The Miranda-Sayago batholith (NW Iberia): Implications on mantle-crust hybrid zone generation during Variscan collision

Abstract

The Miranda-Sayago Batholith (MSB) is a large NW-SE trending intrusive body primarily composed of granodiorites and monzogranites, with minor intrusions of K-rich mafic-intermediate rocks (vaugnerites-sanukitoids), extending from Miranda do Douro in Portugal to northern Zamora in Spain (Sayago region). A geochemical and geochronological analysis reveals complex relations between granodiorites and vaugnerites. Key findings that shed light on the origin of the magmas forming the MSB include: (1) Vaugnerites and granodiorites represent two distinct but genetically-linked and coeval magma series. (2) Vaugnerites were emplaced over a period of approximately 13 million years, from ca. 320 to 307 Ma. (3) Contamination with the surrounding metamorphic and granitic host rocks is recognized in the granodiorites. Also, the autolithic mafic microgranular enclaves, which are isolated from contamination with host migmatites, as well as some vaugnerites, display A/CNK >1, suggesting an Al-rich material involved in the hybridization of the putative mantle source of the intermediate magmas. We propose that granodiorites in the MSB originated by melting of a heterogeneous source rooted in a hybrid mantle that was fluxed by crustal, peraluminous, components during Variscan collision through continental subduction and/or mantle-crust imbrication. Initial Sr and Nd isotopic ratios point to the Cambro-Ordovician “Ollo de Sapo” Formation as the crustal end-member of the hybrid magmas. Parental vaugnerites with values about ε_Nd = −4 and initial ⁸⁷Sr/⁸⁶Sr within a narrow range from 0.705 to 0.709 (at the reference age of 307 Ma), likely originated from melting of mantle material contaminated by 10–20 % crustal components. Granodioritic and vaugneritic magmas ascended through shared channels, promoting further hybridization between magmatic pulses. We conclude that collision played a key role in conditioning fertile mantle sources for post-collisional batholith generation in this sector of the Iberian massif.