|
Armorican Massif |
1 School of Geological Sciences, Kingston Polytechnic, Penrhyn Road, Kingston upon Thames, KT1 2EE, UK
Department of Geology, University of Maryland, College Park, MD 20742, USA
2 Department of Geology, Portsmouth Polytechnic, Burnaby Building, Burnaby Road, Portsmouth PO1 3QL, UK
3 Department of Geology, Oxford Polytechnic, Gipsy Lane, Headington, Oxford OX3 OBP, UK
This review concerns intrusive magmatism related to the Cadomian orogeny in the northeastern part of the Armorican Massif of France. The Cadomian orogeny is interpreted to represent tectono-thermal activity in a continental margin above a subduction zone. The North Armorican Shear Zone (NASZ) separates two major Cadomian terranes, the North Armorican Composite Terrane (NACT) and the Central Armorican Terrane (CAT). The amalgamation of three terranes within the NACT (the St Brieuc Terrane (SBT), the St Malo Terrane (SMT) and the Mancellian Terrane (MT)) around 540 Ma by sinistral transpression is a key element in the Cadomian orogeny. Cadomian plutonic complexes intrude the c. 2000 Ma Icartian Gneisses and form the local basement to, and are emplaced into the Brioverian succession. Cadomian magmatism spans approximately 275 Ma, from c. 700 Ma to c. 425 Ma.
Early Cadomian foliated plutonic complexes occur within the Guernsey-La Hague structural block of the SBT; they represent volcanic arc granites derived in a subduction zone environment. Late Cadomian post-tectonic plutonic complexes within the Guernsey—La Hague structural block were emplaced c. 500 Ma, exhibit a variety of ‘mixed magma’ features and have a normal to mature continental arc geochemistry. To the southeast, the Jersey structural block within the SBT represents a higher structural level as well as a greater distance away from the probable trench site. Here, the late Cadomian posttectonic plutonic complexes were emplaced during Cambrian and Ordovician-Silurian times and represent mature continental arc activity, probably associated with the decay of the Cadomian subduction zone system after transpressional terrane accretion.
The St Malo Terrane, to the southeast, represents an inverted intra- or behind-arc basin. The migmatite belts within this terrane developed by crustal anatexis. Emplacement of homogeneous diatexites and leucogranites synkinematically with sinistral strike-slip movement along shear belts generated by the transpressive terrane accretion allows this event to be dated at c. 540 Ma.
The Mancellian Terrane, further to the southeast, comprises low-grade Brioverian metasediments into which has been emplaced the Mancellian Batholith, a late Cadomian post-tectonic series of predominantly granite complexes. These complexes have the geochemical features of volcanic arc granites.
Overall, Cadomian magmatism represents c. 275 Ma of plutonic activity broadly related to subduction beneath a continental margin. The early part of the Cadomian orogenic cycle includes volcanic-arc plutonic suites and basaltic volcanic rocks with associated feeder dykes. The peak of the Cadomian orogeny involves intra- or behind-arc basin inversion with concomitant mid-crustal anatexis and sinistrally transpressive terrane accretion c. 540 Ma. The late evolution begins with andesite-rhyolite volcanism and calcalkaline plutonism occurs over a period of c. 100 Ma during the decay of the Cadomian subduction zone system.
This article has been cited by other articles:
 |
|
 |
|
  R. D. Nance, J. B. Murphy, R. A. Strachan, J. D. Keppie, G. Gutierrez-Alonso, J. Fernandez-Suarez, C. Quesada, U. Linnemann, R. D'lemos, and S. A. Pisarevsky Neoproterozoic-early Palaeozoic tectonostratigraphy and palaeogeography of the peri-Gondwanan terranes: Amazonian v. West African connections Geological Society, London, Special Publications, 2008; 297: 345 - 383. [Abstract] [Full Text] [PDF]
|
|
