By P. A. Floyd, C. S. Exley, M. T. Styles (auth.)
This quantity illustrates the various major facets of magmatic job from Devonian (408 million years in the past) to early Permian (270 million years in the past) occasions in SW England. this era covers the innovative 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 many 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 background of the orogen from initiation to stabilization. a few of the earliest volcanic task within the Devonian is represented through submarine basaltic and rhyolitic lavas constructed in subsiding basins, attributable to the attenuation of the present continental crust. every now and then, large rifting and attendant magmatism produced slim zones of precise oceanic crust, while in different places basaltic volcanism is expounded to fractures within the continental crust on the margins of the basins. After the filling of the sedimentary basins, and their deformation attributable to crustal shortening (late Carboniferous Period), extra job is manifested through the emplacement of the Cornubian granites and later minor basaltic volcanism within the early Permian. money owed of the constituent components of this background have enriched geological literature from the 19th century onwards, and feature contributed to the development and knowing of magmatic and tectonic processes.
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Extra info for Igneous Rocks of South-West England
Isostatic uplift may have played a part Note: Salinity, lack of kaolinite and change in stress direction. Low temperature metalliferous mineralization Further input of magmatic heat Main phase of metalliferous mineralization much of the batholith. g. Castle-an-Dinas (W) Biotite granite which now forms eastern part of the St Austell granite Comments Geological framework to form a gabbroic, dioritic or minette magma body at the base of the crust, and that this then differentiated to form the main granite magma.
Volcaniclastics, pillow lavas, and massive intrusives mainly within the Greystone Nappe (between Bodmin Moor and Tavistock). 3. Volcaniclastics, minor lavas and massive intrusives within the Blackdown Nappe (west Dartmoor). 4. Minor volcaniclastics and pillow lavas, with dominant massive intrusives within the autochthon (Teign Valley, north-east Dartmoor). Carboniferous-Permian plutonic activity The geography of Cornwall and west Devon is dominated by the granite batholith which constitutes the spine of the peninsula.
There are also fractionated rocks such as hornblende diorites in the north. Leake et al. ( 1990) have described a second type of gabbro from the Trelan area which is a more fractionated type with iron-rich clinopyroxenes, a more sodic plagioclase and often considerable enrichment in Ti and Fe. Overall, the Crousa gabbros have low 39 incompatible-element contents and light REE depletion which indicate derivation from a depleted mantle source and are consistent with an oceanic origin. In this respect they differ from the coarse-grained dolerites and gabbros in the rest of south Cornwall (Floyd, 1984).