Refertilization of serpentinites in the Sulu UHP terrane, Eastern China: constraints from geochronology, mineral composition, geochemistry, and Re–Os isotopes

ABSTRACT

Serpentinites from Junan (JN), Rizhao (RZ), and Rongcheng (RC) in the Sulu ultra-high-pressure (UHP) terrane, China, were analysed for U–Pb zircon geochronology, mineral chemistry, whole-rock major and trace element chemistry (including rare-earth elements (REEs) and platinum-group elements (PGEs)), and Re–Os isotopes, in order to better constrain their petrogenesis and geodynamic process. The serpentinite zircons yield two age groups: 731 ± 10 to 780 ± 10 Ma for relic magmatic zircon cores, which may indicate early crystallization and emplacement of the peridotite in the Yangtze crust, and 209 ± 2 to 218 ± 3 Ma for metamorphic zircon, which coincides with Triassic UHP metamorphism. The spinels in the serpentinites exhibit significant Cr# variation (0.6–0.91) and have undergone multi-stage metamorphism. The serpentinites are characterized by enrichment in incompatible trace elements, low Ni and IPGE concentrations, and high Pd/Ir ratios, and the bulk-rock major elements plot in the ultramafic cumulate region. Their Re and Os concentrations are similar to those of typical orogenic peridotite, but they have high ¹⁸⁷Os/¹⁸⁸Os ratios (0.12433–0.14423). We believe that the serpentinite’s protolith consisted of cumulates from an asthenosphere-derived melt that intruded into the continental crust of the Yangtze craton in the Neoproterozoic. These cumulates were later subducted and metamorphosed during the subduction of the Yangtze craton in the Triassic. The serpentinites underwent melt–rock interactions and fluid enrichment, both prior to and during serpentinization.

KEYWORDS:

• Sulu ultra-high-pressure terrane
• serpentinites
• geochronology
• Re-Os isotope system
• PGEs

Acknowledgements

We appreciate Xiaoqing Zhu, Jun Sun, and Yanfang Xin for their assistance with the fieldwork. We are also grateful to colleagues at the Guiyang Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China, for assistance with the analyses of major elements, trace elements, REEs, PGEs, and Re–Os isotopes and to the Testing Centre of the Shandong Bureau of Chinese Metallurgy and Geology for the electron microprobe and Zircon U–Pb age analyses.

Disclosure statement

No potential conflict of interest was reported by the authors.

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Funding

This work was supported by the Marine geological survey of the 1:250000 Rizhao Sheet and Lianyungang Sheet [grant number GZH201400206].