Constraining deformation timing of orogenic systems by fluid inclusion 40Ar/39Ar geochronology of syn-kinematic tourmaline

Dating discrete deformation events is essential for reconstructing orogenic histories but is often hindered by polyphase tectonism, thermal overprinting, and a lack of suitable geochronometers. This study evaluates the feasibility of applying 40 Ar/ 39 Ar geochronology to fluid inclusions within syn-kinematic tourmaline, a chemically and thermally robust borosilicate. We systematically compared 40 Ar/ 39 Ar stepwise crushing and step-heating on tourmaline from leucogranite dykes and quartz veins from the Chinese Altai, which formed during the Chinese Altai-Junggar arc convergence (termed D3 phase). Stepwise crushing on tourmaline samples oriented parallel to D3 fold axial planes, randomly oriented within syn-D3 leucogranite dykes, and occurring as undeformed crystals in late-D3 quartz veins, yielded 40 Ar/ 39 Ar ages of 322 Ma, 274 Ma, and 261 Ma, respectively, from their primary fluid inclusions. These ages, in line with the regional tectonic framework, tightly constrain the initiation, peak melting, and termination of D3 reworking. In contrast, conventional step-heating of tourmaline grains and crushed tourmaline powders can be compromised by contamination from non-radiogenic argon or by potential mixing with secondary sources, leading to geologically inconsistent results. Our findings validate the 40 Ar/ 39 Ar stepwise crushing of fluid inclusions within syn-kinematic tourmaline as a robust geochronological tool to date deformation time. Compared to accessory mineral U-Pb systems that reflect broader magmatic histories, the 40 Ar/ 39 Ar ages derived from tourmaline fluid inclusions prove to be more sensitive recorders of discrete deformation events. This approach circumvents the problems of excess argon and thermal resetting, offering a direct window into dating fluid-mediated deformation and deciphering the evolution of reworked crustal terranes.