PROVENANCE ANALYSIS OF EOCENE TO MIOCENE SEDIMENTARY STRATA DERIVED FROM THE CENTRAL ALPS - INSIGHTS FROM DETRITAL ZIRCON LASER ABLATION SPLIT STREAM (LASS)-ICP-MS DEPTH-PROFILING (Invited Presentation)

Cenozoic sedimentary strata of the pro-wedge Swiss Molasse and Italian/Adriatic retro-wedge foreland basins record the exhumation of the central Alpine orogen from ~34 Ma to 13.5 Ma and 32 to 18 Ma, respectively. Given the paucity of syn-orogenic Alpine-age magmatism, detrital zircon (DZ) are dominated by Variscan and older age modes, limiting more traditional detrital methods, especially as thin metamorphic zircon rims from the metamorphic domes are difficult to recover. This study employed laser ablation depth-profile DZ U-Pb analysis to samples from both sedimentary basins to systematically recover the youngest magmatic and metamorphic rims and Laser Ablation Split Stream (LASS)-ICP-MS techniques to select samples to enhance provenance interpretations through additional geochemical fingerprinting of the analyzed ages.

The DZ U-Pb ages from strata in both basins provide important new constraints on provenance and help reveal the exhumation/unroofing history of the central Alps. Depth-profiling proved powerful in identifying DZ grains characterized by multiple distinct magmatic and metamorphic growth events and for the systematic detection of thin metamorphic overgrowths. The lack of widespread Alpine-aged zircon overgrowths in both basins provides insight into the exposure levels and progressive exhumation of the high-grade metamorphic Alpine orogenic cores. However, LASS was able to identify characteristically flat heavy rare earth element profiles of metamorphic rims from Rupelian samples in the Adriatic foredeep (Aveto Formation). These Cenozoic metamorphic zircon ages greatly enhanced provenance interpretations and pinpointed unique source regions in the central Alps. In contrast, in the Swiss Molasse Basin, the geochemistry of the DZ did not yield significant changes to interpretations other than confirming the scarcity of metamorphic zircon. Depth-profiling and LASS-ICP-MS analysis of DZ has the potential to refine provenance analysis in orogenic systems that lack significant magmatism by systematically recovering and identifying unroofing of different metamorphic domains.