Silurian A-type and I-type rhyolites from the Huay Kham Formation in the Loei tectonic belt, northwestern Laos: Implications for post-collisional extension after closure of the Proto-Tethys ocean

The timing and geodynamics of the transition from Proto-Tethyan subduction and collision to post-collisional extension in the Indochina Block remain poorly constrained. Silurian felsic magmatism in the Loei tectonic belt records the late-stage evolution of the Proto-Tethys Ocean, yet whether it reflects continued subduction or post-collisional extension remains controversial. Here, we present zircon U-Pb geochronological, Lu-Hf isotopic, and whole-rock geochemical data for rhyolites of the Huay Kham Formation in northwestern Laos. Zircon U-Pb ages of ca. 435–424 Ma, together with published data, define a Silurian magmatic belt extending from northeastern Thailand to northwestern Laos. The Ban Xor A-type rhyolites formed by partial melting of juvenile high-K mafic lower crust, whereas the Sung Thong rhyolites were generated by shallow-level melting of low-K tholeiitic amphibolite. Positive zircon εHf(t) values indicate derivation from isotopically juvenile Meso- to Neoproterozoic crustal sources. The coexistence of these suites suggests lithological and vertical heterogeneity within the crust. Integrated regional zircon geochemical proxies reveal progressive crustal thickening during the Ordovician–earliest Silurian, followed by pronounced Silurian crustal thinning. These observations indicate that the Huay Kham rhyolites formed in a post-collisional extensional setting following closure of the Proto-Tethys Ocean, possibly driven by lithospheric delamination and asthenospheric upwelling. This closure event was broadly coeval with Proto-Tethys closure in southwestern Yunnan and central Qiangtang, suggesting broadly synchronous closure along the northern Gondwanan margin. Its temporal correspondence with the Kwangsian crustal evolution suggests that Kwangsian Orogeny may record an intracontinental response to far-field stresses generated by Proto-Tethyan convergence and collision.