Chilled margins in igneous rocks

Abstract

When hot magma contacts cold solid rock, a chilled margin forms because the heat flux from the magma cannot balance the very large initial conductive flux in the rock. However, the conductive flux decreases with time and can eventually be exceeded by the heat flux from the magma. In these circumstances, the chilled margin begins to remelt and can even disappear. This process is modelled quantitatively for the case of a constant heat flux from the magma. The results are applied to the turbulent steady flow of komatiite magma in dykes and as lavas. The initial chilled margin with cold rock only survives for a few tens of seconds and so does not inhibit thermal erosion significantly. The theory can also be applied to magmatic intrusions where the heat flux varies with time. Results are presented for the remelting of a chilled margin formed at the roof of a basaltic sill. For sills with thicknesses from 10 to 1000 m, the remelting of initial chilled margins from a few tens of centimetres to a few tens of metres is possible. Three kinds of contact are predicted at the margins of intrusions. First, contacts are developed where the chilled margin grows continually without any meltback. Second, meltback can occur, but does not completely redissolve the initial chilled margin. This causes an internal contact to form within the intrusion. Third, meltback is complete and the country rock can then be melted and assimilated. The marginal rocks of such intrusions need not be fine grained.