Telescoping of three distinct magmatic suites in an orogenic setting: Generation of Hercynian igneous rocks of the Albera Massif (Eastern Pyrenees)

Abstract

The petrogenesis of the Hercynian magmatic rocks of the Albera Massif (eastern Pyrenees) is investigated in this work, in order to establish spatial and temporal relationships between deformation episodes, high temperature–low pressure metamorphism and igneous activity, by using fieldwork together with petrological and geochemical methods. Hercynian magmatic rocks comprise (i) calc-alkaline granitoids of La Jonquera composed mainly by biotite, and biotite–hornblende granodiorites; (ii) the leucogranitic pluton of El Castellar; (iii) small mafic complexes composed mainly by amphibole-bearing gabbros and diorites, and (iv) stocks of anatectic leucogranites. Hercynian deformation structures can be classified in two phases, D1 and D2, both reflecting progressive deformation (first compressional, then transpressional) closely linked to the development of regional metamorphism. Spatial relationships between igneous and metamorphic rocks suggest that the emplacement of magmatic bodies was a syn-D2 event, and was concomitant with the metamorphic climax of the country rocks. The compositional and textural variation of the rocks of the mafic complexes is interpreted to reflect fractional crystallization of hydrous mafic magma with a composition similar to that of calc-alkaline basalts, documenting the involvement of basaltic magmas during the generation of Hercynian crustal granitoids. The anatectic leucogranites crystallized from peraluminous magmas produced by partial melting of metasediments and orthogneisses similar to those found in the local migmatite zone. The similar Sr and Nd isotope signatures of the El Castellar leucogranitic pluton, some anatectic leucogranites, and the orthogneissess supports the hypothesis that these leucogranites were generated by partial melting of the orthogneisses. Based on field, petrographic, geochemical and isotopic lines of evidence, La Jonquera granodiorite can be clearly distinguished from the gabbros, and from the anatectic and El Castellar leucogranites. The high CaO content of the granodiorites precludes an origin by partial melting of local metamorphic rocks, and it is suggested that the calc-alkaline granitoids (La Jonquera) were produced by fractional crystallization of a granodioritic magma generated by partial melting of metatonalites in the lower crust. Although the magmatic rocks were emplaced contemporaneously (at about 280 My based on Rb–Sr whole-rock data), they reflect the occurrence of three genetically distinct igneous suites: one granodioritic, one leucogranitic and one mafic. The metamorphic and igneous evolution documented in the Albera Massif bears strong similarities with those described in the neighbouring Hercynian domains from the Pyrenees. The Hercynian regional HT–LP metamorphism and magmatism in the Pyrenees are two processes linked to the same deep-seated thermal anomaly that was generated in a transpressive context.