| Přispěvatelé: |
Froitzheim, N., Bousquet, R., Fügenschuh, B., Schmid, S., Tomljenovic, B. |
| Popis: |
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. |
