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The Cadomian Orogeny (1990)

Geological Society (London, U.K.) ; D’Lemos, Richard S. ; Strachan, R. A. ; [et al.]

London : The Geological Society

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Keywords: Assyntische Faltungsphase ; Armorican Massif ; Cycle cadomien - Congrès ; Cycle cadomien - France (Nord-ouest) ; Geofisica ; Geology, Stratigraphic ; Geotectonica ; Orogeny ; Orogenèse - France - Armoricain, Massif (France) ; Precambrian ; Précambrien - France - Armoricain, Massif (France) ; Roches métamorphiques

Description / Table of Contents: Armorican Massif --- R. S. D’Lemos, R. A. Strachan, and C. G. Topley: The Cadomian orogeny in the North Armorican Massif: a brief review / Geological Society, London, Special Publications, 51:3-12, doi:10.1144/GSL.SP.1990.051.01.01 --- C. Guerrot and J. J. Peucat: U-Pb geochronology of the Upper Proterozoic Cadomian orogeny in the northern Armorican Massif, France / Geological Society, London, Special Publications, 51:13-26, doi:10.1144/GSL.SP.1990.051.01.02 --- M. M. Shufflebotham: The geology of the Penthièvre crystalline massif: a reappraisal of the type-Pentevrian area, northern Brittany / Geological Society, London, Special Publications, 51:27-39, doi:10.1144/GSL.SP.1990.051.01.03 --- R. A. Roach, G. J. Lees, and M. M. Shufflebotham: Brioverian volcanism and Cadomian tectonics, Baie de St Brieuc, Brittany: stages in the evolution of a late Precambrian ensialic basin / Geological Society, London, Special Publications, 51:41-67, doi:10.1144/GSL.SP.1990.051.01.04 --- G. K. Taylor: A palaeomagnetic study of two Precambrian-Cambrian dyke swarms from the Armorican Massif / Geological Society, London, Special Publications, 51:69-80, doi:10.1144/GSL.SP.1990.051.01.05 --- D. Rabu, J. Chantraine, J. J. Chauvel, E. Denis, P. Balé, and Ph. Bardy: The Brioverian (Upper Proterozoic) and the Cadomian orogeny in the Armorican Massif / Geological Society, London, Special Publications, 51:81-94, doi:10.1144/GSL.SP.1990.051.01.06 --- J. P. Brun and P. Balé: Cadomian tectonics in northern Brittany / Geological Society, London, Special Publications, 51:95-114, doi:10.1144/GSL.SP.1990.051.01.07 --- L. Dupret, E. Dissler, F Doré, F. Gresselin, and J. Le Gall: Cadomian geodynamic evolution of the northeastern Armorican Massif (Normandy and Maine) / Geological Society, London, Special Publications, 51:115-131, doi:10.1144/GSL.SP.1990.051.01.08 --- R. A. Strachan and R. A. Roach: Tectonic evolution of the Cadomian belt in north Brittany / Geological Society, London, Special Publications, 51:133-150, doi:10.1144/GSL.SP.1990.051.01.09 --- P. J. Treloar and R. A. Strachan: Cadomian strike-slip tectonics in NE Brittany / Geological Society, London, Special Publications, 51:151-168, doi:10.1144/GSL.SP.1990.051.01.10 --- D. Gapais and P. Balé: Shear zone pattern and granite emplacement within a Cadomian sinistral wrench zone at St Cast, N. Brittany / Geological Society, London, Special Publications, 51:169-179, doi:10.1144/GSL.SP.1990.051.01.11 --- Michael Brown, G. M. Power, C. G. Topley, and R. S. D’Lemos: Cadomian magmatism in the North Armorican Massif / Geological Society, London, Special Publications, 51:181-213, doi:10.1144/GSL.SP.1990.051.01.12 --- G. M. Power, T. S. Brewer, M. Brown, and W. Gibbons: Late Precambrian foliated plutonic complexes of the Channel Islands and La Hague: early Cadomian plutonism / Geological Society, London, Special Publications, 51:215-229, doi:10.1144/GSL.SP.1990.051.01.13 --- Pierrick Graviou and Bernard Auvray: Late Precambrian M-type granitoid genesis in the Cadomian belt of NW France / Geological Society, London, Special Publications, 51:231-244, doi:10.1144/GSL.SP.1990.051.01.14 --- C. G. Topley, M. Brown, R. S. D’Lemos, G. M. Power, and R. A. Roach: The Northern Igneous Complex of Guernsey, Channel Islands / Geological Society, London, Special Publications, 51:245-259, doi:10.1144/GSL.SP.1990.051.01.15 --- G. M. Power, T. S. Brewer, and R. S. D’Lemos: The post-tectonic Cadomian plutonic complex of La Hague, Manche, N. France / Geological Society, London, Special Publications, 51:261-272, doi:10.1144/GSL.SP.1990.051.01.16 --- G. J. Lees: The geochemical character of late Cadomian extensional magmatism in Jersey, Channel Islands / Geological Society, London, Special Publications, 51:273-291, doi:10.1144/GSL.SP.1990.051.01.17 --- David Went and Michael Andrews: Post-Cadomian erosion, deposition and basin development in the Channel Islands and northern Brittany / Geological Society, London, Special Publications, 51:293-304, doi:10.1144/GSL.SP.1990.051.01.18 --- J. Cogné: The Cadomian orogeny and its influence on the Variscan evolution of western Europe / Geological Society, London, Special Publications, 51:305-311, doi:10.1144/GSL.SP.1990.051.01.19 --- Related Areas --- Wes Gibbons and Jana Horák: Contrasting metamorphic terranes in northwest Wales / Geological Society, London, Special Publications, 51:315-327, doi:10.1144/GSL.SP.1990.051.01.20 --- F. C. Murphy: Basement-cover relationships of a reactivated Cadomian mylonite zone: Rosslare Complex, SE Ireland / Geological Society, London, Special Publications, 51:329-339, doi:10.1144/GSL.SP.1990.051.01.21 --- J. C. Pauley: Sedimentology, structural evolution and tectonic setting of the late Precambrian Longmyndian Supergroup of the Welsh Borderland, UK / Geological Society, London, Special Publications, 51:341-351, doi:10.1144/GSL.SP.1990.051.01.22 --- Cecilio Quesada: Precambrian successions in SW Iberia: their relationship to ‘Cadomian’ orogenic events / Geological Society, London, Special Publications, 51:353-362, doi:10.1144/GSL.SP.1990.051.01.23 --- R. Damian Nance: Late Precambrian-Early Palaeozoic evolution of part of the Avalon terrane in southern New Brunswick, Canada / Geological Society, London, Special Publications, 51:363-382, doi:10.1144/GSL.SP.1990.051.01.24 --- J. B. Murphy, J. D. Keppie, J. Dostal, and A. J. Hynes: The geochemistry and petrology of the Late Precambrian Georgeville Group: a volcanic arc-rift succession in the Avalon terrane of Nova Scotia / Geological Society, London, Special Publications, 51:383-393, doi:10.1144/GSL.SP.1990.051.01.25 --- R. Damian Nance and J. Brendan Murphy: Kinematic history of the Bass River Complex, Nova Scotia: Cadomian tectonostratigraphic relations in the Avalon terrane of the Canadian Appalachians / Geological Society, London, Special Publications, 51:395-406, doi:10.1144/GSL.SP.1990.051.01.26 --- Wes Gibbons: Transcurrent ductile shear zones and the dispersal of the Avalon superterrane / Geological Society, London, Special Publications, 51:407-423, doi:10.1144/GSL.SP.1990.051.01.27

Pages: Online-Ressource (VII, 423 Seiten) , Illustrationen, Diagramme, Karten

ISBN: 0903317478

URL: http://sp.lyellcollection.org/content/51/1

Language: English

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The origin of the diorites and associated rocks of Chouet, north-western Guernsey, Channel Islands (1980)

Brown, M. ; Topley, C. G. ; Power, G. M.

Mineralogical Society of Great Britain and Ireland

In: Mineralogical Magazine. 1980; 43(331): 919-929. Published 1980 Sep 01. doi: 10.1180/minmag.1980.043.331.17.

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Publication Date: 1980-09-01

Description: SynopisA number of diorite complexes occur within the Channel Islands region, notably on Jersey, Alderney, and particularly Guernsey. Much of northern Guernsey is made up of the largest of these complexes (fig. S1), the Bordeaux diorite. In the north-western part of this diorite, around Chouet, a complicated association of plutonic rocks occurs. Although the field relationships in this area are sometimes difficult to interpret—this is often the case in diorite complexes—three separate groups of rocks may be distinguished within the association: a diorite group; a granodiorite group; and an inhomogeneous suite of rocks (fig. S2).The widespread diorite group consists predominantly of an even-grained diorite, which is relatively homogeneous but which occasionally grades into an acicular diorite, the latter often containing pods and veins of appinite. The granodiorite group is the least common, occurring as bodies which are interpreted as intrusive sheets and bosses within the even-grained diorite, but occurring as angular blocks within the inhomogeneous suite of rocks. The granodiorite invariably contains rounded diorite xenoliths. The inhomogeneous suite consists of a variety of rocks from patchy, dark diorite, through quartz diorite to tonalite. Commonly, these rock types are intimately associated, often showing gradational contacts with each other and frequently with the more basic portions occurring as ‘xenolithic’ material within the more acidic portions. At contacts between the inhomogeneous suite and the even-grained diorite certain features (e.g. lobate margins and pipe-like structures) indicate that the diorite must have been close to its solidus temperature at the time of emplacement of the inhomogeneous suite. The field relationships between the three groups are interpreted as indicating that the diorite group was emplaced first, followed by the granodiorite group, with both of these clearly pre-dating the inhomogeneous suite.Fifty-nine specimens, chosen to give a representative sample of each of the three rock groups, have been analysed for major, minor, and a selection of trace elements and thirteen of these specimens have been analysed for REE. The chemistry of the analysed rocks confirms the division into three groups, with each group showing distinctive characteristics. Furthermore, chemical plots (e.g. Al2O3, P2O5, Cr, and Ni v. SiO2) show discontinuities and areas of overlap between each group which cannot be explained within the constraints of a single genetic model relating the three groups to each other. This argument is particularly strong for the relationship between the diorite group, which spans the range 50 to 59% SiO2, and the inhomogeneous suite, spanning the range 53 to 68% SiO2. In the area of overlap (53 to 59% SiO2) the two groups are geochemically different. Therefore, for a variety of reasons, including emplacement order, the geochemical characteristics of the groups and the lithological inhomogeneity which is associated only with the chemically intermediate members of the association (the inhomogeneous suite), three quite different and genetically unrelated liquids are required to generate the three groups of rocks.The even-grained diorite shows chemical variation (e.g. with increasing SiO2, decreasing Al2O3, MgO, CaO, Sc, V, Cr, and Ni, and increasing Na2O, La, Nd, and Y) consistent with amphibole + plagioclase fractionation up to 55% SiO2. At 55% SiO2 several elements show a change of slope (e.g. FeO + Fe2O3, TiO2, Rb, Ba, and Zr) indicating the introduction of biotite as a fractionating phase. Increasing total REE content with increasing SiO2 throughout the even-grained diorite supports the contention that amphibole is an important fractionating phase. The higher TiO2, P2O5, Sr, La, Ce, Nd, and Y contents and negligible Cr and Ni contents of the acicular diorite suggest an origin by delayed crystallization of volatile-enriched portions of the diorite group magma.The granodiorite group shows little geochemical variation. Members of this group contain detectable amounts of Cr and Ni, unlike virtually all members of the inhomogeneous suite. For this reason, and because of the field relationships, the granodiorite is considered to be genetically unrelated to members of the inhomogeneous suite and a separate liquid is thus required for its genesis. This liquid may have been the fractionated derivative of some other magma (though if this is so the ‘parent’ is entirely unrepresented at the present erosion level) or it may represent a direct crustal melt. Diorite xenoliths within the granodiorite are chemically similar to the even-grained diorite.Despite the lithological complexity of the inhomogeneous suite, its geochemical unity is clearly established in that, for instance, virtually none of the members of the suite (including even the most SiO2-poor) contain detectable Cr and Ni. Moreover, geochemical variation within the group is rational (with the possible exceptions of Sr, Zr, and Ba) and may be explained in terms of a crystal fractionation model. However, the fractionation must have acted on a liquid itself unrelated to either the diorite or granodiorite group magmas. An additional complication is that later derivative liquids intrude into and partly digest earlier-formed semi-solids of the suite to produce much of the observed inhomogeneity. The phases which have controlled fractionation within the suite include plagioclase (established petrographically as well as geochemically) and hornblende. The role of apatite is uncertain. The fractionation of hornblende is particularly useful in explaining the change in REE contents within the inhomogeneous suite. Total REE contents increase from the dark diorite to the quartz diorite, but decrease from the quartz diorite to the tonalite with concomitant relative HREE depletion. This is taken to be a reflection of the changing hornblende/liquid partition coefficients for REE with increasing SiO2, which are less than one for liquids of basaltic and andesitic composition but greater than one for liquids of dacitic composition.

Print ISSN: 0026-461X

Electronic ISSN: 1471-8022

Topics: Geosciences