Early Palaeozoic P-T-t-D paths of the eastern Shangdan Shear Zone in Qinling Orogen, central China

Comprehensive field, petrographic, mineral chemical, zircon U-Pb geochronological, phase equilibrium modelling, and microstructural and crystallographic preferred orientation (CPO) analyses of garnet gneisses and garnet amphibolites from the eastern Shangdan Shear Zone (SSZ) in the Qinling Orogen reveal a three-stage Early Paleozoic P-T-t-D evolution. The garnet-bearing lithologies collectively define a clockwise P-T trajectory within the mid- to lower-crust, characterized by progressive burial heating, near-isothermal decompression, and subsequent near-isobaric cooling. The three metamorphic-deformational stages are as follows: Stage 1 (ca. 510–480 Ma) records high- to ultrahigh-pressure metamorphism ( ≥30 kbar, ≥700°C) in mantle-depth slivers triggered by northward subduction of the Shangdan oceanic plate, coeval with high-pressure granulite-facies metamorphism (14–15 kbar, 755–775°C) in the lower crust. A steeply dipping penetrative S1 foliation developed during this stage, consistent with NNE–SSW compressional tectonics. Stage 2 (ca. 480–450 Ma) exhibits rapid exhumation-driven retrograde paths characterized by near-isothermal decompression (14–15 kbar → 5.8–7.6 kbar at 725–758°C), with tectonic quiescence facilitating vertical transport of metamorphic complexes along the SSZ. Stage 3 (ca. 420–400 Ma) marks near-isobaric cooling to amphibolite-facies conditions (4.5–6.8 kbar, 550–620°C) during continental collision between the north and south Qinling belts, where syn-kinematic sinistral shearing was dynamically coupled with retrograde metamorphism under transpressional regimes. This study establishes genetic linkages between discrete metamorphic stages and regional deformation events, reconstructs a coherent Early Palaeozoic metamorphic-deformational chronology, and elucidates the coupling mechanisms between deformation and metamorphism during the evolution of the SSZ.