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Triassic rifting magmatism within Zagorje-Mid-Transdanubian zone, examplified by pillow lavas of Hruškovec, Mt. Kalnik, N Croatia

A large pile of pillow lavas in Hruškovec quarry, Kalnik Mt., is a basalt cryptodome intruded and extruded into wet, unconsolidated Triassic carbonate and siliciclastic sediments. It consists of variety of volcanic facies with a huge, a few km long igneous body, thick several hundred meter. It is tektonicaly dismembered and inplaced into Cretaceous melange formation. The presented research deals with variety of volcanic facies: 1.Coherrent pillow lavas. 2."Closely packed pillows 3."In situ" hyaloclastite 3.1.Isolated pillow breccia 3.2. Pillow fragment breccia 3.3. Peperitic hyaloclastites. Peperitic hyaloclastites are formed at the marginal zones where intrusive, synvolcanic, pillowed sills or lava flows override or borrow into wet unconsolidated sediments. It is evidenced by presence of protruding basaltic pods, penetrating, small-scale, basaltic dike-sills, into the micritic, limey sediments. During emplacement the igneous body encountered hydrothermal activity evidenced by veins, filled out with secondary minerals calcite, quartz, hyaloclastite cement and infillings of cavities inside pillow lava pipes (calcite, quartz, pumpellyite, chlorite, epidote, etc.). Fluid inclusion study of quartz from hydrothermal veins, which cut the pillow lava formation and calcite inside the cavities of pillow lavas, recognized following types: 1. Monophase (L), 2. Liquid-rich, (L+V), 3. Vapour-rich, (V+L), 4. Multiphase with gypsum (L+V+SGYSUM), 5. Multiphase with carbonate (L+V+SCARBONAT) and 6. Multiphase with gypsum and carbonate (L+V+SGYSUM+SCARBONAT). Continuous spread of salinity between two end members, low salinity (NaCl-H2O) and CaCl2-rich, high salinity, suggests mixing of sea water and fluids formed during spilitization of igneous body. Minimum depth of extrusion on the sea bottom, determined on the available measurments of Th and salinity and the appropriate pressure of the homogenizing fluid exceeds 100 meter (10 bars). Extensive masses of cherts and shales of Carnian-Norian age, within the melange, as determined on the basis of radiolarian assemblages (Halamić and Goričan, 1995) implied possible Triassic time of extrusion. Peperitic hyaloclastites within micritic limestone, however, offered the appropriate prove to confirm the Triassic time of extrusion. Samples of carbonate rocks from the peperites were collected and treated for conodonts. A standard conodont techniques was used, followed gravity-enrichment. Only one sample proved to yield conodont elements of fragmentary preservation. Conodonts are microscopic hard parts of marine animals (phylum Conodonta). They can be found in strata of Paleozoic to late Triassic age when they became extinct (Clark, 1983). Several individual conodonts, different in shape and form, constituted a conodont apparatus. Certain conodont elements, especially those of platform type, are good guide fossils. All recovered conodonts are the ramiform elements. They are of minor stratigraphic importance and they do not allow precise dating of the sample. The recovered conodonts are opaque white and their value CAI is 6,5 (Epstein et al., 1977). The Color Alteration Index (CAI) of conodont elements is a valuable tool for assessing organic metamorphism (Harris, 1981). The surface of conodont fragments is texturally altered, whereas the deformation of the elements is not perceivable due to poor preservation. The conodont alterations characterize low grade metamorphism (sensu Winkler, 1979), namely the anchizone or epizone sensu Kovacs & Arkai (1987). Discovery of conodonts within the micritic limestone of peperitic hyaloclastites confirms Triassic time of the intrusion, what is synchronous with the extensive rifting magmatism within the Vardar zone and Meliata unit. The basalt cryptodome of Hruškovec, Mt. Kalnik, is not a dismembered ophiolitic fragment, but the product of Triassic, advanced rifting magmatism. It does not imply, however, absence of the ophiolites in the closer or wider surroundings of the Hruškovec locality and Kalnik Mt. (Pamić, 1997), but probably of Jurassic age. Similar findings on Medvednica Mt. with conodonts in the micritic parts of the peperites (Halamić et al., 1998) supports regional significance of the discovery, but not confirms the presence of the Triassic oceanic crust, represented with the dismembered ophiolitic sequence, if such ever existed in Triassic time within Dinaric realm.

Triassic rifting magmatism; pillow lavas; conodonts; Zagorje-Mid-Transdanubian zone

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