Post-collisional potassic rocks in Western Kunlun, NW Tibet Plateau: Insights into lateral variations in the crust-mantle structure beneath the India-Asia collision zone

Abstract

The collision between India and Asia created the most significant orogenic system of the Cenozoic. Although numerous collision-related igneous rocks formed, many post-collisional rocks were emplaced but have received less attention. Recently, geophysical investigations and field observations have suggested significant lateral variations in crust-mantle structure beneath the collisional zone. However, there have been few attempts so far to reveal the possible relationship between the lateral variations in crust-mantle structure and the widely distributed post-collisional rocks. To reveal the possible linkage, we investigate the Pulu volcanic rocks in Western Kunlun, NW Tibet Plateau, through a combined analysis involving whole-rock geochemistry and Sr-Nd-Pb isotopes, and mineralogy. The similar profiles (e.g., Fo and An values) of olivine, pyroxene, and plagioclase phenocrysts, along with relatively consistent whole-rock isotopic compositions argue against mixing and significant crustal contamination. The sub-continental lithospheric mantle source (SCLM) was enriched by the input of sedimentary materials as indicated by Sr-Nd-Pb isotopic compositions (87Sr/86 Sr(t) < 0.708884, 143Nd/144Nd > 0.512344, and 204Pb/207Pb < 15.6831). The Pulu volcanic rocks in Western Kunlun display high Rb/Sr ratios (> 0.05) and low Ba/Rb ratios (〈23), indicating an amphibole-free phlogopite residual source at a depth of >70 km. In addition, high LREE/HREE (LaN/YbN > 49) and Nb/Ta ratios (> 20), and olivine geothermometer confirm a garnet-bearing mantle source at >970 °C under eclogite-facies conditions. In contrast to other post-collisional rocks formed under low pressure and widely variable temperatures associated with the delamination and thinning of SCLM in the eastern and southern Tibet Plateau, post-collisional rocks in Western Kunlun formed under the high pressure and a narrow range of temperature are attributed to lithospheric fractures at great depths before delamination. Such contrasting P-T conditions of the post-collisional rocks in Western Kunlun and eastern and southern Tibet Plateau record lateral variations in crust-mantle structures beneath the India-Asia collisional zone.