An Appinitic Facies Associated with Certain Granites in Jersey, Channel Islands

Publisher Website
Google Scholar
Abstract
Summary: The rocks here referred to as appinites form part of the essentially dioritic facies marginal to both the north-western and south-eastern complexes in Jersey. Mineralogically they are characterized by abundant long prismatic amphiboles, rich in titanium. Many of these contain cores of mica or felsic material, the nature and origin of which are discussed. The country rock commonly occurring in association with the appinitic rocks is dioritic in both complexes; and at Ronez in the north of the island it contains relicts which prove it to be, at least in part, metasomatized gabbro. The diorites and appinites both consist of essentially the same mineral assemblage and, whilst the latter are not metasomatic in the generally accepted sense of the word, there is no doubt that a large proportion of their substance has been derived from gabbro by way of a series of changes initiated by the nearby granites. The changes by which gabbroic material was converted into dioritic have been continued and accentuated, presumably by emanations from the granite, with the local building up of strong flux concentrations, which led to the formation of " pockets " consisting of crystal mush with interstitial liquid. It was from this " magma " that the appinitic rocks crystallized, under essentially pegmatitic conditions. It was demonstrably mobile, and a turbulent fluxional structure is highly characteristic. Textural interpretation receives critical attention in the paper, particularly in so far as it concerns replacement phenomena. The observed changes in mineral contents are consistent with crystallization from a magma, and follow the lines of Bowen's discontinuous reaction series, which must be extended at the low temperature end to include phases usually regarded as secondary, but here unquestionably primary. An unusual feature, occasioned by the special environmental conditions, has been the growth of amphibole and apatite crystals of a hollow-shell type, in which crystallization starts on the outside and progresses towards the centre. In the sequence of paramorphic changes, the inheritance by a later phase of the internal structure of the earlier one which it is replacing, in accordance with the principle of least disturbance of the original Si-0 lattice, is strikingly displayed in these rocks. Problems of classification and nomenclature are involved and receive due attention in the paper.

This publication has 10 references indexed in Scilit: