engleski

Petrology and geochemistry of the A-type granites and related rocks in the Western Srijem area (Eastern Croatia)

Among many different lithological units, including different types of magmatic, metamorphic and sedimentary rocks, alkali feldspar granite, microgranite, alkali feldspar syenite and mylonitic gneiss are important constituents of the Neogene basement of the Western Srijem area (Eastern Croatia). These rocks belong to Upper Cretaceous bimodal magmatism of the Sava suture zone. The magmatic rocks display similar mineral compositions - alkali feldspar granite is composed of perthite, quartz, albite, biotite and amphibole, microgranite contains antiperthite, perthite, quartz and albite, while the alkali feldspar syenite is composed of perthitic orthoclase and microcline, albite, quartz and amphibole. Mylonitic gneiss is composed of quartz, K-feldspar, plagioclase, biotite, muscovite and garnet. Zr-saturation temperatures of 830-850°C and pressure values estimated by Al-in-hornblende geobarometer <2, 5 kbar obtained for the alkali feldspar granite indicate shallow intrusive origin, which is in accordance with petrographic characteristics. A high FeOt/(MgO+FeOt) ratio along with high contents of K2O, Na2O, REE3+, Rb, Zr, Nb and Y, accompanied by low contents of CaO, MgO, P2O5, Ba, Sr and Eu in alkali feldspar granite suggest an A-type affinity. It is in accordance with the mineral chemistry of the ferromagnesian phases – annitic biotite showing clear alkaline affinity and ferro-edenite. The high Fe/(Fe+Mg) ratios of biotite and amphibole point to crystallization under low fO2. Geochemical characteristics of the mylonitic gneiss indicate a protolith comparable to alkali-feldspar granite. Alkali feldspar syenite, otherwise similar to the alkali-feldspar granite, has no Eu anomaly in chondrite-normalized REE patterns, and is characterized by positive K and Zr anomalies in primitive mantle-normalized trace element patterns. These observations indicate a cumulate origin for the alkali-feldspar syenite. The microgranite, on the other hand, has low K2O and Rb contents and high Na2O and Sr contents compared to the alkali feldspar granite. This could be explained by magma separation during its late stage of evolution, which was controlled by crystallization of K-feldspar and biotite. The overall evolution of the magma is controlled by fractional crystallization. Partial melting was followed by fast magma ascent to shallow levels (about 8 km deep). A-type magmatism is related to extensional tectonics, indicating an Upper Cretaceous extensional event probably located in the continental crust. A possible geodynamic setting is a fore-arc basin located on the upper European plate.

A-type magmatism, Neogene basement, Sava suture zone

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