Ringrazio Paola Bonazzi, il Gruppo Nazionale di Mineralogia, il Dipartimento di Scienze della Terra e l’Università di Firenze per avermi dato questa opportunità di portarvi oggi una breve testimonianza in onore di Filippo Olmi. Pur se un po’ indirettamente, in quanto ero già a Cagliari a quel tempo, posso annoverare Filippo tra i miei allievi di Mo...
Complex reaction textures in coronitic metagabbros and retrograded eclogites of the KTB pilot and an adjacent drilling provide evidence for a multistage metamorphic history in the Variscan basement of the NW Bohemian Massif. The eclogites show complete metamorphic recrystallization leaving no textural or mineral relics of their igneous precursors. In contrast, textural relics of the igneous protolith are still preserved in the metagabbros where the metamorphic overprint under high pressure conditions achieved only partial replacement of the initial assemblage plagioclase + augite + amphibole (+olivine or orthopyroxene?) + ilmenite to form the eclogite facies assemblage garnet + omphacite + kyanite + zoisite + quartz+rutile. The garnets in the metagabbros occur in the typical necklace fashion at the borders between the original plagioclase and mafic phase domains. In the same rocks, omphacite formed by a topotactic reaction mechanism replacing igneous augite as well as in smaller grains at the margins of the texturally igneous clinopyroxene where it occurs without fixed orientation with respect to the relict phase. Both eclogites and metagabbros show a partial breakdown under high pressure granulite (transitional to high pressure amphibolite) facies conditions during which omphacite broke down to vermicular symplectites of diopside + plagioclase. A later pervasive medium pressure metamorphism under amphibolite facies conditions led to the development of assemblages dominated by hornblende + plagioclase+titanite: phases prevailing in the overwhelming majority of the surrounding metabasites. Subsequent vein-associated retrogression produced minerals typical of the greenschist to zeolite facies. All metamorphic stages may be represented in a single thin section but although the overall reaction sequence is apparent, the obvious disequilibrium in the rocks makes the use of conventional geothermobarometry difficult. However, calculations made by assuming an approach to domainal equilibrium show that both the eclogite facies and early breakdown occurred above 10 kb. As the metamorphic unit hosting these particular metabasites is generally characterized by pressures below 10 kb these results have important implications for understanding the tectonometamorphic evolution of the region. The relationship between the studied rocks and other units in the NW Bohemian Massif exhibiting a multistage metamorphic evolution is discussed and possible tectonic models evaluated.
The metagabbro-amphibolite sequences in the KTB pilot hole contain intercalations of talc-chlorite-amphibole felses (or ''hosbachites''), which show transitional contacts to the adjacent metagabbros. The hosbachites are characterized by relics of a primary igneous texture and still contain igneous minerals like clinopyroxene, biotite and pseudomorphs after olivine, while brown Ca-amphibole was presumably formed in a late-magmatic stage. The geological, textural, mineralogical and geochemical evidence indicates that the hosbachites were derived from ultramafic cumulates, differentiated from a basaltic magma, either in the inner parts of dolerite sills or in small gabbro intrusions. A pervasive metamorphic overprint under medium-pressure, amphibolite-facies conditions which was accompanied by penetrative deformation led to assemblages with green Ca-amphibole +/- anthophyllite +/- cummingtonite +/- tremolite/actinolite + clinochlore + talc + olivine + ilmenite +/- Cr-bearing spinel + sulfides. Phase relationships are consistent with a prograde P-T path leading to the formation of anthophyllite from olivine + talc at peak metamorphic temperatures of 640-700-degrees-C, at assumed pressures of 8-10 kbar, similar to those derived from mineral assemblages in the adjacent metabasites and metasediments. High-pressure relics locally present in coronitic metagabbros and retrograded eclogites of the KTB pilot hole were not recognized in the hosbachites. A retrograde overprint under greenschist-facies conditions led to the total replacement of igneous or metamorphic olivine by aggregates of antigorite + magnetite, chloritization of biotite and the formation of late tremolite/actinolite.