Emplacement mechanisms of the calc-alkaline Variscan magamtism and its prevailing regional tectonic regime in the Eastern Pyrenees

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. 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.

The interference pattern resulting from the superposition of Variscan (F2) and Alpine (F3) folding in the Eastern Pyrenees gives an exceptional field example to infer the 3D geometry of the SL–LJ pluton and its associated igneous rocks. The intrusion feeder zones are located in the northern flank of the antiform where the mafic complexes crop out, cutting the deeper structural levels of the Roc de Frausa and L'Albera series. The floor of the pluton is located above the Upper Proterozoic – Mid- Ordovician sequence, which is largely parallel to the S1 foliation, and the roof is slightly oblique to the Upper Ordovician-Silurian sequence (S0). This parallelism together with a well-developed magmatic and magnetic fabric parallel to S1 suggests that initial phases of intrusion of SL–LJ magmas took place at the end of D1, at ca. 314–311 Ma. The lack of stratigraphic continuity above and below the pluton suggests that the stratigraphic succession of the L'Albera massif was laterally displaced, and the intruding magma progressively grew while cutting through the entire sequence and filling the available space. This placement of the magmas is compatible with a local extensional setting that favored the ascent of the SL–LJ magmas from a lower crustal reservoir through vertical feeder zones in the footwall of the extensional faults where lithostatic pressure was minimal. The coeval development of NW-SE to NNW-ESE extensional faults with the NE-SW trending D2 contractional structures and the horizontal attitude of the mineral lineations, once restored the Alpine deformation, is compatible with a regional dextral strike-slip tectonic setting that took place during and after the emplacement of the igneous bodies. This strike-slip system is consistent with late-Variscan shear zones displacing Gondwana to the west with respect to Laurasia during the orogenic collapse.