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Ultramafische Einschaltungen in Metabasiten der KTB-Vorbohrung

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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.
Article
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.
Article
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Clinochlore, which is, within the limits of error, the thermally most stable member of the Mg-chlorites, breaks down at \(P_{{\text{H}}_{\text{2}} {\text{O}}} \) = P tot to the assemblage enstatite+forsterite+spinel+H2O along a univariant curve located at 11 kb, 838 ° C; 15kb, 862 ° C; and 18 kb, 880 ° C (±1 kb ±10 ° C). At water pressures above that of an invariant point at 20.3 kb and 894 ° C involving the phases clinochlore, enstatite, forsterite, spinel, pyrope, and hydrous vapor, clinochlore disintegrates to pyrope+forsterite+spinel+H2O. The resulting univariant curve has a steep, negative dP/dT slope of −930 bar/ °C at least up to 35 kb. Thus, given the proper chemical environment, Mg-chlorites have the potential of appearing as stable phases within the earth's upper mantle to maximum depths between about 60 and 100 km depending on the prevailing undisturbed geotherm, and to still greater depths in subduction zones. However, unequivocal criteria for mantle derived Mg-chlorites are difficult to find in ultrabasic rocks.
Article
New experimental data on the upper thermal stability of chlorite in the system H2O-MgO-Al2O3-SiO2 are reported here and are combined with other experimental data, molar volumes of the solid phases, heat capacities, H2O fugacities, and activity expressions for cordierite, chlorite and orthopyroxene to obtain an array of univariant curves about an invariant point at 720 + or - 10oC and 2.75 + or - 0.3 kbar where chlorite, cordierite, forsterite, orthopyroxene, spinel and water are in equilibrium.-J.A.Z.
Article
The pure magnesium orthorhombic amphibole, anthophyllite, has been synthesized and its upper and lower stability limits have been established by reversible hydrothermal experiments. The synthetic mineral has refractive indices n x=1·587±0·001, n y=1·602±0·005, n z=1·613±0·001, and unit-cell dimensions of a 0=18·61±0·02 Å, b 0=18·01±0·06 Å, c 0=5·24±0·01 Å. The mineral is stable over a narrow temperature range in the presence of the phase H 2O. At a P H2O of 1,000 bars the upper stability limit is 745°±10° C and the lower stability limit is 667°±8° C.Rate studies indicate that anthophyllite can nucleate at temperatures above its upper stability limit by disintegration of talc sheets into strips of double chains. The activation energy for this process is 150±30 kcal mol -1.Application of the data to rocks of the Balmat area, New York, suggests that the equilibrium pressure of water during the metamorphism was significantly less than the total pressure. The data indicate that monomineralic zones of anthophyllite in ultramafic rocks are due to the presence of a steep gradient in the activity of H 2O, or a steep gradient in temperature, or both, across the zones.
Article
Mineral equilibria in the MgO--SiOâ--HâO system were investigated, with emphasis on the stability relations of talc, antigorite, forsterite, anthophyllite, and enstatite. Thermal dehydration temperatures at 1 kb HâO are at 514° +- 12°C (Ta--Antig--Fo), 627° +- 7°C (Ta--Fo--Anth), 670° +- 7°C (Anth--Fo--En), 675° +- 7°C (Ta--Anth--Q), 643° +- 8°C (Ta--Fo--En), 690° +- 8°C (Ta--En--Q), 729° +- 8°C (Anth--En--Q). Derived G°/sub f/ values in kcal/gf are -928.756 +- 1.1 (Antig, based on Mgâ.âââSiâOâ (OH)â.ââ), -2711.320 +- 1.7 (Anth) and -348.955 +- 0.6 (EN). Anthophyllite has a relatively narrow temperature stability range at 1 kb, but it increases as P/sub HâO/ increases. The Ta--Fo--En and Ta--En--Q reactions are low-pressure changes, metastable at 1 kb. The (Q) and (Fo) invariant points are at approximately 603°C, 500 bars, and 601°C, 175 bars, P/sub HâO/, respectively. Higher P/sub total/ raises and spreads the P/sub HâO/ values of the invariant points. The log SiOâ /sub aq/ versus 1/T diagrams utilized in this study are applied to metasomatic and isochemical processes involving this system. Aspects of contact metamorphism and metasomatism, the process of serpentinization, and compositional controls and relative stabilities of the serpentine phases are discussed. Isochemical cooling in the subsolidus at finite, though not necessarily constant, P/sub HâO/ results in a mineral reaction series analogous to that of the classical reaction series of silicate melts. The chemical constraints revealed in the various equilibria studied help to clarify and more rigorously to define the isochemical PT relationships of the system. The geological application of activity-activity or chemical potential diagrams is also discussed, and the necessity of considering reactions that correspond to or approximate the true process involved is illustrated.
Article
New experimental data on the upper thermal stability of chlorite in the system H'O- MgO-AlrOr-SiO, are reported here and are combined with other experimental data, molar volumes of the solid phases, heat capacities, HrO fugacities, and activity expressions for cordierite, chlorite, and orthopyroxene to obtain an array of univariant curves about an invariant point at 720 + l0"C and2.75 + 0.3 kbar where chlorite, cordierite, forsterite, orthopyroxene, spinel, and water are in equilibrium. A crystallochemical study of chlorite synthesized from three different bulk compositions at 3 and 14 kbar and over the tem- perature range of 650-850"C suggests that Mg-chlorite attains a composition of (Mg.rAl'.r) (Sir 8Alr ,)Oro(OH)8 at its upper thermal stability at all pressures. Thermochemical data for clinochlore were derived from the experimental data on the dehydration of chlorite above 3.5 kbar, yielding values of -8220.452 + 27 kJ/mol and -2186.2 + 33 J/(K'mol) for
Article
Ultrabasic metamorphic rocks with typical mineral assemblage of cummingtonitic hornblende+Mg-chlorite+talc (described byMatthes u.Okrusch, 1965, and called “hösbachit”) were found for the first time in Odenwald/Germany. Three steps of metamorphic development can be described: the primary magmatic ultrabasic rock consisted of (1) orthopyroxene (bronzite), clinopyroxene (diallage), poikilitic olivine, and some hornblende. This is indicated by textural relicts, structural and geochemical investigations. A regional metamorphic process under conditions of the staurolite-almandine-subfacies ofWinkler’s andTurner andVerhoogen’s almandine-amphibolite facies transformed this assemblage to (2): cummingtonitic hornblende I+Mg-chlorite I (clinochlore)+talc. The third step followed under more diaphthoritic conditions and brought mineral assemblage (3): tremolitic hornblende II+Mg-chlorite II (pennine)+talc.