Objective The Dadaoerji region in the South Qilian Orogen hosts medium−sized ophiolite−type chromite deposits. However, the formation age, petrogenesis and tectonic evolution of the Dadaoerji ophiolite suite are still unclear.

Methods Hence, in this study, we conducted systematic LA−ICP−MS zircon U−Pb for pyroxene peridotive dating and whole−rock geochemical studies of both pyroxene peridotite and gabbros within the upper part of this ophiolite suite.

Results Zircon U−Pb dating show ²⁰⁶Pb/²³⁸U spot ages that are concordant with the main concentration range of 471~420 Ma, and also yielded a weighted mean age of 460±15 Ma (MSWD=3.8), suggesting that the timing of formation of the Dadaoerji ophiolite suite could be constrained in the Late Middle Ordovician. These gabbros belong to low potassium tholeiitic series, both pyroxene peridotites and gabbros are enriched in large ion lithophile elements (e.g., Rb, Ba, U, Sr) and depleted in high field strength elements (e.g., Nb, Ta, Zr, Hf), with Th negative anomalies. Moreover, these gabbros also display a near flat and slightly right pattern ((La/Yb)_N = 1.54~2.43) and slight Eu positive anomalies. These together indicate that the magma of gabbros was derived from a depleted spinel Lherzolite mantle via metasomatism of subduction fluids, and evolved by partial melting of 20%~30%. Furthermore, the magma source of gabbros might have underwent crustal contamination. Accordingly, we also infer that the older zircons in pyroxene peridotite likely inherited from magma source.

Conclusions In combination with the regional geological settings, this study suggests that the Dadaoerji ophiolite suite formed under the extensional environment of the back−arc basin caused by the subduction of the North Qaidam Ocean during the late Middle Ordovician. The Dadaoerji ophiolite was also classified as a subduction zone type (SSZ) ophiolite, which provides new evidence for the subduction evolution of the South Qilian Orogen during the early Paleozoic.