RHYTHMIC LAYERING OF THE KIVAKKA TYPE: GEOLOGY, PETROGRAPHY, PETROCHEMISTRY, AND A HYPOTHESIS FOR ITS FORMATION

Abstract

Structural relationships in the Kivakka intrusion were established, and rhythmic phase layering (Kivakka type layering) was quantitatively characterized. Rhythmic layering occurs in the Kivakka intrusion as contrasting alternation of melanocratic and leucocratic layers, a few meter thick (meter-scale layering). The meter-scale layering persists along strike over distances comparable with the size of the intrusion. The adjacent melanocratic and mesocratic layers have different associations of cumulus phases. The former are bronzite cumulates, and the latter are bronzite-plagioclase or, occasionally, bronzite-plagioclase-augite cumulates. The boundaries between the layers are sharp but undisturbed. The rhythmic units consist of two parts, including the cases when the mesocratic layer is composed of the three-mineral cumulus assemblage (without a transitional two-mineral cumulate). More fractionated mesocratic cumulates may occur in the section below less fractionated ones (for instance, three-mineral cumulates occur in the middle of the layered sequence). A schematic multilayer suspension model was proposed for the formation of phase layering of the Kivakka type. This scheme provides a basis for the construction of a numerical testable model.