This study presents detrital muscovite 40Ar/39Ar data from the Mesoproterozoic Roper Group and overlying informally named successions, in the Beetaloo Sub-basin, northern Australia. Detrital muscovite chronology reveals tectono-thermal processes within source regions and provides new constraints on the basin provenance, revising previous interpretations based on detrital zircon data. Detrital thermo- and geochronology, together demonstrate three main periods when the basin paleogeography was altered that correspond to the evolving tectonic history of the North Australia Craton (NAC) through the Mesoproterozoic. The first is characterised by an increased sediment contribution from source regions that lay along the eastern margin of Proterozoic Australia. These source regions are interpreted to have formed the uplifted rift-shoulders between Proterozoic Australia and Laurentia at ca 1.45 Ga. After that, sediments derived from eastern Proterozoic Australia sources become less voluminous up-section. The youngest analysed formation from the Roper Group, the Kyalla Formation, was predominately from sources to the south of the basin, representing another modification of basin geography. This is interpreted to result from the closure/subduction of the Mirning Ocean as the West Australian Craton (WAC) approached and collided with the NAC, resulting in an uplift of the southern margin of the NAC, at ca 1.35–1.31 Ga. The uppermost Mesoproterozoic to lower Neoproterozoic sandstone successions that overlie the Roper Group were derived from the Musgrave Province. Coupled detrital zircon and muscovite data imply a rapid cooling at ca 1.20–1.15 Ga that is interpreted to reflect syn-orogenic exhumation during the Musgrave Orogeny. Furthermore, data from the Beetaloo Sub-basin suggest that the changed basin tectonic settings reshaped basin geography and result in distinctive detrital zircon and muscovite geochronology records. In this study, we used the detrital U–Pb zircon and muscovite 40Ar/39Ar age data from the Beetaloo Sub-basin and a range of other basins deposited in different tectonic environments, including the convergent, collisional and extensional settings, to reconstruct the basin tectonic geography and illustrate various tectonic controls on basin formation in different tectonic backgrounds.KEY POINTSDetrital muscovite 40Ar/39Ar data provide thermochronological constraints on basin provenance, complementing previous interpretations based on detrital zircon data.Spatial and temporal variation of provenance reconstructs the basin tectonic geography, reflecting the Mesoproterozoic tectonic history of the North Australia Craton.Coupled thermo- and geochronology constrain tectonic settings of basin deposition. Detrital muscovite 40Ar/39Ar data provide thermochronological constraints on basin provenance, complementing previous interpretations based on detrital zircon data. Spatial and temporal variation of provenance reconstructs the basin tectonic geography, reflecting the Mesoproterozoic tectonic history of the North Australia Craton. Coupled thermo- and geochronology constrain tectonic settings of basin deposition.
AbstractThe 40 Ar/ 39 Ar dating technique is based on the knowledge of the age of neutron fluence monitors (standards). RecentinvestigationshaveimprovedtheaccuracyandprecisionoftheagesofmostofthePhanerozoic-agedstandards(e.g.FishCanyonTuffsanidine(FCs),AlderCreeksanidine,GA1550biotiteandLP-6biotite);however,nospecificstudyhasbeenundertakenontheolderstandards (i.e. Hb3gr hornblende and NL-25 hornblende) generally used to date Precambrian, high Ca/K, and/or meteoritic rocks.In this study, we show that Hb3gr hornblende is relatively homogenous in age, composition (Ca/K) and atmosphericcontamination at the single grain level. The mean standard deviation of the 40 Ar⁎/ 39 Ar K (F-value) derived from this study is 0.49%,comparable to the most homogeneous standards. The intercalibration factor (which allows direct comparison between standards)between Hb3gr and FCs is R FCsHb3gr =51.945±0.167. Using an age of 28.02 Ma for FCs, the age of Hb3gr derived from the R-value is1073.6±5.3Ma (1σ; internal error only) and ±8.8Ma (including all sources of error). This age is indistinguishable withinuncertainty from the K/Ar age previously reported at 1072±11Ma [Turner G., Huneke, J.C., Podosek, F.A., Wasserburg, G.J.,1971.
Wider use of radiogenic isotopes in geosciences has been enabled by developments in massspectrometry at the beginning of the 21st century. Nowadays, radiogenic isotope geochemistry forms an integrated part of geosciences in a range of applications, starting from formation of planetary systems, genesis, and the evolution of Earth's lithosphere and associated mineral and oil deposits, as well as environmental tracers. Two primary types of information are available from radiogenic isotopes studies: age determination and isotopic source tracing. In this chapter, the range of isotope systematics currently used in geosciences and their applications are reviewed, together with progress in analytical technologies. The chapter brings together internationally recognised researchers whohave been at the forefront of analytical technologies in the field of geochemistry of radiogenic isotopes.
The Marapicu Alkaline Massif is an intrusion into the Marapicu-Gericinó-Mendanha Igneous Complex that is part of the Cretaceous Poços de Caldas-Cabo Frio magmatic lineament located in the Southeastern region of Brazil. Nepheline syenites and phonolites are the most abundant rocks in the massif that also include syenites forming an alkaline series SiO2- undersatured. Chemically this series is predominantly metaluminous and to a lesser extent peralkaline. This series presents both potassic and sodic suites being the first one in greater content. The data show that both basic and intermediary rocks with parental composition sampled in this area have no genetic relationship with the other rocks of the body. Geochemistry data shows that evolution processes involved fractional crystallization with or without continental crust assimilation and also indicates that this alkaline magma was generating from an enriched mantle source. The 40Ar/39Ar age of hornblende (extracted of nepheline syenite) from Marapicu massif is 80.46 ± 0.58 Ma, which it is contrasting, with the idea of age decrease of the hotspot track from west to east on the Poços de Caldas-Cabo Frio magmatic lineament.
This chapter contains sections titled: Introduction Geological Setting and Sampling Analytical Methods Petrography and Mineralogy AR/AR Geochronology Whole Rock Geochemistry Petrogenesis of Kerforne Basalts The Mantle Source of Kerforne Basalts Conclusions
The 40Ar/39Ar technique is the most commonly used technique to date basaltic rocks. For basaltic rocks older than about 30 Ma, the dating of plagioclase separates is preferred over groundmass as the latter is susceptible to containing cryptic alteration due to fluid circulations, difficult if not impossible to remove during sample preparation. Alteration under such metamorphic conditions progressively forms K-rich sericite after plagioclase. Owing to its transparency, plagioclase allows a distinction to be made optically between partially–completely altered grains and fresh grains. However, practice shows that grains that contain less than about 1% of sericite are hard to identify under the stereomicroscope. Owing to the high K2O content (c. 10 wt%) of sericite, such compromised grains can have dramatic effects on the age determination of plagioclase. Here, we investigate and quantify the effect of sericite on the 40Ar/39Ar age determination of plagioclase using a numerical model with multiple variable parameters. We show that the most influential parameter is the time difference between the crystallization of plagioclase and the sericitization event. We also show that for some continental flood basalts, even 0.1 wt% of sericite can bias the apparent age of a plagioclase separate by several hundred thousand years. The presence of sericite can be identified using a combination of Ca/K ratios, age spectra, and 39Ar and 37Ar degassing curves obtained during a conventional 40Ar/39Ar step-heating procedure. When the age of the fresh plagioclase and its Ca/K ratio are known, the percentage of sericitization and the age of the alteration event can be estimated. Ultimately, above approximately 65% of sericitization, the apparent age measured on the altered plagioclase is within ±1% of the age of the alteration event, with implications for accurately dating low-temperature metamorphism and mineral deposit formations.
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