Cretaceous Research 30 (2009) 575–586 Contents lists available at ScienceDirect Cretaceous Research journal homepage: www.elsevier.com/locate/CretRes Age constraints on the Late Cretaceous alkaline magmatism on the West Iberian Margin Rui Miranda a,b,d,*, Vasco Valadares c,a, Pedro Terrinha c,b, Joa˜o Mata a,d, Maria do Rosa´rio Azevedo e, Miguel Gaspar a,f, Jose´ Carlos Kullberg g, Carlos Ribeiro h a Fac. de Cieˆncias da Univ. de Lisboa, Depto. Geologia , Campo Grande, 1749-016 Lisboa, Portugal b LATTEX, IDL, Univ. de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal c INETI, Depto. Geologia Marinha, Estrada da Azambuja, 2720-866 Amadora, Portugal d Centro de Geologia da Univ. de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal e Depto. Geocieˆncias da Univ. de Aveiro, Santiago Campus, 3810-003 Aveiro, Portugal f CREMINER, Campo Grande, 1749-016 Lisboa, Portugal g Depto. Cieˆncias Terra, Fac. Cieˆncias Tecnologia., Univ. Nova de Lisboa and CICEGe Quinta da Torre, 2829-516 Caparica, Portugal h Depto. Geocieˆncias Univ. E´vora and Centro de Geofı´sica de E´vora, Rua Roma˜o Ramalho, 59, 7000 E´vora, Portugal article info abstract Article history: The onshore sector of the West Iberian Margin (WIM) was the locus of several cycles of magmatic activity Received 21 November 2007 during the Mesozoic, the most voluminous of which was of alkaline nature and occurred between 70 and Accepted in revised form 100 Ma. This cycle took place in a post-rift environment, during the 35 counter-clockwise rotation of 13 November 2008 Iberia and initiation of the alpine compression. It includes the subvolcanic complexes of Sintra, Sines, and Available online 30 November 2008 Monchique, the volcanic complex of Lisbon and several other minor intrusions, covering an area of approximately 325 km2. Previous cycles were tholeiitic and transitional in nature, occuring around Keywords: 200 Ma and 130–135 Ma, respectively. Geochronology 40 39 Alkaline magmatism New LA-ICP-MS U-Pb, Ar/ Ar, K-Ar and Rb-Sr ages on several intrusions distributed along the onshore West Iberian Margin WIM are presented, which combined with previously published data allows us to constrain the duration Late Cretaceous of the Late Cretaceous alkaline cycle to circa 22 Ma (94–72 Ma) and define two pulses of magmatic activity. The first one (94–88 Ma) occurred during the opening of the Bay of Biscay and consequent rotation of Iberia and clusters above N38200. The second pulse (75–72 Ma) has a wider geographical distribution, from N37 to N39. This final pulse occurred during the initial stages of the Alpine orogeny in Iberia that led to the formation of the Pyrenees and Betics and to tectonic inversion of the Mesozoic basins. Isotope and trace element geochemistry point to a sublithospheric source for the alkaline magmatism that clearly distinguishes it from the previous cycles which had an important lithospheric mantle component. Also, it allows the discrimination between the two different alkaline pulses in terms of trace element abundance and residual mantle minerology. It is speculated that these differences might be the result of distinct magma ascent rates due to either more or less favourable tectonic settings that avoided or allowed the interaction with metasomatized lithosphere and equilibration with K rich minerals like amphibole and/or phlogopite. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction 130–135 Ma (Ferreira and Macedo, 1979), respectively. The last cycle was the most voluminous, shows an alkaline nature, and took The onshore sector of the West Iberian Margin (WIM) was the place between 70 and 100 Ma (Ferreira and Macedo, 1979). locus of several cycles of magmatic activity during the Mesozoic. This cycle includes the NNW-SSE aligned subvolcanic complexes Occurrences related to the first two cycles display tholeiitic and of Sintra, Sines, and Monchique, the volcanic complex of Lisbon and transitional affinities (Martins, 1991; Martins et al., 2008) and ages several other minor intrusions (Fig. 1). These rocks are discontin- around 200 Ma (e.g. Dunn et al., 1998; Verati et al., 2007) and uously exposed from parallels 39 Nto37 N and cover an area of approximately 325 km2 (Fig. 1). With the exception of the Mon- chique alkaline complex, all the alkaline rocks were emplaced * Corresponding author. within the Mesozoic Lusitanian and Algarve rift basins, developed E-mail address: [email protected] (R. Miranda). in relation to the opening of the Atlantic. 0195-6671/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.cretres.2008.11.002 576 R. Miranda et al. / Cretaceous Research 30 (2009) 575–586 Fig. 1. The Late Cretaceous alkaline magmatism in the onshore sector of the West Iberian Margin. Circles indicate all of the sampled intrusions. Portuguese Geology and batimetry adapted from data available at http://www.iambiente.pt/atlas/est/index.jsp. Spanish geology adapted from the 1:1000000 geological map of the Iberian peninsula (Alvaro et al., 1994). Batimetry contours every 100 m. Geographic coordinates, WGS 84. The Cretaceous alkaline rocks were grouped along with other Ribeiro et al. (1979) suggested the massifs were emplaced along smaller intrusions from the Pyrenees in the Late Cretaceous Alka- a dextral strike-slip fault during the rotation of Iberia and Mouge- line Igneous Province of Iberia (Rock, 1982). Despite their older age, not (1980) proposed that the massifs intruded along crustal pull- Lustrino and Wilson (2007) included these occurrences in the apart strain domains between en echelon faults during tectonic Circum-Mediterranean Cenozoic Anorogenic Igneous Province. inversion of the WIM. Terrinha (1998) and Kullberg (2000) showed During the Late Cretaceous, the offshore sector of the WIM and the existence of Early-Middle Jurassic age syn-rift faults along the adjacent oceanic crust were also the site of important alkaline lineament and proposed that these faults controlled the magma magmatic activity that produced a series of seamounts like the ascent in the Late Cretaceous. Ormonde peak of the Gorringe Bank (Auzende et al., 1978; Cornen, The main goal of this work is to better constrain the chronology 1982; Fe´raud et al., 1982; Bernard-Griffiths et al., 1997) and the of emplacement of the onshore Late Cretaceous intrusions of the Northern section of the Madeira-Tore rise (Geldmacher et al., 2006; WIM by presenting new LA-ICP-MS U-Pb, 40Ar/39Ar, K-Ar, and Rb-Sr Merle et al., 2006). ages. These data, combined with new geochemical analyses and The alignment of the Sintra, Sines, and Monchique Late Creta- other geological information, bring new insights on the tectono- ceous alkaline massifs has been a matter of debate in the past. magmatic processes of the WIM during the Late Cretaceous. R. Miranda et al. / Cretaceous Research 30 (2009) 575–586 577 2. Geological setting 3. The Late Cretaceous alkaline cycle in the WIM: previous geochronological work In the Mesozoic Lusitanian and Algarve Basins, the last phases of rifting took place during the late Early Cretaceous (Rasmussen et al., The Late Cretaceous alkaline igneous activity in the onshore 1998; Terrinha, 1998) while the initiation of seafloor spreading in section of the WIM took place in a post rift setting, 30 My after the the Tagus an Iberian abyssal plains started at 133 Ma–128 Ma beginning of oceanization in the Tagus Abyssal Plain and, partially (Pinheiro et al., 1996; Russell and Whitmarsh, 2003)(Fig. 2). The contemporaneous with the Pyreneean continental collision in tectonic inversion of these basins was a consequence of the Northern Iberia and the onset of tectonic inversion on the Mesozoic contemporaneous onset of rapid collision and even subduction basins (Fig. 2), thus making the Late Cretaceous alkaline magma- between the Iberian, European, and African plates. This inversion tism a key episode in the history of this passive margin. These rocks started in the Late Cretaceous (Mougenot, 1980; Terrinha, 1998; have been object of several previous dating attempts, summarized Rosembaum et al., 2002) but reached its climax during the Miocene in Table 1. in the Lusitanian Basin and offshore the WIM (Ribeiro et al., 1990; As shown in Table 1, the chronology of the main magmatic Kullberg et al., 2000; Neves et al., in press). However, the Algarve events associated with the Late Cretaceous alkaline cycle has been Basin experienced the compressive effects of the Africa-Iberia established on the basis of a large number of K-Ar ages and Rb-Sr convergence mainly during the latest Cretaceous through Oligo- whole rock isochrons. These two isotopic systems are extremely cene-early Miocene times (Terrinha, 1998). sensitive to post-emplacement disturbance which may explain, at The opening of the Bay of Biscay and consequent rotation of least to some extent, the significant scatter of the published Iberia started between magnetic anomalies M3 and M0 (130 and geochronological data (sometimes exceding 15 Ma for the same 118 Ma, respectively) and ended at magnetic chron A33o (80 Ma) occurrence using one single method, Table 1). The use of more (Sibuet et al., 2004)(Fig. 2). Based on paleomagnetic data, this reliable dating techniques (e.g. incremental release 40Ar/39Ar dating counter-clockwise rotation of Iberia is estimated at about 35 (e.g. and/or U-Pb zircon geochronology) is therefore fundamental to Storetvedt et al., 1987; Galdeano et al., 1989; Moreau et al., 1997; obtain precise age constraints. In order to refine the geochronology Juarez et al., 1998; Ma´rton et al., 2004). It created a sinistral of the Late Cretaceous alkaline cycle, and if possible, detect any transtensive regime along the North Pirenaic Fault, synchronous geochemical variation with age and/or location, several of the with high temperature/low pressure metamorphism, formation of exposed occurrences were selected for further geochronological new sedimentary basins and magmatic activity.