Differentiation and genesis of the Middle Triassic mafic volcaniclastic facies in NW Croatia, a case study from Vudelja quarry

The Middle Triassic period represents a very dynamic time in a broader Tethyan region. Tectonic movements related to disintegration of Pangea and opening of the Neotethys gave rise to the formation of volcanic and volcaniclastic deposits. In the NW Croatia, these rocks outcrop in approximately 60 km long intra-Pannonian Mountain chain (Mts. Kalnik, Ivanščica, Strahinjščica, Kuna Gora, Desinić Gora and Ravna Gora), representing the junction between the Southern Alps and Internal Dinarides. Among these mountains, Mt. Ivanščica represents a complex structure built of shallow-marine to pelagic successions originating off the passive continental margin of Adria microplate. These deposits are found in a tectonic contact with ophiolitic mélange containing remnants of Neotethys. Middle Triassic volcanic and volcaniclastic rocks from this area include basic/intermediate to acidic effusive and pyroclastic lithologies and are interfingered with marine sediments.

The specific area of Vudelja quarry, situated in the central part of the northern Ivanščica slopes, is composed mainly of mafic lithologies and their volcaniclastic derivates. Volcanic and volcaniclastic rocks of basaltic composition were studied in detail to distinguish different facies and their spatial distribution. Facies characteristics enabled the interpretation of their genesis. Three different facies were recognized: 1) effusive coherent facies; 2) pyroclastic flow facies, and 3) resedimented autoclastic facies. Effusive coherent facies is composed of hyaline basalts, with incorporated clasts of the same lithotype. This facies was formed by effusion of basaltic magma. Basaltic clasts were likely incorporated by the primary effusive flow during magma ascent, or following effusion while flowing over the fragmented basaltic material. Quenching fragmentation of a basaltic effusion in the marine environment is another possible process explaining the formation of basalt clasts. Pyroclastic flow facies is composed of plagioclase crystalloclasts, basaltic lithoclasts and scoria fragments, with the flow texture indicated by clast arrangement and glassy matrix with flow features. This facies occurs only locally and its geographical extent is limited thus indicating a small volume of the flow, characteristic for basaltic pyroclastic flows of scoria and ash type. Resedimented autoclastic facies is dominant at the outcropping quarry front and is composed of several lithotypes with various grain sizes and types of matrix. Clast dimensions vary from block to ash size. Some samples exhibit common sedimentary features such as horizontal lamination. The facies was formed by the autofragmention processes of hot basaltic magma in contact with sea water, and subsequent resedimentation of newly formed volcaniclastic particles. Due to an intense tectonic disruption, spatial organization of determined facies is not clear, though the alienation from the primary source can generally be followed from south to north.

Studied locality presents a portion of Middle Triassic volcanic and volcano-sedimentary formations well known from the Southern Alps, Transdanubian Range and the Dinarides. Intense volcanic activity, related to the rifting between the future Adria microplate and southern edge of Laurasia, fed the material to a complicated pattern of sedimentary environments formed along the future Adria passive margin. Extensional tectonics created deep faults within the continental crust which might have served as conduits for submarine basaltic extrusions.