Naslov
The role of complex sedimentological research in solving general problems of the Dinaric ophiolite zone

Autori
Tišljar, Josip

Izvornik
Rad Hrvatske akademije znanosti i umjetnosti (1330-0849) 486 (2003), 27; 111-129

Vrsta, podvrsta i kategorija rada
Radovi u časopisima, članak, znanstveni

Ključne riječi
Dinaric ophiolite zone; chert; grauwacke; shale; Middle/Late Triassic to Late Jurassic; Croatia

Sažetak
Ophiolite complex or ophiolites ("diabase-chert formation") is a group of basic, ultrabasic and ultramafic plutonic rocks (gabbros, peridotites and pyroxenites), hypoabissal (diabases) and more or less altered effusive rocks (spilites-keratophyres-basalts) of oceanic crust's origin. There are also significant quantities of siliceous and fine-grained clastic sediments associated with them. Bedded cherts, shales and fine-grained greywacke sandstones are dominant sedimentary rocks of the ophiolite complex that spreads from Pokuplje and Banovina in central Croatia, through north-western and central Bosnia, western and east-western Serbia, Montenegro, Albania, Macedonian, to the Asian continent. The ophiolite complex of Croatia, Bosnia and Herzegovina, Yugoslavia and Macedonian consists of two regions: Dinaric ophiolite belt and Ophiolite belt of Vardar region. In the paper by PAMIĆ et al. (2000) the term "Dinaric ophiolite zone" has been used instead of the term "Dinaric ophiolite belt". Dinaric ophiolite zone, together with Upper Cretaceous-Paleogene flysch, Jurassic-Cretaceous sequence and Hercine progressive metamorphic sequence, occupies a relatively narrow but long area between the southern part of the Panonian basin and the Adriatic carbonate platform. Ophiolite rocks have been formed in the period of Middle/Late Triassic to Late Jurassic and/or the beginning of Cretaceous. Investigations of general and special petrologic and diagenetical characteristics, of conditions of sedimentation, as well as deposition environments of cherts, sandstones, pelites and carbonate rocks of ophiolite complexes have an exceptionally important role in solving the general problems of ophiolites. Therefore, a considerable amount of attention has been paid to these investigations in the world, which is not, however, the case in Croatia. Whereas basic, ultrabasic and ultramafic rocks of Dinaric ophiolite zone are well investigated, systematic complex exploration of sedimentary rocks is almost complete missing. In Croatian literature, for cherts of the Dinaric ophiolite zone there is no data whatsoever whether or not they contain opal-CT and what kind of interrelation there is between opal-CT, quartzine (= cryptocrystalline, short-fibber variety of chalcedony incompletely recrystallized to quartz) and quartz. Today, in Croatia there exist both qualified personnel and equipment for such research. It really is a pity that such investigations have not been undertaken so far, since by their systematic implementation one could get important answers to the questions on genesis of cherts and diagenetic processes inside them. A similar assertion holds for investigations of geochemical properties of cherts. This pertains firstly to the contents and the mutual portions of Mn, Fe and Al, whose interrelations indicate if cherts are closer to those deposited on mid-ocean ridges or if they show geochemical affinities, especially with regard to the contents of Al, to those deposited on continental margins. In the same manner, on the basis of petrological and geochemical data, special the low content of TiO2 and Al2O3 and the enrichment of contents of Ba, As, Sb, Hg and Se, as well as mutual Al-Fe-Mn relations in the ternary diagram, exact information can be obtained. As to whether or not these values fall into the "hydrothermal field"? That is, whether or not the investigated cherts could be interpreted as hydrothermal products? However, such information for cherts of Dinaric ophiolite zone is not available, so that the interpretations so far have only been founded upon petrological features of magmatic rocks and upon correlation with similar occurrences of such rocks from foreign literature. The mineral composition of clastic rocks of the Dinaric ophiolite zone is rather homogeneous. As a rule, a relatively high portion of mica-chlorite matrix. As essential or significant ingredients occur monocrystalline and policrystalline quartz grains, fragments of effusive rocks (most frequently diabases and spillites), fragments of quartzites, granitoides and gneisses, crystalline shists (mostly mica, chlorite and quartz-sericite shists, slate, phyllite), pelite and sandy sediments and cherts. The feldspars (acid to intermediate plagioclase, microcline, in some places orthoclase), calcite, micas and mica-chlorite matrix, whereas, as accessory ingredients, there occur hematite, goethite, pyrite, siderite, granates, staurolite, rutile, ilmenite, chromite, titanite, amphibole, chloritoide and others. According to the sedimentological characteristics known up to now, blocks and limestone bodies ("olistoliths" or irregular bodies of limestones and limestone breccia in Banovina, Pokupsko and north-western Bosnia) belong to three, from the sedimentological point of view, essentially different deposition environments. There are: a) deep-sea pelagic limestones, b) shallow-water peritidal-lagoon limestones, and c) shallow-water bioclastic perireef limestones. The stratigraphy of the big blocks is very different. In the region of Stupnica, Žirovica, Bojna, Gvozdanski, Ljeskovica, Klasnić, Brezovo Polje, Hrvatsko Selo, Kostajnica and the surroundings of Velika Trepča, Lasinja in Croatia, as well as in north-western Bosnia (the region of Pastirevo, Cetingrad and Velika Kladuša), the big blocks are the Lias, Dogger and Malm age. The shallow-water peritidal-lagoon and bioclastic perireef limestone blocks or olistholits are incorporated in undisturbed (non-melanged) deep-sea pelites and sandy sediments clearly indicate intensive synsedimentary tectonics. The presence of steep submarine slopes along which, in the course of tectonic impacts, proceeded the movement of large defragmented blocks from carbonate shoals and reefs to deeper, tectonically trenched submarine sedimentary basins (deep trokes, deep fault trenches). There has been no systematic sedimentological and petrological research carried out for the region of the Dinaric ophiolite zone, regarding the occurring mode of bedded and ribbon cherts associated with shales, graywacke sandstones, calcitic mudstones, with or without pelagic limestones. Hence, there is no accurate available data about their sedimentary system. However, there are interpretations for some localities that these rocks have characteristics of siliceous turbidites. The chert members are generated by diagenetic changes of accumulated siliceous material deposited in the deeper parts of the basin from submarine platform slopes or seamounts together with small amount of siliciclastic and carbonate (matrix) material with/without limestone olistholits. For example, about fifteen years ago, without systematic sedimentological field research, siliceous turbidites with limestone (oospharite) members (Ta-b) have been noticed by the railroad cut near Nemila, north of Zenica (Bosnia and Herzegovina). However, only Ladinian-Carnian ribbon radiolarian cherts of Medvednica Mt. and Karnian-Norian ribbon radiolarian cherts of Kalnik Mt. (northern Croatia) have been identified as certainly belonging to the distal, fine-grained siliceous turbidites deposited from low-density debris flows, just below, and partly above the CCD line. Many data about litological and structural characteristics and the occurring modes of graywacke sandstones and shales in Pokupsko and Banovina in western Croatia indicate the possible deposition from low-density turbidite currents. However, exact sedimentological data for turbidite, and possibly contourite, involvement should be collected by additional field research. It should be expected that systematic research in the region of Banovina and Pokuplje to be carried out for the purposes of Fundamental geological map of the Republic of Croatia, 1:50000. Will provide a foundation for solving some of the above mentioned problems and the uncertainties regarding petrological-sedimentological characteristics of sedimentary rocks together with igneous and associated metamorphic rocks of ophiolite complex from that area.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

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