Geochemistry of Archean quenched immiscible liquids,Noranda,Quebec,Canada: With reference to magmatic liquid structure.

Abstract

Some lavas south of Noranda are composed of relatively mafic matrix and rhyolitic varioles, representing quenched immiscible silicate liquids. Studies of major elements, rare earth elements (REE), Zr, Y, Ni, etc. indicate that the host magmas, derived by varying degrees of partial melting of mantle material at shallow depths, split into two fractions during and/or after fractional crystallization. The two fractions respectively formed the mafic matrix and rhyolitic varioles. Although the varioles are rhyolitic in major element composition, the contribution of this liquid immiscibility process to the origin of the widespread Noranda rhyolitic volcanic rocks is interpreted to be insignificant. This inter-pretation is based on ΣREE-SiO2 correlations which is on the one hand negative between the matrix and varioles but, on the other hand positive between the various volcanic rocks of the area. The structure of the magmatic liquid is sought using available data on the partitioning of elements between immiscible liquid pairs and mineral-liquid pairs. In brief, wavy twoliquid partition coefficient-ionic radius patterns imply the presence of structural sites in the mafic liquid for cations with ionic radius of _??_0.7 A and _??_0.95 A The systematic variation, with the varying SiO2 content of liquid phase, of the partition coefficient-ionic radius pattern for apatite-liquid pairs indicates that the above-mentioned 0.95 A site decreases in amount with increasing SiO2 content of liquid.