Abstract

The Mount Johnson intrusion is a small vertical plug-like body belonging to the Monteregian hills. It consists of a sequence of rocks that grade imperceptibly from a core of olivine essexite to a margin of pulaskite. Past hypotheses explaining this differentiation have invoked the migration of early crystallizing phases to the center of the conduit up which the magma was flowing, and the diffusion of low-melting components to the cooler margins of the intrusion. However, a variety of depositional and erosional features in the well-layered rocks of this intrusion indicate that the first-formed rocks are at the margin of the intrusion and the youngest in the core.Experiments with a model in which two immiscible liquids convect in a vertical pipe suggest that the zonation from pulaskite at the margin to essexite in the core could result from the solidification of a converting magma column in which a syenitic liquid had separated from an essexitic one and floated to the top.

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