A structural–metamorphic study of the Gubaoquan eclogites and enveloping rock units in the Beishan Orogenic Collage, NW China, with emphasis on the structural evolution, nature of juxtaposition and exhumation

The Gubaoquan area is located in the southern part of the Palaeozoic Beishan Orogenic Collage of the Central Asian Orogenic Belt in northwest China. Structural analysis of the Gubaoquan area has documented multiple phases of deformation, which challenge previous tectonic models and suggest that the eclogites formed through in-situ metamorphism. The study area is a highly deformed belt of metamorphic tectonites, intruded by syn-tectonic granitoids. The belt largely consists of D3 fabrics that developed after peak metamorphism, with only local preservation of D1 eclogite-facies fabrics in the cores of mafic boudins. Fabrics associated with D1–D4 document a clockwise metamorphic trajectory, characterised by eclogite–facies metamorphism (D1) followed by a large decrease in pressure and small decrease in temperature (D2), high-temperature low-pressure conditions (D3) and greenschist-facies metamorphism (D4). D2–D4 is associated with widespread N–S shortening and local extension. We propose that the metamorphic tectonite belt hosting the Gubaoquan eclogite represents Meso- to Neo-Proterozoic crust that underwent Ordovician–Silurian, north-directed subduction to various depths. The eclogite and its host were detached from the down-going slab, probably due to slab break-off and leading to extensive syn-tectonic magmatism. Exhumation probably occurred in an extruding-wedge-type setting. Convergence continued until the emplacement of the Silurian–Devonian granitoids.