MAGMATIC FABRICS OF THE GANGDESE BATHOLITH, SOUTHERN TIBET

Magmatic fabrics in plutons are foliations and lineations defined by the shape preferred orientations of primary igneous minerals. The presence of magmatic fabrics is indicative of incremental strain related to chamber flow dynamics and/or regional tectonic deformation at the time of crystallization. We conducted a field study of the Gangdese batholith, a magmatic product of Tethyan slab subduction and the subsequent India-Asia collision. Regional bedrock pressure increases longitudinally from 90^o E to 94.5^o E, providing a tilted crustal section to investigate the nature of magmatic fabrics and its change with depth and time. We report magmatic foliation (n = 287) and lineation (n = 54) measurements from 45 dated plutons in the Lhasa to Nyingchi transect. Fabric data are plotted against pluton age, longitude, and emplacement depth to identify spatial and temporal patterns. Our preliminary results show (1) 100-70 Ma plutons with dominantly orogen-parallel fabrics, (2) 70-40 Ma plutons with more orogen-perpendicular fabrics, and (3) <40 Ma plutons with both orogen-parallel and perpendicular fabrics. Magmatic lineations are generally subvertical. We hypothesize these magmatic fabrics reflect incremental stain at the time of pluton emplacement thus can be used to deduce the directions of regional principal stress σ₁ as well as the expected response in crustal deformation. This allows us to constrain the strain field during subduction, continental collision, and post-collision phases, and explore how incremental strain varies with depth.