VOLCANIC ROCKS AND THEIR MINOR INTRUSIVES, EASTERN JERSEY, CHANNEL ISLANDS.
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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.
AuthorsThomas, GEORGE MATTHEW
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