Thermal History of Central Australia: Cooper Basin, South Australia & Anmatjira Range, Northern Territory: Insights from Apatite Fission Track and U-Pb Thermochronology

Abstract

A large number of geological studies around the globe have used radiogenic isotopes in accessory minerals to thoroughly investigate upper and lower crustal processes. Minerals such as zircon, monazite, rutile and apatite are some examples of high and low temperature geochronometers which have been applied to various geological problems. Heavy mineral geochronometers have been commonly used to date high temperature igneous and metamorphic processes. Further developments extended their use to dating the depositional ages of sedimentary successions and being able to make conclusions about sediment provenance. Advances in the more recent fission track method have extended the use of these minerals to low-temperature processes such has heating due to burial and cooling due to uplift allowing scientists to date orogenic systems, brittle fault reactivation and basin development. Advances in LA-ICP-MS technology and double dating apatite for fission track and U-Pb chronometers increases the minerals efficiency and applicability to investigatng multiple temperature ranges on a single sample. This study utilizes apatite as the primary tool to investigate and assessa variety of geological settings. Detrital apatite was sampled from the oil and gas rich Cooper-Eromanga Basin to assess and test Cretaceous heating, charge timing and subsequent late Cretaceous cooling profiles from a number of well bores in the region. In this case time-temperature paths were derived by collecting samples over a vertical profile from a range of present-day downhole temperatures, modelled and compared to the results from previous fission track and vitrinite reflectance studies. Apatite U-Pb from was used to make conclusions about the provenance of the stratigraphic unit from which it was sampled and assessed in conjuction with published zircon U-Pb. A secondary assessment was conducted on the cratonic Anmatjira Range, Central Australia. The stability of cratonic regions around the globe has begun to be brought into question due t1o the development of low temperature thermochronometres. In this case apatite fission track was used to assess the assumed Mesozoic stability of the Palaeoproterozoic differentially metamorphosed granitoids of the Anmatjira Range. The actual cause of Mesozoic cooling in Central Australia remains inconclusive but evidence suggests long-wavelength tectonism from either mantle dynamics or synchronous far field orogenic events. Apatite U-Pb, trace and rare earth element analysis was applied to the same samples to assess the effect that prograding greenschist facies to amphibolite facies metamorphism has on the U-Pb chronometer and diffusion characteristics of Mn, Sr and REEs. The result exhibits significant variations in closure temperature between the measured isotopes, indicating a decoupling between U-Pb chronometers, trace and rare earth elements which could have strong implications for sedimentary provenance. Application of multiple apatite chronological methods gives an indication of the minerals flexibility and can be utilized across a range of geological environments. Through the assessment of apatite behavior from recently deformed basin environments to cratonic metamorphosed granitoids, this study investigates some of the uses apatite has and its possible applications to future chronological studies.