Rare occurrences of Early Jurassic radiolarian cherts in the Dinaric-Hellenic Belt, an open problem

The Dinaric-Hellenic mountain belt formed through the closure of the Maliac-Vardar Ocean and the collision of Adria with microplates (Tisia, Dacia) positioned between Eurasia and the main branch of the Neo-Tethys. The ophiolites derived from the Maliac-Vardar Ocean are now scattered over different tectono-stratigraphic units (Bortolotti et al., 2013): i) a melange below the major ophiolite nappe (sub-ophiolitic melange); ii) a unit of Triassic ocean-floor ophiolites; iii) metamorphic soles underlying the major ophiolite nappes; within the major ophiolite assemblage; iv) a Jurassic fore-arc ophiolite unit (with MOR and SSZ magmatic sequences); v) a Jurassic intra-oceanic-arc ophiolite unit (with only SSZ magmatic sequences) and vi) a Jurassic back-arc basin ophiolite unit (with BABB and CAB magmatic sequences). During the last 30 years, several authors studied the Dinaric-Hellenic ophiolite sequences where radiolarian cherts are overlying or interbedded with basalts in several locations, e.g. Serbia (Zlatibor), Albania (Mirdita), Greece (northern Pindos Mountains, Vourinos, Othrys, Koziakas, Argolis, Evvia, Guevgueli), aiming at reconstructing the geodynamic history of the Dinaric-Hellenic belt. Numerous sections were studied and rich faunas of Triassic and Jurassic age were found in the radiolarian cherts; however, only in two stratigraphic sections, at Angelokastron and Vothiki (Argolis), radiolarites of Early Jurassic age were found. At Angelokastron, the Early Jurassic cherts were found in a small quarry, where, included in a matrix of dark reddish-brown cherty shales, fragments of chert and chert nodules impregnated by ferro-manganese oxides occur (Chiari et al., 2013). The examined samples indicate four age groups for the nodules, and a Middle Jurassic (middle Bathonian) age for the siliceous matrix in which they are embedded. The first age group includes radiolarians of Late Triassic (late Norian‐Rhaetian) age; the second group of Early Jurassic (late early to late Pliensbachian and probably middle-late Toarcian) age; the third group comprises species of early Middle Jurassic (Aalenian-Bajocian) age; the fourth group finally consists of late Middle Jurassic (Bajocian-Bathonian) taxa. The Mn-impregnated chert nodules indicate that from Late Triassic to Middle Jurassic a deep oceanic basin existed in the area. Only another small outcrop in the Argolis Peninsula gave a very poorly preserved radiolarian assemblage of probable Early Jurassic age. The sample comes from a thin, tectonized chert horizon intercalated in a massive basalt in a little quarry near the village of Vothiki, and has been dated as ?Sinemurian. The scarcity of Early Jurassic cherts in the entire Dinaric-Hellenic belt poses some significant questions: is the extreme scarcity of Early Jurassic radiolarites due to stratigraphic or tectonic causes? More specifically, is this scarcity due to a lack of sedimentation or secondary tectonic elimination? In the first hypothesis, we should look for paleoenvironmental and/or paleoceanographic changes that may have controlled Early Jurassic radiolarian productivity and deposition. In the second case, the tectonic evolution of the basin may have caused subduction of some Early Jurassic parts of the Vardarian oceanic basin, with the possible incorporation of the Early Jurassic portions of the radiolarite sequences into the Middle Jurassic metamorphic soles, where radiolarites could not thus far be dated.