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Geological Society, London, Special Publications; 1990; v. 51; p. 231-244;
DOI: 10.1144/GSL.SP.1990.051.01.14
© 1990 Geological Society of London

Armorican Massif

Late Precambrian M-type granitoid genesis in the Cadomian belt of NW France

Pierrick Graviou & Bernard Auvray

Institut de Geologie, LP CNRS 4661, Universite de Rennes I, Avenue du General Leclerc, 35042 Rennes Cedex, France

A subduction model has been proposed for the geodynamic evolution of the North Armorican Massif during Upper Proterozoic times. An early period of plutonism (600 ± 20 Ma) is characterized by M-type granitoids with largely intermediate composition. Based on petrological, geochemical and isotopic (Rb-Sr) data from the best exposed batholith (North Tregor Batholith, NTB), the following major conclusions can be proposed.

(a) Various rock-types have resulted from fractional crystalization processes occurring in granodioritic parent magmas.
(b) The Lower Proterozoic basement (Icartian/Pentevrian) has no major role in the genesis of these granitoids.
(c) The mantle wedge above the subducted oceanic slab constitutes one possible source region for the magmas. Fluids escaping from the downgoing slab induced melting of a garnet lherzolite mantle which was also slightly metasomatized. During the ascent of these primary magmas, fractionation of olivine and spinel produced the parent magmas of the Tregor plutonic suite thus explaining the low transition element abundances of these rocks (e.g. Ni, Co contents).