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Triassic evolution of the Adriatic-Dinaridic platform’s continental margins – insights from rare dolerite subvolcanic intrusions in External Dinarides, Croatia

This contribution reports new findings on petrogenesis and possible geotectonic setting of rare subvolcanic Triassic(?) dolerite cropping out in External Dinarides with a goal to contribute to the debate on geodynamic evolution of the continental margin of the Adriatic-Dinaridic platform(s) at the onset of the Mesozoic. Petrography of studied rocks revealed the crystallization order: clinopyroxene → plagioclase → alkali-feldspar ± amphibole ± Fe-Ti oxides. The geochemistry of calc-alkaline dolerite suggested magma generation from a spinel-garnet bearing peridotite mantle source (60sp:40grt). Two mechanisms explaining the origin of dolerite are here suggested. The first involves a low-grade partial melting of the altered lithospheric mantle and the subduction-modified oceanic lithosphere accompanied with subducted/recycled sediments. The second proposes a melting of the subduction- altered heterogeneous lithospheric (subcontinental) mantle contaminated and/or metasomatized during an earlier Hercynian subduction in the Late Paleozoic. Accordingly, two geodynamic concepts have been suggested: (i) Middle Triassic north-vergent active subduction of the Paleotethys beneath the Andean-type active Laurassian continental margin coupled by the formation of a pericontinental volcanic arc and subsequent extension and back-arc rifting, within the Adria Plate, which led to the formation of locally aborted rift systems and subsidence of the platform and (ii) Middle Triassic extension, unaffected by contemporaneous active subduction, of the High Karst nappe unit of the Greater Adria as a result of an ephemeral syn-rift volcanic stage which featured the formation of the Dinaridic aborted rift system followed by the platform subsidence and emergence of local sedimentary basins.

Middle Triassic(?), calc-alkaline subvolcanic rocks, active continental margin, External Dinarides, dolerite

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