Middle Triassic basaltic pyroclastic rocks from the Mt. Medvednica ophiolitic mélange (NW Croatia): petrology, geochemistry and tectono-magmatic setting

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Damir Slovenec
https://orcid.org/0000-0003-3251-827X
Josip Halamić
Branimir Šegvić

Abstract

Hectometric blocks of Middle Triassic mafic pyroclastic rocks, represented by volcanic agglomerates/breccias  and lapilli tuffs, form part of the ophiolitic mélange of Mt. Medvednica, situated in the southwestern segment of  the Zagorje-Mid-Transdanubian Zone. These rocks share petrochemical characteristics with pyroclastic  derivatives of alkali, within-plate basaltic lavas of Mts. Medvednica, Samoborska Gora, and Kalnik, indicating the  occurrence of explosive events preceding the dominant effusive submarine volcanism during the Middle Triassic (Illyrian-Fassanian?) stages. The formation of these pre-ophiolitic pyroclastics is associated with an  intracontinental rift setting and reflects melts derived from an OIB-type enriched mantle plume source. These  pyroclastics represent uncontaminated melts that erupted through a highly thinned continental crust. In  geodynamic terms, the formation of pyroclastites occurred during the Late Anisian-Early Ladinian along the  continental margin of Palaeotethys through the proto back-arc rifting of continental lithosphere (Adria Plate), leading to the formation of the Maliak/Balkan Neotethys Rift, in the emerging northwestern segment of  Neotethys. The investigated pyroclastic rocks of Mt. Medvednica document the extension in an evolved  intracontinental rift basin, which immediately preceded the generation of the initial Neotethyan oceanic  lithosphere during the Upper Triassic. 

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Original Scientific Papers