Age constraints on the Late Cretaceous alkaline magmatism on the West Iberian Margin

Abstract

a b s t r a c tThe onshore sector of the West Iberian Margin (WIM) was the locus of several cycles of magmatic activity during the Mesozoic, the most voluminous of which was of alkaline nature and occurred between 70 and 100 Ma. This cycle took place in a post-rift environment, during the 35 counter-clockwise rotation of Iberia and initiation of the alpine compression. It includes the subvolcanic complexes of Sintra, Sines, and Monchique, the volcanic complex of Lisbon and several other minor intrusions, covering an area of approximately 325 km 2 . Previous cycles were tholeiitic and transitional in nature, occuring around 200 Ma and 130-135 Ma, respectively. New LA-ICP-MS U-Pb, 40 Ar/ 39 Ar, K-Ar and Rb-Sr ages on several intrusions distributed along the onshore WIM are presented, which combined with previously published data allows us to constrain the duration 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 N38 20 0 . 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.

Europe's rare earth element resource potential: An overview of REE metallogenetic provinces and their geodynamic setting

“…The Central Atlantic Magmatic Province, associated with rifting of the Pangaea supercontinent and formation of the central Atlantic Ocean, comprises extensive flood basalts erupted around the Triassic-Jurassic boundary at c. 200 Ma (Marzoli et al, 1999;Blackburn et al, 2013). As the ocean opened, subsequent late Cretaceous rifting cycles (100-70 Ma) emplaced alkaline magmas along the Iberian margin in Spain and Portugal and also in the Pyrenees (Solé et al, 2003;Miranda et al, 2009;Grange et al, 2010). REE mineralisation is not currently known in association with this alkaline magmatic phase, but there may be potential for further investigation.…”

The tectono-stratigraphic evolution of an Atlantic-type basin: an example from the Arrábida sector of the Lusitanian Basin

“…20) is about 8 m thick and can be seen from Praia do Seixalinho, to the south of Praia de Rebenta Bois. From a petrographic perspective, the sill consists of dolerite or, more precisely, a microgranular tephrite (Kullberg et al, 2006;Miranda et al, 2009). An 40Ar/39Ar isotopic age of 93.8 ± 3.9 Ma (Miranda et al, 2006(Miranda et al, , 2009) performed on amphibole (kaersutite) indicates a CenomanianTuronian age, revealing that this outcrop is the first known to the south of the Tagus River that may be correlated with the Mafra Complex, and is therefore part of the third alkaline magmatic cycle (Kullberg, 2000).…”

“…According to recent research (Miranda et al, 2009), the scarcity of the sedimentary record may be related to the occurrence of important magmatic activity between 100 and 60 Ma, as revealed by the existence of a number of different structures and massifs located fairly close to Arrábida: 1) the Mafra radial dyke complex, dated at ~100 Ma (~50 km to the NNW of Arrábida); 2) the Sintra Massif (around 40 km to the NW), dated at 85-72 Ma, and coeval with two other massifs to the south of Arrábida (the Sines and Monchique massifs); and 3) the Lisbon Volcanic Complex (25 km to the N), dated at ~60 Ma. Although smaller in size, some outcrops of magmatic rocks are found in Arrábida, which, as seen at Stop 7 (Foz da Fonte beach), are relevant to understanding the emplacement of those major intrusions.…”

Paleomagnetism of the Central Atlantic Magmatic Province in the Algarve basin, Portugal: First insights

“…Globally, two magmatic episodes affected the West Iberian Margin during the Cretaceous alkaline magmatic episodes (Miranda et al, 2009): the first one (94-88 Ma) occurred during the opening of the Bay of Biscay and consequent rotation of Iberia and clusters above N38 º 20 ´; the second pulse (75-72 Ma) has a wider geographical distribution, from N37 º to N39 º, and includes the Monchique intrusion (Fig. 1C).…”