Late Palaeozoic two-cycle orogenesis of the Qinling orogenic belt: constraints from the metamorphic P–T evolution and geochronology of the Wuguan Complex and Liuling Group in eastern Qinling, China

The Late Palaeozoic tectonic evolution of the Qinling Orogen is hotly debated. The Wuguan Complex and Liuling Group, situated near the Shangnan-Danfeng suture zone (SDSZ), provide crucial constraints on this evolutionary history. This study presents petrological, mineral chemical, and geochronological analyses of one garnet amphibolite and two metapelitic schists from the Wuguan Complex, and one metapelitic schist from the Liuling Group, to characterize the nature, type, and age of metamorphism, as well as the metamorphic evolution of these two units. The results indicate that the Wuguan Complex underwent two distinct metamorphic events, whereas the Liuling Group recorded only one. The estimated P – T conditions of the earlier metamorphism in the Wuguan Complex range from 10.1 to 11.0 kbar and 628–658°C for the garnet amphibolite, to 4.4–6.2 kbar and 565–602°C for the metapelitic schists. The later metamorphism recorded by the metapelitic schists from both the Wuguan Complex and the Liuling Group yielded P – T conditions of 6.2–7.9 kbar and 560–617°C. Both metamorphic events belong to the amphibolite facies and exhibit clockwise P – T paths, with corresponding geothermal gradients mainly ranging from ~15–30°C/km, characteristic of Barrovian metamorphism. Hornblendes from the garnet amphibolite yielded a ⁴⁰Ar/³⁹Ar plateau age of 335.5 ± 1.1 Ma, while muscovites from three metapelitic schist samples yielded plateau ages of 250.4 ± 0.6 Ma, 250.5 ± 0.5 Ma, and 252.8 ± 0.6 Ma, respectively. Combined with previous research, we propose a two-cycle orogenic model for the Qinling orogenic belt during the Late Palaeozoic. The earlier metamorphism occurred at the end of the Early Carboniferous and was related to the continental subduction of the SQT beneath the NQT following the closure of the Palaeo-Qinling Ocean. The subsequent medium-pressure amphibolite-facies metamorphism was likely associated with the initial collision between the SCB and the SQT during the Late Permian.