Using volcaniclastic rocks to constrain sedimentation ages: To what extent are volcanism and sedimentation synchronous?

Volcaniclastic rocks are commonly used to date sedimentary series since absolute ages are routinely obtained from several geochronological methods. In this work, we present five selected geochronological studies (U-Pb on zircon) on late Paleozoic to Mesozoic volcaniclastic series from Asia, with the aim of discussing if absolute ages obtained on volcaniclastic rocks can be used directly to date sedimentation. For all these volcaniclastic series, volcanism and sedimentation are reputedly coeval and zircon grains have been dated to obtained depositional ages. Nevertheless, among the five volcaniclastic series, only two provide U-Pb/zircon ages that are demonstrably representative of the sedimentation ages (Guandao Section, south China, and Luang Prabang Basin, Laos). In the three other series (Wusu Section, northwest China, Chahe and Daxiahiou sections, south China), U-Pb/zircon ages are not suitable to constrain depositional ages. Perhaps more troubling, all the zircon grains collected from a volcanic layer in the Wusu Section are shown to exhibit ages that are much older than the sedimentation age of the deposits in which the volcanic layer is interbedded. These five examples highlight two prerequisites that must be fulfilled in order to date the deposition of volcaniclastic sediments using geochronological methods: (i) at least some of the dated minerals must have crystallized during or just before one of the last eruptions that provided the volcaniclasts, and (ii) volcanic activity and sedimentation must have been coeval. Accurate sedimentation ages from radiometric dates can only be assessed if these prerequisites are fully demonstrated. A reliable method to verify this consists in systematically comparing the maximum depositional ages obtained from a set of samples along a section with their relative position in the section. Indeed, maximum depositional ages getting younger upwards in a section likely demonstrate that the volcaniclasts were produced during sedimentation and contain grains that crystallized immediately before each eruption. In such conditions, orders of magnitude for time scales of reworking can be estimated. They relate to the episodic and different rates of development of the volcanic edifices and fields, competing with the time necessary for erosion, transport and sedimentation.