Secondary mineral paragenesis in the mafic extrusive rocks from the Mt. Medvednica ophiolite melange

• Published in: Rudarsko-geološko-naftni zbornik 2012-12, Vol.25 (1), p.33-46
• Main Authors: Slovenec, Damir, Lugovic, Bosko, Slovenec, Dragutin
• Format: Article
• Language: English
• Subjects: basaltic extrusives; hydrothermal alterations; Mt. Medvednica; ophiolite mélange; secondary minerals
• ISSN: 0353-4529; 1849-0409

The samples of partly or completely altered basaltic extrusive rocks from Mt. Medvednica ophiolite melange were analysed by the optical microscopy and by X-ray powder diffraction (XRD). The chemical composition of minerals from different paragenetic assembleges were determinated by electron probe microanalysis (EPMA) and the measured microdomains were documented by back-scattered electrons (BSE) imagery. The most widespread alteration processes that affected analysed rocks are albitization, chloritization, epidotization, prehnitization, pumpellyitization, zeolitization and titanitization. Subordinate processes that include formation of limonite, calcite, zoisite, siderite, hematite, Mn-oxydes, pyrite and Si[O.sub.2]-phases were not constrained in this paper. Albitization is fairly dominant alteration process in all rock and its progradation throughout a plagioclase grain is clearly documented by BSE microphotographs and microprobe chemical analysis. Prehnite and laumontite are sporadic alteration products of plagioclases. The chemical components released during the alterations of plagioclases and/or clinopyroxenes along with components from circulating and percolating heated see water and fluids enable formation of chlorites, epidote and pumpellyite. Chlorites and pumpellyite formed also by devitrification of volcanic glass in vitric rim of pillow lavas. Small patches of leucoxene, that is abundant in all samples, represent dense aggregates of titanite nanocrystals replacing ilmenite and/or Ti-magnetite. Epidote-bearing paragenesis in the analyzed rocks record comparatively the highest alterations grade that correspond to the middle grade of greenschist facies metamorphism typical of upper sequence of a see floor crust. Key words: secondary minerals, hydrothermal alterations, basaltic extrusives, ophiolite melange, Mt. Medvednica Uzorci djelomicno ili potpuno izmijenjenih bazaltnih ekstruzivnih stijena iz ofiolitnog melanza Medvednice analizirani su primjenom opticke mikroskopije i rendgenske difrakcije u prahu. Kemijski sastav minerala u razlicitim paragenezama odreden je pomocu elektronske mikrosonde, a strukture analiziranih mikrodomena su dokumentirane slikama unatrag rasprsenih elektrona. Glavni alteracijski procesi ukljucuju albitizaciju, kloritizaciju, epidotizaciju, prehnitizaciju, pumpellyitizaciju, zeolitizaciju i titanitizaciju. Sporedni procesi su limonitizacija, kalcitizacija, zoisitizacija, sideritizacija, hematitizacija, manganizacija, pirit