The metamorphic behaviour and petrogenetic significance of zinc in amphibolite facies, staurolite-bearing mica schists, Puolankajärvi Formation, Central Finland

Abstract

Whole rock analyses and mineral analyses were performed on 24 samples of mica schists from the Early Proterozoic Puolankajärvi Formation (PJF). The whole rock Zn content (36–127 ppm, mean 77 ppm) was at the normal level for pelitic and crustal rocks, staurolite being the only mineral phase to contain zinc in measurable amounts (0.0–1.33 wt% ZnO, mean 0.28 wt%). When staurolite was present it contained nearly all of the zinc of the rock. The Zn content does not vary between the rocks in which staurolite has never been generated, those in which staurolite is present and those from which staurolite has been destroyed. This suggests, that despite the fact that staurolite is a very strong acceptor of Zn, the generation of staurolite under reducing conditions (log f O 2 ≤ −22.5, T ≤ 545° C, P = 3.7 kbar) did not increase the content of Zn in the whole rock, neither did the destruction of staurolite under more oxidizing conditions (log f O 2 = −22.2−21.0, T = 500–520° C, P = 2.1–3.5 kbar) reduce it. Because there is evidence of considerable fluid flow through the rock during certain stages of the metamorphism of the PJF, it is concluded that Zn is an immobile element in amphibolite facies metamorphism in petites. Zinc should be treated as an additional component in metamorphic reactions of staurolite which increases the degrees of freedom of these reactions. Because of the immobility of Zn and the increase in degrees of freedom, these reactions will become dependent upon the concentration of Zn in the rock.