Paleozoic Tectonic Transition from Arc–Continent Collision to Post-Orogenic Extension in the East Kunlun Orogen: Constraints from Metamorphic Events

Abstract

The tectonic evolution of the trench–arc–back-–arc basin system—encompassing back-arc spreading, subduction, basin closure, and the subsequent transition from arc–continent collision to post-collisional extension—represents a fundamental process in the evolution of the Proto-Tethyan Ocean. The East Kunlun Orogen, a critical segment of the Proto-Tethyan domain, preserves a complete trench–arc–back-arc system, offering a unique opportunity to investigate this tectonic cycle. This study presents an integrated analysis of petrology, mineral thermobarometry, phase equilibrium modeling, and zircon U–Pb geochronology of garnet–sillimanite–biotite gneisses and garnet–biotite migmatitic gneisses from the Central Kunlun Belt to provide metamorphic insights into the Early Paleozoic evolution of the Proto-Tethyan trench–arc–back-arc basin system in East Kunlun. Our results reveal two distinct metamorphic events: garnet–sillimanite–biotite gneisses record a clockwise P–T path with peak conditions of ~763°C and 10.3 kbar, dated to 422–424 Ma for the retrograde age, which constrains the timing of crustal thickening during arc–continent collision. In stark contrast, garnet–biotite migmatitic gneisses document high-temperature, low-pressure conditions (>830°C and ~4.4 kbar) at 402 ± 2 Ma, followed by near-isobaric cooling persisting to at least 369 Ma. We interpret this pronounced thermal pulse and subsequent isobaric cooling as the direct result of lithospheric delamination and asthenospheric upwelling, marking a definitive tectonic switch from compression to extension. Our findings provide robust P–T–t constraints that chronicle a complete orogenic cycle in the East Kunlun Orogen, from Silurian arc–continent collision to Early Devonian delamination-driven orogenic collapse, offering new insights into the Proto-Tethyan evolution.