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Petrographic and Geochemical Characteristics of the Middle Triassic Volcaniclastic Deposits from Lika, (External Dinarides, Croatia)

Different types of volcaniclastic rocks are reported in several localities in the External Dinarides. Many authors dealt with magmatic rocks and associated volcaniclastics from the External Dinarides (Poljak & Tajder, 1941 ; Pamić, 1985 ; Lugović & Majer, 1981 ; Marci et al., 1991 ; Aljinović et al., 2010.). Hyaloclastic deposits associated with basalts have never been described so far. Therefore, this paper will present the Middle Triassic hyaloclastic deposits and associated basalts that crop out in vicinity of Donje Pazarište - DP. The deposits are surrounded with shales and brown volcanogenic sandstones, crystaloclastic feldspar rich tuffs, vitroclastic tuffs, and grey pelagic limestones with chert interlayers and chert nodules. Investigated basalts and hyaloclastic deposits are in contact, while their relation to surrounding lithologies is not clearly visible. Nevertheless, they are all assigned to Middle Triassic. Petrographic analysis revealed 3 lithotypes present in the recorded succession: 1) pillow basalts ; 2) faint bedded hyaloclastites with basalt clasts and 3) matrix dominated shard rich basalt hyaloclastite. Hyaloclastic rocks are formed by autoclastic processes by quenching at the edges of coherent basalt flows and cold water (McPhie et al., 1991). Occurrence of pillow basalts and laterally present pelagic limestones suggest that the deposition of basalts and hyaloclastites took place in the deep/pelagic marine environment. Mixing of olivine basalt, limestone and highly altered basalt clasts in the hyaloclastic lithotypes indicate the existence of the older magmatic effusions, and resedimentation processes.

Middle Triassic; volcaniclastic deposits; autoclastic; pyroclastic; External Dinarides; Croatia

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