Petrographical, mineralogical and geochemical features of miocene dyke from Podkraj pri Mežici, Slovenia

Mineraloško, petrografsko in geokemično smo raziskali dajk miocenske starosti iz Podkraja pri Mežici. Za raziskavo kamnine smo uporabili naslednje raziskovalne metode: optično mikroskopijo v presevni svetlobi, elektronsko mikroskopijo (SEM-EDS), rentgensko fluorescenčno analizo (XRF), rentgensko praškovno difrakcijo (XRD) in multielementno geokemično masnospektrometrično analizo (ICP-MS). Makroskopsko je raziskovani dajk debelozrnati porfirski andezit, z do 2 cm velikimi vtrošniki plagioklazov, do 4 mm velikimi biotiti in do 6 mm velikimi vtrošniki rogovače v zelo drobnozrnati do mikrokristalni osnovi, ki vsebuje enklave epidotiziranega in sausuritiziranega rogovačnega diorita. Za raziskavo pod optičnim in elektronskim mikroskopom smo naredili 13 zbruskov, za geokemične raziskave pa je bilo narejenih 11 vzorcev. Mineraloška analiza je pokazala, da v andezitu prevladuje plagioklaz, ki po sestavi ustreza andezinu. Preiskani vzorci vsebujejo v osnovi veliko količino primarnega in sekundarnega kremena ter sekundarnih mineralov (kaolinit, montmorillonit, klorit in vermikulit), ki so posledica hidrotermalne spremembe. Kot vključki v plagioklazih in osnovi so zrnca granatov, ki po sestavi ustrezajo almandinu. Osnova kamnine je drobnozrnata do mikrokristalna z veliko manjšimi zrnci kremena, kar je za značilno za dacite, za andezite pa je taka sestava zelo nenavadna. Plagioklazi so po robovih spremenjeni - karbonatizirani in kaolinitizirani, kar je verjetno prav tako posledica hidrotermalnih sprememb. Geokemične raziskave so pokazale, da se elementna sestava nekoliko razlikuje po profilu dajka. Andezit je po sestavi alkalen, z veliko kalijevega oksida (do 2,34% K2O), in sodi v kalcijevo alkalno serijo. Tovrstni andeziti so glede na literaturo sicer značilni za cone subdukcije, kjer nastajajo orogeni, vendar menimo, da je vzrok za povečano vsebnost kalija njegova asimilacija in s tem kontaminacija magmatske taline z njenim stikom z apnenci.We researched mineralogical, petrographical and geochemical features of Miocene dyke from Podkraj pri Mežici, Slovenia. Several different researching methods were used: optical microscopy, electron microscopy (SEM-EDS), x-ray fluorescence analysis (XRF), x-ray powder diffraction (XRD) and inductively coupled plasma mass spectrometry (ICP-MS). In hand specimen, the rock is coarse grained andesite porphyry, with up to 2 cm long plagioclase phencrysts, up to 4 mm long biotite grains and up to 6 mm long hornblende phenocrysts in fine grained groundmass. There are also enclaves of hornblende diorite which shows signs of epidotisation and saussuritization. 13 thin sections were made for microscopic research and 11 samples were made for geochemical research. Mineralogical analysis showed that the plagioclase in the rock is andesine. There is a lot of primary and secondary quartz in the groundmass which also contains secondary minerals like kaolinite, montmorillonite, chlorite and vermiculite, because the rock is altered hydrothermally. The carbonation and kaolinitization of plagioclase grains also indicates hydrothermal alteration. Garnets, which belong to almandine group, are found as inclusions in plagioclase grains or independently in the groundmass. The groundmass itself is fine grained with a lot of small grains of quartz, which is typical for dacitic rocks and very unusual for andesitic rocks. Geochemical research showed that the elemental composition slightly differs along the profile of the rock. Andesite is acidic, with a lot of potassium oxide (up to 2,34% K2O) and is a part of calc-alkaline series. Andesites that belong to the high-K calc-alkaline series are usually typical for subduction zones, but we believe that this much potassium is caused by potassium assimilation and melt contamination because of melt – limestone contact