The Late Triassic-Jurassic magmatic belt and its implications for the double subduction of the Neo-Tethys Ocean in the southern Lhasa subterrane, Tibet

Abstract

The southern Lhasa subterrane (SL), located between the Yarlung-Tsangpo Suture Zone and the Luobadui-Milashan Fault, preserves a complete Late Triassic-Jurassic magmatic record related to Neo-Tethys Ocean evolution. Therefore, the SL is key to understanding the geodynamic processes occurring during the controversial early-stage evolution of this ocean. Here, we present new zircon U–Pb data and a detailed synthesis of geochronological, geochemical, and Sr-Nd-Pb-Hf isotope data from Late Triassic-Jurassic igneous rocks in the SL to constrain the early-stage evolution of the Neo-Tethys Ocean. The data suggest that these igneous rocks can be divided into two groups. Group 1 magmatism lasted from ca. 243 Ma to 145 Ma, with three stages at ca. 243–200 Ma, 199–165 Ma, and 164–145 Ma, and is characterized by high positive εHf(t) and εNd(t) values and low (206Pb/204Pb)t, (207Pb/204Pb)t, and (208Pb/204Pb)t values. Group 2 magmatism lasted from ca. 225 Ma to 160 Ma, with two stages at ca. 225–190 Ma and 189–160 Ma, and is characterized by lower εHf(t) and εNd(t) values and higher (206Pb/204Pb)t, (207Pb/204Pb)t, and (208Pb/204Pb)t values than Group 1. We suggest that the Triassic-Jurassic magmatic belt in the SL was controlled by double subduction; the Group 1 igneous rocks formed in an intraoceanic arc setting, while the Group 2 igneous rocks formed in a continental margin arc setting. An integrated interpretation of magmatism, ophiolites, sedimentary rocks, and metallogenesis within the SL constrains the geodynamic processes that occurred during the early-stage tectonic evolution of the Neo-Tethys Ocean as follows. (1) During the Middle-Late Triassic, the initiation of intraoceanic subduction between Gondwana and the SL produced the ~243–200 Ma magmatism of Group 1. Subsequently, a new continental margin subduction system formed on the southern margin of the SL, which induced the ~225–190 Ma magmatism of Group 2. (2) With ongoing subduction, slab roll-back of the intraoceanic subduction system resulted in the ~199–165 Ma magmatic flare-up of Group 1. Additionally, slab roll-back of the continental margin subduction system resulted in extension of the overriding plate and formed the ~189–160 Ma magmatic flare-up of Group 2. (3) Finally, the slab roll-back associated with intraoceanic subduction transitioned to flat subduction and produced the 164–145 Ma magmatism of Group 1, and porphyry Cu-Au mineralization developed during the stress transition period. In summary, the double subduction system in the early-stage evolution of the Neo-Tethys Ocean controlled the tectonic, magmatic, and metallogenic events in the SL.