In the Central Iberian Zone synorogenic (syn-and late-D 3 , the last deformation phase) composite batholiths of biotite-rich granitods with peraluminous character are well represented. We report here U-Pb zircon and monazite ages, geochemical, Sr-Nd-O isotopic data and Anisotropy of Magnetic Susceptibility (AMS) studies for the late-D 3 Vieira do Minho pluton, in northern Portugal. AMS reveals the paramagnetic behaviour of these granites and the magnetic fabric enhances the major role of NW-SE regional anisotropies on controlling the ascent and emplacement of the magmas. Magnetic anisotropy and medium temperature microstructures also point out the evolution of regional tectonics during crystallization of the magma. U-Pb zircon and monazite analyses yield consistent ages of 310 ± 2 Ma and 312 ± 2 Ma, interpreted as emplacement age. The Vieira do Minho pluton consists of two peraluminous monzogranites, the VMG and the MRG. Both have similar ε Nd values (VMG= −5.2 to −5.7; MRG =−4.98 to −5.96) but the VMG is slightly enriched in 87 Sr/ 86 i = 0.7087-0.7098 as well as in δ 18 O in the range of 10.6‰ to 11.0‰ than the MRG with 87 Sr/ 86 i =0.7064-0.7075 and δ 18 O=9.9‰-10.5‰. These granites are associated with coeval scarce grabroic intrusions and/or mafic microgranular enclaves which are not considered as mafic precursors of the associated granitic magmatism. Instead, a lower metaigneous crustal source, at different levels, is proposed based on the available data set.
<p>This work focuses on the magnetic fabric of 20 variscan granitic massifs from northern and central Portugal and considers the Anisotropy of Magnetic Susceptibility (AMS) results obtained in about 750 sampling sites. In the northern and central Portugal, three main ductile deformation phases were recognized and described: D<sub>1</sub>, D<sub>2</sub> and D<sub>3</sub>, being the variscan magmatism events mainly related to D<sub>3</sub> phase. D<sub>3</sub> produced wide amplitude folds with NW-SE subhorizontal axial plane and subvertical dextral and sinistral ductile shear zones, forming obtuse angles with the maximum compression direction, &#963;1, NE-SW oriented. The post-D<sub>3</sub> brittle phase was responsible for the development of conjugate faults (NNW-SSE, NNE-SSW and ENE-WSW), related to a N-S maximum compression. The studied granites were subdivided according to U-Pb dating, field observations and considering the chronology of their emplacement relative to the D<sub>3</sub> phase of Variscan orogeny. Therefore, the studied granites are subdivided into: (1) syn-D<sub>3</sub> two-mica granites, ca. 311 Ma; (2) late-D<sub>3</sub> monzogranites, biotite-rich and two-mica granites, ca. 300 Ma; (3) post-D<sub>3</sub> monzogranites and biotite-rich granites, ca. 299 &#8211; 297 Ma. Magnetic fabric gives two types of directional data, magnetic foliations and magnetic lineations, which provide important information regarding the orientation of the magmatic flow, feeder zone location, relationship between the magma emplacement and tectonics and, also, the stress field. The data obtained for the magnetic fabric, based on AMS technique, allowed concluding: (i) syn-D<sub>3</sub> granites show magnetic foliations and lineations consistent with the syn-D<sub>3</sub> variscan structures ca. N110&#176;-120&#176;E, related to a NE-SW maximum stress field . The foliations are, mainly, subvertical (> 60&#186;), which may indicate a high thickness of the granitic body and deep rooting; on the other hand, the magnetic lineations exhibit variables plunges. (ii) Late-D<sub>3</sub> granites are characterized by foliations and lineations, dominantly NNW-SSE to NNE-SSW oriented. The foliations are subvertical dips (> 60&#186;) and the lineations have, generally, soft plunges. (iii) Post-D<sub>3</sub> granites have, in general, magnetic foliations and lineations associated with important regional post-D<sub>3</sub> brittle structures, which display NNE-SSW and ENE-WSW trending. The subhorizontal fabric may suggest a small thickness of the granitic bodies. In all granite sets under study there is a dominance of weakly dipping lineations (slope <60&#186;), indicating that the feeding zones are deep, which supports the idea of an emplacement at high structural levels.</p><p>Acknowledgments: The authors thank Department of Geosciences, Environment and Spatial Planning at Faculty of Sciences of the University of Porto and the Earth Sciences Institute (Porto Pole, Project COMPETE 2020 (UID/GEO/04683/2013), reference POCI-01-0145-FEDER-007690).</p>
<p>The magnetic susceptibility (K<sub>m</sub>) of granites is an important characteristic and it is mainly controlled by the presence of certain oxide minerals like magnetite and/or ilmenite, as well as ferromagnesian phyllosilicates such as biotite. The abundance of magnetite or ilmenite can be explained by different redox conditions in the magma chamber and distinct magma sources. The presence of magnetite or ilmenite as accessory minerals represents oxidized- or magnetite-type granites and reduced- or ilmenite-type granites, respectively.</p><p>This work focuses on the K<sub>m</sub> of 20 Variscan granitic massifs from northern and central Portugal and considers the results obtained in about 750 sampling sites, in order to deduce the redox conditions in the magma system. These granites are essentially two mica mesocrustal and biotite-rich basicrustal/infracrustal in origin and their emplacement was related to Variscan orogeny. In the northern and central Portugal, three main ductile deformation Variscan phases were recognized and described: D<sub>1</sub>, D<sub>2</sub> and D<sub>3</sub>. The studied granites were subdivided in three main groups according to U-Pb dating, field observations and emplacement relative to the D<sub>3 </sub>phase. Therefore, the studied granites are subdivided as following: (1) syn-D<sub>3</sub> two-mica (mosc=biot) granites, ca. 311 Ma; (2) late-D<sub>3</sub> monzogranites, biotite-rich and two-mica granites (biot>mosc), ca. 300 Ma; (3) post-D<sub>3</sub> monzogranites and biotite-rich granites, < 299 Ma.</p><p>The evaluation of the K<sub>m</sub> variation of the different granite groups shows that, as granites become progressively younger, the K<sub>m</sub> parameter tends to increase as a result of the increasing in the mantellic contribution to the genesis of the magmas. Syn-D<sub>3</sub> granites display K<sub>m</sub> between 17.8 &#956;SI and 186 &#956;SI. This variation is due to the high textural and compositional diversity, including two-mica granites with different relative proportions of muscovite and biotite. Late-D<sub>3</sub> granites are represented by two-mica and biotite-rich granites with calcium plagioclase, with several degrees of post-magmatic alteration, implying iron leaching processes. These processes also promote the crystallization of secondary muscovite, which implied a decrease in the K<sub>m</sub> values, and also a wide dispersion of K<sub>m</sub> values ranging between 7.3 &#956;SI and 276 &#956;SI. Post-D<sub>3</sub> granites are mostly represented by biotite-rich granites with calcium plagioclase close to I-type granites. This granite group is divided into two subgroups: (i) post-D<sub>3</sub> ilmenite-type granites with K<sub>m</sub> values of ca. 113 &#956;SI, typical of biotite-rich granites; and (ii) post-D<sub>3</sub> magnetite-type granites with K<sub>m</sub> values between 2078 &#956;SI and 11676 &#956;SI representing magnetite-type granites. In N and Central Portugal, these magnetite-type granites can occur in homogeneous plutons or in composite plutons constituted by ferromagnetic and paramagnetic facies.</p><p>As a conclusion, mostly of the granites from northern and central Portugal exhibit average K<sub>m</sub> values below 1000 &#956;SI and are characterized by a paramagnetic behavior corresponding to reduced- or ilmenite-type granites. Among the all studied granites, only one pluton showed to be a truly oxidized- or magnetite-type granite, with K<sub>m</sub> of the order of 11676 &#956;SI.</p><p><strong>Acknowledgements: </strong>This work was funded by the Funda&#231;&#227;o para a Ci&#234;ncia e a Tecnologia (FCT) under UIDB/04683/2020 project.</p>
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