Tectonic transition from continental subduction to orogenic collapse recorded by ultrahigh-pressure to ultrahigh-temperature metamorphism

Multiple high-temperature (HT) to ultrahigh-temperature (UHT) overprinting events on continental ultrahigh-pressure (UHP) terranes offer valuable insights into the tectonic evolution from subduction to orogenic collapse. However, the tectonic mechanisms driving the metamorphic evolution remain poorly understood. Herein, we present a detailed study integrating petrography, phase equilibria modeling, and zircon dating of a suite of eclogite, granulized eclogite, and mafic granulite samples from the Eastern Kunlun domain in the Kunlun-Qaidam Massif, North Tibet. Our work reveals three episodes of metamorphism: (1) peak-pressure UHP eclogite-facies metamorphism (>2.7–2.9 GPa, 700–740 °C) at ca. 450–430 Ma; (2) post-peak isothermal decompression under fluid/melt-absent conditions with the formation of symplectites, followed by decompression-heating with evolving P–T conditions from 1.3 to 1.2 GPa/750–780 °C to ∼0.9 GPa/840–870 °C at 423–410 Ma; and (3) continued decompressional heating culminating in UHT conditions of 0.7–0.4 GPa/∼980 °C at ca. 410–400 Ma. Combined with regional magmatic data, a four-stage tectonic model is proposed to explain our results: (a) the Proto-Tethys Ocean subduction during 537–450 Ma; (b) continental subduction and crustal thickening during 450–430 Ma; (c) tectonic underplating and crustal thinning during 430–410 Ma resulting from exhumation and lateral flow of the subducted material beneath a broad arc-backarc region after slab breakoff; and (d) post-collisional extension and orogenic collapse during 410–380 Ma triggered by lithosphere delamination and mantle upwelling. Our model implies that the transition from continental subduction to orogenic collapse occurred within 20 Myr, and also portrays that the formation of a unique diffusive cryptic suture zone characterized by tectonic mixtures of subducted materials and upper-plate sequences across the Kunlun-Qaidam Massif.