Petrofabric patterns of two contrasting plutons: example of Penedos and Borralha granites (Montalegre, Northern Portugal)

The Penedos-Borralha area is located inside the Galícia-Trás-os-Montes Zone where Silurian metasediments and Variscan granites outcrop. Three Variscan ductile deformation phases were recognized: D₁ (360-337Ma); D₂ (337-320Ma) and D₃ (320-310Ma). D₃implied a regional subvertical crenulation (N120ºE) and the deformation of syn-D₃ granites (321–312Ma). This contribution results from a multidisciplinary approach (viz., fieldwork, petrography, and anisotropy of magnetic susceptibility (AMS)) for the understanding of the magnetic anisotropy patterns of Penedos (PG, post-D₃) and Borralha (BG, syn-D₃). PG occurs as circumscribed outcrop in a triple point marked by the contact between BG, Borralha tonalite and metasediments. PG is a leucocratic, medium- to coarse-grained granite with garnet. BG occurs as a WNW-ESE outcrop and is composed of biotite-rich, medium- to coarse-grained porphyritic granite. At the outcrop scale, oriented patterns were not observed in PG; however, the BG exhibits K-feldspar megacrysts and phyllosilicates N120ºE oriented. The main mineralogical features observed in BG are quartz displaying strong undulose extinction and subgranulation, K-feldspar with poikilitic texture, plagioclase presenting curved twins and well-developed kinked phyllosilicates. In turn, PG presents quartz displaying slightly undulose extinction, slightly stretched plagioclase, euhedral garnet crystals and phyllosilicates occurring as clustered flakes. The petrofabric studies were obtained using AMS providing scalar (magnetic susceptibility, K_m and paramagnetic anisotropy, P_para) and directional (magnetic foliation, ⊥K₃ and lineation, K₁) parameters. The K_mindicated paramagnetic behaviour for both BG and PG (53.9 and 30.39µSI, respectively) classifying them as ilmenite-type granites.BG exhibits the highest P_para values (4.4%), result of the strong K-feldspars and biotite alignment. In contrast, PG exhibits the lowest P_para values (1.89%) compatible with the no-oriented patterns. The ⊥K₃ in the PG are very heterogeneous ranging from NW-SE to E-W; generally, the ⊥K₃ are subvertical in the E side, where the PG is intrusive in the metasediments and subhorizontal in the W side, where the PG cuts the BG. Concerning the K₁ the PG display NNE-SSW to W-E azimuths with subhorizontal to intermediate dips. In the BG, the ⊥K₃, essentially subhorizontal, tends to be parallel to the contacts with the regional rocks and the K₁are strongly subhorizontal with azimuths ranging from WNW-ESE to ENE-WSW. Considering this multidisciplinary approach is clear that the petrofabric obtained for both granites resulted from distinct phenomena. The PG petrofabric was inherited from magmatic stages, where the K₁ trajectories suggest the location of a feeder zone in the SE border and continuous magmatic flow to NW. The evidence of subvertical ⊥K₃ in the E side of PG suggests a tongue-shaped intrusion thicker on this side. In contrast, the BG petrofabric was acquired in the subsolidus and resulted from tectonic processes. The obtained petrofabric agrees with the proposed classification of BG and PG as a syn-D₃ and post-D₃ granites, respectively. These analyses applied to two contrasting intrusions allowed us to verify that the AMS depend on several parameters and must be interpreted with caution.

Acknowledgments: This work was supported by national funding awarded by FCT - Foundation for Science and Technology, I.P., projects UIDB/04683/2020 and UIDP/04683/2020.