ABSTRACT

The contested origin of pink, untwinned sodic plagioclase grains in the Cambrian Charny sandstones of the northern Appalachians near Quebec, first studied by Middleton and discussed by Lajoie. has been re-evaluated by detailed petrographic, X-ray and electron microprobe analyses. The majority of the grains exhibit a perthire-like lamellar structure; the coexisting feldspars are An_8-12 (host) and An₀ (lamellae). We contend that these represent antiperthitic feldspars derived from Grenville anorthositic massifs and transformed in situ, during diagenesis. The K-feldspar lamellae were efficiently albitized during earlier diagenesis, whereas the original andesine host composition failed, even at higher grade conditions, to reach the equilibrium composition and degree f Si-Al order of albite. This is a reflection of the profound changes required in the proportion of tetrahedrally coordinated cations in the plagioclase and the low solubilities of these constituents in the pore fluids. Twinned grey sodic plagioclase, also An_8-12 but free of albite lamellae, may represent albitized detritus from the ubiquitous grey gneisses that surround the Grenville anorthositic complexes.