Geochemical Characteristics, Zircon U-Pb Ages and Lu-Hf Isotopes of Pan-African Pegmatites from the Larsemann Hills, Prydz Bay, East Antarctica and Their Tectonic Implications
Laixi Tong
Round 1
Reviewer 1 Report
Comments and Suggestions for Authors
General comments: This article focuses on the detailed study of the Pan-African pegmatites from the Larsemann Hills, East Antarctica, mainly including petrography, major and trace element geochemistry, zircon U-Pb ages and Lu-Hf isotopes of these pegmatites, and finally inferred their possible tectonic setting and formation process. The tectonic nature of Pan-African event in Prydz Bay has been long debated, there are two points of view existing as to the tectonic nature of Pan-African event in the region, namely, either related to a collision event or reflected an intraplate reworking. Because there is no occurrence of Pan-African ophiolite and high-pressure metamorphic rocks (e.g. eclogite or blueshscist) indicating plate subduction and collision, more and more workers believe that Pan-african event actually should be an intraplate reworking event rather than a collision event. However, real Pan-African suture zone might be located in inland Grove mountains area, where Pan-African high-pressure metamorphic rocks have been reported from there. Therefore, just based on the geochemistry and isotopes of pegmatites in the Larsemann Hills, the authors infer that the pegmatites reflect a Pan-African subduction-collision process, and this inference is not supported by above petrological evidence in the study region. Despite the inference, the paper presented important and detailed studies of the pegmatites in the area, and I would like to suggest that the authors undertake a revision for this point so that this important study results can be published. Furthermore, the paper belongs to a very nice research topic, and the text is mostly organized well, the figures are clear, the tables are compiled well, and the conclusion needs to be more convincing. In addition, since the title does not meet with the study contents, I suggest a revision for the title (pls see detail below). Moreover, some sentence expression also needs to be revised (pls see details below).
Consequently, the paper is significant, and I would like to recommend it for a publication in the journal Minerals after a moderate revision.
Further detailed comments:
(1) Title:
Lines 2-4, As mentioned above, I suggest that the title should be revised to “Geochemical characteristics, zircon U-Pb ages and Lu-Hf isotopes of Pan-African pegmatites from the Larsemann Hills, Prydz Bay, East Antarctica and their tectonic implications”.
(2) Abstract:
Line 9: “Gondwana assembly” is not precise here, and the first sentence should be revised to “The Prydz Bay is an important part of the Pan-African high-grade tectonic mobile belt”.
Line 10, “adjacent rocks” should be revised to a geological specialty term “country rocks”, and similar cases in the text should also be changed. In addition, “and the conditions under which they formed by applying” is suggested to change to “and the formation conditions by applying”.
Line 18: “the primary subduction-collision event” should be revised, because the pegmatite dykes are generally related to an extension event according to structural geology theory.
Line 20, “and mantle” is suggested to be deleted, because pegmatites are mostly crustal-derived rather than mantle-derived in the study area according their mineral assemblage such as sillimatite.
Line 22, “Hills of Pan-Africa, the pegmatites formed” is suggested to change to “Hills, the Pan-African pegmatites formed”. In addition, “subduction, collision,” is also suggested to be revised as mentioned above in line 18.
(3) Text:
Lines 29-30: “Greenville” should be changed to “Grenvillian”.
Line 38: “Grenville” should be revised to “Grenvillian”.
Lines 51-52, “the scarcity” and “limited” and “restricted” are not precise, and they should be revised, because Dirks et al (1993) have done detailed descriptions for a series of pegmatite dykes in the study area, please see the details in that reference.
Lines 55-56, “adjacent rocks” should be changed to “country rocks”. This sentence is suggested to revised according to Dirks et al (1993).
Line 58, “measurements” should be changed to “techniques”.
Line 84, “instances” should be changed to “features”.
Line 86, “found” should be changed to “preserved”.
Line 87, “in lens, lentil, and sausage-shaped formations” should be changed to “in lens-, lentil- and sausage-shaped forms”.
Line 91, “perhaps” and “mass” should be deleted, .
Line 93, “formations” should be changed to “types”, “And that” should be deleted.
Line 98, “first phase, regional gneissic deformation (D1), occurred” should be changed to “first phase of regional deformation (D1) occurred”.
Line 99, “Grenville” should be changed to “Grenvillian”, and “Peak metamorphism event (M1)” should be changed to “associated with peak metamorphic event (M1)”. “second phase, D2, resulted” should be changed to “second phase of deformation (D2) resulted”.
Line 100, “third phase, D3, exhibited” should be changed to “third phase of deformation (D3) exhibited”.
Line 105, “Wang Yanbin, 2002” should be changed to “Wang, 2002”.
Lines 331-332, the reference [63] is about oceanic basalts rather than pegmatites, so the geochemical characteristics of oceanic basalts should not be applied to the pegmatites. Therefore, the inference “associated with plate subduction/collision” is not supported, and this point should be revised.
Lines 342-346, likewise, garnet’s stable pressures during the melting of basaltic rocks cannot be applied to the pegmatites, thus the inference for the formation pressure of the pegmatites are problematic! Therefore, this point should also be modified.
Lines 370-372, from the petrographic descriptions and field relations, some pegmatite contains sillimanite and contact with metapelites, indicating a possible origin as a result of partial melting of the metapelites.
Lines 413-421, as mentioned above, the inference of subduction-collision orogeny is problematic, and this inference should be modified.
Lines 433-438, likewise, the inference of subduction-collision orogeny should be modified.
Line 455, “Wang, Wei R Z….” should be changed to “Wang, W R Z, …..”.
Line 493, likewise, the reference format should be revised.
Line 514, the authors are complete, and “Tong, L; Ren, L.” should be added.
Line 529, likewise, the reference format should be revised.
Line 534, likewise, the reference format should be revised.
Lines 553-534, “(Dirks et a, 1993; Fitzsimons , 1997; Grew et al, 2012……”should be deleted.
Line 557, a few reference “(Carson et al, 2010; Grew et al, 2012; Tong et al, 2019)” should be confirmed or deleted.
Line 349, “granite type” should be changed to “granite”.
(4)Figures:
Figure 12, as mentioned above, the inference of subduction-collision orogeny from Pan-African pegmatite dykes in the study area is very problematic, thus the Fig. 12b-d model might be wrong and should be deleted or modified.
Comments on the Quality of English Language
English may be improved by further some polishing.
Author Response
Dear reviewer,
thank you very much for your valuable revision suggestions. Because I am participating in an Antarctic scientific expedition, it is difficult for me to access more information at the moment. I apologize for any incomplete modifications. Based on your feedback, I have made revisions sentence by sentence, and the modified parts have been marked in different colors in the text.
The relevant modification details are as follows:
Lines 2-4, The title was revised to “Geochemical characteristics, zircon U-Pb ages and Lu-Hf isotopes of Pan-African pegmatites from the Larsemann Hills, Prydz Bay, East Antarctica and their tectonic implications”.
Line 9: “The first sentence was revised to “The Prydz Bay is an important part of the Pan-African high-grade tectonic mobile belt”.
Line 10, “adjacent rocks” was revised to a geological specialty term “country rocks”, and similar cases in the text were changed. and” is change to “and the formation conditions by applying”.
Line 18 “and are associated with the primary subduction-collision event” was deleted.
Line20, “and mantle” was deleted.
Line 22, “Hills of Pan-Africa, the pegmatites formed” was change to “Hills, the Pan-African pegmatites formed”. “subduction, collision,” was deleted.
Lines 29-30: “Greenville” was changed to “Grenvillian”.
Lines 51-52: “Due to the scarcity of Pan-African related samples from the Larsemann Hills and the limited available data, our understanding of pegmatites in this region remains restricted.” was replaced by “The region is defined by the presence of numerous coarse-grained, light-colored pegmatite veins that have a composition similar to that of granitic K-feldspar. These pegmatites were formed at different stages during the deformational history and are found either as flat bodies or as separate lenses that generally occupy structurally con-trolled positions, such as at fold hinges, shear zones, and boudin necks [2].”.
Line 54, “have predominantly focused on Pan-African granites,” was deleted.
Lines 55-56: The path was replaced by “It is necessary to further strengthen the comprehensive correlation analysis between pegmatite veins, country rocks, and their tectonic context. Previous studies on the absolute age, and isotopic composition of Pan-African pegma-tites are week.”
Line 58, “measurements” was changed to “techniques”.
Line 84, “instances” was changed to “features”.
Line 86, “found” was changed to “preserved”.
Line 87, “in lens, lentil, and sausage-shaped formations” was changed to “in lens-, lentil- and sausage-shaped forms”.
Line 91, “perhaps” and “mass” were deleted.
Line 93, “formations” was changed to “types”, “And that” was deleted.
Line 98, “first phase, regional gneissic deformation (D1), occurred” was changed to “first phase of regional deformation (D1) occurred”.
Line 99, “Grenville” was changed to “Grenvillian”, and “Peak metamorphism event (M1)” was changed to “associated with peak metamorphic event (M1)”. “second phase, D2, resulted” was changed to “second phase of deformation (D2) resulted”.
Line 100, “third phase, D3, exhibited” was changed to “third phase of deformation (D3) exhibited”.
Line 105, “Wang Yanbin, 2002” was changed to “Wang, 2002”.
Lines 331-332, “The depletion of Nb, Ta, and Ti indicates the geochemical characteristics of magmatic rocks associated with plate subduction/collision [63]” was deleted.
342-346, “Garnet’s stable pressure during the melting of basaltic rocks is estimated to be greater than 1.0-1.2 GPa, and the lower limit of its stability decreases when melting in the intermediate-acid source region [64-65]” was deleted.
Lines 370-372, “from the petrographic descriptions and field relations, some pegmatite contains sillimanite and contact with metapelites, indicating a possible origin as a result of partial melting of the metapelites.” Was added.
Lines 413-421, This erroneous conclusion (the subduction-collision orogeny) has been completely revised. The part was modified with “Combining rock geochemical and zircon characteristics, the Pan-African pegmatites in the Larsemann Hills likely originated from the Pan-African high-grade tectonic mobile belt of the Gondwana paleocontinent (Figure 12a), forming the D2-4 stage pegmatites (562-517 Ma). This was following by lithospheric thinning (Figure 12b). These results provide new insights into the origin of pegmatites in the D2-D4 stage and effectively confirm the presence of a critical Gondwana paleocontinental extension zone near the Prydz Bay.”.
Lines 433-438, The conclusions were modified with “(1) According to geological data, the pegmatites in the Larsemann Hills formed during the Pan-African D2-D4 stage. The D2 pegmatite formed between 546-562 Ma, the D3 Pegmatite around 534 Ma, and the D4 pegmatite between 517-521 Ma. These pegmatites were formed during the Pan-African tectono-thermal event in an environ-ment marked by an extension. It is plausible that a prominent breakup zone may be situated near the Larsemann Hills. (2) The Pan-African period pegmatites discovered in the Larsemann Hills have different sources of material. Pegmatites from the D2-D3 period are considered to have primarily originated from Paleoproterozoic crustal materials. In contrast, pegmatites from the D4-1 stage (~521 Ma) have sources from both Paleo-Mesoproterozoic crustal material sources and mantle material sources. pegmatites from the D4-2 stage (~517 Ma) have sources from originated from the crust layers.”.
Line 455, “Wang, Wei R Z….” was changed to “Wang, W.R.Z.; Zhao, Y.; Wei, C.J.; Daczko, N.R.; Liu, X.C.; Xiao, W.J.; Zhang, Z.Y.”.
Line 493, the reference was replaced by “Grew, E.S.； Edward S.; Carson, C.J.; Christy, A.G.; Maas, R.; Yaxley, G.M.; Boger, S. D.; Fanning, C.M.”.
Line 514, “Tong, L; Ren, L.” was added.
Line 534, the reference was replaced by “Spreitzer, S.K.; Walters, J. B.; Cruz-Uribe, A.; Williams, M.L.; Yates, M. G.; Jercinovic, M.J.; Grew, E. S.; Carson, C. J.”.
Lines 553-534, “(Dirks et a, 1993; Fitzsimons , 1997; Grew et al, 2012……”was deleted.
Line 557, “(Carson et al, 2010; Grew et al, 2012; Tong et al, 2019)” was deleted.
Figure 12, the Fig. 12b-d model was deleted.
Reviewer 2 Report
Comments and Suggestions for Authors
This is an interesting study. A lot of useful data is generated. There are some problems with the correlation between the zircon and whole rock data. But otherwise it is very well written. Some minor comments are below.
146-148: Not needed here. Petrography of only relevant rocks are needed. Do the same for all others.
Explain the high Al2O3 composition in two pegmatite samples.
328-331: Is that not typical subduction zone phenomenon? Just like 331-332?
344-345: Any lower limit?
351-352: Not sure if just the major element data can justify such an inference
411-412: Post collisional extension?
If not granites, what is the source of these pegmatites? Which sediments? Are they exposed? Can you do any melting modelling?
The Hf data is not well interpreted. Please rephrase the section.
Author Response
Dear reviewer,
thank you very much for your valuable revision suggestions. Because I am participating in an Antarctic scientific expedition, it is difficult for me to access more information at the moment. I apologize for any incomplete modifications. Based on your feedback, I have made revisions sentence by sentence, and the modified parts have been marked in different colors in the text.
The relevant modification details are as follows:
146-148: “The surrounding rock is magmatic gneiss, and the mineral exhibit a coarse texture mostly larger than 5 mm, with a medium- to coarse-grained texture and massive structure.” was deleted.
Explain the high Al2O3 composition in two pegmatite samples.
Response: The two samples contain a large amount of sillimanite and tourmaline, resulting with high Al2O3, indicating a possible origin as a result of partial melting of the metapelites.
328-331: The other reviewer holds a view that the reference [63] is about oceanic basalts rather than pegmatites, so the geochemical characteristics of oceanic basalts should not be applied to the pegmatites. Therefore, the inference “associated with plate subduction/collision” is not supported. We agree the view, and the wrong conclusion of the article was revised.
344-345: Plagioclase will disappear if the pressure exceeds 2.0 GPa [66]. The plagioclases in the samples are stable, so we take the 1.5GPa as a reference value.
351-352: Such a conclusion is really unconvincing and we have deleted it from the paragraph.
411-412: “supporting that the Pan-African period of the Prydz belt underwent a subduction, col-lision, tension, and post-collision tectonic cycle.” was deleted.
If not granites, what is the source of these pegmatites? Which sediments? Are they exposed? Can you do any melting modelling?
Response: from the petrographic descriptions and field relations, some pegmatite contains silli-manite and contact with metapelites, indicating a possible origin as a result of partial melting of the metapelites. Relevant conclusions have been added to the article. Sorry, I am weak in establish the pegmatite melting modelling, I will carry out relevant studies in the later stage.
The Hf data is not well interpreted. Please rephrase the section.
Response: “Previous studies have shown that the genesis of magmatic rocks has two modes: (1) partial melting of preexisting crustal material, in which the εHf(t) value of the zircons in magmatic rocks will be lower than that in chondrite; (2) new crustal material or mantle material partially melted, in which the zircon εHf(t) value of magmatic rocks will be higher than that in chondrite [9].” Was added in this section.
