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the Creative Commons Attribution 4.0 License.
| 21 Oct 2024
Silurian syn- and post-collision granitic magmatism in the western section of the North Qinling Orogen: Implications for collisional orogenic processes
Abstract. The Liqiao and Xianping plutons can provide crucial evidence for the collision-orogeny process of the Proto-Tethys Ocean in the western section of the North Qinling Orogen. In this study, we present petrological, zircon U-Pb geochronological, geochemical, and zircon Lu-Hf isotopic data for these plutons. Both the Liqiao and Xianping plutons are characterized as high-K, calc-alkaline, metaluminous to weakly peraluminous granites, with ages of 429 Ma and 421 Ma, respectively. The Liqiao pluton was classified as I-type granite, displaying positive εHf(t) values ranging from -0.1 to +3.4, and high Mg# values from 37.86 to 48.25. We interpret this to indicate that it was generated by the partial melting of juvenile felsic lower crust, with a contribution from mantle-derived material. In contrast, the Xianping pluton exhibited lower Mg# values (20.40 to 35.11) and negative εHf(t) values (-18.0 to -13.9), consistent with the geochemical characteristics of highly fractionated I-type granite. This suggests that the Xianping pluton formed through the partial melting and extensive fractional crystallization of ancient felsic crust. We propose that the Liqiao pluton originated in a syn-collisional setting, while the Xianping pluton formed in a post-collisional environment. Both plutons are products of the collisional orogeny between the Yangtze Block and the North Qinling Orogen, which were associated with the closure of the Wushan-Shangdan Ocean, the northern of the Proto-Tethys Ocean.
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RC1: 'Comment on egusphere-2024-2100', Mark Allen, 23 Oct 2024
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Review of Silurian syn- and post-collision granitic magmatism in the western section of the North Qinling Orogen: Implications for collisional orogenic processes
By Lin et al.
This paper analyses samples from the Liqiao and Xianping plutons in the Qinling orogenic belt. The geochronology and geochemistry studies are good, and make a case for the plutons having similar chemistry - I-type granite, with ages of 429 Ma and 421 Ma respectively. But, at present there is a bit of a “so what?” feel to the paper – the results are not a surprise, and the tectonic interpretation has problems which I’ll cover later in the review.
The regional review is okay, but any and all attempts to summarise the geology of the Qinling are open to criticism simply because we don’t yet have a unified understanding of the range and its evolution.
Have another go at drafting figures 1a and 1b – the boundaries don’t quite match between the two parts. Also, be clearer about the correlations that are intended. There is a broad match of colours between the Qilian/Qaidam/Kunlun regions to the west and the units of the Qinling, but it is not clear what is meant to be correlated and what is not. The Qinling map can be improved to add more of the important tectonic units, such as the Erlangping Unit and the Shangdan Suture Zone – both of these are wide enough to be marked on Figure 1, and they are important in the regional tectonic evolution.
There are few comments or criticisms to make on the geochronology and geochemistry parts of the paper. They are done well in my view. It would be good to have more description of the field relations and contacts. Are the samples foliated in any way? This is not mentioned, so presumably not, but there are hints of grain alignments in figures 3b and 3f.
The tectonic interpretation needs more thought, but I don’t expect full agreement with everything I suggest here.
First, please define what you mean by “syn-collision” and “post-collision” – this is central to the paper as the phrases appear in the title. But, it is not clear what is meant by each term. Also think about “orogeny”. Collisions can last for 10s of millions of years, and orogenies for even longer as they typically include a phase of pre-colllisional, oceanic, subduction. The India-Eurasia and Arabia-Eurasia examples show us that plate convergence can last for 10s of millions of year after initial continental collision. Presumably all the magmatism and deformation associated with this overall convergence is still “syn-collisional”. But, during this time the overriding plate can experience episodes of extension (See Tibet), tectonic escape (Anatolia) and possibly slab break-off, or delamination – all while convergence continues. So, how can we define “post-collisional”? I’ll argue that the term should be kept for processes and events that take place after overall plate convergence has stopped, and should relate to a different tectonic cycle and setting. I think “post-collisional” commonly gets applied to magmatism and deformation that is very much part of the orogeny and continued convergence. See Şengör’s papers for more discussion of these issues, e.g. “Plate tectonics and orogenic research after 25 years: Synopsis of a Tethyan perspective”. In this paper, there are two similar plutons in terms of chemistry and timing (only 8 Myr) apart, and yet one is assigned to a “syn-collisional” setting and the other to a “post-collisional” setting. The older Liqiao pluton is shown in figure 13a and being linked to slab break-off, the younger Xianping pluton is shown in figure 13b as being linked to delamination. Note that there is not independent evidence for either of these popular tectonic processes having taken place at these times. Given that the broadly I-type chemistry of both plutons is typical of Andean-type subduction zones, it is a simpler interpretation to assume that both plutons took place in this tectonic setting, without the need for further complications. There is an obvious objection to this scenario, in that there is evidence for collision that pre-dates both plutons, e.g. UHP metamorphism of continental protoliths in other parts of the North Qinling. But, these events can be accommodated by models where collision of microcontinents along the Proto-Tethyan margin did not terminate subduction. See Allen et al (2023) and Li Sanzhong et al (2018) for alternative scenarios, that are not simply the 2D cross-sections typically adopted by studies of local parts of the Central China Orogenic Belt. There are surely other and better models waiting to be developed.
I don’t fully understand Figure 13. If the labelled accretionary complex in the south relates to the oceanic plate subduction, what is the unlabelled continental tract immediately to its north and to the south of the remnant slab shown in 13a? How and why does this remnant slab disappear in the delamination event shown in 13b? As noted above, there seems to be no independent evidence for either slab break-off or delamination at the times and places shown. Given the simplicity of the chemistry of the I-type granites in the study, why are they not evidence for active oceanic subduction at this time – similar to the widespread magmatism of ~430-420 Ma in many other parts of the Central China Orogenic Belt.
References:
A couple of test references at the start of this section need to be deleted.
“Mark” et al should be “Allen” et al – here and in the main text.
Figures:
Clear and appropriate. The tectonic cartoons need a re-think, as described above.
Citation: https://doi.org/10.5194/egusphere-2024-2100-RC1 -
AC1: 'Reply on RC1', Hao Lin, 04 Nov 2024
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I am deeply grateful to Mark Allen for your thoughtful comments on this article, as well as for the invaluable advice you have shared. Your feedback has been extremely constructive and instrumental in enhancing our work. We have addressed all the points that we found insightful, which will undoubtedly strengthen our paper. Our detailed responses to each of the reviewer's comments and suggestions are included in the attached file.
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AC1: 'Reply on RC1', Hao Lin, 04 Nov 2024
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