Syn-orogenic tectonomagmatic evolution of the Qilian Orogen: Insights from the Lumanshan gabbro–granite association in the Qilian Block, Northwest China

Abstract

Gabbro–granite associations provide critical geological clues for unraveling crust–mantle interactions, regional tectonics and geodynamic processes. Many Late Ordovician–Early Devonian granitoids occur in the Qilian Block, display high-K calc-alkaline affinities but have less well-constrained geodynamic mechanisms. Here, we report the newly discovered Late Ordovician-Early Devonian Lumanshan (LMS) gabbro–granite association in the Qilian Block that is composed mainly of syenogranites and alkali-feldspar granites, with minor gabbro. Zircon U Pb dating yields crystallization ages of ca. 440 Ma for the gabbros and ca. 450 Ma and 414 Ma for the syenogranite and alkali-feldspar granite, respectively. The gabbros are calc-alkaline series and display enriched mid-ocean ridge basalt ( E -MORB)-like rare earth element (REE) patterns; U, Cs, and Rb enrichments; Nb, Ta, Zr, and Hf depletions; and depleted Sr–Nd–Hf isotope compositions. The results indicate that the gabbros were possibly derived from partial melting of an asthenospheric mantle wedge metasomatized by aqueous fluids. Two granite types within the LMS pluton are peraluminous and have a transitional geochemical affinity between S- and I-type, low Mg# values, and similarly evolved Sr Nd isotope compositions, which overlap with those of Neoproterozoic basement rocks exposed in the study area. The syenogranites show adakitic geochemical characteristics with high La/Yb and Nb/Ta ratios, whereas the alkali-feldspar granites are non-adakitic granites with flat heavy REE patterns and significant Ba, Sr, and Eu depletions, suggesting that they formed at lower and middle crustal levels, respectively. The LMS ca. 450–440 Ma gabbro and syenogranite were emplaced during a back-arc extensional regime associated with roll-back of the northward-subducting South Qilian Ocean slab. Synthesized data from this and previous studies suggest that the final closure of the South Qilian Ocean and continental collision between the Qilian and Qaidam blocks likely occurred at ca. 435 Ma, and the ca. 414 Ma LMS alkali-feldspar granites generated in syn -collisional setting related with exhumation of the subducted continental crust. The LMS alkali-feldspar granites also show decoupled Nd Hf isotopic systems, which should result from significant disequilibrium melting of metasedimentary materials with the apparent abundance of inherited zircons in their source due to a less intense thermal regime. • A gabbro–granite association in the Qilian Block formed in early–middle Paleozoic. • The ca. 472–435 Ma magmatism formed in an extensional setting related to slab roll-back. • The ca. 435–400 Ma magmatism occurred in syn-collisional setting.