Silurian crustal thickening in the Alxa Block: Implications for closure of the Proto-Tethys

Igneous rocks from the Alxa Block preserve critical records of the Proto-Tethys Ocean's tectonic evolution， yet the early Silurian dynamics remain contentious. This study presents integrated geochemical and geochronological analyses of two granitic intrusions （Luquan and Zhoujiajing） to elucidate the tectonic evolution. Zircon U-Pb dating yields ²⁰⁶Pb/²³⁸U ages of 428-418 Ma with elevated Th/U ratios （> 0.1）， documenting extensive granitic magmatism during the Silurian to earliest Devonian. These granites exhibit high silica contents （> 70 wt.%） and metaluminous to weakly peraluminous characteristics （A/CNK = 0.97-1.10）. Distinctive adakitic signatures are evidenced by elevated Sr concentrations （338-888 ppm）， high Sr/Y （77.2-264） and La/Yb （16.9-140） ratios， coupled with depleted Yb （0.23-0.70 ppm） and Y （2.16-9.13 ppm） contents. Enriched isotopic compositions are revealed by initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7131-0.7146， ε_Nd（t） values of -9.7 to -8.0， and zircon ε_Hf（t） values of -12.5 to -3.0. Our findings demonstrate that the Silurian granites originated through partial melting of meta-igneous sources within thickened lower crust （> 50 km）， which contrasts with previous-thought thinner crust （< 40 km） at Silurian. Notably， the obvious increase in crustal thickness at Silurian likely associated with closure of the Proto-Tethyan oceanic basin which separated the Alxa and Qilian blocks.