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

Abstract

ABSTRACTSerpentinites 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.