Petrogenesis of an A2-type intrusion associated with the late-post-collisional calc-alkaline Variscan granites from central-northern Portugal (Central Iberian Zone)

The Lusinde fine-grained biotite granite is a small pluton intruded into the metamorphic basement of the Central Iberian Zone during the waning stages of the Variscan Orogeny. LA-ICP-MS zircon U Pb ages constrain pluton emplacement to ca. 297–295 Ma. Mineralogical and geochemical data reveals that the intrusion corresponds to a strongly ferroan, peraluminous, reduced A 2 -type granite (∆FMQ = −3.4 to −1.7, zircon oxybarometry), characterised by a narrow compositional range (SiO 2 = 67.7–68.8 wt%), elevated FeO T /FeO T + MgO (0.90–0.92) and 10,000 × Ga/Al (3.1–3.4) ratios, high REE, LILE and HFSE contents and strong depletions in Ba, Sr, Nb, P, Eu and Ti. Decoupling between “mantle-like” Nd and zircon Hf (εNd i = −0.40 to +0.58; T DM2 = 0.81–0.95 Ga; zircon εHf i = −0.41 to +3.01; T DM2-Zrn = 1.08–1.30 Ga) and more “crust-like” Sr ( 87 Sr/ 86 Sr i = 0.7097–0.7100) isotopic signatures suggests derivation from small degrees of partial melting of old juvenile mafic/intermediate rocks interlayered in the lower crust, with minor input from more evolved crustal sources. The only studied ferrodioritic enclave shares the A 2 -type affinity of the host granite and seems to have derived from a mixed source (enriched subcontinental lithospheric mantle and juvenile mafic crust), before injection into the silicic magma chamber. We propose that the generation of the Lusinde A-type granite magma and enclosed enclave occurred in a deep hot zone located at the mantle-lower crust interface, at about 7–10 kb and 850–950 °C, in an extensional post-collisional setting.