Pages: 123 - 136
Abstract: The Middle Jurassic (uppermost Bajocian–lower Bathonian to upper Bathonian–lower Callovian) radiolarian cherts in the Medvednica Mt (NW Croatia) have a high content of SiO2 (average 90.87 %). Most of the silica is of biogenic origin as is indicated by a high Si/Si+Al+Fe+Ca ratio (0.83–0.97). The Al/Al+Fe+Mn ratio (average 0.59) and relatively low contents of Fe and Mn suggest that the sedimentation of the radiolarian cherts was not influenced by hydrothermal volcanisms. The high correlation coefficient between the lithophile elements Ti, K, Al, Th, Zr, Hf and Rb implies that the detrital component in radiolarian cherts for the most part has a terrigenous provenance. The MnO/TiO2 ratio and Lan/Cen vs. Al2O3/(Al2O3+Fe2O3) diagram show that the investigated cherts were derived from two different, but not necessarily strongly separated, sedimentation areas: (1) continental shelf and slope or marginal sea, and (2) deep ocean floor, trench or basaltic plateau. According to the proposed sedimentation model the radiolarian cherts in the Medvednica Mt were deposited in a relatively narrow basin. The detrital material was derived from two source areas: (1) from a continent (terrigenous input) and (2) from an accretionary wedge (undifferentiated magmatic arc-like input). During the Late Jurassic–Early Cretaceous the radiolarian cherts were incorporated into the tectonic melange (accretionary prism) along with other fragments: Triassic radiolarian cherts and carbonate rocks; Jurassic shales, siltites and sandstones and basic and ultrabasic magmatic rocks.
Keywords: Jurassic, Croatia, south-western Pannonian region, major and trace elements, radiolarian chert geochemistry, REEDownload PDF document Show Text Version