Abstract

An early event in the history of most or all of the intrusive centres in the British Tertiary province was probably the uprise to a high level of an acid diapir. This fact, combined with the idea that the relative densities of magmas and crust play a major role in the localisation of igneous intrusions, forms the basis for a new interpretation of the mechanism of formation of the intrusions and enables a model to be developed for the evolution of each centre.

Firstly it is postulated that the emplacement of cone sheets is governed by a tendency of rising magma to move in the direction of maximum excess hydrostatic pressure, PHe (this being the amount by which the hydrostatic pressure in the magma exceeds the lithostatic pressure). It is suggested that the perturbation of surfaces of equal PHe near a high level diapir of low-density acid magma can cause uprising magma to be diverted to form cone sheets instead of dykes. Secondly by a similar mechanism some batches of basaltic magma are diverted into a magma trap situated at the base of each diapir and there they gradually build up the great cylinder of basic intrusions the existence of which has been inferred from gravity anomalies. This cylinder marks the upward course of the diapir, and heat supplied by this basic magma aided its uprise.

A late stage in the development of an intrusive centre is the elimination of the diapir because of the escape of successive batches of acid magma from it in a near-surface environment. Subsidence then takes over as a dominant process and ring dykes are emplaced. Intrusions of bell-jar or “curved flange” type also occur and a mechanism involving downsagging of the basic cylinder is invoked for their emplacement.

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