The Provenance of the Lithospheric Mantle in Continental Collision Zones: Petrology and Geochemistry of Peridotites in the Ulten-Nonsberg Zone (Eastern Alps)
Résumé
We report petrographic descriptions, modal mineralogy estimates and major and trace element analyses of whole-rocks and minerals for 36 spinel and garnet–spinel peridotites from the Ulten–Nonsberg Zone (UNZ) in the Eastern Alps in Italy. We seek to constrain the origin and evolution of their source region in the mantle using a comprehensive geochemical dataset on representative, large, modally homogeneous samples from six UNZ sites. This complements earlier work on metamorphism, exhumation history and trace element residence. The samples range from coarse- to dominant fine-grained peridotites affected by syn-tectonic recrystallization and formation of amphibole ± chlorite as well as late-stage alteration (loss on ignition from 0·3 to 8·6 wt %). The UNZ rocks show a rather limited major oxide range (e.g. 1·8–2·8 wt % Al2O3 in ∼80% of the samples) and include neither very fertile nor highly refractory peridotites. Their range and average composition are distinct from those in several massifs from the western Alps, further indicating that the mantle beneath the Alps is heterogeneous, consistent with the tectonically active, plate boundary setting in which mantle domains of different origins may be juxtaposed. Comparison of the Al–Fe–Mg relationships in the UNZ peridotites with experimental data on melting of fertile mantle, together with modelling of REE contents in bulk-rocks, indicates that their mantle protoliths were formed by ∼10–20% polybaric melting in upwelling mantle that began at 2–4 GPa and ended close to the surface, possibly in an oceanic setting. The melting may have started in the presence of garnet, but mainly proceeded in the spinel stability field. Many UNZ peridotites are enriched in silica relative to continental off-craton xenoliths and experimental dry melting trends at similar Al2O3 and MgO. These enrichments are similar to those observed in suprasubduction-zone peridotites, suggesting their potential origin and/or evolution in a subduction-related setting. Modal and cryptic metasomatism is widespread in the UNZ suite, with a broad range of enrichments in incompatible trace elements. It took place mainly in the mantle wedge above a subduction zone, consistent with low high field strength elements and high light rare earth elements, Th, U, Ba and Pb, and probably incorporated slab components. Amphibole is the major host of highly incompatible trace elements whereas garnet, texturally equilibrated with the amphibole, hosts much of heavy rare earth elements and Zr and shows broad grain-to-grain variations of these elements consistent with its growth during tectonic recrystallization and hydrous modal metasomatism.