Corrensite in metabasalts and metagabbros from Mt. Medvednica, Croatia

Complex mineral paragenetic and mineral structural and -chemical studies were carried out on metabasite samples from the ophiolitic mélange unit of Mt. Medvednica, Croatia using pétrographie microscopic, XRPD and EMP techniques. According to XRPD studies of the <2 pm and < 0.6 |xm grain-size fractions, chlorite and corrensite (regularly interstratified chlorite/smectite) are the predominant phyllosilicate components. Randomly interstratified chlorite/smectite and discrete smectite are also found in minor amounts. On the basis of the EMP analyses chemical composition of "chloritic materials" from different textural positions reveals that they contain not only chlorite layers but certain amount of trioctahedral smectite (saponite) interlayers also occur being responsible for their relatively high Ca and/or Na contents, large excess of A1(V1) relative to Al(IV) and high octahedral vacancy. The formation temperature of the "chloritic materials" is suggested to be in the range of ca. 160- 200 °C using the geothermometer of Cathelineau (1988). Metabasalt samples from the ophiolitic mélange could be affected by minimum alteration, and the appearance of corrensite, together with mixed-layered chlorite/smectite and discrete smectite suggest "intermediate" temperature conditions (Shau and Peacor, 1992). According to Proust (1982) and Proust et al. (1986) corrensite as chlorite/vermiculite forms as an intermediate product of the weathering but the occurrence of chlorite/smectite in the studied samples may indicate that rather the diageneticincipient metamorphic alteration is the process that may be the main mechanism responsible for its formation

Prográd és retrográd metamorf folyamatok jellemzése a filloszilikátok és más kőzetalkotó ásványok szerkezeti és kémiai változásai alapján = Characterization of prograde and retrograde metamorphic processes on the basis of structural and chemical changes of phyllosilicates and other rock-forming minerals

Kimutattuk, hogy koegzisztens K- és Na-csillámok esetén az ammónium a K-csillámot preferálja. A dioktaéderes világos csillámok rétegközi K, Na és Ca tartalma megoszlási törvényszerűségeinek, rendeződési folyamatainak leírására - nemzetközi együttműködésben - ún. domén modellt dolgoztunk ki. A modell értelmezi a világos csillámok növekvő hőmérséklettel végbemenő diagenetikus és kezdeti metamorf fejlődését. Meghatároztuk a kőzetkémizmus, a hőmérséklet, a nyomás és a mikrotextura viszonyok szerepét a paragonit és prekurzor fázisa képződésében metapélitekben, hangsúlyozva a tényleges és effektív kémiai összetétel különbségeinek szerepét. A dioktaéderes csillámok és a klorit retrográd átalakulási folyamatait átmeneti képlékeny-rideg nyírásos deformációs rendszerben (Kelet-Bükk) és epizónás metapélitek posztmetamorf hidrotermális és mállási folyamatainak (Szendrői-hegység, Gadna) példáján értelmeztük. Kimutattuk, hogy a röntgendiffraktométeres módszerrel meghatározott kalcit "kristályossági" index és átlagos doménméret a filloszilikátok hasonló paramétereivel pozitív korrelációt mutat, és felhasználható a metamorf fok kiegészítő jellemzésére. A metamorf fok jelző szervetlen (ásványtani) mutatók és a szénült szerves anyag "érettségi" mutatóinak korrelációját a szerves anyag koncentrátumok Raman spektrumainak termometriai értékelésével egészítettük ki. Összehasonlítottuk a horvátországi Medvednica hegység és a vele rokon É-magyarországi képződmények metamorf fejlődéstörténetét. | It was shown that in case of coexisting K- and Na-white micas ammonium had been preferentially enriched in K-mica. For describing the partitioning and ordering processes of the interlayer cations (K, Na and Ca) in white micas, a domain model was elaborated. The model explains the diagenetic and incipient metamorphic evolution of white micas and their precursor phases. The effects of bulk rock major element chemistry, pressure, temperature and microtexture in formation of paragonite and its precursor phase were evaluated for metapelites, emphasizing the role of differences between the as measured and effective rock chemistries. Retrograde processes of white micas and chlorite were interpreted in a transitional ductile-brittle shear system (Eastern Bükk Mts.) and on the example of an epizonal slate (Szendrő Mts, Gadna) affected by post-metamorphic hydrothermal activity and weathering. Positive correlations were found between the calcite 'crystallinity' index, mean crystallite size determined by XRPD and the same parameters of phyllosilicates. Thus, calcite 'crystallinity' may be used for metamorphic petrogenetic purposes as a complementary tool. The correlation between the metamorphic grade (temperature) indicating parameters of inorganic (mineral) and organic (coalified matter) rock constituents was improved by thermometry based on the Raman spectra of organic matter. The metamorphic histories of the Medvednica Mts. (Croatia) and the related NE-Hungarian regions were compared

Dioctahedral mixed K-Na-micas and paragonite in diagenetic to low-temperature metamorphic terrains: bulk rock chemical, thermodynamic and textural constraints

Abstract Metamorphic mineral assemblages in low-temperature metaclastic rocks often contain paragonite and/or its precursor metastable phase (mixed K-Na-white mica). Relationships between the bulk rock major element chemistries and the formation of paragonite at seven localities from Central and SE-Europe were studied, comparing the bulk chemical characteristics with mineral assemblage, mineral chemical and metamorphic petrological data. Considerable overlaps between the projection fields of bulk chemistries of the Pg-free and Pg-bearing metaclastic rocks indicate significant differences between the actual (as analyzed) and effective bulk chemical compositions. Where inherited, clastic, inert phases/constituents were excluded, it was found that a decrease in Na/(Na+Al*) and in K/(K+Al*) ratios of rocks favors the formation and occurrence of Pg and its precursor phases (Al* denotes here the atomic quantity of aluminum in feldspars, white micas and “pure” hydrous or anhydrous aluminosilicates). In contrast to earlier suggestions, enrichment in Na and/or an increase in Na/K ratio by themselves do not lead to formation of paragonite. Bulk rock chemistries favorable to formation of paragonite and its precursor phases are characterized by enrichment in Al and depletion in Na, K, Ca (and also, Mg and Fe2+). Such bulk rock chemistries are characteristic of chemically “mature” (strongly weathered) source rocks of the pelites and may also be formed by synand post-sedimentary magmatism-related hydrothermal (leaching) activity. What part of the whole rock is active in determining the effective bulk chemistry was investigated by textural examination of diagenetic and anchizone-grade samples. It is hypothesized that although solid phases act as local sources and sinks, transport of elements such as Na through the grain boundaries have much larger communication distances. Sodium-rich white micas nucleate heterogeneously using existing phyllosilicates as templates and are distributed widely on the thin section scale. The results of modeling by THERMOCALC suggest that paragonite preferably forms at higher pressures in low-T metapelites. The stability fields of Pg-bearing assemblages increase, the Pg-in reaction line is shifted towards lower pressures, while the stability field of the Chl-Ms-Ab-Qtz assemblage decreases and is shifted towards higher temperatures with increasing Al* content and decreasing Na/(Na+Al*) and K/(K+Al*) ratios

Stratigraphy, facies and geodynamic settings of Jurassic formations in the Bükk Mountains, North Hungary: its relations with the other areas of the Neotethyan realm.

Jurassic mélange complexes related to the subduction of the Neotethys Ocean occur in the Bükk Mountains, North Hungary. This paper characterizes the sedimentary sequence of basin and slope facies that occur in the southwestern part of the Bükk Mountains, placing special emphasis on the redeposited sedimentary rocks (olistostromes, olistoliths: Mónosbél Group) in order to obtain information on the provenance of the clasts, and the mode and time of their redeposition. The series of formations studied shows a general coarsening-upwards trend. Based on radiolarians and foraminifera, the Mónosbél Group formed in Early to Late Bathonian time. The lower part of the complex is typified by a predominance of pelagic carbonates, shale and radiolarite with andesitic volcaniclastic intercalations. The higher part of the succession is characterized by polymictic olistostromes. Large olistoliths that are predominantly blocks of Bathonian shallow marine limestone (Bükkzsérc Limestone) appear in the upper part of the sequence. Based on the biostratigraphic and sedimentological data, results of analyses of the redeposited clasts and taking into consideration the concepts of the development of the western Neotethys domain, the evolutionary stages of the sedimentary basins were defined. The onset of the compressional stage led to initiation of nappe stacking that led to the formation of polymict olistostromes and then to the redeposition of large blocks derived from out-of-sequence nappes of the former platform foreland