scholarly journals Complex <sup>40</sup>Ar/<sup>39</sup>Ar age spectra from low metamorphic grade rocks: resolving the input of detrital and metamorphic components in a case study from the Delamerian Orogen

2021 ◽  
Author(s):  
Anthony Reid ◽  
Marnie Forster ◽  
Wolfgang Preiss ◽  
Alicia Caruso ◽  
Stacey Curtis ◽  
...  
Keyword(s):  
South Australia ◽  
Shear Zones ◽  
Metamorphic Grade ◽  
Degree Of Deformation ◽  
Two Samples ◽  
Step Heating ◽  
Detrital Mineral ◽  
High Metamorphic Grade ◽  
Apparent Age

Abstract. Low metamorphic grade rocks contain both detrital minerals and minerals newly grown or partly recrystallised during diagenesis and metamorphism. However, rocks such as these typically yield complex 40Ar/39Ar age spectra that can be difficult to interpret. In this study, we have analysed a suite of variably deformed rocks from a region of low metamorphic grade within the c. 514–490 Ma Delamerian Orogen, South Australia. The samples analysed range from siltstone and shale to phyllite and all contain either muscovite or phengite determined by hyperspectral mineralogical characterisation. Furnace step heating 40Ar/39Ar analysis produced complex apparent age spectra with multiple age components. Using the concept of asymptotes that define minimum and maximum ages for different components, we interpret the age spectra to preserve a range of detrital mineral ages, along with younger components related to either cooling or deformation- induced recrystallisation. Two samples contain Mesoproterozoic detrital age components, up to c. 1170 Ma, while the c. 515 Ma Heatherdale Shale which has both c. 566 Ma and c. 530 Ma detrital components. All samples contain younger lower (younger) asymptotes in the age spectra defined from multiple heating steps that range from c. 476 to c. 460 Ma. One interpretation of these younger ages is that they are caused by post-metamorphic cooling. However, the shape of the age spectra and the degree of deformation in the phyllites suggest the ages may record recrystallisation of detrital minerals and/or new mica growth during deformation. Potentially these c. 476 to c. 460 Ma ages suggest deformation in the upper portion of the orogen was facilitated by movement along regional faults and shear zones up to around 20 million years after the cessation of deformation in the high-metamorphic grade regions of the Delamerian Orogen.

40Ar/39Ar age of shear zones in the southwest Meguma Zone between Yarmouth and Meteghan, Nova Scotia

1997 ◽  
Vol 34 (6) ◽  
pp. 848-853 ◽  
Author(s):  
Nicholas Culshaw ◽  
Peter Reynolds
Keyword(s):  
Shear Zone ◽  
Shear Zones ◽  
Published Data ◽  
Combined Effects ◽  
Excess Argon ◽  
Two Samples ◽  
Facies Metamorphism ◽  
Apparent Age ◽  
Greenschist Facies Metamorphism ◽  
Isotope Correlation

The Cape St. Marys, Cranberry Point, and Chebogue Point shear zones lie within a zone of D2 deformation that overprinted D1 fabrics and reactivated F1 folds between the shear zones. D2 was accompanied by greenschist-facies metamorphism that variably overprinted earlier metamorphism of varying grade. A collection of single muscovite grains from pressure shadows defining L2 in a sample from the Cape St. Marys shear zone yields an average apparent age of 316 ± 5 Ma. The age obtained for these grains from an isotope correlation diagram is 320 ± 5 Ma. Ages for muscovite separates for two samples from the Cranberry Point shear zone are 325 ± 4 and 319 ± 4 Ma. An amphibolite from this zone yields a discordant spectrum interpreted to reflect the combined effects of Acadian metamorphism and excess argon. The muscovite ages approximate the time of syntectonic deformation because muscovite closure temperatures approximate the temperature of metamorphism during D2. Thus, together with published data from the Chebogue Point shear zone, our data indicate a Mid-Carboniferous age, ca. 320 Ma, for D2 within this system of three shear zones and reactivated folds.

scholarly journals Meta-analysis of Cryogenian through modern quartz microtextures reveals sediment transport histories

2021 ◽  
Vol 91 (9) ◽  
pp. 929-944
Author(s):  
Jocelyn N. Reahl ◽  
Marjorie D. Cantine ◽  
Julia Wilcots ◽  
Tyler J. Mackey ◽  
Kristin D. Bergmann
Keyword(s):  
Meta Analysis ◽  
South Australia ◽  
Principal Component ◽  
Field Observations ◽  
Aeolian Transport ◽  
Transport Mode ◽  
Two Samples ◽  
Glacial Environments ◽  
Scanning Electron

ABSTRACT Quantitative analysis of quartz microtextures by means of scanning electron microscopy (SEM) can reveal the transport histories of modern and ancient sediments. However, because workers identify and count microtextures differently, it is difficult to directly compare quantitative microtextural data analyzed by different workers. As a result, the defining microtextures of certain transport modes and their probabilities of occurrence are not well constrained. We used principal-component analysis (PCA) to directly compare modern and ancient aeolian, fluvial, and glacial samples from the literature with nine new samples from active aeolian and glacial environments. Our results demonstrate that PCA can group microtextural samples by transport mode and differentiate between aeolian transport and fluvial and glacial transport across studies. The PCA ordination indicates that aeolian samples are distinct from fluvial and glacial samples, which are in turn difficult to disambiguate from each other. Ancient and modern sediments are also shown to have quantitatively similar microtextural relationships. Therefore, PCA may be a useful tool to constrain the ambiguous transport histories of some ancient sediment grains. As a case study, we analyzed two samples with ambiguous transport histories from the Cryogenian Bråvika Member (Svalbard). Integrating PCA with field observations, we find evidence that the Bråvika Member facies investigated here includes aeolian deposition and may be analogous to syn-glacial Marinoan aeolian units including the Bakoye Formation in Mali and the Whyalla Sandstone in South Australia.

scholarly journals Investigation into the Cause of Iron-Related Clogging of Groundwater Bores Used for Viticulture in the Limestone Coast, South Australia

Water ◽  
2021 ◽  
Vol 13 (5) ◽  
pp. 683
Author(s):  
Birte Moser ◽  
Meruyert Beknazarova ◽  
Harriet Whiley ◽  
Thilini Piushani Keerthirathne ◽  
Nikki Harrington ◽  
...  
Keyword(s):  
Groundwater Resources ◽  
South Australia ◽  
Water Levels ◽  
Clear Correlation ◽  
Problem Analysis ◽  
Persistent Problem ◽  
Root Cause ◽  
Complex Interactions ◽  
Iron Manganese

Iron-related clogging of boreholes, pumps and dripper lines is a significant and costly problem for irrigators worldwide. The primary cause of iron-related clogging is still debated. Previous studies have described complex interactions between biological clogging and inorganic iron/manganese oxide precipitation. This case study examined groundwater bores used for viticulture irrigation in the Limestone Coast region, a highly productive wine growing area in the SE of South Australia. Iron clogging of bore screens, pumps and dripper systems has been a persistent problem in the region and the issue is perceived to be growing, with irrigators suggesting the widespread introduction of iron-related bacteria (IRB) through drilling equipment to be the root cause of the problem. Analysis of the groundwater microbiology and inorganic chemistry found no apparent correlation between the presence of IRB and the clogging status of wells. In fact, IRB proved to be widespread throughout the limestone aquifer. However, a clear correlation could be found between clogging affected bores and the redox potential of the groundwater with the most severely affected bores strongly oversaturated in respect to iron oxide minerals. Elevated dissolved concentrations of Fe(II) thereby tended to be found in deeper bores, which also were generally more recently drilled. Following decades of less than average rainfall, a tendency to deepen bores in response to widespread declines in water levels has been documented for the SE of South Australia. The gradually widening clogging problem in the region is postulated to be related to the changes in climate in the region, with irrigators increasingly driven to rely on deeper, anoxic iron-rich groundwater resources.

Flexible Theory and Flexible Regulation: Collaboration and Competition in the McLaren Vale Wine Industry in South Australia

1995 ◽  
Vol 27 (1) ◽  
pp. 41-61 ◽  
Author(s):  
G Haughton ◽  
J Browett
Keyword(s):  
South Australia ◽  
Wine Industry ◽  
Social Regulation ◽  
Regulatory Mechanisms ◽  
Industrial Districts ◽  
Key Features

A case study of the McLaren Vale wine industry is used to challenge four areas of the regulation debate. First, the uniqueness of some of the key features underpinning the periodisations of accumulation regimes and their associated modes of social regulation is questioned. Second, concern is raised over the extent to which ‘new industrial districts’ can really be described as engaging in ‘new’ practices. Third, the importance of local regulatory mechanisms is emphasised. Last, the importance of nonstate, nonlegislative forms of regulation are highlighted.

scholarly journals On some pre-Cambrian metadolerites from the central Ivigtut region SW Greenland

1965 ◽  
Vol 52 ◽  
pp. 1-42
Author(s):  
E Bondesen ◽  
N Henriksen
Keyword(s):  
Shear Zones ◽  
Metamorphic Grade ◽  
Amphibolite Facies ◽  
Dynamic Conditions ◽  
Static Conditions

After their consolidation, the Ketilidian gneisses were transversed by several generations of tensional, doleritic dykes-the Kuánitic dykes. During a later episode (the Sánerutian) these dykes were metamorphosed to varying degrees of alteration which increase in the described area from west to east. Along a specific metadolerite, which can be traced approx. 40 km, the metamorphic grade changes from greenschist to amphibolite facies. In the western parts static conditions and in the eastern parts dynamic conditions, prevailed during the alteration. Sánerutian shear zones in the eastern parts depict the dynamic conditions found here.

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