The greenschist metamorphism of Archaean synvolcanic stocks near Wawa, Ontario, Canada

Abstract

Two Archaean synvolcanic stocks with contact aureoles occur in the Wawa greenstone belt near Wawa, Ontario, Canada. The Gutcher Lake and Jubilee stocks consist mainly of granitoid trondhjemite with feldspar laths mottled by white mica + calcite + epidote and rimmed by clear albite. Biotite IS partly or wholly pseudomorphosed by chlorite + sphene; some epidote is partly altered to calcite + chlorite. The granitoid phase grades into a foliated phase of quartz + albite +white mica +calcite +chlorite near fracture zones traversing the stocks.The alteration of the Gutcher Lake stock along its foliated margin involved addition of K,O, H,O + CO,, MnO, plusRb; loss ofCaO plus Sr; and a shift in Fe' '/Fe, from 0.66 to 0.81. The alteration of the Jubilee stock along the Darwin Shear involved addition of H,O + CO,; loss of Sr; and no significant shift in Fe+2/Fe,. The greenschist alteration also modified the contact aureoles bordering both stocks.One interpretation is that regional metamorphism in the Archaean overprinted a greenschist assemblage on both stocks. The alteration was intense near fracture zones and sporadic remote from fractures. Lower integrated water to rock ratios along the Darwin Shear compared to the margin of the Gutcher Lake stock may explain the comparatively lower perturbation of the element abundances and redox state of iron.