A turning-point in the evolution of the Variscan orogen: the ca. 325 Ma regional partial-melting event of the coastal South Armorican domain (South Brittany and Vendée, France)

Abstract

By drastically reducing the bulk strength of crustal materials, partial-melting is one of the main parameter controlling the rheological behaviour of the continental crust. With more than ca. 50% of the outcropping surface characterised by migmatites and granites, the coastal South Armorican domain, offers an opportunity to study deep-orogenic processes and more particularly, to understand the role of partial-melting for the late-evolution of the Variscan belt. To date, time-constraints are scarce hindering the understanding of this crucial stage in the Variscan belt evolution. This paper provides 29 new U-Th/Pb chemical ages on monazite collected over five sampling areas consisting in migmatite domes and late regional classic plutons. Based on structural, textural and chemical criteria, three main U-Th/Pb age-groups are distinguished. The first group, settled at ca. 335–330 Ma concerns samples of restites and core-domains of the monazite crystals for most of the granite massifs. Its significance is ascribed to inherited crystallisation ages probably recording the crossing of prograde monazite forming reactions (i.e. metamorphic isograds) during increasing P-T conditions in an overall nappe-stacking context. The second group that clusters at ca. 325–320 Ma corresponds to newly formed monazite grains that crystallised from juvenile silicate melts. Ages of this group are interpreted as crystallisation ages of leucosomes after a major partial-melting event that affected the whole domain. The last ca. 320 Ma group corresponds to rim-domains of monazite crystals. It is interpreted as the emplacement age of most of the large-scale granite massifs and therefore fixes the end of the partial-melting event. The inception and drastic generalisation of partial-melting at peak-P conditions therefore coincides with a major change in the tectonic regime recorded at regional-scale. In the lights of these results, this implies that (1) either continuous stacking of continental crustal units, rich in radiogenic elements, led to an increase of temperature within the orogenic wedge provoking partial-melting, the resulting drop in the crustal strength inducing collapse and lateral expansion of the belt, or (2) a drastic change of the boundary conditions has induced hot asthenospheric upwelling which in turn led to coeval extension and partial-melting. At a more local scale, strain benefited of the low-strength of the magmatic bodies prior to complete crystallisation promoting intense strain localisation within the South Armorican domain large-scale laccoliths often referred to as synkinematic plutons.