Combination of single-grain fission-track chronology and morphological analysis of detrital zircon crystals in provenance studies sources of the Macigno Formation (Apennines, Italy)

Fission track (FT) analyses on unannealed detrital minerals provide a power ful too) both for refining provenance models derived from traditional metho ds and for collecting information about erosion rates of the source area, T heir power is increased if they are coupled with the study of zircon morpho logy, This combination of methods is applied to the Chattian-Aquitanian (25 -23 Ma) Macigno turbidite complex. Basin-fill patterns and petrographical s tudies consistently identify the uplifting western Central Alps as the main source region for the Macigno Formation, Most zircon grains fall into a young age cluster (similar to 40-30 Ma), der ived from a rapidly exhuming crystalline source region with a high cooling rate. Within this cluster, two age subgroups can be distinguished at 30 and 40 Ma, In the younger subgroup, the zircon morphology supports the presenc e of two main populations: (1) from igneous rocks (S-type euhedral zircons) , which appear to be partly derived from airborne tuffs; and (2) from metas edimentary units. In huge volumes of these metamorphic rocks, mica Ar-Ar an d zircon fission-track thermochronometers have been reset, because of high geothermal gradients in the vicinity of the Periadriatic intrusives in mid- Oligocene times. At the present surface of the Alps, zircon FT ages around and slightly less than 30 Ma are reported in the Sesia-Lanzo zone, the Gran Paradise Massif, the Upper Pennine nappes, the Monte Rosa Massif, and the Dent Blanche complex, The older subgroup of the Tertiary zircons (40 Ma) ma y have been supplied by metamorphic and migmatitic rocks affected by an Eoc ene high-temperature phase, A Late Cretaceous age cluster (similar to 70-60 Ma) is related to cooling a fter the main Austroalpine metamorphic event at 110-100 Ma. Most of the rec ently exposed Austroalpine nappe complex displays mica cooling ages and zir con FT ages between 95-70 Ma and 99-55 Ma, respectively, Finally, an ill-defined Jurassic age cluster, with a mean in Late Jurassic times, is related to rift-shoulder heating of the Austroalpine/South-Alpine crystalline basement due to rifting of the Pennine oceanic domain. Present ly, the Silvretta nappe complex, situated at the western termination of the Austroalpine realm, and the South-Alpine basement west of the Canavese Lin e, display similar zircon FT ages. Therefore, a westward continuation of th e Silvretta complex prior to deep Neogene erosion is suggested.