Multi-stage metamorphism and deformation of the North Qinling Orogenic Belt: Constraints from petrology, geochronology, and structural analysis of the Qinling Complex

Abstract

The Qinling Complex is the only continental nucleus that outcrops in the North Qinling Orogenic Belt (NQOB) and preserves valuable evolutionary information of NQOB during the early Paleozoic. There are many high-pressure (HP)–ultrahigh-pressure (UHP) metamorphic rocks distributed in the northern and southern parts of the Qinling Complex. Whether the central part (namely the Qihe tectonic slab) underwent a similar HP–UHP metamorphism and what the relationship among different tectonic slabs of Qinling Complex is, are the key to revealing the formation and exhumation process of these HP–UHP rocks. Four episodes of deformation (D1–4) have been recognized in the Qinling Complex. The metamorphic pressure‐temperature conditions of syn-D3 amphibolite in the Qihe tectonic slab are limited to 3.3–6.4 Kbar and 479–538 ℃ based on equilibrium modeling, corresponding to an extensional tectonic setting. The U-Pb dating of the amphibolite yielded a protolith age of 714 ± 46 Ma, an eclogite-facies metamorphic age of 514 ± 4 Ma, and two stages of retrograde metamorphism at 462 ± 3 Ma and 418 ± 5 Ma. The parametamorphic rocks of different tectonic slabs have similar detrital zircon age spectra, which is a very different pattern from adjacent areas of the Qinling Complex. The consistency in the protolith ages, peak metamorphic ages, two stages of retrograde metamorphism, and detrital zircon age from different tectonic slabs in the Qinling Complex indicate that HP–UHP metamorphic rocks from Qinling Complex formed during the same tectonic event. Therefore, the Qinling Complex evolved in deep subduction and underwent eclogite facies metamorphism, followed by superimposed deformation structures and retrograde metamorphism during the two stages of exhumation.