S~MMARV. Isolated pockets of pegmatitic appinite characterized by hollow-shell, prismatic amphiboIes are common in the Pre-Cambrian metagabbros and metasomatic diorites of Jersey. Field relationships and petrography indicate a liquid phase in the formation of these appinitic pockets, which are chemically distinct from the associated gabbros and diorites. Close chemical ties between appinites and host rocks, however, prove a replacive, metasomatic, rather than intrusive origin for the pockets. Significant enrichment in SiOz, KzO, and Na~O suggest that sunounding granite provided the metasomatic agents. The localized changes in composition of the basic rocks resulted in the formation of partially molten pockets from which the appinites crystallized. This mechanism probably necessitates a temperature in the region of 9oo ~ at 2-5 Kb PH20 : Fractured, hollowshell, prismatic amphiboles of the pockets are consistent with quench crystallization, possibly due to the sudden loss of volatiles. An increase in the oxygen fugacity may have played a major role in inducing the rapid crystallization of kaersutitic amphibole. The envisaged conditions tinder which these changes took place are those of a high-level, sub-volcanic environment.A P P I N I T I C diorites occur within the basic plutonic complex of Jersey as discrete pockets or, occasionally, more extensive layers. They form a conspicuous facies of the dioritic rocks because of their coarse, often pegmatitic grain size and their distinctive texture dominated by euhedral, prismatic amphiboles. The term 'appinite' was originally applied to these rocks by Wells and Bishop (I955) to describe diorites characterized by prismatic amphiboles, and it is this feature alone that is in keeping with the original definition of the term proposed by Bailey (I916). In terms of bulk chemistry and mineralogy the Jersey appinites are closer to quartzdiorites or tonalities and bear little resemblance to the basic and ultra-basic rocks of the Caledonian appinitic suite.The origin of small pegmatitic pockets of appinitic diorite presents problems of interpretation. Identical appinites at Bon Repos, Guernsey, have been ascribed by Roach (1964) to pegmatitic crystallization from residual fluids in magmatic diorite. The Jersey diorites, however, have been shown to be the result of the action of granitic metasomatism on an earlier gabbroic mass (Wells and Bishop, I955; Bishop, I963), and Wells and Bishop (I955) have suggested, from field and petrographic evidence, that the appinites represent localized areas of partial melting due to the concentration of volatile fluxes emanating from the granites that now completely surround the basic masses. The present work provides geochemical support for the previous interpretation of appinites and associated diorites and discusses the mechanism by which giant hollow-shell amphiboles were produced. FieM relationships of appinitesThroughout the dioritic complex, both in the south-east at Le Nez Point and in the northwest at Ronez and Sorel Point there occur n...

The Cadomian diorites of SE Jersey contain two principal types of layering: primary gabbroic layering , formed as igneous cumulates in a gabbroic magma, and differentiated diorite sheet layering , which developed from gabbroic layering during metasomatic alteration of gabbro to diorite, probably by hydrous fluids associated with several pulses of granite emplacement. Primary gabbroic layering is preserved only in a small area near Le Nez and, though only a small proportion of relict grains of original pyroxene and calcic plagioclase remain, many of the textures and structures typical of primary layering are still preserved. The differentiated diorite sheets have the same orientation as the primary layers and each grades upwards from meladiorite to leucodiorite or quartz diorite. Intrusive structures at sheet contacts indicate that they, like the gabbroic layers, formed essentially horizontally, though they are now inclined NE at up to 65°, probably as a result of block rotation in granite. Chemical variation within the differentiated sheets suggests that there has been considerable modification of the original gabbro. The most basic part of the sheets occurs several tens of centimetres above the base because of reaction across the quartz diorite-meladiorite boundary. The layered diorites, and probably much of the homogeneous diorites as well, have originated by metasomatic alteration of layered gabbro, rather than having formed as cumulates in a primary dioritic magma.

The plutonic complex in SE Jersey consists of a late Precambrian gabbro-diorite mass which has been intruded by several granites. The status of the diorites which, like the gabbros, often possess a layered structure, is not clear. New geochemical data, including rare earth and trace element analyses, suggest that acid-basic magma mixing was not responsible for the variety of intermediate rocks. Amphibole-controlled fractional crystallization of hydrous basic magma is also unlikely in view of the REE and HFS (high field strength) element distribution. The model which best fits all the available field, petrographic and geochemical evidence is one in which the chemical variation was initially produced by fractional crystallization of anhydrous basic magma and subsequently overprinted by granitic metasomatism. This superimposed calc-alkaline characteristics on the complex and produced hybrid diorites which, because they were above their solidus temperature, recrystallized with textures indicative of a magmatic origin.

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