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Volcanic facies analysis of a subaqueous basalt lava-flow complex at Hruškovec, NW Croatia — Evidence of advanced rifting in the Tethyan domain

The Hruškovec quarry of basaltoid rocks is situated on the northwestern slopes of Mt. Kalnik, within the Zagorje– Mid-Transdanubian zone, a part of the North-western Dinarides. The basaltoids are inter-bedded with radiolarites of the Middle and Upper Triassic age (Langobardian, Carnian– Norian). Spilites, altered diabases and meta-basalts form part of Triassic volcanic-sedimentary sequence, made of sandstones, shales, micritic limestone, altered vitric tuffs and radiolarian cherts, incorporated tectonically into the Jurassic– Cretaceous mélange. The architecture of the 2 km long and 100 m high pile of the extrusive basaltoid rocks is interpreted as a subaqueous basaltic lava flow. The presented research deals with a variety of volcanic facies of the subaqueous basaltic lava flow, which consists of several facial units: 1. Coherent pillow lavas, with massive core ; the bending rims around the massive core, 30– 50 cm thick, are dissected by polygonal columnar joints radiating from the pillow centres ; 2. Closely packed pillows ; densely packed and contorted pillows due to emplacement accommodation, clearly younging upward ; 3. Pillow fragment breccia ; clast supported, matrix poor, monomict breccia, formed proximal to the axis of the extrusion ; 4. Isolated pillow breccia ; matrix supported, clast poor breccia, made of lava pipes and tubes, within a matrix of fine-grained sideromelan granules and shards ; 5. Pyjama-style pillows ; spherical, decimetre to meter size pillow lava balls, grown and chilled in isostatic state (i.e. in a state of diminished density contrast) within water-soaked sediments, named after peculiar alternating basaltic shelves inside the sphere, which are encrusted with white secondary minerals ; 6. Peperite and peperitic hyaloclastites ; blocky and globular peperites developed at the contact of soft, wet sediment and hot intruding magma. Discovery of peperite and peperitic hyaloclastites within the Triassic radiolarian cherts, shales, and micritic limestone, with registration of conodonts (index microfossils), links their origin to magmatism during advanced rifting in a progressively subsiding basin during the Triassic. It precludes an affiliation of the subaqueous basaltic lava flow with the dismembered ophiolite formation, a product of oceanization created in Jurassic– Cretaceous times, which is a commonly stated alternative explanation. The basaltic rocks and their sedimentary hosts may be correlated with advanced Triassic rifting, and related volcanic-sedimentary successions in the Dinarides, Albanides, Helenides and further along the Tethyan orogenic belt.

subaqueous basaltic lava flow; pillow lavas; blocky peperites; globular peperites; Triassic rifting; Dinarides; Tethys; ophiolites; conodonts

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