Crustal recycling and juvenile addition during lithospheric wrenching: The Pontivy-Rostrenen magmatic complex, Armorican Massif (France), Variscan belt

Abstract

The South Armorican Shear Zone (SASZ), in the French Armorican Variscan belt, is a lithospheric wrench fault that acted during the Late Carboniferous as a transition zone between two distinct domains: a thickened domain to the south affected by extension and crustal magmatism, and a weakly thickened domain to the north subjected to dextral wrenching and crust- and mantle-derived magmatism. The Pontivy-Rostrenen complex is a composite intrusion emplaced along the SASZ. To the south, the complex is made of leucogranites whereas, to the north, monzogranites outcrop together with small intrusions of quartz monzodiorite. U-Pb dating of magmatic zircon by LA-ICP-MS reveal that most magmatic rocks were emplaced at ca. 315Ma (between 316.7±2.5Ma and 310.3±4.7Ma), excepted a late leucogranitic intrusion that was emplaced at 304.7±2.7Ma. The leucogranites (−4.8<εNd (T)<2.1; presence of Archean to Paleozoic inherited zircon) are strongly peraluminous (A/CNK>1.1) and formed by partial melting of metasediments and peraluminous orthogneisses. The monzogranite (−4.0<εNd (T)<−3.2; scarce Paleozoic inherited zircon) is moderately peraluminous (1<A/CNK<1.3) and formed by partial melting of an orthogneiss with a probable metaluminous composition whereas the quartz monzodiorite (−3.2<εNd (T)<−2.2; no inherited zircon) is metaluminous (0.7<A/CNK<1.1) and formed by partial melting of a metasomatized lithospheric mantle. The evolution of the magmas was controlled by fractional crystallization, magma mixing and/or peritectic mineral entrainment. At the scale of the Armorican Variscan belt, crustal partial melting, to the south of the SASZ, was triggered by lithospheric thinning and adiabatic decompression during extension. Conversely, to the north, asthenosphere upwelling during strike-slip deformation and subsequent slab tearing, as suggested by tomographic data, induced the melting of both the crust and the mantle fertilized during previous subduction events. This process is likely not exclusive to the Armorican Massif and may be applied to other regions in the Variscan belt, such as Iberia.