the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Jun 2024
Petrogenesis of Early Paleozoic I-type granitoids in the Longshoushan and implications for the tectonic affinity and evolution of the southwestern Alxa Block
Abstract. In the Paleozoic, the Alxa Block was situated between the Central Asian Orogenic Belt and the North Qilian Orogenic Belt, and experienced intense magmatic activity. Thus, the Alxa Block is an important area for understanding the tectonic framework and evolution of these two orogenic belts. However, there has long been debate regarding the tectonic affinity and tectonic evolution of the Longshoushan, located in southwestern margin of the Alxa Block, during the Paleozoic. In this study, we present zircon U-Pb ages, whole-rock major and trace elements, and Hf isotopic data for the granitoids from the east of the Longshoushan to investigate these issues. Bulk-rock analyses show that these granitoids are weakly peraluminous, with high SiO2 and K2O, but low MgO, TFe2O3 and P2O5. They are also characterized by enrichment in LREE and LILE, depletion in HREE and HFSE, and a large range of variation in εHf(t) values (monzogranite: -0.37 to -16.28; K-feldspar granite: 3.53 to -7.74). These geochemical features indicate that these granitoids are highly fractionated I-type granite, which were formed by crust and mantle-derived magma mixing. LA-ICP-MS zircon U-Pb dating constrains that the monzogranite and K-feldspar granite were formed at 440.8 ± 2.1 Ma and 439.4 ± 2.0 Ma, respectively. Combining these results with previous chronological data, the geochronology framework of Paleozoic magmatic events in the Longshoushan is consistent with the North Qilian Orogenic Belt to the south, but significantly differs from other parts of the Alxa Block and the Central Asian Orogenic Belt to the north. This result indicates that the Longshoushan was primarily influenced by the North Qilian Orogenic Belt during the Early Paleozoic. Integrated with previous studies, a three-stage tectonic model is proposed of Early Paleozoic accretion and arc magmatism leading to collision in the Longshoushan. (1) 460–445 Ma: Arc magmatism on an active continental margin with the northward subduction of the North Qilian back-arc basins (NQ bab). (2) 445–435 Ma: Magmatic rocks, dominated by I-type granites, were formed in a continent-continent collision setting. Significant crustal thickening is interpreted to result from compressional stress and/or magmatic additions. (3) 435–410 Ma: The development of abundant A-type granites and mafic dikes in response to intraplate extension, supported by a change in trace element chemistry indicating crustal thinning at this stage. This sequence of events and their timings is similar to other parts of the Central China Orogenic Belt, and requires either a coincidence of several oceanic plates closing at the same time, or an along-strike repetition of the same system.
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RC1: 'Comment on egusphere-2024-1145', Jan Wijbrans, 29 Jul 2024
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Review of: “Petrogenesis of Early Paleozoic I-type granitoids in the Longshoushan and implications for the tectonic affinity and evolution of the southwestern Alxa Block”.
By: R. Zeng, H. Su, M.B. Allen, H. Shi, H. Dua, C. Zhange, and J. Yan.
General comments
This manuscript reports on a geochemical study of two granites from the Longshoushan, part of the Alxa Precambrian basement block that is situated just north of the North Qilian Shan orogenic belt, part of the Central Asian Orogenic Belt.
The study of the two granites yields a comprehensive picture of granite petrogenesis, leading to a hypothesis who these two granites were formed, taking into account the major and trace element concentrations, REEs, and radiogenic isotopes. A detailed evaluation of the zircon Hf- and trace element geochemistry is given.
I feel that this an excellent, state of the art discussion of the topic.
The manuscript is in general well written, and the data presentation and discussion is sufficient detail. Having said this, I am somewhat uncomfortable with the discussion section of the manuscript. This is specified in the specific comments section below.
Specific comments
The second part of this manuscript section 6.4. contains a detailed synthesis of published geochemical data discussing the merits of that wealth of geochemical information for underpinning a tectonic model for the region during the Paleozoic.
in lines 401 – 403, the authors state “In this study, we collected published data for 153 magmatic rock samples of Early Paleozoic age in the Longshoushan, including 8 ultrabasic rock samples, 31 basic rock samples, 45 intermediate rock samples and 88 acidic rock samples.”
The authors come up with a three stage tectonic model which is based on a proposed division in magmatism presented in section 6.1 and summarized in figure 7: with first, second and third stages respectively from 460 -444 Ma, from 444 – 435 Ma, and from 435 – 410 Ma. This division is not further discussed in section 6.1. This discussion only comes in section 6.4.
The discussion in section 6.4 is based exclusively on the published data set. This part of the manuscript is not well integrated with the preceding study of the two granites and their merits for the tectonic interpretation. One would have expected that the outcomes of the geochemical study described in the first part of the manuscript would have been mentioned in section 6.4, where the second stage of the model, the period from 444 – 435 Ma is discussed in lines 455 – 478. Given the fact that the two granites that were studied in detail both had emplacement ages around 440 Ma, their geochemical make up as discussed in sections 6.1 - 6.3, to me seems relevant for the discussion in section 6.4. There is however in section 6.4 no mention of the preceding granite study. And I feel that this is a substantial shortcoming of this discussion section. A better integration of the outcomes of the geochemical study of the two granites in the discussion of the tectonics in section 6.4. is currently missing.
To my mind the discussion section, and in particular the sections 6.1 and 6.4 could do with a substatial revision, including a motivation for the model division into three stages of tectonic and magmatic evolution to section 6.1, and integrating the outcomes of the discussion of section 6.1 to 6.3 better into section 6.4.
Technical comments
Figure 1. In the inset ‘a’ the border between the Alxa block and the North China craton is a Noth-south structure. This North South structure is not shown and cannot be inferred from the detailed map ‘b’, where it seems more appropriate to inferre a NE-SW trending contact between the two units.
Figure 2: inset ‘b’is a blow-up of part of overview map ‘a’, but the location of ‘b’on ‘a’is not well indicated. Please make the link between the two maps more noticeable.
Line 139-140: wavelength dispersive XRF. Isn’t XRF not always wavelength dispersive.
Line 162 – 163. You claim that the samples are in the Calcalkaline series in figure 4b. It seems to me that your data points are ‘straddling’ the Calc-alkalic – alkali-calcic boundary.
Line 179 – 187: what you don’t discuss in this section is the data in fig 5e:the series samples that have no LREE enrichment, which show flat patterns and in a couple of cases even a positive Eu*/Eu peak. Are these really granites? I’d expect such patterns more for diorites or gabbro’s.
Fig 6 b and d, it may be a suggestion to plot the data with the ages going from low to high? Isn’t going to change the outcome of the discussion, but it would make evaluation of the data by eyeballing somewhat simpler.
Line 277: ‘Atype’ to read ‘A-type’
Line 282: ‘SiO2P2O5 ‘ to read ' SiO2-P2O5’.
Line 286 ‘Itype’ to read ‘I-type’
Citation: https://doi.org/10.5194/egusphere-2024-1145-RC1 -
CC1: 'Review of article by Renyu Zeng, Hui Su, Mark B. Allen, Haiyan Shi, Houfa Dua, Chenguang Zhange, and Jie Yan, entitled “Petrogenesis of Early Paleozoic I-type granitoids in the Longshoushan and implications for the tectonic affinity and evolution of the', Adar Glazer, 05 Aug 2024
reply
In their article, the authors investigate the geochemical properties and zircon U-Pb-Hf of granitoids from the Longshoushan area in southwestern Alxa Block to decipher the tectonic affinity and tectonic evolution of the region. By integrating their results with previously published data the authors shed light on how the southwestern Alxa Block relates to its geologic environment and suggest a three-stage, early Paleozoic tectonic model for the region.
The subject tackled in this study is certainly of interest to the geological community, the text runs smoothly, and the figures are informative (although both will benefit from a few modifications). Overall, the manuscript is good and should be published in Solid Earth after minor revision.
Major comments
- Some of the conclusions of this manuscript are based on geochemical analyses for which statistical measures are not provided. For example, linear correlations are sometimes referred as “good” without mentioning the R-squared values. Adding R-squared values would assure the readers that correlations are actually good and improve the reliability of this manuscript. Also, in Figure 13, KDE bandwidth and histogram bin width are not reported.
- Based on the εHf(t)-age pattern of zircons, the authors suggest that “the Longshoushan Complex is most likely the crustal source of the granitoids in this study”. As most of the zircons fall outside the εHf(t)-age field of Longshoushan Complex rocks, more evidence should be provided to support this argument.
- In several cases methods are described outside the ‘Methods’ section making the manuscript a bit complicated and tiring for reading.
- Please avoid the use of acronyms. That would make the article more accessible to the readers.
Minor comments
Line 39: south > southern
Line 40: Add a reference to your map when introducing the study area.
Line 54-55: change ca. to ~ or use one of them throughout the whole manuscript.
Line 105: “with a small amount of gabbro also present” > with subordinate Gabbroic rocks.
Line 109: “The investigated plutons in this study” > In this study we investigated two plutons located…
Line 112: Duplication of “biotite quartz schist”.
Line 147: CJ-1 > GJ-1
Line 153: “has the values of” – values of what? Please state.
Line 154: It would be much more convenient for the reader to omit all these zeros right of the decimal point, e.g. “Plešovice: 0.282482 ± 23”.
Line 185: slight > slightly
Line 191 and in many other cases: use zircons instead of “spots”.
Line 201: “which converts to εHf(t)” > with εHf(t) of
Line 202: what do you mean by “using the weighted mean age”? Each zircon has its crystallization and Tdm age. please clarify.
Line 207: “In this paper, the 206Pb/238U age and 207Pb/206U ages are determined for younger zircons (<1000 Ma) and older grains (>1000 Ma).” – should be moved to methods.
Line 208-211 (and in other cases): referring to spots #1/2/3… is very confusing and not necessary. please consider it again. You can potentially just say “Among them, three zircons are weakly luminescent…”
Line 216: Would be more convenient if zircon Hf data immediately follows zircon U-Pb geochronology. Also, Twelve zircon spots > Twelve zircons
Line 218: “The 176Hf/177Hf ratio of #6 (825 Ma) is 0.281812, which converts to εHf(t) value of -16.07, and TDM2 of 2722.” > One older zircon (825 Ma) has a 176Hf/177Hf ratio of…
Line 234: would be useful to add a Th/U vs. Age diagram when discussing the implications of Th/U values. Also, you state that the ages of zircons from the second group represent the timing of metamorphism. What are their ages? Do their ages correspond to any metamorphism event known in the region?
Line 280: “Thus, the monzogranite and K-feldspar granite are not A-type granites” and are more compatible with being I or S-type granites.
Line 284: Those correlations seem to be very weak. Please provide R-squared values.
Line 293: Also here, I wouldn’t say that these are “clear linear correlations” as they are not so clear. Please provide R-squared values.
Line 311: “The εHf(t) values of the monzogranite” / “while those of the K-feldspar granite” – εHf(t) values are of zircons, not whole rock.
Line 312: “a large range of variation” > a large range of εHf(t)
Line 318: Please provide R-squared values.
Line 320: Add a few words on the Xijing clinopyroxene diorite and Jiling granite and how they relate to the studied plutons. That would make the manuscript more accessible to the international community.
Line 328: mantle-derived and crust-derived > mantle and crust-derived
Line 338: “…Longshoushan Complex, into which the studied plutons are intruded.
Line 340: Actually, most of the zircons from your samples fall outside the εHf(t)-age field of the Longshoushan Complex, so arguing based on εHf(t)-age data that “the Longshoushan Complex is most likely the crustal source of the granitoids in this study” is inaccurate.
Line 356: During the Paleozoic to Mesozoic > During the Paleozoic-Mesozoic.
Line 359: “which includes areas ~2000 km to both the east and west of the Alxa Block” > which extends ~2000 km east and west of…
Line 377: is > are
Line 381: some scholars > various authors
Line 387-389: awkward phrasing. That observation has far-reaching implications for the geology and tectonic evolution of your study area. Please rephrase so it is clearer for the reader.
Section 6.4.2: that section should be modified so it becomes smoother and clearer. Methods should be moved to the ‘Methods’ section or just be cited and the results of your crustal thickness analysis should become more prominent.
Line 425: which statistics?
Line 426: can be divided > can be defined.
Line 441: tectonic background > tectonic setting
Line 457: calc-alkaline granitic magmatism
Figures
The overall quality of the figures should be improved in the final version. Figure captions should be more descriptive.
Figure 1: would be useful to add an inset of the world map for orientation. Please highlight your study area.
Figure 2: add in the figure caption some reference to the geochronology you present. Are these zircon U-Pb ages? Whole rock Rb-Sr? Please describe. Highlight your study area in Figure 2a. ‘Mafic rocks’ appear twice in the legend.
Figure 3: mineral abbreviations and scale bars should be highlighted.
Figure 11: what is the meaning of the red arrow, please describe.
Figure 13: please provide KDE bandwidth and histogram bin width.
Figure 14: add ‘open circles’ in legend.
Figure 15: would be more informative to give a different color to ophiolite belts. Add in legend the black arrows, and highlight the pink cross section paths on map view.
Citation: https://doi.org/10.5194/egusphere-2024-1145-CC1 -
RC2: 'Comment on egusphere-2024-1145', Huan Li, 31 Oct 2024
reply
The Alxa Block is located at the junction of multiple tectonic plates, and its tectono-thermal evolution has long drawn international attention. This manuscript presents new zircon U-Pb ages, whole-rock major and trace elements, and zircon Hf isotopic and trace element data for two Early Paleozoic I-type granitoids in the Longshoushan region of the southwestern Alxa Block. The authors used their new data, and the previously published data on Paleozoic magmatic rocks to attempt to shed light on the tectonic background, crustal thickness variations, and tectonic evolution of the Longshoushan during the Early Paleozoic. They propose a three-stage tectonic evolution model, which includes an active continental margin from 460 to 445 Ma that led to abundant arc magmatism, a continent-continent collision from 445 to 435 Ma resulting in the formation of I-type granites and significant crustal thickening, and intraplate extension from 435 to 410 Ma leading to the formation of abundant A-type granites and crustal thinning. I think the story is clear and well-structured, contributing significantly to the re-establishment of the formation and evolution of the North Qilian orogenic belt and the southern margin of the Alxa Block. In addition, English language is also fine. However, there are still some weaknesses in MS. I recommend that this paper can be accepted by Solid Earth after minor revisions.
The major comments for modification are outlined below.
- Line23: It is no need to display εHf(t) values to two decimal places.
- Line59: The timing of the amalgamation of the Alxa block and North China Craton is in dispute, not unclear. The relationship between the Alxa block and North China have also been discussed by Yuan Wei and Yang zhenyu, 2015 and Li jinyi et al., 2012.
- There are several instances of improper citations, and many important references are missing. Such as line 58, papers by scholars such as Wang Tao and Wu Fuyuan can be referred to; Line 69-73, the authors neglected earlier researchers, who first to make these points, e.g. Shi Xingjun et al., 2014, and Zhang Lei et al., 2023, and these work seems to provide some new ideas for the study of magmatism in the Longshoushan Mountains; Line 78-87, When it comes to the Precambrian magmatism of the Alxa block, the findings of some authors should be considered, e.g. Dan Wei ,2012, Wang zengzhen , 2019, Dong chunyan ,2007,
- Line 110: The size and shape of the pluton have not been described.
- Line 343-345: The geochemical identification of high- fractionated granites relies primarily on trace elements, e.g. Cr, Ni, Co, Sr, Ba, Zr and ratio of Zr/Hf、Nb/Ta and Y/Ho.
- Figure1: The mafic rocks in northern Alxa region are mostly gabbroic not ultramafic ( Fig. 1b), and the research area is not clear in Fig1b.
- Line 401-408: The authors removed a large amount of raw data based on Hu's methodology, can you explain the rationale?
- Whether tectonics or related magmatism of extension have been reported in the region in the third stage? It is lack of evidence for post-collisional environment.
- Some of the references cited in the manuscript are old. Try to updated some latest ones.
Other comments and suggestions:
- Line 55-56: “Therefore, there is still considerable debate regarding the tectonic background and tectonic evolution of the Longshoushan during the Early Paleozoic.” This sentence can be deleted.
- Line54-55: the language need revise.
- Line 134: “exhibited” change to “exhibit”.
- Line 381: “HREE distribution curves” change to “HREE distribution patterns”.
- Line 485: “were developed” change to “intruded”.
- Line 489: “that” change to “than”
Huan Li
Central South University
Citation: https://doi.org/10.5194/egusphere-2024-1145-RC2
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