Diverse P‐T‐t Paths Reveal High‐Grade Metamorphosed Forearc Complexes in NW China.

Subduction zones provide important constraints on reconstruction of tectonic configurations and convergent geodynamics. The timing and evolution of the subduction zones at the interaction between the Paleo‐Asian and Tethyan Oceans remain ambiguous, casting significant uncertainty on the reconstructions of proto‐Asia and subduction dynamics. Here, we report new petrologic and geochronologic data of a high‐grade metamorphic complex in the Dunhuang area, NW China, together with thermodynamic modeling, geothermobarometry, and second ion mass spectrometry U‐Pb chronology to reveal the complex metamorphic structure and history of the Paleozoic subduction‐exhumation channel at the intersection of the Paleo‐Asian and Tethyan domains. The subduction zone contains a diverse collection of eclogite, high‐pressure (HP) granulites, amphibolites, and metasedimentary rocks with a broad spectrum of P‐T‐t paths, which were buried and exhumed at different depths at ∼463–411 Ma. Our first dating of the oldest (∼463 Ma) HP granulite extends the orogenic period earlier to the Middle Ordovician. The uneven change in subduction gradients from ∼18°C/km to ∼10°C/km illustrates the thermal evolution of the subduction zone from infancy to maturity. A dramatic drop of the gradient at ∼420 Ma suggests a potential short‐term switch of the subduction dynamics from "hot" to "cool," possibly due to plate geometry reorganization in response to slab roll‐back. Our data demonstrate that the Dunhuang Complex was chaotically mixed and juxtaposed at different levels in the subduction‐exhumation channel. The southward younging and increase in depth of the HP metamorphism indicate that the Dunhuang Complex was formed by north‐dipping subduction of the Proto‐Tethyan Ocean from the Early Paleozoic. Plain Language Summary: The exhumed metamorphic complex in the Dunhuang area, NW China, provides primary evidence of how a deep subduction zone was constructed and evolved during the interaction between the Paleo‐Asian and Tethyan Oceans. Knowledge of these tectonic relations is essential to reconstruct the history of accretion of proto‐Asia. Our new petrologic and geochronologic data from the Dunhuang Complex reveal the metamorphic history of Paleozoic subduction‐exhumation. We report the oldest Ordovician (∼463 Ma) high‐pressure granulite and demonstrate that the regional thermal gradient during subduction ranged from ∼18°C/km to ∼10°C/km between ∼463 and ∼411 Ma. These results make the Dunhuang Complex one of the finest natural exposures on Earth that highlights how fast and how much a thermal gradient of subduction can evolve over time. Diverse P‐T‐t paths of these high‐grade metamorphic rocks indicate that the Dunhuang Complex was a Paleozoic accretionary forearc that was produced by km‐scale mixing and tectonic juxtaposition of diverse rocks in a subduction‐exhumation channel during closure of the Tethyan Ocean. Key Points: The T/P gradient of the Dunhuang Complex varied unevenly from ∼18°C/km to ∼10°C/km between the Middle Ordovician and Early DevonianA dramatic drop of the gradient at ∼420 Ma highlights a potential short‐term change in the subduction dynamics in response to slab roll‐backDiverse P‐T‐t paths indicate a Paleozoic accretionary forearc produced by km‐scale mixing and juxtaposition of rocks in a subduction channel [ABSTRACT FROM AUTHOR]