|Abstract (english)|| |
The pre-Neogene basement of the Pannonian basin in the eastern Croatia is composed of different types of magmatic, metamorphic and sedimentary rocks. Core samples of granitoids from deep wells have been chosen for petrographic and geochemical analyses, namely 5 samples for mineral chemistry by EMPA, 23 samples for whole rock chemical analysis by ICP-AES and ICP-MS and 3 samples for strontium isotope analysis by TIMS. The aim of this study was to define different types of granitoids, determine the magma source, petrogenetic processes and tectonic setting of magmatic activity, to correlate analysed rocks with a possible equivalents presented in literature and put them into the regional-geological and geodynamic context.
In the Slavonija-Srijem depression several types of rocks have been analysed: 1) alkali feldspar granite, microgranite, alkali feldspar syenite of Privlaka area; 2) mylonitic gneiss of Đeletovci area and 3) porphyritic granite and rhyolite of Županja-3 exploration well.
Alkali feldspar granite contains perthite, quartz, albite, biotite and hornblende. Alkali feldspar syenite is composed of similar mineral assemblage - perthitic orthoclase and microcline, albite, biotite, quartz and hornblende. On the other hand, microgranite contains antiperthite, perthite, quartz and albite, with the sporadic ferromagnesian phase completely altered to chlorite. Alkali feldspar granite represented shallow intrusion, which is indicated by petrographic characteristics and confirmed by Zr-saturation temperatures of 830-850 ⁰C along with pressure values of <2.5 kbar estimated by Al-in-hornblende geobarometer. Biotite and hornblende chemistry is characterised by high Fe/(Fe+Mg) ratio, pointing to crystallization under low fO₂. Whole rock geochemistry of the alkali feldspar granites is characterised by 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, indicating A-type affinity. Alkali feldspar syenite is geochemically similar to alkali feldspar granite, but is characterized by positive K and Zr anomalies in primitive mantle-normalized trace element patterns and by the absence of Eu anomaly in chondrite-normalized REE patterns. These geochemical features indicate a cumulate origin for the alkali-feldspar syenite. Otherwise similar to alkali feldspar granite, microgranite is characterized by lower K₂O and Rb and higher Na₂O and Sr contents. These characteristics could be a consequence of magma separation during its late stage of evolution, which was controlled by crystallization of K-feldspar and biotite. Strontium istotope ratio analysed in alkali feldspar granite (⁸⁷Sr/⁸⁶Sr=0.7202) indicates crustal source of magma. Based on their petrographic and geochemical characteristics, as well as the mode of the occurence, Privlaka granitoids could be correlated with Požeška gora alkali feldspar granites (Pamić, 1987; Schneider, 2017), a part of the Upper Cretaceous bimodal complex of Sava zone (Ustaszewski et al., 2010).
Mylonitic gneiss of Đeletovci is composed of quartz, K -feldspar, plagioclase, biotite, muscovite and garnet. Geochemical characteristics, especially primitive mantle-normalized trace element patterns and chondrite-normalized REE patterns, indicate a protolith comparable to Privlaka alkali feldspar granite. Porphyritic granite of Županja-3 exploration well is composed of orthoclase, albite, and quartz, while the ferromagnesian mineral is completely altered to chlorite. Rhyolite is composed of quartz, feldspar and muscovite. Whole rock geochemical characteristics of these rocks, such as high Ba, low Zr, Nb and Y concentrations, weak negative Eu anomaly and more pronounced LREE/HREE enrichment compared to granitoids of Privlaka, indicate a different origin of Županja-3 porphyritic granite and rhyolite.
In the eastern part of Drava depression different types of rocks have been determined: 1) monzodiorite, granodiorite and hornblende diabase of Obod area; 2) mylonitic gneiss of Marjanci-4 exploration well; 3) monzogranite of Osijek-2A exploration well and 4) leucogranite of Torjanski Rid-1 exploration well.
Monzodiorite and granodiorite, with enclaves of hornblende diabase, occur as shallow intrusion(s). Hornblende diabase is composed of plagioclase and hornblende, with local biotite and quartz close to the contact with the granodiorite. Monzodiorite contains plagioclase, hornblende, orthoclase, quartz and biotite. Granodiorite is composed of plagioclase, quartz, orthoclase and biotite, with hornblende appearing near the contact with the hornblende diabase. Hornblende as the main ferromagnesian phase indicates unusually high water content in melt. Textural characteristics, such as the presence of different kind of mafic microgranular enclaves, point to magma mixing and mingling processes. Whole rock geochemistry of analysed hornblende diabase, monzodiorite and granodiorite is characterized by Cs positive and Ba negative anomaly in primitive mantle-normalized trace element patterns, along with LREE enrichment relative to MREE and HREE in chondrite -normalized REE patterns. These characteristics point to a common source, probably lithospheric mantle metasomatized by subduction processes. Mantle source is also indicated by strontium isotope ratio analysed in monzodiorite sample (⁸⁷Sr/⁸⁶Sr=0.7043). Hornblende diabase and monzodiorite display weak negative Eu anomaly, while granodiorite displays positive Eu anomaly. This feature rules out fractionation of plagioclase during the evolution of magma. Overall characteristics of the analysed Obod magmatic rocks are similar to Paleogene intrusive rocks of Zala basin basement in Hungary (Benedek et al., 2004).
Mylonitic gneiss of Marjanci-4 well is composed of plagioclase, quartz, K-feldspar, biotite and allanite. Zonal plagioclase and allanite indicate the granitic protolith. The presence of magmatic allanite and high Zr-saturation temperature (803 ⁰C), point to high temperature crystallization. This rock is characterized by generally high trace element content, with depletion of HREE and Y in chondrite-normalized REE pattern and primitive mantle-normalized trace element pattern, respectively. In chondrite-normalized REE pattern it displays no Eu anomaly.
Monzogranite of Osijek-2A well contains quartz, zonal plagioclase, muscovite and biotite. It is characterized by high K₂O, P₂O₅. Primitive mantle-normalized trace element pattern displays positive Ba, U and Pb anomalies and negative Nb, Ti and Eu anomalies. Correlation with literature data indicate similarity between this monzogranite and S-granites of Papuk complex (Pamić and Lanphere, 1991; Horvat, 2004).
Leucogranite of Torjanski Rid-1 exploration well is composed of quartz, albite, perthite and muscovite. Whole rock geochemistry is characterized by high SiO₂, K₂O, P₂O₅, low CaO, TiO₂, FEOt, MgO and generally very low concentration of trace elements, besides Cs, Rb, U and Pb, as well as very pronounced Eu anomaly in chondrite-normalized REE pattern. These characteristics point to magma originating by low-degree partial melting of the metasedimentary source, probably by muscovite dehydration melting.