Petrographic characteristics of some Middle Triassic volcanic and volcaniclastic rocks in the External Dinarides (Croatia and Bosnia and Herzegovina)

In the geotectonic unit of the External Dinarides, several volcaniclastic-sedimentary successions of Middle Triassic age have been investigated from outcrops in Croatia (in the vicinity of Sinj and Knin) and in Bosnia and Herzegovina (in the vicinity of Bosansko Grahovo). The appearance of volcanic and volcaniclastic rocks in the External Dinarides have usually been interpreted as the Middle Triassic syn-rift phase in which graben-like depressions had been formed along deep structural fractures. The Middle Triassic rift phase in the External Dinarides is marked by volcanic activity that had been defined as basaltic extrusions at the beginning and more acidic explosive activity that characterised deposition of thick pyroclastic and volcanoclastic successions in the later phases. Volcanic and volcaniclastic successions near Sinj have been described mineralogically and petrographycally by the same authors and interpreted as vitric to crystal tuffs and ignimbrites deposited as pyroclastic flows in intrashelf environment, not far from a subaerially located caldera.
   In our investigation we examined several lithotypes of volcanic and volcaniclastic rocks not previously described. Near Bosansko Grahovo there are occurrences of blocky pepperites. In the angular pepperite type jigsaw-fit texture can be observed suggesting quenching of Mgdepleted basaltic lava on the contacts with unconsolidated pelagic limestones. The Ladinian age of the succession was determined on the basis of conodonts found in limestones intercalations.
   Near Sinj a thick volcaniclastic beds (called “pietra verde”) are interbeded with marine bioclastic, well bedded limestones, cherts and dolomites. Bioclastic limestone and dolomites are characterised by an abundance of calcareous algae, foraminifers, gastropods, bivalves, brachiopods, crinoids, serpulids as well as radiolarians, ammonoids and conodonts, the latter suggesting open marine (pelagic) associations. Limestone beds are strongly silicified and recrystalised. Dolomites exhibit macrocrystaline anhedral texture suggesting a secondary dolomitisation. Lower Ladinian age was inferred on the basis of conodonts and amonoids.
   Volcaniclastic beds (tuffs) near Sinj are massive or evenly laminated. Cross lamination occurs at the top of some beds. Several volcaniclastic lithotypes (tuffs) do not significantly differ in composition. They contain predominant former glassy fragments which are cuspate, platy, or udeformed bubble-wall shards. Pumice wisps occur in some lithotypes showing random orientation and relict tube vesicle texture. Other components are sparse feldspar and quartz crystal fragments. The formerly glassy shards are recrystalised to a fine quartz-rich mosaic. Shards are occasionally well preserved due to carbonate microcrystaline calcite that outlines them.
   The dominance of juvenile pyroclasts, particularly the abundance of various shard types and quartz crystal fragments suggest that they were sourced from explosive, acidic eruptions. The interlayering with carbonates containing pelagic (open marine) fossil assemblages suggests that they have been deposited farther offshore. In this circumstances are these subaqueous deposits not likely to by strictly primary pyroclastic in origin. Bedforms indicate rapid, possibly mass-flow, deposition in offshore environment. Nevertheless the abundance of texturally poorly or unmodified pyroclasts suggests that pyroclastic material was delivered more of less directly to sedimentary transport and deposition systems. Although it has been suggested that the origin had been primary pyroclastic surge deposits from subaerial depositional settings, we thought that pyroclastic material generated from explosive eruptions at marine submerged vents. The dominance of juvenile pyroclasts suggests that fragmentation occurred when lavas emplaced into marine unlithified sediments allow explosive vaporisation of pore fluids or when superheated water flashed to steam that rapidly expands. Thus we interpret deposits near Sinj as pyroclast rich offshore mass-flow deposits.