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An integrated approach to provenance analysis: an example of Cretaceous sandstones from the northwestern Dinarides

Provenance studies which integrate results from multiple analytical approaches can offer an in- depth view into the history of clastic detritus and its source area. In northwestern Croatia, synorogenic clastic formations document the evolution of the Dinaride orogen, including the Dinaride-Alpine transitional area, a region characterized by considerable geological complexity resulting from a severe and long- lasting deformational history. By applying a range of analytical methods to study the siliciclastic detritus of Cretaceous sandstones, our aim is to constrain the composition and dynamics of ophiolitic and continental source terrains being exhumed and eroded in the northwestern 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. Our study concentrates on Cretaceous clastic rocks which outcrop on the mountains and hills surrounding Zagreb, Croatia (Figure 1). The siliciclastic detritus of fine to medium grained sandstones ranging in age from Early to latest Cretaceous was studied using wholerock geochemistry, microprobe analysis of individual heavy mineral grains, and zircon fission track (ZFT) thermochronology. The studied sandstones were sampled from five distinct clastic formations (Figure 1) 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. 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. 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. Metapelites of 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 sandstones of Barremian to Cenomanian age the youngest identified ZFT age populations are 134±14 Ma, 145±36 Ma, 162±43 Ma, and 159±30 Ma (Figure 2). Also present within these sandstones are zircon grains which belong to older, more diffuse ZFT age populations ranging from the Early Jurassic to the Carboniferous. The sources of these zircons were most likely fragments of the Adria basement, segments of which had undergone thermal overprint related to obduction of the Neotethys in the Late Jurassic, and were subsequently exhumed and incorporated into thrust sheets together with ophiolites. 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 from Mt. Medvednica the distributions of single-grain ages are unimodal at 80±18 Ma and 73±23 Ma (Figure 2). This clearly identifies a distinct Eo-Alpine overprint in the source area, indicating that rapidly exhuming basement units either in the Eastern Alps and/or Tisza, or the Dinarides themselves, were supplying the basins of the NW Dinarides with detritus at the end of the Cretaceous. 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. 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. 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.

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

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