Zircon geochemistry of the Xigaze ophiolite in southern Tibet: Insights into the tectonic evolution of the Neotethys Ocean in the Mesozoic

Abstract The petrology and geochemistry of ophiolites provide important records of the tectonic transition from seafloor spreading to oceanic subduction along plate margins. However, it remains intriguing how ophiolites have acquired their crustal signatures at the plate margins. The occurrence of the Xigaze ophiolite in the Yarlung-Zangbo suture zone between the Indian and Asian continents provides an important target for deciphering the formation and evolution of the Neotethys Ocean in southern Tibet. This study has comprehensively investigated the geochemistry of zircon from gabbro and rodingite. Zircon domains in these rocks are categorized into three groups based on their U-Pb ages and trace elements. Group 1 and 2 domains have similar U-Pb ages with an average of 124 ± 1 Ma for their growth. Group 1 domains exhibit steep heavy rare earth element (HREE) patterns, whereas Group 2 domains display flattened HREE patterns. These zircon domains show variable εHf(t) values from 0.1 to 20.6 for the gabbro and from −10.8 to +34.4 for the rodingite, indicating they grew from basaltic magmas with highly heterogeneous Hf isotope compositions. In addition, both Group 1 and Group 2 domains exhibit δ18O values of 4.5‰–5.5‰ for the gabbro and 4.6‰–6.0‰ for the rodingite, mostly falling below the normal mantle zircon δ18O values. Group 3 domains show variably older U-Pb ages of mainly 363–276 Ma, variable εHf(t) values from −11.1 to +14.2, and variable δ18O values from 3.6‰ to 8.6‰, along with steep REE patterns and negative Eu anomalies, suggesting that they are relict zircons of magmatic origin. Taken together, all of these observations suggest that the basaltic magmas would contain the following three components: (1) a highly Lu/Hf fractionated basaltic crust to account for the unusually high εHf(t) values of 20.6–34.4, (2) a high-temperature seawater-hydrothermally altered oceanic crust to account for the low δ18O and variable εHf(t) values, and (3) minor amounts of seafloor sediment to account for some domains with high δ18O and low εHf(t) values. Based on the geochemical compositions of these zircons, we propose that two supracrustal components were incorporated into the source of basaltic magmas during subduction failure of the Neotethys oceanic plate, which would subsequently experience subduction re-initiation upon seafloor re-spreading. This series of processes is associated with the attempted opening and failure of the Neotethys Ocean during the Mesozoic. In this regard, the zircon geochemistry of gabbro and rodingite from the Xigaze ophiolite provides new insights into the evolution of the Neotethys Ocean during a series of tectonic processes from subduction failure through seafloor re-spreading to subduction re-initiation in the Mesozoic.