Jurassic Igneous Activity in the Yuseong Area on the Southern Margin of the Gyeonggi Massif, Korean Peninsula, and Its Implications for the Tectonic Evolution of Northeast Asia during the Jurassic
Round 1
Reviewer 1 Report
The manuscript presents data on ages, major oxides and trace elements of four Jurassic granites and two diorites collected from the Yuseong area located in the central part of the Korean Peninsula at the boundary between the Okcheon belt and Gyeonggi massif. The choice of the samples is based on previous studies of Jurassic granitoids that show migration of magmatic activities across the area. The authors believe that “the igneous rocks in the boundary between the Okcheon belt and Gyeonggi massif are important for interpreting the geotectonic relationship between Early and Middle Jurassic igneous activities in the Korean Peninsula” (lines 54-55). In addition, they hope to get key information for interpreting Jurassic magmatic evolution in adjacent areas of Northeast China and Japan. They introduce a reader into general understanding of plate tectonics in the area that implies the development of subduction of the paleo-Pacific plate after the Permo-Triassic collision between the North China Craton and South China Craton. In interpreting data obtained, they follow only this hypothesis. In Discussion, they focus at 1) petrogenesis of the Jurassic diorite, 2) petrogenesis of the Jurassic granite, and 3) regional evolution of subduction processes in the Jurassic. Although the idea about the Jurassic paleo-Pacific (Izanagi) plate subduction beneath East Asia discussed already since 1990-th (i.e. Maruyama et al., 1997), now it requires proper arguments. Some authors (i.e. Seton et al. [2012]) believe that about 200 Ma, the Izanagi plate was not subjected to subduction, but in the Jurassic, the Izanagi plate could be pushed under the Amuria. In fact, the authors of the manuscript have no enough data to create and substantiate a new hypothesis and try to confirm previously reported ideas.
Comments are given in the attached manuscript. More comments are bellow.
- Only the development of an active continental margin is evidenced by occurrence of the Early-Middle Jurassic gabbro-granitic magmatism in Korean Peninsula. A well-established subduction zone is usually shown by geochemical zonation of sources with a particular zone of predominating eruptions of andesitic magmas. No evidence on specific sources for subduction-related melts in a frontal part of the assumed migrated magmatic arc is presented (or at least mentioned) in the manuscript.
- On Figures 3-6, analytical results show a single group of compositions that is characteristic of Jurassic igneous rocks in Korean Peninsula in a whole [Kim et al., 2015].
- A shift of a magmatic front in Figure 9, shown from a geochronological data set presented mostly by Kim et al. [2015], was not additionally substantiated by new geochronological data presented by the authors. The presented geochronological data on the Yuseong area are obtained by low-precision LA-MC-ICPMS technique. The mean zircon 238U/206Pb values of dates are calculated assuming their concordant relations with 235U/207Pb ones. These zircon dates should be considered as a rough approximation to the real timing of magmatism. The indicated errors of dates should be confirmed yet by classical zircon dating. Two magmatic stages, designated in the paper from the data obtained (Early Jurassic – ca. 177-178 Ma and Middle Jurassic – 168-169 Ma), designate the age that fall within a magmatic interval of ca. 203–166 Ma defined by the same and another low-precision (SHRIMP) techniques for geochemically uniform Jurassic granites and related rocks from Korean Peninsula [Kim et al., 2015] or 225–166 Ma for similar magmatic rocks from the Ergun Massif in Northeast China [Li et al., 2021]. The latter are spatially related to closing of the Mongol-Okhotsk Bay of the paleo-Pacific. Similar contemporaneous magmatic events, widespread in the whole Asia, cannot be related solely to subduction processes.
- Principles of the rock classification accepted by authors are unclear. On classification diagrams of Figure 3, data on analyzed samples are given together with data on rocks from the YM published by Kee et al. [15] and Kim et al. [18] that are subdivided into groups of gabbro, diorites, and granites. Points of these groups and new samples fall in different fields of the classification diagrams. Names of the rocks do not correspond to those in diagrams. For example, in line 196, the rock is monzodiorite (not diorite).
- Different hypotheses presented in Discussion imply different sources for Jurassic granitoids. The preferential ones have not been determined by authors. Diorites show trace-element signatures of crustal sources (Figures 4-7), but this kind of signatures contradicts to the sentence of lines 284-286 “The Early Jurassic gabbro and diorite in the YM show trace element characteristics similar to those in the Yuseong area, and they are also considered to have formed by melting of enriched mantle sources [15,18]”. On Figure 7e,f, diorite compositions correspond to those of Proterozoic gneisses, i.e. the latter is a potential source for the former. Why is discussed and accepted a hypothesis on melting of pelitic and basaltic materials in a source region? To define crustal or mantle signatures of sources more definitely, isotopic (Sr or Nd) analyses are still required.
- Some minor mistakes are marked in the text. On Figure 7a, Mg#, calculated from oxides, is not correct. In line 275, the ratio is presented as Mg# in molar Mg2+/(Mg2++Fe2+) × 100. This is an accepted designation of the Mg number.
Comments for author File: 
Comments.pdf
Author Response
Thank you for your consideration!
The manuscript is thoroughly revised as considering reviewer's comments.
We responded for your comments as point-by-point.
Please see the attachment file(response for reviewer.docx)
Author Response File: 
Author Response.docx
Reviewer 2 Report
The manuscript reports useful U-Pb zircon age and geochemical data of major and trace elements of the igneous activity which developed in the Jurassic time in the Yuseong area at the southern margin of the Gyeonggi massif in the Korean Peninsula. This is a part, up to now poorly investigated, of the magmatic activity that developed in the framework of the tectonic evolution of Northeast Asia (the Korean Peninsula, North China Craton and Japan) during the Jurassic after the collision event that triggered the subduction of the paleo-Pacific plate beneath the Northeast Asian plate.
The investigated products are diorites and two types of granites (I-type and S-type) that the new U-Pb zircon age places at the time of Middle Jurassic (178-177Ma and 169-168Ma). The numerous tectonic/series discrimination diagrams used to investigate the tectonic setting and the petrogenetic aspects related to origin of the igneous products, give with sufficient detail a plausible reconstruction of the various magmatic phases and mode of magma genesis in the study area, also in relation with the flat-slab segments that began to occur at 175 Ma (although the Sr-Nd-Pb isotope systematics could have been useful for a deeply investigation of sources involved in the genesis of magmas in the area!).
The relative figures and tables are sufficiently clear.
However, although I am not a mother tongue, in my opinion the manuscript suffer the occurrence of several confused sentences that make reading difficult at time. As a consequence, those sentences has to be rewritten to make the concepts clearer. In the uploaded text some of these phrases are highlighted in some cases specific comments for the Authors are reported.
In summary, in my opinion, the topic of the manuscript is of interest for the scientific community and suitable to be published in Minerals, after a minor revision consisting in a partial re-organization of the text.
Comments for author File: Comments.pdf
Author Response
Thank you for your consideration!
Following reviewer's comments in the uploaded pdf file, we re-wrote several confused sentences into more clearly.
We have reviewed whole text in manuscript and corrected several mistake parts into clearly.
Round 2
Reviewer 1 Report
Response to Reviewer Comments
Point 1: Only the development of an active continental margin is evidenced by occurrence of the Early-Middle Jurassic gabbro-granitic magmatism in Korean Peninsula. A well-established subduction zone is usually shown by geochemical zonation of sources with a particular zone of predominating eruptions of andesitic magmas. No evidence on specific sources for subduction-related melts in a frontal part of the assumed migrated magmatic arc is presented (or at least mentioned) in the manuscript.
Response: In the Korean Peninsula, the upper crust and sedimentary rocks on it were disappeared by erosion and/or tectonic uplift. As a result, the volcanic rocks including andesite can not be observed. However, Jurassic diorite which have similar geochemical composition to andesite are well distributed, indicating that there was andesite on the surface during Jurassic. The geochemical characteristic of the Jurassic mafic to felsic plutonic rocks in Korean Peninsula show similar with those of igneous rocks formed at typical volcanic arc tectonic setting as are shown in Fig 4 and 5.
No zoning, however.
Point 2: On Figures 3-6, analytical results show a single group of compositions that is characteristic of Jurassic igneous rocks in Korean Peninsula in a whole [Kim et al., 2015].
Response: The Kim et al. (2015) reported chemical composition of whole Jurassic igneous rocks in Korean Peninsula. Among them, we used only early Jurassic igneous rocks in Fig 3-6 and middle or late Jurassic compositions were not used.
Geochemically, I do not see any grouping implying spatial-temporal variations of arc-type magmatism.
Point 3: A shift of a magmatic front in Figure 9, shown from a geochronological data set presented mostly by Kim et al. [2015], was not additionally substantiated by new geochronological data presented by the authors. The presented geochronological data on the Yuseong area are obtained by low-precision LA-MC-ICPMS technique. The mean zircon 238U/206Pb values of dates are calculated assuming their concordant relations with 235U/207Pb ones. These zircon dates should be considered as a rough approximation to the real timing of magmatism. The indicated errors of dates should be confirmed yet by classical zircon dating. Two magmatic stages, designated in the paper from the data obtained (Early Jurassic – ca. 177-178 Ma and Middle Jurassic – 168-169 Ma), designate the age that fall within a magmatic interval of ca. 203–166 Ma defined by the same and another low-precision (SHRIMP) techniques for geochemically uniform Jurassic granites and related rocks from Korean Peninsula [Kim et al., 2015] or 225–166 Ma for similar magmatic rocks from the Ergun Massif in Northeast China [Li et al., 2021]. The latter are spatially related to closing of the Mongol-Okhotsk Bay of the paleo-Pacific. Similar contemporaneous magmatic events, widespread in the whole Asia, cannot be related solely to subduction processes
Response: The problem mentioned by reviewer should be considered. However, in many previous studies using LA-MC-ICPMS for age dating, the ages obtained using LA-MC-ICPMS were successfully used to constrain the time of igneous activities. And, in this and previous studies, the error is less than 5 Ma for ages of Jurassic igneous rocks in the Korean Peninsula. Therefore we think that there is no serious problem to distinguish 200-190 Ma igneous activity from 180-170 igneous activity, which gives important evidence of shifting of a magmatic front in Figure 9. Furthermore, the spatial distribution tendency of age for the Jurassic igneous rocks in Korean Peninsula is clear as discussed in discussion chapter 5.3; especially, 200-190 Ma igneous rocks occur only in the Yeongnam massif, southeastern part of Korean Peninsula while 180-160 Ma igneous rocks are widely distributed in the Okcheon belt, the Gyeonggi and Nangrim massifs in the Korean Peninsula excluding Yeongnam massif. As the error range of igneous ages in this study was 0.27 Ma-4.4 Ma, there may be no serious problem to distinguish between 178-177 Ma and 168-169 Ma. As reviewer mentioned, Triassic-Jurassic igneous activities were common in the whole Asia, but the subduction related Jurassic igneous activities were reported along the eastern margin of East Asia in many previous studies. Thus, there is no doubt about the existence of Jurassic subduction zone along the eastern margin of Asia including Korea and Japan.
Why do you prefer the interpretation of Jurassic granitoid magmatism as a result of subduction. It is only one possible setting of granitoids on active continental margin. If you accept only this hypothesis, then one of the major unsolved issues in modern geology is why and how subduction begins. Perhaps subduction really began in the Jurassic in the continental margin of Korea. Geochemistry is not convincing. Other arguments should be provided to prove Jurassic subduction process. What kind of argumentation was given in previous works? It is not enough to write “According to previous studies, Early to Middle Jurassic igneous rocks are widely distributed in the Korean Peninsula and formed due to northwestern subduction of the paleo-Pacific plate beneath Northeast Asia [4,16,19,31,38,51] (lines 398-399).
Point 4: Principles of the rock classification accepted by authors are unclear. On classification diagrams of Figure 3, data on analyzed samples are given together with data on rocks from the YM published by Kee et al. [15] and Kim et al. [18] that are subdivided into groups of gabbro, diorites, and granites. Points of these groups and new samples fall in different fields of the classification diagrams. Names of the rocks do not correspond to those in diagrams. For example, in line 196, the rock is monzodiorite (not diorite).
Response: Considering reviewer’s comment, names of igneous rocks are revised based on their chemical composition.
Line 493 “diorite and granitoids” does not sound good. Diorite belongs to granite-like rocks.
Point 5: Different hypotheses presented in Discussion imply different sources for Jurassic granitoids. The preferential ones have not been determined by authors. Diorites show trace-element signatures of crustal sources (Figures 4-7), but this kind of signatures contradicts to the sentence of lines 284-286 “The Early Jurassic gabbro and diorite in the YM show trace element characteristics similar to those in the Yuseong area, and they are also considered to have formed by melting of enriched mantle sources [15,18]”. On Figure 7e,f, diorite compositions correspond to those of Proterozoic gneisses, i.e. the latter is a potential source for the former. Why is discussed and accepted a hypothesis on melting of pelitic and basaltic materials in a source region? To define crustal or mantle signatures of sources more definitely, isotopic (Sr or Nd) analyses are still required.
Response: Because the diorite in the Yuseong area have higher Mg# (47-50) and MgO (4-6 wt %) contents than those in the typical melt from mafic lower crust, mantle can be considered as the origin of diorite instead of crust. The crustal signature in trace element composition of the diorite can be considered as a crustal contamination as shown in Fig. 7e, f. We used most enriched one among data of Paleoproterozoic gneisses for the calculation of contamination degree; to avoid confusion of readers we deleted data of other Paleoproterozoic gneisses in Figure e, f. In this paper, the pelitic and basaltic materials were not considered as a source material of diorite but source material of granite in the Yuseong area.
For the diorite, you have some specific source, but I do not think that the source has mantle signatures.
Point 6: Some minor mistakes are marked in the text. On Figure 7a, Mg#, calculated from oxides, is not correct. In line 275, the ratio is presented as Mg# in molar Mg2+/(Mg2++Fe2+) × 100. This is an accepted designation of the Mg number.
Response: In this paper, Mg# represents a molar ratio of Mg/(Mg+Fe2+) × 100. In Table 1, we corrected mistake; MgO(MgO+FeO) is changed into Mg/(Mg+Fe2+) × 100.
Some comments in the previously attached PDF version of the manuscript should be taken into consideration. On the map of Fig. 1, I asked to show the numbers of sites from which samples were selected. Without this information, Supplementary data is meaningless. In Fig. 4, I pointed out that it makes no sense to plot data points of granitoids on the discriminant diagram initially constrained for basalts (only one sample of gabbro can be plotted). I advise to omit this diagram.
It is clear for me that a setting of Jurassic granitoid magmatism of Korean Peninsula is ambiguous. After minor corrections, this manuscript can be published as similar articles already settled subduction-related interpretation.
Comments for author File: Comments.docx
Author Response
We thank to the reviewer for the helpful comments and suggestions. We revised our manuscript following comments by reviewer. Please see the attachment file of detail responses.
Author Response File: Author Response.docx
