Subduction of the Proto-Tethys Ocean along the Northern Margin of Gondwana: Evidence from the Middle Cambrian High-Mg Dioritic Pluton in the Northern Lhasa Terrane, Tibet

Late Neoproterozoic and Early Paleozoic igneous events in the Gondwana continental margin remain enigmatic. In this study, we examined U-Pb ages and Hf isotopic compositions of zircon, whole-rock major and trace elements, and Sr-Nd isotope data for diorites from the Laren pluton. Zircon grains from the diorites have U-Pb ages of 509–503 Ma. Diorites are compositionally similar to high-Mg andesites with high SiO2 (54.3–55.8 wt%), MgO (5.08–8.06 wt%), Cr (83–159 ppm), and Ni (31.9–56.4 ppm) contents and Mg# (68–73). They have low FeOT/MgO (0.78–0.98), TiO2, and P2O5 contents and are strongly calc-alkaline. These diorites are rich in Rb, Th, Pb, and Sr in mantle-normalized trace elemental spider diagrams and have chondrite-normalized rare earth element (REE) patterns enriched in light rare earth elements (LREEs). They also have minor Eu anomalies and are extremely depleted in Nb, Ta, and Ti. Geochemically, the Laren pluton resembles arc magmatism. It is also characterized by variable but commonly high whole-rock 87Sr/86Sr(t), with εNd(t) values ranging from –2.70 to 1.84 and zircon εHf(t) values ranging from –0.2 to –1.1. Detailed analyses of these geochemical characteristics indicate that the primary Middle Cambrian diorite was derived from the interaction between the partial melting of the subducted sediment and overlying mantle peridotites. In combination with the regional geology, the Laren dioritic pluton was most likely placed in an Andean-type active margin related to the subduction of the Proto-Tethys Ocean. Therefore, the results can provide direct evidence of sediment melting during the subduction of the Proto-Tethys Ocean and constrain the Cambrian tectonic evolution of the Lhasa Terrane.