Naslov
Petrochemical and geotectonic characteristics of amphibolites from the Zagorje-Mid-Transdanubian shear Zone (Mt. Kalnik, Croatia)

Autori
Šegvić, Branimir ; Boško Lugović ; Sanja Ignjatić

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
Abstract Book / Velić, Ivo ; Vlahović, Igor ; Biondić, Ranko - Zagreb : Hrvatski geološki institut, 2005, 143-144

Skup
Third Croatian Geological Congress

Mjesto i datum
Opatija, Hrvatska, 29.09.2005. - 01.10.2005

Vrsta sudjelovanja
Predavanje

Vrsta recenzije
Domaća recenzija

Ključne riječi
amphibolites; metamorphic sole; BARB and IAT protolithes; Mt. Kalnik

Sažetak
Mt. Kalnik is located at the southwestern tip of the Zagorje-Mid-Transdanubian shear Zone and is largely composed of ophiolite mélange (i. e. the Kalnik Unit) which discontinuously extends to the SW into the Medvednica Mt. Conodont paleontology in sedimentary cover overlying MORB-type pillow lavas from the mélange suggests Illyrian-Fassanian to Bathonian-Early Callovian age of topmost layers of relict oceanic crust. The Kalnik Unit originated from a trench accretionary prism setting during the Middle Jurassic to Hauterivian as indicated by palynomorph assemblages documented in the mélange matrix. Remnants of an intraoceanic arc is recognised in the greenschist facies metabasites of the Medvednica Mts. obducted onto Adria platform in the Early Cretaceous implying the existence of an island arc during Middle Jurasic to Hauterivian time. Amphibolites were sampled from the scarce outcrops representing a few meters thick slices composed of amphibolites and overlaying serpentinised mantle peridotites, and from amphibolite-bearing debris of the nearby streams. These slices were tectonically emplaced within Upper Cretaceous-Paleogene sedimentary succession. Amphibolites, previously assumed as blocks of ophiolite mélange are fine- to medium-grained, well foliated with nemathoblastic texture. Primary assemblage comprises light green amphibole, plagioclase, accessory titanite and apatite ± ; ; garnet ± ; ; clinopyroxene ± ; ; zoisite-clinozoisite. Retrograde paragenesis consists of epidote, zoisite-clinozoisite, titanite, chlorite and actinolite. Reaction rim around remnant garnet cores and patches comprises albite (An0.1-0.3), actinolite, chlorite and quartz. The mineral paragenesis decipher an amphibolite to lower granulite facies prograde metamorphic event followed by a greenschist facies retrograde metamorphism. The amphibolites overlain by serpentinites are sheared and show penetrative deformation, all others are undeformed. Lenses and veins filled by calcite and prehnite are common in the deformed amphibolites. All characteristics allow us to interpret these amphibolites as sub-ophiolite metamorphic sole. Clinopyroxene is homogeneous ferroan diopside with Mg/(Mg+Fe2+) ratio around 73.5. Amphibole representing peak-metamorphism displays wide compositional range stretching from Ti-rich magnesio-hornblende (0.037-0.195 i.p.f.u. Ti) through tschermakite to pargasite. Plagioclase ranges in composition from An29.8 to An48.6. The garnet is almandine rich (Pyp10.8-19.7Alm52.1-56.7Sps2.9-8.4Grs16.5-25.0) and shows prograde compositional variation across the grains. Epidotes have the range of 100*Fe3+/(Fe3++AlVI) ratios between 14.3 and 23.8, and show the highest values in the secondary assemblages indicating high oxygen fugacity during the retrogression. High grade titanite has abundant Ti (up to 0.957 i.p.f.u.) apart from the retrograde compositions (around 0.777 i.p.f.u. Ti). Peak equilibrium temperatures of the Mt. Kalnik amphibolites were estimated for rim to rim compositions of coexisting minerals using different geothermometers. The geothermometers based on coexisting amphibole-plagioclase pairs in silica-saturated systems and on garnet-clinopyroxene Fe-Mg exchange equilibrium have given the most consistent results (580-620˚ C and 680˚ C, respectively). Pressure estimation was performed by the various geobarometers (0.59-1.05 GPa). Amphibolites from the Mt. Kalnik originated from igneous mafic protoliths. REE patterns of the amphibolites and their plots in discrimination diagrams decipher geochemical affinity of the back-arc ridge basalts (BARB) and tholeiitic island arc (IAT) mafic magmas, respectively. Spidergrams of the arc related magmas reveal geochemical signatures similar to MORB but show slight flavour of subducted slab fluids recorded by negative Ta-Nb and other HFSE anomalies. Some IAT samples show strong positive Eu anomaly suggesting that cumulate rocks were also involved in the metamorphism. In the absence of clear field relations between the amphibolites and other rocks from the Kalnik Unit we use the geochemical data as plausible substitute. Overall signatures of the protoliths indicate two geochemical types of metamorphic sole which were formed in a suprasubduction setting. We suppose that subduction was initiated by detachment of oceanic lithosphere at the slow rate back-arc spreading ridge. The hot oceanic crust was heated by the overridden mantle sequence peridotites to form the high grade amphibolites of tholeiitic BARB affinity. Closely latter, the peridotites were obducted over an active or a while ago dormant intraoceanic magmatic arc yielding metamorphic sole amphibolites with IAT geochemical signatures. The greenschist facies metamorphic overprint of the metamorphic sole took place during the exhumation of composite peridotite-metamorphic sole slices before their emplacement into continental margin Upper Cretaceous-Paleogene sedimentary succession. Proposed geodynamic model implies the existence of a separated marginal basin during the Early Cretaceous located more to the north from the Central Dinaridic oceanic strand. On account of overall characteristics of the metamorphic sole the correlation between these two oceanic domains seems not viable since the protolithes of metamorphic sole from the Central Dinaridic Ophiolite Belt show exclusively MORB affinity and were metamorphosed around 172 Ma ago.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

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