How faithfully do the geochronological and geochemical signatures of detrital zircon, titanite, rutile and monazite record magmatic and metamorphic events? A case study from the Himalaya and Tibet

Detrital geochronology, generally focused on zircon only, is widely used not only to obtain qualitative information on source areas but also in quantitative provenance analysis aimed at calculating sediment yields and erosion rates under the assumption that age spectra broadly reflect the proportions of areal exposures of source rocks. The principal aim of this article is to put this assumption to test. The Tibetan Plateau and Himalayan belt, in which the age of magmatic and metamorphic events has been accurately determined through decades of geological and geochronological studies, represent an excellent natural laboratory for provenance analysis. In this study, we focus on sand carried by three major Tibetan rivers - including the largest tributaries of the Yarlung Tsangpo draining either the Lhasa block or the Himalaya exclusively - and couple classical petrographic and heavy-mineral analyses with geochronological and geochemical fingerprinting of detrital zircon, monazite, titanite, and rutile as witnesses of episodes of crustal growth in their catchment. Zircon, rutile, monazite, and titanite provide different responses to magmatic and metamorphic events and are highly to moderately durable minerals that can be preserved in ancient sandstones, thus representing precious tracers in provenance analysis. Our results show that largely euhedral zircon and titanite grains in Lhasa River sand reflect the multiple magmatic events that affected the Lhasa block through the Mesozoic and Cenozoic, and together with detrital monazite and rutile highlight post-collisional metamorphic events in the Nyainqentanglha Range. Mostly rounded detrital minerals generated in the Nianchu and Pumchu catchments are instead largely recycled from Tethys Himalayan sedimentary rocks, but young ages of monazite and titanite grains reflect the widespread Miocene thermal event associated with leucogranite intrusions both at the top of the Greater Himalayan metamorphic unit and in Northern Himalayan gneiss domes. The geochemical signature of most rutile grains in all studied samples indicates ultimate provenance from metapelites. Most important, the age spectra of different datable detrital minerals reflect poorly and in very different proportions the areal exposure of source-rock lithologies (and in the Lhasa River catchment also the areal exposure of granitoids of different age). We conclude that, although detrital geochronology does provide essential information in qualitative provenance reconstructions, the age distributions of single minerals cannot be reliably converted in terms of proportional sediment yield, and thus cannot be used to calculate sediment budgets or trace erosion rates unless fertility bias is accurately quantified and taken into due account.