The Guomangco ophiolitic mélange is situated in the middle part of the Shiquanhe–Yongzhu–Jiali ophiolitic mélange belt (SYJMB) and possesses all the subunits of a typical Penrose-type ophiolite pseudostratigraphy. The study of the Guomangco ophiolitic mélange is very important for investigating the tectonic evolution of the SYJMB. The mafic rocks of this ophiolitic mélange mainly include diabases, sillite dikes, and basalts. Geochemical analysis shows that these dikes mostly have E-MORB major and trace element signatures; this is the first time that this has been observed in the SYJMB. The basalts have N-MORB and IAB affinities, and the mineral chemistry of harzburgites shows a composition similar to that of SSZ peridotites, indicating that the Guomangco ophiolitic mélange probably originated in a back-arc basin. The Guomangco back-arc basin opened in the Middle Jurassic, which was caused by southward subduction of the Neo-Tethys Ocean in central Tibet. The main spreading of this back-arc basin occurred during the Late Jurassic, and the basalts were formed during this time. With the development of the back-arc basin, the subducted slab gradually retreated, and new mantle convection occurred in the mantle wedge. The recycling may have caused the metasomatized mantle to undergo a high degree of partial melting and to generate E-MORBs in the Early Cretaceous. E-MORB-type dikes probably crystallized from melts produced by about 20%–30% partial melting of a spinel mantle source, which was metasomatized by melts from low-degree partial melting of the subducted slab.