Origin of Appinitic Pockets in the Diorites of Jersey, Channel Islands

Colin H. Key1
Department of Geology, Queen Mary College, University of London, Mile End Road, E1 4NS
1Present address: Department of Geology, Goldsmiths' College, University of London, New Cross, SE14 6NW.

Summary: Isolated pockets of pegmatitic appinite characterized by hollow-shell, prismatic amphiboles 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 SiO2, K2O, and Na2O suggest that surrounding 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 900 °C at 2–5 Kb PH2O: Fractured, hollow-shell, 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 under which these changes took place are those of a high-level, sub-volcanic environment.

Mineralogical Magazine; June 1977 v. 41; no. 318; p. 183-192; DOI: 10.1180/minmag.1977.041.318.05
© 1977, The Mineralogical Society
Mineralogical Society (www.minersoc.org)