Discovery of Permian N-MORB in the Garze-Litang Ophiolitic Mélange Belt and Its Implications for the Evolutionary Transition of MOR Mantle

The Garze-Litang ophiolitic mélange belt is located at the junction of the Tethyan tectonic domain and the Eurasian Plate. The age, petrogenesis, and tectonic setting of the Garze-Litang Ocean remain controversial, primarily due to the lack of studies on Late Paleozoic ophiolites. This study presents detailed field investigations, as well as petrological, geochemical, Sr-Nd isotopic, and U-Pb geochronological analyses of the newly identified Permian Zhuqing ophiolite. The Zhuqing ophiolite consists of olivine pyroxenite, diabase, basalt, breccia lava, tuff, and siliceous rocks, forming lens- or clast-shaped blocks within a strongly schistose silty slate matrix. Zircon U-Pb dating indicates basalt formed at 266 ± 2 Ma, during the Middle Permian. The volcanic rocks belong to the low-K tholeiitic series, exhibiting typical N-MORB characteristics with LREE depletion, no negative Nb, Ta, or Eu anomalies, and positive ε_Nd（t） values. Geochemical analysis suggests a depleted spinel lherzolite mantle source, with 8%-15% partial melting and fractional crystallization of olivine and clinopyroxene, without contamination from crustal or subducted material. Based on these findings and previous data, a multi-stage evolutionary model for the Garze-Litang Ocean is proposed. During the Carboniferous initial expansion, the mantle source exhibited E-MORB characteristics, influenced by ridge expansion, mantle plume, and Paleo-Tethys Ocean subduction. In the Permian to Early Triassic, the mantle source showed N-MORB characteristics. During the Middle to Late Triassic intra-oceanic subduction, the mantle source was contaminated and exhibited E-MORB characteristics.