Abstract

Mr C. G. Topley & Dr M Brown write: Bishop & Key (1983) have proposed an unnecessarily complex model for the origin of the layered and sheeted diorites of SE Jersey. The central theme of their model is that layering within the diorite, together with some relict mineralogy, is inherited from what was originally a layered gabbro, and that this gabbro was metasomatized, largely in the solid state, during the emplacement of the surrounding SE granite complex to produce the dioritic rocks and to develop differentiated diorite sheet layering. In contrast, it is our contention that most, and perhaps all of the characteristics of these diorites have a primary igneous origin. In this respect we wish to question a number of the lines of argument used by Bishop & Key, and to propose a more plausible interpretation of the evidence, evidence which we believe to be more consistent with an origin involving primary igneous processes.

In an attempt to establish that the diorites were originally gabbros, subsequently altered by metasomatism, Bishop & Key use three main lines of argument based on evidence from field relationships, textures and geochemistry. We will discuss each of these in turn and demonstrate that a simpler interpretation, consistent with all the evidence, is that the diorites are primary igneous rocks.

In our view, many of the field relationships of the sheeted diorites give clear indications that the full sequence of dioritic rocks was above solidus temperature simultaneously and that the diorites crystallised from what was

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