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

Autori
Aljinović, Dunja ; Kolar-Jurkovšek, Tea ; Jurkovšek, Bogdan ; Hrvatović, Hazim

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
Geologica Balcanica, Abstract volume / Chatzipetros, Alexanros ; Melfos, Vasilios ; Machev, Peter ; Lakova, Iskra - Solun, 2010, 24-25

Skup
XIX Congress of the Carpathian-Balkan Geological Association

Mjesto i datum
Solun, Grčka, 23.09.2010. - 26.09.2010

Vrsta sudjelovanja
Poster

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
Middle Triassic; volcaniclastic rocks; External Dinarides; Croatia; Bosnia and Herzegovina

Sažetak
In the geotectonic unit of the External Dinarides, several volcaniclastic-sedimentary successions of Middle Triassic age have been investigated at the outcrops in Croatia (vicinity of Sinj and Knin) and in Bosnia and Herzegovina (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 along deep structural fractures graben-like depressions had been formed. The Middle Triassic rift phase is in the External Dinarides 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 vitro- to crystaloclastic tuffs and ignimbrites deposited as pyroclastic flows in intrashelf environment, not far from subaerially located caldera. In our investigation we examined several lithotypes of volcanic and volcaniclastic rocks not described before. Near Bosansko Grahovo there are occurrences of blocky peperites. In the angular peperite type jigsaw-fit texture can be observed suggesting quenching of Mg-depleted basaltic lava on the contacts with unconsolidated pelagic limestones. The Ladinian age of the succession was discerned according to 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 ammonoids, radiolarians and conodonts suggesting open marine (pelagic) associations. Limestone beds are strongly silicified and recrystalised. Dolomites exhibit macrocrystaline anhedral texture suggesting a secondary dolomitisation. On the base of conodonts and amonoids the Lower Ladinian age was inferred . Volcaniclastic beds (tuffs) near Sinj are massive or evenly laminated. Cross lamination occur at the top of some beds. Several volcaniclastic lithotypes (tuffs) do not significantly differ in composition. They dominantly contain formerly glassy fragments which are cuspate, platy or udeformed bubble-wall shards. Pumice wisps occur in some lithotypes in random orientation, showing 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 interlay ring with carbonates containing pelagic (open marine) fossil assemblages suggest that they have been deposited farther offshore. In this circumstances are these subaqueous deposits not likely to by strictly primary. 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 flashes to steam that rapidly expands. Thus we interpret deposits near Sinj as pyroclast rich offshore mass-flow deposits.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

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