Globina kristalizacije pohorskega granodiorita

Sotelšek, Tim (author), Vrabec, Mirijam (mentor), Vrabec, Marko (comentor)

Kamnine granodioritne intruzije na Pohorju se pojavljajo v večih različkih. V splošnem se spreminja od bolj debelozrnate strukture na jugovzhodnem delu, do drobnozrnate proti severozahodnem delu. Te razlike so opazne tako makroskopsko kot tudi mikroskopsko. Delni vzrok za to je lahko, da na površju izdanjajo različni deli intruzije. JV deli lahko predstavljajo globlje dele, SZ pa plitvejše dele intruzije. Tako geometrijo je možno razložiti z nagnjenjem magmatskega telesa po njegovem vtisnjenju, kar bi se moralo odražati v tlakih, pri katerih so kristalili posamezni deli. V poliranih preparatih kamnin smo prepoznali kremen, ki nastopa v treh generacijah; K-glinence, ki po kemični sestavi ustrezajo ortoklazom; od plagioklazov se pojavljata večinoma oligoklaz in andezin. Povprečno vsebujejo 31 % anortitske komponente. Biotiti se po sestavi najbolj približujejo annitom. Med amfiboli se pojavljajo tschermakiti, fero-tschermakiti in Mg rogovače. Kot akcesorni minerali nastopajo titanit, allanit-(Ce), epidot, cirkon, apatit in granat. Slednji je najverjetneje magmatskega nastanka in ni metamorfni restit. Tlake in temperature smo določali s pomočjo konvencionalnih geotermobarometrov. Tlake smo izračunali na podlagi vsebnosti aluminija v amfibolih. Izračunani tlaki se nižajo od JV dela, kjer dosegajo 6,6 kbar, proti SZ, kjer znašajo 2 kbar. To potrjuje predpostavko, da je bilo magmatsko telo po vtisnjenju nagnjeno, zaradi česar so danes na površju razgaljeni različni globinski nivoji. Temperature smo določili s pomočjo treh termometrov, in sicer s termometrom amfibol plagioklaz, biotitovim termometrom in termometrom K-glinenec-plagioklaz. Temperature, z izjemo biotitovega termometra, kažejo enak trend padanja proti SZ delu intruzije. S 724 ? padejo na 670 ?. Z vmesnim dvigom tlakov v sicer padajočem trendu proti SZ, smo potrdili tudi obstoj normalnega preloma, ki v zahodnem delu seka intruzijo in spušča JV blok.

• Language: Slovenian
• Keywords: pohorski granodiorit, tlak, temperatura, geotermobarometrija
• Work type: Bachelor thesis/paper
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Year: 2019
• PID: 20.500.12556/RUL-109816
• COBISS.SI-ID: 1525598
• Publication date in RUL: 08.09.2019

Secondary language

• Language: English
• Title: Depth of crystallisation of granodiorite from Pohorje Mountains

Abstract

Granodiorite in the Pohorje intrusion occurs in several varieties. Its texture changes from coarse-grained in the southeastern part to fine-grained in the northwest. The difference can be seen both microscopically and macroscopically. A possible explanation of this observation is that different levels of intrusion are exposed at the surface, which would imply post-intrusional tilting of the magmatic body. Southeastern parts could represent deeper levels of intrusion whereas the northwestern part belongs to its shallower domains. Consequently it is to be expected that pressures of crystallisation will vary systematically across the presently outcropping pluton. Quartz was recognised in three generations in polished thin sections; K-feldspars chemically correspond to orthoclases. Among plagioclases oligoclase and andesine were recognised. Plagioclases consist of 31 % of anorthite component in average. Biotites are close to annite in chemical composition. Amphiboles occur as tschermakite, ferro-tschermakite and Mg-hornblende. Among accessory minerals titanite, allanite-(Ce), epidotes, zircon, apatite and garnets were identified. The latter are most probably of magmatic origin and not a metamorphic restite. Pressures and temperatures were determined using conventional geothermobarometers. Pressures were calculated on the basis of Al- amount in amphiboles. Calculated pressures decrease from southeast to northwest from 6.6 kbar to just 2 kbar. This supports our idea that the intrusion was tilted after emplacement. Temperatures were deduced using amphibole-plagioclase, biotite and K-feldspar-plagioclase thermometers. Temperatures follow the same decreasing trend as pressures with the exception of biotite-calculated temperatures, and exhibit a decrease from 724 ? in the southeastern to 670 ? in the northwestern part of the granodiorite body.

• Keywords: Pohorje granodiorite, pressure, temperature, geothermobarometry