Characteristics of ∼40 Ma Mafic Rocks in North Tibet and Their Tectonic Implications

Abstract Several magmatic belts are present in the Tibetan Plateau. Among these, the Qiangtang magmatic belt in North Tibet remains elusive. In particular, the petrogenesis, magma source, and tectonic implications of the ultramafic to mafic rocks in the North Qiangtang area are underinvestigated due to their sparsity and remoteness. Here we report the age and geochemistry of newly discovered mafic rocks in North Qiangtang. Zircon U‐Pb analysis on gabbros revealed a crystallization age of ca. 40 Ma with inherited zircon ages of 803 Ma and 425 Ma. The geochemical characteristics of the 40 Ma zircons show that they are MORB‐type gabbro with positive Sr‐Nd‐Hf isotopic ratios (0.0 ∼ +6.4). Geochemical data indicate that the mafic rocks originated from the asthenosphere at a depth of over 250 km. The primary magma of the gabbros experienced fractional crystallization and ancient continental contamination as it exploited trans‐lithospheric faults during its upwelling process. The gabbros in the North Qiangtang have distinct geochemical and isotopic features from the intermediate‐felsic rocks in the same magmatic belt, which is different from the relationship between mafic and intermediate‐felsic rocks in South Tibet. We interpret that the magmatic activities in North Tibet are derived from mantle delamination processes assisted by some trans‐lithospheric faults in North Tibet. Moreover, the inherited zircon ages of 802 Ma and 426 Ma support the previous interpretation that the North Qiangtang terrane is of Cathaysian affinity, different from the South Qiangtang terrane of Gondwana affinity.