engleski

Geochemistry, Geochronology and Metamorphic Evolution of the Moslavačka Gora Massif (Croatia)

The Moslavačka Gora Massif is located about 50 km east-south-east of Zagreb. It covers an area of about 180 km2 and represents one of the major surface exposures of crystalline basement within the Tertiary sediments of the Pannonian Basin (PAMIĆ, 1990 ; PAMIĆ et al., 2002). The central part of the massif is made up of different types of granites: the fine-grained undeformed Pleterac type and the slightly deformed Garić-grad type. The peripheral parts are mainly built up by migmatites and various types of metamorphic rocks (gneisses, amphibolites, metapelites). Geochemistry of the granitoids The Pleterac type (PT) granites are slightly more peraluminous than the Garić-grad type (GGT) granites, whereas the later have on average a slightly higher SiO2 (70-75 vs 73-76 wt.%). Clear chemical differences exist in the Ba, Y and Zr contents. The GGT granites are probably derived from metaigneous lower crustal sources. With reference to their high barium (543-788 ppm) and yttrium (44-94 ppm) contents, they resemble fractionated I-type granites (CHAPPELL, 1999). The PT granites correspond chemically rather to S-type than to I-type granites. They show A/CNK ratios slightly above 1.1 and much lower yttrium (15-30 ppm) and barium (201-444 ppm) at higher rubidium concentrations (184-323 ppm). The PT and the GGT granites can also be distinguished by their zircon morphologies (see STARIJAŠ et al., 2005, this volume). Geochronology For a long time the Moslavačka Gora Massif has been considered as a major outcrop of Variscan crystalline basement of the South Tisia unit, like the Slavonian Mountains (Papuk, Psunj and Krndija). However, BALEN et al. (2001, 2003) reported Ar-Ar mica and amphibole cooling age < 90 Ma for various rocks of the massif. Also, a systematic electron-microprobe-based dating campaign on monazites from the granitic and the metamorphic rocks, which was carried out recently at Salzburg University, provided almost exclusively Cretaceous ages. Just in one sample of a metapelite, relics of monazites with a Permian age were found (STARIJAŠ et al., 2004). Metamorphic evolution The Cretaceous metamorphism in the Moslavačka Gora Massif was of the low-pressure type and reached, at least in places, granulite facies grade (ca. 700-750 °C, 3-4 kb), with the formation of the paragenesis garnet + cordierite + K-feldspar + An-rich plagioclase + quartz (± ; sillimanite, ± ; biotite) in metapelitic lithologies. For a retrograde metamorphic event, which involves the growth of biotite at the expense of garnet, the replacement of cordierite by muscovite and biotite intergrowths, the replacement of sillimanite through muscovite + quartz symplectites, the growth of andalusite and the formation of a new, less calcic plagioclase, the thermobarometric estimates are ca. 550 °C and 3 kb. This retrograde overprint may be related to the intrusion of the Pleterac granite. Due to the large error of the electron-microprobe-based monazite dating method (ca. ± ; 20 Ma), the precise timing of the different Cretaceous events (prograde low-P/high-T metamorphism, retrograde overprint, granitic plutonism) could not be resolved yet. Ar-Ar plateau ages of muscovites from a pegmatite and a deformed granite of around 75 Ma (BALEN et al., 2001) provide a lower age limit for the Cretaceous high-T metamorphism and the intrusion of the Moslavačka Gora granites, respectively. In amphibolitic lithologies, mineral relics of a (possibly pre-Mesozoic?) Barrovian type metamorphosis are preserved but are not yet dated. A possible relationship with the Banatite magmatic belt? Due to the newly discovered Cretaceous formation ages of the granitic rocks, a correlation of the Moslavačka Gora Massif with the Banatite magmatic belt of southeastern Europe may be possible. According to NEUBAUER (2002) the Banatite belt may have formed as a consequence of post-collisional slab break-off, representing a long but narrow zone, with increased heat input from the astenospheric mantle. The high-T/low-P metamorphism recorded in the Moslavačka Gora Massif indicates a position within a Late Cretaceous high-heat-flow zone, and thus would be compatible with such a model. Chemical data indicate that the Moslavačka Gora granites are most likely derived from crustal sources, representing mainly slightly peraluminous, felsic granites (s.s.) to granodiorites. They may have formed as secondary magmas in the contact aureole of hot mafic mantle melts ponding at the base of the crust. References BALEN, D., SCHUSTER, R., GARAŠIĆ, V. (2001): A new contribution to the geochronology of Mt. Moslavačka Gora (Croatia).- PANCARDI 2001Abstracts, DP-2, Sopron. BALEN, D., SCHUSTER, R., GARAŠIĆ, V., MAJER, V. (2003): The Kamenjača olivine gabbro from Moslavačka Gora (South Tisia, Croatia). Rad HAZU, 486, 27, 57-76. CHAPPELL, B. (1999): Aluminium saturation in I- and S-type granites and the characterization of fractionated haplogranites. Lithos, Volume 46, Issue 3, 535-551. NEUBAUER, F. (2002): Contrasting Late Cretaceous with Neogene ore provinces in the Alpine-Balkan-Carpathian-Dinaride collision belt. Geological Society Special Publications No. 204, 81-102. PAMIĆ, J. (1990): Alpine granites, migmatites and metamorphic rocks from Mt. Moslavačka Gora and the surrounding basement of the Pannonian Basin (Northern Croatia, Yugoslavia).- Rad JAZU 10, 7-121, Zagreb. PAMIĆ J., BALEN D., TIBLJAŠ D. (2002): Petrology and geochemistry of orthoamphibolite from the Variscan metamorphic sequences of the South Tisia in Croatia – an overview with geodynamic implications. Int J Earth Sci (Geol Rundsch) 91, 787-798. STARIJAŠ, B., BALEN, D., TIBLJAŠ, D., SCHUSTER, R., HUMER, B., FINGER, F. (2004): The Moslavačka Gora Massif in Croatia: part of a Late Cretaceous high-heat-flow zone in the Alpine-Balkan-Carpathian-Dinaride collision belt. Ber. Inst. Erdwiss. K.-F.-Univ. Graz, Band 9 Anhang, 453-454, Graz, Austria, 24.-26. September 2004.

cretaceous ; granites ; metamorphism ; geochemistry ; geochronology ; Moslavačka Gora ; Croatia

nije evidentirano