Esther Schmädicke

Eclogite-facies rocks in the Saxonian Erzgebirge, Germany: high pressure metamorphism under contrasting P-T conditions

The metamorphic sequences of the Saxonian Erzgebirge were thoroughly overprinted by a Variscan medium-pressure event under amphibolite facies conditions. However, eclogitic relics documenting an older high-pressure event are widespread. P-T conditions of the eclogite-facies metamorphism systematically decrease, over a distance of 50 km, from about >29 kbar/850°C, in the central part, to 20–24 kbar/650°C, in the westernmost part of the Erzgebirge crystalline complex. A distinct gap in P-T conditions exists between the central and the western Erzgebirge coinciding with the fault zone of the Flöha syncline. Therefore, the eclogitebearing sequences are assumed to represent at least two different nappe units. The lower-grade eclogite assemblages in the western Erzgebirge display a continuous metamorphic zonation with a gradual decrease of peak metamorphic temperatures towards the west. Assemblages formed in the stability field of coesite and thus indicating a regional ultra-high pressure metamorphism, are restricted to the central Erzgebirge, where they are widespread in the eclogites, but also present in metaacidic country rocks. The same high-temperature/high-pressure conditions, testifying to a burial of at least 100 km, were independently recorded for the ultramafic garnet pyroxenites associated with the eclogites of the central Erzgebirge. Mineral relics included in the eclogite phases and mineral assemblages formed by retrograde reactions permit reconstruction of the prograde and retrograde P-T paths in the different parts of the Erzgebirge crystalline complex.

A first find of retrogressed eclogites in the Odenwald Crystalline Complex, Mid-German Crystalline Rise, Germany: evidence for a so far unrecognised high-pressure metamorphism in the Central Variscides

Abstract Metabasic rocks were recently found in the Bollsteiner Odenwald, being part of the Variscan Mid-German Crystalline Rise (MGCR), that give evidence of a so far unrecognised eclogite-facies metamorphic event and testify, for the first time, to high-pressure metamorphism in the MGCR, the assumed suture zone of the European Variscides. Eclogite-facies metamorphism is indicated by both widespread clinopyroxene–plagioclase symplectites—interpreted as breakdown products of omphacite—and the composition of symplectitic clinopyroxene with measured jadeite contents of up to 27 mol%, extending into the omphacite field. Reintegration of numerous clinopyroxene–plagioclase symplectites implies minimum jadeite contents of the former omphacite of at least 38 mol%. For the eclogite stage, the four-phase assemblage omphacite-garnet-quartz-rutile can be reconstructed. A post-eclogitic overprint led to the formation of symplectitic intergrowths of clinopyroxene and plagioclase, amphibole–plagioclase coronas around garnet and domains with recrystallised amphibole and plagioclase. Preliminary P–T estimates for the eclogite-facies metamorphism indicate minimum pressures of some 16–17 kbar and temperatures of approximately 700±50 °C. Geothermobarometry for the subsequent symplectitic breakdown of omphacite yields some 14 kbar and 700 °C. P–T estimates on retrograde amphibolite-facies domains and on prograde mineral assemblages preserved in garnet cores point to a clockwise P–T path experienced by these rocks. The eclogites formed from a tholeiitic protolith, that may have been genetically linked to a continental extension zone or a young oceanic ridge or back-arc environment.

Intergrowths between anthophyllite, gedrite, calcic amphibole, cummingtonite, talc and chlorite in a metamorphosed ultramafic rock of the KTB pilot hole, Bavaria

A metamorphosed ultramafic rock, penetrated by the KTB pilot hole at a depth of 1382.36 m, contains the assemblage Ca-amphibole — anthophyllite — chlorite — talc, formed at about 630°C/10 kbar under conditions of the high-P amphibolite facies. EMP analyses of the Ca-amphibole yielded magnesiohastingsite (to pargasite) compositions with (Al+Fe 3+ +Cr+Ti) [6] of 1.0-1.2 p.f.u. and (Fe 2+ +Mn)/(Mg+Fe 2+ +Mn) ratios of 0.10-0.16, while coexisting anthophyllite has (Al+Fe 3+ +Cr+Ti) [6] ≤ 0.16 and (Fe 2+ +Mn)/(Mg+Fe 2+ +Mn) ratios of 0.21-0.29. Chlorite is clinochlore with Al [4] /(Al [4] +Si) of 0.27-0.30 and Fe 2+ /(Fe 2+ +Mg) of 0.12-0.14. TEM investigations revealed, for the first time, a complex lamellar intergrowth in the sequence talc → anthophyllite → anthophyllite/cummingtonite intercalations → anthophyllite → Ca-amphibole. This lamellar intergrowth is about 15 μm in width, but individual cummingtonite lamellae intercalated with dominant anthophyllite are only 1 μn wide. All the amphiboles share the directions b * and a * ; anthophyllite and the monoclinic amphiboles are intergrown along (100); anthophyllite and talc are intergrown with (100) Ath //(001) Tlc . In other areas anthophyllite and chlorite are intergrown with a * Ath // c * Chl and b * Ath // b * Chl and Ca-amphibole exsolves lamellae of cummingtonite //(100). With one possible exception, cummingtonite has the space group C 2/ m . Anthophyllite displays chain multiplicity faults //(010), anthophyllite and cummingtonite chain arrangement faults //(100). Microstructures suggest that anthophyllite was formed at the expense of cummingtonite that is interpreted as a high-T precursor phase testifying to a possible earlier, granulite-facies metamorphic stage. During the retrograde P-T path, on cooling presumably below 500-550°C, anthophyllite exsolves platelets or lamellae of gedrite with {hk0} composition planes. Orientations close to {230} and {110} have been recorded, so far not described in the literature. Alteration products of biotite are composed of clinochlore and a sheet silicate intermediate in composition between chlorite and biotite, i.e. , with partly high K contents, but with chlorite metric.

THE METAMORPHOSED MAFIC-ULTRAMAFIC COMPLEX OF MOKURO, ILESHA SCHIST BELT, SOUTHWESTERN NIGERIA

Abstract In the Proterozoic Schist Belt of Nigeria, lenticular bodies of metabasites and meta-ultramafics are frequently intercalated within staurolite bearing metapelitic schists. Such a metamorphosed mafic-ultramafic complex is particularly well exposed in the Mokuro riverbed between the towns of Ife and Ilesha. These outcrops display contact relationships with the surrounding metasediments, as well as between the individual mafic and ultramafic rock types. The most common mafic rocks are indistinctly layered amphibolites, accompanied by apatite rich amphibolites and massive amphibolites, in part rich in ilmenite and pyrrhotite. Among the generally massive ultramafic rocks, nearly monomineralic amphibole rocks predominate, while chlorite-amphibole, talc-chlorite-amphibole and talc bearing olivine-chlorite-amphibole rocks occur in subordinate amounts. Field, textural and geochemical evidence suggest that the mafic-ultramafic complex derived from a thick, structurally differentiated basaltic sill that contained doleritic portions in its interior. Slow cooling rates in these inner parts enabled crystal settling with the formation of ultramafic cumulates. Due to the enrichment of volatiles during the crystallisation process, higher amounts of apatite and sulphides, as well as late magmatic amphibole, were formed in parts of the mafic-ultramafic body. Mineral assemblages in the mafic-ultramafic complex testify to a metamorphic overprint under amphibolite-facies conditions. Thermodynamic modelling in the system CMFASH leads to an estimated P–T range of 1.5–3 kbar and 550–620°C for the metamorphic peak assemblage talc-olivine-chlorite-Ca amphibole-orthoamphibole.

Phase relations of calcic amphibole–orthoamphibole–chlorite- talc assemblages, with applications to ultramafic rocks from the KTB pilot hole, Bavaria

Abstract Lenses of ultramafic rocks intercalated within a metagabbro–amphibolite sequence were encountered in the KTB pilot hole. A pervasive metamorphic overprint formed the dominating assemblage calcic amphibole–orthoamphibole–chlorite–talc. In this study the phase relations of ultramafic rocks are investigated in order to (a) constrain the stability field of this assemblage in general, and (b) define the equilibrium pressure–temperature (P–T) conditions of this assemblage in the ultramafic rocks from the KTB borehole. For that purpose, phase equilibria were calculated in the model systems CaO–MgO–Al2O3–SiO2-H2O (CMASH) and CaO–MgO–FeO–Al2O3–SiO2-H2O (CMFASH). Thereby, the continuous compositional change of solid solutions with pressure and temperature was modeled, including the Tschermak’s substitution and the MgFe-1 exchange. Based on these results, petrogenetic grids were constructed, revealing that calcic amphibole–orthoamphibole–chlorite–talc assemblages cover a stability field of <650–770 °C/1→ 14 kbar (CMASH) and <550–650 °C/1→14 kbar (CMFASH), respectively. This explains the widespread occurrence of the considered assemblage. Based on the bulk rock composition of the KTB samples, a special P–T diagram was constructed, limiting the stability field of the calcic amphibole–orthoamphibole–chlorite–talc assemblage. At 580 °C the stability field extends from 6 to 14 kbar pressure, and shrinks to 10–11 kbar at 630 °C. Conventional estimates using the mineral compositions of the KTB samples yield a temperature around 630 °C, at which the calculated stability field of calcic amphibole–orthoamphibole–chlorite–talc extends from 10 to 11 kbar.