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

L. González-Menéndez, G. Gallastegui, A. Cuesta, P. Montero, A. Rubio-Ordóñez, J.F. Molina, F. Bea

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 among which the Porriño pluton studied here is one of the best examples. The Porriño pluton 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 290-295 Ma 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 scatter 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). The observed geochemistry results from 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 protholiths 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 syntectonic granites that left restites and less fertile rocks. Late orogenic new melting affected the previously un-melted 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.


Keywords


Post-orogenic magmatism; Variscan orogen; postectonic granites; monzogranites; enclaves

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DOI: http://dx.doi.org/10.1344/%25x

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Geologica Acta (ISSN-1695-6133)
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