Melt inclusions in detrital spinel from the SE Alps (Italy-Slovenia): a new approach to provenance studies of sedimentary basins

Detrital spinel is a widespread heavy mineral in sandstones from the Maastr ichtian-Middle Eocene sedimentary basins in the SE Alps. Chemistry of detri tal spinels from the Claut/Clauzetto and Julian Basins (N Italy and NW Slov enia) is used to constrain petrological and geochemical affinities and tect onic provenance of the source rocks. In addition, we have analysed melt inc lusion compositions in the detrital volcanic spinels to better constrain th e nature of their parental magmas. This is the first study of melt inclusio ns in detrital spinels. Two principal compositional groups of detrital spin els are recognised based on their TiO2 and Fe2+/Fe3+; one derived from peri dotites, the other from basaltic volcanics. Peridotitic spinels are more ab undant and have TiO2 < 0.2 wt% and high Cr/Cr + Al(40-90), characteristic o f suprasubduction zone harzburgites. Significant chemical variations among volcanic spinels (TiO2 up to 3 wt%, Al2O3 12-44 wt%) suggest multiple sourc es, with geochemically distinct characteristics, including MORE-type and ba ckarc basin basalts, subduction-related magmas and tholeiites produced duri ng early continental rifting. Compositions of homogenised melt inclusions i n spinels with TiO2 > 0.2 better distinguish the differences between the co mpositions of their host spinels and help to further clarify the geodynamic provenance of extrusive source rocks. Several compositional groups of melt inclusions have been recognised and represent diverse magmatism of margina l basins, including MORB- and subduction-related geochemical types, as well as magmas characteristic of early continental rifting. These results, comb ined with the data on regional ophiolitic complexes and tectonic reconstruc tions favour the Internal Dinarides of Yugoslavia as a possible source area for the SE Alps sediments.