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 the geodynamic evolution of the continental margin of the Adriatic-Dinaridic platform(s) at the onset of the Mesozoic. Petrography of the 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). Twomechanisms explaining the origin of dolerite are here suggested. The first one involves a low-grade partial melting of the altered lithospheric mantle and the subduction-modified oceanic lithosphere accompanied with subducted/recycled sediments. The second one proposes a melting of the subduction-altered heterogeneous lithospheric (subcontinental) mantle contaminated and/or metasomatized during an earlier Variscan subduction in the Late Paleozoic. Accordingly, two geodynamic interpretations are suggested: (i) Middle Triassic north-vergent active subduction of the Paleotethys beneath the Andean-type active Laurasian 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