SEDIMENT PROVENANCE OF THE TRIASSIC VIEUX EMOSSON FORMATION, SOUTHWEST SWITZERLAND: INSIGHTS FROM U-PB DETRITAL ZIRCONS GEOCHRONOLOGY
U-Pb dating (LA ICP-MS) of detrital zircons from two Triassic samples (above and below a prominent erosional surface in the lower unit) and two samples from L. Paleozoic metasedimentary basement units was utilized to further constrain source areas. All samples yielded abundant Paleozoic and Neoproterozoic, and minor amounts of earlier Precambrian ages. Neoproterozoic and E. Paleozoic ages match with the Cadomian and Pan African orogenies and reflect the northern Gondwana margin origin of the Alpine basement blocks. L. Paleozoic ages are from Variscan (Hercynian) metamorphic and igneous basement events. Similar age distributions from the L. Paleozoic and lower Triassic samples suggest similar provenance and/or significant reworking of the metasediments into the Triassic unit. In both, the presence of abundant Silurian and Devonian ages is significant; M. Paleozoic metamorphic or magmatic events are not recognized in Alpine basement. Possible interpretations are that the source rocks for these zircons are either not yet discovered, were eroded away, or were hidden by Alpine tectonism. Alternatively, a northern source area of Avalonian (Caledonian orogeny) basement may have supplied the zircons during the Variscan event and were subsequently reworked into the Triassic. The younger Triassic sample bore no M. Paleozoic or E. Neoproterozoic zircons, few Variscan zircons, but abundant Ordovician ages, indicating a significant change in provenance for the upper sandstones.