Thermal convection below the solidus in a mantled gneiss dome, Fungwi Reserve, Rhodesia

Publisher Website
Google Scholar
Abstract
Previously unrecorded structures inside a mantled gneiss dome are described and interpreted as having important geological implications. The gneisses which make up this basement structure were domed on at least three scales near the thermal acme of the post-Umkondo 500 m.y. orogeny in the Zambezi orogenic belt. Groups of domes on one scale occur inside those of successively larger scales, forming a hierarchy of structures which are interpreted as being due mainly to gravitational, rather than lateral forces. The rocks involved have an essentially uniform density at the present and it is argued that a concentration of the regional orogenic thermal gradient would have induced unstable density gradients capable of driving thermal convection below the range of temperatures at which the rocks would have melted. For the domes (or convection cells) to have been preserved in the manner in which they are found the convection can only have been due to marginal mechanical instability and must have been stabilized after the first half-cycle of motion. Heat is conducted faster parallel to a gneissose foliation than normal to it and, in restricted ranges of thermal gradients, the mechanism of heat transfer, conduction or convection-with-conduction, may depend solely on the attitude of the foliation in relation to the thermal gradient. The hierarchy of structures is interpreted as a chain reaction of alternations of conduction and convection caused by the rotation of this thermal anisotropy as the structures formed. The genetic model suggested is tested and found to be geologically acceptable in termsthe thicknesses of the convecting layers, their effective viscosities, and the time necessary for each structural pattern to develop. It is argued that had the regional orogenic thermal gradient persisted or been exceeded the incipient convection preserved in Fungwi would have become cyclic. This would have resulted in the homogenization of the granite- syenitic gneiss to what would subsequently look like a normal magmatic rock in a very short time geologically. Other basement domes in the region are interpreted as recording a predictable sequence of normal gneiss domesto completely ‘igneous’ plutons. The production of homogeneous rocks from a gneissose basement by convective overturn may be anticipated as a reasonably common orogenic phenomenon inside local thickenings of mechanically uniform rock sequences. It is suggested that this simple mechanism is a link often assumed or alluded to by earlier writers who have described migma and magma domes in close proximity.