engleski

Petrology and tectonic significance of greenschists from the Medvednica Mts, (Sava unit, NW Croatia

The Early Cretaceous (124.0-113.6 Ma) low-grade metamorphic complex of the Medvednica Mts. located at the SW Mid-Transdanubian shear zone consists of ortho- and para-schists showing single stage metamorphic event and increase of metamorphic grade from prehnite-actinolite to greenschist facies. Massive to foliated green rocks from the central part of the mountain show mineral paragenesis typical of greenschist facies metamorphic rocks derived from igneous protolithes. Electron microprobe analyses were carried out for albite actinolite, epidote, chlorite, and titanite in order to yield information about metamorphic conditions, and chemical composition of the representative samples were determined to characterize geochemical affinity of the igneous precursors. Metamorphic P-T conditions inferred by comparison with experimentally calibrated phase reactions producing observed paragenesis are estimated to pressure maximum of 0.27 GPa and temperature minimum of 330 °C for the iron-free system, and are consistent with temperature range of 305-350 °C determined by chlorite geothermometry. Metamorphic precursors have compositions of evolved basalts and andesites, which were hydrothermally altered, at least albitized, prior to the metamorphism. They show geochemical characteristics similar to suprasubduction tholeiites, and an intraoceanic island arc was inferred as most plausible geotectonic setting of the precursor eruptions. Structural analysis suggests that the greenschists of the Medvednica Mts were formed during southwestern vergent obduction of the Jurassic to Early Cretaceous ophiolites onto the Dinaridic continental margin composed of Ordovician to Upper Triassic sedimentary and volcano-sedimentary successions. The petrologic and geochemical investigation showed that an island arc was involved in the tectonic movements which formed Medvednica Mts. low-grade metamorphic complex.

Medvednica Mts ; orthogreenschists ; metamorphic conditions ; tholeiitic island arc protolithes ; ophiolite obduction

nije evidentirano