the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Sep 2023
New age constraints supporting the existence of protracted deformation in the Delamerian Orogen: 40Ar/39Ar geochronology on fabric forming minerals of the Kanmantoo Group metasediments
Abstract. The final assembly of Gondwanaland following Rodinia breakup during the Neoproterozoic-Cambrian period was marked by tectonic mode switch leading to the transformation of a passive margin in the east to an active subduction margin. This period was marked by subduction of the Pacific plate leading to the initiation of the Delamerian Orogeny in South Australia at c. 515 Ma, culminating at c. 490 Ma. Most studies have attempted to constrain the timing of deformation through dating of magmatic intrusions as proxies for deformation, however, relatively few studies strived to date minerals in fabrics that developed during deformation. As opposed to existing age constraints on the timing of cessation of the Delamerian Orogeny at c. 490 Ma, the work presented here identifies new protracted periods of active regional tectonics in the eastern Mt. Lofty Ranges. This was first observed in the underlying Backstairs Passage Formation overlain by the Tapanappa Formation of the Cambrian Kanmantoo Group through ultra-high vacuum 40Ar/39Ar geochronology and 39Ar diffusion experiments conducted on mica separates. It is observed that younger ages up to c. 468 Ma are recorded in the stratigraphically older Backstairs Passage Formation, while the overlying Tapanappa Formation bears no 40Ar/39Ar apparent ages younger than c. 486 Ma. In addition, age data reveals a period of fluid activity at c. 497–493 Ma resulting in micaceous intrusions locally and regionally. The methodology adopted here involves compilation of the age information with microstructural analyses where the age interval of c. 493–487 Ma is interpreted as post peak-metamorphism garnet growth event. This period is likely associated with retrograde metamorphism and is co-eval with Cu-mineralization in the region at c. 492–485 Ma. A second generation of retrogressed biotite growth at the expense of garnet has been identified. Importantly, there are differences in metamorphic grade and timing of deformation between the stratigraphically older the Backstairs Passage Formation and the younger Tapanappa Formation. This can be explained through macroscopic and/or microscopic processes. Two interpretations are proposed here- first involving the lower activation energy of the Backstairs Passage Formation as a measure of argon retentivity, and therefore younger ages, and the second considering proximity of the two formations to the metamorphic core of the belt at Reedy Creek and the effect of dissipated heat affecting record of argon ages in each unit. These newly identified young ages indicate the protracted nature of the Delamerian Orogeny than previously documented.
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Interactive discussion
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CC1: 'Comment on egusphere-2023-1839', Alan Collins, 11 Oct 2023
Thanks for submitting this lovely paper - I've gone through it and really enjoyed reading it. I've a few comments that are really about the bigger picture (as well as updating with some recent published work by Hong et al. and Reid et al. on the region). Numbers refer to line numbers in the manuscript.
55-56 – The Delamerian orogeny is not really part of the so-called Pan-African orogens (that term is also meaningless as it variously refers to 300-400 million years of orogens around the world). The Delamerian occurs after the youngest of the Gondwana-forming orogens (Collins and Pisarevsky, 2005; Merdith et al., 2021; Schmitt et al., 2018) and has been associated with the initiation of circum-Gondwana subduction (Foden et al., 2006; Foden et al., 2020) – the so-called Terra Australia Orogen (Cawood, 2005).
63 – the best dating of this deposition is reported in Betts et al (2018).
67-74 Hong et al. (2023)needs to be incorporated into this work. I understand that this probably wasn’t published when your paper was submitted, but it is a thorough geochronological analysis of the timing of mineralisation in the Delamerian that needs to be incorporated into your manuscript.
90 – again, I think Betts et al. (2018) is an important reference here for the timing of the Kanmantoo
117 – quote Lloyd et al. (2020) when you first mention the Adelaide Superbasin – this is the reference where this was defined (and defined to include the Kanmantoo trough).
205 – ‘Microstructural’ not ‘microstructure’
Fig 3 and Fig 4 – The Tectonic Sequence Diagram notation needs to be referred to as meeting these acronyms here before they are described in the text, leads to confusion. Perhaps say something in the caption like ‘TSD = Tectonic Sequence Diagrams; for explanation see text.’ Also, the mineral acronyms are not spelt out – many can be guessed, but they need to be defined.
Fig 6g – this doesn’t look like a ‘late stage static garnet’ as it has biotite inclusion trails that form obliquely to the surrounding matrix fabric – suggesting that the garnet has rotated since it formed…
384 – ‘important’ instead of ‘imperative’
386, 387 ‘the Backstairs Passage Formation’, ‘the Tapanappa Formation’ – this needs checking and correcting through the manuscript.
637 – ‘…tectonics were…’
Thanks for writing up a lovely study! I think with your incorporation of Hong et al. and Ried et al.’s (Reid et al., 2022) recent work, this will make a really key study of the mineralisation timing in the Delamerian Orogen.
Sincerely
Alan Collins
Betts, M. J., Paterson, J. R., Jacquet, S. M., Andrew, A. S., Hall, P. A., Jago, J. B., Jagodzinski, E. A., Preiss, W. V., Crowley, J. L., Brougham, T., Mathewson, C. P., Garcia-Bellido, D. C., Topper, T. P., Skovsted, C. B., and Brock, G. A., 2018, Early Cambrian chronostratigraphy and geochronology of South Australia: Earth-Science Reviews, v. 185, p. 498-543.
Cawood, P. A., 2005, Terra Australis Orogen: Rodinia breakup and development of the Pacific and Iapetus margins of Gondwana during the Neoproterozoic and Paleozoic: Earth Science Reviews., v. 69, p. 249-279.
Collins, A., and Pisarevsky, S., 2005, Amalgamating eastern Gondwana: The evolution of the Circum-Indian Orogens: Earth-Science Reviews, v. 71, no. 3-4, p. 229-270.
Foden, J., Elburg, M. A., Dougherty-Page, J., and Burtt, A., 2006, The timing and duration of the Delemerian Orogeny: correlation with the Ross Orogen and implications for Gondwana assembly: Journal of Geology, v. 114, p. 189-210.
Foden, J. D., Elburg, M., Turner, S., Clark, C., Blades, M. L., Cox, G., Collins, A. S., Wolff, K., and George, C., 2020, Cambro-Ordovician magmatism in the Delamerian orogeny: Implications for tectonic development of the southern Gondwanan margin: Gondwana Research.
Hong, W., Fabris, A., Wise, T., Collins Alan, S., Gilbert, S., Selby, D., Curtis, S., and Reid, A. J., 2023, Metallogenic Setting and Temporal Evolution of Porphyry Cu-Mo Mineralization and Alteration in the Delamerian Orogen, South Australia: Insights From Zircon U-Pb, Molybdenite Re-Os, and In Situ White Mica Rb-Sr Geochronology: Economic Geology, v. 118, no. 6, p. 1291-1318.
Lloyd, J. C., Blades, M. L., Counts, J. W., Collins, A. S., Amos, K. J., Wade, B. P., Hall, J. W., Hore, S., Ball, A. L., Shahin, S., and Drabsch, M., 2020, Neoproterozoic geochronology and provenance of the Adelaide Superbasin: Precambrian Research, v. 350.
Merdith, A. S., Williams, S. M. E., Collins, A. S., Tetley, M. G., Mulder, J. A., Blades, M. L., Young, A., Armistead, S. E., Cannon, J., Zahirovic, S., and Muller, R. D., 2021, Extending full-plate tectonic models into deep time: Linking the Neoproterozoic and the Phanerozoic: Earth-Science Reviews, v. 214.
Reid, A., Forster, M., Preiss, W., Caruso, A., Curtis, S., Wise, T., Vasegh, D., Goswami, N., and Lister, G., 2022, Complex 40Ar ∕ 39Ar age spectra from low-grade metamorphic rocks: resolving the input of detrital and metamorphic components in a case study from the Delamerian Orogen: Geochronology, v. 4, no. 2, p. 471-500.
Schmitt, R. d. S., Fragoso, R. d. A., and Collins, A. S., 2018, Suturing Gondwana in the Cambrian: The Orogenic Events of the Final Amalgamation, in Siegesmund, S., Basei, M. A. S., Oyhantçabal, P., and Oriolo, S., eds., Geology of Southwest Gondwana: Cham, Springer International Publishing, p. 411-432.
Hong et al. 2023 – abstract
Paleozoic porphyry-style hydrothermal alteration and mineralization has previously been recognized within the Delamerian orogen, South Australia, where porphyry prospects include Anabama Hill, Netley Hill, and Bendigo. However, limited exploration due in part to thick postmineralization cover hinders the understanding of the temporal context, metallogenic setting, and mineral potential of the porphyry systems along the Proterozoic
continental margin of Australia. In this study, we have characterized the hydrothermal alteration and mineralization of these porphyry occurrences. Zircon U-Pb, molybdenite Re-Os, and white mica Rb-Sr ages have been determined to constrain the timing for emplacement of magmatic intrusions, precipitation of metal-bearing sulfides, and duration of hydrothermal alteration in the Delamerian orogenic belt. Zircon U-Pb laser ablationinductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of nine granitoids reveal that the intrusive rocks were emplaced mostly between 485 and 465 Ma, whereas three intrusions at Bendigo have zircon U-Pb ages of 490 to 480 Ma. Molybdenite isotope dilution-negative thermal ion mass spectrometry (ID-NTIMS) Re-Os dating of the four prospects identifies two porphyry Cu-Mo mineralization events at 480 and 470 to 460 Ma, respectively. Nineteen white mica Rb-Sr LA-ICP-MS/MS (tandem mass spectrometers) analyses return an age range between 455 and 435 Ma for phyllic alteration at the Anabama Hill and Netley Hill prospects, whereas intense white mica-quartz-pyrite alteration at Bendigo prospect appears to have developed between 470 and 460 Ma. These geochronologic results indicate that the Delamerian porphyry systems postdated subduction-related magmatism in the region (514–490 Ma) but instead formed within an inverted back-arc regime, where mineralized magmas and fluids ascended along favorable lithospheric-scale structures, probably due to asthenospheric upwelling triggered by mafic delamination. Porphyritic stocks, dikes, and aplites with ages of 470 to 460 Ma are the most likely hosts to porphyry-style mineralization in the Delamerian orogen that appears to have formed simultaneously with the oldest known porphyry systems in the intraoceanic Macquarie arc (e.g., Marsden, E43, and Milly Milly; 467–455 Ma). These results emphasize the significance and potential of Early-Middle Ordovician intrusive systems to host such a type of magmatic-hydrothermal mineralization in the Delamerian orogen.
Citation: https://doi.org/10.5194/egusphere-2023-1839-CC1 -
RC1: 'Comment on egusphere-2023-1839', Anonymous Referee #1, 28 Nov 2023
This manuscript by Goswami uses a combination of field observations, microstructural and chemical analyses, along with 40Ar/39Ar step heating experiments, to suggest that the Delamerian Orogen was more protracted than previously thought. I have two main concerns for this manuscript, the first of which makes it a little tricky to fully review. The paper does not seem to have been carefully proofread by the author – Figure 1 is a good example of this. Figure 1 is modified by Reid et al. 2022, but there are so few modifications that it is confusing – the key shows the location of a cross-section apparently in Figure 3, which appears to actually be Figure 3 from Reid et al. 2022. Given this, I have not gone through and listed all the specific errors (of which unfortunately there are quite a few) and will keep the remaining comments general.
The second issue relates to the 40Ar/39Ar work itself. Firstly, I am not sure as to how robust the results of muscovite and biotite mixtures are. The release spectra for these are difficult to interpret (Figure 8a for eg.), and I would urge the author to be more cautious when assigning ages based on release spectra such as these. Best practice for how to interpret release spectra were laid out recently in a community paper: Schaen et al. 2021 GSA Bull. Secondly, I am not convinced by the calculation of the closure temperatures for the mica mixtures using the Arrhenius plots. While Figure 9b does appear to show different domains, the results are potentially showing a mixture of the biotite and muscovite degassing, meaning that the calculated kinetics and closure temperatures are not meaningful. I also am not convinced by some of the regressions chosen (e.g. in Figure 9b). It is important to note too that muscovite and biotite are hydrous silicates and therefore will not remain stable throughout the step heating experiments (unless they are hydrothermal experiments – which unless I have missed it, I don’t believe these are). This means that the resulting diffusion kinetics are not the diffusion kinetics of the muscovite and biotite in question – and the closure temperature estimates are not geologically meaningful. The interpretation of these dates, especially in section 6.1 needs a lot more detail.
While I think a lot of further work is needed on this paper before it can be published, I do want to end this review on a positive note. I think that there is a lot of good research that has been conducted by the author. The microstructural analyses for example have been carefully done, and the evidence is clear and well documented in the paper. Not all of the 40Ar/39Ar results are difficult to interpret – inverse isochron plots of the pure splits of mica could be illuminating if the author has not already tried that. I like Figures 10 onwards, they lay out clearly the different tectonic events and the evidence behind them. With careful examination and interpretation of the Ar results, along with proof reading, this paper has potential to be a good addition to our knowledge of the Delamerian Orogen.
Citation: https://doi.org/10.5194/egusphere-2023-1839-RC1 -
RC2: 'Comment on egusphere-2023-1839', Christopher Barnes, 20 Dec 2023
Review of “New age constraints supporting the existence of protracted deformation in the Delamerian Orogen: 40Ar/39Ar geochronology on fabric forming minerals of the Kanmantoo Group metasediments.” by N. Goswami.
Summary
The manuscript focuses on two outcrops belonging to subgroups of the Kanmantoo Group of the Delamerian orogeny, combining structural analysis, petrography, and 40Ar/39Ar geochronology (using the step-heating method) to investigate the tectonic evolution of the two outcrops, related to orogenesis and regional mineralization. The study utilizes to good approach to provide a lithological/structural/mineralogical foundation for interpreting the 40Ar/39Ar dates. However, the resulting step-heating spectra are used to extract multitudes of dates from single samples and may be over-interpreted. I will not necessarily dismiss the interpretations that are made by the author, but a lot of processes/phenomena that can contribute to the patterns observed in the step-heating spectra are overlooked, and not enough characterization of the samples and the mica have been done to directly support the interpretations made. Therefore, the study has potential to be a valuable contribution for understanding the tectonics of the Delamerian orogeny, but significant work is required before it can be recommended for publication. Major issues are outlined below, and more minor comments are provided according to the manuscript sections/lines, some of which may reiterate themes of the major issues.
Major Issues
- Focus of the study
The focus of the study is not entirely clear. In particular, the Introduction and Discussion both shift their focus between a) understanding the deformation associated with mineralization of the Kanmantoo mine (which would only require studying the Tapanappa Formation), b) resolving the regional tectonic evolution of the Delamerian orogeny, and c) exploring the systematics of 40Ar in mica. All three topics are related and can be discussed throughout the manuscript, but what is the overarching theme? From the title, and that both outcrops from the Tapanappa Formation (hosting the Kanmantoo mine) and the Backstairs Passage Formation are involved in the study, I would assume that the tectonics of the Delamerian orogeny is the primary focus and the other two themes are secondary. This needs to be clearly defined for the reader and echoed in how the Discussion is structured.
- 40Ar/39Ar step-heating spectra
Firstly, the supplementary data is not adequate and more information is needed in the results of 40Ar/39Ar geochronology. I was unable to independently investigate the dating results because I do not have access to the eArgon software (nor could find it online), and the isotope ratios and uncertainties that are needed to plot the data in IsoplotR are not presented. Therefore, I had to evaluate all of the data visually from the figures showing the step-heating spectra and could calculate dates from the spectra myself. A reference needs to be provided to locate the eArgon software online for the reader, and more information needs to be provided in the supplementary data file, i.e., the different Ar isotope ratios and associated uncertainties should be provided so the data can be input into IsoplotR.
The results themselves may be over-interpreted. Several of the spectra do not provide well-defined plateau dates, yet specific segments of the spectra are extracted to calculate dates. The rationale for the approach is not clearly explained. For example, sample -270 is used to extract dates of 492 ± 1.7 Ma and 495 ± 1.1 Ma from degassing steps that individually yield apparent dates that appear to be statistically overlapping. In contrast, several dates from sample -272 are extracted (496 ± 1.7 Ma, 488 ± 1.6 Ma, 506 ± 2.3 Ma, and 498 ± 1.6 Ma) from segments of the spectra that represent ≤ 10 % of 39Ar released with inconsistent apparent dates from the individual steps that are grouped together. Even though some of the dates represent low % of released 39Ar, they are interpreted to represent distinct geological events related to the Delamerian orogeny.
Other factors for why some of the spectra do not define plateaus needs to be discussed. For example, sample -268 shows a sinuous pattern. Two dates are extracted from this pattern, 476 ± 1.7 Ma and 468 ± 1.4 Ma, which are interpreted in the context of distinct geological events. However, the segment of the spectra that the latter date was extracted from correlates with a rise in Ca/K ratio, which could indicate 37Ar recoil is partly responsible for this younger part of the spectra, not necessarily growth of a more retentive mica at c. 468 Ma. A discussion of the potential effects of intergrown biotite and white mica also needs to be provided, as these two mineral phases inherently represent distinct domains.
Overall, I would suggest that the recent publication of Schaen et al. (2021; GSA Bulletin) should be consulted for understanding and extracting dates from the 40Ar/39Ar spectra. In this paper, they detail approaches for interpreting spectra and factors that may produce stair-cased or sinuous patterns (i.e., their Figure 8). If the current approach for extracting dates from the spectra is used, then the rationale needs to be explained and the results appropriately discussed with regards to 40Ar behaviour in mica and the tectonic history of the Delamerian orogeny.
- Petrographic and chemical analysis
This issue is related to the previous topic, in that interpretations of the spectra are not supported by physical/chemical evidence of the rocks. Although the structures of the outcrops are well-studied, only two of the five samples were examined in thin section. For such detailed interpretations to be made from the 40Ar/39Ar geochronology results, more petrography and chemical analysis (etc.) of micas in the dated rocks is critical to provide physical/chemical evidence of potentially different 40Ar reservoirs in the micas. For example, the two dates from vein sample -268 are interpreted as two generations of mica (Lines 581-585: “The age data for the muscovite from quartz muscovite vein shows the growth of a more-retentive white mica domain at c. 468 Ma as it degasses in the higher temperature part of the experiment subsequent to the degassing of c. 476 Ma domain.”). No petrography was conducted on this sample, thus, there is no evidence for two generations of mica and this interpretation may not be valid. Thin sections should be studied for all 5 sections, even if it shows that mica define a single generation and are undeformed, chemically homogeneous, etc. because this is very important information to link the well-detailed structural evolution of the outcrops to 40Ar behaviour in the mica and reveal potentially distinct 40Ar domains.
For the two metasedimentary rocks, thin sections have been presented but chemical analysis was only conducted for one sample (-272), whereas the other sample (-269) was not studied due to the small size of the mica. However, the grain size is both rocks looks similar and I think that chemical analysis of both rocks can be done. I would suggest that at least high-contrast BSE images are acquired that can show potential chemical zoning in the mica grains, or e.g., for comparing the chemistries of different mica generations in sample -269 with the crenulation cleavage. The mica chemistry that is presented is not explained very well, i.e., if there is a large dispersion of chemistry for mica in different textural positions or within single grains, and I believe only biotite chemistry is presented from a sample with both biotite and white mica.
- Arrhenius plots
The Arrhenius plots need to be better explained and related to the 40Ar/39Ar spectra, especially the portions of the spectra that are used to extract individual dates. It should not be assumed that the reader will understand how these plots are constructed and what information they provide. What is the input data required to construct the plots? What are the criteria for defining the lines that calculate the diffusion parameters of potentially different mica domains? I understand that the plots are constructed from the same heating steps that the 40Ar/39Ar spectra are constructed from, so how do the data points on the plots correspond to the 40Ar/39Ar spectra, and in particular, with the segments of the spectra that are used to extract 40Ar/39Ar dates?
From what I can understand after reading Forster and Lister (2010), the points from right to left correspond to the heating steps from low-temperature to high-temperature. However, in the Arrhenius plots shown, the first few steps are used to calculate diffusion parameters and closure temperature for a “distinct” diffusion domain but the corresponding steps in the 40Ar/39Ar spectra are stair-cased and do not provide reliable 40Ar/39Ar dates. Are the diffusion parameters calculated from these first few steps actually useful? Some of the lines constructed from the Arrhenius plots also seem to violate the Fundamental Asymmetry Principal (if I understood this correctly from Forster and Lister, 2010), and some of the lines seem to be calculated using very few points. Also, how reliable is this approach with the Arrhenius plots for samples that mix biotite and muscovite? These two phases inherently have different diffusion parameters.
Moderate/Minor Comments
Abstract
-I think that the abstract really needs to reflect how the study was carried out. The types of lithologies, structures, etc. that were dated at the outcrop scale are not mentioned. The interpretations of the dates (cooling, deformation, crystallization) are not really provided either. Instead, the abstract uses vague terminology, such as “It is observed that younger ages up to c. 468 Ma are recorded in the…Backstairs Passage Formation” but no indication of what this date actually represents in terms of the tectonic history. Thus, statements like this do not provide much information. Another example in lines 26-28 “The methodology…garnet growth event.” does not provide information if this is recorded in the Tapanappa Formation or the Backstairs Passage Formation. It is clear these two formations underwent different tectonic histories, thus, the results for the two should be distinct.
-The language is in some places a bit confusing and can be written better. For example, lower activation energy of the Backstairs Passage Formation does not make sense. Rather, “the lower activation energies for mica in the samples obtained from the Backstairs Passage Formation” better describes the idea put forth.
-What is meant by 39Ar diffusion experiments? And why conduct such experiments using 39Ar that is produced due to irradiation of 39K? Is this meant to indicate the step-heating method was used for 40Ar/39Ar geochronology?
Introduction
-The main issue needs to be clearly stated in this introduction. The introduction jumps between the themes of applying mica geochronology to date deformation, the history of the Delamerian Orogeny, focuses on understanding mineralization of the Kanmantoo Mine (which only pertains to the Tapanappa Formation and disregards the Backstairs Passage Formation), and understanding 40Ar/39Ar dates, but the main problem is not well-defined. What does this study aim to resolve, why, and how? What are secondary objectives/themes?
-Dating of magmatic bodies can be useful for bracketing deformation events, but this is not a conventional approach for dating deformation. More often, many studies that date deformation use minerals that define rock structures (as is done in the present study). Perhaps what is meant is that studies trying to constrain deformation of the Delamerian orogeny have conventionally relied on dating magmatic bodies, and the present study instead aims to focus on using mica that define rock structures to directly date tectonic events, which is a more conventional approach than using magmatic rocks (which is a good rationale for the study). Thus, the use of mica and why it is so important to use needs to be highlighted as well.
-The second and third paragraphs partly read as geological background information rather than introducing the focus of the study, the approaches used, and the rationale for the approaches. What is missing from the understanding of the Delamerian orogeny that the current study aims to investigate?
Geological Setting
-It would be useful to know where the study of Foden et al. (2020) was located with respect to the current study locations.
-The last paragraph of 2.1 should be presented first to discuss the sequence of events in time, otherwise, it at first reads like the Kanmantoo Group was deposited after the orogeny.
-What is the relationship between the Keynes subgroup (containing the Backstairs Passage Formation) and the Bollaparudda subgroup (containing the Tapanappa Formation)? Is this a primary depositional contact, or a tectonic contact? If it is tectonic, any information as to when they were juxtaposed?
-Is the Backstairs Passage Formation metamorphosed? The current description suggests these are sedimentary rocks, but later in the manuscript they are presented as metamorphic rocks. A lot more background on the Backstairs Passage Formation needs to be provided… there is no information about structural generations, metamorphic history, previous geochronology (if any) on these rocks. They seem to record a distinct history from the Tapanappa Formation (as is apparent from the 40Ar/39Ar results), yet, the histories of the two formations are ultimately mixed. The histories and results from the two formations need to be distinguished.
Kanmantoo Mine area
-This section focuses on the details of the Tapanappa Formation, but the Backstairs Passage Formation lacks such detailed background. This suggests the main focus of the paper is about events associated with mineralization rather than the Delamerian orogeny itself.
-Where are the Mt. Lofty Ranges located relative to the study area? Is it the entire study area (according to the map in Figure 1). I think it should be introduced earlier in the manuscript in the geological setting.
-What types of intrusions are found in the region? Why are these intrusions important? Are the intrusions surrounding the Kanmantoo mine (in the second and third paragraphs of 3.2) the same as the post-Delamerian intrusions (in the first paragraph), in terms of age or lithology? Or are there different generations?
-It seems that the terminology of ‘intrusions’ and ‘veins’ are being mixed, the former often used to define igneous rocks derived from melt, and the latter from e.g., hydrothermal fluids. Are the veins presented in the study actual veins, or related to the regional igneous intrusions?
-The D2 and D3 events are detailed for the regional geology of the mine, but what represents D1? And what about the Backstairs Passage Formation? Does it show the same structural evolution in terms of D1, D2 and D3?
Methodology
-Thin sections from all five rocks need to be studied. It is not adequate that only two samples were studied petrographically and three were ignored, yet intricate interpretations are made from the 40Ar/39Ar results of the three understudied samples. Furthermore, the chemistries of mica in all five samples should be studied, ideally with high-contrast BSE imaging and WDS analysis.
-Is the Backstairs Passage vein concordant or discordant to the foliation? Same for the non-boudinaged Tapanappa vein, is it concordant or discordant?
-Please check that the supplementary methods for 40Ar/39Ar analysis are correct and pertinent to the current study. The methods refer to separation processes and analysis for K-feldspar, which was not dated. This gives the impression that this text was copied-and-pasted from another source without proofreading.
Results
-How is the bedding defined in the Tapanappa Formation?
-How does the schistosity of the outcrop vary with respect to bedding, i.e., lines 271-272. Please elaborate on this.
-Are the studied veins actual veins, or igneous intrusions?
-Bt-rich selvedges provides evidence of a melt-host rock interaction, yet the presence of melt (either partial melting or intrusive in nature) is not really presented. What is the cause of the selvedges?
-How do the late veins show compression of the host rock/earlier veins (i.e., lines 293-294)
-Section 5.2.1 is not necessary here. Perhaps it can be incorporated into the methods.
-The Backstairs Passage phyllite looks to be suitable for chemical analysis, provided the images in Fig. 5.
-If only the top half of the section is defined by the crenulation cleavage, how is the bottom half defined? This is not explained.
-If there is microstructural variation in the metasedimentary rocks that is noticeable on the thin section scale, which portions of the rocks were selected for crushing and mica extraction?
-How does the crenulation cleavage merge and transition into Scren on the outcrop scale? This is not obvious from the field photos.
-Why do the decussate white mica need to be late? Is there evidence that they overgrew the crenulate cleavage planes, or are they only found in the microlithons?
-Figure references for Figure 6 are out of order in 5.2.3
-For the Tapanappa schist, the thin section photos do not really support a strong distinction between four generations of mica, I see perhaps two (white mica 1 vs. white mica 2, 3, decussate). Can you provide more evidence for four different generations, e.g., with chemical data?
-The “non-fractured, unaltered garnet” are fractured in Fig. 6h.
-How can secondary minerals be “around” the garnet fractures (i.e., lines 354-356).
-The Tectonic Sequence Diagram section does not provide much clarity for the overall important structural and metamorphic features of the rocks. This needs to also be summarized in words, highlighted/summarized the key features of the rocks and which generations of mica have likely been dated and what formed them (in which metamorphic conditions). This is the most important information that needs to be provided from the petrography to set up the interpretations of the 40Ar/39Ar results.
-For chemical analysis, how many points were obtained for garnet and mica? How were they obtained, using profiles or single point analysis according to mineral zones observed in BSE images? Are there chemical variations in single grains, or between grains in different textural positions or supposed generations for garnet and white mica?
-The Arrhenius plots need to be explained better. What is the input data? What is the Fundamental Asymmetry Principal and how is it used to define diffusion domains/parameters from the Arrhenius plots? What do the dots represent on the plots? Etc.
-How do the different activation energies from the Arrhenius plots relate to the dates extracted from the 40Ar/39Ar spectra since these two methods are constructed using the same step-hearing results. Are these activation energies applicable to the dates extracted from the spectra? I.e., the first few lowest-temperature steps are used to calculate activation energy and closure temperature in the Arrhenius plots, but these corresponding heating steps in the spectra are not used to calculate 40Ar/39Ar dates.
-It would be nice to see Arrhenius plots for all five samples in the main text.
-Are the reported uncertainties for the dates at 1 or 2 sigma?
-Ideally, Cl/K and Ca/K ratios should be provided with the 40Ar/39Ar spectra in the main text.
-Mixing biotite and white mica inherently mixes different 40Ar diffusion domains. How does this potentially manifest in the 40Ar/39Ar spectra and Arrhenius plots (perhaps a topic for the Discussion)?
-It may potentially be useful to examine the results of step-heating using an isochron (see Schaen et al., 2021) to scrutinize the data more.
Discussion
-This section needs to start with a synthesis of the structural and metamorphic evolutions of the two outcrops. This would follow the overall approach to the study and help provide the foundation for the interpretations of the 40Ar/39Ar dates later in the section.
- The evolutions of the Backstairs Passage and Tapanappa formations need to be distinguished and discussed separately in more detail as they seem to have distinct histories.
-One very important issue with the discussion is the lack of references provided for statements regarding 40Ar behaviour in rocks. For example, the second paragraph in 6.1 provides a lot of statements regarding 40Ar behaviour in mica but does not support any of these statements with references. The proper references need to be provided.
-The rim and core concept is not the only means by which mica may preserve distinct 40Ar/diffusion domains. This can also be achieved by localized deformation in single grains (i.e., bent tips, kinking), different generations of mica related to sequential rock structures (e.g., crenulation cleavage, inclusions vs. matrix grains), different mica species (e.g., biotite vs. white mica), localized fluid interaction with mica, which may not necessarily produce a core-rim chemical geometry. More possibilities need to be discussed for how mica may produce variable 40Ar/39Ar ages, even in a single grain.
-The basis for many of the interpretations are not clear. For example, the c. 484-481 Ma interval from sample -269 is interpreted to reflect formation of the crenulation cleavage, but evidence for this as opposed to e.g., cooling ages are not provided. Furthermore, there is no petrography nor chemical information provided to support the interpretation of two distinct mica generations in the Backstairs quartz vein (sample -268).
-What is the evidence that the Backstairs rocks were “substantially cooled” before intrusion of the quartz vein at c. 476-468 Ma? Cooled to what temperatures?
-What caused growth of the potential second generation of mica in the vein?
-What is meant by “geological episodes” in the final paragraph of 6.1, and why is it distinguished from cooling ages? Previously in the section, it is stated “geological episodes of cooling and/or deformation”. The language used and terminology is confusing and vague.
-Why is c. 506 Ma interpreted as the formation of the rock fabric and not e.g., excess 40Ar. This needs to be explained better. It is also unclear how the rock fabric formed at c. 506 Ma but the boudinaged dyke/vein intruded at c. 497 Ma (c. 495 Ma?)? The boudinage is evidence of host rock deformation post-intrusion, so I would not expect the dominant rock structure to be c. 506 Ma in age. However, perhaps it is possible that the c. 506 Ma is evidence of a partially reset 40Ar/39Ar record, and that deformation and fabric development post-495 Ma partially reset the previous c. 506 Ma (or older) history. Is that what is meant by “the maximum formation age of schistosity at c. 506 Ma”?
-The metasedimentary rocks in the Tapanappa outcrop were deforming after c. 495 Ma (intrusion of boudinaged dyke/vein), so this foliation is likely not that same as the foliation related to D2 dated by Foden et al. (2006), but may seem to be a younger deformation event overprinting the c. 506 Ma D2. It would be good to explain earlier in the Discussion how the structures in the examined outcrops relate to the regional D1, D2, D3, etc. to understand better these geochronological relationships.
-How did intrusion of the second vein in the Tapanappa outrcrop reset the mica in the boudinaged vein? Is there evidence of heating or fluid percolation in the rocks? Did it effect mica in the host rock as well? It would be good to see evidence of this in thin section in the boudinaged vein.
-What is the evidence for regional fluid flow events in the outcrops?
-I am surprised that garnet would form at such low T conditions, i.e., not exceeding the ~280 °C closure temperature calculated for the mica in the sample. Is there evidence in the literature for such low temperature garnet forming from hydrothermal fluids? Are the closure temperatures perhaps underestimated by the Arrhenius plots? Or perhaps garnet is not that late in the overall metamorphic sequence of the outcrop?
-What brackets the growth of garnet to c. 493-486 Ma?
Figures
-In general, the figures (especially of the map and outcrops) are a bit small and blurry, but perhaps this is a consequence of the file upload. Please ensure that the final forms are high-quality, as right now the structural relationships in some of the outcrop photos are not entirely visible.
-In Figure 1 there is a cross-section presented that is not presented in the manuscript.
-It would be beneficial to highlight on the overview outcrop photos the locations of the close-up images in the subsequent figures.
-Showing the Ca/K and Cl/K ratios with the 40Ar/39Ar spectra would be beneficial.
-It would be useful to see all Arrhenius plots in the main text.
-References for Figures 10, 11, and 12 are needed.
-The construction of Figure 11/12 is a bit confusing. The acronyms should be explained in the figure captions. Perhaps Figure 12 is only needed in the manuscript to summarize the sequences of events as both figures provide a lot of repetition of the same information.
-The same external constraints are used for the Backstairs Passage and Tapanappa formations in Figures 11 and 12 (and in Figure 10 for that matter), yet it is not evident in the geological setting, nor in the discussion that the structural and metamorphic histories of the two formations are shared. Rather, it seems they have distinct tectonic histories, as highlighted by the data compilation in Figure 10. I would be surprised if they experienced the exact same histories in this case, but if so, it needs to be clearly presented earlier in the manuscript.
I hope that my comments are useful for improving the study. I think that there is potential for a very nice contribution for understanding the evolution of the Delamerian orogeny, but more work is required to properly scrutinize the data and support the interpretations/conclusions.
Regards,
Christopher J. Barnes
Citation: https://doi.org/10.5194/egusphere-2023-1839-RC2 -
EC1: 'Editor Comment on egusphere-2023-1839', Brenhin Keller, 16 Apr 2024
The author has requested that this manuscript be withdrawn
Citation: https://doi.org/10.5194/egusphere-2023-1839-EC1
Interactive discussion
Status: closed
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CC1: 'Comment on egusphere-2023-1839', Alan Collins, 11 Oct 2023
Thanks for submitting this lovely paper - I've gone through it and really enjoyed reading it. I've a few comments that are really about the bigger picture (as well as updating with some recent published work by Hong et al. and Reid et al. on the region). Numbers refer to line numbers in the manuscript.
55-56 – The Delamerian orogeny is not really part of the so-called Pan-African orogens (that term is also meaningless as it variously refers to 300-400 million years of orogens around the world). The Delamerian occurs after the youngest of the Gondwana-forming orogens (Collins and Pisarevsky, 2005; Merdith et al., 2021; Schmitt et al., 2018) and has been associated with the initiation of circum-Gondwana subduction (Foden et al., 2006; Foden et al., 2020) – the so-called Terra Australia Orogen (Cawood, 2005).
63 – the best dating of this deposition is reported in Betts et al (2018).
67-74 Hong et al. (2023)needs to be incorporated into this work. I understand that this probably wasn’t published when your paper was submitted, but it is a thorough geochronological analysis of the timing of mineralisation in the Delamerian that needs to be incorporated into your manuscript.
90 – again, I think Betts et al. (2018) is an important reference here for the timing of the Kanmantoo
117 – quote Lloyd et al. (2020) when you first mention the Adelaide Superbasin – this is the reference where this was defined (and defined to include the Kanmantoo trough).
205 – ‘Microstructural’ not ‘microstructure’
Fig 3 and Fig 4 – The Tectonic Sequence Diagram notation needs to be referred to as meeting these acronyms here before they are described in the text, leads to confusion. Perhaps say something in the caption like ‘TSD = Tectonic Sequence Diagrams; for explanation see text.’ Also, the mineral acronyms are not spelt out – many can be guessed, but they need to be defined.
Fig 6g – this doesn’t look like a ‘late stage static garnet’ as it has biotite inclusion trails that form obliquely to the surrounding matrix fabric – suggesting that the garnet has rotated since it formed…
384 – ‘important’ instead of ‘imperative’
386, 387 ‘the Backstairs Passage Formation’, ‘the Tapanappa Formation’ – this needs checking and correcting through the manuscript.
637 – ‘…tectonics were…’
Thanks for writing up a lovely study! I think with your incorporation of Hong et al. and Ried et al.’s (Reid et al., 2022) recent work, this will make a really key study of the mineralisation timing in the Delamerian Orogen.
Sincerely
Alan Collins
Betts, M. J., Paterson, J. R., Jacquet, S. M., Andrew, A. S., Hall, P. A., Jago, J. B., Jagodzinski, E. A., Preiss, W. V., Crowley, J. L., Brougham, T., Mathewson, C. P., Garcia-Bellido, D. C., Topper, T. P., Skovsted, C. B., and Brock, G. A., 2018, Early Cambrian chronostratigraphy and geochronology of South Australia: Earth-Science Reviews, v. 185, p. 498-543.
Cawood, P. A., 2005, Terra Australis Orogen: Rodinia breakup and development of the Pacific and Iapetus margins of Gondwana during the Neoproterozoic and Paleozoic: Earth Science Reviews., v. 69, p. 249-279.
Collins, A., and Pisarevsky, S., 2005, Amalgamating eastern Gondwana: The evolution of the Circum-Indian Orogens: Earth-Science Reviews, v. 71, no. 3-4, p. 229-270.
Foden, J., Elburg, M. A., Dougherty-Page, J., and Burtt, A., 2006, The timing and duration of the Delemerian Orogeny: correlation with the Ross Orogen and implications for Gondwana assembly: Journal of Geology, v. 114, p. 189-210.
Foden, J. D., Elburg, M., Turner, S., Clark, C., Blades, M. L., Cox, G., Collins, A. S., Wolff, K., and George, C., 2020, Cambro-Ordovician magmatism in the Delamerian orogeny: Implications for tectonic development of the southern Gondwanan margin: Gondwana Research.
Hong, W., Fabris, A., Wise, T., Collins Alan, S., Gilbert, S., Selby, D., Curtis, S., and Reid, A. J., 2023, Metallogenic Setting and Temporal Evolution of Porphyry Cu-Mo Mineralization and Alteration in the Delamerian Orogen, South Australia: Insights From Zircon U-Pb, Molybdenite Re-Os, and In Situ White Mica Rb-Sr Geochronology: Economic Geology, v. 118, no. 6, p. 1291-1318.
Lloyd, J. C., Blades, M. L., Counts, J. W., Collins, A. S., Amos, K. J., Wade, B. P., Hall, J. W., Hore, S., Ball, A. L., Shahin, S., and Drabsch, M., 2020, Neoproterozoic geochronology and provenance of the Adelaide Superbasin: Precambrian Research, v. 350.
Merdith, A. S., Williams, S. M. E., Collins, A. S., Tetley, M. G., Mulder, J. A., Blades, M. L., Young, A., Armistead, S. E., Cannon, J., Zahirovic, S., and Muller, R. D., 2021, Extending full-plate tectonic models into deep time: Linking the Neoproterozoic and the Phanerozoic: Earth-Science Reviews, v. 214.
Reid, A., Forster, M., Preiss, W., Caruso, A., Curtis, S., Wise, T., Vasegh, D., Goswami, N., and Lister, G., 2022, Complex 40Ar ∕ 39Ar age spectra from low-grade metamorphic rocks: resolving the input of detrital and metamorphic components in a case study from the Delamerian Orogen: Geochronology, v. 4, no. 2, p. 471-500.
Schmitt, R. d. S., Fragoso, R. d. A., and Collins, A. S., 2018, Suturing Gondwana in the Cambrian: The Orogenic Events of the Final Amalgamation, in Siegesmund, S., Basei, M. A. S., Oyhantçabal, P., and Oriolo, S., eds., Geology of Southwest Gondwana: Cham, Springer International Publishing, p. 411-432.
Hong et al. 2023 – abstract
Paleozoic porphyry-style hydrothermal alteration and mineralization has previously been recognized within the Delamerian orogen, South Australia, where porphyry prospects include Anabama Hill, Netley Hill, and Bendigo. However, limited exploration due in part to thick postmineralization cover hinders the understanding of the temporal context, metallogenic setting, and mineral potential of the porphyry systems along the Proterozoic
continental margin of Australia. In this study, we have characterized the hydrothermal alteration and mineralization of these porphyry occurrences. Zircon U-Pb, molybdenite Re-Os, and white mica Rb-Sr ages have been determined to constrain the timing for emplacement of magmatic intrusions, precipitation of metal-bearing sulfides, and duration of hydrothermal alteration in the Delamerian orogenic belt. Zircon U-Pb laser ablationinductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of nine granitoids reveal that the intrusive rocks were emplaced mostly between 485 and 465 Ma, whereas three intrusions at Bendigo have zircon U-Pb ages of 490 to 480 Ma. Molybdenite isotope dilution-negative thermal ion mass spectrometry (ID-NTIMS) Re-Os dating of the four prospects identifies two porphyry Cu-Mo mineralization events at 480 and 470 to 460 Ma, respectively. Nineteen white mica Rb-Sr LA-ICP-MS/MS (tandem mass spectrometers) analyses return an age range between 455 and 435 Ma for phyllic alteration at the Anabama Hill and Netley Hill prospects, whereas intense white mica-quartz-pyrite alteration at Bendigo prospect appears to have developed between 470 and 460 Ma. These geochronologic results indicate that the Delamerian porphyry systems postdated subduction-related magmatism in the region (514–490 Ma) but instead formed within an inverted back-arc regime, where mineralized magmas and fluids ascended along favorable lithospheric-scale structures, probably due to asthenospheric upwelling triggered by mafic delamination. Porphyritic stocks, dikes, and aplites with ages of 470 to 460 Ma are the most likely hosts to porphyry-style mineralization in the Delamerian orogen that appears to have formed simultaneously with the oldest known porphyry systems in the intraoceanic Macquarie arc (e.g., Marsden, E43, and Milly Milly; 467–455 Ma). These results emphasize the significance and potential of Early-Middle Ordovician intrusive systems to host such a type of magmatic-hydrothermal mineralization in the Delamerian orogen.
Citation: https://doi.org/10.5194/egusphere-2023-1839-CC1 -
RC1: 'Comment on egusphere-2023-1839', Anonymous Referee #1, 28 Nov 2023
This manuscript by Goswami uses a combination of field observations, microstructural and chemical analyses, along with 40Ar/39Ar step heating experiments, to suggest that the Delamerian Orogen was more protracted than previously thought. I have two main concerns for this manuscript, the first of which makes it a little tricky to fully review. The paper does not seem to have been carefully proofread by the author – Figure 1 is a good example of this. Figure 1 is modified by Reid et al. 2022, but there are so few modifications that it is confusing – the key shows the location of a cross-section apparently in Figure 3, which appears to actually be Figure 3 from Reid et al. 2022. Given this, I have not gone through and listed all the specific errors (of which unfortunately there are quite a few) and will keep the remaining comments general.
The second issue relates to the 40Ar/39Ar work itself. Firstly, I am not sure as to how robust the results of muscovite and biotite mixtures are. The release spectra for these are difficult to interpret (Figure 8a for eg.), and I would urge the author to be more cautious when assigning ages based on release spectra such as these. Best practice for how to interpret release spectra were laid out recently in a community paper: Schaen et al. 2021 GSA Bull. Secondly, I am not convinced by the calculation of the closure temperatures for the mica mixtures using the Arrhenius plots. While Figure 9b does appear to show different domains, the results are potentially showing a mixture of the biotite and muscovite degassing, meaning that the calculated kinetics and closure temperatures are not meaningful. I also am not convinced by some of the regressions chosen (e.g. in Figure 9b). It is important to note too that muscovite and biotite are hydrous silicates and therefore will not remain stable throughout the step heating experiments (unless they are hydrothermal experiments – which unless I have missed it, I don’t believe these are). This means that the resulting diffusion kinetics are not the diffusion kinetics of the muscovite and biotite in question – and the closure temperature estimates are not geologically meaningful. The interpretation of these dates, especially in section 6.1 needs a lot more detail.
While I think a lot of further work is needed on this paper before it can be published, I do want to end this review on a positive note. I think that there is a lot of good research that has been conducted by the author. The microstructural analyses for example have been carefully done, and the evidence is clear and well documented in the paper. Not all of the 40Ar/39Ar results are difficult to interpret – inverse isochron plots of the pure splits of mica could be illuminating if the author has not already tried that. I like Figures 10 onwards, they lay out clearly the different tectonic events and the evidence behind them. With careful examination and interpretation of the Ar results, along with proof reading, this paper has potential to be a good addition to our knowledge of the Delamerian Orogen.
Citation: https://doi.org/10.5194/egusphere-2023-1839-RC1 -
RC2: 'Comment on egusphere-2023-1839', Christopher Barnes, 20 Dec 2023
Review of “New age constraints supporting the existence of protracted deformation in the Delamerian Orogen: 40Ar/39Ar geochronology on fabric forming minerals of the Kanmantoo Group metasediments.” by N. Goswami.
Summary
The manuscript focuses on two outcrops belonging to subgroups of the Kanmantoo Group of the Delamerian orogeny, combining structural analysis, petrography, and 40Ar/39Ar geochronology (using the step-heating method) to investigate the tectonic evolution of the two outcrops, related to orogenesis and regional mineralization. The study utilizes to good approach to provide a lithological/structural/mineralogical foundation for interpreting the 40Ar/39Ar dates. However, the resulting step-heating spectra are used to extract multitudes of dates from single samples and may be over-interpreted. I will not necessarily dismiss the interpretations that are made by the author, but a lot of processes/phenomena that can contribute to the patterns observed in the step-heating spectra are overlooked, and not enough characterization of the samples and the mica have been done to directly support the interpretations made. Therefore, the study has potential to be a valuable contribution for understanding the tectonics of the Delamerian orogeny, but significant work is required before it can be recommended for publication. Major issues are outlined below, and more minor comments are provided according to the manuscript sections/lines, some of which may reiterate themes of the major issues.
Major Issues
- Focus of the study
The focus of the study is not entirely clear. In particular, the Introduction and Discussion both shift their focus between a) understanding the deformation associated with mineralization of the Kanmantoo mine (which would only require studying the Tapanappa Formation), b) resolving the regional tectonic evolution of the Delamerian orogeny, and c) exploring the systematics of 40Ar in mica. All three topics are related and can be discussed throughout the manuscript, but what is the overarching theme? From the title, and that both outcrops from the Tapanappa Formation (hosting the Kanmantoo mine) and the Backstairs Passage Formation are involved in the study, I would assume that the tectonics of the Delamerian orogeny is the primary focus and the other two themes are secondary. This needs to be clearly defined for the reader and echoed in how the Discussion is structured.
- 40Ar/39Ar step-heating spectra
Firstly, the supplementary data is not adequate and more information is needed in the results of 40Ar/39Ar geochronology. I was unable to independently investigate the dating results because I do not have access to the eArgon software (nor could find it online), and the isotope ratios and uncertainties that are needed to plot the data in IsoplotR are not presented. Therefore, I had to evaluate all of the data visually from the figures showing the step-heating spectra and could calculate dates from the spectra myself. A reference needs to be provided to locate the eArgon software online for the reader, and more information needs to be provided in the supplementary data file, i.e., the different Ar isotope ratios and associated uncertainties should be provided so the data can be input into IsoplotR.
The results themselves may be over-interpreted. Several of the spectra do not provide well-defined plateau dates, yet specific segments of the spectra are extracted to calculate dates. The rationale for the approach is not clearly explained. For example, sample -270 is used to extract dates of 492 ± 1.7 Ma and 495 ± 1.1 Ma from degassing steps that individually yield apparent dates that appear to be statistically overlapping. In contrast, several dates from sample -272 are extracted (496 ± 1.7 Ma, 488 ± 1.6 Ma, 506 ± 2.3 Ma, and 498 ± 1.6 Ma) from segments of the spectra that represent ≤ 10 % of 39Ar released with inconsistent apparent dates from the individual steps that are grouped together. Even though some of the dates represent low % of released 39Ar, they are interpreted to represent distinct geological events related to the Delamerian orogeny.
Other factors for why some of the spectra do not define plateaus needs to be discussed. For example, sample -268 shows a sinuous pattern. Two dates are extracted from this pattern, 476 ± 1.7 Ma and 468 ± 1.4 Ma, which are interpreted in the context of distinct geological events. However, the segment of the spectra that the latter date was extracted from correlates with a rise in Ca/K ratio, which could indicate 37Ar recoil is partly responsible for this younger part of the spectra, not necessarily growth of a more retentive mica at c. 468 Ma. A discussion of the potential effects of intergrown biotite and white mica also needs to be provided, as these two mineral phases inherently represent distinct domains.
Overall, I would suggest that the recent publication of Schaen et al. (2021; GSA Bulletin) should be consulted for understanding and extracting dates from the 40Ar/39Ar spectra. In this paper, they detail approaches for interpreting spectra and factors that may produce stair-cased or sinuous patterns (i.e., their Figure 8). If the current approach for extracting dates from the spectra is used, then the rationale needs to be explained and the results appropriately discussed with regards to 40Ar behaviour in mica and the tectonic history of the Delamerian orogeny.
- Petrographic and chemical analysis
This issue is related to the previous topic, in that interpretations of the spectra are not supported by physical/chemical evidence of the rocks. Although the structures of the outcrops are well-studied, only two of the five samples were examined in thin section. For such detailed interpretations to be made from the 40Ar/39Ar geochronology results, more petrography and chemical analysis (etc.) of micas in the dated rocks is critical to provide physical/chemical evidence of potentially different 40Ar reservoirs in the micas. For example, the two dates from vein sample -268 are interpreted as two generations of mica (Lines 581-585: “The age data for the muscovite from quartz muscovite vein shows the growth of a more-retentive white mica domain at c. 468 Ma as it degasses in the higher temperature part of the experiment subsequent to the degassing of c. 476 Ma domain.”). No petrography was conducted on this sample, thus, there is no evidence for two generations of mica and this interpretation may not be valid. Thin sections should be studied for all 5 sections, even if it shows that mica define a single generation and are undeformed, chemically homogeneous, etc. because this is very important information to link the well-detailed structural evolution of the outcrops to 40Ar behaviour in the mica and reveal potentially distinct 40Ar domains.
For the two metasedimentary rocks, thin sections have been presented but chemical analysis was only conducted for one sample (-272), whereas the other sample (-269) was not studied due to the small size of the mica. However, the grain size is both rocks looks similar and I think that chemical analysis of both rocks can be done. I would suggest that at least high-contrast BSE images are acquired that can show potential chemical zoning in the mica grains, or e.g., for comparing the chemistries of different mica generations in sample -269 with the crenulation cleavage. The mica chemistry that is presented is not explained very well, i.e., if there is a large dispersion of chemistry for mica in different textural positions or within single grains, and I believe only biotite chemistry is presented from a sample with both biotite and white mica.
- Arrhenius plots
The Arrhenius plots need to be better explained and related to the 40Ar/39Ar spectra, especially the portions of the spectra that are used to extract individual dates. It should not be assumed that the reader will understand how these plots are constructed and what information they provide. What is the input data required to construct the plots? What are the criteria for defining the lines that calculate the diffusion parameters of potentially different mica domains? I understand that the plots are constructed from the same heating steps that the 40Ar/39Ar spectra are constructed from, so how do the data points on the plots correspond to the 40Ar/39Ar spectra, and in particular, with the segments of the spectra that are used to extract 40Ar/39Ar dates?
From what I can understand after reading Forster and Lister (2010), the points from right to left correspond to the heating steps from low-temperature to high-temperature. However, in the Arrhenius plots shown, the first few steps are used to calculate diffusion parameters and closure temperature for a “distinct” diffusion domain but the corresponding steps in the 40Ar/39Ar spectra are stair-cased and do not provide reliable 40Ar/39Ar dates. Are the diffusion parameters calculated from these first few steps actually useful? Some of the lines constructed from the Arrhenius plots also seem to violate the Fundamental Asymmetry Principal (if I understood this correctly from Forster and Lister, 2010), and some of the lines seem to be calculated using very few points. Also, how reliable is this approach with the Arrhenius plots for samples that mix biotite and muscovite? These two phases inherently have different diffusion parameters.
Moderate/Minor Comments
Abstract
-I think that the abstract really needs to reflect how the study was carried out. The types of lithologies, structures, etc. that were dated at the outcrop scale are not mentioned. The interpretations of the dates (cooling, deformation, crystallization) are not really provided either. Instead, the abstract uses vague terminology, such as “It is observed that younger ages up to c. 468 Ma are recorded in the…Backstairs Passage Formation” but no indication of what this date actually represents in terms of the tectonic history. Thus, statements like this do not provide much information. Another example in lines 26-28 “The methodology…garnet growth event.” does not provide information if this is recorded in the Tapanappa Formation or the Backstairs Passage Formation. It is clear these two formations underwent different tectonic histories, thus, the results for the two should be distinct.
-The language is in some places a bit confusing and can be written better. For example, lower activation energy of the Backstairs Passage Formation does not make sense. Rather, “the lower activation energies for mica in the samples obtained from the Backstairs Passage Formation” better describes the idea put forth.
-What is meant by 39Ar diffusion experiments? And why conduct such experiments using 39Ar that is produced due to irradiation of 39K? Is this meant to indicate the step-heating method was used for 40Ar/39Ar geochronology?
Introduction
-The main issue needs to be clearly stated in this introduction. The introduction jumps between the themes of applying mica geochronology to date deformation, the history of the Delamerian Orogeny, focuses on understanding mineralization of the Kanmantoo Mine (which only pertains to the Tapanappa Formation and disregards the Backstairs Passage Formation), and understanding 40Ar/39Ar dates, but the main problem is not well-defined. What does this study aim to resolve, why, and how? What are secondary objectives/themes?
-Dating of magmatic bodies can be useful for bracketing deformation events, but this is not a conventional approach for dating deformation. More often, many studies that date deformation use minerals that define rock structures (as is done in the present study). Perhaps what is meant is that studies trying to constrain deformation of the Delamerian orogeny have conventionally relied on dating magmatic bodies, and the present study instead aims to focus on using mica that define rock structures to directly date tectonic events, which is a more conventional approach than using magmatic rocks (which is a good rationale for the study). Thus, the use of mica and why it is so important to use needs to be highlighted as well.
-The second and third paragraphs partly read as geological background information rather than introducing the focus of the study, the approaches used, and the rationale for the approaches. What is missing from the understanding of the Delamerian orogeny that the current study aims to investigate?
Geological Setting
-It would be useful to know where the study of Foden et al. (2020) was located with respect to the current study locations.
-The last paragraph of 2.1 should be presented first to discuss the sequence of events in time, otherwise, it at first reads like the Kanmantoo Group was deposited after the orogeny.
-What is the relationship between the Keynes subgroup (containing the Backstairs Passage Formation) and the Bollaparudda subgroup (containing the Tapanappa Formation)? Is this a primary depositional contact, or a tectonic contact? If it is tectonic, any information as to when they were juxtaposed?
-Is the Backstairs Passage Formation metamorphosed? The current description suggests these are sedimentary rocks, but later in the manuscript they are presented as metamorphic rocks. A lot more background on the Backstairs Passage Formation needs to be provided… there is no information about structural generations, metamorphic history, previous geochronology (if any) on these rocks. They seem to record a distinct history from the Tapanappa Formation (as is apparent from the 40Ar/39Ar results), yet, the histories of the two formations are ultimately mixed. The histories and results from the two formations need to be distinguished.
Kanmantoo Mine area
-This section focuses on the details of the Tapanappa Formation, but the Backstairs Passage Formation lacks such detailed background. This suggests the main focus of the paper is about events associated with mineralization rather than the Delamerian orogeny itself.
-Where are the Mt. Lofty Ranges located relative to the study area? Is it the entire study area (according to the map in Figure 1). I think it should be introduced earlier in the manuscript in the geological setting.
-What types of intrusions are found in the region? Why are these intrusions important? Are the intrusions surrounding the Kanmantoo mine (in the second and third paragraphs of 3.2) the same as the post-Delamerian intrusions (in the first paragraph), in terms of age or lithology? Or are there different generations?
-It seems that the terminology of ‘intrusions’ and ‘veins’ are being mixed, the former often used to define igneous rocks derived from melt, and the latter from e.g., hydrothermal fluids. Are the veins presented in the study actual veins, or related to the regional igneous intrusions?
-The D2 and D3 events are detailed for the regional geology of the mine, but what represents D1? And what about the Backstairs Passage Formation? Does it show the same structural evolution in terms of D1, D2 and D3?
Methodology
-Thin sections from all five rocks need to be studied. It is not adequate that only two samples were studied petrographically and three were ignored, yet intricate interpretations are made from the 40Ar/39Ar results of the three understudied samples. Furthermore, the chemistries of mica in all five samples should be studied, ideally with high-contrast BSE imaging and WDS analysis.
-Is the Backstairs Passage vein concordant or discordant to the foliation? Same for the non-boudinaged Tapanappa vein, is it concordant or discordant?
-Please check that the supplementary methods for 40Ar/39Ar analysis are correct and pertinent to the current study. The methods refer to separation processes and analysis for K-feldspar, which was not dated. This gives the impression that this text was copied-and-pasted from another source without proofreading.
Results
-How is the bedding defined in the Tapanappa Formation?
-How does the schistosity of the outcrop vary with respect to bedding, i.e., lines 271-272. Please elaborate on this.
-Are the studied veins actual veins, or igneous intrusions?
-Bt-rich selvedges provides evidence of a melt-host rock interaction, yet the presence of melt (either partial melting or intrusive in nature) is not really presented. What is the cause of the selvedges?
-How do the late veins show compression of the host rock/earlier veins (i.e., lines 293-294)
-Section 5.2.1 is not necessary here. Perhaps it can be incorporated into the methods.
-The Backstairs Passage phyllite looks to be suitable for chemical analysis, provided the images in Fig. 5.
-If only the top half of the section is defined by the crenulation cleavage, how is the bottom half defined? This is not explained.
-If there is microstructural variation in the metasedimentary rocks that is noticeable on the thin section scale, which portions of the rocks were selected for crushing and mica extraction?
-How does the crenulation cleavage merge and transition into Scren on the outcrop scale? This is not obvious from the field photos.
-Why do the decussate white mica need to be late? Is there evidence that they overgrew the crenulate cleavage planes, or are they only found in the microlithons?
-Figure references for Figure 6 are out of order in 5.2.3
-For the Tapanappa schist, the thin section photos do not really support a strong distinction between four generations of mica, I see perhaps two (white mica 1 vs. white mica 2, 3, decussate). Can you provide more evidence for four different generations, e.g., with chemical data?
-The “non-fractured, unaltered garnet” are fractured in Fig. 6h.
-How can secondary minerals be “around” the garnet fractures (i.e., lines 354-356).
-The Tectonic Sequence Diagram section does not provide much clarity for the overall important structural and metamorphic features of the rocks. This needs to also be summarized in words, highlighted/summarized the key features of the rocks and which generations of mica have likely been dated and what formed them (in which metamorphic conditions). This is the most important information that needs to be provided from the petrography to set up the interpretations of the 40Ar/39Ar results.
-For chemical analysis, how many points were obtained for garnet and mica? How were they obtained, using profiles or single point analysis according to mineral zones observed in BSE images? Are there chemical variations in single grains, or between grains in different textural positions or supposed generations for garnet and white mica?
-The Arrhenius plots need to be explained better. What is the input data? What is the Fundamental Asymmetry Principal and how is it used to define diffusion domains/parameters from the Arrhenius plots? What do the dots represent on the plots? Etc.
-How do the different activation energies from the Arrhenius plots relate to the dates extracted from the 40Ar/39Ar spectra since these two methods are constructed using the same step-hearing results. Are these activation energies applicable to the dates extracted from the spectra? I.e., the first few lowest-temperature steps are used to calculate activation energy and closure temperature in the Arrhenius plots, but these corresponding heating steps in the spectra are not used to calculate 40Ar/39Ar dates.
-It would be nice to see Arrhenius plots for all five samples in the main text.
-Are the reported uncertainties for the dates at 1 or 2 sigma?
-Ideally, Cl/K and Ca/K ratios should be provided with the 40Ar/39Ar spectra in the main text.
-Mixing biotite and white mica inherently mixes different 40Ar diffusion domains. How does this potentially manifest in the 40Ar/39Ar spectra and Arrhenius plots (perhaps a topic for the Discussion)?
-It may potentially be useful to examine the results of step-heating using an isochron (see Schaen et al., 2021) to scrutinize the data more.
Discussion
-This section needs to start with a synthesis of the structural and metamorphic evolutions of the two outcrops. This would follow the overall approach to the study and help provide the foundation for the interpretations of the 40Ar/39Ar dates later in the section.
- The evolutions of the Backstairs Passage and Tapanappa formations need to be distinguished and discussed separately in more detail as they seem to have distinct histories.
-One very important issue with the discussion is the lack of references provided for statements regarding 40Ar behaviour in rocks. For example, the second paragraph in 6.1 provides a lot of statements regarding 40Ar behaviour in mica but does not support any of these statements with references. The proper references need to be provided.
-The rim and core concept is not the only means by which mica may preserve distinct 40Ar/diffusion domains. This can also be achieved by localized deformation in single grains (i.e., bent tips, kinking), different generations of mica related to sequential rock structures (e.g., crenulation cleavage, inclusions vs. matrix grains), different mica species (e.g., biotite vs. white mica), localized fluid interaction with mica, which may not necessarily produce a core-rim chemical geometry. More possibilities need to be discussed for how mica may produce variable 40Ar/39Ar ages, even in a single grain.
-The basis for many of the interpretations are not clear. For example, the c. 484-481 Ma interval from sample -269 is interpreted to reflect formation of the crenulation cleavage, but evidence for this as opposed to e.g., cooling ages are not provided. Furthermore, there is no petrography nor chemical information provided to support the interpretation of two distinct mica generations in the Backstairs quartz vein (sample -268).
-What is the evidence that the Backstairs rocks were “substantially cooled” before intrusion of the quartz vein at c. 476-468 Ma? Cooled to what temperatures?
-What caused growth of the potential second generation of mica in the vein?
-What is meant by “geological episodes” in the final paragraph of 6.1, and why is it distinguished from cooling ages? Previously in the section, it is stated “geological episodes of cooling and/or deformation”. The language used and terminology is confusing and vague.
-Why is c. 506 Ma interpreted as the formation of the rock fabric and not e.g., excess 40Ar. This needs to be explained better. It is also unclear how the rock fabric formed at c. 506 Ma but the boudinaged dyke/vein intruded at c. 497 Ma (c. 495 Ma?)? The boudinage is evidence of host rock deformation post-intrusion, so I would not expect the dominant rock structure to be c. 506 Ma in age. However, perhaps it is possible that the c. 506 Ma is evidence of a partially reset 40Ar/39Ar record, and that deformation and fabric development post-495 Ma partially reset the previous c. 506 Ma (or older) history. Is that what is meant by “the maximum formation age of schistosity at c. 506 Ma”?
-The metasedimentary rocks in the Tapanappa outcrop were deforming after c. 495 Ma (intrusion of boudinaged dyke/vein), so this foliation is likely not that same as the foliation related to D2 dated by Foden et al. (2006), but may seem to be a younger deformation event overprinting the c. 506 Ma D2. It would be good to explain earlier in the Discussion how the structures in the examined outcrops relate to the regional D1, D2, D3, etc. to understand better these geochronological relationships.
-How did intrusion of the second vein in the Tapanappa outrcrop reset the mica in the boudinaged vein? Is there evidence of heating or fluid percolation in the rocks? Did it effect mica in the host rock as well? It would be good to see evidence of this in thin section in the boudinaged vein.
-What is the evidence for regional fluid flow events in the outcrops?
-I am surprised that garnet would form at such low T conditions, i.e., not exceeding the ~280 °C closure temperature calculated for the mica in the sample. Is there evidence in the literature for such low temperature garnet forming from hydrothermal fluids? Are the closure temperatures perhaps underestimated by the Arrhenius plots? Or perhaps garnet is not that late in the overall metamorphic sequence of the outcrop?
-What brackets the growth of garnet to c. 493-486 Ma?
Figures
-In general, the figures (especially of the map and outcrops) are a bit small and blurry, but perhaps this is a consequence of the file upload. Please ensure that the final forms are high-quality, as right now the structural relationships in some of the outcrop photos are not entirely visible.
-In Figure 1 there is a cross-section presented that is not presented in the manuscript.
-It would be beneficial to highlight on the overview outcrop photos the locations of the close-up images in the subsequent figures.
-Showing the Ca/K and Cl/K ratios with the 40Ar/39Ar spectra would be beneficial.
-It would be useful to see all Arrhenius plots in the main text.
-References for Figures 10, 11, and 12 are needed.
-The construction of Figure 11/12 is a bit confusing. The acronyms should be explained in the figure captions. Perhaps Figure 12 is only needed in the manuscript to summarize the sequences of events as both figures provide a lot of repetition of the same information.
-The same external constraints are used for the Backstairs Passage and Tapanappa formations in Figures 11 and 12 (and in Figure 10 for that matter), yet it is not evident in the geological setting, nor in the discussion that the structural and metamorphic histories of the two formations are shared. Rather, it seems they have distinct tectonic histories, as highlighted by the data compilation in Figure 10. I would be surprised if they experienced the exact same histories in this case, but if so, it needs to be clearly presented earlier in the manuscript.
I hope that my comments are useful for improving the study. I think that there is potential for a very nice contribution for understanding the evolution of the Delamerian orogeny, but more work is required to properly scrutinize the data and support the interpretations/conclusions.
Regards,
Christopher J. Barnes
Citation: https://doi.org/10.5194/egusphere-2023-1839-RC2 -
EC1: 'Editor Comment on egusphere-2023-1839', Brenhin Keller, 16 Apr 2024
The author has requested that this manuscript be withdrawn
Citation: https://doi.org/10.5194/egusphere-2023-1839-EC1
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Naina Goswami
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