Palaeomagnetism and isotopic age data from Upper Cretaceous igneous rocks of W. Portugal; geological correlation and plate tectonic aspects

Abstract

Summary Palaeomagnetic results and K–Ar age data for the Sintra and Sines intrusive complexes (W. Portugal), and further details on the palaeomagnetic structure of the Lisbon volcanics are reported. The Sintra complex consists of two main intrusive phases having been emplaced in the Upper Cretaceous at around 90 Ma and 75 Ma respectively. The radiometric results show that the Sines complex formed concurrently with the second Sintra magmatism. The early (main) Sintra pluton has a characteristic magnetization of D = 358°, I = 27° (α95= 3.3°). This remanence direction is defined by gabbros and diorites as the granitic rocks (constituting the bulk of the complex) are shown to possess stable secondary magnetization imposed during Quaternary weathering. The characteristic magnetization of the younger intrusive event, as defined by the Sines rocks, has a mean direction of D = 041°, I = 41° (α95= 3.3°). These two Upper Cretaceous palaeomagnetic directions are significantly different at the 95 per cent probability level. The Lisbon volcanics show the presence of the ‘75 Ma’ magnetization suggesting that also this volcanic complex dates from the Upper Cretaceous. In addition, the palaeomagnetic results from the Lisbon complex have given further substance to a previously reported magnetization component with shallow inclination and north-northwest declination, now defined by D = 333° I = 14° (α95= 7.4°). It is inferred that this magnetization which has a dual-polarity structure formed through low temperature oxidation in late Cretaceous to Lower Tertiary time. The declination difference between the ‘90 Ma’ and ‘75 Ma’ magnetizations may be interpreted in terms of a two-phase rotation of Iberia in the Upper Cretaceous prior to the assumed late Cretaceous–early Tertiary secondary magnetic imprint, implying a counterclockwise rotation of minimum 40° followed by a clockwise rotation of similar magnitude. However, if the relevant palaeomagnetic reference frame for Iberia, before and after its independent tectonic movements in the late Cretaceous, is the African plate, then the clockwise rotation is about 20° larger than the counterclockwise one, apparently accounting for the compressive tectonism in the Bay of Biscay.