PROBING COMPOSITION VARIATIONS WITH DEPTH IN THE TILTED GANGDESE BATHOLITH, SOUTHERN TIBET

The Gangdese Batholith in southern Tibet represents the magmatic product of the subduction of the Tethyan plate during the Mesozoic-early Cenozoic and the India-Eurasia collision since ~55 Ma. The Gangdese crust (90-94.5E) is an eastward tilted crustal section in which supracrustal rocks are exposed in the west, and deeper mid-lower crustal rocks crop out in the east. Here, we combine published geochemical data with bedrock pressures to investigate how magmatic compositions of the batholith vary with depth. The bedrock pressures were obtained by spatial interpolation based on barometry-determined values. We divide the crust into an upper (0-4 kbar) and a mid-to-lower (4-8 kbar) section. During the continental arc phase (110-55 Ma), Kernel Density Estimation (KDE) of the bulk crust displays a weak bimodal distribution consisting of two peaks: the mid-lower crust reveals a peak at 54 wt.% SiO₂, compared to a more silicic upper crust with a peak at 67 wt.% SiO₂. During the India-Asia collision phase (55-0 Ma), the bulk crust is more silicic and shows a peak at ~66 wt.% SiO₂, a trend driven by the more silicic upper crust that has a major peak at 67 wt.% SiO₂ as well as some silicic lithologies in the mid-lower crust (peak at ~70 wt.% SiO₂) that contribute to a bimodal distribution. Low silica composition makes up the other peak (~47 wt.% SiO₂). Our results show that during the continental arc phase, the bulk crust SiO₂ distribution resembles the one of the Central Andean continental arcs. The continental collision caused the increased production of evolved magmas in both upper and lower crusts. The bimodal magmatism in the mid-lower crust could be attributed to the break-off of the Tethyan slab and/or the delamination of the Asian mantle, which facilitated the upwelling of the asthenospheric mantle and the subsequent magmatism. Our study shows how subduction and collision can modify the composition of a continental crust. We will compare the Gangdese crust with other crustal sections to better understand the compositional variations in different magmatic orogens.