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Published September 1, 2022 | Accepted Version + Supplemental Material
Journal Article Open

Whole-rock geochemistry and zircon O-Hf isotope compositions of ca. 2.35 Ga strongly peraluminous granites: Implications for increase in zircon δ¹⁸O values during the Paleoproterozoic

Abstract

Zircon oxygen isotope ratios have been used to trace the incorporation of sedimentary rocks into magmas. The dramatic increase in maximum zircon δ¹⁸O values in the Paleoproterozoic observed in global databases coincides with changes in surface environments (e.g., the rise of subaerial and oxidative weathering), implying a connection between elevated zircon δ¹⁸O and these changes. Zircon δ¹⁸O between 2.5 and 2.2 Ga, however, is relatively under-constrained owing to limited available data in this age range. To augment data from this critical time period and understand potential causes for the elevated zircon δ¹⁸O values, we report U-Pb zircon ages and δ¹⁸O values of zircon, as well as, whole-rock major and trace element geochemistry of Paleoproterozoic strongly peraluminous granites (SPGs) from the southwestern margin of the Yangtze Block (China). Our geochronological data demonstrate that these SPGs crystallized at ∼2.35 Ga and that inherited zircon with ages of 2428–2721 Ma are present in these granites, indicating the source rocks of these granites were deposited, subsequently metamorphosed, and partially melted between 2.43 and 2.35 Ga. Synmagmatic zircon from samples dated in this study have εHf(t) values of −6.4 to −0.9 and high δ¹⁸O values of 7.6–9.9‰, elevated above the maximum value observed in Archean zircon (∼7‰). These granites can be divided into two groups based on whole-rock geochemistry. Both Group 1 and Group 2 granites were derived from a similar high δ¹⁸O, metapelitic source, but were generated by dehydration melting and hydrous melting, respectively. Our results demonstrate that the fine-grained sedimentary rocks from which the SPGs were derived had relatively high δ¹⁸O (as compared to older sedimentary rocks) by 2.43–2.35 Ga. The depositional time interval of the high-δ¹⁸O sedimentary sources for SPGs studied here coincides with the emergence of continental crust above sea level and the Great Oxidation Event. Supporting the findings of previous studies, the contemporaneity of our dataset with these changes in Earth's surface environments suggests that subaerial and potentially oxidative weathering contributed (at least partially) to the elevation of δ¹⁸O of fine-grained sedimentary rocks. Recycling of these high-δ¹⁸O sedimentary rocks into magmas contributed to the dramatic change in δ¹⁸O of magmatic zircon in the earliest Paleoproterozoic. In addition, although this study is focused on a single locality, our results suggest that the abrupt shift observed in global zircon δ¹⁸O data sets likely occurred by 2.35 Ga. Last, a literature compilation of zircon δ¹⁸O data from SPGs suggested that zircon δ¹⁸O values may have also experienced a stepwise increase in the Neoproterozoic to Phanerozoic from 12 to 14‰. The coincidence of these increases in zircon δ¹⁸O values with global oxygenation events suggests that atmospheric oxygenation may have contributed to the increase in δ¹⁸O of sedimentary rocks.

Additional Information

© 2022 Elsevier. Received 30 August 2021, Accepted 21 June 2022, Available online 25 June 2022. We thank Christopher Spencer, Justin Payne and an anonymous reviewer for their detailed comments that helped us to improve the manuscript substantially. Editor Weidong Sun and Jeffrey G. Catalano are thanked for editorial assistance and for constructive comments. This research was supported by National Natural Science Foundation of China (grant numbers 91855206, 41772193, 42172219, 41402167), the Fund from the Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources (grant numbers J1901-20-9, J1901-8, J1901-20-2), and the Key Scientific and Technological Innovation Project of Shandong Province. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Accepted Version - 1-s2.0-S0016703722003179-main.pdf

Supplemental Material - 1-s2.0-S0016703722003179-mmc1.doc

Supplemental Material - 1-s2.0-S0016703722003179-mmc2.doc

Supplemental Material - 1-s2.0-S0016703722003179-mmc3.xls

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Additional details

Created:
August 22, 2023
Modified:
October 24, 2023