Microstructures and deformation mechanisms of the Yuka eclogites in the North Qaidam UHP belt, NW China

This study investigates the relationships between eclogite-facies mineral assemblages and deformation microstructures in the Yuka terrane, part of the North Qaidam ultrahigh-pressure (UHP) metamorphic belt in NW China. The analysis focuses on understanding the mineralogical and microstructural evolution during subduction and exhumation processes. Eclogites from the study area were found to exhibit distinct mineral assemblages and deformation features, reflecting multiple stages of metamorphism.

During prograde metamorphism, garnet, omphacite, and phengite were predominantly deformed by intracrystalline plasticity, indicative of dislocation creep as the primary deformation mechanism. These minerals contributed to the development of well-defined foliations and lineations in the rock, shaped by the alignment of omphacite and phengite grains. Garnet grains often displayed concentric zoning with inclusion-rich cores and inclusion-free rims, recording growth under varying pressure-temperature conditions. Omphacite showed evidence of dynamic recrystallization, highlighting the mechanical and chemical adjustments during progressive subduction.

In contrast, amphibole, which formed through the topotactic replacement of omphacite under fluid-present conditions, exhibited features associated with diffusional flow, such as dissolution-precipitation creep. This retrograde mineral is thought to have crystallized during amphibolite-facies retrogression, marking the exhumation of the eclogites. The lack of significant deformation microstructures in amphibole, such as subgrain boundaries or undulose extinction, supports its formation during a late-stage metamorphic environment.

The Yuka eclogites contain a range of mineral assemblages, including garnet + omphacite, garnet + omphacite + phengite, and garnet + omphacite + phengite + amphibole, reflecting diverse pressure-temperature paths. The compositional variability of these assemblages is tied to the complex geodynamic history of the North Qaidam UHP belt, which underwent subduction, continental collision, and exhumation. This work highlights the significance of integrated petrographic and microstructural studies for deciphering the metamorphic and tectonic evolution of UHP terranes.