By P. A. Floyd, C. S. Exley, M. T. Styles (auth.)
This quantity illustrates a number of the major facets of magmatic job from Devonian (408 million years in the past) to early Permian (270 million years in the past) instances in SW England. this era covers the revolutionary improvement of the Variscan mountain-building episode, from preliminary basin formation to ultimate deformation and the next improvement of a fold mountain belt - the Variscan Orogen. either extrusive (volcanic) and intrusive (plutonic) rocks are present in the orogen, and chart some of the phases of its magmatic improvement. The websites defined during this quantity are key localities chosen for conservation simply because they're consultant of the magmatic heritage of the orogen from initiation to stabilization. a number of the earliest volcanic job within the Devonian is represented via submarine basaltic and rhyolitic lavas constructed in subsiding basins, brought on by the attenuation of the prevailing continental crust. from time to time, vast rifting and attendant magmatism produced slender zones of real oceanic crust, while somewhere else basaltic volcanism is said to fractures within the continental crust on the margins of the basins. After the filling of the sedimentary basins, and their deformation because of crustal shortening (late Carboniferous Period), additional job is manifested through the emplacement of the Cornubian granites and later minor basaltic volcanism within the early Permian. money owed of the constituent elements of this background have enriched geological literature from the 19th century onwards, and feature contributed to the development and figuring out of magmatic and tectonic processes.
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Extra resources for Igneous Rocks of South-West England
The rare-earth-element geochemistry of Lizard peridotites has been studied by Frey (1969) and Davies (1984). 4). This is rather unusual as they have major-element chemistry similar to undepleted mantle where a relatively flat REE pattern would be expected. The anhydrous and hydrous recrystallized types were progressively less depleted with close to chondritic abundances and slight light-REE depletion. Davies (1984) also found a plagioclase lherzolite type that had five times chondrite abundances and slight light-REE enrichment.
Overall, dyke Groups 1 and 2 (with La/Nb < 1, ZrlNb c. 30 and depleted light REE patterns) are chemically similar to oceanic basalts (Kirby, 1984), whereas Group 3 (considered to represent the earliest dyke phase) is more enriched than either group 1 or 2 and was probably derived from a 'recently' light REE-enriched or metasomatized source, similar to that of the recrystallized peridotites (Davies, 1984). The Kennack Gneiss is a series of interbanded acid and basic gneissose rocks that occur along the south-east coast of the Lizard, roughly at the base of the Goonhilly Downs structural unit.
The distribution of the two types of hornblende schist has been more or less accepted by all subsequent workers. The Landewednack type occurs largely around the southern tip of the Lizard and the Traboe type around Traboe, Mullion and Predannack. Green (1964b) suggested a radically different hypothesis for their origin. He proposed that all the hornblende schists had originated as basaltic lavas which had undergone amphibolite-facies regional metamorphism. Those close to the peridotite (Traboe type) had suffered an additional contact metamorphism during intrusion of the hot peridotite to form brownhornblende granulites, whereas those immediately adjacent to the peridotite developed twopyroxene granulites.