Preprint Article Version 2 Preserved in Portico This version is not peer-reviewed

The Early Silurian Gabbro in the Eastern Kunlun Orogenic Belt, Northeast Tibet: Constraints on the Proto-Tethyan Ocean Closure

Version 1 : Received: 13 August 2020 / Approved: 14 August 2020 / Online: 14 August 2020 (09:02:26 CEST)
Version 2 : Received: 29 August 2020 / Approved: 31 August 2020 / Online: 31 August 2020 (09:57:20 CEST)

A peer-reviewed article of this Preprint also exists.

Zhou, W.; Li, H.; Chang, F.; Lv, X. The Early Silurian Gabbro in the Eastern Kunlun Orogenic Belt, Northeast Tibet: Constraints on the Proto-Tethyan Ocean Closure. Minerals 2020, 10, 794. Zhou, W.; Li, H.; Chang, F.; Lv, X. The Early Silurian Gabbro in the Eastern Kunlun Orogenic Belt, Northeast Tibet: Constraints on the Proto-Tethyan Ocean Closure. Minerals 2020, 10, 794.

Journal reference: Minerals 2020, 10, 794
DOI: 10.3390/min10090794

Abstract

The early Paleozoic is a crucial period in the formation and evolution of the Eastern Kunlun Orogenic Belt (EKOB), and is of great significance for understanding the evolutionary history of the Proto-Tethyan Ocean. This paper presents new petrography, geochemistry, zircon U–Pb dating, and Lu–Hf isotopic research on the Yuejingshan gabbro from the eastern segment of the EKOB. Zircon U–Pb data suggests that the gabbro formed in the Early Silurian (435 ± 2 Ma). All samples have relatively low TiO2 contents (0.45–2.97%), widely varying MgO (6.58–8.41%) and Mg# (58–65) contents, and are rich in large ion lithophile elements (LILE such as Rb, Ba, Th, and U) and light rare earth elements (LREE). This indicates that it has a similar geochemical composition to island arc basalt. The major element features indicate that the formation of this gabbro underwent fractional crystallization of clinopyroxene, olivine, and plagioclase. The depletion of high field strength elements (HFSE, such as Nb, Ta, and Ti), and a slightly positive Hf isotope (with εHf(t) ranging from 1.13 to 2.45) may be related to the partial melting of spinel-bearing peridotite, led by slab fluid metasomatism. The gabbro likely represents magmatic records of the latest period of the early Paleozoic oceanic crust subduction in the Eastern Kunlun. Therefore, the final closure of the Proto-Tethyan Ocean and the beginning of collisional orogeny occurred before the Early Silurian.

Subject Areas

northeast Tibet; Proto-Tethyan Ocean; early Silurian; eastern Kunlun Orogenic Belt; gabbro; zircon U–Pb dating

Comments (1)

Comment 1
Received: 31 August 2020
Commenter: Wenxiao Zhou
Commenter's Conflict of Interests: Author
Comment: The English language level of this manuscript needs some professional revision. I have detected numerous grammatical errors and poor choice of vocabulary. I corrected wherever I could, but this is not the role of a reviewer to do. I would suggest that the authors revise this manuscript with the help of a native English speaker, or seek the assistance of a language editing expert before publication.The manuscript has sent to a language editing expert and ask for the language editing. I was not able to see Fig.3 in the paper due to some file/computer error. This is unfortunate and I hope this could be fixed. In the version I examined all the figures are of a rather poor resolution, and I hope this will not be the format in which they will be published.The fig.3 has been resized and well show in the revised manuscript. My biggest concern and scientific comment on this paper is the seeming discrepancy between the title and what is actually presented in the paper. While the results displayed by the authors could shed light on the Proto-Tethyan evolution and the significance of the EKOB and their gabbros in that respect, the manuscript dives into the geochemical details with very little focus on the grand tectonic setting and the connection between the gabbros and the Proto-Tethys Ocean and its closure are poorly presented in the text. I would strongly suggest that a series of tectonic sketches would be added to the manuscript that will detail the early Paleozoic tectonic setting in the study area and the role of the studied gabbros within it, along with the Proto-Tethys connection.A series of new tectonic sketches has been added in the tectonic discussion section of the manuscripts. The data tables are mostly okay. However, I would like to see a column added to the U-Pb data table that would include the signal in counts-per-second (cps) of 207Pb, that would allow to asses the quality of measurement. For a Proper format of U-Pb and Hf isotopic data tables I would suggest the authors follow the format in Linnemann et al., (2014).A column of counts-per-second (cps) of 207Pb has added to the U-Pb data tables followed the format in Linnemann et al., (2014). Are the reported zircon ages the only zircon grains detected in gabbros? the authors mention zircon inheritance in some points in the text. Why are they not reported in the data tables? inheritance can have significance in deciphering the tectonic history of these gabbroic intrusions.Yes, the reported zircon ages are from the gabbros. The inheritance zircons are analyzed from the CL images, and not detected the U-Pb ages with LA-ICP-MS. The authors claim that the slightly positive εHf(t)suggest they were sourced from the lithospheric mantle. This is unsupported. The Mesoproterozoic Hf-TDM model ages reported by the authors suggest the gabbros result from the Ordovicio-Silurian reworking of juvenile Mesoproterozoic crust. This possibility is not discussed by the authors and it deserves a proper discussion and attention in the text.This part has been rewritten as “,indicating that the gabbros are the product of reworking of Mesoproterozoic(1.16-1.23 Ga) juvenile crust the lithospheric mantle.”Other than the issues presented above all other comments I have are included in the annotated PDF. I recommend publication after a major revision, and after all issues raised here are answered and corrected by the authors.All the comments added in the PDF are corrected as the reviewers required, such as the references missed in the old version. 
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