Relaminated subduction mélange beneath southern Tibet: A key to Lhasa ultrapotassic magmatism

Ultrapotassic rocks are widely considered to originate from a metasomatized mantle source enriched by crustal components; however, mechanisms controlling the crustal inputs remain poorly understood. To address this issue, the petrogenesis of Miocene ultrapotassic volcanics, a vital lithoprobe, from the Lhasa terrane, southern Tibet, is investigated. Their geochemistry shows (1) ultrahigh Th/La ratios, indicating recycling of crustal material at shallow depths (≤80 km), and (2) evolved Sr-Nd isotopic signatures, consistent with contributions from Himalayan sediments and Yarlung Zangbo ophiolite. Phase equilibria modeling of such mixtures at 25 kbar and 900−1100 °C—consistent with xenolith pressure-temperature constraints—produces 10−20 vol% melts resembling the natural ultrapotassic rocks. Integrating geological and geophysical evidence, we propose a tectonic model in which subducted sediments and oceanic crust form a mélange, are relaminated beneath the Tibetan lithosphere, and subsequently melt to generate ultrapotassic magmas. This model provides new insights into crustal recycling in subduction systems in general.