CHAP. 28. TRIASSIC MAGMATISM AND JURASSIC OPHIOLITES AT THE MARGINS OF THE ADRIATIC MICROPLATE

Triassic magmatic rocks, spatially associated with Jurassic ophiolites, are distributed along the Apennine-Alpine-Dinaride-Hellenide orogenic belts around the Adria continental plate. They can be related to a rifting phase which is precursor of the Jurassic oceanization and show different affinities. Alkaline to transitional anorogenic magmatism charactetises at the western margin of the Adria plate, from Calabria to Tuscany, while rocks of the calc-alkaline/shoshonite orogenic series are widespread at the northern and eastern margins, from the Alps to Dinarides and Hellenides. The apparent discrepancy between the “orogenic” character of the latter and the general “anorogenic” setting of the former can be explained if mantle sources at the northern and eastern Adria margins inherited subduction-related geochemical components from the Hercynian orogenic cycle. The subsequent Jurassic oceanization produced multiple oceanic basins: the Western Tethyan (Ligure-Piemontese) basin at the western margin, and the Serbian, Mirdita-Pindos basins of the Subpelagonian Zone at the eastern Adria margin. The western ophiolites of the Alpine-Apennine orogenic belt display exclusively MORB affinities and structural features which indicate discontinuous oceanic crust generation along a “slow-spreading” system characterized by passive lithospheric extension and tectonic denudation of large sectors of subcontinental mantle peridotites. In contrast, the Subpelagonian Zone ophiolitic complexes are characterized by the juxtaposition of two subparallel belts, of MORB-type to the west, and of suprasubduction-type to the east, implying the development of intra-oceanic subduction processes within a pristine oceanic basin. The hypothesised original location of this basin between the Adria and Pelagonian continental blocks is to be preferred with respect to a possible location in the Vardar-Almopias oceanic basin.