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Leucogranite from Srednja Rijeka (Moslavacka Gora, Croatia)

The Mt. Moslavacka Gora represents crystalline crust exposed over an area of 180 km2 in the southwestern part of the Pannonian Basin. It consists of a S-type granitoid pluton surrounded by migmatites and metamorphic rocks of amphibolite to granulite facies grade. Geochronological data indicate a Cretaceous emplacement of the pluton and contemporaneous metamorphism in the country rocks (Lanphere & Pamic, 1992 ; Garasic, 1993 ; Balen, 1999 ; Balen et al., 2001 ; Starijas et al., 2004). The investigated leucogranite of Srednja Rijeka is located in the northern part of Mt. Moslavacka Gora. It forms dikes within a peraluminous two-mica granite, suggesting an extensional regime during its emplacement. Mafic enclaves are missing in the leucogranite whereas they are present in the two-mica granite. The leucogranite is characterized by subequal proportions of quartz, K-feldspar (microcline and orthoclas) and plagioclase (Ab93-95) with variable contents of muscovite, biotite, garnet, andalusite and tourmaline. Four types of muscovite (A-, B-, C- and D-typ) were recognized on the basis of textural and chemical characteristics. Consistently high AlIV (2.65-2.90 apfu) in biotite (annite) indicates its equilibration with an Al-rich phase. Garnet occurs as anhedral, interstitial grains. It is chemically homogeneous (XFe=0.6 ; XMn=0.4). The Fe3+ content in andalusite decreases from the pinkish core (0.011 apfu) to the colourless rim (0.007 apfu). Tourmaline (schorl-foitite) shows chemical zonation too, with Na+ and F- contents increasing from core (0.488 ; 0.287 apfu) to rim (0.623 ; 0.389 apfu). The leucogranite is strongly peraluminous (ASI = 1, 25), contains 75 wt.% SiO2 and low concentrations of Fe2O3 (0.71-0.79 wt.%), MgO (0.06-0.08 wt.%), CaO (0.45-0.55 wt.%) and TiO2 (0.03-0.05 wt.%). It has also low contents of Ba (7-50 ppm) and Sr (4-21 ppm) and a low Sr/Ba (0, 4-0, 5) ratio. The high Rb content (209-234 ppm) as well as high Rb/Ba (4-36) and Rb/Sr (10-67) ratios pointing to K-feldspar rich melting residue and muscovite dehydration melting at low aH2O. Additionally, the leucogranite shows low concentrations of Zr (31-44 ppm) and REE, which are usually explained by zircon and monazite retention in the residue. The chondrite normalized REE patterns are not fractionated [(La/Yb)N = 2.02] and have a prominent negative Eu anomaly (Eu/Eu* = 0.09). Field relations, mineral assemblage and chemistry of the investigated rocks are consistent with MPG type granitoids and indicate its formation by melting of continental crust in a collisional environment.

leucogranite; Moslavačka gora; peraluminous two mica granite; andalusite; REE pattern

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