Tectono-Thermal Evolution of the Laojunshan Gneiss Dome in Yunnan, China: Constraints from Multi-Mineral Deformation and Composition

Gneiss domes are typical tectonic types related to deep crustal exhumation, and their polyphase superimposed deformation characteristics make them ideal natural laboratories for studying deep crustal exhumation processes. The Laojunshan gneiss dome in Yunnan, China, lies at the junction of the Tethyan and Circum-Pacific tectonic domains, as well as the boundary between the South China Block and Indochina Block. Its tectonic setting, deep crustal tectono-thermal evolution and exhumation history are closely linked to the kinematic evolution of the two blocks, making it a key site for investigating the Tethyan tectonic domain’s spatiotemporal evolution and inter-block interactions. Based on systematic field investigations combined with microstructural analysis, stress field inversion, electron backscatter diffraction (EBSD) analysis, geochemical and geochronological analyses, significant findings on the dome’s exhumation-related tectono-thermal evolution are obtained. The Laojunshan tectonic units comprise a core dominated by high-grade metamorphosed-deformed rocks and granites, an arcuate detachment fault system, and a sedimentary cover. Regional stress field inversion reveals two distinct regimes (compressional and extensional), with the latter predominant and radially distributed, reflecting late exhumation tectonics. EBSD analysis of major exposed minerals indicates the core underwent high-temperature plastic deformation (620–710 °C). Mylonite parameters (fractal dimension, differential stress) in the detachment fault zone reflect transitions between high and medium-high temperature deformation. Epidote EBSD constrains late exhumation P-T conditions to 350–500 °C, which, combined with geochemical data, divides late exhumation into three stages: deep compression, uplift transition and shallow extension. Geochronological data show the Caledonian (445–420 Ma) as the main formation period of granitic gneiss protolith (synchronous with coeval magmatism), core leucogranite emplacement at 416–411 Ma, and metamorphic zircons in plagioclase constrain Indosinian high-temperature metamorphism and shortening deformation to 241–220 Ma. An exhumation model is proposed: the dome initiated with early Caledonian (445–420 Ma) regional extension and magmatism, followed by 420–410 Ma compressional orogeny, crustal thickening and anatexis. Indosinian (241–230 Ma) compression induced thrusting, folding and detachment faults. Yanshanian (144–80 Ma) extension and magmatism accelerated exhumation, and Cenozoic (33–21 Ma) strike-slip faulting drove rapid exhumation to the surface.