ma) was dominated by small-volume eruptions that predominantly produced alkali basalts. From oldest to youngest, these included the Bald Hill, Mount Ridley, Tullamarine, Crowe Hill, Spring Hill, Summerhill Rd, Redstone Hill and Aitken Hill lava flows. After ~3.8 Ma, large-volume eruptions dominated, with Fenton Hill, Mount Kororoit, Tulloch Hill and Mount Fraser producing lavas generally of tholeiitic composition. This progression from alkali basalt to tholeiitic volcanism over time is contrary to the purported province-wide progression from tholeiitic to alkali-rich lavas, indicating either that geochemical evolution in Melbourne was distinct from that of the NVP, or that the conclusions of province-wide studies are based on an unrepresentative sample set. An age of 7.931 +/- 0.038 Ma for the Bald Hill Lava Flow far exceeds the generally accepted maximum age of NVP activity (ca. 4.6 Ma). This lava flow, along with those erupted from Pretty Sally, Green Hill and Mount Cooper, is also geochemically distinct from other Victorian NVP products, but is not dissimilar to lavas of the Cosgrove Leucitite suite, the purported products of a long-lived mantle plume. This raises the possibility that the ‘Cosgrove Plume’ traversed the latitude of Melbourne just after 7.9 Ma, casting doubt on its possible role in the initiation of NVP activity some 3.3 million years later. 11 of the 38 samples selected for 40Ar/39Ar dating produced concordant results, with the remainder exhibiting varying degrees of discordance. The underlying causes of this discordance and implications for accurate age determinations are examined and modelled in age spectra and inverse isochron space. A correlation is found between the proportion of radiogenic 40Ar (40Ar*) released and the nature of discordance exhibited by a sample. 39Ar recoil is suggested as the most likely cause of discordance in high-40Ar* samples, whereas low-40Ar* samples exhibit discordance consistent with the modelled effects of mass-dependent fractionation. Based on modelling results, isochron rotation is the main impact of isotopic disturbance on an inverse isochron plot (39Ar/40Ar vs 36Ar/40Ar), leading to a negative correlation between 40Ar/39Ar ages and (40Ar/36Ar)i values. A new framework for the treatment of 40Ar/39Ar data from basaltic rocks is submitted, optimising the interpretation of inverse isochrons and informing the allocation of age constraints. Finally, a new method of 40Ar/39Ar data treatment, here named the multi-isochron approach to 40Ar/39Ar dating, is introduced. A multi-isochron regression utilises the combined output of all possible inverse isochrons from data of a single aliquant to determine its ideal isochron and eruption age. This method has the potential of transforming 40Ar/39Ar data treatment, allowing eruption ages to be calculated even when isotopic disturbance is severe, and to be reported in cases where this was previously not possible.
-
ItemThermochronology of Tasmania and the South Tasman Rise: implications for the dynamic evolution of a complex rifted continental block( 2013)The crustal architecture of previously adjacent basement terranes in SE Australia, Tasmania and northern Victoria Land, Antarctica is a legacy of late Neoproterozoic-Paleozoic subduction along the east Gondwana margin, highlighting the Cambro-Ordovician Delamerian-Ross orogeny. Structures in this ancient crust were reactivated during late Mesozoic-Cenozoic Gondwana breakup. Tasmania and the offshore South Tasman Rise (STR) lay in a crucial location at the centre of these continental fragments and potentially contain clues regarding the nature of Gondwana dispersal. This study reports results of a systematic thermochronological study that has been carried out on STR dredge samples and Tasmanian dolerites to uncover the history of the ancient subduction system and the post-break up thermal history recorded in their crust. Oceanic investigations (ODP, IODP, magnetic and seismic surveys and swath mapping, etc) have been conducted across the STR and the western transform boundary over the past few decades. Important results have been previously reported and form the foundation towards understanding the tectonic significance of this region. In particular, the interpreted seismic profiles along with basement dredge materials provide essential structural, stratigraphic and petrological controls as in situ investigations are not possible. Nevertheless, only limited geochronological data are available and the regional thermal history is still sparse. In this study a systematic low-temperature thermochronological study has been carried out, applying 40Ar/39Ar, AFT, ZHe and AHe dating techniques to the STR dredge samples to disclose the tectonic evolution of study area. Results suggest that the western and eastern terranes of the STR (W-STR and E-STR, respectively) evolved differently prior to east-Gondwana breakup. While the 40Ar/39Ar data from the W-STR suggest late Cambrian-early Ordovician ages (~495-460 Ma), results from the E-STR suggests a much younger early Carboniferous (Mississippian) age range (325-357 Ma). The data allow temporal comparisons to be made between existing 40Ar/39Ar and K-Ar datasets obtained from S-SE Australia and NVL, and results obtained here from the thermochronologically less studied offshore STR region. The data support reconstructions, which indicate that W-STR shared provenance with Wilson Terrane prior to the continental breakup, and furthermore, refine its paleo-position. Based on the age pattern as well as petrological evidence, it is proposed that it was most likely situated west of the Lanterman Fault Zone (LFZ) and extended further west to the western flank of the pop-up structure bounded by Wilson and Exiles Thrust. E-STR,40Ar/39Ar ages are synchronous with a phase of major granite emplacement and mineralization, which occurred in western Tasmania and are correlated with deformation post Tabberabberan Orogeny in Tasmania and the Victoria. Results from this study provide more solid time-temperature constraints for late Neoproterozoic-Cambrian subduction-related processes and the more recent evolution of the transform Tasman Fracture Zone during separation between Antarctica and Australia. The thermal history models suggest a strong correlation between rapid cooling and tectonic activity in the STR block. Since mid-Cretaceous time these can be summarized as follows: (1) onset of the opening of the Tasman Sea at ~80 Ma, (2) amalgamation of the W- and E-STR blocks and a shift in the relative motion between Australia and Antarctica and (3) final clearance of continental breakup and onset of the opening of the Tasman Gateway. This study is also reports results from the first application of apatite (U-Th-Sm)/He (AHe) thermochronometry to the Tasmanian region. The data not only provide further spatial and temperature constraints, but also examine the quality of AHe ages obtained from mafic lithologies with lower U and Th content than felsic rocks and yielding less age dispersion. Mid Jurassic (~175-180 Ma) dolerite is widely distributed across onshore Tasmania making such a study possible. The dolerite forms part of the Ferrar Group continental flood basalt (CFB) emplaced prior to eastern Gondwana breakup. Its spatial/temporal significance and chemical composition make it an ideal rock-type to aid in documenting the regional post-continental breakup history and to test the influence of different parameters such as -radiation damage and U and Th zonation on AHe age dispersion from a low eU perspective. Low-temperature thermal modelling reveals two distinct cooling episodes. (1) Mid-Cretaceous cooling, which involved km-scale denudation (~3-4 km) in response to continental extension prior to the actual seafloor spreading in the Tasman Sea. (2) Late Cretaceous-early Tertiary cooling, restricted to the west margin of Tasmania in response to transform margin tectonism to the west. In addition, no correlation could be found between AHe ages and potential factors influencing age dispersion such as radiation damage, grain size, U-Th zonation. However, zircons derived from Tasmanian dolerites having high eU suggest an effective upper dosage limit (0.2-0.31018/g) for obtaining meaningful ZHe ages when studying Tasmanian dolerites or possibly other similar mafic lithologies.
-
ItemCalibration of the cosmogenic 21Ne exposure dating technique for application to Quaternary volcanic chronology( 2012)The use of terrestrial cosmogenic nuclides to investigate surface exposure histories continues to gain popularity due to the applicability of the technique to many different minerals and rock types. However, errors associated with cosmogenic exposure ages are typically either poorly specified or relatively high compared to other more established dating methods. This is dominantly due to large systematic uncertainties. This study focused on the stable cosmogenic nuclides 21Ne and 3He, and investigated their production in olivine and clinopyroxene from young (<150 ka) basalt flows. The primary aims of this study were to (i) improve the cosmogenic 21Ne dating technique for application to these minerals, (ii) develop in-house methods for dating young basalt flows by the 40Ar/39Ar step-heating technique, and (iii) inter-calibrate cosmogenic 21Ne surface exposure ages with corresponding 40Ar/39Ar eruption ages, in order to test available cosmogenic nuclide production rate scaling models. Alkali basalt flows (~30 – 130 ka) from three mid-latitude localities were studied: Mount Porndon (Victoria, Australia), Amboy Crater (Mojave Desert, California, USA), and the Nave flow, Mount Etna (Sicily, Italy). Helium and neon isotopic compositions of olivine (n = 13) and clinopyroxene (n = 7) separates from the above localities were determined from two-step heating experiments (900 ̊C and 1600 ̊C) using a VG5400 mass spectrometer. All samples showed evidence of variable degrees of helium loss and isotope mass fractionation, which compromised the calculation of cosmogenic 3He (3Hec) ages. Although minor neon loss was also apparent in most samples, it did not affect calculation of cosmogenic 21Ne (21Nec) ages. Overall, olivine separates yielded more consistent cosmogenic 21Ne ages than corresponding clinopyoxene separates extracted from the same samples. Crushing experiments on selected samples demonstrated an atmospheric composition for the 21Ne/20Ne ratio of the trapped neon component. Apparent weighted mean 21Nec exposure ages were calculated at: 64 ± 10 (2σ, Mount Porndon), 105 ± 23 ka (2σ, Amboy Crater), 29.0 ± 7.6 ka (2σ, Nave flow), respectively. Prior to analysing the very young (<150 ka) basalts from the selected calibration sites by the 40Ar/39Ar technique, studies of slightly older samples (300-600 ka) collected from the Newer Volcanic Province of southeastern Australia (Mount Rouse basalt (n = 4) and Mount Warrnambool basalt (n = 2)) were carried out on a VG3600 mass spectrometer in order to establish appropriate sample preparation procedures and analytical protocols. Multi-aliquot 40Ar/39Ar furnace step-heating analyses of whole-rock samples from several (n = 4) Mount Rouse basalt flows yielded weighted mean ages of: 309 ± 20 ka (2σ, NVP19), 382 ± 24 ka (2σ, NVP06); 301 ± 27 ka (2σ, NVP20) and 280 ± 19 ka (2σ, NVP21). Samples from Mount Warrnambool (n = 2) yielded weighted mean ages of: 547 ± 23 ka (2σ, NVP03) and 535 ± 27 ka (95% CI. NVP04). The internal concordance of results and agreement with previous K-Ar ages enabled validation of the experimental protocols adopted. Basalt samples younger than 150 ka from Mount Porndon, Amboy Crater and Mount Eccles were analysed on a new generation multi-collector ARGUSVI mass spectrometer via laser step-heating. This system yielded significantly more precise isotopic measurements than was possible using the older VG3600 system. Two to four groundmass aliquots were analysed per sample (n = 4), giving weighted mean 40Ar/39Ar ages of: 105 ± 22 ka (2σ; Mount Porndon, NVP14), 121 ± 8 ka (2σ; Amboy Crater, 08010), 92 ± 8 (2σ; Mount Eccles, NVP18)), 84 ± 13 (2σ; Mount Eccles, NVP12) and 293 ± 7 ka (95% CI; Mount Rouse, NVP21). Although lava surfaces showing apparent primary flow textures were sampled for cosmogenic dating, comparison of the cosmogenic 21Ne and 40Ar/39Ar ages indicates that significant erosion has occurred at the outcrops sampled for exposure dating from the Mount Porndon lava flows. Erosion modelling assuming an eruption age of 105 ± 22 ka (2σ, 40Ar/39Ar) suggests an erosion rate ranging from 2 - 7 mm/ka, depending on the value assumed for the scaled 21Nec production rate. As estimated 21Nec production rates for Mount Porndon are at the lower end of the range predicted by latitudinal production rate scaling models, it is possible that available scaling models may require further refinement for application to Southern Hemisphere localities. For the Amboy Crater flow, an erosion rate of 1 mm/ka is estimated, consistent with previous studies. In the case of the Nave flow, the agreement of the 21Nec exposure age for sample 08025 with the weighted mean K-Ar age (32 ± 8 ka (2σ)) of Blard et al. (2005) is interpreted as evidence for minimal erosion at this outcrop. However, the other two samples collected from adjacent outcrops on the Nave flow yielded slightly younger results, suggesting that erosion cannot be excluded for this flow. In order to refine estimates for 21Nec production rates in olivine and clinopyroxene, studies involving step-heating of large (several grams) mineral separates from young (ca. < 30 ka) basalt flow surfaces that have undergone negligible erosion and have independently constrained eruption ages are recommended. For the purposes of investigating muon-induced production of cosmogenic neon in olivine, 21Nec concentrations were studied in samples spanning an 11 m vertical profile through an Eocene basalt flow located in the Monaro Volcanic Province, NSW, Australia. A 40Ar/39Ar age of 55.52 ± 0.02 Ma (2σ) is calculated for this flow from step-heating (n = 4) experiments on whole-rock samples. It is demonstrated that excess 21Ne in the deepest samples can be accommodated solely by nucleogenic neon production, meaning that muogenic neon production in olivine is insignificant, even over million year time-scales. This justifies the assumption implicit in previous studies that 21Nec production in olivine (and likely clinoproxene) is purely the result of spallation reactions. Preliminary modelling based on 21Nec concentrations in the studied profile suggests an appreciably low average Cainozoic erosion rate of ca. 1 m/Ma.