Differences between Southalpine and Austroalpine quartzphyllite complexes Eastern Alps: results from zircon chemical compositions and Pb/Pb dating

摘要

We applied the zircon evaporation method and zircon typological and compositional studies to detrital-zircon populations of quartzphyllite complexes of the Eastern Alps, including the Southalpine (Vetriolo and Recoaro) and the Austroalpine (Goldeck, Gailtal, and Thurntaler) complexes. Combined zircon morphology and geochemical and geochronological analyses (single-zircon evaporation technique) provided significant constraints for the age of the studied protoliths. The morphological study points out that zircons from Austroalpine and Southalpine quartzphyllites show substantial differences in terms of shape, colour, size, type of inclusions, cracks, and turbidity. Electron microprobc analyses (Si, Zr, Hf, P: HREE) indicate homogeneous compositions, suggesting common source areas characterized by granitoid rocks, whereas typology indicates the existence of two distinct groups. Zircon ages from the quartzphyllites are rather different, those from Austroalpine samples being younger than Southalpine ones. Pb-Pb zircon ages define two principal evolutionary sequences. The Austroalpine complexes are characterized by an older evolution typical of subduction-rclated magmatism. This started by 598 Ma N-MORB-type, mainly involving depleted mantle sources, and finished with 379-341 Ma acid suites typical of a crustal source. Southalpine complexes zircon ages show an evolution characterized by tholeiitic and 473 Ma alkaline within-plate basalt-type suites in both pre- and post-Early Ordovician units. It involved intraplate mantle metasomatism and an enrichment trend along multicomponent sources. These magmatic evolution lines can be related to a plate-tectonic scenario that involved terrane accretion in a progressively mature Neoproterozoic to Ordovician active margin and a subsequent Palaeo-Tcthys passive margin along the north Gondwanan periphery.