New age constraints on the Lan Sang gneiss complex, Thailand, and the timing of activity of the Mae Ping shear zone from in-situ and depth-profile zircon and monazite U-Th-Pb geochronology

The Mae Ping shear zone (MPSZ) is one of four major strike-slip systems in Thailand and a key structural element in some models of the Cenozoic tectonic evolution of Southeast Asia. In northwestern Thailand, the MPSZ exposes a ∼5-km-wide core of intensely deformed high-grade metamorphic and minor undeformed magmatic rocks, the Lan Sang gneiss complex (LSGC). Despite previous efforts to determine the chronology of the LSGC and, ultimately, the MPSZ, the evolution of this gneiss complex remains incompletely resolved. Here we present new zircon and monazite U-(Th-)Pb LA-(MC-)ICP-MS ages and whole-rock geochemical analyses from three peraluminous, high-K (meta)igneous granitic rocks (augen gneiss, sheared granodiorite, undeformed subvolcanic dike) of the LSGC. Our zircon U-Pb dating suggests that the granitic protolith of the augen gneiss intruded metasedimentary basement of Sibumasu as early as ∼235–220 Ma. Zircon rim and monazite U-(Th-)Pb ages from the same sample attest to periods of metamorphic recrystallization and overgrowth in the Late Triassic-Early Jurassic, Early Cretaceous and Eocene. The latter event had started by ∼45 Ma and culminated in partial melting of the basement gneisses and the intrusion of granodiorite magma into the LSGC at ∼35 Ma, contemporaneous with ductile deformation along the MPSZ. A late undeformed subvolcanic dike that crosscuts the mylonitic fabric of the calc-silicate and marble core of the MPSZ, dated in this study at ∼22 Ma, may indicate that ductile shearing had ceased by that time. Thus, our study provides the first evidence for Late Eocene-Early Miocene magmatism in the LSGC.