Petrology and geochronology of the Porriño late-Variscan pluton from NW Iberia. A model for post-tectonic plutons in collisional settings

Abstract

The Variscan orogen of NW Iberia contains abundant syn- and post-tectonic granitoids. The post-tectonic granitoids are metaluminous to slightly peraluminous, I-type granites, monzogranites ± granodiorites ± tonalites. The Porriño pluton studied here is a representative example. It consists of two units: i) a pink-red, peraluminous, biotite granite and ii) a gray, metaluminous to peraluminous, biotite (± amphibole ± titanite) monzogranite, including mafic-intermediate enclaves. SHRIMP U-Pb dating yielded 290-295Ma ages for all the units. The mineralogy and geochemistry show that the pink-red granite has features of I- and A-type granites, whereas the gray monzogranite and enclaves are I-types. Sr isotopes show scattered values for the pink-red granite (87Sr/86Sr295Ma ≈ 0.702-0.710) and uniform values for the gray monzogranite and enclaves (87Sr/86Sr295Ma ≈ 0.705-0.706). Geochemical results indicate a peritectic entrainment of clinopyroxene + orthopyroxene ± Ca-plagioclase ± ilmenite ± garnet, and minor accessory phases (± zircon ± titanite ± apatite) into a melt similar to the leucocratic gray monzogranite. A mafic-intermediate source is proposed for the gray monzogranite and its enclaves. Restitic protoliths generated granitic melts with A-type features such as the pink-red granite. The I-type nature of many post-tectonic granitoids could be explained by the previous extraction of S-type syn-tectonic granites that left restites and less fertile rocks. Late orogenic new melting affected the previously unmelted and more mafic lithologies of the lower-middle crust, and gave rise to I-type granitoids. Repeated melting events affecting such lithologies and previous restites could have generated granitic melts with A-type features.