Deciphering Iberian Variscan Orogen Magmatism Using the Anisotropy of Magnetic Susceptibility from Granites
Abstract
:1. Introduction and Objectives
- To understand the geologic significance of magnetic susceptibility data, considering the presence of biotite and/or muscovite, along with the magnetic behavior of these granites. This includes examining the paramagnetic and/or ferromagnetic (s.l.) mineralogy and establishing correlations with the redox conditions during magma genesis.
- To correlate magnetic anisotropy fabrics with macro- and microstructures observable in the granites and to interpret them in the context of the emplacement of these granites during the Variscan orogeny.
- To map the magnetic fabric of plutons with different ages within the Variscan orogen and to investigate the magma strain history recorded by these granites.
- Finally, to analyze the timing of the pluton’s emplacement with relation to the Variscan orogeny, identifying the transitions between regional stress fields during the evolution of the orogen.
2. Anisotropy of Magnetic Susceptibility (AMS)
2.1. Theorethical Framework and Applications
2.2. AMS Methods
2.3. AMS Parameters
3. Iberian Variscan Belt
4. Variscan Magmatism
5. AMS Data Integration
5.1. Studied Granite Plutons
5.2. Variability of Magnetic Susceptibility
5.3. Magnetic Anisotropy, Field Observations, and Microstructures
5.4. Magnetic Fabric and Magmatic Strain
- (i)
- Plutons emplaced during high compression (pure shear)
- (ii)
- Plutons emplaced during moderate compression level (pure shear)
- (iii)
- Plutons emplaced during extension (pure shear)
6. Understanding the Magmatic History of the Iberian Variscan: AMS Contribution
6.1. Pluton Classification Using Tectonic Emplacement Settings
6.2. Magnetic Susceptibility and Magnetic Anisotropy Geologic Meaning
7. Conclusions
- The Variscan studied granites exhibit a magnetic susceptibility ranging from 30.4 × 10−6 to 10,436.1 × 10−6 SI units. The distribution of magnetic susceptibility values by classes shows that granites mostly have Km values lower than 1000 × 10−6 SI, showing a paramagnetic behavior, with the biotite being the main Fe carrier. The two-mica (muscovite > biotite) granites show magnetic susceptibility values ranging between 30 × 10−6 SI and 70 × 10−6 SI, which are lower than the values displayed by the biotite-rich facies (biotite > muscovite), with magnetic susceptibility values higher than 110 × 10−6 SI. The composite plutons, with both facies, have magnetic susceptibility values ranging between 70 × 10−6 SI and 110 × 10−6 SI.
- The dominant paramagnetic behavior of the granite plutons studied in this work reflects the presence of biotite as the more important ferromagnesian phase. This feature indicates the reduced conditions involved in the granite melt formation during the Variscan orogeny, suggesting a dominant graphite-bearing (i.e., reducing) source for these collisional magmas. The Lavadores-Madalena pluton, among the granites studied, is the only example of a truly Variscan magnetite-type granite, suggesting a deep magma origin and the presence of melt-oxidized conditions, controlled by the source region.
- Magnetic anisotropy fabrics are related to structures observed in the granites at different scales. Granites that are anisotropic in the field, with a visible orientation of biotite and/or K-feldspars and have magnetic anisotropies > 3%, while granites with anisotropies < 2%, are isotropic, based on field observations. Syn to late-C3 granites show essentially high-to-medium-temperature solid-state deformation microstructures with magmatic-to-submagmatic microstructures being less commonly observed. The magnetic anisotropy in these granites exceeds 3.3%. Syn-E2 granites display almost ubiquitous magmatic-to-submagmatic microstructures and a magnetic anisotropy < 3.3%.
- The studied set of granitic intrusions shows AMS fabrics that record the stress affecting the granites, expressing the chronologic evolution of the stress status during the orogeny. This chronologic approach highlights the magmatic events occurring between around 330 Ma and 315 Ma, attending an extensional regime. The Borralha pluton serves as an example of a suite that recorded an extensional AMS fabric and is, therefore, considered a syn-E1 granite. Plutons with ages between 315 Ma and 305 Ma show AMS fabrics defining their emplacement in a compressional tectonic regime related to the Variscan collision and can, therefore, be considered as syn-C3 granites. Plutons younger than 305 Ma record AMS fabrics indicating a tectonic setting changing from a wrench regime (late-C3 granites) to an extensional one (syn-E2 granites), at the end of the collision stage. The tectonic wrench and the extensional regimes show some chronological overlap, because plutons of similar ages show AMS fabrics recording extension and moderate compression tectonic settings.
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Pluton | Granites | Age (Ma) | Mineralogy/Texture | References |
---|---|---|---|---|
Borralha | Borralha | 320 * | Medium-to-coarse-grained porphyritic biotite granite | [63,64] |
S. Mamede | S. Mamede | 310 * | Medium-grained two-mica granite | [19] |
Vila Verde | Medium-to-coarse-grained two-mica granite | |||
Vila Real-Gralheira | Vila Real | 311 ± 1 | Medium-to-coarse-grained two-mica granite | [9,65] |
Minheu-Lagoa | Medium-grained porphyritic two-mica granite | |||
Gralheira | Medium-to-coarse-grained two-mica granite | |||
Porto | Porto | 311 ± 7 | Medium-grained porphyritic two-mica granite | [9,66] |
Castelo Branco | Alcains | 310 ± 1 | Porphyritic two-mica granite | [9,67] |
Castelo Branco | Coarse-grained porphyritic monzogranite | |||
Vieira do Minho | Vieira do Minho | 310 ± 2 | Coarse-grained porphyritic monzogranite | [68] |
Moreira de Rei | 307 ± 3.5 | Medium-grained porphyritic monzogranite | ||
Freixo de Numão | Freixo de Numão | 306 ± 2 | Medium-to-coarse-grained porphyritic two-mica granite | [64,69,70] |
Frei Tomé | Fine-grained two-mica granite | |||
Serra da Estrela | Seia | 304.1 ± 2.9 | Coarse-grained porphyritic biotite granites | [71,72] |
Covilhã | ||||
Caria-Vila da Ponte | Caria | 301.2 ± 1.2 | Medium-grained porphyritic biotite granite | [12,73] |
Vila da Ponte | Medium-to-coarse-grained porphyritic biotite granite | |||
Valpaços | Valpaços | 305 ± 17 | Medium-to-coarse-grained porphyritic muscovite-biotite granite | [70,74] |
Lagoas | Fine-grained muscovite granite | |||
Capinha | Capinha | 301 ± 3 | Medium-grained two-mica porphyritic granite | [64,75] |
Castro de Aire | Calde | 294.1 ± 3.5 | Coarse-grained porphyritic biotite granites | [72,76] |
Alva | Fine-grained two-mica granite | |||
Lamas de Olo | Lamas de Olo | 297.19 ± 0.73 | Medium-to-coarse-grained porphyritic biotite granite | [13,18,77,78] |
Alto dos Cabeços | Medium-to-fine-grained porphyritic biotite granite | |||
Barragem | Fine-to-medium-grained biotite porphyritic leucogranite | |||
Vila Pouca de Aguiar | Pedras Salgadas | 297 ± 14 | Medium-to-fine-grained porphyritic biotite granite | [5,79] |
Vila Pouca de Aguiar | 298 ± 9.1 | Medium-to-coarse-grained porphyritic monzogranite | ||
Águas Frias-Chaves | Águas Frias | 299 ± 3 | Coarse-grained porphyritic biotite granite | [5] |
Sto. António de Monforte | Medium-grained two-mica granite | |||
Lavadores-Madalena | Madalena | 298 ± 11 | Medium-to-coarse-grained porphyritic biotite granite | [80,81] |
Lavadores | Medium-grained porphyritic biotite granite | |||
Esmolfe-Matança | Esmolfe-Matança | 298 ± 11 to 298 ± 13 | Medium-grained porphyritic biotite granite | [77,82] |
Penedos | Penedos | 298 * | Medium-grained leucogranite with garnet | [34,63,82] |
Monção | Monção | 296 ± 3 | Coarse-grained porphyritic biotite granite | [83] |
Pluton | Km (×10−6 SI) | Km (×10−6 SI) | P% | P% | K1d | K1i | K1 95% c.a. | Magnetic Lineation | K3d | K3i | K3 95% c.a. | Magnetic Foliation | |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Borralha (n = 7) | 53.9 | 25.8 | 4.4 | 1.8 | 101 | 5 | 5 | 5°/N101° | 242 | 69 | 3 | N152°, 21°NE | |
S. Mamede (n = 8) | 84.2 | 35.4 | 4.7 | 1.12 | 338 | 13 | 23 | 13°/N338° | 16 | 27 | 9 | N106°, 63°SW | |
Vila Real-Gralheira (n = 20) | 57.2 | 1.5 | 3.6 | 0.5 | 224 | 78 | 10 | 78°/N224° | 38 | 12 | 4 | N128°, 78°SW | |
Porto (n = 6) | 48.7 | 3.9 | 5.9 | 4.9 | 128 | 79 | n.a | 79°/N128° | 13 | 6 | n.a | N103°, 84°SW | |
Castelo Branco (n = 84) | 71.6 | 28.4 | 4.6 | 2.0 | 137 | 56 | 25 | 56°/N137° | 51 | 2 | 11 | N141°, 88°SW | |
Vieira do Minho (n = 13) | 161.6 | 37.8 | 4.2 | 0.9 | 147 | 1 | 20 | 1°/N147° | 61 | 27 | 13 | N151°, 63°SW | |
Freixo de Numão (n = 40) | 123.9 | 23.8 | 2.6 | 2.0 | 112 | 36 | n.a. | 36°/N112° | 12 | 37 | n.a. | N102°, 53°SW | |
Serra da Estrela (Seia and Covilhã) (n = 201) | 107.0 | 47.5 | 4.1 | 1.1 | 173 | 33 | 43 | 33°/N173° | 267 | 17 | 18 | N177°, 73°NE | |
Caria-Vila da Ponte (n = 80) | 76.0 | 15.9 | 1.8 | 0.5 | 340 | 10 | 15 | 10°/N340° | 110 | 62 | 11 | N20°, 28°NW | |
Valpaços (n = 34) | 56.7 | 10.1 | 3.1 | 0.8 | 110 | 4 | 11 | 4°/N110° | 5 | 63 | 5 | N95°, 27°S | |
Capinha (n = 30) | 73.4 | 12.3 | 2.0 | 1.3 | 208 | 8 | 10 | 8°/N208° | 319 | 87 | 3 | N59°, 3°SE | |
Castro Daire (n = 105) | 83.0 | 2.9 | 3.4 | 0.9 | 354 | 22 | 46 | 22°/N354° | 253 | 12 | 19 | N163°, 78°NE | |
Lamas de Olo (n = 48) | 1719.0 | 6350.6 | 5.1 | 2.4 | 169 | 28 | 30 | 28°/N169° | 258 | 2 | 14 | N168°, 88°NE | |
Vila Pouca de Aguiar (n = 105) | 117.2 | 40.7 | 1.3 | 0.6 | 359 | 1 | 14 | 1°/N359° | 129 | 83 | 11 | N39°, 7°NW | |
Águas Frias-Chaves (n = 13) | 78.1 | 22.9 | 2.1 | 1.3 | 296 | 7 | 15 | 7°/N296° | 181 | 80 | 6 | N91°, 10°N | |
Lavadores-Madalena (n = 14) | Lavadores | 10,436.1 | 2983.6 | 18.4 | 4.9 | 219 | 78 | 12 | 78°/N219° | 17 | 12 | 9 | N107°, 78°SW |
Madalena | 98 | 8 | 11 | 8°/N98° | 4 | 16 | 8 | N94°, 74°SW | |||||
Esmolfe-Matança (n = 59) | 64.4 | 10.3 | 3.2 | 3.7 | 330 | 6 | 8 | 6°/N330° | 110 | 87 | 3 | N20°, 3°NW | |
Penedos (n = 6) | 30.4 | 7.0 | 1.9 | 2.0 | 352 | 44 | 13 | 4°/N352° | 62 | 36 | 6 | N152°, 51°SW | |
Monção (n = 3) | 117.4 | 34.3 | 1.4 | 1.5 | 260 | 12 | n.a | 12°/N3260° | 153 | 57 | n.a | N63°, 33°NW |
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Sant’Ovaia, H.; Cruz, C.; Gonçalves, A.; Nogueira, P.; Noronha, F. Deciphering Iberian Variscan Orogen Magmatism Using the Anisotropy of Magnetic Susceptibility from Granites. Minerals 2024, 14, 309. https://doi.org/10.3390/min14030309
Sant’Ovaia H, Cruz C, Gonçalves A, Nogueira P, Noronha F. Deciphering Iberian Variscan Orogen Magmatism Using the Anisotropy of Magnetic Susceptibility from Granites. Minerals. 2024; 14(3):309. https://doi.org/10.3390/min14030309
Chicago/Turabian StyleSant’Ovaia, Helena, Cláudia Cruz, Ana Gonçalves, Pedro Nogueira, and Fernando Noronha. 2024. "Deciphering Iberian Variscan Orogen Magmatism Using the Anisotropy of Magnetic Susceptibility from Granites" Minerals 14, no. 3: 309. https://doi.org/10.3390/min14030309