Geochemistry and Mineralogy of Peridotites and Chromitites from Zhaheba Ophiolite Complex, Eastern Junggar, NW China: Implications for the Tectonic Environment and Genesis

Round 1

Reviewer 1 Report

This is a good paper to represent an ophiolite complex located the north-westernmost part in China where is pretty far to access by foreign researchers. They showed the results of geochemistry and mineralogy of peridotites and chromitites with figures in good forms and discussed the petrogenesis as a two-state evolution from MOR to SSZ setting. I enjoyed reading through the manuscript. This is a good contribution for the journal.

However, before accepting this submission, there are two issues to be considered in their discussions.

1. They should discuss about the possibility of a back-arc origin. Recently, petrologies of ocean-floor peridotites have been published. I wonder if the authors could compare their results with the ocean-floor peridotites from Parece Vela Basin, Shikoku Basin and Westernmost Mariana Trench etc.

2. Concerning to calculated equilibrium temperature based on the olivine-spinel thermometry, I do not agree that the temperature values they calculated are those of magma systems for chromites in peridotites. It should preserve the lowest equilibrium temperatures during cooling (e.g., Fig. 9 of Satsukawa and Michibayashi, 2013 Lithos). This must be a critical error. Since the temperature values are not so important in their discussion, they should modify interpretations of temperature values. 

Author Response

Response 1: The Zhaheba-Armantai ophiolite is likely the product of back-arc basin caused by the southward subduction of the palaeo-Asia Ocean [42,43] or in oceanic island setting and showed SSZ affinities according to the volcanic rocks with alkaline signature [41,44], as well as ocean-floor peridotite in MOR setting [45]. Notwithstang the above studies, according to the results of conodont, radiolarian in chert [32], peridotite Sm-Nd dating (479±27Ma) [55], sphene U-Pb dating in gabbro, zircon U-Pb dating in layered gabbro (489±4Ma) [35], zircon U-Pb dating in cumulate gabbro (485.8±2.5Ma) [40, 41], Zhaheba ophiolite is a typical ocean relic and dated at about 495~460Ma. Our research are focused on the ophiolitic peridotites and their related chromitite.

From the Late Ordovician to Silurian, Zhaheba initial oceanic arc developed, and from Devonian to middle Carboniferous, the Dulate arc formed to the north of the Zhaheba–Aermantai ophiolite [41]. As for the back-arc, In the Early Carboniferous, continuous subduction of Paleo-Asian Ocean resulted in back-arc spreading and the formation of a back-arc basin [52]. The northern Erqis ophiolites, with zircon SHRIMP U–Pb ages of 372 ± 19 Ma and 352 ± 4 Ma, might represent the formation of a back-arc basin in the Zhaheba area during the Late Devonian–Early Carboniferous [43]. This subduction and spreading can be compared with the Parece Vela Basin, Shikoku Basin and Westernmost Mariana Trench etc.. But this back-arc origin and related petrology are not relevant to this study.

Response 2: We have modified the interpretations of temperature values. The calculated temperature values may not be those of magma systems for chromites in peridotites, as peridotite may preserve the lowest equilibrium temperatures during cooling [91] or the equilibrium temperatures may represent reequilibration during the postmagmatic stage [92].

Reviewer 2 Report

General comments:

The study contains interesting results worth to publish after some minor corrections. Chapter 4 (Sample descriptions and analytical techniques) should precede chapter 3 as this latter belongs to the observed results already. However, it should be extended as well. Sampling strategy should be introduced (with some details like number of taken and analysed samples) to justify the attribute "representative". It should be indicated if (as it looks like in Supplement 1) analyses taken from other sources, made not necessarily with the same method as in this study, were also used for interpretation. It also looks like that not the same samples were used for different types of analyses. Most importantly, the sampling sites should be given to ensure reproducibility. I suggest to include a table showing coordinates of the sampling sites (supplemented also with identifiers shown on the map of Fig. 1c), the codes of samples taken from these sites, and indicating the type of mineralogical and chemical tests carried out on these samples or subsamples.

Specific comments:

Abstract: Please do not compile the abstract by copying sentences from the conclusions in the same form! Try to summarize the essential points instead.

Figure 1b: Names used in Chapter 2 should be clearly shown (Eastern Junggar Terrane, Erqis Fault, Kalamaili Fault).

Figure 1c: Two profile traces are indicated, but no profile is shown in this article, although at least one could be useful. Misspelling in the legends has to be corrected ("Herzolite", "Quarts"); "predicted fault" is rather inferred fault. The dark brown color is missing from the legends. If tuff & andesite is the cover as in the text, it should stand at the top of the legends. The legends should separate the rocks belonging to Zhaheba Ophiolite from rocks of other genetic units.

Rows 116-117: "diabase dyke and interlayered carbonate cut through" - did you mean carbonatite, or veins? I cannot imagine sedimentary carbonate rocks within a dyke cutting through something.

Rows 119-121: This sentence is incomplete and cannot be understood. Maybe the ophiolite and sedimentary rocks in the basement are overthrust by a nappe comprising Devonian volcanics?

Rows 129-130: several small scale bodies of massive chromitites?

Row 133: "petrography characters" - petrographic characteristics

Rows 134-135: The first sentence has to be reformulated, it is not clear.

Figure 2: Caption 'a' and 'd' are missing.

Row 150: "cumulate structural amphibolites" - This is something which neither can be seen on the image nor is it explained in the text. Did you mean cumulate gabbro protolith altered to amphibolite?

Subchapters 3.1,2,3: Dunite and its alteration should be also introduced in a separate paragraph.

Rows 170-171: The first sentence is incomplete (occurs as what?)

Row 184: Before making a difference between properties of chromian spinel crystals, those of the grains in the massive chromitite also should be described.

Figure 3: Is "Mag" magnetite?

Row 253: MgO content

Rows 259-260: Enrichment has to be compared to a basis (relatively to what?)

Rows 275-276: Orthopyroxene data as well. This sentence should precede the subchapter caption as it refers to all following parts of 5.2

Row 297: Table 1 does not exist: Supplement 2

Rows 310-311: The first sentence is superfluous, it was stated at the beginning of 5.2 already.

Row 337: The chapter caption should be preceded by Figure 8.

Rows 415-419: This is a part of a figure caption (of Fig. 10? It does not perfectly match) embedded in the text instead.

Row 444: "relevant" - related?

Rows 513-517: The last sentence of the conclusions should be reformulated; the very last part about two-stage evolution should be separated in a closing sentence, as it is based not only on the calculated melt composition but on all of the described results.

There are several mistypes and grammar errors, which I do not list extensively (e.g. "dissminated" instead of disseminated at several positions), and even if these do not hamper understanding the text, nevertheless, all should be corrected. Rock names shouldn't be used in plural regularly. Acronyms like SSZ and BON used in the abstract and later as well should be resolved at the first mention as MOR is resolved already. Unnecessary acronyms like EJT which is used for a name which is written out completely in all 5 times should be eliminated. However, the name appears in the form "East", "Easter" and "Eastern" Junggar Terrane as well; it should be uniformized. The unit names in English should be written with capitals, also the rock names and words like terrane if these are parts of the names. Supplement tables should be also edited and the errors corrected (e.g. eliminate Chinese characters from Supplement 3) - maybe it would be better to use a single xlsx file with 5 sheets for each supplement table.

 

Author Response

Response 1: 

The name of Chapter 4 was modified with “Analytical techniques” and the paragraph about sample descriptions was inserted ahead of subchapter 3.1.

Response 2:

We have labeled the analysed samples in figure1c. In Supplement (sheet 1), data 11-13 are from [41]; 19-22 are from [104]；23-26 are from[105], other data are from this study.

Response 3:

We have modified the abstract by summarizing the essential points instead of copying the conclusions.

Response 4:

We have shown the location of “Eastern Junggar Terrane (EJT), Erqis Fault and Kalamaili Fault” in Figure 1b. The Eastern Junggar Terrane (EJT) contains Dulate Arc and Yemaquan Arc.

Response 5: 

We have deleted the profile traces, for the samples are not taken from the profiles.

Response 6:  

The misspellings ("Herzolite", "Quarts") are corrected with "Lherzolite, Quartz " and the “predicted fault" is replaced by “inferred fault” in the legends of Figure 1c.

The legend for the dark brown color is same as that of “the Middle Devonian tuff,andesite”. We have changed the legends sequence of the cover rocks and the ophiolitic rocks.

Response7: 

The interlayered carbonates are carbonatized–silicified–serpentinized and ophiolite-related, named listvenite or ophicarbonate, (Buisson and Leblanc,1985). The carbonates mainly compose of magnesite, quartz, dolomite, calcite, serpentine, chromite, fuchsite, occasionally ankerite, albite and intruding the serpentinized peridotites typifie listvenite and reflects carbonation of peridotite.

 Response 8:

Rows 119-121: Affected by thrust nappe structureor erupting  volcanic rocks in late period, named early Devonian Tuoranggekuduke Formation and middle Devonian Beitashan Formation along the northeast margin of the ophiolite, the ophiolite is overthrust and strongly foliated.

Response 9:

129-130: The small scale massive chromitites bodies are brecciated and massive chromitites occured in the carbonated dunite (Figure2 g, h).

Response 10:

Row 133: We have corrected "petrography characters" with “petrographic characteristics”.

 Response 11:

Rows 134-135: We have reformulated the first sentence with “As the major part of the ophiolite, the peridotites are orientated nearly in NNW-SSE trending and show ~7 km length and 1–2 km width”.

 Response 12:

Figure 2 a. Harzburgites are intruded by ophicarbonate; d, e. Serpentinized dunites.

 Response 13:

Row 150: "cumulate structural amphibolites" are “cumulate structural diopsidites” which can be seen the diopside with cumulate texture in the microphotographs.

 Response 14:

We have added subchapter “3.3 Dunite” as a separate paragraph ahead of the subchapter “Chromitities”.

 Response 15:

Rows 170-171: The first sentence is modified with “Lherzolite occurs as ribbon”.

 Response 16:

Row 184: The massive ores contain up to 80 vol % chromite. The chromite grains in the massive chromitite show polygonal or angular crystal appearances and pull-apart fractures, cataclastic textures.

 Response 17:

Figure 3: The "Mag" is magnetite.

 Response 18:

Row 253: We have added “content” after MgO.

 Response 19:

Rows 259-260: Relative to primitive mantle-normalized trace element patterns, peridotites contain enriched levels of large-ion lithophile elements (LILE) (like Cs), high field strength elements (HFSE) (U, Zr, Hf), and Er.

 Response 20:

Rows 275-276: We have removed the sentence ahead of the subchapter 5.2.1 and added “orthopyroxene” in the sentence.

 Response 21:

Row 297: Table 1 has been substituted by Supplement (sheet 2).

 Response 22:

Rows 310-311: The sentence was deleted.

 Response 23:

Row 337: The chapter caption are preceded by Figure 8.

 Response 24:

Rows 415-419: We have removed this paragraph to the caption of Fig. 10.

 Response 25:

Row 444: The "relevant" has been replaced by “related”.

 Response 26:

Rows 513-517: A two-stage evolution model for the chromites was proposed, disseminated chromites first formed in a MOR environment and were then modified by later-stage melts and fluids and formed massive chromites in a SSZ setting during intra-oceanic subduction.

 Response 27:

We have modified the mistypes errors, e.g. " disseminated " instead of “disseminated”. We have unified acronyms like EJT, MORB, SSZ and BON at the first mention. The five supplement tables are merged into a single xlsx file with 5 sheets and we have corrected the errors in the sheet.

Reviewer 3 Report

-in fig. 1c lherzolite instead of herzolite

-Lin 171-173 It is stated that lherzolites contain app.60% olivine, 4-10% opx, and 4-8% cpx, but even if the maximum values of opx and cpx ratios are taken their total is much less than 100%. Rates should br reexamined.

-Chromite and boninite are formed by high degree partial melting of upper mantle residues. If chromite and boninite exist, they must have been formed by high-degree partial melting of peridotites in the SSZ. In this case, shouldn't we also see the SSZ peridotites, which have undergone highly partial melting? However, it is stated in all diagrams that all peridotites were formed in the Abysal environment. How can the absence of SSZ peridotites be explained? Aren't the host rocks in which the chromites are found to be dunite and harzburgite? Do these dunites and harzburgites also belong to the abyssal peridotites? This issue needs clarification.

 

 

Author Response

Response 1: We have modified the "herzolite"with "lherzolite".

Response 2: We have reexamined about ten thin sections of lherzolite under the microscope, the compositions of orthopyroxene and clinopyroxene have wide variation, with 8–20% orthopyroxene, 8–15% clinopyroxene.

Response 3: Although ophiolites are often associated with boninites (Gillis and Banerjee, 2000; Ishikawa et al., 2002), boninite is not identified in the Zhaheba ophiolites but in the neighbouring Saerbulake area in the Dulate arc [46, 48]. We aslo can not find the direct relationships of petrology between the boninites and peridotites or chromites. In the diagrams, all the peridotites (lherzolites and harzburgite) and minerals of linopyroxenes and orthopyroxenes are indeed fall in the field of Abysal. A possible reason is that dunite is scarce in the Zhaheba ophiolites and the association of the only dunite and chromitites is not clear. We can not confirm if the dunite is formed in the SSZ environment or belongs to abyssal peridotite.

Round 2

Reviewer 1 Report

I think that this manuscript is ready for the journal.