An albite-anorthite assemblage in low-grade amphibolite facies rocks

Poikiloblastic amphibole schists from an ophiolite series in the vicinity of the Tertiary Bergell granite contain the assemblage albite (An 0)-anorthite (An 95), actinolite-pargasite, together with clinozoisite. TEM illustrates how large andesine crystals with a periodic APB (anti-phase boundaries) structure of e plagioclase change over a few hundred angstroms to pure albite and pure PT anorthite. Anorthite formation from andesine is accompanied by changes in the APB microstructure. Adjacent APB's combine, form loops, and finally annihilate to reduce strain energy along the boundaries. The continuous transition from e to D plagioclase and the chemical phase separation are cooperative processes. APB's provide favorable surfaces for large-scale diffusion. The TEM investigation documents that anorthite is definitely not a relict from an earlier high-grade metamorphic episode and thus appears to be stable at low metamorphic grade together with albite, which puts new constraints on the plagioclase phase diagram. Albite-anorthite assemblages are restricted to amphibole schists with pargasitic hornblende coexisting with actinolite. A striking correlation of (Na + K) and Alrv suggests that the amphibole assemblage also is the result of decomposition. Introduction temperature? There is no clear documentation for Intergrowths of plagioclase with different composithis except for a short abstract by Voll (1971) which tions are an expression of the complex ,ourorii'#rtun::. ne.ver been elaborated' During a study of ambility relations of this mineral ,"ri... S"u","J"riilphibolites,in the Central Alps I recently found alcibitity gaps have been established, with i;;;l;t bite-anorthite assemblages in fibrous amphibole structures forming by exsolution during """li"g ;;;schists :l th" border zone between amphibolite and neous rocks, and compositional zoning, "h;;;t& sreel.sctril facies' Even though I am far from undergrowth structures, and two-feldspar assembffiffi standing all implications either for regional geology metamorphic rocks (for general reviews'.""" ;.;: :t p119l::t"se phase relations' the observation seems Smith, 1974; champness and Lorimer, 1976). At ;;important enough to report it in some detail' dium metamorphic grade albite coexists with oligo^ clase (An 15-20) (e.g. Evans, 1964), andesine (An 30t'eological setting and petrography 35) with labradorite (An 60-70), and labradorite (An The Tertiary Bergell granite has been emplaced 60-70) with anorthite (An 90-95) (e.g.E. Wenk and for the most part conformably in the Upper Pennine H. R. Wenk, lg77). nappes. However, along its NE edge there are some In general there is a maximum An content for a pronounced discordant contacts (Fig. l). This area is givenhetamorphic grade which is usually attained characteized by contact metamorphism with anin calcareous schists. At higher normative An content dalusite-cordierite hornfelses. To the west metaor lower temperature other minerals form instead of morphism was more of a regional type (H. R. Wenk calcic plagioclase, most importantly clinozoisite. This et at., 1914). Along the granite contact amphibolite holds true for most regilnafly metamorphic rock series dominate. They are associated with lenses of suites (e.g. E. Wenk and H. R. Wenk, 1977) and has ultramafic rocks and locally display pillow structures. also been confirmed experimentally (Goldsmith, Further south they grade into tonalites which were 1978). But a crucial qrr"riion has always remained mobilized along with the Bergell granite (H. R. unsolved. Is the endmimber anorthite stable at low Wenk et al., 1977). All these calcic rocks, together 0N3-N4X/79/lll2-1294$02.0