engleski

Sedimentary provenance analysis in an orogenic setting: unraveling clues to the early history of the northwestern Dinarides

Basin remnants of Cretaceous synorogenic sediments which are exposed in the Medvednica, Ivanščica, Žumberak and Samobor Mountains of northern Croatia record the early orogenic history of the NW Dinarides. The provenance of fine to medium grained litharenites to sublitharenites which cover a time span from the Early to the latest Cretaceous was studied by combining petrography, whole-rock geochemistry, heavy mineral chemistry and detrital zircon fission track (ZFT) dating. By applying a range of analytical methods we aim to constrain the composition and dynamics of ophiolitic and continental source terrains being exhumed and eroded in the NW Dinarides and neighboring regions during the Mesozoic. Such integrated data can greatly contribute to a better understanding of the major events which marked the early tectonic history of the Dinarides. Furthermore, it allows direct comparisons to be drawn with neighboring Alpine, Tisza and other Dinaride regions. The studied sandstones were sampled from five distinct clastic formations which have been extensively studied and described in the literature (see references in Lužar- Oberiter et al., 2009): the Oštrc, Bistra, Kravljak, Vivodina, and Glog formations. Differences in Fe2O3/K2O ratios between the different formations reflect the variable amount of contribution from mafic and ultramafic minerals, which agrees with the observed abundance of ophiolitic lithoclasts in the Oštrc Fm, their presence in the Bistra, Kravljak and Vivodina Fms, and lack thereof in the Glog Fm. In sandstones from the lower Cretaceous Oštrc formation (Ivanščica Mt.), values of Cr/V range between 20 and 30, considerably higher than in other analyzed samples. Together with very low Y/Ni values, this is indicative of a very large contribution of material from ophiolites (McLennan et al., 1993). The Bistra (Medvednica Mt.), Kravljak and Vivodina (Žumberak Hills) samples have Cr/V and Y/Ni values indicative of some ophiolite contribution, however, much more limited than is the case for the Oštrc. Meanwhile, the high Y/Ni values (and low Cr/V) in the Glog samples (Medvednica Mt.), similar to those found in felsic rocks, suggest a purely continental source. The chemistry of detrital Cr-spinel has helped to further reveal the petrogenesis of the ophiolite source rocks being obducted and eroded along the margin of Adria during Late Jurassic-Cretaceous times (Lužar-Oberiter et al., 2009). During both the Early and Late Cretaceous the source ophiolites were predominantly composed of harzburgite peridotites and associated cumulate rocks (Lužar- Oberiter et al., 2009). A distinct regional similarity in detrital Cr-spinel compositions is evident between the Oštrc Fm. and contemporaneous Cr-spinel rich formations in the North Calcareous Alps and Transdanubian Central Range (Pober & Faupl, 1988, Árgyelán, 1996). The make-up and dynamics of continental sources terrains can be constrained by the chemical composition of detrital tourmaline, rutile and garnet, and ZFT thermochronology. Heavy mineral chemistry data suggest that metapelites of low to medium grade were likely the most widespread continental non- carbonate lithology in the source region during the entire Cretaceous. Metamafic rocks also presented a significant source, while high grade metasediments and granitoid rocks occurred more sporadically. In lower Cretaceous to Cenomanian sandstones the youngest identified ZFT age populations are 134±14 Ma (Oštrc Fm), 145±36 Ma (Oštrc Fm), 122±45 Ma (Bistra Fm) and 159±30 (Kravljak Fm). The sources of these zircons were most likely fragments of Adria basement, segments of which had undergone thermal overprint related to the obduction of the Neotethys in the Late Jurassic, and were subsequently exhumed and incorporated into thrust sheets together with ophiolites during the Early Cretaceous. Also present within these sandstones are zircon grains which belong to older, more diffuse ZFT age populations ranging from the Jurassic to the Late Paleozoic, probably deriving from various thermally unreset basement units. In the latest Cretaceous sandstones, detrital ZFT ages are notably younger. In the Maastrichtian Vivodina sample (Žumberak Mts.) a well defined ZFT age population lies at 76±13 Ma, while in the Maastrichtian Glog samples (Mt. Medvednica) the distributions of single-grain ages are unimodal at 80±18 Ma and 73±23 Ma. This clearly identifies a distinct Eo-Alpine overprint in the source area, indicating that newly and rapidly exhuming basement units were supplying the basins of the NW Dinarides with detritus at the end of the Cretaceous. It is likely that the Maastrichtian erosion generating Eo-Alpine metamorphic detritus occurred to a great extent on the upper plate Tisza-Dacia Unit, either already during the subduction stage or during the initial stages of the continent-continent collision with Adria (Ustaszewski et al., 2010). Literature: Árgyelán, G.B. (1996): Geochemical investigations of detrital chrome spinels as a tool to detect an ophiolitic source area (Gerecse Mountains, Hungary). Acta Geol. Hung., 39, 341-368. Dunkl, I. & Székely, B. (2002): Component analysis with visualization of fitting ; PopShare, a Windows program for data analysis (abstract). Geochim. Cosmoch. Acta, 66, 201. Lužar-Oberiter, B., Mikes, T., von Eynatten, H., Babić, Lj. (2009): Ophiolitic detritus in Cretaceous clastic formations of the Dinarides (NW Croatia): evidence from Cr-spinel chemistry. Int. J. Earth Sci., 98, 1097-1108. McLennan, S.M., Hemming, S., McDaniel, D.K. & Hanson, G.N. (1993): Geochemical Approaches to Sedimentation, Provenance and Tectonics. In: Johnsson, M.J. & Basu, A. (Eds.): Processes Controlling the Composition of Clastic Sediments. Geol. Soc. Am. Spec. Paper, 284, 21– 40. Pober, E. & Faupl, P. (1988): The chemistry of detrital chromian spinels and its implications for the geodynamic evolution of the Eastern Alps. Geol. Rundsch. 77, 641-670. Ustaszewski K, Kounov A, Schmid SM, Schaltegger U, Krenn E, Frank W, Fügenschuh B (2010) Evolution of the Adria-Europe plate boundary in the northern Dinarides: From continent- continent collision to back-arc extension. Tectonics, 29: TC6017. doi:10.1029/2010TC002668

Provenance ; Cretaceous ; Dinarides ; Sandstone geochemistry ; Heavy mineral chemistry ; Zircon fission track thermochronology

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