Physical and chemical interactions between coexisting acid and basic magmas at Elizabeth Castle, Jersey, Channel Islands
Elizabeth Castle forms part of the South-East Granite Complex of Jersey, Channel Islands
and is one of several multi-magma complexes in the region. The rocks have calc-alkaline
signatures indicative of a subduction zone setting. In the western half of the Elizabeth
Castle complex, the outcrops are wholly granophyre, while to the east, granophyre and
minor monzogranite are intimately associated with diorite. The dioritic rocks form part of a
layered series which is preserved at several localities. The layered diorites were initially
intruded by multiple sub-horizontal granitic sheets. All contacts between the diorite and the
granitic sheets are crenulate, indicating that the two were present as coexisting magmas.
Fine-grained, dark margins in the diorites contain quench textures such as spherulitic
plagioclase and acicular apatite, and are interpreted as chilled margins.
At many contacts a narrow tonalitic marginal zone, with acicular amphiboles, is present.
Field relationships suggest that this is a hybrid produced by interaction between coexisting
dioritic and granitic magmas and this is confirmed by modelling based on geochemical data.
It is proposed that within the marginal zones the presence of volatile-rich fluids, increased
temperatures and a decrease in viscosity promoted chemical diffusion across the dioritegranite
interface. The transfer of elements, together with the presence of volatiles,
promoted the growth of hydrous mafic phases and suppressed crystallization of alkali
feldspar. At the same time, fluid infiltration modified the composition of the dioritic magma.
Field evidence indicates that these processes took place in a narrow time frame prior to
further granitic intrusion. Parts of the sheeted complex were extensively disrupted by the
later granitic intrusions, producing large areas rich in dioritic enclaves. Within these
disrupted areas a grey inhomogeneous rock is encountered. Field and petrographic evidence
suggest that this is a hybrid rock produced by the physical mixing of dioritic and granitic
magmas. Linear chemical trends confirm this interpretation. Minor intrusions comprising
red granite dykes, basic dykes, composite dykes and aplite sheets cut the complex.