Cambrian mid-ocean ridge basalt-like mafic rocks and associated boninites in the Dachadaban ophiolite from the North Qilian Block, northwest China: Petrogenesis and implications for back-arc extension

The early Paleozoic Qilian Orogen is ideal for investigating ancient plate tectonic processes and can be divided into the north, middle, and south Qilian blocks. There is a consensus that North Qilian oceanic lithosphere was subducted northward beneath the Alxa Block. However, the details of this subduction in the North Qilian Ocean (e.g., whether an intra-oceanic back-arc basin was developed) are poorly constrained. In this study, we undertook petrological observations, zircon UPb geochronological and Hf isotopic analyses, and whole–rock and mineral major and trace element, and SrNd isotopic analyses of the Cambrian (513–506 Ma) lower series mid-ocean ridge basalt (MORB)-like mafic rocks and upper series (503–490 Ma) boninites in the Dachadaban ophiolite in the North Qilian Block, northwest China. The lower series mafic rocks can be further divided into two types. Type 1 mafic rocks have uniform SiO₂ contents of 45.1–49.8 wt% and slightly enriched light rare earth element (REE) and relatively flat heavy REE patterns, similar to enriched-type MORBs. The type 2 mafic rocks and boninites have a wider range of SiO₂ contents of 49.3–59.2 and 48.1–56.6 wt%, respectively, and are depleted in light REEs and have relatively flat heavy REE patterns, similar to normal-type MORBs. In primitive-mantle-normalized element diagrams, the type 1 and 2 mafic rocks exhibit no or negligible NbTa depletion, while the boninites exhibit obvious NbTa depletion. The lower series type 1 and 2 mafic rocks have depleted whole-rock Nd (ε_Nd[t] = +2.6 to +6.5) and zircon Hf (ε_Hf[t] = +2.8 to +14.8) isotopic compositions, while the upper series boninites have relatively enriched NdHf (ε_Nd[t] = +2.8 to +4.7; ε_Hf[t] = +3.4 to +13.6) isotopic compositions. We suggest that the lower series type 1 and 2 mafic rocks were formed by partial melting of garnet lherzolite (>70 km depth; type 1) and spinel harzburgite (<70 km; type 2) mantle sources, respectively. The upper series boninites were derived from a refractory harzburgitic mantle source that had been metasomatized by subducted-sediment-derived melts and minor slab-derived fluids. Based on these results and regional geological data, we suggest that the North Qilian Ocean experienced three stages of expansion, in the Precambrian–early Cambrian (550–520 Ma), middle–late Cambrian (513–497 Ma), and Middle–Late Ordovician (458–449 Ma). In addition, intra-oceanic subduction beneath the North Qilian Ocean and subsequent back-arc extension occurred during the middle–late Cambrian (513–490 Ma).