Protolith Characteristics of the Maobei and Qinglongshan High-Al Eclogites: Implications on the Tectonic Architecture of the Yangtze Craton During 600~850 Ma

The Sulu ultrahigh-pressure (UHP) eclogites were formed by northward subduction of the Yangtze Craton beneath the North China Craton during ~220 Ma. Protoliths of most Sulu eclogites are contemporaneous with Neoproterozoic magmatisms (600~850 Ma) in the Yangtze Craton. Therefore, their geochemical characteristics have implications on the tectonic architecture of the Yangtze Craton during 600~850 Ma. In this study, major and trace element abundances and Nd isotope ratios of high-Al eclogites from Maobei and Qinglongshan at southern Sulu UHP terrane were used to constrain their protolith characteristics. The negative ɛNd(220 Ma) values of the Maobei (-2.8~-5.4) and Qinglongshan (-2.6~- 13.4) eclogites have continental affinities. The Maobei eclogites are characterized by high Al2O3 (23.7~29.5%), low trace element contents, positive Sr anomalies and Nb-Ta-Zr-Hf depletions, similar to the Neoproterozoic gabbros in the margins of the Yangtze Craton, for which arc origins have been proposed. In contrast, the Qinglongshan eclogites are distinct from Maobei eclogites for higher trace element contents with 16.5~18.5% Al2O3 and 45.5~49.7% SiO2. These are characteristics of high- alumina basalts that mostly occur in subduction zones. The low Zr/Y (0.83~4) and Zr/Sm (3.6~23.2) ratios of the Maobei and Qinglongshan eclogites further confirm their arc origins. The similarities between the compositions of the studied eclogites and unmetamorphosed mafic rocks indicate limited element mobility during cold and dry continental subduction. Therefore, the geochemical signatures of the Maobei and Qinglongshan high-Al eclogites reflect their protolith characteristics, which are similar to those of modern continental arc magmatisms. These results are consistent with the existence of active continental arcs along the margins of the Yangtze Craton during 600~850 Ma implying that the Yangtze Craton was isolated from other continents by subduction zones.