Journal of Petrology | Volume 17 | Number 4 | Pages 440-471 | 1976
© Oxford University Press 1976
research-article |
Pumpellyite-Actinolite Facies Schists of the Taveyanne Formation near Loèche, Valais, Switzerland
Geology Department, University of Otago Dunedin, New Zealand (D.S.C.)
Geological Institute, University of Tokyo Japan (Y.N.)
Départment de Minéralogie, Université de Genève ( M.V.)
Received October 28, 1975; Revised November 14, 1975; ABSTRACT
Electron microprobe analyses are presented for new-formed minerals from a small exposure of semi-schistose Taveyanne Formation of the pumpellyite-actinolite facies near Loèche, Valais. Comparisons are drawn with minerals of other low-grade metamorphic areas, especially in southern New Zealand. Sphene shows considerable substitution of Ca(Al,Fe)SiO4(OH) for CaTiSiO5. Epidotes are sharply divided into early pistacitic (Ps = 0.28–0.37) and later clinozoisitic varieties (Ps = 0.11–0.19). Pumpellyites range from pumpellyite-(Fe) to pumpellyite-(Al) and are generally less Fe-rich than those of zeolite and prehnite-pumpellyite facies. Pumpellyite inclusions in albitized plagioclase are particularly low in Mg. Actinolites are low in A12O3, TiO2, and Na2O, essentially identical compositions being nucleated on detrital augite, hornblende, and in the matrix. Phengites are also extremely low in Na2O and TiO2. Chlorites are ripidolites. Albitized clastic plagioclase has the composition An0.7–1.6 and albite in clinozoisite-calcite-albite-phengite-chlorite veins An2.1–2.3. Calcites carry minor Mn > Fe » Mg. New-formed iron oxides are absent, whereas pyrrhotite and minor pyrite occur in one rock, buffering fs2 and indicating low fo2.
Ratios Mg: Fe* (Fe* = total Fe) in coexisting chlorites and A1, Na-poor actinolites vary sympathetically both in the Loèche and southern New Zealand rocks here considered, giving KD = (Mg/Fe*) actlnolIte/(Mg/Fe*)chlorle = 1.72. Mg/Fe* ratios in pumpellyites tend to vary sympathetically with those of coexisting chlorites and actinolites but are more variable. Substitution of (Fe, Mg)Si for A12 in phengitic micas and chlorites varies sympathetically in the same suites between mafic volcanic and more pelitic extremes. Various minor elements also behave in a consistent fashion, indicating an encouraging tendency towards equilibrium. Variable (though small) A12O3 contents of actinolite, Fe: Al ratios in epidotes and pumpellyites, and Mg: Fe* ratios in phengites, even within a single grain, are evidence of short-range disequilibrium; metamorphic equilibration is evidently easier between some crystal structures and structural sites than between others.
In phase rule analysis of assemblages in such rocks it is commonly necessary to treat Fe2O3, FeO, and MgO as separate components and it may also be necessary to regard CO2 as an inert component and/or to interpret observed assemblages as of low variance. The presence of the Ca-Al silicates and sphene indicates very low Xco2 in the metamorphic fluids in all rocks examined except an albite-chlorite-calcite-quartz-anatase assemblage. But higher An in albites than in isofacial and in greenschist facies rocks of southern New Zealand can be ascribed to significantly higher Xco2 at Loèche, especially in the veins, than in New Zealand.
Pumpellyite and epidotes of the pumpellyite-actinolite facies tend to be lower in Fe and richer in Al than those of lower grade facies. Important reactions include those of the form pumpellyite-(Fe3+)+chlorite+quartz+H2=pumpellyite-(Al)+actinolite, and pumpellyite+chlorite+quartz- ‘epidote’+actinolite+water. Careful selection of pumpellyite and chlorite compositions is required for experimental and chemographic analysis of pumpellyite stability. In the absence of critical data, temperatures of about 250–350° and pressures of several kilobars are provisionally suggested for the Loèche metamorphism.
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