The role of mantle and crust in the generation of calc-alkaline Variscan magmatism and its tectonic setting in the Eastern Pyrenees

Abstract

The lower structural levels of the Variscan orogen exposed in the Eastern Pyrenees reveal three genetically associated magmatic suites: (i) a batholitic sized calc-alkaline granitoid (Sant Llorenç - La Jonquera, SL-LJ); (ii) minor mafic intrusions with local ultramafic cumulates (Ceret and Mas Claret mafic complexes); and (iii) peraluminous leucogranite bodies. These suites were emplaced in a syn- to post-collisional setting during the Late Carboniferous - Early Permian (ca. 315-290 Ma) in an Upper Proterozoic-Upper Ordovician metasedimentary sequence. We carried out field and petrographic work and whole-rock geochemistry (including Sr-Nd radiogenic isotopes) in order to constrain the petrogenetic relationships between them and to determine the interplay of the igneous suites with the country rock. We compare geochemical data to those from the neighbouring massifs of the Pyrenees and Catalan Coastal Ranges. The granitoids and the mafic complexes underwent variable degrees of lower crustal assimilation as demonstrated by the Sr and Nd isotopic ratios of SL-LJ granitoids and mafic rocks. Contaminated gabbro-diorites are high in Fe and Zr and contain magmatic garnet in equilibrium with an Fe-Mg amphibole. A supra-subduction metasomatized mantle source for the mafic complexes is inferred. The magma that formed the SL-LJ granitoids was of intermediate composition and may have formed by differentiation of magmas derived from partial melting of a subduction-metasomatized mantle caused by active subduction or mantle delamination or by partial melting of the lower crust triggered by underplating of mantle-derived mafic magmas. Leucogranite magmas formed later by partial melting of crustal rocks with compositions similar to the outcropping metapelites and orthogneisses. We deduce a retrograde cooling decompression path from 0.75 GPa down to 0.55 GPa from phase equilibria for a garnet-bearing diorite. This path is compatible with a local extensional context linked to a regional dextral strike-slip regime that provided space for the ascent and emplacement of the pluton. This strike-slip system is consistent with late-Variscan shear zones displacing Gondwana to the west with respect to Laurasia during the orogenic collapse.