Cachapuz, P.1*, Chichorro, M. 3,Bento dos Santos, T. 1,2, Carvalho, D.R. 1, Solá, A.R.4 1 FCUL - Faculdade de Ciências da Universidade de Lisboa, Dep.de Geologia, Campo Grande, C6, 1749-016 Lisboa, Portugal 2 IDL - Instituto Dom Luiz, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal,
3 FCT-UNL - Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portuga 4 LNEG – Laboratório Nacional de Energia e Geologia, 2720-866 Amadora, Portugal
* pedrofilipecachapuz@hotmail.com
Resumo: O complexo vulcânico de Viana do Alentejo apresenta rochas vulcânicas félsicas e máficas, geradas no Paleozoico inferior, tendo este estudo se focado na análise geoquímica das rochas máficas. Todas as 12 rochas analisadas apresentam valores de SiO2 entre 42 e 51 % e um caracter fortemente metaluminoso. O padrão do diagrama multi-elementar é em tudo semelhante ao padrão típico da crosta, sendo que a grande amplitude de valores da razão (La/Lu), bem como o padrão “em leque” para as LREE, sugere diferentes graus de contaminação crustal na génese destas rochas. Por sua vez, nos diagramas geotectónicos verifica-se que as amostras máficas analisadas formam uma sequência transicional entre um ambiente intra-placa (WPT) e um ambiente oceânico (OIT), evidenciando diferentes graus de enriquecimento em Th, Ti e P2O5.
Assim sendo pode-se afirmar que os dados geoquímicos obtidos nas rochas máficas deste complexo e a sua relação litosestratigráfica com magmatismo félsico e carbonatos evidenciam que tais rochas se terão gerado durante um episódio de rifting.
Palavras-chave: Ossa-Morena, rochas máficas, geoquímica, rifting.
Abstract: The Lower Paleozoic volcanic-sedimentary complex of Viana do Alentejo comprises a succession of mafic and felsic volcanic rocks, associated with clastic and carbonate sedimentary sequences, variably deformed and metamorphosed during variscan orogeny. In this study we will focus on the geochemical analysis of the metabasites. All 12 samples analyzed present SiO2 values between 42 and 51 wt% and a strongly metaluminous character. The pattern of the multi-element diagram is similar to the typical pattern of the crust, with large range of values of (La/Lu), as well as the “fan like” pattern for the LREE, suggesting different degrees of crustal contamination. In the geotectonic diagrams, the analysed mafic samples form a transitional sequence between a within-plate environment (WPT) and an oceanic environment (OIT), showing different degrees of enrichment in components such as Th, Ti and P2O5. The geochemical data and related lithostratigraphy (bimodal magmatism and carbonates) evidences that the protholiths of the metabasites were generated during Rift Stage.
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1.
Introduction
and
Geological
setting
Some of the felsic and basic volcanic successions of Viana do Alentejo, located in the Montemor-Ficalho Sector of the Ossa-Morena Zone (OMZ) in SW Iberia, are considered to be part of the “Série de Moura” (Carvalhosa, 1971), also known as Xistos de Moura (Carvalhosa, 1999) or Moura Phyllonitic Complex (Araújo, 1995; Araújo et al., 2005). The Xistos de Moura lithological sequence consist of imbricated micashists interlayered by acid and basic volcanic rocks, with few lidites, black shales and rare carbonated levels (Araújo et al., 2006). The entire sequence is strongly deformed, showing some levels of high- pressure metamorphic rocks, namely blue schists and eclogites. The Moura Phyllonitic Complex is currently considered to have an upper Ordovician to lower Devonian age (Piçarra, 2000). However, recent stratigraphic and geochemical studies, supported by U-Pb zircon geochronology carried out on adjacent domains, (Chichorro, 2006) suggest that the metasedimentary and metavolcanics representative of Viana do Alentejo can be included in the Lower Paleozoic successions integrated in Ficalho-Moura volcanic-sedimentary complex (Évora- Aracena metamorphic belt). This Lower Paleozoic volcanic-sedimentary complex first described by Ribeiro et al., (1992) and Carvalhosa and Zbyszwesky (1994) comprises a succession of mafic and felsic volcanic rocks variably deformed and metamorphosed during the Variscan Orogeny. The metavolcanic series are generally associated with clastic and follows carbonate sedimentary sequences that unconformably overlain the Ediacaran metasedimentary succession (Série Negra – SN).
In this study, we will focus on the geochemical analysis of the basic rocks of the Viana do Alentejo volcanic complex, with the main goal of understanding the geodynamic environment in which these rocks were formed.
2. Methods
Samples were prepared at DG-FCUL. The geochemical analyses were done in
Activation Laboratories (Ontario, Canadá), using standard procedures for ICP-OES analysis of major elements and ICP-MS of trace elements.
3. Whole rock Geochemistry
The 12 analysed samples show SiO2
values between 42 and 51 wt%, corresponding mostly to basalts in the TAS diagram (Middlemost, 1994; Fig. 1b), with only 2 samples being plotted in the picro- basalt field and one sample in the basaltic trachy-andesite field. All mafic samples are strogly metaluminous plotting around the boundary between the tholeiitic and the calc-alkaline series (Irvine & Baragar, 1971; Fig. 1a). These samples also form a linear trend in the Eu vs Hf diagram (Fig. 1c).
Fig. 1 – Classification of the metabasic rocks according to the AFM diagram (a), the TAS diagram
(b) and the Eu vs Hf ratios (c).
In both Primitive Mantle and N-MORB (Fig. 2a) normalized spidergrams, the analised samples display irregular patterns for the LILE, being slightly enriched in such elements, while the HFSE display a flat pattern, showing good agreement with the
N-MORB reference line. Both spidergrams also present a large positive anomaly of Pb and a slightly smaller Sr positive anomaly. Regarding the REE patterns (Fig. 2b), the analised samples show varying degrees of fractionation, with some samples presenting very steep patterns, with enrichment in LREE, while others display flat pattern. Some samples also show HREE enrichment compared to LREE (overall (La/Lu)n = 0.57 to 13.46).
The Ti-Zr diagram (Pearce, 1982) shows although almost every sample is plotted in the MORB field, with some of the samples being enriched in Ti and Zr, thus plotted in the MORB/WPL field, whereas others are plotted in the MORB/IAL field, as shown in Figure 3a. The Th-Hf-Nb diagram (Wood, 1980) also shows a transitional setting from N-MORB to E-MORB, similarly to data obtained by Pedro (2004).
Fig. 2 – Spidergrams normalized to the Primitive Mantle (McDonough & Sun, 1995) (a) and to the N-
MORB (McDonough & Sun, 1995) (b). c) - Chondrite-normalized (McDonough & Sun, 1995)
REE patterns.
4. Discussion and conclusions
The analysed metabasic rocks seem to derive from protholits with tholeiitic basalt compositions. Eu/Hf ratios suggests that
these rocks are cogenetic, as they form a clear linear trend (Fig. 1c). La/Lu ratios show a very large range. SiO2 vs La/Lu(Fig.
4), show that the samples with the largest and lowest La/Lu do not follow the trend displayed by the remaining samples. This suggests that fractional crystallization is not the cause for the the large variation of La/Lu ratios. However, two other scenarios are still possible: distinct degrees of crustal contamination or heterogeneity in the mantle source.
Both spidergrams present positive Pb anomalies and a negative P anomaly, which is typical of crustal contamination. Crustal influence in these rocks is indeed very plausible, but source heterogeneity and/or different degrees of partial melting cannot be excluded.
Fig. 3 – a) Ti-Zr diagram (Pearce, 1982), b) Th-Hf- Nb ternary diagram (Wood, 1980).
As for the geodynamic diagrams, all of them display different degrees of enrichment in elements such as Th and Ti, with Th being a great proxy for crustal contamination. This combined with all that was said above seems to indicate that our mafic samples were subjected to varying degrees of crustal contamination during emplacement. Although these rocks have not been dated yet, our data suggest that these cogenetic basic rocks were formed in
86 a geotectonic scenario controlled by extension in a continuously thinning crust. Such a rifting model had already been proposed in order to explain the genesis of other mafic rocks in the OMZ (e.g. Araújo et
al., 2005; Chichorro, 2006). Such tectonic
scenario controlled by transtension is corroborated by the Rift Stage Cambrian felsic magmatism, which is always spatially associated with the metabasites (Chichorro et al., 2008), as well as by the synchronous development of clastic and carbonate basin sedimentary sequences (Oliveira et al., 1991).
Fig. 4 – La/Lu vs SiO2 diagram References
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