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Miocene syn-rift evolution of the North Croatian Basin (Carpathian-Pannonian Region): 1. Regional magmatic activity and Mts. Kalnik and Požeška gora pyroclastic record

Pannonian Basin, being part of the Carpathian-Pannonian Region (CPR, Alpine–Carpathian–Dinaridic orogenic system ; HANDY et al., 2014), represents a “back-arc” basin formed due to Oligocene–Miocene subduction roll-back in the Carpathians and Dinarides (HORVÁTH et al., 2015 ; BALÁZS et al., 2016 ; van GELDER, 2017). Valuable information concerning the North Croatian Basin (NCB ; PAVELIĆ & KOVAČIĆ, 2018) syn-rift evolution, interpreted to have developed due to Early–Middle Miocene extension in the SW margin of the Pannonian Basin, are archived in Mts. Kalnik and Požeška gora Lower–Middle Miocene pyroclastic and sedimentary rocks. However, no state-of-the-art multi-proxy approach has been applied so far on the most promising localities (Croatian Geological Survey data) in order to resolve many crucial local and regional open questions and refine currently available reconstructions. Complex post-collisional (SEGHEDI & DOWNES, 2011) tectonic processes (e.g., core-complex and wide-rift types of extension) in the CPR generated, together with evolution of magmas in the crustal environment, Lower Miocene to Recent magmatic rocks of highly diverse composition. Magmatic activity shows a distinct migration in time from west to east (PÉCSKAY et al., 2006 ; SEGHEDI & DOWNES, 2011). The most complex and abundant group of CPR magmatic rocks is the calc-alkaline group (SEGHEDI & DOWNES, 2011 ; LUKÁCS et al., 2018), with LUKÁCS et al. (2018) constraining the lasting (as well as petrogenesis and geodynamic setting) of large silicic volcanic activity recorded as Bükalja Volcanic Field (BVF, Hungary) silicic pyroclastics from 18.2 to 14.4 Ma. Calc-alkaline magmatic activity (20–16 Ma) associated with core-complex formation related to Pannonian Basin extension is recorded in the Vardar Zone (Internal Dinarides ; SEGHEDI & DOWNES, 2011 ; SCHEFER et al., 2011 ; ANDRIĆ et al., 2018). Early–Middle Miocene volcanic activity was also recorded in the NCB in the form of volcanics and pyroclastics occurring intercalated with penecontemporaneous alluvial, lacustrine and marine sediments (PAMIĆ et al., 1995 ; PAMIĆ, 1997 ; PAMIĆ & BALEN, 2001a, b ; MANDIĆ et al., 2012 ; MARKOVIĆ, 2017). Mt. Kalnik pyroclastics were dated (Ar/Ar) at 18.07 ± 0.07 Ma (Early Miocene, ~ Eggenburgian/Ottnangian transition) by MANDIĆ et al. (2012), constraining the timing of the initial rifting tectonics of the NCB (since they occur intercalated with alluvial sediments representing the base of NCB continental serie). TIBLJAŠ et al. (2002) and MANDIĆ et al. (2012) correlated Mt. Kalnik pyroclastics with Upper Oligocene–Lower Miocene (Egerian–Eggenburgian) andesites and dacites, as well as pyroclastics, outcropping in north Croatia (Hrvatsko Zagorje Basin-HZB, AVANIĆ, 2012). HZB volcanic rocks are interpreted to be related to the easternmost part of the PFZ and/or slab breakoff magmatic activity (PALINKAŠ & PAMIĆ, 2001 ; PAMIĆ & BALEN, 2001a, b). However, Periadriatic (PFZ) magmatic rocks sensu stricto of the Alps (as well as magmatic belt within the Western Carpathians north of the Mid Hungarian fault Zone), are represented by Paleogene (mostly Oligocene) plutons with age span of 28–42 Ma (ROSENBERG, 2004 ; NEUBAUER et al., 2018). Geodynamic setting and formation processes of the eastern younger magmatic (intrusive and volcanic) and pyroclastic rocks, ranging from ~ 20–14 Ma and located north and south of PFZ, is unresolved (e.g., Mt. Pohorje magmatism ; FODOR et al., 2008 ; NEUBAUER et al., 2018). The most reliable age ascription of Middle Miocene (Badenian) pyroclastics intercalated with NCB lacustrine and marine deposits is given by MANDIĆ et al. (2012) and MARKOVIĆ (2017). This age ascription is based on high-precision Ar/Ar dating of sanidine. Age span of intermediate to acid pyroclastics provided by these authors, occurring in several NCB localities, is 16.03–14.40 Ma. Mt. Papuk Nježić locality (14.40 ± 0.03 Ma) and Mt. Medvednica Čučerje locality (14.81 ± 0.08 Ma) have been related (LUKÁCS et al., 2018) to CPR BVF Harsány and Demjén ignimbrites, respectively. In order to make reliable tephrochronological and volcanic provenance (as well as petrogenetic and geodynamic setting) reconstructions of Mts. Kalnik and Požeška gora (intercalated with Lower–Middle Miocene marine deposits) pyroclastics, state-of-the-art volcanological, petrological, high-precision geochronological (EARTHTIME initiative), geochemical and isotopic analysis are being conducted. Such high-resolution data will also provide an absolute age constraint of NCB syn-rift evolution and biostratigraphy, as well as add valuable new data for more comprehensive understanding of the regional (i.e., CPR-related) magmatic and geodynamic evolution (and their mutual relationship).

Miocene, North Croatian Basin, syn-rift, volcanism, Carpathian-Pannonian Region

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