Középső-triász platformkarbonátok és vulkanitok vizsgálata a Latemar környékén (Dolomitok, Olaszország) [Middle Triassic platform carbonates and volcanites in the Latemar area (Dolomites, Italy)]

Abstract

Sponsored by the T043341 project of the Hungarian Research Found (OTKA) we have investigated the buildup of the Middle Triassic Latemar platform (Western Dolomites), its volcanic stuctures and also basinal successions of the surrounding area. Our leader on the field trip was Nereo Preto from the Padova University. The relatively small isolated Middle Triassic carbonate platform of the Latemar is one of the most famous Triassic platform of the Dolomites. It is surrounded by pelagic sediments of coeval interplatform basins. The main part of the platform is built up by cyclic peritidal to subtidal lagoon facies. Within this succession more than 600 shallowing upward basic cycles were reconstructed and several higher cyclostratigraphic units were defined. These cycles have been interpreted as orbitally forced Milankovič cycles. Based on their concept therefore 14 My were estimated for the duration of the deposition of the whole carbonate sequence. However, on the basis of new biostratigraphic and radiometric data that yielded a duration only between 2 and 4,7 My a group of researchers questioned this interpretation. The Sciliar (Schlern) Formation has been only slightly dolomitized on the Latemar, this causes unique preservation of the fossils. Dasycladacean algae and ammonoids occure together in the platform succession. Correlation of algae and ammonoid zonation can give a chance to use a more accurate and detailed chronostratigraphic chart within the Middle Triassic platforms not only in the Dolomites but in the whole area of the Western Tethys. Ammonite bearing limestone lying above the "lower edifice" belongs to the Avisianum Subzone of the Reitzi Zone (sensu VÖRÖS 1998). Based on our field observations this facies can be interpreted as infilling of a channel which connected the platform-lagoon with the pelagic basin. Within the overlying platform carbonate succession the Crassus and Serpianensis Subzone (sensu MIETTO, MANFRIN 1995) can be detected while in the uppermost part of the section the basal Curionii Zone was also indicated. Preliminary investigations of algae-assemblage show (Fig. 5) that the last occurence of Diplopora annulatissima can be correlated with the base of Secedensis (Nevadites) Zone, while the first occurence of Teutloporella herculea fits to the base of Curionii Zone. The later change in the flora may give the opportunity to fix the newly difined Anisian/Ladinian boundary within thick platform carbonate successions, as well. Being calibrated with ammonite zones the Dasycladacean zonation of the Latemar provides useful tool for chronostratigraphic subdivision of the Budaörs Dolomite (lithostratigraphic counterpart of the Schlern Formation) and for reconstruction of the evolution of Middle Triassic platforms in the Transdanubian Range. A series of dyke swarm cross cuts the carbonate platform of the Latemar. The individual dykes are mafic, aphanitic to microholocrystalline, and they vary in thickness between dm to tens of metres. The dyke margins are generally straight, however, long wavelength and small amplitude undulations have been recognized in case of thick dykes. The dykes have chilled margin up to 10 cm in width. Quench crystals of plagioclase are common in mm-size range and their size increases toward the centre of the dykes. The dykes are more weathered than the surrounding carbonate material, and therefore their location is represented by sharp irregularities in the otherwise flat top of the platform. Along the dyke strikes in the centre of the Latemar three pyroclastic breccia zone have been identified. The southernmost is a complex association of tilted limestone beds that are surrounded by a coarse-grained pyroclastic breccia. They form funnel shape disturbed zone in the carbonate platform with an average width of 150 m. In the pyroclastic breccia, angular shape basaltoid lapilli are common. Basaltoid lapilli are abundant in lherzolite nodules with angular shape and few cm in diameter. Similar, but larger nodules form cumulate zones in the basal region of the exposed volcaniclastic succession. The volcanic clasts are generally altered, but their shape is still angular, and closely resembling their non-abraded, primary origin due to fragmentation of the magma by an explosive volcanic eruption. The volcanic clasts, as well as the intruded irregularly shaped dykes in the lower part of the volcanic pipe are chilled indicating sudden cooling by magma-water interaction as an inferred fragmentation process. In the pyroclastic breccia hosts there are large angular shape blocks of debris that have been derived from the surrounding carbonate platform units. These clasts range cm to few m size in diameter and they are always angular, indicating that they must have been consolidated and hard by the time they have been disrupted by an eruption. In the upper part of the volcanic breccia pipe mega-blocks up to few tens of metres across have been identified. They are tilted, rotated, and form a chaotic zone in the pyroclastic breccia host all indicates that they have been derived from a former conduit/crater wall. The general architecture of the volcanic breccia pipe is inferred to be a diatreme that is an exposed and exhumed volcanic conduit of a former phreatomagmatic volcano. North of the above described diatreme two other pyroclastic breccia body forms a few tens of metres wide semicircular zone. Each of them is rich in angular limestone fragments, gravels, and bedded red lapilli tuff fragments. Carbonate clasts often form trains of clasts indicative of some sort of movement through the pyroclastic breccia zone inferred to be a result of a fluidization through the volcanic pipe. Each pyroclastic breccia pipe shows angularity, chilled margins, microlite-free textural features on the juvenile clasts that are indicative for fragmentation by sudden cooling of magma by magma/water interaction. The identified three pyroclastic breccia pipes are inferred to be diatremes, root zones of former small to medium volume, mafic, phreatomagmatic volcanoes. K/Ar dating of the sample from the diatreme gave an age of 204±7.8 My (Balogh Kadosa pers. comm.). Near to the Latemar on the Dos Capel, a thick succession of pelagic basin fcies (Livinallongo Formation, partly heteropic with the Sciliar Formation) crops out which is interbedded with several dm thick pyroclastic beds of typical "pietra verde". The grading, sorting, lower and upper bed contacts indicate that these beds were predominantly deposited by ash turbidites that carried volcanic material into the basin. Thin layers of fall beds also exist. Near the top of the Dos Capel sequence a well-exposed thickly to thinly bedded, accidental lithic-rich, cross-bedded or stratified, occasionally dune-bedded lapilli tuff and tuff succession crops out. These beds are rich in angular limestone clasts that occasionally form shallow impact sags on the underlying bed surface. The juvenile clasts are angular, chilled, and low in vesicularity, characteristic for juvenile fragments fragmented by phreatomagmatic explosive interaction of melt and water. The large volume of the accidental lithic clasts in the pyroclastic rock units indicates that the magma fragmentation must have occurred in subsurface environment and/or the volcanic conduit was partially closed. On the basis of the preliminary field study and comparison of different volcanic facies in and around the Latemar highlight the possible facies relationships between diatremes that cut through the platform and their eruption fed tephra falls deposited in the pelagic basin and/or produced pyroclastic density currents that may initiated volcaniclastic turbidites transported pyroclasts deep into the basin around the platforms. It is also inferred that in a shallow water environment pyroclastic mounds and associated tuff cones may have produced volcanic islands on top of platforms and an entire lateral facies transition could be expected to be identified in the near future via systematic mapping and interpretation of the pyroclastic successions in the region.