Origin of appinitic pockets in the diorites of Jersey, Channel Islands

Abstract

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 geochemical character of late Cadomian extensional magmatism in Jersey, Channel Islands

“…The lack of structural complexity in the supracrustal sequence (Helm 1986), their low metamorphic grade, and the features shown by the igneous complexes (Salmon 1987;Wells & Wooldridge 1931;Wells & Bishop 1955;Bishop & Key 1983;Key 1977;Bland 1984), as well as their young ages, suggest that Jersey forms a structurally high-level and late segment of the Cadomian orogenic belt.…”

“…South Hill (650476) Bishop 1964a); mesocratic hornblende lamprophyres, characterized by long, often cored, bladed brown hornblendes and acicular apatites, (hereafter referred to as appinites (cf. Wells & Bishop 1955;Key 1977), though classified as camptonites in Bishop & Bisson 1989) in composite dykes off Le Croc (673462) (see below); and fine-grained plagioclase-rich rocks with a trachytic texture (e.g. an early sill on the west side of Portelet Bay (596468)).…”

“…A spectacular example, some 10 m thick, occurs between Le Croc (673462), Le Nez (678462), and Green Island (674459), in which small pegmatitic patches occur (cf. Bishop & Key 1983;Key 1977).…”

Cadomian magmatism in the North Armorican Massif

“…The Southeast Granite Complex (SEGC), in contrast with the other two, contains a much larger amount of more basic rocks of gabbroic and dioritic compositions (Bishop 1954;Wells & Bishop 1955;Bishop 1963;Key 1977Key , 1985Key , 1987Bishop & Key 1983; these rocks show layering the origin of which has been debated. They form part of an older element to the SEGC, and are intruded by the megacrystic Dicq Granite which passes northwards, with reduction in megacrysts, into the Longueville Granite.…”

Interpretation of field relationships of Diorites and associated rocks with particular reference to northwest Guernsey, Channel Islands

“…northwest and southeast Jersey (Wells and Wooldridge 1931;Wells and Bishop 1955;Key 1977), northern Guernsey (Drysdalll957; Elwell er al. 1960, 1962Roach 1966; Brown er af.…”