Cotrim, B.1*, Bento dos Santos, T. 1, Mata, J. 1, Benoit, M. 2, Jesus, A.P. 1 1 IDL – Instituto Dom Luiz / Departamento de Geologia, Faculdade de Ciências, Universidade de Lisboa, Portugal. FCUL - Campo Grande Edifício C1, Piso 1 1749-016 Lisboa
2 GET – Géosciences Environnement Toulouse, OMP – Observatoire Midi Pyrénées, Université Paul Sabatier. 14 Avenue Edouard Belin, 31400 Toulouse, França
* beatriz_cotrim_@hotmail.com
Resumo: Na região de Montebelo (Viseu) afloram anfibolitos e metagabros espacialmente associados ao Super Grupo Dúrico – Beirão, posicionados a sul da Faixa Metamórfica Porto-Viseu, considerada como limite provável entre o Grupo do Douro e o Grupo das Beiras. Este contexto geodinâmico é peculiar, necessitando de uma melhor compreensão que pode ser obtida através de um estudo petrológico, geoquímico e isotópico destas litologias.
A geoquímica de rocha total revela que os metagabros e os anfibolitos não são cogenéticos, embora ambos pertençam à série toleítica e estão associados a ambientes tectónicos extensivos. Os anfibolitos apresentam uma assinatura geoquímica MORB-E e os metagabros MORB-N. A geologia isotópica reforça esta ideia, pois os anfibolitos apresentam εNdi entre +4.6 e +6.2 e 87Sr/86Sri entre 0.7052 e 0.7065 enquanto os metagabros apresentam εNdi entre +5.8 e +7.7 e 87Sr/86Sr
i entre 0.7040 e 0.7051. Estes dados implicam baixos graus de contaminação crustal com geração em ambiente distensivo e intrusão em crosta continental adelgaçada na zona axial da Zona Centro-Ibérica.
Verifica-se assim que estas rochas metabásicas são o reflexo dos estágios de rifting que, pelo seu posicionamento paleogeográfico, são um importante marcador da separação entre a Bacia do Douro e das Beiras. Neste contexto, os metagabros representam uma fase de rifting mais avançada que os anfibolitos.
Palavras-chave: Maciço Varisco Ibérico, rifting, MORB, geoquímica
Abstract: In the region of Montebelo (Viseu) amphibolites and metagabbros outcrop spacially associated to the Beiras – Douro Supergroup, positioned south of the Porto – Viseu Metamorphic Belt, considered the likely limit between Douro’s Group and Beira’s Group. This geodynamical context is peculiar and warrants a further investigation which was obtained by a petrological, geochemical and isotopic study of these lithologies.
The whole rock geochemistry shows that the metagabbros and the amphibolites are not cogenetic, although both belong to the tholeiitic series and are associated to extensional environments. The amphibolites present an E-MORB geochemical signature and metagabbros show a N-MORB signature. The isotopic geochemistry reinforces this idea: the amphibolites have εNdi between +4.6 and +6.2 and 87Sr/86Sri between 0.7052 and 0.7065 while the metagabbros show εNdi between +5.8 and +7.7 and 87Sr/86Sr
i between 0.7040 and 0.7051. These data imply low degrees of crustal contamination generated in an extensional environment and intrusion on thinned continental crust on the axial zone of the Central- Iberian Zone.
We propose that these metabasic rocks were formed during the rifting stages and, due to its paleogeographic setting, might delimit the separation of Douro and Beiras’ Basins. In this context, the metagabbros would represent an advanced rifting stage compared to the amphibolites.
104 (ppm)
1. Introduction
In the Central-Iberian Zone (CIZ) of the Variscan orogen, the Porto – Viseu Metamorphic Belt has recently been a target of interest by several authors (Valle Aguado & Azevedo, 2006; Rodrigues et al., 2013). This anatectic complex has been considered as the most probable limit between the Douro and Beiras Group (Villaseca et al., 2014).
Immediately to the South of this metamorphic belt, there are several syn- and tardi-orogenic granitoids, metasediments of the Douro – Beiras Supergroup (Sousa & Sequeira, 1989) and metabasic rocks, namely metagabbros and amphibolites (Godinho et al., 2010). These metabasic rocks are the focus of this work, as their geological significance contribute to a better understanding of the CIZ’s geodynamic evolution during the Variscan Cycle.
2. Geochemical Characterization
Taking into account the metamorphosed character of the studied samples, we will focus our attention mostly on the use of HFSE (High Field Strength Elements). The studied samples are characterized by Nb/Y ratios lower than 0.7, emphasing their sub-alkaline affinity which is more obvious in the metagabbros (Y/Nb ≤ 0.16) than for the amphibolites, characterized by Nb/Y = 0.43 - 0.68. It should be noticed that the amphibolite sample BJT-12 is always associated to the metagabbros in every diagram. Yet plotted in all diagrams, this sample will not be considered in the following discussion. These differences on the alkalinity degree reflect in distinct ratios between incompatible elements as depicted in the Nb vs. Zr diagram (Fig.1), where metagabbros present significantly higher ratios. Such difference is also evident on REE patterns (Fig.2) with amphibolites being clearly more fractionated [(La/Lu)N ≥ 3.8] than those of
the metagabbros [(La/Lu)N ≤ 1.2].
Considering that amphibolites correspond to less evolved magmas (higher Mg# and Ni contents) such differences cannot be attributed to magma evolution processes, instead suggesting that both groups of samples are not cogenetic. This
interpretation is further supported by distinct isotope signatures (see below).
Fig. 1 – Nb vs Zr ratios show that there are two sets of cogenetic basic rocks.
2.1. Geoctectonic Environment
As already mentioned, on the chondrite-normalized REE patterns (Fig. 2), the amphibolites present a slight enrichment of LREE when compared to HREE [(La/Lu)N = 3.8 – 4.5], while the
metagabbros present a flatter pattern [(La/Lu)N = 0.7 – 1.2]. Moreover, while
metaggabros are characterized by (La/Sm)N down to 0.78, amphibolites
present for the same ratio values clearly above 1 [(La/Sm)N = 1.98 – 2.04].
Fig. 2 – Rare earth elements diagram of all samples using the primitive mantle of McDonough
& Sun (1995) as a normalization factor. (La/Sm)N ratios < 1 in basic rocks are
indicative of a source depleted in incompatible elements. For the studied
0 2 4 6 8 10 12 14 0 20 40 60 80 100 120 Nb Z r Nb/Zr =21 Nb/Zr = 8.6
Symbols and colours by group
Amphibolite Metagabbro
rocks such depletion is likely an ancient feature as indicated by εNdi ranging from
+5.7 to +7.7. Such values are similar to those characteristics of N-MORB type rocks, yet island arcs can also present similar depleted signatures.
The distinction between those two types of magma affinities (MORB vs. IAT (Island Arc Tholeiits)) can be elucidated through the use of the Nb/Yb vs Th/Yb diagram (Fig.3; Pearce, 2008). Indeed, all the metagabbros plot on the OIB (Ocean Island Basalts) - MORB mantle array, showing no evidence of Th enrichment as is typical from supra- subduction environments and/or crustal contamination. The amphibolites also plot on the OIB-MORB array, but their relative enrichment in incompatible elements places them close the E-MORB field.
Fig. 3 – Discriminant diagram Nb/Yb vs Th/Yb of Pearce (2008) that separates MORB-OIB rocks
from vulcanic arc rocks.
These affinities are also evident in E-MORB normalized spidergrams (Fig. 4), wherein amphibolites tend to present a flat pattern (close to 1), the same applying to metagabbros when normalized to N- MORB. Similar information can be extracted using other discriminant diagrams, such as the proposed by Wood
et al. (1980), where the metagabbros plot
on N-MORB field, while amphibolites are placed on the E-MORB/Within Plate Tholeiites.
Nb and Ta normalized concentrations in the studied rocks are enriched to the same level of the LILE, excluding Cs which is extremely mobile. Additionally, the lack of
Ti negative anomalies is suggestive of negligible crustal contamination or a subduction-related fingerprint. However, crustal contamination cannot be completely discarded given the observed P negative anomalies, which are more significative in the metagabbros.
Fig. 4 – Multivariation diagram of all samples using the primitive mantle of Sun & McDonough (1989)
as a normalization factor.
The metagabbros exhibit negative anomalies of Ba and K and a positive anomaly in Sr. The amphibolites have the same negative anomalies. The positive anomaly of Sr may thus be explained by accumulation of plagioclase, the second most abundant mineral of these rocks or, along with Ba anomaly, might result of element mobility by later Variscan contact metamorphism.
3. Further on Isotopic data
Isotopic analysis of Rb – Sr and Sm – Nd were performed in selected samples. Both rocks types occur within the Douros- Beiras Supergroup, and thus have a lower Cambrian minimum age. Considering the intrusive character of the metagabbros, initial isotope ratios were calculated assuming 480 Ma as the age of magmatic cystallization. However, given the large half-life periods of both isotopic systems, even a 50 Ma age variation would not affect significantly the obtained results.
On the 87Sr/86Sr
i vs εNdi diagram (Fig. 5) we
can assess the affinity of the studied samples to different geochemical reservoirs. For 480 Ma, the 87Sr/86Sr ratios
106 respectively, 0.7040 - 0.7051 and 0.7052 - 0.7065. Regarding εNd, the values range between +5.8 - +7.7 and +4.6 - +6.2 for metagabbros and amphibolites, respectively.
Fig. 5 – 87Sr/86Sr vs εNd diagram. The studied samples were recalculated for 480 Ma and the metabasic rocks of Villaseca et al. (2015) for 475
Ma.
The positive (always higher than +4) values of εNdi in the studied samples shows that
both lithological groups are derived from depleted mantle domains, i.e. with low Sm/Nd ratios. Such depletion is more notorious for metagabbros. The highly positive values of εNd are similar to those of MORB. The dispersion of 87Sr/86Sr is
most likely due to Variscan metamorphism, since both Rb and Sr are mobile.
4. Conclusion
The analysed metagabbros and amphibolites are not cogenetic but both correspond to tholeiites not related with subduction. Both their elemental and isotopic compositions are compatible with an extensional environment. Although there is no independent data pointing to the existence of an oceanic crust in the region, we emphasize that the amphibolites and metagabbros have affinity to E-MORB and N-MORB, respectively.
Thus, data allow to consider the hypothesis of rifting in this setting, since the samples are South of the Porto – Viseu Metamorphic Belt, considered as the probable limit between the Douro and Beiras’ Basins, which may have evolved separatedly (Villaseca et al., 2014).
If we assume the amphibolites as interstratified into the sedimentary sequence and the gabbros intrusives on it, the increasing sub-alcalinity of magmas
point to a progressively stretched/thinner crust during a process of continental rift. On this perspective, the studied rocks and those described by Villaseca et al. (2015) should represent the initial stages of the extensional process that formed the Rheic Ocean.
Acknowledgements
This abstract was supported by FCT - Project UID/GEO/50019/2019 - Instituto Dom Luiz.
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