Noncarnbonate mineral phases in marbles from Pohorje

Pohorski marmor je metamorfna kamnina, ki izdanja v obliki leč in manjših teles zlasti na vzhodnem in južnem delu pohorskega hribovja. Za razliko od ostalih metamorfnih kamnin na Pohorju za marmor še ne vemo natančno, pri kakšnih metamorfnih pogojih je nastajal. Delni vzrok za to je izjemna čistost pohorskih marmorjev z malo nekarbonatnimi minerali, ki pa so ključnega pomena za določitev metamorfnih pogojev v času nastajanja kamnin. Določitve teh nekarbonatnih mineralnih faz v pohorskem marmorju smo se lotili s pregledom poliranih zbruskov na optičnem mikroskopu pri polarizirani svetlobi. Bolj natančne preiskave nekarbonatnih mineralnih faz smo opravili z vrstično elektronsko mikroskopijo v kombinaciji z energijsko disperzijsko spektroskopijo rentgenskih žarkov (SEM/EDS) in z elektronsko mikrosondo v kombinaciji z valovnodolžinsko disperzijsko spektroskopijo rentgenskih žarkov (EPMA/WDS). Nekarbonatne mineralne faze, ki smo jih določili v marmorju, so: glinenci, pirokseni, amfiboli, olivini, kremen, epidot, muskovit, biotiti, flogopit, fengit, kloriti, serpentini, titanit, apatit, zoisit, cirkon, turmalini, skapolit, rutil, barit, ilmenit, hematit in pirit. Pirokseni so Ca-Mg sestave z 0,5 do 10,3 ut. % FeO in različnim deležem CaO in MgO, pri čemer smo določili predvsem različke endiopsid, diopsid, avgit in wollastonit.Glinenci so zelo različne sestave od skoraj popolnoma kalijevih glinencev s 93 mol. % ortoklazove komponente do povsem anortitovih razločkovnajdemo še plagioklaze s sestavo bytownita, labradorita, andezina in oligoklaza. Med amfiboli prevladujejo Ca amfiboli tremolit, aktinolit, edenit, fero-edenit in fero-pargasit, ki vsebujejo od 3,8 do 14,1 ut. % CaO, 4,5–24,1 ut. % MgO, 0,6–31,2 ut. % FeO ter do 0,2 ut. % MnO in do 1,5 ut. % Na2O. Od olivinov je prisoten forsterit s 85,2–97,4 mol. % forsteritove komponente. Glede na mineralno paragenezo kalcit + kremen + biotit + tremolit + forsterit + diopsid + plagioklaz ali kalcit + dolomit + kremen + biotit + tremolit + forsterit + diopsid + plagioklaz sklepamo, da so marmorji s Pohorja dosegli visoko stopnjo metamorfoze, ki sega v območje regionalne metamorfoze amfibolitnega faciesa.Pohorje marble is a metamorphic rock that forms lenses and smaller bodies and is outcropping predominantly on the eastern and southern parts of the Pohorje Mountains. Unlike for other metamorphic rocks from Pohorje, for marbles, we still do not know precisely what were the metamorphic conditions during the peak of the metamorphism. The partial reason for this is the exceptional purity of Pohorje marbles with very seldom non-carbonate minerals, which are crucial for determining metamorphic conditions at the time of rock formation. The determination of these non-carbonate mineral phases in the Pohorje marbles was carried out by examining the polished thin sections using the optical microscope under plain polarized light. More precise investigations of non-carbonate mineral phases were carried out using electron microscopy in combination with X-ray diffraction spectroscopy (SEM / EDS) and electron microprobe in combination with X-ray diffraction spectroscopy (EPMA / WDS). Recognized non-carbonate mineral phases in marbles are: feldspars, pyroxenes, amphiboles, olivines, quartz, epidot, muscite, biotites, phlogopite, fengite, chlorites, serpentines, titanite, apatite, zoisite, zircon, turmalins, scapolite, rutile, hematite and pyrite. Pyroxene are Ca-Mg pyroxenes with 0,5 do 10,3 wt % FeO and various proportions of Ca and Mg oxide, and are determined mainly as endiopside, diopside, augite, and wollastonite.The feldspars are very different in composition varying from almost pure potassium feldspar with 93 mol% of the orthoclase component to completely pure anorthite, and in addition there are plagioclases with the composition of bytownite, labradorite, andesite and oligoclase. Among the amphiboles, Ca amphibole tremolite, actinolite, edenite, ferro-edenite and ferro-pargasite are predominant, containing from 3.8 to 14.1 wt % CaO, 4.5-24.1 wt % MgO, 0.6-31.2 wt % FeO and up to 0.2 wt % MnO and up to 1.5 wt % Na2O. Forstite is present with 85.2–97.4 mol. % of forsterite component. Based on mineral paragenesis calcite + quartz + biotite + tremolite + forsterite + diopside + plagioclase or calcite + dolomite + quartz + biotite + tremolite + forsterite + diopside + plagioclase we conclude that marbles from Pohorje reached high matamorphic grade in the area of the regional metamorphism of the amphibolite facies