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.
Zhang et al. (Tue,) studied this question.