Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/185 Comunicações Geológicas (2014) 101, Especial III, 1449-1453 IX CNG/2º CoGePLiP, Porto 2014 ISSN: 0873-948X; e-ISSN: 1647-581X

The Central Atlantic Magmatic Province (CAMP) in Portugal, high eruption rate in one short-lived volcanic pulse A província magmática do Atlântico Central (CAMP) em Portugal, alta taxa eruptiva num curto pulso vulcânico S. Fernandes1*, E. Font1, M. Neres2, L. Martins3, N. Youbi3,4, J. Madeira3, A. Marzoli5

© 2014 LNEG – Laboratório Nacional de Geologia e Energia IP

Abstract: The Central Atlantic Magmatic Province (CAMP) is one of the largest igneous provinces of the Phanerozoic and its formation is believed to be coeval with the biological crisis of the Triassic-Jurassic boundary (~201Ma). Other coeval lavas have been extensively studied in the United States and Morocco, however relatively little attention has been given to the CAMP lavas from the south of Portugal for which geochemical data have been recently published. Here we provide new rock magnetic and magnetostratigraphic data on the CAMP lavas cropping out in the Algarve basin, Southern Portugal, in order to better constrain their age and eruptions rates. Despite severe superficial alteration that characterizes the area, the selected rocks preserved their primary magnetic mineralogy, represented by an assemblage of fine grained Ti-poor titanomagnetite and accessorily hematite. After cleaning by alternating field, all samples show normal (positive) characteristic remanent magnetization confirming the absence of any negative polarity that might correspond to the well-known E23r subchron. VGP’s directions vary slightly within the lava pile (>5 m) suggesting quite rapid eruptions and a single magmatic pulse. Keywords: CAMP, Iberia, Magnetostratigraphy, Rock-magnetism, Triassic-Jurassic Boundary. Resumo: A província magmática do Atlântico Central (CAMP) é uma das maiores províncias magmáticas do Fanerozóico e pensa-se ser contemporânea da crise biológica do Triássico-Jurássico (200 Ma). As lavas da CAMP têm sido extensivamente estudadas nos Estados Unidos e em Marrocos, mas pouca importância tem sido dada às lavas da CAMP que afloram no Sul de Portugal para as quais recentes dados geoquímicos foram publicados. Neste trabalho, novos dados de mineralogia magnética e dados magnetostratigráficos são fornecidas das lavas da CAMP que afloram na bacia do Algarve, com o propósito de melhor se constranger a sua idade e taxas de erupção. Os resultados mostram que apesar da forte oxidação que afecta a maioria das amostras da bacia do Algarve, a magnetização remanescente é estável nas amostras selecionadas e é essencialmente portada por grãos finos de titanomagnetite e acessoriamente hematite. Todas as amostras submetidas à desmagnetização em campo alternado (AF) apresentam padrões de desmagnetização estáveis, todas elas com polarização normal (positiva) comprovando a inexistência da E23r sugerida em estudos anteriores nas lavas de Marrocos. Os dados magnetostratigráficos sugerem taxas de erupção relativamente rápidas. Palavras-chave: CAMP, Ibéria, Magnetostratigrafia, Magnetismo de rocha, Crise do Triássico-Jurássico. 1IDL-UL, Instituto Dom Luís, Faculdade de Ciências da Universidade de Lisboa, Portugal.

2IPMA, Av. Brasília 1449-006 Lisboa, Portugal. 3Departamento de Geologia da Faculdade de Ciências da Universidade de Lisboa, Portugal. 4Department of Geology, Cadi Ayyad University, Marrakech, Morocco. 5Dipartimento di Geoscienze, Università degli Studi di Padova, via Gradenigo 6, 35131 Italy. *Corresponding author / Autor correspondente: su05_89@hotmail.com

1. Introduction

The Central Atlantic Magmatic Province (CAMP) is one of the largest Phanerozoic continental Large Igneous Provinces (LIP) and is apparently coeval with one of the five most severe mass extinctions of the Phanerozoic, the end-Triassic Mass Extinction (ETME) 201 Ma ago (Deenen et al., 2010; Marzoli et al., 2004; Ruhl et al., 2010). The synchronism between the CAMP onset and the ETME has been established indirectly, by comparison of radio-isotopic (Verati et al., 2007), magnetostratigraphic (Font et al., 2011; Knight et al., 2004) and chemostratigraphic (carbon isotope composition; (Deenen et al., 2010)) data of lavas with marine/continental sections. Previous works on CAMP lavas from Morocco suggested that CAMP volcanism preceded the Tr- J boundary and thus could be responsible for the associated biotic crisis (Knight et al., 2004). Such hypothesis is based on the presence of short geomagnetic reversals recorded in the intermediate unit of the Moroccan CAMP lava and correlated to the E23r anomaly of the Newark basin, USA (Olsen & Kent, 1999) and on palynological data suggesting an end-Triassic onset of CAMP volcanism in Morocco and Canada (Cirilli et al., 2009; Marzoli et al., 2004). However, the primary origin of these reversals has been recently challenged and proved to be the product of remagnetization and hydrothermal processes (Font et al., 2011).

Here, we conduct a new rock magnetic and paleomagnetic study on CAMP lava flows from the Algarve basin in order to: i) verify if the magnetic mineralogy is preserved and suitable for paleomagnetic investigation, ii) check for the presence of any geomagnetic reversal as possible chronological marker and

Artigo Curto Short Article

1450 S. Fernandes et al. / Comunicações Geológicas (2014) 101, Especial III, 1449-1453

infer possible flows and eruption rates. To reach these objectives, approximately 300 samples were collected from nine sites distributed along the E-W South Portugal margin. However, only seven paleomagnetic sites gave reliable magnetostratigraphic data. Ayamonte (AY) and Rio de Alte (RL) were disregarded due to unreliable paleomagnetic data and dubious structural orientations (Fig. 1).

Fig. 1. A) Sketch map showing the paleo-position and distribution of CAMP volcanism worldwide (modified from Font et al., 2014); B) Simplified geological map of Iberia showing the representative CAMP lavas in Messejana dyke (modified from Martins et al., 2008); C) Sampling locations in Southern Portugal (modified from Martins et al., 2008). Fig. 1. A) Mapa da distribuição e paleo-posição do vulcanismo da CAMP no mundo (modificado de Font et al., 2014); B) Mapa geológico simplificado da Ibéria mostrando as lavas da CAMP representadas essencialmente pelo dique da Messejana (modificado de Martins et al., 2008); C) Locais de amostragem no Sul de Portugal (modificado de Martins et al., 2008).

2. Geological setting and sampling

The Central Atlantic Magmatic Province (CAMP) is one of the largest flood basalt provinces known in the Phanerozoic, with an average volume greater than 2×106 km3(Marzoli et al., 1999). It is associated with the breakup of the Pangea supercontinent that precludes the opening of the Atlantic Ocean. CAMP volcanism is represented by continental flood basalts, sills and dykes, which crop out across four continents: North and South America, Northwestern Africa and Southwestern Europe (Fig. 1A). In Portugal, CAMP volcanism is mainly represented by the Messejana dyke which actually correspond to the unique high-quality paleomagnetic pole for Iberia at around 200 Ma (Ortas et al., 2006). CAMP volcanism is also expressed by lava flows cropping out in the Algarve and Santiago do Cacém basins for which paleomagnetic data are still lacking.

The Algarve and Santiago do Cacém basins comprises a basal sequence of continental sediments, namely siltstones and limestones of Hettangian age, capped by CAMP continental lava flows, pyroclastic deposits and peperites (Martins et al., 2008). Comparison with the Moroccan sequence suggests that Algarve basin outcrops are relatively homogeneous and similar to the Moroccan intermediate and upper unit (Marzoli et al., 2004; Rapaille et al., 2002). Recent 40Ar/39Ar data on the Portuguese volcanic units yields an age close to the Tr-J boundary

with a mean value of 198.1 ± 0.4 Ma (Verati et al., 2007). This age is correlated to the recurrent unit of the Moroccan basins and suggests a synchronous rifting and volcanism on both Southwestern Europe and Northeastern Africa (Marzoli et al., 2004; Verati et al., 2007).

Samples were collected along the E-W transect in the Algarve basin using a gasoline powered rock drill (Fig. 1C). All samples were oriented on the field by a magnetic and solar compass. We also measured structural orientations (magmatic walls and joints) of the lava flows and, when possible, the underlying sediments.

3. Methods

All samples were progressively demagnetized with an alternating field (AF) using a LDA-3A demagnetizer (AGICO). At each demagnetization step, the remanence was measured with a JR6 spinner magnetometer.

Principal Component Analysis (Kirschvink, 1980) and Fisher statistic (Fisher, 1953) were used to calculate the direction of the Characteristic Remanent Magnetization (ChRM) using the REMASOFT software (Chadima & Hrouda, 2006). Representative samples, previously demagnetized, were submitted to a progressive uniaxial and constant magnetic field up to 1 T at constant temperature, using a pulse magnetizer (ASC Scientific IM-10-30). Data were subsequently treated by the cumulative log-Gaussian function by using the Kruiver et al. (2001) software.

Thermomagnetic curves were performed in an Argon controlled atmosphere using a CS-L cryostat apparatus for the low temperature measurements and a CS4 furnace for the high temperature measurements in a MFK1 Kappabridge (AGICO). Data were treated by the Cureval 8.0 (AGICO) software. Curie temperatures were determined by the inverse susceptibility method (Petrovský & Kapricka, 2006).

4. Rock magnetism Raw IRM curves conducted on more than 30 samples, for which two representative samples are illustrated at figure 2. Approximately 90% of the saturation is reached at fields below 100 mT. After unmixing raw IRM curves by using the CLG function ( Robertson & France, 1994; Kruiver et al., 2001), results show the dominance of a low coercivity phase (<0.3 T). This low coercivity phase is present in all measured samples and is interpreted to be magnetite. Few samples also display the presence of a higher coercive phase with B1/2 values up to 250 mT, which is thought to be hematite (Fig. 2).

Under low temperature (LT) thermomagnetic curves, all samples display the Verwey transition at about -150ºC, characteristic of structural changes of magnetite (Dunlop and Özdemir 1997, Özdemir et al., 1993). Under high temperature (HT) treatment most samples are characterized by the presence of a single magnetic phase with Curie temperatures ranging from 554ºC to 577ºC which is interpreted to be Ti-poor titanomagnetite in agreement with the presence of the Verwey transition (Fig.

Paleomagnetism of the CAMP in Algarve, Portugal 1451

2). Some samples also display a second magnetic phase, characterized by a hump at ~240ºC to 320ºC during the heating process and no longer observed on the cooling process (Fig. 2). These more weathered samples likely contain titanomaghemite, which seems to be the result of low-temperature oxidation of titanomagnetite (Dunlop & Özdemir, 1997).

Fig. 2. A) Isothermal remanent magnetization (IRM) acquisition treated by cumulative log-Gaussian function (Kruiver et al., 2001) showing the dominance of a low to medium coercive phase; B) Thermomagnetic susceptibility curves at low and high temperatures showing magnetite as principal magnetic carrier. Fig. 2. A) Aquisição de magnetização remanescente isotérmica (MRI) tratada por uma função log- Gaussiana cumulativa (Kruiver et al., 2001) mostrando a dominância de uma fase de coercividade média a baixa; B) Curvas de susceptibilidade termomagnética a temperaturas baixas e altas mostrando a magnetite como o principal portador magnético.

5. Magnetostratigraphy

A total of 231 specimens were demagnetized using AF treatment for which 221 samples yield stable demagnetization patterns and were used to calculate the characteristic magnetic component (ChRM). Samples carry a natural remanent magnetization varying between 0.4 and 13.5 A/m (Fig. 3). Most of the remanence was removed below 30-40 mT, suggesting a low to medium coercive phase as the dominant magnetic carrier. Zijderveld diagrams suggest the presence of two magnetic components: a viscous component, successfully removed with fields lower than 20 mT, and a high field component reaching the origin (Fig. 3). All samples display a positive (normal) remanent magnetization, confirming previous magnetostratigraphic data conducted on CAMP’s lava worldwide (Kent et al., 1999; Deenen et al., 2011; Font et al., 2011). Considering the recent radio-isotopic ages conducted on Algarve outcrops (Verati et al., 2007), the positive interval found in all samples is correlated to the E24n chron. Mean VGP’s were calculated for each cylinder for all seven paleomagnetic sites (Fig. 4). Mean VGP’s latitude per unit with a95 higher than 10º were disregarded and not taken into account for the mean paleomagnetic direction of the site (Chenet et al., 2008). Paleomagnetic directions have statistically identical magnetic directions within sites (σ<10º and <10º) except for Hortas do Tabual (HT) where we can clearly distinguish two different paleomagnetic directions (Fig. 4).

Fig. 3. Paleomagnetic results for two representative samples. Stereographic, orthogonal projections and remanence intensity versus AF demagnetizing field plots show well-defined bimodal components carried by a low coercive phase (magnetite). Fig. 3. Resultados paleomagnéticos para duas amostras representativas. Projeções estereográficas, ortogonais e um gráfico de intensidade de remanescência vs campo de desmagnetização AF mostram a presença de componentes bimodais bem definidas portados por uma fase coerciva baixa (magnetite).

6. Discussion and conclusion Magnetic mineralogy results on CAMP lavas from South Portugal suggest that despite the severe superficial alteration that affected the Algarve region, the original magnetic mineralogy of the selected basalt has been preserved, and is represented by fine Ti-poor titanomagnetite and accessorily titanomaghemite and hematite. Also, stable demagnetization patterns at high AF demagnetization fields, together with well-defined magnetic directions significantly distinct from the present-day field (or any younger magnetization) are suggestive of primary magnetization (Fig. 3).

Magnetostratigraphy has played an important role in the definition of CAMP and its link with the Triassic-Jurassic mass extinction. The main problematic resides in the difficulty to date precisely the lava flows and to correlate their ages with that of the Triassic-Jurassic boundary recorded in marine sections (Ruhl et al., 2010). In Morocco, Knight et al. (2004) found a negative polarity in the Intermediate unit of the Moroccan CAMP lavas, correlated by the authors to the E23r anomaly of the Newark basin (Kent & Olsen, 1999), which precedes the Triassic-Jurassic boundary by ~20 Kyr (Deneen et al., 2010). However, recent studies on these Moroccan lavas demonstrated that the negative polarities found in the Intermediate unit are a result of chemical remagnetization (Font et al., 2011). Here, we show that CAMP lavas from Portugal also carry a positive magnetization, which may be correlated to the E24n chron. This correlation seems to be supported by the nearly indistinguishable 40Ar/39Ar ages

1452 S. Fernandes et al. / Comunicações Geológicas (2014) 101, Especial III, 1449-1453

for the CAMP in Portugal (Verati et al., 2007) and in the U.S.A. basins (Marzoli et al., 2011). However, the magmatic history of CAMP in Portugal is badly known and the basaltic sequences are rarely complete in the field.

Despite these limitations, we attempt to evaluate the number of volcanic pulses in the studied sections by using i) morphological, ii) sedimentological and iii) magnetic criteria. Morphologically, a single lava unit is defined as a cooling unit bound by quenched margins on all sides (Self et al., 1998). In Morocco, basalt flow show the typical lava core and lava core sequence which is not observed in Portugal. Here, lavas are built by thick and massive basalts. From a sedimentological point of view, magmatic flows can also be distinguished by the presence of interbedded sedimentary layers and paleosols which represent minor periods of volcanic quiescence. In Portugal, there is no evidence of the existence of paleosols or sedimentary layers interbedded within the lavas, suggesting the presence of a single volcanic pulse, except

for Hortas do Tabual (HT) where a thin dolomite layer is observed (Fig. 4). From a magnetic point of view, it has been proved that successive lava units with undistinguishable paleomagnetic directions, (directional group, DG) have not extend over a sufficient time to record secular variation which is a robust indicator of a single eruptive event (Knight et al., 2004). Our data suggest homogeneous paleomagnetic directions within sites except for HT where two directional groups can be distinguished. Interestingly, lava thickness in HT is considerably thinner than in the central and eastern sector of the Algarve basin that can reach 30-50 meters of massive lavas (Sao Bartolomeu, SB) apparently of the same magmatic pulse. We weren’t able to calculate eruption rates due to the difficulty to infer the geometry of lavas deposition probably due to the strong oxidation and regional movements. However, the very weak VGP’s latitude variation strongly suggests quite rapid and massive eruption in the central part of the Algarve basin.

Fig. 4. Magnetostratigraphic data of the studied sections: all sequences carry a positive magnetization assigned to correspond to E24n chron except for Hortas do Tabual where two lava flows are separated by a dolomitic layer. Undistinguished VGP’s latitude variation suggests a rapid and single eruptive event. Fig. 4. Dados magnetostratigráficos das secções estudadas: todas as sequências são portadoras de uma magnetização positive correlacionada com o crone E24n com a excepção de Hortas do Tabual onde existem duas sequências de lava separadas por uma fina camada de dolomites. As variações de VGP’s por unidade são praticamente indistinguíveis o que sugere um único e rápido evento eruptivo.

Paleomagnetism of the CAMP in Algarve, Portugal 1453 Acknowledgments

This work was supported by FCT (PTDC/CTEGIX/110205/2009 and PTDC/CTE_GIX/110205/2010) and IDL (Pest-OE/CTE/LA0019/2011-IDL). We thanks Celia Lee and Ana Sousa for administrative supply.

References Cirilli, S., Marzoli, A., Tanner, L., Bertrand, H., Buratti, N., Jourdan,

F., Renne, P.R., 2009. Latest Triassic onset of the Central Atlantic magmatic province (CAMP) volcanism in the Fundy basin (Nova Scotia): new stratigraphic constraints. Earth and Planetary Science Letters, 286, 514-525.

Deenen, M.H.L., Ruhl, M., Bonis, N.R., Krijgsman, W., Kuerschner, W.M., Reitsma, M., van Bergen, M.J., 2010. A new chronology for the end-Triassic mass extinction. Earth and Planetary Science Letters, 291(1-4), 113-125.

Deenen, M.H.L., Krijgsman, W., Ruhl, M., 2011. The quest for chron E23r at Partridge Island, Bay of Fundy, Canada: CAMP emplacement postdates the end-Triassic extinction event at the North American craton. Canadian Journal of Earth Sciences, 48, 1282-1291.

Dunlop, D.J., Özdemir, Ö.,1997. Rock Magnetism: Fundamentals and Frontiers. Cambridge University Press, New York, London and Cambridge, 573 p.

Fisher, R., 1953. Dispersion on a sphere. Proceedings of the Royal Society of London Series a-Mathematical and Physical Sciences, 217(1130), 295-305.

Font, E., Fabre, S., Nédélec, A., Adatte, T., Keller, G., Veiga-Pires, C., Ponte, J., Mirão, J., Khozyem, H., Spangenberg, J., 2014. Atmospheric halogen and acid rains during the main phase of Deccan eruptions: Magnetic and mineral evidence. In: G. Keller, A.C. Kerr, (Eds). Volcanism, Impacts, and Mass Extinctions:Causes and Effects. Geological Society of America Special Paper, 505, 353–368.

Font, E., Youbi, N., Fernandes, S., El Hachimi, H., Kratinova, Z., Hamim, Y., 2011. Revisiting the magnetostratigraphy of the Central Atlantic Magmatic Province (CAMP) in Morocco. Earth and Planetary Science Letters, 309(3-4), 302-317.

Kent, D.V., Olsen, P.E., 1999. Astronomically tuned geomagnetic polarity timescale for the Late Triassic. Journal of Geophysical Research-Solid Earth, 104, 12831-12841.

Kirschvink, J.L., 1980. The least-squares line and plane and the analysis of paleomagnetic data. Geophysical Journal of the Royal Astronomical Society, 62(3), 699-718.

Knight, K.B., Nomade, S., Renne, P.R., Marzoli, A., Bertrand, H., Youbi, N., 2004. The central Atlantic magmatic province at the Triassic-Jurassic boundary: paleomagnetic and Ar-40/Ar-39 evidence from Morocco for brief, episodic volcanism. Earth and Planetary Science Letters, 228(1-2), 143-160.

Kruiver, P.P., Dekkers, M.J., Heslop, D., 2001. Quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation. Earth and Planetary Science Letters, 189(3-4), 269-276.

Martins, L.T., Madeira, J., Youbi, N., Munha, J., Mata, J., Kerrich, R., 2008. Rift-related magmatism of the Central Atlantic magmatic province in Algarve, Southern Portugal. Lithos, 101(1-2), 102-124.

Marzoli, A., Bertrand, H., Knight, K. B., Cirilli, S., Buratti, N., Verati, C., Bellieni, G., 2004. Synchrony of the Central Atlantic magmatic province and the Triassic-Jurassic boundary climatic and biotic crisis. Geology, 32(11), 973-976.

Marzoli, A., Jourdan, F., Puffer, J.H., Cuppone, T., Tanner, L.H., Weems, R.E., Bertrand, H., Cirilli, S., Bellieni, G., De Min, A., 2011. Timing and duration of the Central Atlantic magmatic province in the Newark and Culpeper basins, eastern U.S.A. Lithos, 122, 175-188.

Marzoli, A., Renne, P. R., Piccirillo, E.M., Ernesto, M., Bellieni, G., De Min, A., 1999. Extensive 200-million-year-old continental flood basalts of the Central Atlantic Magmatic Province. Science, 284(5414), 616-618.

Olsen, P.E., Kent, D.V., 1999. Long-period Milankovitch cycles from the Late Triassic and Early Jurassic of eastern North America and their implications for the calibration of the Early Mesozoic time-scale and the long-term behaviour of the planets. Philosophical Transactions of the Royal Society a-Mathematical Physical and Engineering Sciences, 357(1757), 1761-1786.

Ortas, A.P., Osete, M.L., Vegas, R., Silva, P., 2006. Paleomagnetic study of the Messejana Plasencia dyke (Portugal and Spain): A lower Jurassic paleopole for the Iberian plate. Tectonophysics, 420(3-4), 455-472.

Rapaille, C., Marzoli, A., Bertrand, H., Feraud, G., Reisberg, L., Fontignie, D., 2002. Geochemistry and age of the European CAMP basalts. Goldschmidt Conference Abstracts, A626.

Robertson, D.J., France, D.E., 1994. Discrimination of remanence-carrying minerals in mixtures, using Isothermal Remanent Magnetization acquisition curves. Physics of the Earth and Planetary Interiors, 82(3-4), 223-234.

Ruhl, M., Deenen, M.H.L., Abels, H.A., Bonis, N.R., Krijgsman, W., Kurschner, W.M., 2010. Astronomical constraints on the duration of the early Jurassic Hettangian stage and recovery rates following the end-Triassic mass extinction (St Audrie's Bay/East Quantoxhead, UK). Earth and Planetary Science Letters, 295(1-2), 262-276.

Self, S., Keszthelyi, L., Thordarson, T., 1998. The importance of pahoehoe. Annual Review of Earth and Planetary Sciences, 26, 81-110.

Verati, C., Rapaille, C., Feraud, G., Marzoli, A., Bertrand, H., Youbi, N., 2007. (40)Ar/(39)Ar ages and duration of the Central Atlantic Magmatic Province volcanism in Morocco and Portugal and its relation to the Triassic-Jurassic boundary. Palaeogeography, Palaeoclimatology, Palaeoecology, 244(1-4), 308-325.