VOLCANIC ROCKS AND THEIR MINOR INTRUSIVES, EASTERN JERSEY, CHANNEL ISLANDS.
Abstract
Volcanic rocks which outcrop over 40 sq. km in
eastern Jersey and which have a maximum thickness of 25 km
have been mapped in detail. Basaltic and andesitic lavas,
tuffs and agglomerates, which have been affected by
keratophyric alteration, are overlain by numerous ignimbrite
cooling units, the youngest of which are generally aphyric,
the oldest being porphyritic. Local rhyolites, tuf'fs and
andesites occur among the igniinbrites. The volcanics are
essentially conformable upon Brioverian sediments and are
regarded as being late Precambrian in age. Cadomian E-W and
N-B fold phases, with associated faulting, are largely
responsible for the present outcrop pattern.
The petrology and geochemistry of these volcanics
have been studied. Together with the regional distribution
of similar late Precambrian volcanism they indicate that
the caic-alkaline suite was generated soon after the
initiation of the global tectonic movements which eventually
produced the Caledonides. The basalts and andesites are
thought to result from the partial melting of a descending
lithospheric plate within the mantle, the magmas thus
produced being modified en route to the surface by
fractionation and crustal contamination under a variable
The rhyolitic maginas were produced by crustal
anatexis and collected in high-level magma chambers prior
to their eruption. Post.-orogenic Cadomian plutonics which invade
the volcanics are also caic-alkaline but are chemically
distinct from the extrusives. The petrology and geochemistry
of a dyke swarm which invades the volcanics and just
postdates emplacement of the plutonics is also discussed.
This appears genetically related to the plutonic complexes
and was generated close to the base of the crust.
Authors
Thomas, GEORGE MATTHEWCollections
- Theses [4121]