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Geochronological, geochemical and metamorphic framework of the Moslavačka Gora Massif (Croatia)

The Moslavačka Gora Massif (MGM) in Croatia, located about 50 km east-south-east of Zagreb, is a major exposure of crystalline basement within the Tertiary sediments of the Pannonian Basin. The central part of the massif is built up of a finegrained, two-mica S-type granite. The peripheral parts are formed by a metamorphic complex which includes mainly felsic anatexites and orthogneisses of granitic composition, some metapelites (paragneisses and micaschists) and rare amphibolites. In terms of the Alpine plate tectonic framework, the MGM has been considered as a part of the Tisia block. Recently, the MGM has been related to the so-called “Sava Zone”, which is believed to host the suture between Tisia and the Dinarides. Reliable information concerning the geological evolution of the massif was as yet hardly available, so that any correlation with neighboring basement units remained largely speculative. For a long time it was a common belief that the MGM represents Variscan crystalline basement. However, our new geochronological data indicate that this massif is a Cretaceous high-temperature/low-pressure metamorphic dome that was intruded in its center by a Cretaceous granite pluton. The age of the LP/HT metamorphism is estimated at ca. 90–100 Ma using the method of electron microprobe based Th-U-Pb dating on monazites. U-Pb dating of zircons by means of LA-SF-ICP-MS method for the Central Granite gave a concordant age of 82 ± 1 Ma. Zircons from three different samples of metagranites were dated at 486 ± 6 Ma, 483 ± 6 Ma and 491 ± 1 Ma, suggesting that most of the metamorphic complex of the MGM represents an Early Ordovician granitic series. In order to better characterize the different metagranites of the MGM, a geochemical study was carried out. The volumetrically dominant Jelen grad type defines a coarse grained, K-feldspar phyric, S-type granite suite. Fine grained metagranites (Garić-grad type) show the characteristics of A-type magma. The metamorphic evolution of the MGM was studied on metapelites. From the observed peak paragenesis (Crd-Grt-Kfs) and electron microprobe data it can be concluded that the Cretaceous metamorphism was of the low-pressure type reaching granulite facies conditions of ca. 750 °C and 3–4 kbar. A retrograde metamorphic event, mainly caused by heat and fluid input from the Central Granite, occurred at lower amphibolites facies conditions (500–600 °C, 1–2 kbar). As yet there are no clear evidences for a Variscan tectonothermal event in the MGM. However, Permian magmatism or anatexis in the lower crust is indicated by inherited Permian zircons in the Central Granite. Relics of Permian monazite found in one sample of metapelite document a Permian phase of metamorphism in the metamorphic series of the MGM. The high heat flow regime of Cretaceous age is unique in the basement of the Pannonian Basin and may be a local feature triggered by a mafic intrusion in the lower crust.

Moslavačka gora; geochronology; geochemistry

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